Synthesis of zinc oxide-montmorillonite composite and its effect on the removal of aqueous lead ions

Davidson E.Egirani·Nanfe R.Poyi·Napoleon Wessey

Abstract Lead adsorption of zinc oxide-coated ACOR montmorillonite was investigated in batches and under reducing conditions at ambient temperature.The presence of zinc oxide coating significantly enhanced the adsorption of Pb2+ions by ACOR montmorillonite.Characterization of adsorbents involved the use of X-ray diffraction,sodium saturation techniques,coulter laser analysis,scanning electron microscopy,and electron dispersive spectroscopy.Synthesis involved the trimetric process,activation of the ACOR montmorillonite and reacting of the same with zinc nitrate to produce a zinc oxide composite solid at 450°C.The reaction mechanism indicated less than one proton coefficient,and higher mass transfer rates,when compared with bare montmorillonite.Intraparticle diffusion was higher than the value recorded for the bare montmorillonite.Reactions based on initial Pb2+concentration indicated thatcoated montmorillonite gradually became saturated as the concentration was increased.Reactions based on solid concentration demonstrated a complex change in the capacity of adsorption over different Pb2+concentrations(10-40 mg L-1)and solid concentrations(2-10 g L-1).The specific surface area reduction,particle size increase,mineral aggregation,and concentration gradient effect controlled the complex changes in adsorption.

Keywords Synthesis·Characterization ·Zinc oxidemontmorillonite composite·Adsorption,Pb2+ions

1 Introduction

Lead is known to be a cumulative toxicant that affects several body organs.When ingested,it is transmitted to the brain,liver,kidney,and bones.The effects of lead in humans range from permanent brain damage to increased risk of high blood pressure.Lead used in the manufacture of pigments for lead paints,and lead pipes that deliver potable water may contaminate water systems(Abdullahi 2013;Khodadadi et al.2017).

Therefore,research that could result in smart and costeffective lead(Pb2+)removal from the aquatic environment has gained considerable attention.The release of lead into the aquatic environment is predominantly through the extractive industry.The recovery of heavy metals from ore is through hydrometallurgical leaching at low pH.This process leads to the release of leaching muds containing oxides of heavy metals and Pb2+ions as composites(Tkacova et al.1993;Tipre and Dave 2004).The toxic nature of these effluents to the ecosystem has attracted scientific interest(Dastoora and Larocpue 2004;Eric et al.2010;Stephan et al.2010;Arancibia-Miranda et al.2016).The waste migrates into shallow wells by infiltration(Kuncoro et al.2018;Zhang et al.2018).

The eh-pH of hydrometallurgical waste controls the leaching of elements.Therefore,before discharge into the aquatic environment,hydrometallurgical waste containing Pb2+ions and other metals is contained in sealed plastic tanks under reducing conditions.This results in the lowering of the metal load in the slurries and in mud leaching(Plescia and Maccari 1996;Tavares et al.2017).Under reducing conditions,leached out metals and compounds including zinc oxide(ZnO)-ACOR montmorillonite are negligible(van der Sloot 1991;Jha et al.2001).Therefore,there is a need to test the removal of lead from hydrometallurgical-contaminated slurries using standard laboratory procedures.

There are several methods for the removal of heavy metals from aqueous solution(Strange and Onwulata 2002;Ravichandran 2004;Elouear et al.2008;Olivaa et al.2011;Cataldo et al.2013a,b;Eze et al.2013;Jiang et al.2015;Chen et al.2016,2017,2018;Uddin 2017;Taraba and Bulavová2018;Wang et al.2018).However,some of these new techniques have limited applications.In addition,reaction mechanisms involved in these processes are not clear and require further study.

Some of the well-known innovative techniques for treatment of water containing heavy metals include:ion exchange(Fu and Wang 2011),membrane filtration(Blocher et al.2003),coagulation-flocculation-sedimentation(Ndabigengesere and Narasiah 2010),and adsorption(Husein 2013;Uddin 2017;Zaki et al.2017).However,these methods have setbacks,such as the large volume of sludge(Haydar and Aziz 2009),difficulty in maintenance(Hom et al.1982;Haydar and Aziz 2009),and interfering of competing ions.

