Racheal Aigbe,Doga Kavaz

1 Department of Environmental Science,Cyprus International University,Nicosia,Mersin 10,Turkey

2 Environmental Research Centre,Cyprus International University,Nicosia,Mersin 10,Turkey

3 Department of Bioengineering,Cyprus International University,Nicosia,Mersin 10,Turkey

Keywords:Zinc oxide nanoparticles Carbonized saw dust Matrix Batch process Reusability potential

ABSTRACT Zinc oxide nanoparticles(ZnOnp)are molecular nanoparticles synthesized by a chemical precipitation method from zinc nitrate tetrahydrate and sodium hydroxide.Carbonized sawdust(CSD)was prepared from sawdust obtained from a local wood mill.The matrix of both provides a better material as an adsorbent.The present study applied the functionality of ZnOnp,CSD,and ZnOnp-CSD matrix as adsorbent materials for the removal of Pb(II)ions from aqueous solution.The method of batch process was employed to investigate the potential of the adsorbents.The influence of pH,contact time,initial concentration of adsorbate,the dosage of adsorbents,and the temperature of adsorbate-adsorbent mixture on the adsorption capacity were revealed.The adsorption isotherm studies indicate that both Freundlich and Langmuir isotherms were suitable to express the experimental data obtained with theoretical maximum adsorption capacities(qm )of 70.42,87.72,and 92.59 mg·g?1 for the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix,respectively.The separation factors(RL )calculated showed that the use of the adsorbents for the removal of Pb(II)ions is a feasible process with RL ＜1.The thermodynamic parameters obtained revealed that the processes are endothermic,feasible,and spontaneous in nature at 25–50°C.Evaluation of the kinetic model elected that the processes agreed better with pseudo-second order where the values of rate constant(k2 )obtained for the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix are 0.00149,0.00188,and 0.00315 g·mg?1·min?1,respectively.The reusability potential examined for four cycles indicated that the adsorbents have better potential and economic value of reuse and the ZnOnp-CSD matrix indicates improved adsorbent material to remove Pb(II)ions from aqueous solution.

1.Introduction

An increase in industrial activities has a great influence in the continuous contamination of freshwater resources with a variety of heavy metal pollutants,which are the major challenge for hygienic water supply.Due to the toxicity and non-biodegradable nature of heavy metals,they pose health risks to living organisms including humans.For example,lead has made a remarkable detrimental effect on both animals and plants which is recognized according to US public health as a dangerous pollutant in the air,soil,and water body with potent poison that causes serious illness at short-time exposure.Long-time exposure leads to inhibition of certain enzymes such as acetylcholine which resulted in neurological problems such as brain damage and central nervous system disorder[1].Also,exposure to lead metal resulted in a very high medical ailment[2],infertility in both men and women[3],Alzheimer[4]and birth congenital abnormality[5].Therefore,a versatile method is required for water treatment to remove lead(heavy metal).

Though many technologies had been in existence for efficient water treatment such as ion-exchange[6],coagulation[7],and complexationultrafiltration[8]but required high energy which makes them economically disadvantage and generation of toxic sludge without complete removal of toxic metal [9].While the process of adsorption has been recognized as an effective and efficient wastewater treatment technology due to its eco-friendly,low energy requirement and readily available materials which require excellent adsorbents for the removal of toxic materials in aqueous solution.Researchers focused on the potential of different adsorbents for wastewater remediation whereby nano-material emerged as an essential adsorbent among their numerous applications for wastewater treatment which are capable of the mremoval of heavy metals from aqueous solution due to their affinity to the metals and high surface area [10].Implausible potential has been posed by nano-material as adsorbent with advancement in nanoscience where all the surface chemistry processes are in nanoscale level[11,12].

Zinc oxide nanoparticles have been reported as an efficient and effective adsorbent with a high surface area having high metal removal capacity with low cost which makes it economically valuable[13].Zinc oxide nanoparticle with different applications such as photo catalyst has shown high efficiency in the removal of Zn(II),Cd(II)and Hg(II),Pb(II),Cu(II)and Cr(II)ions[14–17].Besides,doped ZnO nanoparticles demonstrated more efficient adsorbent for the removal of both inorganic and organic pollutants from aqueous solution[18,19],and nanocomposite of ZnO nanoparticles gives a synergetic improved value in the efficiency of the removal of heavy metal with increased surface area[20,21].Sani et al.[22]experimented on the effect of ZnO/talc nanocomposite in adsorption efficiency evaluated for Pb(II)removal.The result was compared with the adsorption characteristics of ZnO and talc material.The study showed that the adsorption capacity (Qm) of the ZnO/talc nanocomposite was 48.3 mg·g?1.The comparative analysis of the study also showed that ZnO/talc nanocomposite had a better efficiency of Pb(II)removal of 95%,as compared with 45% and 50% of the talc and ZnO respectively.The study explained the reason for this improvement as a result of new properties formed due to the interaction of the materials.A similar study was carried out by[23]which showed the efficiency of nanocomposite in adsorption of heavy metals.The study utilized nanocomposite of Fe3O4/talc in the removal of Cu(II),Ni(II),and Pb(II).The study stated the reason for improved adsorption efficiency of nanocomposite as that the nanoparticle,due to its higher surface,increases the ability of the talc in heavy metal removal.A study by[24]analyzed the adsorption efficiency of sawdust evaluated for Cu(II).The result showed that the efficiency of sawdust was dependent on the amount of sawdust,pH of the metal,and contact time.Hence as stated in past report[23],the adsorption efficiency of sawdust for heavy metal removal can be further improved,if synthesized with nanoparticle,as it would increase its contact time.

