Mohamed A.El-Nemr ,Ibrahim M.A.Ismail,2 ,Nabil M.Abdelmonem ,Ahmed El Nemr *,Safaa Ragab

1 Department of Chemical Engineering,Faculty of Engineering,Cairo University,Giza,Egypt

2 Renewable Energy Program,Zewail City of Science and Technology,Egypt

3 Environmental Division,National Institute of Oceanography and Fisheries,Kayet Bey,El-Anfoushy,Alexandria,Egypt

Keywords:Waste treatment Powder technology Citrullus lanatus Hexavalent chromium Biochar amination Adsorption

ABSTRACT Watermelon peel residues were used to produce a new biochar by dehydration method.The new biochar has undergone two methods of chemical modification and the effect of this chemical modification on its ability to adsorb Cr(VI)ions from aqueous solution has been investigated.Three biochars,Melon-B,Melon-BO-NH2 and Melon-BO-TETA,were made from watermelon peel via dehydration with 50% sulfuric acid to give Melon-B followed by oxidation with ozone and amination using ammonium hydroxide to give Melon-BO-NH2 or Triethylenetetramine(TETA)to give Melon-BO-TETA.The prepared biochars were characterized by BET,BJH,SEM,FT-IR,TGA,DSC and EDAX analyses.The highest removal percentage of Cr(VI)ions was 69%for Melon-B,98%for Melon-BO-NH2 and 99%for Melon-BO-TETA biochars of 100 mg·L?1 Cr(VI)ions initial concentration and 1.0 g·L?1 adsorbents dose.The unmodified biochar(Melon-B)and modified biochars(Melon-BO-NH2 and Melon-BO-TETA)had maximum adsorption capacities(Qm)of 72.46,123.46,and 333.33 mg·g?1,respectively.The amination of biochar reduced the pore size of modified biochar,whereas the surface area was enhanced.The obtained data of isotherm models were tested using different error function equations.The Freundlich,Tempkin and Langmuir isotherm models were best fitted to the experimental data of Melon-B,Melon-BO-NH2 and Melon-BO-TETA,respectively.The adsorption rate was primarily controlled by pseudo-second–order rate model.Conclusively,the functional groups interactions are important for adsorption mechanisms and expected to control the adsorption process.The adsorption for the Melon-B,Melon-BO-NH2 and Melon-BO-TETA could be explained for acid–base interaction and hydrogen bonding interaction.

1.Introduction

Heavy metals are naturally occurring compounds usually present in trace amounts in waters but most of them are toxic at very low contaminations[1].However,various anthropogenic activities such as industrialization and urbanization are introducing heavy metals into the environment[2].Increasing of heavy metals concentration in our resources is becoming a serious issue of concern,especially since a large number of industries including electroplating,leather tanning,cement,mining,dyeing,photography and fertilizer are discharging their effluents into fresh water without any effective treatment[3–6].Most of heavy metal contaminations end up accumulating into the soil and sediments through water bodies causing a number of negative impacts on public health and causing ecosystem disorder [7].Hexavalent chromium has a priority metal pollutant due to its highly toxicity and carcinogenic properties even in minor quantity [8].The removal of toxic heavy metals from metal contaminated water has been achieved by several processes such as ion exchange[9],sedimentation[10],electrochemical processes[11,12],cementation[13],biological operations [14],coagulation/flocculation [15],filtration and membrane processes[16,17],chemical precipitation,solvent extraction[18,19]and adsorption[20–28].Most of these remediation technologies are very expensive since they are required handling of large amounts of harmful secondary wastes [29].Adsorption is regarded as the most economically effective and widely used for the removal of heavy metals from aqueous solution.Biochar,carbon aerogel,mesoporous carbon,activated carbon,and graphene as adsorbates are of great interest to the most of scientists working in wastewater treatment[30].Among these adsorbent materials,biochar has different applications in the environment and agricultural[31].

Recent studies have found that biochar refers to highly aromatic,hollow-structured pore networks and C-rich residues,thus,it has a potential to absorb and store chemical species including some of the common water pollutants such as heavy metal ions and hydrocarbon molecules [32–34].Biochar can be produced from many biomaterial wastes including agricultural byproducts and manures of animals through combustion at relatively low temperatures(e.g.300–800°C)under oxygen-limited conditions[35,36].The biochar is recognized as an important adsorbent material for wastewater treatment.However,the using of biochar in the field of metal contaminated water treatment was limited as compared to activated carbon due to its lower surface area and limited porosity but the biochar present more functional groups on the carbonaceous surface than activated carbons[37].Therefore,with the introduction of a number of functional groups containing heterozygos,they are considered urgent adjustments to impose a chemical amendment on the surface of biochar to enhance the ability of biochar to remove minerals from aqueous solutions and improving its potential practical applications.Generally,four modification methods can be proposed to improve the adsorption capacity of biochar including impregnation with minerals,nano-scale formation,oxidation and reduction of biochar surface [38].Impregnation with mineral elements process was utilized as a method to enhance the functionality of modified biochar via introduce amino groups to the pore of the adsorbent[39].In nano-scale metals assistance,the biochar was loaded with nano-metals to enhance the affinity of biochar to remove the heavy metals from water,via increasing the specific surface area,thermal stability,resistance to oxidation and the number of adsorption sites[40].Recent studies have also demonstrated that,the precipitation of nano-metals onto biochar surface can efficiently prevent the aggregation of nanoparticles and increased the dispersion performance of the biochar[41,42].In surface oxidation method,the modification refers to the use of oxidants such as acids(HNO3,H2SO4/HNO3,or H3PO4),bases (NaOH or KOH) and certain oxidant reagents (H2O2,KMnO4,NH3·H2O,NaClO or(NH4)2S2O8)[43–45]to improve the number of acidic functional groups [46].Surface reduction modification was utilized to improve the surface of biochar,especially nitrogenous functional groups such as primary amines,secondary amines,tertiary amines,quaternary ammonium and imidazole.The NH3.H2O,Na2SO3,FeSO4,aniline and H2are commonly used as reducing agents.Based on literatures,amine groups are considered as a potential source of basic sites for acidic chromium adsorption,which lead to increase the adsorption capacity of biochar [47,48].Chromium metal exists in many valence states ranging from ?2 to +6,with Cr3+and Cr(VI)being the major two oxidation states in aqueous solution in natural waters.Cr(VI)ion is highly soluble and mobile in aqueous solution and is of significant environment concern due to its carcinogenic,mutagenic,and teratogen behavior in biological systems.The principal Cr(VI)Cr(VI)ion species areand.Overall,,anddominate at pH<6.0 whiledominates at pH>6.0[20–28].

Watermelon(Citrullus lanatus)is a vine-like flowering plant species in the family Cucurbitaceae,originally domesticated in West Africa and is a worldwide highly cultivated fruit.There is evidence from seeds in Pharaoh Tombs of watermelon cultivation in Ancient Egypt.Watermelon is grown in favorable climates from tropical to temperate regions worldwide for its large edible fruit.No study has been done on Watermelon peels waste materials as a suitable precursor for the preparation of biochar by dehydration using H2SO4followed by ozonation in water and amination with Triethylenetetramine (TETA) or ammonium hydroxide with significant adsorptive performance toward Cr(VI)ions.This study aims to investigate the role of mesopore structure and amino modification on Cr(VI)ions adsorption.The biochar properties,the existence functional groups on the biochar surface,morphology of biochar surface and the biochar thermal stability before and after modification were characterized and their effects on Cr(VI)ions adsorption were tested.Triethylenetetramine and ammonium hydroxide were used to introduce nitrogen functional groups to the surface of biochar.In addition,adsorption isotherm and kinetics were applied to the experimental data.Usually a pyrolyzed material under a lack-of-oxygen atmosphere (dehydration under heating almost at 400–700 °C)is necessary to produce biochar.However,in this work we tried to prepare biochar via dehydration using H2SO4as dehydrating agent at 100°C in order to have a good functional group on the surface of produced biochar.This method does not give a complete combustion of the organic matter and thus gives an opportunity to form functional groups on the surface of the achieved biochar.The novelty of this work is the formation of new functional group on the surface of prepared biochar and the adsorption capacity of prepared biochar for removal of Cr(VI) ions compared to literature reported data.This study provides an insight into the preparation of biochar materials and the modification of the surface chemistry of biochar as it shows some beneficial uses for it,as well as providing an effective approach to the use of solid waste materials in preparing materials that have important applications.

2.Materials and Methods

2.1.Materials and equipment

Watermelon peels were collected from a local market and were used as a raw material for the preparation of biochar.Sulfuric acid(H2SO4,Mw=98.07 g,Assay(acid-metric)99%),ammonia solution(NH4OH,Mw=35 g,Assay 25%)and Triethylenetetramine(TETA)were purchased from Sigma Aldrich.Standard stock of potassium dichromate(K2Cr2O7,Mw=294.19 g,Assay (99%)) was made from potassium dichromate of Sigma Aldrich.1,5–Diphenyl Carbazide as reagent for Cr(VI)was obtained from BDHZ chemicals LTD Poole-England.Digital spectrophotometer (Analytic Jena (SPEKOL1300 UV/Visible spectrophotometer))with matched glass cells of 1 cm optical path,Shaker (A JS shaker(JSOS-500)),PH meter JENCO(6173).