On the other hand,adsorption is considered a safe technique that is highly effective and low cost(Qiu et al.2009).This process can eliminate pollutants without generating harmful by-products(Crini 2005;He et al.2017).The use of composites in adsorption is novel,groundbreaking,and low-cost.One such composite-emerging method is the use of oxide coatings on clay minerals to enhance adsorption.In testing the remediation of leadcontaminated water using ZnO-ACOR montmorillonite composite,comprehension of the reaction mechanism and application of appropriate kinetic models to support reaction patterns is required(Dzombak and Morel 1990;Lopez-Munoza et al.2010;Chiew et al.2016).

Factors that control lead and the mechanisms involved in Pb2+from the aquatic environment include metal speciation,metal mobility,adsorbent size,adsorbent surface charge,adsorbent surface area,and chemistry of protonmetal ratio(Arancibia-Miranda et al.2016).Also,contact time and pH are regulatory factors in the hydrolysis of lead ions(Maruthupandy et al.2017;Zhang et al.2018).In addition,contact time or aging enhances the reorganization of mineral surfaces in aqueous solution(Wang et al.2018).Particle concentration,particle size,and chemistry of the adsorbent control the degree of lead removal from the aquatic environment(Akpomie et al.2015).Outer sphere complexation is linked to increased lead removal as adsorbent particle concentration increases.However,increase in particle size and particle concentration does not necessarily lead to increase in Pb2+uptake(Eze et al.2013).

Lead uptake is controlled by the presence of surface area and surface-active sites(Kuncoro et al.2018;Zhang et al.2018).Based on cost and simplicity of design,adsorption is considered a simple technique for water treatment(Amiri et al.2018;Rwiza et al.2018;Seema et al.2018).The quantitative decrease in adsorption is dictated by an increase in metal concentration and vice versa(Hua et al.2012;Akpomie et al.2015).Four successive steps have been identified as a pathway for a solid-solution system undergoing adsorption.These include externalmass transfer,intra-particle diffusion,protonation,and adsorption of molecules of adsorbate(Uddin,2017;Shen et al.2017,2018).

The use of reaction mechanism models is necessary to determine the reactions involved(Dhal et al.2013).The fast process of intra-particle diffusion and slow process of outer-sphere complexation are components of reaction mechanism involved in lead uptake in aqueous solution(Bonnet et al.2017;Khodadadi et al.2017).In addition,lead removal from the aquatic environment is controlled by solution concentration and exchange of ions(Egirani and Wessey 2015a;Wang et al.2016).The use of untreated ACOR montmorillonite in lead removal has been a focus of previous studies(Egirani and Wessey 2015a,b).ACOR montmorillonite adsorbs heavy metals by cation exchange and aluminol and silanol sites(Akpomie et al.2015;Allahdin et al.2017).Other conservative techniques used in the removal of lead from aqueous solution have been reported(Bouabidi et al.2018;Maity and Ray 2018).

The effect of bare ACOR montmorillonite on the removal of Pb2+ions under similar experimental conditions has been studied before(Egirani and Wessey 2015a,b).As a follow-up to that study,this paper highlights the significance of using ACOR montmorillonite as a composite with ZnO nanoparticles as a component in lead removal.Experiments were conducted using batch mode related techniques under reducing conditions at ambient temperature.These were done with the variable pH,contact time,adsorbent concentration,and initial dosage of Pb2+ions.ZnO coating on ACOR montmorillonite is a novel adsorbent for Pb2+ion removal.It has been applied in the treatment of Pb2+ions in aqueous solution and is useful in the treatment of hydrometallurgical wastewater.The ZnOACOR montmorillonite composite significantly enhanced adsorption capacity of Pb2+ions.The synthesis of the adsorbent,characterization of the adsorbent,and testing of ZnO-coated ACOR montmorillonite to remove lead ions are discussed below.

