Abdul Samad ,Muhammad Imran Din ,Mahmood Ahmed,Saghir Ahmad

1 Institute of Chemistry,University of the Punjab,Lahore 54590,Pakistan

2 Renacon Pharma Limited,Lahore 54600,Pakistan

Keywords:Adsorption Isotherm Kinetics Silty clay Thermodynamic

ABSTRACT The aim of the study was to synthesize zinc oxide nanoparticles(ZnONPs)composite with clay by a novel route and then to explore the capability of composite of ZnONPs and silty clay (SC) as adsorbents for Pb(II)eradication from aqueous media by batch adsorption method.The effect of different operating factors like temperature,pH,dose and time of contact on the adsorption process were studied to optimize the conditions.Langmuir,Freundlich,Dubinin–Radushkevich (D-R) and Temkin isotherms were applied for the interpretation of the process.The R2 and q values obtained from Langmuir model suggested that the process is best interpreted by this model.The values of adsorption capacity (qm) noted were 12.43 mg﹒g-1 and 14.54 mg﹒g-1 on SC and ZnONPs-SC respectively.The kinetic studies exposed that pseudo second order (PSO) kinetics is followed by the processes suggesting that more than one steps are involved to control the rate of reactions.Various thermodynamic variables such as change in free energy (ΔG?),change in enthalpy (ΔH?) and change in entropy (ΔS?) were calculated.Thermodynamic data suggested that Pb (II) adsorption on SC and ZnONPs-SC are spontaneous,endothermic and feasible processes.

1.Introduction

Waste water produced as a result of industrial operations carries certain toxic heavy metals.Mining,battery production,metal coatings and some others are the key industries discharging these toxic metals in waste water[1].These metals are considered are as hazardous pollutants because of their toxic nature and their characteristic to accumulate in food chain.As they are not biodegradable,their accumulation in environment harm ecosystem and hence,human life[2–5].Among the heavy metals,Pb (II) is recognized as a major hazardous environmental pollutant.As Pb is present in automobiles fuel so its main entry to environment is from exhaust gases produced by automobiles.Besides,it enters to environment and water through effluents discharged by different industries[6].It is quite necessary to keep heavy metals concentration within the permissible limit in waste water,drinking water and water used for agriculture.Different approaches are being practiced to decontaminate waste water from toxic metals concentration.These methods are,ion exchange methods,chemical extraction,photo extraction,reverse osmosis,ultrafiltration and electro dialysis[7,8].The said methods carry certain shortcomings like high cost,energy,complex treatment system and labor [9].Adsorption is one of important and versatile techniques used for eradication of toxic metals from polluted media.Natural adsorbents have substantial adsorption capacity and strong affinity for toxic metals removal [10,11].To eradicate toxic metal ions from waste system,certain adsorbents like activated carbon,clays,natural and synthetic zeolites are most often utilized [12–18].Clay is an earthy,naturally occurring,fine grained material mainly made of crystalline mineral groups.It occupies a key position among industrial materials.Clay has been used for different purposes including adsorption of different pollutants.Clay is considered as a suitable adsorbent as it is not only a cheap material but also possess efficient removal capacity.Its effective sorption capacity is due to its greater surface area and exchange characteristics.Its surface structure makes it appropriate candidate to be utilized as sorbent for the metal ions removal [19].In our present research project,clay was obtained from around Purana Chaman Karaiz in Khojak mountain ranges near Chaman (34°53′1′′N,72°51′44S),Baluchistan-Pakistan.The clay collected was termed as silty clay(SC) as it contains some quantity of silt beside clay.The clay was modified by synthesizing zinc oxide nanoparticles (ZnONPs) composites within it by in situ method.The potentials of the said silty clay and its composite with ZnONPs were examined for Pb (II)removal from aqueous media.Different physico-chemical factors like pH,time,temperature and dose were optimized and afterwards,adsorbate concentration factor was evaluated.Adsorption isotherms,kinetic studies and thermodynamics were seen to know the mechanism and the process of Pb(II)ion adsorption on SC and ZnONPs modified SC.

2.Materials and Methods

2.1.Preparation of SC as adsorbent

The SC was used as adsorbent in the present study.It was obtained fromBaluchistan province of Pakistan nearChaman cityinthe Khojak mountains ranges.Firstly,the SC was kept inoven for 24 h at 100°C to remove all water content present in it.The said clay was sieved through 80 μm.The clay obtained after sieving was used as such in research without any modification and further treatment.