2.2.Methods

2.2.1.Melon-B preparation

The carbonaceous precursor used for preparation of biochar was watermelon peels which were collected from the local market.Watermelon peels were thoroughly washed with tap water several times to remove dust and the cleaned watermelon peels were dried in an oven at 105°C for 48 h,then milled and crushed.The crushed watermelon peels were boiled in a refluxed system using 25 g in a 100 ml solution of 50%H2SO4for 2 h,then the samples were filtered and washed with distilled water until washing solution become neutral followed by washing with ethanol.The final product of biochar was dried in an oven at 70°C,then its mass is determined.The obtained biochar from this reaction was labeled as Melon-B.

2.2.2.Chemical surface modification of Melon-B biochar

The prepared Melon-B biochar was subsequently subjected to a two stages of modification.At the first stage,the Melon-B biochar was oxidized by ozone in water for 2 h[49,50].Then the ozonated biochar was filtered and washed by distilled water,ethanol and allowed to dry in an oven at 70°C for 48 h.The products were labeled as Melon-BO biochar.This stage is urgently to increase the content of oxygen surface groups as the linking agent for grafting amine functional groups.At the second stage,two amine functionalized biochars were obtained by grafting Triethylenetetramine (TETA) and ammonium hydroxide(NH4OH).For functionalized Melon-B biochar with NH4OH,20 g of asprepared Melon-BO biochar was boiled in a refluxed system in a 100 ml solution of 25%NH4OH for 2 h,then cool,filter,and washed with distilled water followed by ethanol.Finally,the amine functionalized biochar was dried in the oven at 70°C for 48 h,and then its weight is determined to give 25 g and labeled as Melon-BO-NH2.For functionalization with TETA,a certain amount of Melon-BO(20 g)was boiled in a refluxed system in a 100 ml solution of TETA for 2 h then the reaction mixture was cooled,filtered and washed with distilled water and ethanol.The final product was dried at 70°C for 48 h,and then its weight is determined to give 28.5 g.The product was labeled as Melon-BO-TETA biochar(Fig.1).

Fig.1.Graph report the preparation of Melon-B,Melon-BO,Melon-BO-NH2,and Melon-BO-TETA.

2.3.Characterization of adsorbent biochar

The adsorption–desorption isotherm of nitrogen gas on biochar was determined at boiling point of N2gas.The BET surface area(SBET)measurements of the biochar were made by nitrogen adsorption at 77 K using surface area and pore analyzer(BELSORP-Mini II,BEL Japan,Inc.)[51,52].Analysis of the isotherm was carried out by applying the BET plot to obtain mono layer volume(Vm)(cm3?g?1),the surface area(SBET)(m2·g?1),total pore volume(Vp)(cm3·g?1),(C)energy constant and mean pore diameter(nm).Also the micropore surface area(Smi)and micropore volume(Vmi)as well as the mesopore surface area(Smes) and mesopore volume (Vmes) of biochar were determined by Barrett–Joyner–Halenda (BJH) methods,respectively,according to BELSORP analysis program software.Pore size distribution is calculated from desorption isotherm by applying BJH method [53].The biochar surface morphology was investigated by the Scanning Electron microscope(SEM)(QUANTA 250)that coupled with Energy Dispersive X-ray spectrometer (EDAX) to carry out elemental analysis.The functional groups on the biochar surface were studied using Fourier Transform Infrared (FTIR) spectroscopy VERTEX70 connected with Platinum ATR unit model V-100 to detect the IR-observable functional groups on the biochar surface,in the wave number (400–4000 cm?1).Thermal analyses were performed using SDT650-Simultaneous Thermal Analayzer instrument(50–1000°C)using 5°C per min as ramping temperature.

2.4.Adsorption measurement for hexavalent chromium

A stock solution of Cr(VI) ions (1000 mg·L?1) was prepared by dissolving 2.8289 g of K2Cr2O7in 1000 ml of distilled water,and this solution was diluted to have the required concentration for removal test and the standard curve.A batch adsorption experiments were employed to evaluate the adsorption capability,thermodynamic and kinetic parameters of Melon-B,Melon-BO-NH2,and Melon-BO-TETA which were prepared from watermelon peels.A series of Erlenmeyer flasks(300 ml)containing 100 ml of different concentrations of Cr(VI)ions solution and different amounts of biochar were shaken at 200 r·min?1for a certain time.The sample pH was adjusted to the desired values with 0.1 mol·L?1HCl or 0.1 mol·L?1NaOH.About 0.1 ml of the solution in the Erlenmeyer flask was then separated from the adsorbent,and the concentration of Cr(VI)ions was detected at different interval times and at the equilibrium.The concentration of Cr(VI)ions was determined by spectrophotometry using the method of 1,5-diphenylcarbazide as chromogenic agent (λmax=540 nm) [20,21].The adsorption capacities at equilibrium (qe) were calculated from Eq.(1):

where qeis the amount of adsorbent in the solid phase(mg·g?1)at equilibrium;C0and Ceare the initial and equilibrium concentrations in the liquid phase (mg·L?1),respectively;V is the volume of the solution(L)and W is the mass of adsorbent(g).

2.4.1.Effect of solution pH

The effect of pH was studied for biochars by using 100 ml of 100 mg·L?1of initial Cr(VI)ions concentration using solution pH at 1.51,2.47,3.54,5.34,6.98 and 9 for Melon-BO-NH2(100 mg),pH values at 1.49,2.56,3.57,5.19,7.01 and 9.09 for Melon-B(100 mg)and pH values at 1.53,2.49,3.57,5.25,7.23 and 9.04 for Melon-BO-TETA.The percentage of removal was calculated by the Eq.(2):

where C0and Ctare the initial and interval chromium concentrations at time t in the liquid phase(mg·L?1),respectively.

2.4.2.Effect of initial Cr(VI)ions concentration

The isotherm study for Watermelon biochar was performed using various initial concentrations of Cr(VI)ions solution(50,75,100,150,200 and 300 mg·L?1)using different doses(3,4,5,6 and 7 g·L?1)for Melon-B and(1,1.5,2,2.5 and 3 g·L?1)for Melon-BO-NH2as well as various concentration of Cr(VI)ions(100,150,200,250,300 and 400 mg·L?1)using different doses(1,1.5,2,2.5 and 3 g·L?1)for Melon-BO-TETA.The reaction mixture was shaken at 200 r·min?1,and the Cr(VI) concentration was analyzed at different time intervals at room temperature((25±2)°C).

2.4.3.Effect of adsorbent dosage

The effect of adsorbent dose on Cr(VI)ions removal was studied by shaking 100 ml of initial Cr(VI) ions concentration of (50,75,100,150,200 and 300 mg·L?1) for Melon-B and Melon-BO-NH2and initial Cr(VI) ions concentration of (100,150,200,250,300 and 400 mg·L?1) for Melon-BO-TETA using different adsorbent doses (1,1.5,2,2.5 and 3 g·L?1)of Melon-BO-NH2and Melon-BOTETA,and (3,4,5,6 and 7 g·L?1)of Melon-B at room temperature((25±2)°C).

2.4.4.Effect of contact time

In the kinetics study of Melon-B of doses(3,4,5,6 and 7 g·L?1),and doses of (1,1.5,2,2.5 and 3 g·L?1) for Melon-BO-NH2and Melon-BO-TETA,respectively,were added to Erlenmeyer flasks containing 100 ml of(50,75,100,150,200 and 300 mg·L?1)of initial Cr(VI) ions solution for Melon-B and Melon-BO-NH2biochars and(100,150,200,250,300 and 400 mg·L?1)of initial Cr(VI)ions solution for Melon-BO-TETA.The suspensions were shaken at 200 r·min?1and samples were taken from the solution at different interval times for analysis of the residual concentration of Cr(VI)ions.

3.Theoretical Background

3.1.Isotherm models

The Langmuir isotherm model[54,55]can be presented as the linear Eq.(3)[56,57].

where,Qmis a maximum capacity of monolayer adsorption(mg·g?1);Kais constant (L·mg?1) that is related to the apparent energy of sorption.

The Freundlich model[58]is the earliest known equation describing the adsorption process and can be written as linear Eq.(4):

where,KFis Freundlich constant represents the relative adsorption capacity of biochar and 1/n is a constant represents the intensity of the adsorption of Cr(VI) ions onto the biochar.The adsorption process becoming more heterogeneous if 1/n value gets closer to zero.1/n<1 indicates a normal Langmuir isotherm while 1/n>1 indicates cooperative adsorption.

The Freundlich maximum adsorption capacity Qmcan be calculated following Eq.(5)[59].

The Tempkin isotherm model[60,61]can be applied in the linear form as in Eq.(6)[62–64].

where β=(RT)/b,T is the absolute temperature in Kelvin and R is the universal gas constant,8.314 J·mol?1·K?1.The constant b is related to the heat of adsorption[65,66].

3.2.Kinetic models

The Lagergren first-order model [67] is generally expressed as in Eq.(7):

The values of lg(qe?qt)were linearly correlated with t from which k1and predicted qecan be determined from the slope and intercept of the straight line,respectively.

The pseudo-second-order model given by Ho et al.[68] can be applied in its linear form as Eq.(8):

where k2(g·mg?1·min?1)is the second-order rate constant of adsorption,which were used to calculate the initial sorption rate,h,following Eq.(9).