2 Materials and experimental methods

2.1 Materials and reagents

Analytical grade reagents were used.Acros Organics(Belgium)provided ACOR montmorillonite.Double distilled water was used to wash the ACOR montmorillonite.Merck company in Germany provided lead nitrate stock solution.An AAS standard solution of 1000 mg L-1lead nitrate was prepared using a volumetric flask.As per Merck guidelines,the content of the reagents was filled to the mark and stored for use.The diluted stock solutions were used to prepare the working solutions of different concentrations.Zinc nitrate was used as a precursor,and KOH purchased from Sigma-Aldrich company in Belgium was used as a precipitating agent to synthesize ZnO nanoparticles.

2.2 Preparation of anaerobic suspensions

To create reducing conditions for the experiments,all solutions were prepared using de-aerated and deionized water.The deionized water was obtained from a Millipore Milli-Q system(18.2 MΩ cm at room temperature).The experimental content was bubbled through continuously for 24 h using purified nitrogen gas.The content was securely sealed and stored in airtight containers in an anaerobic chamber in the dark until use(Dunnette et al.1985).

2.3 Adsorbent characterization

X-ray diffraction(XRD)was used to verify the ACOR montmorillonite and the ZnO nanoparticles.A model 3340 Jenway ion meter was used to determine the pH of ACOR montmorillonite suspensions and reacting solutions.Cation exchange capacity(CEC)and specific surface area were determined using the Na saturation method and the standard volumetric Brunauer,Emmett,and Teller(BET)method,respectively(Brunauer et al.1938).Determination of the latter was achieved by measuring the adsorption of the N2gas on the mineral solid phase at the boiling point of liquid nitrogen(Lowell and Shields 1991).Determination of particle size and spectral analysis were done using coulter laser,and JEOL JSM 5900 LV Scanning Electron Microscopy(SEM)with Oxford INCA Energy Dispersive Spectroscopy(EDS),respectively(Tournassat et al.2016).Secondary electron images were acquired at low vacuum control pressure after viewing the samples.The point of zero salt effect(PZSE)-synonymous with the point of zero charge(pHzpc)-of the adsorbent was determined using the titrimetric method(Karickhoff and Bailey 1973;Janaki et al.2014).Potentiometric titration was conducted by equilibrating 1%(by mass)ACOR montmorillonite suspensions.The pH range near the PZSE was used as a reference(Bolan et al.1986;Alves and Lavorenti 2005).

2.4 Synthesis of zinc oxide-coated ACOR montmorillonite

We used the method reported by Eren(2009)and modified by Egirani et al.(2017a,b).Double distilled water was used to prepare a solution of zinc nitrate(Zn(NO3)2·6H2O)of 0.2 M concentration and KOH of 0.4 M concentration.Here,0.20 g of ACOR montmorillonite was mixed with 100 mL 1 M Zn(NO3)2solution and 180 mL of 2 M KOH solution(Eren 2009;Phiwdang et al.2013).This process activated the ACOR montmorillonite.The activated ACOR montmorillonite was dispersed into 150 mL of 0.2 M Zn(NO3)2solution.Subsequently,300 μL 0.4-M KOH aqueous solution was titrated at a rate of 1 mL/h.The content was subjected to vigorous stirring under nitrogenflow at ambient temperature(Maruthupandy et al.2017;Liu et al.2018).A double-distilled water was used to wash the white precipitate.This was centrifuged and finally washed with absolute alcohol.This was done to free the content from NO3-ions.Finally,the solid was heated at 450°C for 3 h in air,thus leading to the formation of ZnOACOR montmorillonite composite.