This research was aimed at synthesis and characterization of zinc oxide nanoparticle(ZnOnp),preparation of carbonized sawdust(CSD),and preparation of ZnOnp-CSD matrix as shown in the Graphical Abstract.The synthesized ZnOnp was characterized with available analytical instruments such as scanning electron microscopy(SEM),Energy Dispersive X-Ray Analysis(EDX),Fourier transform infrared spectroscopy(FTIR),UV–Visible spectroscopy.The synthesized and prepared materials were utilized as adsorbents for the removal of Pb (II) ions from aqueous solution of its salt.Batch process was carried out to examine the equilibrium parameters,adsorption isotherm,thermodynamic and kinetic model for the adsorption of Pb(II)ions from aqueous solution onto the adsorbents.

2.Materials and Methods

2.1.Materials

All chemical reagents were of analytical grade from Sigma Aldrich and were used as received from the supplier without further purification.Sawdust was obtained from a local wood mill in North Cyprus.

2.2.Synthesis of Zinc oxide nanoparticle

Zinc oxide nanoparticle was synthesized by direct precipitation method using zinc hydroxide tetrahydrate (Zn (NO3)2·4H2O) as a precursor material and sodium hydroxide (NaOH) as a precipitating agent.The solution of 0.2 mol?L-1Zn(NO3)2·4H2O was prepared by dissolving 13.05 g of the hydrated salt in 250 ml distilled water and stirred continuously for 30 min to achieve the complete dissolution of the salt.The solution of 0.4 mol?L-1NaOH was also prepared by dissolving 4.0 g of NaOH in 250 ml distilled water and stirred continuously for 30 min until the complete dissolution of solid NaOH.Then 150 ml of Zn(NO3)2·4H2O solution was transferred into 250 ml Erlenmeyer flask and placed on magnetic stirrer under vigorous stirring at 70°C,the precipitating agent(NaOH)solution was added from the burette dropwise at a temperature of 80°C.A color change observed from a clear solution to white cloudy solution indicates the formation of precipitate and zinc hydroxide solution.

The precipitating solution was left on the magnetic stirrer for 2 h,the solution was then placed in an ice bath and allowed to settle.The white precipitate solution formed was centrifuged at 5000 r?min-1for 15 min.The supernatant obtained was decanted carefully and the precipitate was washed several times with double de-ionized water to remove impurities or possible absorbed ions.The product obtained was calcined at 100°C,300°C,and 500°C for 3 h each to obtain different samples labeled a,b and c respectively.The calcined product was crushed into fine powder mechanically and then collected for characterization.

2.3.Preparation and carbonization of sawdust

Sawdust was collected from a local wood mill in North Cyprus.The sawdust was washed with de-ionized water to remove surface adhere particles and water-soluble impurities from the sawdust and dried at 60°C for 2 h to remove water content and moisture.The dried sawdust was mechanically homogenized and sieved with the size range of 150–200 μm and placed in a muffle furnace where it was carbonized at a temperature range of 700°C within the heating period of 3 h.The carbonized sawdust(CSD)was ground to a fine powder and used for further procedure.

2.4.Preparation of ZnOnp-CSD matrix

The zinc oxide nanoparticle-carbonized sawdust(ZnOnp-CSD)matrix was prepared by adding 10 g of synthesized ZnOnp into 100 ml distilled water and stirred vigorously on a magnetic stirrer for 30 min.Then,5 g of CSD was added into the solution with continuous stirring for 30 min at 100°C.The product was then filtered and washed several times with double de-ionized water to remove soluble impurities.It was then calcined for 2 h at 200 °C.The calcined product was crushed to powder using agate mortal.