Elovich kinetic equation is expressed in its linear form as in Eq.(10)[69–71].

where α is the initial adsorption rate(mg·g?1·min?1)and β is the de-sorption constant(g·mg?1).

The intra-particular diffusion model is known as Eq.(11)[72,73].

where C is the intercept and Kdifis the intra-particle diffusion rate constant.

3.3.Error analysis models

The average percentage errors(APE)calculated according to Eq.(12)indicated the fit between the experimental and predicted values of adsorption capacity used for plotting isotherm curves[74].

The hybrid error is given as Eq.(13)[75,76].

The chi-square error,X2,[77]is given as Eq.(14).

The Marquart's percentage standard deviation(MPSD)[74]is given by the following Eq.(15).

The sum of the absolute errors(EABS)[74]is given by the following Eq.(16).

Fig.2.FTIR analysis of Melon peel,Melon-B and Melon-BO.

The root mean square errors(RMS)[74]are given by the following Eq.(17).

4.Results and Discussion

4.1.Characterization of biochars

The functional groups of the prepared biochars were investigated by Fourier Transform Infrared Spectroscopy(FT-IR).The raw Watermelon peel was compared with the untreated melon biochar(Melon-B)and the three treated biochars(Melon-BO,Melon-BO-NH2and Melon-BO-TETA)as shown in Figs.2 and 3.The FT-IR spectra of all samples demonstrate some similarities.The band at 3274–3062 cm?1represents the O—H stretching vibration which existed in Watermelon peel,Melon-B and Melon-BO biochars (Fig.2).The broad adsorption peak around 3033 and 3190 cm?1is inductive of the existence of the–OH and–NH groups in Melon-BO-NH2and Melon-BO-TETA biochars(Fig.3).These proved that TETA and NH4OH were successfully reacted with biochar to form nitrogen groups onto its surface.The adsorption peak at 2921 cm?1can be assigned to C—H stretching.The strong adsorption peak at 1702–1711 cm?1can be assigned to C=O stretching of carboxyl group which was existed in Melon-B and Melon-BO,while it is weak in Watermelon peel and it is completely disappeared in Melon-BO-NH2and Melon-BO-TETA biochars (Figs.2,3).However,the strength at 1702–1711 cm?1of Melon-BO treated biochar was enhanced when compared with the raw Watermelon peel or Melon-B biochar,indicating the C=O functional group might be increased by ozone treatment of Melon-B.The bands at 1598–1601 cm?1imply the C=O stretching vibration of β-ketone and almost existed in Watermelon peel,Melon-B and Melon-BO biochars(Fig.2).The appearance of new peak at 1550–1554 cm?1is attributed to bending of N-H in fatty amine or aromatic secondary amine in Melon-BONH2and Melon-BO-TETA biochars.The peak at 1405–1418 cm?1represents the C—O functional group which was moderate in Watermelon peel,weak in Melon-B,and very weak in Melon-BO while it is completely disappeared in Melon-BO-NH2and Melon-BO-TETA biochars,indicating that the refluxing in 25%NH4OH solution or TETA might have strong effect on the C—O functional group of biochars(Figs.2,3).Also,the modification leads to the formation of new functional groups,in the FT-IR spectra of Melon-BO-NH2and Melon-BO-TETA biochars.New peak appeared at 1354–1372 cm?1is due to the N=C=O group stretching vibration(Fig.3).These new peaks proving the formation of the amino groups onto the biochar surface after the treatment with TETA and ammonium hydroxide.The presence of oxygenated carbon chains peak at 1239–1084 cm?1represents an increase of C—O—C asymmetric stretching functional group for Melon-B and Melon-BO,while it is very weak in Watermelon peel,Melon-BO-NH2and Melon-BO-TETA biochars.The peak at 1026–1039 cm?1is due to the primary hydroxyl group which has strong intensity in Watermelon peel and weak intensity in other samples(Figs.2,3).

Fig.3.FTIR analysis of Melon-BO,Melon-BO-NH2 and Melon-BO-TETA.

Fig.4.(a) Adsorption Desorption,(b) BET analysis,(c) BJH analysis plot of Melon-B,Melon-BO,Melon-BO-NH2 and Melon-BO-TETA biochars.

Fig.5.SEM image of(a)Melon-Peel,(b)Melon-B,(c)Melon-BO,(d)Melon-BO-NH2 and Melon-BO-TETA(e).

The nitrogen adsorption–desorption isotherms of the biochars prepared from Watermelon peel were investigated to study the influence of the O3,NH4OH and TETA reagents on the surface characteristics.The specific surface area and mesopore area were calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)methods.The textural properties including the BET specific surface area,total pore volume,mean pore diameter,mono layer volume,mesopore area,mesopore volume and mesopore distribution peak for the Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars are presented in Fig.4.The Textural properties varied widely among these biochars (Fig.4).In general the BET-specific surface area of biochars declined as Melon-BO-NH2(14.16 m2·g?1) >Melon-BOTETA (13.64 m2·g?1) >Melon-B (1.79 m2·g?1) >Melon-BO (1.22 m2·g?1).Notably,the modification increased the surface area of Melon-B and Melon-BO biochars.The mono layer volume values of Melon-BO-NH2,Melon-BO-TETA,Melon-BO and Melon-B biochars were 3.24,3.13,2.81 and 0.41 cm3·g?1,respectively,which demonstrated similar trend with surface area.The total volume values of Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars were 0.006,0.017,0.018 and 0.021 cm3·g?1,respectively.The mean pore diameters of Melon-B,Melon-BO,Melon-BO-NH2and Melon-BOTETA biochars were 12.47,5.70,5.17 and 6.05 nm(mesopores),respectively.This result revealed that the modification process reduced the pore size of Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars.The meso surface area of biochars declined as Melon-BO-TETA(13.49 m2·g?1) >Melon-BO-NH2(12.97 m2·g?1) >Melon-BO (11.75 m2·g?1)>Melon-B(1.82 m2·g?1).The meso pore volume values of Melon-BO-TETA,Melon-BO-NH2,Melon-BO and Melon-B biochars were 0.023,0.021,0.021 and 0.006 cm3·g?1,respectively,which demonstrated similar trend with surface area.The mesopore distribution peak values of Melon-B,Melon-BO,Melon-BO-NH2and Melon-BOTETA biochars were 1.22 nm,1.66 nm,1.66 nm and 1.22 nm,respectively.As expected,the surface area,total pore volume,monolayer volume were increased via modification due to the evolution of new pores.The mean pore diameter was decreased;this result revealed the modification process control the morphology of biochar surface.

Scanning electron microscope(SEM)photographs for raw Watermelon peel,Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars shown in Fig.5.As shown in Fig.5(b),the Melon-B biochar appears clean and free of any particulate impurities as well as no damage in the pores of the raw watermelon peel was observed as a result of treatment with heated sulfuric acid.Fig.5(b)and(c)showed that a little pores were existed at both Melon-B and Melon-BO biochars which reflects the small surface area for Melon-B and Melon-BO biochars.SEM images of Melon-BO-NH2and Melon-BO-TETA biochars are given in Figs.5(d)and(e),respectively.It is obvious that new pores of different size are created,thus the surface areas of the modified biochars increased and pore diameter decreased.

Fig.6.(a)TGA;(b)DTA and(c)DSC analyses of raw Melon peel and Melon-B,Melon-BO,Melon-BO-NH2 and Melon-BO-TETA biochars.

Chemical compositions of the samples were analyzed with Dispersive X-ray spectrometer(EDAX).This analysis was carried out for the treated and untreated biochars.The elements percent of Melon-B,Melon-BO,Melon-BO-NH2,and Melon-BO-TETA were examined and reported in Table 1,which show the absence of nitrogen peak before the modification by NH4OH and TETA reagents.The EDAX analysis of Melon-BO-NH2and Melon-BO-TETA biochars proved the presence of about 13.36%and 19.56%nitrogen of sample weight,respectively.

Table1 EDAX analysis data of Melon-B,Melon-BO,Melon-BO-NH2 and Melon-BO-TETA biochars

Thermal gravimetric analysis(TGA)was performed on the selected Melon biochars in order to detect the influence of their structural differences on the degradation behavior and to define operating temperature.Each sample was heated from room temperature to 1000°C under N2atmosphere.Fig.6(a)shows TGA analysis curves related to raw melon peel,Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars.Before 100°C,all samples reported their first weight loss peaks which are attributed to H2O volatilization.For raw Watermelon peel,Melon-B and oxidized Melon-BO biochars,the second weight loss peak(>100°C)is due to the decomposition of different acidic oxygen functional groups.However,acidic groups such as carboxylic,anhydrides and lactones may be decomposed at lower temperatures while acidic groups such as phenol may be decomposed at higher temperature.In the case of Melon-BO-NH2and Melon-BO-TETA biochars,the second weight loss peaks appeared above 180°C.In the temperature up to 450°C,oxidized and amine modified samples continue to gradually lose weight,whereas raw Melon peel and Melon-B biochar exhibit a slight plateau of weight losses.All TGA curves became similar at the temperature above 450°C as a result of carbon decomposition in the biochar structure.At the final teperature1000°C,different weight losses with an order of Melon-BO-NH2

Figure Fig.6(b)shows the Differential thermal analysis(DTA)of Watermelon peel,Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars.As revealed in Fig.6(b),the DTA curve of the Watermelon-peel(Green)sample exhibits six peaks at temperature of flow(Tf,76.93°C,177.63°C,250.50°C,291.80°C,425.89°C,and 871.64°C),while the DTA curve of Melon-B(Red)sample exhibits only three peaks at Tf(75.11°C,214.56°C,and 488.88°C).As shown from the DTA curves,the pyrolysis of both of Watermelon-peel(Green)and Melon-B(Red)samples show six and three well-resolved degradation peaks,respectively.The degradation peaks decreased from six to three peaks after treatment with sulfuric acid.These results imply that the degree of degradation of the Watermelon-peel(green)is much affected after acid treatment dehydration.As presented in Fig.6(b),the DTA curves of the Melon-BO,Melon-BO-NH2and Melon-BO-TETA samples showed manly similar two wellresolved degradation peaks at temperature of flow(Tf)(78.76°C,419.43°C),(76.73 °C,476.97 °C),and (76.73 °C,351.53 °C),respectively and onset points at(55.42 °C,173.35 °C),(55.34 °C,257.87°C),(56.04 °C,251.68°C),respectively.This proved that the stability of Melon-biochar samples increased by modification with ozone and amine.However,these results also proved that the degree of crystallinity of the Melon biochar products is affected by the reagent used in modification process.