2.5 Batch mode adsorption experiments

The detailed experimental set up involving ACOR montmorillonite is from Egirani et al.(2015a,b).Therein,Pb2+concentration from 10 to 40 mg L-1at pH 4-8 was equilibrated with ACOR montmorillonite from 2 to 10 g L-1.This content was used to determine parametric effects of solid concentration,pH,aging,and reaction mechanisms.Therefore,no ACOR montmorillonite control experiments are provided in this paper.Here,the same experimental procedure was followed to determine all parametric effects including the effect of initial Pb2+concentration.The suspension was made to 50 mL and thereafter equilibrated for 24 h at pH 4-8.One percent each of ZnO-ACOR montmorillonite composite suspension was reacted with Pb2+solution(10-40 mg L-1).Solutions containing Pb2+ions from 10 to 40 mg L-1,were reacted with solid concentrations of ZnO-ACOR montmorillonite composite from 2 to 10 g L-1,made to 50 mL and equilibrated for 24 h at pH of 4-8.This content was used to determine the particle concentration effect(Cp).The Pb2+concentrations from 10 to 40 mg L-1were reacted with 1%ZnO-ACOR montmorillonite composite at pH 4-8.Suspensions made to 50 mL and aged from 24 to 720 h were used to determine the effect of aging.All the experiments involving the adsorbent were done in triplicate under reducing conditions,and at ambient temperature.

To predict the reaction mechanisms,the proton coef ficient(otherwise known as the proton exchange isotherm)was derived from the change of pH versus LogKd plot.The Freundlich isotherm was used to describe adsorption of Pb2+as it is suitable for heterogeneous surfaces over a wide range of solute concentrations(Egirani and Wessey 2015b;Wang et al.2016)as given by Eqs.(2)and(3).

whereSOHrepresents the mineral surface-reactive site,SO-the surface-bound Pb2+,log Kpthe apparent equilibrium-binding constant,and α the coefficient of protonation-the number of protons displaced when one mole of Pb2+binds to the mineral surface(Pirveysian and Ghiaci 2018).One percent ZnO-ACOR montmorillonite composite suspension was regulated to the required pH and to 50 mL.Finally,this was reacted with a Pb2+ion solution of 10 mg L-1.The mass transfer rate and intraparticle diffusion were derived from Eqs.(3),(4),and(5):

whereCois initial lead concentration(mg L-1)at timet=0,Ctthe concentration(mg L-1)at timet,Vthe total ZnO-ACOR montmorillonite composite suspension volume,andmthe weight(g)of the adsorbent(Contescu et al.1993;Arshadi et al.2014;Egirani and Wessey 2015a).The kinetics of Pb2+ion adsorption to the mineral surface binding sites were controlled by the mass transfer constantKf.Here,Ct/Coversus time provided the slope of the curve derived from Eq.(4)(Egirani and Wessey 2015a):

whereC0andCtdenote the initial concentration of Pb2+and concentration at timet,Ssthe exposed specific surface area of ZnO-ACOR montmorillonite composite,andKfthe coefficient of mass transfer(Shi et al.2017).These models-as reviewed previously(Qiu et al.2009)and derived from the Freundlich isotherm-were adopted to describe adsorption of Pb2+ions(Egirani and Wessey 2015a;Uddin 2017;Liu et al.2018).To investigate the role of intraparticle diffusion in the adsorption process,the Weber-Morris model was used(Feng et al.2004;Hua et al.2012;Egirani and Wessey 2015a;Tavares et al.2017;Uddin 2017)as given in Eq.(5):

whereKiis the intraparticle diffusion constant(mg g-1min)and the intercept(C)represents the effect of the layer boundary.Kiwas derived from the slope of the plots of Qtversus t0.5.A linear plot of Qtversus t0.5indicated that diffusion of intraparticle was involved in the process of adsorption(Nguyen et al.2015;Taghipour et al.2018).For the study of these reaction mechanisms,10 mg L-1Pb2+ion solution was reacted with 1%ZnO-ACOR montmorillonite composite.The content was made to 50 mL and regulated to the required pH.The amount of Pb2+ions remaining in solution was determined after 2,4,6,8,12,18,and 24 h.A 0.2 μm pore size cellulose acetate filter was used on the supernatant and content analysed for Pb2+ions,using a Hitachi Atomic Absorption Spectrophotometer(HG-AAS).The percent of Pb2+removed from solution was calculated from Eq.(6):

whereC0andCeare the initial and equilibrium concentrations(mg L-1),respectively,of Pb2+in solution.