2.2.Preparation of zinc oxide nanoparticles reinforced SC

Literature reported procedure was adopted for the preparation of ZnONPs-SC composite with a bit modification [20,21].Briefly,10 g of SC was taken in a conical flask and 15 ml of 20 % of zinc nitrate solution was added to it.This mixture was stirred on hot plate for 30 min.The mixture was poured in china dish and was heated at 600 °C for 4 h in oven.The dried mixture was grounded by mortar and pestle.

2.3.Characterization of adsorbents

FTIR analysis was performed to know the composition and nature of SC and ZnONPs-SC composite.The synthesis of and ZnONPs was also confirmed by UV spectrometry.

2.4.Adsorbate solutions– synthetic waste solutions

Pb (II) solutions were prepared from lead nitrate,Pb (NO3)2of analytical grade from Sigma Aldrich (USA).Firstly,for 1000×10-6solution of Pb (II),1.598 g of Pb (NO3)2was dissolved in one liter of solution.The said solution was used as stock solution for other dilute solutions.AA Analyst 100 atomic absorption spectrometer of Perkin Elmer,USA instruments was used for the determination of the metal ion concentrations in the solution.

2.5.Adsorption scheme of Pb (II) on SC and ZnONPs-SC

In the present research scheme,50 ml of 50×10-6Pb(II)solutions were taken and a fixed amount of adsorbent(SC and ZnONPs-SC separately) was added to it.The said mixtures were shaken by means of isothermal shaker at 150 r﹒min-1to obtain the equilibrium.The solutions were then left to rest for a short while.Afterwards,these mixtures were carefully filtered to separate the adsorbent from the adsorbate.The filtrate obtained after filtration,was investigated for the quantity of metal ions in the system.The absorbance,Af,of the solutions were measured by atomic absorption spectrometer.For better and reliable results,all experimental steps were performed in triplicate.From the data of absorbance,we calculated two important adsorption parameters i.e.,adsorption capacity,q,and percentage removal,R,using following formulas.

In the above two equations,Coand Cerepresent initial concentration and equilibrium concentration of metal solution respectively in mg﹒L-1,V shows volume of solution (liter) while ‘‘m”shows mass of adsorbent (grams).The role of different operating factors such as dose,temperature,pH,time and concentration were studied to understand their influence on removal process of Pb (II).

3.Results and Discussion

3.1.Characterization

The adsorbents,SC and ZnONPs-SC composite,were characterized by FTIR spectroscopy as depicted in Fig.1a.The clay minerals,in general,contain variety of constituents like alumino-silicate,tetrahedral silicates and aluminate ions,octahedral metal cations,hydroxyl groups,and various diffused layer cations.While analyzing FTIR spectra,in the range of 3400–3750 cm-1,O—H stretching modes are observed.Around wavenumber 600–950 cm-1,bending modes of metal—O—H are noticed.Si—O and Al—O stretching give rise to peaks between 700 and 1400 cm-1and the bending vibrations of the same bonds are found from 150 to 600 cm-1.In case of SC,FTIR spectra recorded contains peaks of bending vibrations for Fe-Al-OH at 877 cm-1and for Al-O-H at 987 cm-1.The stretching vibrations due to O—H groups has given peaks around 3600 cm-1and 3400 cm-1.The peak due to Si—O stretching appeared at 1426 cm-1in the spectra while peak appearing at 779 cm-1is because of Al—O bending vibration[22].The FTIR spectra of ZnONPs-SC composite has also the same peaks with slight shifting due to SC modification with nanoparticles.

The UV–Visible spectra of ZnONPs-SC was recorded from 200 to 800 nm range.The absorbance spectra with lambda max at 365 nm was noted showing the successful formation of ZnONPs as depicted in Fig.1b.The value was in close agreement with the literature[23].The scanning electron microscope images depicted that ZnONPs are successfully imbedded in clay matrix and evenly distributed in the composite.In comparison to SC (Fig.2a),the composite contains small particles which are due to the inclusion of nanoparticles of zinc oxide (Fig.2b).

3.2.Adsorption studies

For the present adsorption process,various physico-chemical factors like dose,temperature,time of contact and pH were optimized after varying experimental conditions.