Differential scanning calorimetry(DSC)can be utilized to compare materials via thermal transitions.Fig.6(c) illustrate the differential scanning calorimetry profile of the raw material of Watermelon peel,Melon-B,Melon-BO,Melon-BO-NH2and Melon-BO-TETA biochars,respectively.All samples showed their crystallization temperatures TCbefore 100°C which are attributed to crystallization of water molecules except Melon-B showed another crystallization temperatures TCat 225.10 °C and 818.96 °C.Melon-BO,Melon-BO-NH2and Melon-BOTETA biochars do not exhibit other phase transitions.As the temperature increased the raw Watermelon peel and Melon-B go through the melting temperature.However,raw Watermelon peel undergoes melting temperatures Tmat 297.62 °C and 422.87 °C while Melon-B undergoes melting temperature at 486.76 °C.Raw Watermelon peel showed lower transition temperature while Melon-B biochar showed the highest.The higher transitional temperatures resulted in a higher degree in the crystal,providing structural stability and making the grains more gelatin resistant.

4.2.Adsorption of Cr(VI)ions on Melon-biochars

4.2.1.Effect of pH

Change in the pH of solution impacts the behavior of functional groups on the biochar surface such as carboxyl,hydroxyl,and amino groups.The removal of Cr(VI)ions and the amounts of metal adsorbed at equilibrium (qe) by the three types of biochars (Melon-B,Melon-BO-NH2and Melon-BO-TETA)at varied pH range from 1.0 to 9.09,at an initial Cr(VI)ions concentration of 100 mg·L?1at room temperature((25±2)°C)and adsorbents dose 1.0 g·L?1were studied and shown in Fig.7 indicates the impact of pH changes on Cr(VI)ions percentage of removal at pH range 1.0–9.09 and 3.5 h contact time.Fig.7(a)shows that the highest efficiency of removal was obtained at pH 1.49 as removal percentages of Cr(VI) ions for Melon-B,Melon-BO-NH2,and Melon-BO-TETA were 69%,98%,and 99%,respectively.

Fig.7.(a)Effect of solution pH on the removal%;(b)Effect of solution pH on the qe(mg·g?1)of Cr(VI)ions(100 mg·L?1)using Melon-B,Melon-BO-NH2,and Melon-BO-TETA of adsorbent dose(1.0 g·L?1)at(25±2)°C.

Fig.7(b)shows the relationship between the amounts of Cr(VI)ions adsorbed at equilibrium(qe,mg·g?1)onto Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars at pH values varied from 1 to 9.09.The results indicate that,the amounts of Cr(VI)ions adsorbed at equilibrium(qe)for the Melon-BO-NH2and Melon-BO-TETA biochars were nearly similar at pH 1.49.By changing pH values from 1.49 to 9.09 the(qe)decreased from 99 to 55 mg·g?1for Melon-BO-TETA and to 45 mg·g?1for Melon-BO-NH2biochars.The amount of Cr(VI)ions adsorbed at equilibrium(qe)(mg·g?1)at pH 1.49 was 69 mg·g?1and highly decreased to 35 mg·g?1at pH 9.09 for Melon-B biochar.

The results showed that,the pH 1.49 is the optimal pH for the Cr(VI)ions uptake for the three adsorbents.In this study,two mechanisms have been proposed for Cr(VI)ions adsorption:(1)electrostatic attraction between negatively charged Cr(VI) ions species and positively charged biochar and(2)reduction of Cr(VI)to Cr(III)ions followed by Cr(III)ions complexation by hydroxyl and carboxyl groups on biochar.The Melon-BO-NH2and Melon-BO-TETA biochars were effective in Cr(VI)ions adsorption,which is mainly through electrostatic attraction between the HCrO4?,and Cr2O72?forms of Cr(VI)ions,which are the predominant species between pH 1 to 4,is adsorbed preferentially on the adsorbents.This result probably occurred due to the formation of amino group and completely disappeared of carboxyl group on biochar surface after treatment of Melon-B biochar.The Melon-B biochar was effective in Cr(VI) ions adsorption through Cr(VI) ions reduction to Cr3+ions and then complexation by hydroxyl and carboxyl groups on biochars.The complexation and electrostatic attraction are important mechanisms for Cr(VI)ions removal by biochar,with functional groups being the dominant property governing Cr(VI)ions adsorption.While at the alkaline medium,there was an excess of HO?ions which competing with CrO42?,the predominant species in solution for adsorption.Also,in the alkaline medium,the surface sites on the biochar were negatively charged which did not favor the adsorption of chromium heavy metal due to electrostatic repulsion.

4.2.2.Effect of contact time

The contact time required achieving a specific removal of Cr(VI)ions and it is important for applying biochar to Cr(VI)ions adsorption applications.The effect of contact time on adsorption of Cr(VI)ions at 3.0 g·L?1of Melon-B biochar concentrations at different initial Cr(VI)concentrations ranged from 50 to 300 mg·L?1and pH 1.49 has been shown in Fig.8(a).As can be seen from Fig.8(a),more than(20%–50%)of Cr(VI) ions adsorption occurred in the first 15–30 min,and thereafter the rate of removal increased gradually with increasing contact time,since the largest amount of Cr(VI) ions attached to adsorbent was(60%–89%)within 3.5 h at initial concentrations of Cr(VI)ions(300,200,150,100,75 and 50 mg·L?1).

Fig.8(b)shows the percentage of Cr(VI)ions removal at different initial Cr(VI)ions concentrations ranged from 100 to 300 mg·L?1and pH 1.49 using Melon–BO-NH2adsorbent biochar(3.0 g·L?1).For initial Cr(VI)ion concentrations(50 and 75 mg·L?1),the removal occurred after 15 min and reaches the maximum value(96 and 95%,respectively)after 60 min,after that the removal of Cr(VI)ions became nearly constant within increasing time until 3.5 h.For initial Cr(VI)ions concentrations(100,150,200 and 300 mg·L?1),it can be seen from the Fig.8(b),the percentage of removal of Cr(VI)ions is about(65%,56%,55%and 55%)in the first 15 min,after that the removal of Cr(VI)ions increase gradually with increasing time until reaches(96%,95%,94%and 90%)at 3.5 h which is optimum time to attain the equilibrium.

Fig.8.Removal of Cr(VI) ions (50–300 mg·L?1) using (a) Melon-B (3.0 g·L?1) and(b)Melon-BO-NH2 adsorbent dose(3.0 g·L?1)at(25±2)°C;(c)Removal of Cr(VI)ions(100,150–400 mg·L?1)using Melon-BO-TETA adsorbent dose(3.0 g·L?1)at(25±2)°C.

Plot of the percentage removal of Cr(VI)ions versus contact time of initial Cr(VI)ions concentrations(100,150,200,250,300 and 400 mg·L?1)on 3.0 g·L?1of Melon-BO-TETA biochar at pH 1.49 was presented in Fig.8(c).The results show that the equilibrium time required for the adsorption of Cr(VI)ions on Melon-BO-TETA biochar is almost 3.5 h.These results also indicate that the removal process can be considered to be very rapid,since the largest amount of Cr(VI)ions attached to the adsorbent within the first 60 min of adsorption.The removal of Cr(VI) ion was(83%–93%) just after 15 min.The percentage of Cr(VI) ions removal slightly increases with time,but after 60 min the removal reached(92%–98%)and became nearly constant for all cases.