2.5.Characterization of ZnO nanoparticle

XRF spectrometry is widely used for the determination of both major and trace elements in natural and geological materials.To obtain the Xray fluorescence(XRF)patterns of sawdust and carbonized sawdust,the samples were measured with an X-ray fluorescence(Rigaku ZSX Primus II).Subsequently,the Fourier-transform infrared(FTIR)spectra were collected(using an IR Prestige–21 Shimadzu,Japan)and used to investigate the properties of nanoparticles;significant peaks were observed and recorded over the range of 400–4000 cm?1.This analysis isimportant in the determination of the functional groups that are present in the nanoparticles.To determine the constituting elements of the nanoparticles,it was necessary to conduct an energy-dispersive X-ray(EDX)analysis.The EDX analysis(Oxford Instruments AZTEC EDS)generates X-rays using a beam of electrons focused on the specimen in the SEM analysis(JSM-6610LV,JEOL,Japan);this makes it insufficient to give a full representation of the sample.To determine the composition and level of crystallinity of the nanoparticles,an X-ray diffractometer(Bruker D8 Advance model)(XRD)pattern was applied to a powder X-ray diffraction using Cu Kαradiation,k=0.154 nm at 25 °C.The XRD data for the analysis of barley grass was collected from 10°to 80°of 2θ at a scan rate of 2(°)?min-1.

Table 1 Conditions for adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix

Table 2 Percentage elements present in saw dust and carbonized saw dust

2.6.Batch adsorption experiment

Synthesized solution of Pb (II) ions (100,200,300,400,500 and 600 mgL?1)was prepared separately through dilution of stock solution with an appropriate volume of stock solution and de-ionized water.Different concentrations of adsorbate(Pb(II)ions)prepared were used to evaluate the potential of the adsorbents(ZnOnp,CSD,and ZnOnp-CSD matrix)to remove Pb(II)ions from the aqueous solution.The experiments were conducted in a 250-ml Erlenmeyer flask containing 40 ml of synthetic solution of Pb (II) ions,and 0.05 g of adsorbents was added into the solution.The mixture was agitated at a speed of 150 r·min-1for a given time (min).After the agitation,the mixture was filtered with Whatmann No 3 filter paper and the adsorbate(Pb(II)ions)in the filtrate was analyzed using Atomic Absorption Spectrometer(AAS)to determine the equilibrium concentration of the adsorbate(concentration of unadsorbed adsorbate) denoted as Ce.The influence of change in pH,contact time,initial concentration of adsorbate,dosage of the adsorbent,and temperature of adsorbateadsorbent mixture were determined(with other conditions fixed as indicated in Table 1)concerning the efficiency of adsorption of Pb(II)ions onto the adsorbents.The pH was varied by adding 0.1 mol?L-1oxalic acid and 0.1 mol?L-1NaOH into the adsorbate-adsorbent mixture to reduce and raise the pH respectively.

Fig.1.SEM image of ZnOnp at calcination temperature of(a)100°C(b)300°C and(c)500°C(d)EDX image of the synthesized ZnOnp at calcination temperature of 500°C.

High initial concentration of adsorbate(Pb(II)ions)was used to replicate the concentration of heavy metal in contaminated wastewater from industrial effluent.

The efficiency of adsorption(%)was measured using eq.(1).

where Ciand Ceare the initial concentration of adsorbate(mg·L?1)and equilibrium concentration of adsorbate(mg·L?1)respectively.

And the adsorption capacity(qe)which is the quantity of adsorbate adsorbed per unit mass of adsorbent was calculated as follows:

where V and m are volume of synthetic solution used (L) and amount of adsorbent(g)used respectively.

The adsorption capacity at a specific contact time(t),qt(mg·g?1)is given as:

where Ctis the equilibrium concentration at a given time(t).

2.7.Adsorption isotherm model

Validation and analysis of experimental data were investigated by adsorption isotherm models which described the phenomenon of solid–liquid adsorption interaction in the system.The adsorption isotherm was described by Freundlich,Langmuir,and Tempkin models that gave useful information about the adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix.Where their respective linear equations are given by:

where qeand Ceare as stated earlier and kFis the Freundlich constant representing the adsorption capacity and n represent the adsorption intensity(dimensionless)indicating that if n=1,the partition between the two phases are independent of the concentration,n ≤1 indicate unfavorable process and 1 ＜n ＜101 indicates favorable adsorption process[10].Freundlich applies to both monolayer(chemisorption)and multilayer or physiosorption[25].

Fig.2.Fourier transform infrared(FTIR)spectrum of ZnOnp at calcination temperature of(a)100°C(b)300°C and(c)500°C.

Fig.3.UV–visible spectrum of ZnOnp at calcination temperature of(a)100°C(b)300°C(c)500°C.

