Widi Astuti,Achmad Chafidz,Ahmed S.Al-Fatesh,Anis H.Fakeeha

1 Chemical Engineering Department,Universitas Negeri Semarang,Semarang 50229,Indonesia

2 Chemical Engineering Department,Universitas Islam Indonesia,Yogyakarta 55584,Indonesia

3 Center for Material Science and Technology Studies,Chemical Engineering Department,Universitas Islam Indonesia,Yogyakarta 55584,Indonesia

4 Chemical Engineering Department,King Saud University,Riyadh 11421,Saudi Arabia

Keywords: Modified coal fly ash Adsorption Dual-sites adsorbent Pb(II)Zn(II)Aqueous solution

ABSTRACT Coal fly ash (CFA) is composed of minerals containing some oxides in crystalline phase (i.e.,quartz and mullite),as well as unburned carbon as mesoporous material,thus enabling CFA to act as a dual-sites adsorbent with unique properties.This work focused on the adsorption of Pb(II) and Zn(II) from binary system,a mixture containing two metal ion solutions present simultaneously,onto NaOH-modified CFA(MCFA).Several adsorption tests were conducted to evaluate the effect of several parameters,including pH and contact times.The experiment results indicated that chemical treatment of CFA with NaOH increased pore volume from 0.021 to 0.223 cm3·g-1.In addition,it could also enhance the availability of functional groups on both minerals and unburned carbon,resulting in almost 100% Pb(II) and 97%Zn(II) adsorbed.The optimum pH for adsorption system was pH=3 and quasi-equilibrium occurred in 240 minutes.Equilibrium data from the experimental results were analyzed using Modified Extended Langmuir (MEL) and Competitive Adsorption Langmuir-Langmuir (CALL) isotherm models.The analysis results showed that the CALL isotherm model could better describe the Pb(II) and Zn(II)adsorption process onto MCFA in binary system compared with MEL isotherm model.

1.Introduction

Heavy metal ions present in natural surface water (e.g.,river,lake) are considered as one of the major environmental issues worldwide due to their non-biodegradability,harmful effects,tendency to reach the food chain and accumulate in living organisms[1–4].Usually,they are present in the environment through waste discharges of industries,e.g.,metallurgy(metal coating,metal processing,alloy),paint,mining,smelting,and battery industries[5,6,7].Heavy metals can affect the quality of natural water resources,and also threaten the human life as well as the water ecosystem because of their carcinogenic,toxicity,immunogenic,mutagenic,and teratogenic action [8].Among the common heavy metals (i.e.Co,Cd,Hg,Cr,Mn,Cu,Ni,and Pb),Pb(II) has a high bioaccumulation degree and is responsible for many deadly diseases,such as cancer,renal blood dysfunction,neurotoxic,immune disorders,severe anemia,kidney failure,lung disease,and semi-permanent brain stroke[2,3,7,9].The report by WHO(World Health Organization)mentioned that Pb(II)could potentially cause around 140 million deaths especially in the developing countries.Therefore,removal of Pb(II) from the aquatic environment (e.g.river,lake) is very important to safe human life [10].

Additionally,even though Zn (II) is one of the essential micronutrient needed for life,it can also cause anemia,neurologic symptoms,pancreas damage,and lethargy if accumulated above its permissible limit[5].The concentration of Pb(II)and Zn(II)ions in wastewater vary greatly depending on the source of wastewater.The industrial wastewater containing Pb(II) ions ranges between 200 and 500 mg·dm-3while the average concentration of Zn(II)in wastewater discharged from electroplating industry is around 35 mg·dm-3[11,12].On the other hand,Pb(II) and Zn(II) contents in environments are strictly restricted by many standards and guidelines.For example,the maximum allowable limit of Pb(II)and Zn(II) in potable water suggested by WHO are about 0.01 and 15 mg·dm-3,respectively [5,6]while the permissible level of Pb(II) and Zn(II) in potable water suggested by the Ministry of Health of the Republic of Indonesia are 0.05 and 5.0 mg·dm-3,respectively.Due to the harmful effect of Pb(II)and Zn(II)pollution in surface water,there is an increasing demand to eliminate or reduce the content of Pb(II) and Zn(II) in wastewater before discarded into the stream [13].Numerous methods for removal of Pb(II) and Zn(II) from the aqueous solution have been studied,including ion exchange [14,15],membrane separation [16,17],and adsorption [2,5,6,18,19].Among those methods,adsorption has been the most popular and extensively applied for Pb(II) and Zn(II) removal from wastewater due to its efficient method with simple design and operation[4,20].Activated carbon[21,22],polymeric material[23,24],MCM-41[25],and composite materials[26]have been utilized as adsorbents.In this sense,the existence of specific functional groups and surface area were thought to affect adsorption capacity.Nevertheless,these materials are commonly expensive,though they could be regenerated.

