S.I.Moussa,M.M.S.Ali,Reda R.Sheha

1 Nuclear Chem.Dept.,Hot Lab Center,Atomic Energy Authority,Cairo 13759,Egypt

2 Analytical Chem.Dept.,Hot Lab Center,Atomic Energy Authority,Cairo 13759,Egypt

Keywords:Activated carbon Nickel ferrite Composite Mechanism

ABSTRACT A novel magnetic activated carbon composite(AC/NiF)was synthesized by a precipitation method and applied in retention of Cu(II),and Zn(II)ions from aqueous solutions.The impact of different sorption parameters such as:equilibration time,solution pH value,competing cations and ionic strength on the amount sorbed of Cu(II),and Zn(II)was clarified.Results illustrated that the magnetic composite had retention ability towards both metal ions significantly higher than that of activated carbon(AC).The magnetic composite exhibited an affinity to adsorb Cu(II)higher than Zn(II)ions.The maximum sorption capacities(Qmax )of the applied magnetic composite(AC/NiF)towards Cu(II)and Zn(II)were 105.8 and 75.1 mg·g?1,respectively.Retention of Cu(II)and Zn(II)was proposed to be achieved though an ion exchange and surface adsorption in neutral conditions,while precipitation was believed to be the relevant mechanism in their removal from basic solutions.The kinetic studies showed that sorption process followed the kinetics of pseudo-second-order reactions with rate constant of 3×10?3 and 2×10?3 min?1 for sorption of Cu(II)and Zn(II)onto AC/NiF composite.Removal of Cu(II)slightly decreased with increasing the ionic strength of aqueous solution,using NaCl as a background electrolyte.In contrast,presence of Mn(II),Mg(II)and Co(II)in reaction solutions highly depressed the sorption of Cu(II)and Zn(II)with a competing efficiency followed the order:Mg(II)>Mn(II)>Co(II).The magnetic composite was rapidly recovered from aqueous solution by an external magnetic field,and effectively regenerated using 0.1 mol·L?1 HCl and 0.1 mol·L?1 FeCl3 as eluents.Sorption of Cu(II)and Zn(II)onto the surface of AC/NiF composite occurred via a spontaneous reaction.And thermodynamically favorable process had ΔHo values of 30.9 kJ·mol?1 and 19.7 kJ·mol?1,respectively.The results confirm that the magnetic composite can be viewed as a promising novel composite opens new opportunities for the attainment of required adsorption and operative magnetic separation.

1.Introduction

Heavy metals are natural components of Earth's crust usually associated with toxicity[1].The wide application of heavy metals in many industries has resulted in an increased presence of different metallic materials in water sources [2,3].The presence of heavy metals such as Cu(II),Cd(II),Hg(II),Zn(II),Pb(II),and others metals in aqueous environment has an increasing significance and prime interest[4].Environmentally,they pose a serious threat to plants,animals and even human beings owing to their bioaccumulation,non-biodegradable properties and toxicity even at low concentrations[5].Water pollution,that caused by heavy metals,has harmful effects on the environment all over the world.It was reported that water pollution is the major reason for death of about 14,000 person everyday[6].Actually,the acute problems related with water pollution have paid many countries,as Egypt,to start strict environmental regulations associated with several industries and funded research institutes to get cost-effectively and feasible water treatment methods.

Many technologies were applied for removal of heavy metals from waste water.They involve:1)chemical precipitation,2)ion exchange,3)membrane filtration,4)electrochemical treatment.5)adsorption,etc.[7–10].Among these technologies,adsorption is widely used and effectively applied in removal of different metal ions,using various adsorbents.This is owing to its high efficiency,low coast and simple operation[11,12].The conventional adsorbentsaszeolites,clays,biomass,polymeric materials,metal oxides and activated carbon (AC) were widely applied in retention of many heavy metals[13–15].

Metal oxide based materials have been extensively used as sorbents,for metal ions removal,due to their high natural abundance,low cost and environmentally benign.Metal oxides,especially iron counterparts,are nanoscale materials have high specific surface area and display enhanced adsorption with rapid kinetics.Ferrites are soft magnetic materials well known with their cubic spinel structure.They have excellent chemical stability and high saturation magnetization.Recently,the utilization of MnFe2O4and CuFe2O4ferrites,in treatment of polluted water,has been studied and exhibited admirable adsorptive properties with an effective recovery[16].Among ferrites,nickel ferrite(NiFe2O4)is an emotive material has a moderate saturation magnetization,good chemical stability and mechanical hardness[17].One of the problems accompanied with nanosized ferrite particles is their tendency to form agglomerates due to their high surface energy and magnetic dipolar interactions [18].To address these issues,iron oxides have been combined with varied matrixes as carbonaceous materials to fabricate magnetically responsive composites[19].

Activated carbon is a common adsorbent composed from graphene sheets randomly replaced with hetero-atoms.The characteristics of any prepared activated carbon depend,in particular,on the raw materials used and the activation process applied in their manufacturing[20].AC has a well-developed internal pore structure,a large surface area,and featured with presence of several functional groups on its surface.Hence,it has been used as an effective and economical adsorbent,has high sorption capacity for removal of different metal ions from their aqueous solutions[21–23].Experimentally,application of AC in separation processes is limited due to some disadvantages.Of these limitations,the difficulties that encountered in separation and regeneration of spent powdered AC sorbents.In addition,the traditional method for separating the exhausted powdered AC sorbents,as filtration,could cause the blockage of filters and/or loss of carbon sorbents[24].The increased demand on activated carbon has encouraged many researchers to improve its adsorption properties.Hence,the research efforts were recently directed to overcome the disadvantages of powdered activated carbon though acquiring it a magnetic property.The modification of activated carbon with ferrite particles is a new and good way to prepare promising novel magnetic activated carbon composites can be easily separated using a magnetic technology[25].