The linearized form of the Langmuir model is given as[26]:

where qmis the maximum amount of adsorbate require to form a monolayer or Langmuir monolayer capacity (mg·g?1) and b is the Langmuir equilibrium or adsorption constant(mg·L?1).

Also,the linear equation of Tempkin is given as[27]:

Fig.4.XRD pattern of ZnOnp at calcination temperature of(a) 100°C (b) 300°C and(c)500°C.

Fig.5.Influence of change in(a)pH,(b)contact time,(c)initial concentration of Pb(II)ions,(d)adsorbent dosage and(e)temperature on adsorption capacity of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix.

where B is the constant related to heat of adsorption(J·mol?1),ATis the Tempkin constant related to binding energy (L·mol?1),bTis the Tempkin isotherm constant,T is the absolute temperature (Kelvin)and R is the universal gas constant(8.314 J·mol?1·K?1).

2.8.Thermodynamic studies

The driving force and fundamental parameters for the spontaneity of adsorbate-adsorbent interaction is given by the value of change in Gibbs free energy(ΔG)of the adsorption process.The negativity of ΔG indicates that the process is spontaneous.The following expression gives the relationship between the free energy and the thermodynamic equilibrium constant(Kc).

where R,T and Kcare the same as expressed earlier.

The thermodynamic equilibrium constant(Kc)can be calculated as:

where C is the concentration of adsorbate adsorbed onto the adsorbent and Ceis the equilibrium concentration.

Using Van't Hoff equation as expressed in Eq.(10),other thermodynamic parameters were calculated.

Eq.(11)shows that the plot of ΔG versus T gives ΔH as the intercept and ?ΔS as the slope[18].

Fig.6.Depict the plot of(a)Freundlich,(b)Langmuir,and(c)Tempkin isotherms for adsorption of Pb(II)ion onto ZnOnp,CSD and ZnOnp-CSD matrix.

Table 3 Adsorption isotherms parameters for adsorption of Pb (II) ions onto ZnOnp,CSD and ZnOnp-CSD matrix

2.9.Kinetic(Dynamic)model

The experimental data were analyzed using different kinetic models to determine the reaction mechanism and rate-determining step of the adsorbate–adsorbent interface[27].Therefore,the experimental data obtained from adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix at optimum conditions were examined according to Lagergren pseudo-first order,Ho's pseudo-second order and intraparticle diffusion models to determine the suitable reaction mechanism and rate-determining step of the adsorption process.

The linearized form of pseudo-first order kinetic model equation is given as:

where qeand qtare the amount of adsorbate adsorbed onto unit mass of adsorbent(mg·g?1)at equilibrium and specific time(t)respectively,k1is the pseudo-first order rate constant(min?1).The linear plot of lg(qe?qt)versus t give the k1=2.303×(slope)and lg qe=intercept.

While the linearized form of the pseudo-second order model[28]is as folows:

where k2is the pseudo-second order rate constant (g·mg?1·-min?1).A plot of t/qtversus t gives slope=1/qeand intercept=

Also,the model of intra-particle diffusion[29]which is applicable to study the rate determining step of adsorption process is written as:

where qtis the amount of adsorbate adsorbed onto unit mass of the adsorbent(mg·g?1)at given time(t).kwis the intra-particle rate constant (mg·g?1·min?0.5).The plot of qtversus t0.5shows the slope equal to kwassuming the intercept is zero.

2.10.Reusability efficiency of the adsorbent

The economic value of the adsorbents was examined by a process of regenerating the adsorbents and reusing the regenerated adsorbents.The loaded adsorbents(used adsorbents)were regenerated by the process of desorption.The desorption process involved the removal of adsorbate from the loaded adsorbents to restore the adsorbents close to the original condition for reuse.0.1 mol?L-1NaOH was used for desorption of adsorbate from loaded adsorbents while the recovered adsorbents were reused to adsorbed Pb (II) ions.Similar optimum conditions were applied and the cycle was repeated four times on given adsorbents.The process was carried out at each cycle with optimum adsorption conditions of pH=8,an initial concentration of adsorbate of 100 mg·L?1,contact time of 100 h,and an adsorbent dose of～0.05 g at room temperature.

3.Results and Discussion

3.1.XRF analysis for saw dust and carbonized saw dust

Analysis of sawdust and carbonized sawdust was presented in Table 2 demonstrating the evidence of the elemental components of the nanoparticles.Both carbonized sawdust and saw dust show a high percentage of carbon and oxygen in its elemental form while other elements such as Fe,Cr,Ca,Si,Cl,Ni,and Fe contributed to the trace percentage of the composition.