Coal fly ash(CFA),a by-product of a coal combustion in thermal power plant,is a very low-cost material and abundantly available.It is in the form of glass like fine grey powdered,which is usually collectedfromthefluegasbyelectrostaticprecipitatororotherdustcollectors[27,28].About 40%of power plants in the world are still using coal-firedbasedthermalpowerplant,whichgeneratingconsiderable amount of CFA.With increasing demand for energy,especially coalbased energy,the amount of CFA released annually also increases rapidly.Currently,CFA is mostlyused in constructionmaterials,such as additives for cement.However,its current utilization is still low,which is only about 20%-30% on average globally [27,28].The CFA consists of minerals containing several oxides in crystalline phase(i.e.,quartzandmullite)[28–30],aswellasunburnedcarbonasmesoporous material[31],thus allowingit to be used as metals adsorbent.In this sense,adsorption process can take place on both minerals and unburned carbon,thus enabling CFA to act as a dual-sites adsorbent[32,33].Several research studies have reported the effectiveness of CFAintheremovalofPb(II)ion[27,34–36]andZn(II)ion[34,36]from aqueoussolutions.ForimprovingtheadsorptioncapacityofCFA,severalmodificationsandactivationprocesseshavealsobeenpreviously studied,such as modification using alkalis[35,37–39],and mechanical activation[34].

So far,most researchers have reported metals adsorption in a single system.However,the real effluents correspond to multicomponent system(more than one metal present simultaneously)with competitive process.In this sense,most of studies on the metals adsorption have used Langmuir and Freundlich isotherm model to predict adsorption capacity for single system[38,40–42],binary system[43],and ternary system[43–45]while extended Langmuir isotherm model have used to predict adsorption capacity for binary system[46–48],a mixture containing two metal ions solution present simultaneously.However,these models may not be appropriate for metals adsorption onto CFA in binary system due to the presence of dual adsorption sites in the CFA (i.e.,minerals and unburned carbon).Therefore,the objectives of this study were to find and analyze an appropriate model for metals adsorption in binary system(i.e.,a mixture containing Pb(II)and Zn(II)solution)onto CFA acting as a dual-sites adsorbent.Several adsorption tests were conducted to evaluate the effect of several parameters,including pH and contact times.In this study,the CFA was chemically modified using NaOH (namely MCFA) to decrease the crystallinity of minerals,increase the surface area,and increase the availability of functional groups that provide required sites for the adsorption process.

2.Experimental

2.1.Production and characterization of MCFA

The CFA was taken from the dust collector unit of coal-based thermal power plant,which located in Tanjungjati,Jepara city,Indonesia.Prior to the modification process,the CFA was carefully washed using distilled water and subsequently dried in an oven(Model UN55 Memmert,Germany) at approx.temperature of 105 °C for one hour to remove the water content.Afterward,the dried CFA was crushed and sieved.The fraction with the particle size <0.149 mm was used for the experiment,namely sieved-CFA(SCFA).For modification,about 50 g of the SCFA was refluxed with 300 cm3of NaOH 3 mol·dm-3at 60 °C for 2 hours,filtered using Whatman paper No.5,washed using distilled water for several times until the filtrate reached pH of about 7,and then dried using oven at temperature of 110 °C for one hour,namely modified-CFA(MCFA).Both CFA and MCFA were further characterized.The chemical composition of CFAs(i.e.CFA and MCFA)was analyzed using XRay Fluorescence (XRF),namely ZSX Primus II from Rigaku,Japan.Whereas,the existence of functional groups in the CFAs was analyzed by Shimadzu Fourier Transform Infrared Spectrometer(FTIR)(Spectrum 100,PerkinElmer,USA)at wave number 400–4000 cm-1using KBr pellet method.A Scanning Electron Microscopy (SEM)equipped with Energy-Dispersive X-ray (EDX) feature was carried out to investigate the surface morphological characteristics and elemental mapping analysis of the CFAs.Whereas,the pore structure of the CFAs was characterized by a gas sorption apparatus(QuantaChrome,Nova 1200,USA) using N2adsorption–desorption measured at 77 K.Specific surface area of CFAs was determined using Brunauer Emmet Teller(BET)equation while pore size distribution and pore volume was determined using Barrett-Joyner-Halenda (BJH) method.