3.2.1.Effect adsorbents dose

The dose of adsorbent is an integral parameters in adsorption studies as it helps to know the adsorption capacity of a particular sorbent against the metal ion solution at specific working conditions.In the present research,the effect of dose effect was explored in the range 0.1–1.0 g for both sorbents against 50×10-6solution of Pb(II).The mass of adsorbent(g)was plotted against removal as is depicted in Fig.3a.The plot depicted that as the quantity of adsorbent increases,the adsorption also increases.This trend is seen up to 0.2 g after which no significant rise in adsorption is observed.The same trend has been seen for both adsorbents.The rise in removal of metal with dose addition is correlated to the greater surface area of sorbent.Upon further increase in dose quantity,no more rise in removal is due to agglomeration of dose particles which reduces surface area and hence binding sites.The same removal trend was observed for Pb (II) adsorption on expanded perlite [24].While determining the effect of dose other variables were remain constant like contact time(30 min),temperature (318 K) and pH,6.0 for Pb (II) adsorption.

3.2.2.Contact time effect

Fig.1.FTIR spectra of ZnONPs-SC and SC (a) and UV spectrum of ZnONPs-SC (b).

Fig.2.SEM images of SC (a) and ZnONPs-SC.

Contact time refers to the adsorption of the quantity of adsorbate by a definite quantity of sorbent with varying time span.The time factors in adsorption studies helps to find out the equilibrium time of the process of adsorption[25,26].In our research,the contact time effect was seen on the removal of Pb (II) as is seen in Fig.3b.It is inferred from the plot that the percentage removal of Pb(II)ion on SC and ZnONPs-SC composite increases with time up to 30 min.in case of both adsorbents.Afterwards,no appreciable increase in percentage removal is observed.Thus,30 min.was the optimized time noted for the system to acquire the equilibrium.The increase in adsorption in first 30 min.is because of the presence of greater surface area of sorbent.Once the sites are occupied by metal ions,surface of adsorbent gets saturated.After this stage,the process gets slower and no more adsorption takes place[27].While determining the effect of contact time other variables were remain constant like dose,0.2,temperature(318 K)and pH,6.0.

Fig.3.Factors effecting on the age removal percent of Pb (II) by SC and ZnONPs-SC (a) dose (b) contact time (c) temperature and (d) pH.

3.2.3.Effect of temperature

Temperature is a key factor in adsorption studies as it assists to find out whether the process is endothermic or exothermic.If rise in temperature facilitates the process of adsorption,the process is referred to as endothermic.The increase in adsorption happens because of rise in the speed of metal ions as well as rise of binding sites on adsorbent surface.On contrary,the process would be exothermic if rise in temperature causes to slow down adsorption process and the decrease in adsorption in this case is attributed to lesser active sites and weakening of interaction between sorbate and sorbent molecules [28].In our research project,the influence of temperature on the percentage removal of Pb (II) was investigated by carrying out the experiment at different temperatures i.e.,283–333 K as is shown in Fig.3c.The data revealed that Pb(II)removal by SC and ZnONPs-SC composite increased as the temperature was raised,suggesting the process of adsorption to be endothermic in nature.While determining the effect of temperature other variables were remain constant like dose,0.2 g pH,6.0 and contact time (30 min).

3.2.4.Effect of pH

pH factor is of much significance in adsorption studies.In our experiment of Pb (II) adsorption on SC and ZnONPs-SC composite,pH had remarkable influence on Pb (II) removal from the system.The pH factor was examined by changing pH of the system from 3 to 9 as is depicted in Fig.3d.The optimum pH,on which maximum removal of Pb (II) took place,was 6 in both cases.When pH was further increased,the removal started decreasing.The decrease of removal with rise in pH is correlated to the disturbance of electrostatic attraction between metal ions and the adsorbent clay [29].While determining the effect of pH other variables were remain constant like dose,0.2 g,temperature (318 K) and contact time (30 min).