From Fig.8,we can observed that,there was a rapid adsorption occurred at initial contact time(15 min)by using Melon-B,Melon-BONH2and Melon-BO-TETA biochars,but the most notable difference between the three biochars was the significantly higher removal of Cr(VI)ions using a small doses of Melon-BO-NH2and Melon-BO-TETA biochars at short time.The maximum value of removal was(98%and 100%) for Melon-BO-TETA and Melon-BO-NH2,respectively.After modification of Melon-B biochar obtained by boiling with H2SO4followed by treatment with O3in H2O then refluxed in 25% NH4OH solution or refluxed in TETA.The surface area and pore size distribution may not be the functions of Cr(VI)ions adsorption because they were slightly decreased than would be expected based on BET analysis.The significant difference between the three biochars could be attributed to the surface charge.One hypothesis is the amine sites formation on the surface of biochars during the modification process,this hypothesis was confirmed on the basis of the FT-IR and EDAX analysis.At low pH values (1.49),the protonation of functional groups (carboxyl and amino groups)has been known to give an overall positive charge to the biochar,which is able to adsorb negatively anionic metal ions.Thus the modification of Melon-B biochar might expose more binding sites and therefore,the accessibility of the anionic Cr(VI)ions to the sorption sites might be increased.The slow rate of Cr(VI)ions adsorption after the first 15 min probably occurred due to the slow pore diffusion of the solute ion into the bulk of adsorbent.

4.2.3.Effect of initial Cr(VI)ions concentration

The initial metal concentration plays an important role in the process of adsorption.The influences of the initial Cr(VI)ions concentration on adsorption capacity at equilibrium(qe)were studied.To determine the influence of Melon-B doses on the sorption capacities at equilibrium(qe),the Melon-B mass was varied from 3.0 to 7.0 g·L?1,while initial Cr(VI)ions concentration was 50,75,100,150,200,and 300 mg·L?1at(25±2)°C and pH 1.49.Fig.9(a)shows that,the amount of adsorbed of Cr(VI)ions at equilibrium(qe)decreases with increases of Melon-B doses at the same initial concentration of Cr(VI) ions,while it increases with increases of initial Cr(VI)ions concentration for all studied doses of Melon-B biochar.From Fig.9(a),the results indicate that,the adsorption capacities at equilibrium(qe)increased from 15.78 to 67.30,12.40 to 55.63,9.95 to 49.19,8.29 to 44.50,and 7.10 to 40.79 mg·g?1with the initial Cr(VI) ions concentrations 50,75,100,150,200 and 300 mg·L?1,respectively,using Melon-B doses (3.0–7.0 g·L?1).

The influence of Melon-BO-NH2doses on the sorption capacities at equilibrium (qe) was investigated,the Melon-BO-NH2masses were varied from 1.0,1.5,2.0,2.5 and 3.0 g·L?1,while initial Cr(VI)ions concentrations were 50,75,100,150,200,and 300 mg·L?1for 3.5 h at(25± 2) °C and pH 1.49.The results are represented in Fig.9(b) which shows the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreases with increases of the Melon-BO-NH2doses at the same initial concentration of Cr(VI)ions,while it increases with increase of initial Cr(VI)ions concentrations for all studied doses of Melon-BO-NH2.From Fig.9(b),the results indicate that,the adsorption capacities at equilibrium(qe)increased from 40.34 to 107.12,33.00 to 112.65,24.78 to 116.36,19.80 to 100.32,and 16.52 to 92.61 mg·g?1with an increase in the initial Cr(VI) ions concentrations from 50,75,100,150,200,and 300 mg·L?1using Melon-BO-NH2doses 1.0,1.5,2.0,2.5,and 3.0 g·L?1,respectively.

The influence of Melon-BO-TETA doses on the sorption capacities at equilibrium(qe)was investigated,the Melon-BO-TETA masses were varied from 1.0,1.5,2.0,2.5,and 3.0 g·L?1,while initial Cr(VI)ions concentration was 100,150,200,250,300,and 400 mg·L?1for 3.5 h at(25±2)°C at pH 1.4.From Fig.9(c),the results indicate that,the adsorption capacities at equilibrium(qe)increased from 96.86 to 295.81,66.46 to 236.75,50.00 to 193.06,40.00 to 156.86,and 33.33 to 133.33 mg·g?1with an increase in the initial Cr(VI) ions concentrations from 100,150,200,250,300,and 400 mg·L?1using Melon-BO-TETA doses 1.0,1.5,2.0,2.5,and 3.0 g·L?1,respectively.

Fig.9.Effect of Cr(VI)ions initial concentration (a)(50–300 mg·L?1) using different Melon-B doses (3.0–7.0 g·L?1);(b)(50–300 mg·L?1) using different Melon-BO-NH2 doses (1.0–3.0 g·L?1);(c) (100–400 mg·L?1) using different Melon-BO-TETA doses(1.0–3.0 g·L?1)on qe(mg·g?1)at(25±2)°C.

Fig.10.Effect of Melon-B different doses(3.0–7.0 g·L?1)of different initial concentration(50–300 mg·L?1)of Cr(VI)ions at(25±2)°C(a)on removal%;(b)on qe(mg·g?1).

Fig.11.Effect of Melon-BO-NH2 different doses(1.0–3.0 g·L?1)of different initial concentration(50–300 mg·L?1)of Cr(VI)ions at(25±2)°C(a)on removal%;(b)on qe(mg·g?1).

Fig.12.Effect of Melon-BO-TETA different doses(1.0–3.0 g·L?1)of different initial concentration(100–400 mg·L?1)of Cr(VI)ions at(25±2)°C(a)on removal%;(b)on qe(mg·g?1).

The sorption capacities at equilibrium(qe)onto the three different biochars at a lower initial concentration of Cr(VI) ions were smaller than the corresponding sorption capacities at equilibrium(qe)when higher initial concentrations were used and it is decreased with increasing of adsorbents dose.The decrease in(qe)value was probably due to the concentration gradient between the adsorption vacant sites of the solid adsorbent and the concentration of Cr(VI)ions solutions which get decreased with increasing adsorbent mass leading to decrease in qevalue.Also,the decrease in qevalue was probably due to an aggregation that took place during the adsorption process which lead to a reduction in surface area of adsorbent.This result may be attributed to the formation of reactive amino functional groups on the surface of biochars raised during the modification process that efficiently increased the negatively surface charges.

4.2.4.Effect of adsorbent dosage on metal adsorption

Batch experiments were conducted to know the effect of adsorbent dosage of Melon-B biochar on Cr(VI)ions removal and the amount of adsorption at equilibrium (qe) by using different doses (3.0–7.0 g·L?1) of Melon-B biochar and the experimental conditions were:initial concentration of Cr(VI) ions (50,75,100,150,200 and 300 mg·L?1),adsorption time 3.5 h and pH 1.49 at(25±2)°C,as shown in Fig.10.The experimental results revealed that the percentage ofremoval of Cr(VI)ions increased gradually with increasing doses of the adsorbent,while the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased with increasing of the quantity of adsorbent for all studied doses of Melon-B biochar.

Increasing the quantity of Melon-B biochar from 3.0 to 7.0 g·L?1significantly enhanced the percentage of removal%of Cr(VI)ions from 67%to 93%,69%to 95%,82%to 98%,86%to 99%,91%to 99%,and 95%to 99%for initial Cr(VI)ions concentration at 50,75,100,150,200,and 300 mg·L?1,respectively.On the other hand the amount of Cr(VI) ions adsorbed at equilibrium(qe)decreased from 15.78%to 7.10%,22.65%to 10.66%,28.71%to 14.24%,40.76%to 21.21%,46.40%to 27.76%,and 67.30%to 40.79%mg·g?1with increasing of the quantity of adsorbent from 3.0 to 7.0 g·L?1for initial Cr(VI)ions concentrations 50,75,100,150,200 and 300 mg·L?1,respectively.It is worth to mention that,the maximum percentage of removal and minimum amount of adsorption at equilibrium were achieved using 7.0 g·L?1of adsorbent dose.

The effect of different adsorbent dosage (1.0,1.5,2.0,2.5,and 3.0 g·L?1)on Cr(VI)ions removal%and the amount of adsorption at equilibrium by using Melon-BO-NH2biochar under the following conditions:initial concentrations of Cr(VI)ions were(50,75,100,150,200,and 300 mg·L?1),adsorption time of 3.5 h,and pH 1.49 at(25±2)°C was studied(Fig.11).The results revealed that the percentage of removal of Cr(VI)ions increased gradually with increasing doses of the adsorbent,while the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased with increasing of the quantity of adsorbent for all studied doses of the Melon-BO-NH2biochar.Increasing the quantity of Melon-BO-NH2biochar from 1.0 to 3.0 g·L?1significantly enhanced the percentage of removal of Cr(VI)ions from 35%to 90%,45%to 98%,62%to 99%,63%to 99%,80%to 99%,and 81%to 99%for initial Cr(VI)ions concentrations at 300,200,150,100,75,and 50 mg·L?1,respectively(Fig.11a).On the other hand,the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased from 40.34 to 16.52,59.91 to 24.83,64.42 to 33.17,92.76 to 49.72,91.41 to 65.04,and 107.12 to 92.61 mg·g?1with increasing of the quantity of adsorbent from 1.0 to 3.0 g·L?1for initial Cr(VI)ions concentrations 50,75,100,150,200 and 300 mg·L?1,respectively(Fig.11b).The maximum percentage of removal and minimum amount of adsorption at equilibrium were achieved using 3.0 g·L?1of adsorbent dose.