2.2.Preparation of stock solution of Pb(II) and Zn(II)

Chemicals used for the preparation of stock solution were Zn(NO3)2.6H2O and Pb(NO3)2obtained from Merck,Germany.The stock solution (1000 mg·dm-3) of Pb(II) and Zn(II) were made by dissolving 1.598 g of Pb(NO3)2and 2.907 g of Zn(NO3)2·6H2O in deionized water.The required concentrations of Pb(II) and Zn(II)for adsorption tests were prepared by dilution of the stock solution in aquadest.

2.3.Effects of pH

About 50 cm3each solution of Pb(II)and Zn(II)(100 mg·L-1)and 1 g of MCFA were added into seven Erlenmeyer flasks (250 cm3size).The adsorptions were carried out at various pH (3–9).The pH of the suspensions was controlled by using 0.1 mol·dm-3NaOH or HCl.The suspensions were shook using a shaker apparatus for 240 minutes at(27 ± 2) °C and 120 r·min-1.Afterward,all the suspended solids were separated using Whatman filter paper (No.5).Then,the filtrates were characterized for residual contents of Pb(II)and Zn(II) using an Atomic Absorption Spectroscopy (AAS) Model PinAAcle 900F (PerkinElmer,USA).

2.4.Effects of contact time

About 50 cm3each solution of Pb(II) and Zn(II) (100 mg·dm-3)and 1 g of MCFA were added into ten Erlenmeyer flasks (250 cm3size).All the suspensions were adjusted to pH 3 by adding with 0.1 mol·dm-3NaOH or HCl.The samples were shook at 120 rpm using a shaker equipment for various contact times (i.e.,10–240 minutes) at (27 ± 2) °C.Afterward,all the suspended solids were separated using Whatman filter paper (No.5).Then,the filtrates were characterized for the residual contents of Pb(II) and Zn(II)using an Atomic Absorption Spectroscopy (AAS) Model PinAAcle 900F (PerkinElmer,USA).

2.5.Adsorption equilibrium

Adsorption equilibrium tests were conducted in several 250 cm3Erlenmeyers containing 1 g of MCFA and 50 cm3of each solution of Pb(II) and Zn(II) with different initial concentrations(i.e.10–1000 mg·dm-3).The experiments were performed at pH 3.The pH was controlled with added 0.1 mol·dm-3NaOH or HCl.The Erlenmeyers were shook using a shaker apparatus for 240 minutes at(27±2)°C and 120 r·min-1.Afterward,the suspended solids were separated using Whatman filter paper (No.5).Then,the filtrates were characterized for the residual contents of Pb(II) and Zn(II) using an Atomic Absorption Spectroscopy (AAS) Model PinAAcle 900F (PerkinElmer,USA) for further isotherm analysis.The quantity of Pb(II)and Zn(II)adsorbed by the adsorbent at equilibrium (qe) (mg·g-1) was determined by Eq.(1) [41]:

whereCois initial concentration of Pb(II) (Co,Pb) or Zn(II) (Co,Zn) in mg·dm-3,Ceis concentration of Pb(II) (Ce,Pb) and Zn(II) (Co,Zn) in solution at equilibrium (mg·dm-3),Vis total volume of Pb(II) and Zn(II) solution in dm3,andmis the adsorbent mass (g).