Therefore,the effort in this work was devoted to combine the advantages of activated carbon and magnetic nickel ferrite(NiFe2O4)particles to prepare a promising novel composite opens new opportunities for the attainment of required adsorption and operative magnetic separation.In this paper,a facile co-precipitation method was applied to prepare activated carbon/NiFe2O4magnetic composite via a chemical deposition of NiFe2O4onto AC.The performance of prepared composite to retain Cu(II)and Zn(II)ions from aqueous solution was investigated.The sorption equilibrium at different conditions,mechanisms and thermodynamics was also evaluated.

The evaluation of sorption equilibrium at different conditions,included mechanisms and thermodynamics was investigated as well.

2.Experimental

2.1.Materials

Copper chloride(CuCl2,134.45 g·mol?1)and Zinc chloride(ZnCl2,136.29 g·mol?1)were purchased from Aldrich Chemical Laboratories.Ferric chloride(FeCl3,162.21 g·mol?1)and nickel chloride(NiCl2.6H2O,237.64 g·mol?1)was supplied from Redel,Germany.Phosphoric acid(H3PO4,98 g·mol?1)was supplied from Peking Chemical Works,China.Ammonia solution (NH4OH,33%) was a product of Sigma-Aldrich,Germany.All reagents and chemicals,used in this work,were of analytical grade purity and were used without further purification.Stock solutions of 1000 mg·L?1Cu(II)and Zn(II)ions were prepared by dissolving the required amount of their salts in definite volume of bi-distilled water and were used to prepare different adsorbate solutions have the desired concentrations.The pH value of aqueous solutions was stabilized by addition of 0.1 mol·L?1HCl and/or NaOH solutions.

2.2.Preparations

2.2.1.Activated carbon(AC)

Activated carbon was prepared from corncobs raw materials.Corncobs were firstly collected,dried and crushed.A weight of 40 g dried sample was immersed in 50%H3PO4for 24 h.Then,it was filtered and transferred to a stainless steel cylinder reactor has 4 cm diameter and 60 cm length with narrow ports of 1 cm diameter at both ends.The sample was heated at 600°C for 2 h and then let to cool.The cooled activated mass was continuously washed with hot water to remove the extra acidity till washing solution has pH～6.The washed activated mass was dried at 110°C,and finally stored in tight glass containers.

2.2.2.Nickel ferrite(NiF)

Nickel ferrite(NiFe2O4)was prepared by a precipitation method.A solution of 500 ml 1 mol·L?1FeCl3was stirred and purged with N2gas at room temperature.To this solution,an equal volume of 0.5 mol·L?1NiCl2·6H2O solution was added slowly under continuous stirring and nitrogen flow to have Fe3+∶Ni2+molar ratio 2∶1.The mixture was agitated and purged with N2for 30 min at room temperature,then NH4OH solution(25%)was added until pH attained the value of 11.A brown precipitate of nickel ferrite was formed,washed several times with bi-distilled water till pH～7 was attained.Finally,nickel ferrite precipitate was separated and dried at 70°C for 48 h.

2.2.3.Magnetic composite(AC/NiF)

Magnetic composite was prepared using a known weight of activated carbon,following the same previous preparation steps with the same ingredients.

2.3.Characterization

The specific surface area was determined from N2adsorption isotherm using BET method with Nova 3200,Version 6.08 High Speed Gas Sorption Analyzer.The surface characteristics of the synthesized resins were characterized using Fourier Transform Infrared(FT-IR)spectrophotometer of type Nicolet iS10,Thermo,USA.The powder X-ray diffraction patterns was recoded with Philips X-ray diffractometer model PW1710 equipped with mono-chomatized CuKαradiation(λ=0.154 nm,40 kV and 25 mA) employing a scan rate of 0.02 (°)·s?1in the range from 10°to 70°.Samples morphology was analyzed using Scanning Electron Microscope(SEM)model JSM-6510A from JEOL,Japan.The concentrations of Cu(II)and Zn(II)were determined using an Atomic Absorption Spectrometer (AAS).Analysis was performed using Solaar-II M5 atomic absorption spectrometer from Thermo Fisher Scientific Inc.,Cambridge,UK.The measurements were carried out using Air-C2H2flame with a fuel flow rate 1.2 L·min?1.

2.4.Sorption study

The sorption of Cu(II)and Zn(II)on prepared sorbents was carried out using batch technique.The experiments were performed using 0.1 g of prepared samples and 10 ml of Cu(II)or Zn(II)solutions have initial concentration of 275 mg·L?1and 220 mg·L?1,respectively.The initial pH value was adjusted at (4.5 ± 0.1) for both metals.The samples were shaken at room temperature(25±1)°C in glass vials for a definite time period.After equilibrium,the sorbents were separated from the solutions using a permanent magnet and the final concentrations Cu(II)and Zn(II)were determined,using AAS.