The effect of Melon-BO-TETA biochar dosage on Cr(VI)ions removal%and the amount of adsorption at equilibrium(qe)from aqueous solutions was investigated using five different adsorbent concentrations(1.0,1.5,2.0,2.5,and 3.0 g·L?1)and six different initial chromium concentrations(100,150,200,250,300 and 400 mg·L?1)at pH 1.49(Fig.12).The results revealed that the percentage of removal of Cr(VI)ions increased gradually with increasing doses of the adsorbent,while the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased with increasing of the quantity of adsorbent for all studied doses of the Melon-BO-TETA biochar.Increasing the quantity of Melon-BO-TETA biochar from 1.0 to 3.0 g·L?1significantly enhanced the percentage of removal of Cr(VI)ions to 74%,84%,89%,91%,93.5%,and 97.5%to a complete removal of Cr(VI)ions of initial concentration at 400,300,250,200,150,and 100 mg·L?1,respectively(Fig.12(a)).On the other hand the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased from 96.86 to 33.33,138.86 to 50.00,180.62 to 66.67,223.80 to 83.33,252.06 to 99.84,and 295.81 to 133.33 mg·g?1with increasing of the quantity of adsorbent from 1.0 to 3.0 g·L?1for initial Cr(VI)ions concentration of 100,150,200,250,300 and 400 mg·L?1,respectively(Fig.12(b)).

Fig.13.(a) Linearized Langmuir adsorption isotherm for Cr(VI) ions of initial concentration (50–300 mg·L?1) on Melon-B doses (3.0–7.0 g·L?1);(b) (50–300 mg·L?1)on Melon-BO-NH2 doses (3.0–7.0 g·L?1);(c) (100–400 mg·L?1)on Melon-BO-TETA doses(1.0–3.0 g·L?1)at(25±2)°C.

The maximum percentage of removal and minimum amount of adsorption at equilibrium were achieved using 3.0 g·L?1of adsorbent dose(Fig.12).The results revealed that the percentage of removal of Cr(VI)ions increased gradually with increasing doses of the adsorbent,while the amount of Cr(VI)ions adsorbed at equilibrium(qe)decreased with increasing of the quantity of adsorbent for all studied doses of the three biochars.These results can be attributed to the increased availability of active adsorption site for Cr(VI)ions binding or may be due to the fact that the higher sorbent doses provide the more sorbent surface area and pores volume which will be available for adsorption.

Fig.14.Freundlich adsorption isotherm for Cr(VI)ions of initial concentration(a)(50–300 mg·L?1)on Melon-B doses(3.0–7.0 g·L?1);(b)(b)(50–300 mg·L?1)on Melon-BO-NH2 doses(3.0–7.0 g·L?1);(c)(100–400 mg·L?1)on Melon-BO-TETA doses(1.0–2.5 g·L?1)at(25±2)°C.

The higher removal of Cr(VI)ions by Melon-BO-NH2and Melon-BOTETA than Melon-B biochars were caused by the synergy of amino functionality.The percentage of removal of Cr(VI)ions increased to about 100%with the increase in the adsorbent dose to 3.0 g·L?1,which may be suggested that the economic absorbent dose for the removal Cr(VI)ions in this system is 3.0 g·L?1.

Fig.15.Tempkin adsorption isotherm for Cr(VI)ions of initial concentration(a)(50–300 mg·L?1)on Melon-B doses(3.0–7.0 g·L?1);(b)(50–300 mg·L?1)on Melon-BO-NH2 doses(3.0–7.0 g·L?1);(c)(100–400 mg·L?1)on Melon-BO-TETA doses(1.0–2.5 g·L?1)at(25±2)°C.

4.3.Adsorption isotherms

Study of the isotherm data is important in order to describe the adsorbate molecules fraction that are distributed at equilibriumbetween liquid and solid phases and so is critical in optimizing the use of adsorbent.The data obtained were analyzed with Langmuir,Freundlich,and Tempkin isotherm equations.

The Langmuir model presumes that the maximum adsorption of the surface will be achieved when the surface reached the saturation point and the model assumes uniform energies of adsorption and no transmigration of the adsorbate.Values of Langmuir constants,the saturated monolayer sorption capacity (Qm) and the equilibrium adsorption constant related to the affinity of adsorption sites,KL,are represented in(Table 2)for the sorption of Cr(VI)ions onto Melon-B biochar.

Table2 Isotherm study data of adsorption of Cr(VI)ions of different initial concentration(50–300 mg·L–1)onto Melon-B and Melon-BO-NH2 of different adsorbent doses(3–7 g·L–1),and adsorption of Cr(VI)ions of different initial concentration(100–400 mg·L–1)onto Melon-BO-TETA of different adsorbent doses(1.0–3.0 g·L–1)at(25±2)°C.

The applicability of the linear form of Langmuir model was provided by the high correlation coefficient (R2) ≥0.992 at Melon-B dose 5.0 g·L?1and the maximum monolayer capacity(Qm)was 72.46 mg·g?1of Melon-B biochar.By blotting Ce/qeversus Ce,the 1/QmKLand 1/Qmof the Langmuir model are,respectively,obtained from the intercept and the slope of the line plot of Fig.13(a).The results obtained from Langmuir isotherm model for the removal of Cr(VI) ions onto Melon-B biochar indicated that,the maximum monolayer capacity (Qm) obtained was 72.46 mg·g?1of Melon-B biochar(Table 2).From Table 2,the correlation coefficient for the linear form of Langmuir model(R2)≥0.992 and the equilibrium adsorption constant KLwas ranged between 0.05 and 0.68 L·mg?1,which showed strong positive evidence on the adsorption of Cr(VI)ions onto Melon-B biochar.Table S1 in supplement,represented the relation between qe(mg g?1)of Cr(VI)ions at different initial concentrations(50,75,100,150,200,and 200 mg·L?1)obtained by batch experimental and(qe)mg·g?1theoretical monolayer adsorption capacity of Melon-B biochar using data from different isotherm models and different doses of Melon-B(3.0–7.0 g·L?1)at(25±2)°C.Table S1 in supplement showed that there was a great agreement between the qeof Cr(VI)ions obtained by batch experimental and qecalculated using data obtained from Langmuir isotherm model.

Fig.13(b&c)represented a relationship between Ce/qeand Cefor Melon-BO-NH2and Melon-BO-ETAT biochars,respectively.Table 2 represented the values of Langmuir constants,the saturated monolayer sorption capacity(Qm)and the equilibrium adsorption constant(KL)for the sorption of Cr(VI)ions onto Melon-BO-NH2and Melon-BO-TETA biochars,respectively.The results obtained from the Langmuir isotherm model for the removal of Cr(VI) ions onto Melon-BO-NH2and Melon-BO-TETA biochars indicated that,the maximum monolayer capacity(Qm)mg·g?1are 123.46 and 333.33 mg·g?1for Melon-BONH2and Melon-BO-TETA,respectively.The equilibrium adsorption constant (KL) for the sorption of Cr(VI) ions was ranged between 0.155 and 0.646 L·mg?1for Melon-BO-NH2and ranged between 0.08 and 1.78 L·mg?1for Melon-BO-TETA.The correlation coefficients for the linear form of Langmuir model (R2) ≥1.000 for Melon-BO-NH2and Melon-BO-TETA which showed that the adsorption of Cr(VI)ions onto the two adsorbents biochar are strongly followed the Langmuir model isotherm.

Table S2 in supplement,represented the relation between qeof Cr(VI)ions at different initial concentrations(50,75,100,150,200 and 300 mg·L?1) obtained by batch experimental and (qe) mg·g?1calculated using data from different isotherm models using different doses of Melon-BO-NH2(1.0–3.0 g·L?1) at (25 ± 2)°C.While,Table S3 in supplement represented the relation between qeof Cr(VI)ions at different initial concentrations (100,150,200,250,300 and 400 mg·L?1)obtained by batch experimental and qecalculated using data from different isotherm models using different doses of Melon-BO-TETA (1.0–3.0 g·L?1) at 25 °C.Tables S2 and S3 showed,there were a significant agreement between the qeof Cr(VI)ions experimental data and qeobtained from modeled Langmuir isotherm on various dose of both adsorbents biochar.The results indicate the applicability of Langmuir isotherm model to adsorption of Cr(VI)ions onto Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars.Consequently,the occurring of monolayer adsorption of the Cr(VI)ions on the surface of the three adsorbents biochar is suggested.

Freundlich isotherm model was applied to analyze the adsorption characteristics of Cr(VI)ions on treated and untreated biochars.The linear fitting parameters from the model of Freundlich isotherm are listed in Table 2.Fig.14 represent the plot of lg(qe)versus lg(Ce)with the intercept value of lg KFand the slope of 1/nF,where KFis the Freundlich constant(L·g?1),related to the bonding energy.KFcan be defined as the adsorption or distribution coefficient and represented the quantity of Cr(VI)ions adsorbed onto adsorbent for unit equilibrium concentration.In general,as the KFvalue increases,the adsorption capacity of the adsorbent,for the given adsorbate,increases.It is worth to mention that the KFincreases for Melon-BO-NH2and Melon-BO-TETA than that for Melon-B biochars.In Freundlich model,it is relatively easy for adsorbent to adsorb solute when 1/n is less than 1.All 1/n values were less than 1,which indicated that the three adsorbents biochar were good at adsorbing of Cr(VI) ions (Table 2).Also,value nFindicates the degree of non-linearity between solution concentration and adsorption process.The values of nF>1,indicating that the adsorption of Cr(VI)ions on the three adsorbents biochar is a favorable physical process.It is expected that a physical adsorption within pores and electrostatic attraction on amine groups controls the adsorption mechanism.