The adsorption stability of the adsorbent materials is a significant parameter,which can determine the prospect of the adsorption application.The ability of adsorbent to lock heavy metal ions is detected through desorption experiment at ambient temperature of (27 ± 2) °C.To determine the amount of metal ions physically adsorbed by the adsorbent,1 g CFA or MCFA was added to 50 cm3each solution of Pb(II) and Zn(II).The mixture (pH=3)was shook at 120 r·min-1for 240 minutes at (27 ± 2) °C and then went through the filtration.The filtered solid was dispersed in a 100 cm3distilled water and shook for three days at ambient temperature (27 ± 2) °C.Furthermore,the suspended solid was separated using Whatman filter paper (No.5) and the filtrate was analyzed for the Pb(II) and Zn(II)content using an Atomic Absorption Spectroscopy (AAS) Model PinAAcle 900F (PerkinElmer,USA).The amount of Pb(II)and Zn(II)desorbed was expressed by Eqs.(2)and (3),respectively [49].

whereCD,Pbis the Pb(II) concentration in the desorption solution(mg·dm-3),CD,Znis the Zn(II) concentration in the desorption solution(mg·dm-3),VDis total volume of the desorption solution(L),mDis the adsorbent mass in the desorption experiment (g),qe,Pbis the quantity of Pb(II)adsorbed by the adsorbent in the adsorption process (mg·g-1) expressed by Eq.(4),and andqe,Znis the quantity of Zn(II)adsorbed by the adsorbent in the adsorption process(mg·g-1)expressed by Eq.(5).

I made my decision there and then. I took my leave, and traveled away from the city. When I returned I had bought myself a cane and wrapped my leg tightly with bandages. I told everyone I had been in a car crash and that my leg would never completely heal again. My dancing days were over. No one suspected the story—I had learned to limp convincingly before I returned home. And I made sure the first person to hear of my accident was a reporter I knew well. Then I traveled to the hospital. They had pushed your grandfather outside in his wheelchair. There was a cane on the ground by his wheelchair. I took a deep breath, leaned on my cane and limped to him.

3.Results and Discussion

3.1.Characterization of CFA and MCFA

The CFA consists of minerals containing several oxides in crystalline phase(i.e.,quartz(SiO2)and mullite(3Al2O3·2SiO2)),as well as unburned carbon as mesoporous material[50,51].The XRF characterization results exhibited that the CFA mainly consists of 36.47% of SiO2,19.27% of Al2O3,and 19.11% of unburned carbon.Whereas,the percentage of SiO2,Al2O3,and unburned carbon in the MCFA were 28.69%,20.51% and 16.68%,respectively.The decrease of SiO2content in the MCFA may be due to the partial conversion of quartz (SiO2) to sodium silicate (Na2SiO3) with the addition of NaOH during the modification process,as can be seen in Eq.(6) [52].

The surface morphological characteristics and elemental scanning of the CFA and MCFA was investigated by using SEM equipped with EDX analysis.Fig.1(a) and (c) exhibits the SEM micrographs of the CFA,which was composed of minerals having a characteristic of spherical like structure with a smooth surface,and unburned carbon having a rough surface with many pores which enable it to act as a dual-sites adsorbent.Whereas,Fig.1(b)and(d)shows the surface morphology of the MCFA.As seen in Fig.1(b) and (d),chemical treatment of CFA with NaOH has resulted in rougher surface and more pores,especially for minerals.Furthermore,Fig.2 exhibits the elemental mapping of CFA analyzed by SEM-EDX.As seen in Fig.2,the Al,Si,O,and C elements were likely well distributed on the CFA surface.

Additionally,the N2adsorption–desorption isotherm of CFA showed III-type isotherm according to Brunauer’s classification[see Fig.3(a)]indicating the interaction of adsorbent-adsorbate was weak,while the N2isotherm of MCFA showed II-type isotherm with hysteresis loop,indicating that the MCFA has mesopores other than macropores [53].The BET specific surface area of CFA and MCFA were 10.4 and 80.3 m2·g-1,respectively,while the pore volume of CFA and MCFA determined using Barret Joiner Halenda(BJH) method were 0.021 and 0.223 cm3·g-1,respectively.As noticed,the pore volume of the MCFA was higher than CFA,indicating that the porosity of the MCFA was also higher than the CFA.CFA and MCFA have pores with a diameter in the range of 1.5–50 nm,as shown in Fig.3(b).The peak of CFA curve was located at a pore radius of approx.1.5 nm,while the MCFA has two peaks located at a pore radius of approx.1.9 and 2.8 nm.It can be concluded that the majority of pores for CFA were 1.5 nm in radius,whereas for MCFA were around 1.9 and 2.8 nm.