3.2.Characterization of zinc oxide nanoparticle(ZnOnp)

The SEM images reveal a morphological aggregated structure of fine crystal of the ZnO nanoparticles as observed in Fig.1.This is caused by the steady increase in calcination temperature,which allows for an increase in the super-saturation of reaction products which further increases the forming of the crystal core in a short duration of time.The continual increase in the calcination temperature causes the“nuclearaggregation”phenomenon causing the nucleus of the crystal to aggregate as the rate of aggregation of particles depends on the morphology of the crystallinity of the products[30].The average particle sizes of individual ZnO nanoparticles from the SEM images based on the scale bar provided are around 70 nm for 500°C.Fig.1d explicitly gave the elemental composition of ZnO nanoparticles using EDX(Energy Dispersive X-ray spectroscopy which reveals similar peaks for all the samples characterized.The spectroscopy test reveals that ZnO nanoparticles were mainly composed of element Zn and O element with no impurity detected.This result is in line with the observed XRD result,thereby further confirming the purity of ZnO nanoparticles.

The FTIR of ZnOnp samples were recorded and studied in the wavelength range of 0–4000 cm?1.The spectrum of the ZnOnp at different calcination temperatures was presented in Fig.2 for calcination temperatures of 100°C,300°C,and 500°C as samples a,b,and c,respectively.The spectrum of sample‘a(chǎn)’showed a weak band peak at 860.25 cm?1corresponding to a compound with stretching and bending vibrations of intercalated C--O species.The band at 1325 cm?1and 1546 cm?1isthat of asymmetric and symmetric stretching of the carbonyl group attached to the ZnOnp during synthesis,while the highest peak observed at 400.7 cm?1corresponds to the vibration of ZnOnp.The sample‘b’spectrum indicates the highest peak at 405.1 cm?1due to the vibration of ZnOnp with low peak observed at a vibrational frequency in the range of 1300–1700 cm?1indicating impurities in the sample.The sample‘c’spectrum demonstrated the highest peak at 401.2 cm?1in the fingerprint region due to the vibration of ZnOnp.Therefore,all the samples indicate the formation of ZnOnp with impurities attached to sample‘a(chǎn)’during the reaction process while the impurities observed in sample‘b’were as a result of a side reaction due to high temperature.Sample‘c’shows a highly pure ZnOnp sample.

Table 4 Comparison of different adsorbents for adsorption of Pb(II)ions from aqueous solution

The UV–Visible spectra were analyzed as shown in Fig.3 for the synthesized ZnOnp using 0.2 mol?L-1zinc nitrate tetrahydrate and 0.4 mol?L-1NaOH at different calcination temperatures of 100°C,300°C,and 500 °C for samples a,b,and c,respectively.Each sample for UV–Visible analysis was prepared by ultrasonic dispersing method in absolute ethanol.The UV–Visible spectra of all the samples were recorded in the wavelength range of 290 nm to 500 nm.Strong absorption bands were exhibited at different wavelengths by each of the samples.Sample‘a(chǎn)’indicates a strong absorption band at 378.2 nm,sample‘b’shows a strong absorption band at 375.8 nm,and sample‘c’shows a strong absorption band at 382.7 nm.The significant sharp absorption of ZnOnp illustrates the monodispersed nature of the nanoparticle distribution[31].Also,there is a redshift in the wavelength with a higher calcination temperature and agglomeration in the sample with lower calcination temperature[32].

UV–visible absorption spectra can be used to calculate the average particle size using effective mass model[33]where the approximation of band gap is given as:

Due to relatively small effective masses of ZnO(me=0.26,mh=0.59),with small mathematical simplification of effective mass model given above [34],the size of the particle from absorbance spectra is given as:

where λpis the peak absorbance wavelength in nm.Therefore,the prepared ZnOnp samples show λpequals 378.2,375.8,and 382.7 nm for samples a,b,and c respectively which correspond to the particle size of 33.6,31.4 and 39.9 nm respectively.

Fig.4 shows the result of the XRD containing three samples with varying calcinated temperature.Pattern analysis of the XRD revealed diffractions peaks at 32.74°,33.49°,36.57°,49.01°,55.87°,63.22°,67.44°,and 69.44°.In comparison relating journals,the FWHM (full full-width at half-maximum) data results revealed the crystalline peaks of ZnO nanoparticles matching with the JCPDS card of 36–1451 range.Further observation of ZnO demonstrates similar characteristics behavior at 20°＜2θ ＜80°.

The result of XRD suggests a peak increase with increasing calcination temperature similar to the JCPDS card.Conclusively,there is a possibility of the crystal quantity being hindered by the calcination temperature settings as observed that local reactant concentrations appear in the crystal as they grow too quickly.

Fig.7.Change in separation factor with change in initial concentration of Pb (II) ion adsorbed onto ZnOnp,CSD and ZnOnp-CSD matrix.