3 Results

ACOR montmorillonite characterization is provided in Table 1.The adsorbents involved in this study have been characterized and summarized in Table 1 and Figs.1,2,3 and 4.The XRD spectrum indicates smectite as the key constituent.The EDS spectrum and SEM morphology indicate the presence of ACOR montmorillonite.The pHzpc was 7.13(the same as PZSE).This determines the positive and negative charge divide on the mineral surface.The specific surface area of the adsorbent controls the quantity of exposed mineral surface available for reaction.Proton coefficient(α)was based on a theoretical framework given by Eqs.(1)and(2),and predicted and derived from Fig.5(Table 2).The value of the plot was0.67.The pH plot had a maximum distribution coefficient(Kd)of 7.5 mg g-1.The intraparticle diffusion was based on a theoretical framework given by Eq.(5)and derived from the plot(Fig.6;Table 3).The intraparticle diffusion constant derived from the slope was 9.07(mg-1)min0.5and the intercept,C,was 582.59,≠0.The plot consisted of three linear parts,and the first part represented external mass transfer.The second and third parts represented the intraparticle diffusion and adsorption inside the adsorbent surface,respectively.The maximum adsorption capacity was 891.61 mg g-1at 24 h.

Table 1 Characteristics of ACOR montmorillonite(Egirani and Wessey 2015a)

Fig.1 X-ray diffraction of ACOR montmorillonite

The mass transfer constants(Kf)were predicted from Eqs.(3)and(4)and derived from Fig.7(Table 4).This also consisted of three linear parts.The second linear part started after 4 h and the third after 8 h.Figure 8 shows a decrease in capacity of adsorption as Pb2+initial concentration increased.The maximum adsorption capacity of 3211.5 mg g-1occurred at 10 mg L-1.Figure 9 consists of a series of non-linear and complex plots over the range of Cp investigated.There was a complex characterization of adsorption as particle concentration was increased.The highest adsorption capacity was 3285.5 mg g-1at 10 g L-1.Adsorption capacity increased with increasing residence time(Fig.10).The adsorption increased from 730 to 860 mg g-1over the range of residence time investigated.Themaximum adsorption capacity was 864 mg g-1at 720 h.The adsorption capacity generally increased with increase in pH(Fig.11).The rate of removal of Pb2+ions as predicted from Eq.(6),was from 90.72 to 94.36%over the range of pH investigated.

4 Discussion

Fig.2 a EDS-SEM for ACOR montmorillonite showing element peaks and particle sizes.b X-ray diffraction of synthetic zinc oxidemontmorillonite composite showing peaks.c EDS for zinc oxide-ACOR montmorillonite composite showing elemental peaks