The increase of the pore size was possibly due to the partial damage of quartz (SiO2in crystalline phase) due to reaction with NaOH [54,55],resulting in amorphous phase and Na2SiO3[38],as explained previously.Scheme of the partial damage mechanism was shown in Fig.4.Moreover,the partial damage caused active sites (i.e.functional groups contained in the minerals) to be more open and easier to bind adsorbate molecules [56].This result showed a good agreement with SEM images that showed a rougher surface and more pores.

Fig.1.SEM micrographs of (a) CFA and (b) MCFA at magnification of 300×,(c) CFA and (d) MCFA at magnification of 1000×.

Fig.2.SEM micrographs with elemental mapping of CFA at magnification of 1000×: (a) morphology,(b) Al,(c) Ca,(d) C,(e) Fe,(f) Mg,(g) Na,(h) O,and (i) Si contents.

Fig.3.(a)Nitrogen adsorption–desorption isotherms and(b)pore size distribution curve of CFAs.

Additionally,FT-IR test was also performed to study the existence of functional groups.The FT-IR spectra of CFA,SCFA and MCFA were depicted in Fig.5(a).The FT-IR spectrum of the CFA (1) showed a broad peak around 3400 cm-1may result from vibration of O-H groups from alcohols or phenols [57]while a peak around 1677 cm-1was due to stretching vibration of C=O bond in the aromatic rings.These peaks show that apart from being a mesoporous material,the surface of unburned carbon also contains functional groups.Furthermore,the peaks around 1135 and 835 cm-1were allotted to the TO4(T represents Al,Si) asymmetric stretching vibrations and Si-O vibration,respectively [39],while the band centered at c.a.597 cm-1was out-of-plane bend vibration of Si-O external connection [58].Comparing the SCFA spectrum with CFA spectrum,there was a change in the intensity of some bands.The intensity of band indicating the existence of (-OH) groups reduced while the band at 1677 cm-1disappeared,which was likely due to the loss of some carbon during the washing process.Conversely,if the MCFA spectrum was compared to the CFA spectrum,there was an increase in the intensity and widening of band at c.a.3500–3400 cm-1.It was likely due to the addition of (-OH) group from NaOH.It can be concluded that chemical reaction using NaOH could enhance the availability of functional groups acting as active sites for the adsorption process.Accordingly,adsorption mechanism on the unburned carbon is likely not only physisorption through van der waals interaction in the pores,but also chemisorption through electrostatic interaction between surface functional groups and the adsorbate molecules.In addition,the chemical reactions also partially damaged the minerals,i.e.,quartz(SiO2) and mullite (3Al2O3·2SiO2),thus pore size increased[52]and minerals become more amorphous.The reaction of quartz and NaOH was described in Eq.(6),whereas the reaction of mullite with NaOH was described in Eqs.(7) and (8).The damage of the quartz and mullite was indicated by the decrease of peak intensity at 1135,835,and 597 cm-1.

The FT-IR spectra before and after adsorption process using MCFA are depicted in Fig.5(b).After adsorption of Pb(II) and Zn(II),some bands decrease in the intensity,disappeared,and shifted.The band at c.a.3500–3400 cm-1decreased in the intensity,while the band at c.a.1500 cm-1disappeared.It might be due to electrostatic bonding between Pb(II) or Zn(II) and -OH or C=O groups located on unburned carbon surface [59].The band at c.a.1050,835,and 505 cm-1shifted to 1095,879,and 547 cm-1,respectively,which might be due to electrostatic bonding between Pb(II) or Zn(II) and Si-O-Si and Si-O-Al groups [59,60]located on minerals surface.It is concluded that chemisorption through electrostatic interaction between adsorbate molecules and MCFA was occurred on both unburned carbon and mineral in the MCFA.In this sense,the functional groups acting as active sites for the adsorption process.