The effect of shacking time was studied at different time intervals ranged from 5 min to 50 h.After each time period,sorbents were magnetically separated and the residual of metals ions concentration was determined.In addition,the sorption was investigated as a function of pH value to explore the role of pH value on the retention Cu(II)and Zn(II)onto prepared sorbent.The initial pH values were adjusted from 1 to 9,using 0.1 mol·L?1HCl and/or 0.1 mol·L?1NaOH solutions.After equilibrium,the sorbents were separated and the metal concentration was determined using AAS.

The influence of ionic strength on Cu(II) and Zn(II) sorption was verified by carrying out a set of experiments at different electrolyte concentrations covered the range from 0.05 mol·L?1to 0.8 mol·L?1NaCl.Also,an additional set of experiments was performed,as previously described,with presence of different concentrations of Mg(II),Mn(II)and Co(II)as competing ions.In all sets,the sorbents were magnetically separated,after equilibrium,and the supernatants were subjected to AAS measurements.

Temperature is an important parameter in thermodynamic studies.To clarify the influence of temperatures on Cu(II)and Zn(II)sorption,experiments were conducted at different temperatures ranged from 25°C to(55±1)°C at pH(4.5±0.1).Finally,Cu(II)and Zn(II)concentrations were determined after equilibrium for all temperature degrees.

2.5.Desorption study

The reversibility of Cu(II)and Zn(II)sorption was investigated by performing desorption experiments using different eluents.Basically,the prepared composite was loaded with Cu(II)and Zn(II)ions by equilibrating them individually with Cu(II)and Zn(II)solutions.The applied magnetic composite was magnetically separated and the residual concentrations of metal ions were determined.Subsequently,10 ml of five kinds of eluents including H2O,0.1 mol·L?1NaOH,0.1 mol·L?1HCl,0.1 mol·L?1NiCl2and 0.1 mol·L?1FeCl3were thoroughly added to the five sets of loaded sorbents and stirred for 24 h at room temperature(25±1)°C .After equilibrium,the supernatants were separated and the concentrations of eluted metals ions were measured.The desorption ratio(D)was calculated using Eq.(1)[26]:

3.Results and Discussion

3.1.Characteristics of synthesized materials

The physic-chemical properties of the prepared samples were examined using scanning electron microscope,surface area and FT-IR spectroscopy and the results were displayed below.

3.1.1.SEM analysis

The SEM images of as-prepared AC,AC/NiF composite and AC/NiF composite loaded with Cu(II)and Zn(II)were presented in Fig.1.The image in Fig.1(a)clarifies that activated carbon prepared from Corncobs have a porous structure with several pores homogeneously pervaded on its surface.When activated carbon was merged with nickel ferrite(NiFe2O4)particles during the synthesis of magnetic composite,ferrite particles were randomly distributed on AC surface and coated it.It is notable that synthesis of AC/NiF magnetic composite consumed a part of the surface porosity of AC.Although,a part of AC porosity was shielded by NiFe2O4aggregates,the synthesized composite possesses a porous structure,could still be seen,Fig.1(b).This surface porosity promoted the retention of Cu(II)and Zn(II)onto the composite's surface.After loading Cu(II)and Zn(II)onto the composite's surface,a significant change in morphology was observed,Fig.1(c,d).The surface morphology was changed from being smooth to rough texture.

3.1.2.Surface area

The surface characteristic of prepared samples was investigated using N2gas adsorption and the obtained nitrogen adsorption–desorption isotherms are shown in Fig.2.The samples were firstly degassed before nitrogen adsorption measurement.The plots of nitrogen adsorption isotherm for both AC and AC/NiF magnetic composite could be classified as type-V with H3 hysteresis loop according to Brunauer-Deming-Deming-Teller(BDDT)classification.This classification indicates the existence of slit-shaped pores.Also,the isotherms showed a hysteresis loop reflecting textural pores formed between plate-like particles[27,28].The large SBET further confirms the porous structure of prepared sorbents,which agreed with the results of SEM images.The textural properties of synthesized materials,such as specific surface are given in Table 1.It is clear from these data that AC/NiF magnetic composite has high SBETand VPvalues compared with that of nickel ferrite.These properties announce the promising surface characteristics of AC/NiF magnetic composite to be used as a novel sorbent in removal of different metal ions and verify the role of AC in changing the texture properties of synthesized sorbent.The pore size distribution curves are given in Fig.S1 of supplementary data(SD).They are quite broad and multimodal with small and large mesopores.The small mesopores reflect porosity in nano-flakes,while large ones could be related to the pores formed between stacked nano-flakes[29,30].

3.1.3.FT-IR analysis

The FT-IR spectra of as-prepared samples and loaded AC/NiF magnetic composite are shown in Fig.3.The spectrum of activated carbon exhibits broad band at 3422 cm?1could be assigned to O—H stretching of carboxylic acid functional group[31],Fig.3(a).The peaks detected at 2923 cm?1and 2854 cm?1may be assigned to asymmetric and symmetric—CH stretching bands.The stretching vibrations peak of C=O group appeared at 1745 cm?1.The peak detected at 1563 cm?1could be attributed the carboxylic and carboxylate anion stretching mode[32,33].The peaks detected at 1461 cm?1and 1431 cm?1were assigned to C=C stretching bands.The peaks between 1212 cm?1and 1081 cm?1was associated with—C—O stretching and—OH bending modes of alcoholic,phenolic and carboxylic groups.These observations indicate that carboxylic group had been successfully introduced on the surface of AC.In Fig.3(b)that displays the spectrum of NiFe2O4.The peak imposed at 1619 cm?1could be assigned to the bending vibration of adsorbed water molecules.The band at 1462 cm?1may be corresponding to the deformation vibration of hydroxyl groups on metal oxides.The peaks observed at 594 cm?1could be referred to the stretching vibration of tetrahedral groups of Fe2O4.In Fig.3(c),the spectrum includes the characteristic peaks of both activated carbon and nickel ferrite which confirms the formation of AC/NiF magnetic composite.In additions,new peaks were detected at 1082 cm?1,1032 cm?1that may be assigned to C—O bond of activated carbon.Also,the new peaks observed at 606 cm?1and 425 cm?1and could be ascribed to Fe—O vibration of octahedral spinel ferrite[32,34].All these features imply a change in the surface functionality of the as-synthesized magnetic composite.