Fig.3 Particle size distribution ofACOR montmorillonite at suspension pH

Fig.4 Plot of net proton charge versus suspension pH for ACOR montmorillonite

In comparison with cited literature data,ZnO-ACOR montmorillonite composite stands out as a novel adsorbent that is useful in the treatment of Pb2+contaminated aqueous solution.The reaction mechanism was determined based on the proton coefficient,intraparticle diffusion,and mass transfer rates.In previous studies using bare ACOR montmorillonite,the proton coefficient α was greater than one(Egirani and Wessey 2015a,b).Here,the α for ZnOACOR montmorillonite composite was 0.67.This was less than one and slightly less than the value recorded in previous studies(Table 2).This suggests that protonation was controlled and attenuated by the ZnO coating on ACOR montmorillonite.This is an indication that protonation was lowered in the presence of ZnO coating.The presence of this coating could mask the acidic sites on the edges and planar surfaces of ACOR montmorillonite.The three steps of mass transfer have been recognized in the reaction mechanism of Pb2+with ZnO-ACOR montmorillonite composite.These include film diffusion,adsorbent surface diffusion,and intraparticle diffusion(Figs.6,7;Tables 3,4).Although the intraparticle diffusion was involved in the adsorption process(Fig.6;Table 3),this was not a ratelimiting reaction.Also,there was an indication of boundary layer control.The slope and intercept of bare ACOR montmorillonite were higher than those of the ZnO-ACOR montmorillonite composite(Egirani and Wessey 2015a,b),suggesting that the presence of ZnO coating enhanced intraparticle diffusion.The mass transfer rates for the bare ACOR montmorillonite of previous studies were lower than results for the coated ACOR montmorillonite(Egirani and Wessey 2015b),suggesting the enhancement of mass transfer of Pb2+to the external layer of the ZnO-ACOR montmorillonite composite(Fig.7).The adsorption of Pb2+ions depended on contact time.This was determined through the plot of adsorption capacity versus time(Fig.6).The assessment was from 2 to 24 h at ambient temperature and initial Pb2+concentration of 10 mg L-1at pH 4-8.Here,only adsorption capacity at pH 4 has been discussed.The reaction pattern increased with increase in contact time and began to plateau at 12 h.Thus,there was evidence of gradual saturation of adsorption sites.This result agrees with earlier reports(Egirani and Wessey 2015a;Al-Farhan 2016)on Pb2+adsorbed on bare and modified ACOR montmorillonite,respectively.The adsorption rate was initially fast,and the capacity of adsorption decreased over time.This agrees with other reports(Al-Farhan 2016).The initial quick adsorption of Pb2+in the first phase may be related to larger numbers of active adsorption sites(Egirani and Wessey 2015a;Wang et al.2016).

Fig.5 Plot of Log Kd(distribution coefficient)versus final pH for proton coefficient

Table 2 Statistical presentation of proton coefficient derived from Fig.5

Fig.6 Adsorption capacity versus time for intraparticle diffusion

Table 3 Statisticalpresentation ofintraparticle diffusion data derived from linear fit of Fig.6

Fig.7 Ct/Coversus contact time for mass transfer rates

Fig.8 Adsorption capacity versus initial arsenite concentration at pH 4 and solid concentration=10 g L-1

Fig.9 Sorption capacity versus Cpat pH 4 for particle concentration effect

The investigation of different Pb2+concentrations was necessary since contaminated aquatic systems present different concentrations of Pb2+.Here,the decrease in adsorption capacity as Pb2+concentration was increased suggests that the mass transfer rate of Pb2+ions between the solid-solution divide was controlled by a concentration gradient(Fig.8).This was similar to previous studies(Egirani and Wessey 2015b).In both cases,a decrease inadsorption capacity for some heavy metals adsorbed on ACOR montmorillonite(100-300 mg L-1)was reported as well as a linear decrease in adsorption capacity of Pb2+on bare ACOR montmorillonite.In this report,the constant linear plot indicated a unified decrease in capacity of adsorption due to saturation of the active and reactive sites.Also,Akpomie et al.(2015)reported a similar trend for adsorption of heavy metals on bare ACOR montmorillonite.

Table 4 Mass transfer rates for Pb2+ions adsorbed on ZnO-ACOR montmorillonite composite derived from Fig.7

Fig.10 Plot of adsorption capacity versus residence time(aging)for mineral systems at pH 4 and 10 mg L-1metal concentration