The synthesized nano powered was also confirmed to be pure and free of impurities with no characteristic XRD peaks aside from the observed ZnO peaks.Debye–Scherer formula was used to calculate the diameter of the ZnO nanoparticle.The Debye–Scherrer formula goes thus;

where 0.89 is Scherrer's constant,λ is the wavelength of X-rays,θ is the Bragg diffraction angle,and β is the full width at half-maximum(FWHM) of the diffraction peak corresponding to plane〈101〉.Using the Scherrer's equation,the crystallite size of the nanoparticles was calculated at the corresponding calcination temperature of 100,300,and 500°C to respectively give 38,47,and 56.The steady increase of crystallinity of the ZnO nanoparticles as a result of increasing calcination temperature as observed by other literature further supports the claim[31].

Fig.8.Thermodynamic studies of the adsorption of Pb (II) ions onto ZnOnp,CSD and ZnOnp-CSD matrix.

Table 5 Thermodynamic studies parameters for the adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix

3.3.Adsorption studies

The batch process was carried out to study the potential of ZnOnp,CSD and ZnOnp-CSD matrix as an adsorbent for the removal of Pb(II)ions in aqueous solution.ZnOnp obtained from calcination temperature of 500°C was used in the process and the preparation of the ZnOnp-CSD matrix.

3.3.1.Effect of pH of solution

Variation of pH of adsorbate-adsorbent mixture was investigated as a factor that affects the adsorption capacity of Pb(II)ions in aqueous solution.It was deduced from Fig.5a that with an increase in pH of the mixture,the adsorption capacity of Pb(II)ion increases up to pH=8 while decreases adsorption capacity observed at higher pH.This may be attributed to competing strength between H+formed by protonation and the positive adsorbate in acidic medium on the surface of the adsorbent while at higher pH,the formation of(Pb(OH)4)2?which competes with the OH?arisen[35]at the adsorption sites which resulted to the decrease in adsorption capacity of Pb(II)ions at higher basic medium.Therefore,pH 8 was determined to be the optimum pH for adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix.This is in good agreement with the removal of Pb (II) ions onto ZnOmontmorillonite composite[20].

3.3.2.Effect of Contact Time

In the batch experiment,one of the major factors affecting the adsorption of heavy metal ions onto the adsorbents is contact time.As illustrated in Fig.5b,at pH 8 the adsorption capacity increases with an increase in contact time between the adsorbate and the adsorbent agitated at 150 rpm.At the initial point,it may be due to the availability of adsorption active sites at the surface of the adsorbents.The slow rate of adsorption at a later stage may be a result of electrostatics hindrance caused by occupied Pb(II)ions at the active sites of adsorption and total occupation on the active sites leads to change in the amount of adsorption.Therefore,at maximum uptake of adsorbate,the rate of adsorption is equal to the rate of desorption and resulted in a decrease in adsorption capacity above equilibrium contact time of 100 min.A similar trend had been reported when bamboo-based activated charcoal and bamboo dust were used for the adsorption of Cd(II)and Pb(II)ions at an optimum time of 90 min[36].The capacity of adsorption of carbonized sawdust is greater than that of ZnOnp particle which may be due to higher surface area in atomic particle CSD compared with molecular nanoparticle ZnOnp.

3.3.3.Effect of initial concentration of adsorbate

When varying the initial concentration of Pb(II)ions to be adsorbed onto a fixed mass of each adsorbent,increases in adsorption capacity are observed with an increase in the initial concentration of Pb(II)ions as depicted in Fig.5c.This may be as a result of multilayer adsorption active sites available within the adsorbent.Also,due to the driving force of adsorbate,ions increase with an increase in the initial concentration of the adsorbate at the solid–liquid interface;therefore,it increases the adsorption capacity of the adsorbent.As reported earlier,silicate porous material was used for the adsorption of Pb(II),Cd(II),and Cu(II)ions where the adsorption capacity increases with an increase in the initial concentration of the adsorbates[37].

3.3.4.Effect of adsorbent dosage

The effect of adsorbent dosage was investigated by varying the adsorbent dosage from 0.02 to 0.2 g using 40 ml of 100 mg·L?1concentrations of Pb (II) ions.As described in Fig.5d,the adsorption capacity initially increases with an increase in adsorbent dosage due to additional available adsorption sites at the surface of the adsorbents.After 0.05 g optimum dosage,the efficiency of adsorption unchanged with additional dosage as a result of saturation of limited amount of the adsorbate onto increasing surface adsorption site and agglomeration of the adsorbent particles which leads to a reduction in adsorption active sites at the surface of the adsorbent.The report shows that the adsorption of Pb(II)ions onto Zinc oxide-coated nanoporous carbon with a variation of adsorbent dosage demonstrated an increase in efficiency of adsorption with an increase in dosage[38].