It is worth to note that sorption of Cu(II)and Zn(II)on the surface of synthesized magnetic composite increased the broadness and intensity of some peaks while other peaks were hardly identified in the spectra of composite,Fig.3(d),(e).This finding could be referred to the probability of overlapping of these peaks due interaction with Cu(II)and Zn(II)ions.Also,AC/NiF magnetic composite loaded with Cu(II)and Zn(II)exhibited new absorption bands at1559 cm?1and 1380 cm?1,respectively.This confirms the formation of new C—O and Fe—O bonds and deformation of H—O,respectively during the sorption process[35].

3.2.Retention study

The sorption behavior of Cu(II)and Zn(II),from aqueous solutions,onto the surface of synthesized materials was investigated under the role of different parameters using batch technique.

3.2.1.Equilibrium time

Fig.1.SEM micrographs of:a)as-prepared activated carbon,b)magnetic AC/NiF composite,c)Cu(II)loaded composite and d)Zn(II)loaded composite.

The variation in uptake of Cu(II)and Zn(II)with time was studied at different agitating periods covered the range from 5 min to 48 h,using batch technique,and the data are exhibited in Fig.4(a),(b).The plots clarify that the rate of Cu(II)uptake was initially fast compared with that of Zn(II),followed by a slower rate until reaching equilibrium state.More than 50% of Cu(II) was removed by both AC/NiF and NiF samples during the first 1 h of reaction,while a small amount not exceed 20%was removed using AC,Fig.4(a).A further increase in contact time had a slight increase on the removed quantity of copper up to 48 h.In contrast,the removal of Zn(II) was quite slow and gradually increased with time till reaching equilibrium after 24 h,Fig.4(b).It is obvious that the samples AC/NiF and NiF exhibit a high ability to retain Zn(II)compared with AC sample.Generally,the time necessary to reach equilibrium was found to be 24 h,where a further increase in contact time up to 48 h did not bring any remarkable effect on the separated quantity of Zn(II).Accordingly,a contact time of 24 h was selected for the rest of experiments.

Fig.2.Nitrogen adsorption–desorption isotherms of:a)as-prepared activated carbon,b)magnetic AC/NiF composite.

The uptake of Cu(II)onto AC,NiF and AC/NiF samples,attained the values 30.3%,77.8% and 92.7% while that of Zn(II) were 16.5%,64.0%and 72.3%respectively.It is pertinent to note that AC/NiF magnetic composite exhibited a high affinity to both Cu(II)and Zn(II)ions and removed them from aqueous solution with fast rates compared with AC.These premium sorptive properties could be attributed to the textural characteristics and enhanced surface functionalization of AC/NiF composite.The combination of activated carbon with nickel ferrite produced a magnetic composite has unique physicochemical properties differ from their bulk counterparts[30].Based on FT-IR analysis,the functional groups present on the surface magnetic composite are oxygenous functional groups,(such as:C—O—C,—COOH,C=O,C—O and O—C=O groups),that promoted from AC surface,and metal hydroxyl groups(as:Ni—OH,Fe—OH groups),that exhibited from the ferrite constituent.Hence,the properties of magnetic composite's surface are upgraded and highly functionalized compared with its initial constituents.Such enhanced surface functionalization of composite's surface increases the sorptive interaction of Cu(II)and Zn(II)analytes with composite,and hence promote its sorption performance compared with its constituents.

It is notable that sorption of Cu(II)ions is higher than Zn(II)ions,although both of them belong to the same first(3d)d-block series of the transition metals in the periodic table.These findings are similar to the sorption trends that reported by many other investigators[36,37].This behavior could be attributed to the change in the ionic radius of both Cu(II)and Zn(II)as well as the types of their interactions with sorbent surface[38].The high affinity of prepared magnetic composite towards Cu(II)ions compared with that for Zn(II)ions could be referred to their ionic radius and electronegativity.The ionic radius(?)and electronegativity(Pauling)of Cu(II)ions have the values 0.73and 1.9 while that of Zn(II)ions have the values 0.74 and 1.6,respectively.The higher electronegativity of a heavy metal the stronger the attraction of its ions on the negatively charged surface's sites.Moreover,the above affinity order was consistent with standard reduction potential of the heavy metal ion.The standard reduction potentials for Cu2+/Cu and Zn2+/Zn are ?0.3419 V and ?0.7618 V,respectively[39].These characteristics promote Cu(II)ions to be highly retained within the surface of magnetic composite compared with Zn(II)ions.

Table 1 The textural properties of synthesized sorbents

Fig.3.FT-IR spectra of as-prepared and loaded samples.