3.2.Adsorption of Pb(II) and Zn(II) by MCFA

In general,the porosity of adsorbent has less effect in the heavy metal ions adsorption than the density of oxygen containing functional groups which affects the surface acidity of the adsorbent[61].The surface acidity of CFA measured at the point of zero charge (pHPZC) is 12.15,suggesting that adsorption of Pb(II) and Zn(II) could be at least performed in a solution with a pH of 12,where the surface of adsorbent has a neutral charge.A solution with lower pH will change the adsorbent surface into positive charge state.In this sense,a decrease in pH solutions led to an increase in the number of protonated functional groups,as can be seen in Eq.(9) [52].

Fig.4.Scheme of the partial damage mechanism of SiO2.

However,in a solution with higher pH (i.e.,pH > 9),precipitation of Pb(II) ions in aqueous phase will results in extraction instead of adsorption [62].According to the condition,the range of pH studied was 3–9 [63].Fig.6(a) shows the effect of pH on the competitive adsorption of Pb(II) and Zn(II).Increasing solution pH from 3 to 4 resulted in an increase of the amount of adsorbed Pb(II)(i.e.,close to 100%),although the increase is insignificant.It was likely due to deprotonation of functional group in the MCFA,as can be seen in Eq.(9).It created an electrostatic attraction between positively charged of Pb(II) (i.e.,Pb2+) and negatively charged of MCFA(i.e.,unprotonated functional groups),increasing the amount of Pb(II) adsorbed [60].The sorption mechanism on the mineral surface of MCFA was explained by the following reactions [50]:

Fig.5.(a)FT-IR spectra of(1)CFA;(2)SCFA;and(3)MCFA,and(b)FT-IR spectra of(1)MCFA after adsorption of Pb(II)and Zn(II)at pH of 3;(2)MCFA after adsorption of Pb(II) and Zn(II) at pH of 4; and (3) MCFA before adsorption.

Conversely,an increasing solution pH from 4 to 9 resulted in the decrease of adsorption uptake.It was likely because of the formation of PbOH+having a larger molecule,and precipitation of Pb(OH)2and litharge (PbO).Actually,Pb(OH)2can be formed and begin to precipitate in pH values higher than 5.7[60].The adsorption uptake of Zn(II) was slightly lower than Pb(II).Selective adsorption of different heavy metal ions by MCFA can be attributed to their different affinity,which was determined by their ionic characters such as hydrated ionic radii and electronegativity [64].Electronegativity of Pb(II)was 2.33 while Zn(II)was 1.65.Whereas,hydrated ionic radii of Pb(II) and Zn(II) was 0.401 and 0.430,respectively [64].Due to Pb(II) had electronegativity higher than Zn(II) and hydrated ionic radii smaller than Zn(II); as a result,Pb(II) had higher adsorption affinity to interact with a functional group in the MCFA through electrostatic attraction.The same results have been reported by Josephet al.,where CFA removal efficiency for Pb(II) was higher than Zn(II) [65].The increase of solution pH from 3 to 4 has resulted in the increase of amount of adsorbed Pb(II),but the adsorption uptake of Zn(II) decreased.It was likely due to Pb(II)that would replace the initially adsorbed Zn(II) during the sorption process.Similar to Pb(II),increasing solution pH from 4 to 9 resulted in a decrease in the amount of adsorbed Zn(II)due to the formation of Zn(OH)and Zn(OH)2.Based on this result,pH 3 was selected for further adsorption experiments.Many researchers have also recorded related findings.Pb(II) and Zn(II) were adsorbed in the pH region 3–4 where Pb2+and Zn2+cations predominate in the solution [66,67].

Fig.6.(a) Effect of pH and (b) contact time on the amount of Pb(II) and Zn(II)adsorbed by MCFA.

The adsorption of heavy metal increases rapidly at the start of the process,usually during the first 10 min [59,65],due to the rapid external mass transfer to the surface of the adsorbent.Furthermore,it is followed by a slower intra-particle diffusion process,until the equilibrium is reached and the adsorption process is stopped.Fig.6(b) illustrates the percentages of Pb(II) and Zn(II)adsorbed by the MCFA as function of the contact time.The initial stages were denoted by a steep increase in the amount of heavy metals adsorbed.After reaching equilibrium time (240 minutes),there were no significant changes in the concentration of the metals (i.e.,Pb(II) and Zn(II)) adsorbed by the MCFA,thus the adsorption process is stopped.At the initial stage,there was an abundance of active sites available for binding,and as it approaching equilibrium,almost all active sites on the MCFA were occupied by Pb(II)and Zn(II),thus the adsorbent became difficult to be filled by the metals due to the repulsive effect between the metal ions attached on the MCFA and the metal ions in the solution.The adsorption equilibrium was attained in 60 and 240 minutes for Pb(II) and Zn(II),respectively.It was probably caused by the affinity of Pb(II),which was stronger than that of Zn(II),as explained previously.