3.3.5.Effect of temperature

Change in temperature has influence in some of the factors involved in the adsorption process such as stability of the adsorbate in aqueous solution,surface porosity of the adsorbent,and ionization of the adsorbent surface[39].As shown in Fig.5e,the adsorption capacity of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix increases with an increase in temperature.The temperature rise resulted to the increase in ionization of the adsorbent surface by creating more adsorption active sites on the surface.The rate of adsorption of Pb(II)ions onto the adsorbents will be more intense than the rate of desorption at high temperatures.This may also have attributed to an increase in the rate of diffusion of Pb(II)ions across the multilayer within the adsorbents.Therefore,temperature rise is required for optimum capacity of the adsorbents.A similar effect was demonstrated using polyaniline/ZnO nanocomposite as adsorbent for the removal of Cr (VI) ions from aqueous solution where an increase in temperature from 30 to 50°C shows corresponding increase in adsorption capacity while 50°C was indicated as the optimum temperature in the experiment[40].

Fig.9.Depict the plot of(a)Pseudo-first order(b)Pseudo-second order(c)Intra-particle diffusion kinetic model for adsorption of Pb(II)ion onto ZnOnp,CSD and ZnOnp-CSD matrix.

Table 6 Kinetic studies parameters for the adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix

3.4.Adsorption isotherms

The relationship between the amount of Pb(II)ions adsorbed per unit mass of adsorbent and the amount of Pb(II)ions unadsorbed at equilibrium was illustrated by adsorption isotherm.The adsorption isotherms explored to investigate the adsorption of Pb(II)ions onto the adsorbents were Freundlich [41,42],Langmuir [26,43]and Tempkin[21,27].The linear plot of each isotherm was presented in Fig.6(a-c)and the parameters derived from the slope and intercept of each plot were presented in Table 3.Both the Freundlich and Langmuir were the most suitable isotherm to explain the adsorption of Pb (II) ions onto ZnOnp,CSD,and ZnOnp-CSD matrix[37]as proven by the value of regression correlation coefficient (R2) which are approximately 1 for both isotherms compared to Tempkin isotherm investigated.This suggested the multilayer adsorption of Pb(II)ions onto the surface of the adsorbents and heterogeneous surface conditions of the adsorption process.It also indicates the reversible process of adsorption of Pb(II)ions.In Table 3,the maximum adsorption capacity (qm) obtained is 70.42,87.72,and 92.59 mg·g?1for adsorption of Pb (II) ions onto ZnOnp,CSD,and ZnOnp-CSD matrix respectively.The value of constant b indicates the affinity of binding of metal ions onto the surface of the adsorbent which is higher when Pb (II) ions adsorbed onto ZnOnp-CSD matrix.The Freundlich constant(KF)described the adsorption capacity of the adsorbent which indicates ease of the uptake of Pb (II)ions onto ZnOnp-CSD matrix compare to other adsorbents.The intensity of adsorption is described by constant n which shows that the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix is a normal adsorption with n ＞1[44].Tempkin isotherm is not fit to the adsorption data obtained with R2＜1.The binding energy as illustrated by ATand the heat of adsorption described by B derived from Tempkin isotherm constants indicate higher binding energy when Pb(II)ions adsorbed onto ZnOnp-CSD matrix with a requirement of higher heat of adsorption.

Fig.10.Reusability potential of adsorbents for the adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix.

This paper reports the different adsorbent materials with initial Pb(II)concentration and its maximum adsorption capacity.The efficiency of adsorption is for metal ions by most especially in the removal Pb2+is compared by other literatures in Table 4.

3.5.Separation factor(RL )

Separation factor (RL) is an essential feature of Langmuir model which determines the feasibility of the process of adsorption of adsorbate onto the adsorbent.The separation factor(RL)is defined thus:

where b is the Langmuir constant and Ciis the initial concentration of the adsorbate.The value of RLindicates favorable process when 0 ＜RL＜1,unfavorable process when RL＞1,linear when RL=1 and irreversible when RL=0[48,49].

The values of separation factors(RL)were plotted against the initial concentration of the Pb(II)ions as illustrated in Fig.7 which indicates the nature of adsorption to be favorable since 0 ＜RL＜1.Therefore,the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix is favorable at all initial concentration and the favorability increases with an increase in the initial concentration of the adsorbates as the RLdecreases in value.