Fig.4.Effect of contact time on uptake of:a)Copper and b)Zinc ions on the prepared samples.(Co Cu(II) =275 mg·L?1,CoZn(II) =220 mg·L?1,V/m=100 ml·g?1,T=25°C,pH=4.5).

Table 2 The maximum sorption capacity of synthesized sorbents towards Cu(II)and Zn(II)ions in comparison with other sorbents

The maximum sorption capacity(Qmax)of both Cu(II)and Zn(II)was experimentally determined though sequential sorption processes.The values of Qmaxof AC,NiF and AC/NiF towards Cu(II)were found to be 31.07 mg·g?1,92.4 mg·g?1and 105.8 mg·g?1,while those for Zn(II)sorption onto AC,NiF and AC/NiF were 17.3,66.4 mg·g?1and 75.1 mg·g?1,respectively.The data clarify that AC/NiF magnetic composite exhibited a sorption capacity much higher than that of other ferrites and conventional sorbents,Table 2.The results has clearly authenticate that the constructive synthesis of the core-shell structure of AC/NiF have introduced a novel and promising performance to the synthesized composite compared with its constituent to retain heavy metals from hazardous aqueous solutions.

3.2.2.Sorption kinetics

To select the optimum operating conditions for batch sorption process,the sorption rate constants were determined and clarified.For this concern,two different kinetic models were used to estimate the different rate constants and investigate the mechanisms of sorption process.The curves of Lagergren equation were obtained by plotting ln(qe–qt)vs.time according to the pseudo-first-order model following Eq.(2)[40]:

where qeand qtare the amounts sorbed from Cu(II)and Zn(II)per unit mass of synthesized sorbents at equilibrium and at any time(t),and k1is the first-order sorption rate constant.The plots exhibit linear relations and the rate(mg·g?1)constants were determine from the slopes,and their values along with correlation coefficients (R2) are given in Table 3,Fig.S2 in supplementary data(SD).The results clarify that the calculated qevalues for Cu(II) sorption on AC,NiF,and AC/NiF were found to be 6.63 mg·g?1,14.97 mg·g?1and 16.55 mg·g?1,respectively while the experimental values were 7.41 mg·g?1,18.7 mg·g?1and 24.1 mg·g?1,respectively.The amount adsorbed(qe)of Zn(II)on AC,NiF,and AC/NiF was found to be 1.94 mg·g?1,12.0 mg·g?1and 13.33 mg·g?1,respectively while the experimental values were 3.68 mg·g?1,14.16 mg·g?1and 16.14 mg·g?1,respectively.It obvious that qevalues obtained from Lagergren plots differed from the experimental values.Although Lagergren plots exhibited linear relations,it did not essentially assure a first-order mechanism.This is mainly due to the inherent falsely in the values of estimated sorption capacity.Therefore,the kinetics of first-order-model are less probable to clarify the rate processes.

Table 3 Pseudo-first-order constants for retention of Cu(II)and Zn(II) ions onto synthesized sorbents

The former deviation has further led to examine the kinetics of interactions using the second-order equation developed by Ho and McKay[41].It could be expressed as the following Eq.(3).

where:k(g·mg?1·h?1)is the pseudo-second-order rate constant,plotting t/qtagainst t,for sorption of Cu(II)and Zn(II)on AC,NiF,and AC/NiF sorbents gave straight lines,Fig.S3 in SD.The values of rate constants were estimated from the slopes of straight terms,and data are listed in Table 4.It is clear that the correlation coefficient(R2)is extremely high and closer to unity(R2=0.999).The values of the second order rate constant for Cu(II) sorption varied from 3 × 10?3to 4 × 10?3g·mg?1·min?1,while it changed from 2.2 × 10?3to 2 × 10?3g·mg?1·min?1for sorption of Zn(II).Also,data indicate that qevalues obtained using pseudo-second-order model are consistent with the experimental values for both Cu(II)and Zn(II).On the light of these data,sorption kinetics could be elucidated more satisfactorily by pseudosecond-order model.So,the rate-determining step in sorption of both metal ions is a chemisorption process[42].

3.2.3.Effect of pH

pH of aqueous solution is one of the most significant parameters that affect the sorption of different metal ions.It changes the speciation of almost metal ions,alters their physic-chemical interactions with the surfaces of solid sorbents and varies the competition of hydrogen ions with these metal ions to the available active sites on sorbents'surfaces.The effect of initial pH value on uptake of Cu(II)and Zn(II)ions using synthesized sorbents is shown in Fig.5(a),(b).Sorption was studied at initial pH values ranged from～1.2 to 8.3 for Cu(II)and from 1.3 to 7 for Zn(II).To authenticate the solubility range of both metal ions under applied experimental conditions,blank experiments were performed in absence of synthesized sorbents.The results illustrate thatCu(II)precipitation started at pH～5.5 when its initial concentration equaled 275 mg·L?1,while Zn(II)precipitation had obviously initiated at pH～ 6 when its initial concentration equaled 220 mg·L?1.This precipitation increased with rising pH over the mentioned values for both metal ions.