The correlation coefficient obtained from Freundlich model was R2>0.989 for Melon-B,0.924 for Melon-BO-NH2and 0.998 for Melon-BOTETA.The correlation coefficient(R2)is wavy for Melon-BO-NH2while it is decreased with increasing the adsorbent dose of Melon-BO-TETA.The Qm(maximum adsorption capacity in the Freundlich equation)was 84.68 for Melon-B,175.23 for Melon-BO-NH2and 467.80 mg·g?1for Melon-BO-TETA.The higher the Qmvalue,the better the adsorption of Cr(VI)ions.It is worth mention that the Qmfor Melon-B is less than the Qmfor the Melon-BO-NH2and for Melon-BO-TETA(Table 2).The relation between qeof Cr(VI)ions obtained from experimental data and qeobtained from modeled Freundlich isotherm on various dose of Melon-B,Melon-BO-NH2and Melon-BO-TETA were reported in Tables S1–S3 in supplement.A reliable value of calculated qewas founded accordance with Freundlich isotherm model for all data collection.It could be concluded that the linear fits Langmuir equation was better than those for Freundlich equation for Melon-BO-NH2and Melon-BO-TETA biochar.While the Freundlich and Langmuir models isotherm fitting and agree with experimental data for Melon-B biochar.Therefore,the adsorption processes of Cr(VI)ions on Melon-BO-NH2and Melon-BO-TETA biochars were complicating and the probability of monolayer adsorption was higher than that of multilayer adsorption.

Fig.16.Pseudo first order kinetic of adsorption of Cr(VI) ions of initial concentration(a)(50–300 mg·L?1)by Melon-B adsorbent dose(3.0 g·L?1);(b)(50–300 mg·L?1)by Melon-B-NH2 adsorbent dose(3.0 g·L?1);(c) (100–400 mg·L?1)on Melon-BO-TETA doses(2.0 g·L?1)at(25±2)°C.

Table4 Pseudo first-order and pseudo second-order results of adsorption of Cr(VI)ions of different initial concentration(50–300 mg·L?1)onto Melon-B of different adsorbent doses(3–7 g·L?1 Cr(VI)ions solution)at(25±2)°C

Table5 Pseudo first-order and pseudo second-order results of adsorption of Cr(VI)ions of different initial concentration(50–300 mg·L?1)onto Melon-BO-NH2 of different adsorbent doses(1–3.0 g·L?1 Cr(VI)ions solution)at(25±2)°C

This model takes into account the effects of indirect adsorbent/adsorbate interactions on the adsorption process.Tempkin isotherm is usually used for heterogeneous surface energy systems(non-uniform distribution of sorption heat).The Tempkin constants[the slope AT=Tempkin isotherm equilibrium binding constant(L·g?1)and intercept BT=Constant related to heat of sorption(J·mol?1)]were determined from the linear relation between qeagainst ln Ce.From the Tempkin plot shown in Fig.15,the following values were estimated and listed in Table 2:AT=0.8,12.29,6.30,8.99,11.88,and 12.26 L·g?1and BT=13.20,8.07,7.53,7.12,and 7.36 J·mol?1for Melon-B of different adsorbent doses 3.0–7.0 g·L?1,respectively.AT=2.28,5.76,11.72,9.51,and 10.8 L·g?1and BT=19.49,17.10,15.39,16.19,and 17.00 J·mol?1for Melon-BO-NH2of different adsorbent doses 1.0–3.0 g·L?1,respectively.Also,AT=1.29,15.54,3.81,9.25 and 0.12 L·g?1and BT=59.88,33.49,50.78,33.15,and ?35.2 J·mol?1for Melon-BO-TETA of different adsorbent doses 1.0–2.5 g·L?1,respectively.Very low values of the heat of sorption obtained in the present study indicates rather weak ionic interaction between the adsorbate and the sorbent and metal removal seems to involve physisorption.The heat of Cr(VI)ions adsorption BTis directly related to coverage of Cr(VI)ions onto the three biochars due to adsorbent-adsorbate interaction.It was decreased with increasing adsorbents doses,as listed in Table 2.The correlation coefficients values obtained from Tempkin isotherm R2>0.987 for Melon-B,0.977 for Melon-BO-NH2and 0.965 for Melon-BO-TETA biochars.The values of R2proved that the Tempkin model fitted well with experimental data of the Cr(VI)ions adsorption on Melon-B and Melon-BO-NH2biochars.Examination of the data shows that the Tempkin isotherm is not applicable to the adsorption of Cr(VI) ions onto the high adsorbent doses of Melon-BO-TETA biochar due to the low correlation coefficients.Tables S1–S3 in supplement showed,there were significant agreement between the qeof Cr(VI)ions obtained from experimental and qeobtained from Tempkin isotherm model on various dose of the three adsorbents.

4.4.Error function studies for Best-fit isotherm model

The experimental equilibrium data of Cr(VI)ions onto the Melon-B untreated and treated biochars were analyzed by the four widely used Langmuir,Freundlich,Tempkin and D-R isotherm models.The best fit isotherm model for the experimental data was studied by several different error functions.The applied error functions are the average percentage errors(APE),Chi-square error(X2),the root mean square errors(RMS),the hybrid error function(HYBRID),the sum of the absolute errors(EABS),and Marquardt's percent standard deviation(MPSD)were used to detect the error distribution between the experimental equilibrium data and predicted isotherms.The calculated isotherm parameters by six error functions between experimental data and theoretical isotherms are given in Table 3 for Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars,respectively.The other three isotherm models are comparable and applicable to the experimental equilibrium data.Also,from Table 3,it is clearly indicates that the error functions(APE),(X2),(RMS),(HYBRID),(EABS)and(MPSD)do not take any much effort in considering the theoretical limits of Freundlich isotherm,because it is the best fit isotherm model for the experimental data of Melon-B biochar.While the Tempkin isotherm is the best fit isotherm for the experimental data of Melon-BO-NH2biochar and the linear-Langmuir isotherm is the best fit isotherm model for the experimental data of Melon-BO-TETA biochar.

Table3 Represent the best-fit isotherm model to the experimental equilibrium data by several different errors functions for Cr(VI)ions onto Melon-B,Melon-BO-NH2,and Melon-BO-TETA at(25±2)°C

4.5.Adsorption Kinetic studies

The pseudo-first-order and pseudo-second-order,Elovich and intraparticle diffusion equations are applied to model the kinetics of Cr(VI)ions adsorption onto Melon-B,Melon-BO-NH2and Melon-BOTETA biochars.The conformity of the models was expressed by the correlation coefficients(R2).

Straight line plots of lg(qe?qt)against t were used to determine the rate constant,k1,and equilibrium adsorption capacity(qe)as shown in Fig.16.Correlation coefficients (R2) for different initial concentrations of Cr(VI)ions and adsorbent doses of Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars shown in Tables 4–6.Based on the values of R2obtained from the plots of pseudo first-order rate equations,it is obvious that although the correlation coefficients obtained from the pseudo-first-order model are found to be mainly high,the calculated qevalues don't agree with experimental values.This indicates that adsorption of Cr(VI) ions onto Melon-B is not an ideal pseudo-first-order reaction (Tables 4–6).The correlation of experimental data to the pseudo-first-order model increased with increasing of initial adsorbent concentration from 1.0 to 3.0 g·L?1,in the case of Melon-BO-NH2biochar.In contrast of,the correlation coefficients R2obtained from the pseudo-first-order model are found to be mainly high at low initial concentration of Melon-BO-TETA adsorbent.Also,the theoretical values of qenot agree very well with the experimental ones.Both facts suggest that the adsorption of Cr(VI)onto Melon-BO-NH2and Melon-BO-TETA biochars,did not follow pseudo-first-order kinetics(Tables 4–6).

Fig.17.Pseudo second order kinetic of adsorption of Cr(VI)ions of initial concentration(a)(50–300 mg·L?1)by Melon-B adsorbent dose(3.0 g·L?1)at 25±2°C;(b)(50–300 mg·L?1) by Melon-B-NH2 adsorbent dose (3.0 g·L?1) at (25 ± 2) °C;(c) (100–400 mg·L?1)on Melon-BO-TETA doses(2.0 g·L?1)at(25±2)°C.

Fig.18.Elovich kinetic of adsorption of(a)Cr(VI)ions of initial concentration(50–300 mg·L?1)by Melon-B adsorbent dose(3.0 g·L?1),(b)Cr(VI)ions of initial concentration(50–300 mg·L?1)by Melon-BO-NH2 adsorbent dose(3.0 g·L?1),and(c)Cr(VI)ions of initial concentration(100–400 mg·L?1)on Melon-BO-TETA doses(2.0 g·L?1)at(25±2)°C.

The kinetics of Cr(VI)ions adsorption over untreated and treated samples of biochars following the pseudo-second-order was studied to investigate the promoting effect of amine groups on the adsorption kinetics.The pseudo-second-order kinetic constant,K2(g·mg?1·min?1),and the amount of Cr(VI)ions adsorbed at equilibrium (qe) can be calculated from the intercept and slope of the plot between t/qeversus t.Fig.17 show pseudo-second-order kinetic plots for the sorption of Cr(VI)ions over untreated and treated Melon-peel biochars.The calculated kinetic constant,K2,the experimental and theoretically predicted qeand the corresponding R2values were shown in Tables 4–6.The relatively higher R2values for a pseudo-second-order kinetics which are equal to unity suggested that pseudo-second-order proposed as a best fit kinetic model.