3.6.Thermodynamic studies

As illustrated in Eqs.(11)–(14),the free energy(ΔG)was plotted against T from the experimental data obtained which is depicted in Fig.8 and the thermodynamic parameters obtained were expressed in Table 5 for the adsorption of Pb (II) ions onto ZnOnp,CSD,and ZnOnp-CSD matrix.The positive value of enthalpy change (ΔH) described the endothermic nature of the adsorption of Pb(II)ions onto the adsorbents while an increase in the degree of disorderliness of solid–liquid interface of the process was described by the positive value of the entropy change(ΔS)obtained which indicates the spontaneity of the adsorption process which are in agreement with the reported work on adsorption of Pb (II) ions onto manganese oxide/activated carbon composite [50]and Pb (II) ions adsorption onto green synthesized ZnO nanoparticle indicate endothermic nature of the process[15].The negativity of ΔG calculated for the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix described the feasibility of the adsorption process.The endothermic nature,spontaneity,and feasibility of the adsorption of Pb(II)ions increase in the order of the adsorbent as ZnOnp-CSD ＞CSD ＞ZnOnp.

From the table it is apparent that the ZnOnp-CSD matrix showed better adsorption efficiency.

3.7.Adsorption kinetic studies

The experimental data of the adsorption of Pb(II)ions onto ZnOnp,CSD,and ZnOnp-CSD matrix were simulated with the Lagergren pseudo-first order,Ho's pseudo-second order and intra-particle diffusion models to determine the suitable reaction mechanism and ratedetermining step of the process.The plot of each model was illustrated in Fig.9(a–c) and the constant parameters obtained for each model were presented in Table 6.By comparison,the regression correlation coefficient(R2)obtained for each model show that pseudo-second order mechanism is the most suitable model to explain the process of adsorption of Pb(II)ions onto the presented adsorbents.The R2of the experimental data obtained for the adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix in simulation with pseudo-second order were 0.992,0.997 and 0.999 respectively which are greater than the R2of other models.Ho's pseudo-second order assumed that the ratedetermining step for adsorption of Pb (II) ions onto each of ZnOnp,CSD and ZnOnp-CSD matrix are chemisorption which may involve sharing of electron between Pb(II)ions and the surface of the adsorbents[40].Since diffusion and adsorption are simultaneous processes,the process of the Pb(II)ions uptake onto the adsorbents includes diffusion and coordinate bond formation which are physical and chemical process respectively.

3.8.Reusability potential studies

The reuse potential of each of the adsorbent was investigated by desorption of the adsorbates from the adsorbent and reuse the adsorbent for adsorption of adsorbate from aqueous solution using constant initial concentration of the adsorbate.The procedure was repeated in four cycles of use and reuse.The results obtained were described in Fig.10 showing little degradation in the potential of the adsorbents with number of cycle of reuse.Therefore,the investigation shows that all the three adsorbents have better reuse potential with reasonable economic value.

4.Conclusions

ZnO nanoparticles were synthesized using a simple chemical precipitation method.The SEM,EDX,FTIR,XRD and UV–Visible characterization show that pure ZnOnp is formed with the method of synthesis and calcination at 500°C.Also,the carbonization of sawdust was carried out by thermal process and was analyzed by XRF.Both carbonized sawdust and saw dust shows a high percentage of carbon and oxygen in its elemental composition.The present study revealed the potential of ZnOnp,CSD,and ZnOnp-CSD matrix as adsorbents for the removal of Pb(II)ions from aqueous solution.The experimental adsorption capacity achieved for adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix were 56.83,60.75 and 65.52 mg·g?1respectively at room temperature and optimum condition of pH=8,100 min of agitation,100 mg·L?1initial concentration of Pb(II)ions and 0.05 g/40 ml of adsorbent–adsorbate mixture concentration.The experimental equilibrium data were best fit with both Freundlich and Langmuir isotherms compared with Tempkin isotherms.RLvalues obtained confirmed the favorability of the process of Pb(II)ions adsorption onto ZnOnp,CSD,and ZnOnp-CSD matrix.The kinetic model were best described by Ho’s pseudo-second order model compared with pseudo-first order and intra-particle diffusion model.The reusability potential exploration gives an attractive results whereby the efficiency of adsorption of Pb(II)ions onto ZnOnp,CSD and ZnOnp-CSD matrix changes after fourth cycles from 71.04%to 51.11%,75.94%to 61.89%and 81.96%to 66.34%respectively.The sorption efficiency of the studied sorbent was greater than the sorption efficiency of most sorbents studied in other research.Therefore,ZnOnp,CSD and ZnOnp-CSD matrix are better adsorbent materials whereby ZnOnp-CSD matrix demonstrated more efficient potential for removal of Pb(II)ions from aqueous solution and wastewater as the studies revealed.

Acknowledgements

The authors would like to extend their appreciation to Hacettepe University Biopolymeric Systems Research Group,Hacettepe University Chemistry Laboratory and HUNITEK laboratories in Ankara,Turkey,for their assistance in the characterization experiments.