Table 4 Pseudo-second-order constants for retention of Cu(II)and Zn(II)ions onto synthesized sorbents

The retention of Cu(II) onto synthesized magnetic composite increased with increasing initial pH up to 4 and attained a constant value at higher pH values,Fig.5(a).The uptake percent of Cu(II)ions onto AC/NiF attained the value 91%at initial pH of 4.04.This value approximately remained constant with rising pH values up to 8.3.Also,data displayed in Fig.5(b)clarify that AC/NiF magnetic composite exhibited a removal percent of 70.5%from Zn(II)ions at pH 2.8.This percent was slightly increased with rising pH value up to 7.It is worthy to note that retention of Cu(II)and Zn(II)ions on all sorbents decreased the final pH values.Such decrease,in final pH,indicates that sorption of Cu(II)and Zn(II)resulted in a liberation of proton from the active sites on the surface of magnetic composite into the aqueous solution[43].

3.2.4.Speciation

Fig.5.Effect of pH value on uptake of:a)Copper and b)Zinc ions on the prepared samples.(Co Cu(II) =275 mg·L?1,CoZn(II) =220 mg·L?1,teq =24 h,V/m=100 ml·g?1,T=25°C).

The change in sorption of Cu(II) and Zn(II) could be assigned to changing the speciation of these metal ions and altering the functionality of the magnetic composite's surface.The distribution of various hydrolyzed ionic species of both Cu(II)and Zn(II)as a function of pH at(25 ± 2) °C was calculated using visual MINTEQ software [44].The modeled data are given in Fig.6(a),(b).It is clear that copper presents in aqueous solution mainly in Cu2+form up to pH 6.The positively charged Cu(OH)+species appear in pH range 5–11 while the hydrolytic productsappear in pH range 6–10.Also,the neutral species Cu(OH)2began to precipitate at pH 6.0 and become prime at pH 9.5.The negatively charged speciesandpredominate at pH values higher than 11,Fig.6(a).

Fig.6(b)clarifies that zinc presents,in aqueous solution,mainly in Zn2+form up to pH～6.The positively charged Zn(OH)+species appear in pH range 5–10.The solids Zn(OH)2were initially formed at pH 6 and become prime at pH 10.Also,the negative speciesand Zn(OH)appear at pH values 9 and 11and become predominant at pH values 12 and 14,respectively.These data avouch that electrostatic attraction between the studied metal ions and the positively charged site of synthesized magnetic composite was unfavorable interaction in acidic pH range,while precipitation could be the main mechanism participating in retention Cu(II)and Zn(II)at pH values over 6[42].

Fig.6.The distribution of:a)Copper and b)Zinc ionic species in aqueous solutions at different pH values.

Table 5 The complexation equilibria and stability constants of copper and zinc hydroxocomplexes

In addition,the formation constants of Cu(II)and Zn(II)hydroxycomplexes are listed in Table 5[45].The data illustrate that the stability constants(lgβ)attained the values ?7.29 and ?7.89 for Cu(OH)+and Zn(OH)+hydroxy-complexes,respectively.These ensure that Cu(II)hydroxy-complexes is stronger than that of Zn(II),and this is in an agreement with Irving William series in which Cu(II)>Zn(II)[46].Although lgβ values increase with increasing the atomic number of the metal ion,Cu(II)ions form hydroxyl-complexes have higher stability compared with that of Zn(II),even the atomic number of copper(29)is smaller than that of zinc(30).The revealed stability is due to the distortion correlated to Jahn-Teller effect which always occurs in Cu(II)complexes[47].

3.2.5.Retention mechanism

At low pH,sorption was low and this may due to the protonation of surface active sites caused by of π-electron rich regions on the surface of magnetic composites and presence of increased concentration of H3O+ions.H3O+ions could establish bonds with the cloud of π-electrons of the aromatic rings of activated carbon constituent in magnetic composite creating positive surface charge [48,49].Thus,the decrease in uptake of Cu2+and Zn2+ions,in highly acidic medium,was due to the positively charged surface site and the competition between H3O+and both metal ions for the available binding site in sorbent's surface [48].The increase in pH up to 6 led to decrease the concentration of H3O+ions and negatively charged the composite's surface.Such changes improved the electrostatic interactions between metal cations and the magnetic composite,that resulted in a higher retention of Cu(II) and Zn(II) species.At pH>6,the retention extent was also enhanced by precipitation of both metal ions on the solid surface of AC/NiF composite as hydroxides.This would drop concentration of metal ions,in the aqueous solution,and lead to higher metal removal.Other investigators have early reported that precipitation highly participate in retention of Cu(II)at pH>8 on other adsorbents[4,20].

The performance of prepared magnetic composite to retain Cu(II)and Zn(II)from their aqueous solution is strongly dependent on speciation of metals ions and function groups in sorbent surface as well as chemical composition of aqueous solution and crystal structure of magnetic composite.The variation in retention mechanism was proposed to be varied with altering solution pH due to the change in characteristic of reactions that could take place onto the solid sorbent's surface.Depending upon solution pH,the oxide groups of NiFe2O4magnetic particles at the surface of magnetic composite can act as a weak acid or base(i.e.it can undergo protonation or deprotonation).Therefore,the following reactions were expected to occur on the surface of a magnetic composite as indicated by Eqs.(4)and(5)in the pH range(2–6).

where—Fe-OH represents a singly protonated oxide site.In acidic solution,the protonated oxide sites—are the predominating surface species giving raise a high positive charge density at composite surface.This surface positivity made the separation of Cu(II)and Zn(II)electrically unfavorable in this region due to the strong electrostatic repulsion.At these conditions,retention of Cu(II)and Zn(II)was suggested to be achieved though an ion exchange process according to the following interactions as in Eqs.(6),(7):