3.3.Adsorption isotherms

The adsorption isotherms relates the sorbate concentration with the sorption capacity at constant temperature.In our present research,various adsorption isotherms such as Langmuir isotherm,Freundlich isotherm,D-R isotherm and Temkin Isotherm models were employed to interpret the data.The main assumption of Langmuir model is that adsorption happens on definite homogenous sites present on the surface of sorbent.The model has been successfully used in monolayer adsorption processes.The Langmuir adsorption Eq.(1) is given by,

In the above equation,qeis the metal ion concentration on sorbent at equilibrium in mg﹒g-1,Ceis metal ion concentration at equilibrium in solution(mg﹒L-1),qmrefers to the adsorbent monolayer sorption capacity in mg﹒g-1while KLis Langmuir constant having units L﹒mg-1.KLis related to free energy.To test Langmuir model,Ce/qewas plotted against Ceas is shown in Fig.4a.The slope of graph was 1/qmand intercept was equal to 1/qmKL.The data obtained from the isotherm is depicted in Table 1.R2values were almost equal to one and the calculated value of q on SC and ZnONPs-SC were 12.43 mg﹒g-1and 14.54 mg﹒g-1respectively which were very enough closer to experimental value.Another important parameter known as separation factor,RL,(dimensionless) was calculated from Langmuir data (Eq.(2)).

Table 1 Isotherm parameters of Pb (II) adsorption on SC and ZnONPs-SC

The value of separation factor(RL)calculated for the adsorption of Pb (II) on SC and ZnO-clay were 0.0365 and 0.028 respectively which showed that Pb (II) removal by SC and ZnONPs-SC were favorable processes.Thus,the data suggested that Langmuir adsorption isotherm is being followed by Pb (II) removal on both sorbents.

Fig.4.Langmuir isotherm (a),Freundlich isotherm (b),D-R isotherm (c) and Temkin isotherm (d).

The Freundlich isotherm assumption is that multilayer adsorption occurs on heterogeneous surfaces.The Freundlich equation(3)in logarithmic form is shown as,

In the above equation,Kfand n are two constants related to adsorption capacity and sorption intensity,respectively.Their values depend on the nature and heterogeneity of material.Freundlich isotherm was applied by plotting a graph between lnqevs ln Ceas is seen in Fig.4b.The n value was obtained from the slope while Kfvalue was obtained from the intercept of plot.The data obtained is shown in Table 1.Based on the data,it is concluded that Freundlich isotherm does not hold well to interpret the data of Pb (II) removal on SC and ZnONPs-SC.Dubinin–Radushkevich (D-R) model was applied to better explain the data(Eq.(4)).

Where ? is called as Polanyi potential and its formula(Eq.(5))is

β represents activity coefficient (mol2﹒J-2) related to mean adsorption energy,E (Eq.(6)).

To test the model,graph was plotted between lnqeand ε2as is shown in Fig.4c.The slope of the graph gave the value of β and intercept gave lnqm.The data obtained from D-R isotherm shown in Table 1 suggested that Pb (II) adsorption on SC and ZnONPs-Clay do not follow this isotherm [30–32].Temkin isotherm narrates that because of interaction of sorbate– sorbent molecules,there happens fall in heat of adsorption for molecules residing in the same layer [33].Temkin isotherm is shown in Eq.(7) as:

where Ktdenotes binding constant while β refers to heat of adsorption [34].A plot of qeagainst lnCegave a straight line with slope β and intercept equal to βlnKtas is shown in Fig.4d.For the Pb (II)adsorption on SC and ZnONPs-Clay,the results indicated that Temkin model is not appropriate to interpret the data.

3.4.Adsorption kinetics

Rate of adsorption of sorbate on sorbent is investigated by applying different kinetic models.These kinetic models include pseudo first order (PFO),pseudo second order (PSO),intra particle diffusion (IPD) model and Elovich kinetic model,etc.Yuh and Ho applied PFO model to explore the adsorption mechanism [35,36].The PFO model is given by Eq.(8),

In the above equation,K1is the rate constant,qtis the amount of metal ion sorbed at time t (mg﹒g-1),while qeis the amount adsorbed at equilibrium (mg﹒g-1).Upon integration,Eq.(9)assumes the form,

This kinetic model was applied by plotting ln (qe-qt) against t as is shown in Fig.5a.Parameters calculated are shown in Table 2.The R2values are not close to unity and also the calculated values of ‘‘q”are far from the experimental value of ‘‘q”.It revealed that the Pb (II) adsorption on SC and ZnONPs-SC does not follow PFO kinetics.

Table 2 Kinetic parameters of Pb (II) adsorption on SC and ZnONPs-SC

Ho and McKay have proposed PSO kinetic model.The PSO model is given by Eq.(10),

Integrating and rearranging Eq.(10) gives below and form new Eq.(11) as

k2is the rate constant having units (g﹒mg-1﹒min-1).