Desorption experiments using distilled water were carried out to determine the amount of metal ions physically adsorbed by the CFA and MCFA(i.e.physisorption)through van der waals interaction in the pores[45].As seen in Table 1,only 2%-6%of Pb(II)and Zn(II)could be desorbed by distilled water,indicating less than 10%of Pb(II) and Zn(II) were physically adsorbed in the pores of CFAs through van der waals interaction.In this sense,chemical interactions on the functional groups of the CFAs(i.e.chemisorption)were considerably strong.As previously explained,chemisorption was occurred on the both mineral and unburned carbon.However,from previously study could be concluded that the adsorption process occurred mostly on the minerals surface [29,68].

Table 1The amount of Pb(II) and Zn(II) desorbed by CFA and MCFA

In addition to metal oxide such as SiO2,Al2O3,Fe2O3,CaO,MgO,Na2O and K2O,CFA also contains trace metals such as Pb,Cd,Cr,Cu,and Zn with concentration of 7.59,0.16,327,1.52,and 118 mg·kg-1,respectively[66,68].However,the leachability results of trace metals from CFA showed that the average leaching capacity of them is around 1.52%.Thus,it can be considered that the trace metals were not leached from CFA.

3.3.Adsorption isotherm model

The isotherm model describes the retention of substance on solid at constant temperature and various concentrations.It is a major tool to describe a mechanism of solute adsorption on the adsorbent surface.To describe the competitive adsorption of Pb(II) and Zn(II) from the binary system onto MCFA as a dual-sites adsorbent,two isotherm models were analyzed including ModifiedExtended Langmuir (MEL) and Competitive Adsorption Langmuir-Langmuir(CALL).Both of the models were modified from Langmuir isotherm model [66,69].The schematic diagrams of the two models above were depicted in Fig.7.

3.3.1.Modified Extended Langmuir (MEL)

In the MEL model,it is assumed that during the Pb(II)and Zn(II)adsorption,the contribution of unburned carbon was very small or negligible.Adsorption only occursviaelectrostatic interaction between surface functional groups having homogeneous energy in the minerals with adsorbate molecules (i.e.,Pb(II) and Zn(II))(see Fig.7(a)).This assumption is based on the results of previously study presenting the effect of unburned carbon in the CFA toward metal adsorption [68].It is also assumed as localized adsorption,and thus,once an adsorbate molecule takes place on an active site,no more adsorption can occur on that site.Additionally,the interaction among the adsorbate molecules is also negligible.Extended Langmuir is developed from Langmuir isotherm,which is defined in Eq.(12) [46,47].

whereCe,PbandCe,Znare concentration of Pb(II) and Zn(II) respectively,in the solution at equilibrium (mg·dm-3).qm,Pbandqm,Znare quantity of Pb(II)and Zn(II)respectively,which required to create a monolayer on a unit mass of the adsorbent at equilibrium(mg·g-1).Whereas,KLis a constant corresponding to the affinity of the binding site toward Pb(II) (i.e.KL,Pb) and Zn(II) (i.e.,KL,Zn)(dm3·mg-1).The Solver function in the Microsoft Excel was used to determineqm,Pb,qm,Zn,KL,Pb,andKL,Znby minimizing the difference between the experimental and predicted data [70].