Such proton liberation from—Fe—OH sites of the magnetic composite,into aqueous solution,clearly elucidates the reduction in final pH values that accompanied with copper and zinc retention progress.In addition,a release of Ni(II)and Fe(II)ions,to aqueous solution,was proposed to occur from the magnetic composite surface,where the retention of copper and zinc were predicted to be attained though a cationic exchange mechanism of Ni(II)and Fe(II)cations with Cu(II)and Zn(II)cations was illustrated as the following Eqs.(8),(9):

To demonstrate this hypothesis,some experiments were carried out to detect the release of Ni(II)and Fe(II)ions,in aqueous solution,after retention of both Cu(II)and Zn(II).The concentrations of Ni(II)and Fe(II) ions released from the synthesized beads after Cu(II) and Zn(II)sorption were experimentally determined using AAS and the values are listed in Table 6.The results show that different concentrations of both Ni(II)and Fe(II)were detected in the solution after Cu(II)and Zn(II)sorption.The high measureable amounts of released cations obviously support ion exchange as a main mechanism participating in Cu(II) and Zn(II)retention.Data show also that retention of Cu(II) was achieved though a participation of ion exchange amounted to 89.3%and 7.9% with Ni(II) and Fe(II) ions beside other mechanisms using NiF sample.These amounts were decreased to 50%and 6.3%using AC/NiF composite.The contribution of ion exchange in the overall retention of Zn(II)from its aqueous solution using NiF sample was determined to be 86.4%and 11.4%due to exchange with Ni(II)and Fe(II)cations,while these values were decreased to 63.5%and 9.4%using AC/NiF composite,respectively.It is clear that the participation of ion exchange in NiF sample is greater than that in AC/NiF magnetic composite to retain the traced analytes.This illustrates that the exchange capacity of NiF towards Cu(II)and Zn(II)is substantially greater than that of AC/NiF composite.Finally,with further increase in pH value,precipitation of copper and zinc as hydroxides was the relevant mechanism in their separation from aqueous solutions.The binding interactions of Cu(II)and Zn(II)on the surface AC/NiF magnetic composite are schematically represented in Fig.7.

3.2.6.Ionic strength

The effect of ionic strength of aqueous solution on sorption of Cu(II) is presented in Fig.8.The plots clarify that removal of Cu(II)slightly reduced with increasing ionic strength using NaCl as a background electrolyte salt.This behavior could be referred to the hindrance in Cu(II) movement from bulk solution towards the sorbent surface.Similar reductions in different metal ions sorptiononto activated carbons were previously observed [50–52].This retardation was induced by presence of Na+ions that forms a positive layer on the surface of applied composite.In addition,the speciation of Cu(II) species became more complex with presence of NaCl,due to the formation of metal complexes with chlorine ions.The chloro-copper complexes have varied charges and masses compared with non-complexed copper ions.This could considerably alert the interaction between Cu(II) species and the active sits on composite surface [20,53].Copper cation forms relatively stable mono-complex (CuCl+) which has weaker affinity to carbon sorbents in comparison with non-complexed metal cations.In addition,Na(I)competes with Cu(II)for the available active sites on the surface of magnetic composite.Therefore,the overall uptake slightly reduced with raising the concentration of NaCl as a background electrolyte.A Similar behavior was observed for sorption of Zn(II) ions.Depending the sorption behavior of a metal on the ionic strength,of an aqueous solution,highlights the minor participation of electrostatic attractive sorption compared to surface complexation in the overall separation.

Table 6 Ion exchange characters of applied samples towards Cu(II)and Zn(II)ions

3.2.7.Competing ions

The role of the presence of different cations on sorptive retention of Cu(II) and Zn(II) was investigated using different concentrations of Mg(II),Mn(II) and Co(II) ions as competitors,with known pH value 4.5.The revealed experimental results are summarized in Table 7.The tabulated data clarify that the uptake of Cu(II) and Zn(II) decreased with presence of increased concentrations up to 0.5 mol·L?1Mg(II),Mn(II) and Co(II) in reaction solutions.The presence of 0.5 mol·L?1Mg(II),Mn(II) and Co(II)decreased Cu(II) uptake from 92.75% to 59.09%,71.38% and 83.85%,respectively.In addition,the sorption of Zn(II) was depressed from 72.13% to 58.78%,63.2% and 68.27%,due to presence of 0.5 mol·L?1Mg(II),Mn(II)and Co(II)respectively.Hence,the competing efficiency followed the order:Mg(II)>Mn(II)>Co(II).It is known that the electronegativity of the cations Mg(II),Mn(II)and Co(II)has the values 1.2,1.5 and 1.8,respectively.The order of the electro negativities for these competing metal ions is:Co(1.8)>Mn(1.5)>Mg(1.2).Hence,the data authenticate that the competing efficiency of Mg(II),Mn(II)and Co(II)ions is inversely related to their electronegativity,i.e.those having lower electronegativity highly depressed the sorption of Cu(II)and Zn(II)and competed against both of them for the available sites onto the magnetic composite.Finally,it could be concluded that the presence of particular foreign ions in reaction solution affected the sorption of metal ions and competed against each other for the available sites onto the sorbent's surface.

Fig.7.Schematic representation for the binding interactions of Cu(II)and Zn(II)on the surface AC/NiF magnetic composite.