Fig.5.PFO (a),PSO (b),IPD model (c) and Elovich model (d) for Pb (II) removal by SC and ZnONPs-SC.

To elaborate the Pb(II)adsorption on SC and ZnONPs-SC,graphs were plotted between t/qtand ‘‘t”as is shown in Fig.5b.

The intercept of the graph gave 1/(k2qe)while the slope of graph is equal to 1/qe.R2values ware closer to unity in both cases.The calculated q value were also in close agreement to experimental value of q as is shown in Table 2.Thus,the data suggested that second order kinetics model is being followed by Pb(II)removal on SC and ZnONPs-SC.Intra particle diffusion model in the form of equation is given by equation (12)

where Kidis the rate constant with units mg﹒g-1﹒min-1/2[37].The model was applied to interpret the data of adsorption of Pb (II) on SC and ZnONPs-SC by drawing plot between qtand t1/2as is shown in Fig.5c.Different parameters were calculated as depicted in Table 2.The data showed that intra diffusion model could not be fitted well to explain the adsorption of Pb (II) on SC and ZnONPs-SC.

Elovich kinetic model was applied to interpret the data (Eq.(13))

where α is sorption rate constant with units’mol﹒g-1﹒min while β is desorption constant with units’ mol﹒g-1﹒min-1.By simplifying and rearranging,the above Eq.(13) in the form of straight line can be written as,

For Pb (II) removal on SC and ZnONPs-SC,qtagainst lnt was plotted as is seen in Fig.5d.The value of R2,very far from unity,suggested that this model does not hold to explain the process.

3.5.Thermodynamic parameters

To know the thermodynamic feasibility of the process,some important thermodynamic quantities such as change in entropy(ΔS?),change in free energy(ΔG?),and enthalpy(ΔH?)were evaluated.Firstly,ΔG?was calculated by following equation (15),

where T is temperature in kelvin,R is gas constant having value 8.314 J﹒mol-1﹒K-1,KDis distribution coefficient.The value of KDis calculated as,

where qeis equilibrium concentration of metal ions on adsorbent(mg﹒g-1) and Ceis equilibrium concentration of metal ion in solution (mg﹒g-1).

Eq.(17)shown below was used to calculate the values of(ΔS?)and (ΔH?).

A graph was plotted between lnKDagainst 1/T as is shown in Fig.6.The slope of plot is equal to -ΔH?/RT while the intercept of the plot was ΔS?/R.The values of different quantities are seen in Table 3.

Table 3 Various parameters of thermodynamic study for Pb (II) removal by SC and ZnONPs-SC

The Gibbs free energy (ΔG?) at different temperatures was calculated by using Eq.(3).The negative values of ΔG?showed that the adsorption of Pb (II) on SC and ZnONPs-SC were feasible and spontaneous processes [38].The positive value of ΔH?calculated suggested that Pb (II) removal by SC and ZnONPs-SC were endothermic processes [39,40].As already discussed under the heading of effect of temperature that increase in sorption with rise in temperature shows the process of removal is endothermic.The positive values of ΔS?refers to the irregular movement of metal ions on surface of SC and ZnONPs-SC [41].

4.Conclusions

The composite of zinc oxide nanoparticles with clay was prepared by relatively simpler and economical method.The results showed that ZnONPs-SC and SC were efficient adsorbents for Pb(II) removal from the aqueous system.Optimum conditions,for the maximum removal of metal ions,of different variables such as temperature 318 K,pH 6,contact time 30 min,and dose 0.2 g were noted in case of both adsorbents.Better values were obtained in case of ZnONPs-SC.The data was well fitted to Langmuir isotherm as the value of R2were 0.998 and 0.995 in case of silty clay and ZnONPs-SC respectively which were closer to unity.Thereby,suggesting the adsorption of metal ion on SC to be monolayer.The adsorption capacity,qm,were 12.43 mg﹒g-1and 14.54 mg﹒g-1for SC and ZnONPs-SC respectively.Kinetic studies suggested that PSO was followed by both processes.The values of thermodynamic variables suggested that adsorption of Pb(II)on clay and ZnONPs-SC are endothermic in nature,as well as spontaneous and feasible processes.

Fig.6.Thermodynamic relation for Pb (II) adsorption on SC and ZnONPs-SC.

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.

Acknowledgements

One of the authors is highly acknowledged of Higher Education Commission of Pakistan for the financial support in the form of indigenous PhD fellowship.