Table6 Pseudo first-order and pseudo second-order results of adsorption of Cr(VI)ions of different initial concentration(100–400 mg·L?1)onto Melon-BO-TETA of different adsorbent doses(1.0–3.0 g·L?1)at(25±2)°C

Table7 Elovich and intraparticle diffusion models results of adsorption of Cr(VI)ions of different initial concentration(50–300 mg·L?1)onto Melon-B of different adsorbent doses(3–7 g·L?1 dye solution)at(25±2)°C

In the present study,from Tables 4–6 it can be observed that the qevalues predicted using the pseudo-second-order plot successes to predict the qevalues for all initial dye concentration studied in which the experimental and theoretically predicted qewere significantly agreed.This confirms the applicability of pseudo-second-order kinetic model in predicting the amount of Cr(VI)ions adsorbed at equilibrium for three biochars.From Tables 4–6 it can be observed that,there did not exist a definite relationship between the initial dye concentration and reaction rate constant(K2).

If the adsorption process is chemisorption,the experimental results may be described by the Elovich model[62].Fig.18 depict the plot of qtversus ln(t).The Elovich constants α and β were calculated from the intercept and slope,respectively,of the straight line and were listed in Tables 7–9.The correlation coefficients R2are high for Melon-B and Melon-BO-TETA except at high initial Cr(VI)ions concentration where the R2values are low(Tables 7,9).The correlation coefficients R2are fluctuating without a definite role,it is high one time and low the other time for Melon-BO-NH2adsorbent biochar(Table 8).The effect of functional groups on the adsorption of Cr(VI)ions and the expected mechanisms were reported[78,79].

Table8 Elovich and intraparticle diffusion models results of adsorption of Cr(VI)ions of different initial concentration(50–300 mg·L?1)onto Melon-BO-NH2 of different adsorbent doses(1–3 g·L?1 dye solution)at(25±2)°C

The Elovich model provided a high degree of correlation with the experimental data for some of the sorption process for Melon-B,Melon-BO-NH2and Melon-BO-TETA adsorbents.Results show that chemical adsorption processes may be the rate-limiting in some cases of the sorption of Cr(VI)ions onto the three biochars during agitated batch contact time experiments.Also,the values of R2obtained in the present study in the other cases indicate the adsorption of Cr(VI)ions seems to involve physisorption.Therefore,the adsorption processes of Cr(VI)ions on Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars were complicating and the probability of chemical adsorption was higher than that of physical adsorption.

In the solid–liquid sorption process,the solute transfer is usually characterized by intra-particle diffusion.In this study,the intraparticle diffusion model was used.Because after treatment,it is necessary to explain and identify the steps involved during adsorption process.Three steps in the adsorption process on solid particles occurred:(i)The diffusion of solute molecules from the solution to the surface of the solid particles through a liquid film.(ii)Diffusion from the surface to the inside adsorbents,which is likely to be a slow process.(iii)Chemical reaction occurred on the active group of the solid particles.The last step is the fastest.The adsorption process is affected by the speed of the three steps,with the controlling the adsorption rate as the slowest step.In general,the adsorption process controlled by inner and film diffusions.

Webber and Morris [72] provided the amount of Cr(VI) ions adsorbed at time t as qt(mg·g?1),the rate constant for intraparticle diffusion as Kdif(mg·g?1?min0.5),and C is related to the thickness of the boundary layer.Webber and Morris thought that if the qtand t0.5have a linear relation that passes through the origin,the adsorption is controlled by intra-particle diffusion,however,if the line does not pass the origin,a larger C which indicates the film diffusion was greatly influences adsorption rate.Fig.19 shows the Webber-Morris adsorption line of Cr(VI)ions adsorption onto the three biochars at different initial Cr(VI) ions concentration using different absorbents dose.The values of C and Kdifare calculated from the intercept and slope of the plot of qtagainst t0.5,respectively,and represented in the Tables 7–9.From Fig.19,we can deduced that the straight lines are not pass through the origin and the intercept C is quite large indicates that within time,the adsorption rate was faster and mainly does not controlled by the intra-particle diffusion,but the film diffusion was the dominant step that controlled theadsorption rate.This may be attributed to the surface area and pore volume which slightly reduced after treatment which show intraparticle diffusion based mechanism cannot account for Cr(VI) ions adsorption enhancement.Also,the values of C increased significantly with increasing of adsorbent doses of Melon-BO-NH2and Melon-BOTETA than that for Melon-B biochars,it is speculated that it could be due to the electrostatic attraction between amino groups on the absorbents surface and the adsorbate ions.

Table9 Elovich and intraparticle diffusion models results of adsorption of Cr(VI)ions of different initial concentration(100–400 mg·L?1)onto Melon-BO-TETA of different adsorbent doses(1.0–3.0 g·L?1)at(25±2)°C

Fig.19.(a)Intraparticle diffusion of adsorption of Cr(VI)ions of initial concentration(50–300 mg·L?1) by Melon-B adsorbent dose (3.0 g·L?1),(b) Cr(VI) ions of initial concentration(50–300 mg·L?1)by Melon-BO-NH2 adsorbent dose(3.0 g·L?1),(c)Cr(VI) ions of initial concentration (100–400 mg·L?1) on Melon-BO-TETA doses (2.0 g·L?1)at(25±2)°C.

4.6.Comparison of the obtained results with some of the results reported in literature

Comparison of adsorption of Cr(VI)ions from aqueous solution by Melon-B,Melon-BO-NH2,and Melon-BO-TETA biochars in the present study with different adsorbent materials indicated that the prepared biochars are effective adsorbents for removal of Cr(VI)as reported in Table 10.The Melon-BO-TETA reported the maximum adsorption capacity (333.33) of Cr(VI) ions compared to the other adsorbents reported in Table 6.

Table10 Comparison of maximum adsorption capacity of Hexavalent chromium by different adsorbents

4.7.Regeneration of biochars

Adsorption by adsorbent materials is vastly used technology for removing of pollutants from wastewater and the economy of this technology extremely depends on the reuse of these adsorbent materials.Chemical regeneration technique are utilized for the regeneration of exhausted Melon-B,Melon-BO-NH2,and Melon-BO-TETA biochars.The Melon-B,Melon-BO-NH2,and Melon-BOTETA biochars were regenerated by washing with 0.1 mol·L?1NaOH solution,0.1 mol·L?1HCl and then followed by hot distilled water [86].The maximum removal of Cr(VI) ions by Melon-B,Melon-BO-NH2,and Melon-BO-TETA biochars was found to be 45.8%,75.2%,and 85.3% after six regeneration cycles,respectively(Fig.20),which proved the regeneration possibility of Melon-B,Melon-BO-NH2,and Melon-BO-TETA biochars.

5.Conclusions

In this study,many goals were achieved;firstly,the various novel synthesis methods for modified biochars production and the effect of these methods on the physicochemical properties of biochar surface,along with the corresponding effect on Cr(VI) ions adsorption were study.Secondly,a small amount of biochar can be used for Cr(VI)ions adsorption was successfully performance which may lead to decrease the cost of waste water treatment.More than 80%of total adsorption was achieved at the first rapid stage,which is the most importance in the practice for industrial applications.Thirdly,making a cost-effective biochar for field application from selective cheap biomass such as Watermelon peels was occurred.Fourthly,enhancing the biochar properties via addition of functional groups was evaluated.Fifthly,the Cr(VI)ions adsorption capacity was significantly enhanced and reached five times greater for Melon-BO-TETA than Melon-B biochar.Melon-B biochar was prepared from Watermelon peels by sulfuric acid,the surface of prepared Melon-B biochar was chemically modified by O3followed by reflux with NH4OH or TETA and the comparison of Cr(VI)ions adsorption by various biochars were reported.The performance of Cr(VI) ions adsorption onto Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars was evaluated at room temperature 25 °C.After modification the amount of Cr(VI) ions adsorption significantly increased probably due to the existence of a great chemical affinity between Cr(VI) ions and amine sites.EDAX and FT-IR analysis data indicated that the modified Melon-BO-NH2and Melon-BO-TETA biochars were successfully functionalized with amino groups.Overall,the highest percentage of removal and the highest adsorption capacity was obtained at a pH 1.49.Experimental data of Cr(VI)ions adsorption by Melon-B,Melon-BO-NH2and Melon-BO-TETA biochars were studied by Langmuir,Feundlich,and Tempkin isotherm models.The error function equations are applied to the data of isotherm models to find the best fit model.The Freundlich isotherm is the best fit isotherm model for the experimental data of Melon-B biochar.While the Tempkin isotherm is the best fit isotherm for the experimental data of Melon-BO-NH2biochar and the linear-Langmuir isotherm is the best fit isotherm model for the experimental data of Melon-BO-TETA biochar.The kinetic study of Cr(VI) ions uptake revealed that,The pseudo-second order kinetic model is the most appropriate to fit the adsorption kinetic data.

Fig.20.Effect of regeneration cycles for(a)Melon-B,(b)Melon-BO-NH2,and(c)melon-BO-TETA biochars on the adsorption and desorption%using adsorbent dose(3.0 g·L?1)and Cr(VI)ions initial concentration(200 mg·L?1)at(25±2)°C.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.020.