Fig.8.Effect of ionic strength on the uptake of Cu(II)ions onto prepared samples.(Co =275 mg·L-1,teq =24 h,V/m=100 ml·g?1,T=25°C,pH=4.5,salt:NaCl).

3.3.Desorption experiment

The facile regeneration of a sorbent is an important demand and a key factor in improving its performance for truthful applications in removal and recovery of heavy metals from waste waters.The desorption behavior of Cu(II)and Zn(II)from loaded AC/NiF magnetic composite using different eluents is depicted in Fig.9(a),(b).The data illustrate that Cu(II)and Zn(II) were hardly eluted from synthesized sorbents using distilled water and 0.1 mol·L?1NaOH compared with other used eluents.The high desorbed amounts of Cu(II) and Zn(II) from synthesized magnetic composite were occurred using 0.1 mol·L?1HCl and 0.1 mol·L?1FeCl3.The amounts 95.8 and 98.3% of Cu(II)and Zn(II)ions were released from loaded AC/NiF magnetic composite using 0.1 mol·L?1FeCl3.This behavior may be attributed to the reversible process of cationic exchange between the traced metal ions and the synthesized magnetic composite's surface[4].The relevant recovery of Cu(II) and Zn(II) revealed with FeCl3could be referred to the chemical properties of Fe(III)ions compared with that of Cu(II)and Zn(II)ions.It is well known that Fe(III)ions have small ionic radius(0.064 nm)compared with that of Cu(II)(0.073 nm)and Zn(II)(0.074 nm)and also have high surface electronegativity.Hence,Fe(III)ions could simply diffuse into interior structure of magnetic composite,effectively replace Cu(II) and Zn(II) ions and finally elute them from loaded sorbent due to its high electronegativity and low ionic radius.The efficiency of the applied eluents to release Cu(II)and Zn(II)from loaded AC/NiF magnetic composite has the order:FeCl3>HCl>NiCl2>H2O>NaOH.This shows that the synthesized magnetic composite can be fully regenerated in a simple and economic process.

Table 7 Retention of Cu(II)and Zn(II)ions onto AC/NiF magnetic composite at presence of different concentration of some metal ions

Fig.9.Desorption of:a)Copper and b)Zinc from loaded sorbents using different eluents.(Co Cu(II) =275 mg·L?1,CoZn(II) =220 mg·L?1,teq =24 h,V/m=100 ml·g?1,T=25°C).

3.4.Thermodynamics

The effect of temperature on Cu(II)and Zn(II)sorption was studied by performing batch experiments at the temperature range 25–55°C.The standard enthalpy change of the overall process is assessed by the well-known Van't Hoff Eq.(10)[54]:

where:Kd:is the distribution coefficient,ΔSo:is the entropy change(J·mol?1·K?1),R:is gas constant,ΔHo:is the enthalpy change(kJ·mol?1) and T:is the absolute temperature in Kelvin.To explore the mechanism involved in Cu(II) and Zn(II) retention,parametersincluding standard Gibbs free energy(ΔGo),enthalpy(ΔHo)and entropy change(ΔSo)for the sorption were graphically estimated from the slope and intercept of lgKdversus 1/T plots,Fig.S4 in SD.The thermodynamic parameters'values were determined,at different temperatures,using the following equations and data are listed in Table 8.

Table 8 Thermodynamic parameters for sorption of Cu(II) and Zn(II) ions onto the applied samples

The positive value of standard enthalpy change(ΔH°)authorizes the endothermic nature of Cu(II)and Zn(II)sorption onto the synthesized magnetic composite and suppose that the transfer the metals ions,from aqueous solution to the composite,consumed a large amount of energy.The negative values of ΔGoindicate that sorption of Cu(II)and Zn(II)onto the surface of applied samples occurs via a spontaneous process and thermodynamically favorable interaction.The depress in ΔGovalues,with rising temperature,suggests that sorption of Cu(II)and Zn(II)became more favorable at higher temperatures.Also,the positive values of ΔSopostulate an increased randomness at the solid/solution interface during sorption process.Further,the positive ΔSovalues announce the increased disorder in the system with changing the hydration of Cu(II)and Zn(II)cations,where the randomness will increase at the solid–solution interface during the sorption process.

4.Conclusions

This work presents a novel magnetic activated carbon composite synthesized from Corncobs,oxidized by phosphoric acid,coated with nickel ferrite as a shell.The synthesis procedures were adapted to have a coreshell structure composed from activated carbon in core and nickel ferrite as a shell.The composite was applied in removal of Cu(II)and Zn(II)from aqueous solution.Experiments were planned to elucidate the speciation of Cu(II)and Zn(II)and the involved sorption mechanisms.The synthesized magnetic composite displayed a higher sorption capacity and better retention ability for studied metal ions higher than that of its precursors.Sorption of Cu(II) and Zn(II) was dependent on both pH and ionic strength.Sorption results were well defined by the kinetics of pseudosecond-order reaction.The results elucidate that ion exchange was the chief mechanism contributing in Cu(II)and Zn(II)retention.Both FeCl3and HCl effectively eluted Cu(II)and Zn(II)from synthesized magnetic composite.Thermodynamically,the sorption process was spontaneous and governed by physic-sorption interaction.On the light of these data,the synthesized composite can be regarded as promising materials for removal of heavy metals from wastewaters,as well as decontamination of hazardous aqueous solutions.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.07.036.