Fig.11 Adsorption capacity versus pH at10 mg L-1 Pb2+concentration

The nonlinear and complex pattern of decrease in adsorption over the range of Cp investigated was different from an earlier report(Egirani and Wessey 2015b),for Pb2+adsorbed on bare ACOR montmorillonite.There,increase in the particle concentration led to increased adsorption capacity.Here,changes in adsorption pattern were more pronounced at a different Pb2+concentration(Fig.9),suggesting that the presence of ZnO coating led to strengthened linkages between adsorbate and adsorbent concentrations as the reaction proceeded.Again,increased Cp suggestively led to low pressure at the solid-solution interface,a subsequent decrease in surface area,reactive sites,and a concentration gradient effect.Thus,there was decreased Pb2+diffusion to reactive sites.In this report,adsorption capacity increased with aging-a linear increase rather than a step-wise decrease in adsorption pattern(Fig.10).This was different from in Egirani and Wessey(2015b)for Pb2+adsorbed on bare ACOR montmorillonite over the same range of aging.Previously,a complex decrease in adsorption pattern has been reported.This was linked to intra-particle diffusion,essentially controlled by inner-sphere complexation(Egirani and Wessey 2015b).In this report,the linear increase in adsorption was essentially controlled by outer-sphere watermolecule bonding,hydrolysis,and reactive support of ZnO coating,together leading to an increased reorganization of active sites(Table 5).

As pH was increased,protonation and hydroxylation of the mineral surface controlled the adsorption process(Fig.10).This was different from a previous report(Egirani and Wessey 2015b)conducted in the absence of ZnO coating.In that case,there was a linear increase in adsorption as pH was increased outside the zpc.In this case,surface charges on the ZnO-coated ACOR montmorillonite surface affected the role of solution pH.The pHzpc of ACOR montmorillonite was approximately 7.13.As pH was increased around zpc,there was a decrease in protonation and enhancement of hydroxylation,favoring Pb2+adsorption.In addition,the stability of ZnO is pH greater than 7,and optimum adsorption of Pb2+was expected above pH 7(Allahdin et al.2017).

Table 5 Linear fit for Fig.10

5 Conclusions

The presence of ZnO coating on ACOR montmorillonite significantly enhanced adsorption of Pb2+ions.The solution provided the highest adsorption capacity of 94.36%at pH 8 and 10 mg L-1metal concentrations.This was followed by initial metal concentration effect with an adsorption capacity of 3211.5 mg g-1.The aging of the solutions provided an adsorption capacity of 864 mg g-1.In this paper,synthesis of ZnO-ACOR montmorillonite composite was done and characterization conducted using usual laboratory techniques.Batch mode techniques were used to test the adsorption of Pb2+on ZnO-ACOR montmorillonite composite.The mechanism of reaction tested included proton coefficient less than one,intraparticle diffusion controlled by boundary layer,and mass transfer rates lower than those of bare ACOR montmorillonite.There was a decrease in adsorption capacity as Pb2+concentration was increased,indicating that the active and reactive sites of the ZnO-ACOR montmorillonite composite were becoming saturated.The non-linear and complex characteristics of adsorption over the range of Cp investigated suggest the following:(1)decrease in surface area,(2)reactive sites,and(3)concentration gradient effect.The decrease in adsorption capacity in this situation suggested an increase in particle size and aggregation of the mineral system as the reaction proceeded.

The adsorption of Pb2+was increased by aging.The maximum adsorption of Pb2+was 880 mg g-1over the range of residence time investigated.The higher magnitude ofadsorption pattern wasessentially controlled by hydrolysis and reactive support of ZnO coating.These included increased reorganization of active sites.As pH was increased,deprotonation and hydroxylation of ZnOACOR montmorillonite composite controlled the adsorption process.As the pH was increased,there was a decrease in protonation and enhancement in hydroxylation,leading to increased adsorption of Pb2+ions.Here,adsorption increased from 90.72 to 94.36%over the pH range investigated.These results supported the need to source different nano-sized clay composites to treat toxic materials in the environment.In comparison with other findings by Akpomie et al.(2015)and Egirani and Wessey(2015a),ZnOACOR montmorillonite composite significantly enhanced the adsorption of Pb2+in aqueous solution.Future work is needed to test removal of toxic materials using other coated clay minerals.

AcknowledgementsThe authors remain grateful to the Niger Delta University for the usual research allowances provided for the running of research projects.

Compliance with ethical standards

Conflict of interestOn behalf of all authors,the corresponding author states that there is no conflict of interest.