3.3.2.Competitive Adsorption Langmuir-Langmuir (CALL)

In the CALL model,the active sites as well as unburned carbon have a similar role during the adsorption.The active sites and unburned carbon can be considered as different patches(see Fig.7(b)) having different levels of energy.Each patch is assumed independent without interaction between one patch to another.The adsorption site has homogeneous energy distribution in the same patch.In this sense,physisorption through van der walls interaction(especially in the unburned carbon pores)is assumed very small due to only 2.63%of Pb(II)and 3.72%of Zn(II)are physically adsorbed in the pores of MCFA(Table 1),so the role of pores as adsorption sites is ignored.Adsorption only occurs via electrostatic interaction between surface functional groups on both minerals and unburned carbon having homogeneous energy with adsorbate molecules(i.e.,Pb(II)and Zn(II)).Additionally,it is assumed as localized adsorption,and thus,once an adsorbate molecule takes place on an active site,no more adsorption can occur on that site.The interaction among the adsorbate molecules is also negligible.The CALL model can be expressed using Eq.(13)below:where subscript ‘1’ indicates site 1 (i.e.minerals) and subscript ‘2’indicates site 2 (i.e.unburned carbon).Thesolverfunction of the Microsoft Excel was used to calculateKL1,Pb,KL1,Zn,KL2,Pb,KL2,Zn,qm1，Pb,qm1，Zn,qm2，Pb,qm2，Znby minimizing the difference between the experimental and predicted data [70].

Fig.7.The schematic of the adsorption mechanism based on (a) MEL model,and (b) CALL model.

3.3.3.Analysis of isotherm models

The sum of square of errors(SSE)deviation was applied to select an appropriate adsorption isotherm model for Pb(II) and Zn(II)removal from the binary system via adsorption onto MCFA acting as a dual-sites adsorbent.The equation is given by Eq.(14) [71].Three isotherm models,i.e.,Langmuir (L) [66],Modified Extended Langmuir (MEL) and Competitive Adsorption Langmuir-Langmuir(CALL) were compared.The values of error measurements (SSE)and average deviation are given in Table 2.

Table 2 shows that the CALL model was best fitted with the experimental data because it has smaller SSE than L and MEL models.It can be concluded that there are enough unburned carbons in the MCFA that can contribute to the adsorption,together with minerals.The Zn(II)and Pb(II)ions could be adsorbed by both minerals and unburned carbon through electrostatic interaction between surface functional groups with the adsorbates.It is in accordance with FTIR result.The values of parameters for CALL isotherm modelis given in Table 3.Whereas,the amount of Pb(II) and Zn(II)adsorbed by minerals and unburned carbon based on the L,MEL,and CALL models are depicted in Fig.8.

Table 2The values of error measurements (SSE) and average deviation

Table 4 shows the comparison of the adsorption capacity of MCFA for Pb(II) and Zn(II) with different adsorbents reported in the literature.It was observed that the adsorption capacity of MCFA was higher than other adsorbents.Therefore,it could be considered that MCFA was a promising adsorbent for Pb(II) and Zn(II)removal.

Table 3Values of parameters for CALL im model

Table 4Comparison of the adsorption capacity of MCFA for Pb(II) and Zn(II) with different adsorbents

Fig.8.The amount of Pb(II) and Zn(II) adsorbed based on experimental data and CALL model.

4.Conclusions

The adsorption test results show that modified coal fly ash by chemical treatment using NaOH (MCFA) could be used effectively as an adsorbent.It could enhance the availability of functional groups that provides sites for the adsorption process.It also partially damaged quartz and mullite and converted into sodium silicate(Na2SiO3)and sodium aluminate(NaAl(OH)4),which dissolved in the solution so that the pore size increases.The Pb(II)and Zn(II)adsorption from an aqueous solution (binary system) onto MCFA was influenced by pH.The effective pH for Pb(II)and Zn(II)removal was found to be pH 3.Additionally,the quasi-equilibrium of Pb(II)and Zn(II)adsorption was reached in 240 minutes,with more than 90% of Pb(II) and Zn(II) was adsorbed.Unburned carbon also contributed to the adsorption of Pb(II) and Zn(II) along with the minerals.Additionally,it was also found that the appropriate isotherm model for the adsorption of Pb(II)and Zn(II)from the aqueous solution (binary system) onto MCFA was Competitive Adsorption Langmuir-Langmuir (CALL) due to it has smaller SSE than that of Modified Extended Langmuir (MEL) model.

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

The authors would like to appreciate the Deanship of Scientific Research (DSR) at King Saud University (KSU),Saudi Arabia for financially supporting this research project (No.RG-1435-078).