Qingrong Zou,Wanyu Wang,Tong Zhang,Yuanyuan Liu,*

1 State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400044,China

2 Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment of Ministry of Education,Chongqing University,Chongqing 400044,China

3 Central-Southern Safety &Environment Technology Institute Co.,Ltd,Wuhan 430061,China

Keywords:Iron sulfide Nanoparticles Multi-heavy metal contamination Simultaneous removal Environment Remediation

ABSTRACT Cadmium(Cd),lead(Pb),and hexavalent chromium(Cr(VI))are often found in soils and water affected by metal smelting,chemical manufacturing,and electroplating.In this study,synthetic iron sulfide nanoparticles (FeS NPs) were stabilized with carboxymethyl cellulose (CMC) and utilized to remove Cr(VI),Cd,and Pb from an aqueous solution.Batch experiments,a Visual MINTEQ model,scanning electron microscopy(SEM),X-ray diffraction(XRD),and X-ray photoelectron spectrometer (XPS)analysis were used to determine the removal efficiencies,influencing factors,and mechanisms.The FeS NP suspension simultaneously removed Cr(VI),Cd,and Pb from an aqueous solution.The concentrations of Cr(VI),Cd,and Pb decreased from 50,10,and 50 mg·L-1 to 2.5,0.1,and 0.1 mg·L-1,respectively.The removal capacities were up to 418,96,and 585 mg per gram of stabilized FeS NPs,respectively.The acidic conditions significantly favored the removal of aqueous Cr(VI)while the alkaline conditions favored the removal of Cd and Pb.Oxygen slightly inhibited the removal of Cr(VI),but it had no significant influence on the removal of Cd and Pb.A potential mechanism was proposed for the simultaneous removal of Cr(VI),Cd,and Pb using FeS NPs.The interactions of the three heavy metals involved a cationic bridging effect on Cr(VI)by Cd,an enhanced adsorption effect on Cd by[Cr,Fe](OH)3,precipitation of PbCrO4,and transformation of PbCrO4 to PbS.Therefore,FeS NPs have a high potential for use in the simultaneous removal of Cr(VI),Cd,and Pb from contaminated aqueous solutions.

1.Introduction

The discharge of heavy metals from industrial and agricultural activities has seriously impacted aquatic and soil ecosystems [1].Cadmium(Cd)and lead(Pb)are typical poisonous cationic metals,and hexavalent chromium(Cr(VI))is a typical anionic metal found in the environment.Cr(VI),Cd,and Pb are often found in soils affected by metal smelting,chemical manufacturing,and electroplating at concentrations ranging from 80 to 1000 mg·kg-1,1 to 50 mg·kg-1,and 10 to 1000 mg·kg-1,respectively[2,3].Heavy metals leached from contaminated farmlands and mining areas then enter the surface and ground water at concentrations ranging from 1 to 100 mg·L-1[4-8].

The remediation of Cd,Pb,and Cr(VI) involves precipitation,adsorption,and redox processes.Cr(VI),including chromateand dichromateis more mobile and toxic than Cr(III).Reducing Cr(VI) to Cr(III) is an effective method in remediating Cr(VI) contamination [9-11].Unlike Cr(VI),Cd and Pb have stable chemical valences and can be precipitated by carbonates,phosphates,and sulfides.Adsorption and chemical precipitation are the primary methods for remediating Cd and Pb contamination[12,13].Currently,few studies have been performed on the simultaneous removal of Cr(VI),Cd,and Pb from aqueous solutions.Nanomaghemite [14],clinoptilolite [15],and agricultural wastes[6]have been used to simultaneously remove the three heavy metals through ion exchange and complexation although the removal efficiencies were quite low(see Supplementary Material Table S1).In addition,there have been a series of studies on the simultaneous removal of Cr(VI)and Pb,Cr(VI)and Cd,and Pb and Cd that utilized two agents in combination.Composite agents include FeS and carboxyl-functionalized ferroferric oxide microspheres[16]as well as biochar and nanoscale zero valent iron [17].The simultaneous removal of two or three metals including Cr(VI),Cd,and Pb,is listed in Table S1.

The interactions of heavy metals make the removal processes of multiple heavy metals more complicated than the removal of a single heavy metal.Effects such as competitive adsorption,enhanced precipitation,and enhanced adsorption are seen during the simultaneous removal of heavy metals.Competitive behavior can be seen during the adsorption of Cd and Pb by wheat straw where Cd is inhibited by Pb [18].Cr(VI) competes with Pb for Fe0,and Cr(VI) has an apparent competitive advantage [19].Cr(VI) and Pb form a PbCrO4precipitate giving Cr(VI) and Pb an antagonistic effect in the growth of cabbage [20].As(V) further enhances the adsorption of Cd(II) on MgZnFe-CO3layered double hydroxides by the anion bridging and the shielding effect,and Cd(II)strengthens As(III) adsorption by the formation of a ternary surface complexation [21].Competitive or enhanced reduction,precipitation,adsorption,and other mechanisms may exist in the simultaneous removal of Cr(VI),Pb,and Cd from aqueous solutions.

FeS is an effective material to immobilize Cr(VI),Cd,and Pb from aqueous solutions [22].FeS nanoparticles can remediate Cr(VI) contaminated groundwater and saturated soil with a high capacity of 1046.1 mg Cr(VI) per gram of FeS NPs [23].Pyrite and synthetic FeS have been used to adsorb Cd with adsorption capacities of 2.08 and 3.05 mg·g-1,respectively [24].FeS nanoparticles have been used to adsorb Pb through surface complexation and chemical precipitation [25].FeS is widely used for the removal of Cr(VI),Cd,and Pb [22],but few studies have been done on the simultaneous removal of Cr(VI),Cd,and Pb from aqueous solutions.

In this study,synthetic FeS NPs were used to simultaneously remove Cd,Pb,and Cr(VI)from an aqueous solution,and the mechanisms of the simultaneous removal of the metals were explored.The specific objectives of this study include (1) evaluating the simultaneous removal efficiencies of Cd,Pb,and Cr(VI) from an aqueous solution by FeS NPs;(2)determining the effects of typical environmental conditions,including pH and oxygen;and(3)elucidating the reaction pathway during the simultaneous removal of Cd,Pb,and Cr(VI) from an aqueous solution.

2.Materials and Methods

2.1.Materials

Stabilized FeS NPs were prepared through a reaction between ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium sulfide nonahydrate (Na2S·9H2O) with chemically pure carboxymethyl cellulose (CMC) as a stabilizer following a previous study [26].The solids obtained through centrifuging the prepared FeS NP suspension were washed with deoxygenated deionized water and freeze-dried,which were called washed FeS NPs (Fig.S1).

Stock solutions of Cd,Pb,and Cr(VI) were prepared by dissolving cadmium nitrate(Cd(NO3)2,lead nitrate(Pb(NO3)2),and potassium dichromate (K2Cr2O7),respectively,in deionized water.All reagents were of analytical or higher grade.All solutions were prepared with deionized water (18.25 MΩ·cm-1).

2.2.Batch experiments

Batch experiments were conducted to study the reaction kinetics and influencing factors between the FeS NPs and heavy metals.Kinetic tests were done to remove single heavy metals in aqueous solutions:Metal ion stock solution was added into a 1-L Erlenmeyer flask to obtain an initial Cr(VI) concentration of 50 mg·L-1,a Pb concentration of 50 mg·L-1,and a Cd concentration of 10 mg·L-1.The dosages of the FeS NPs were 56,21,and 8 mg·L-1with an FeS-to-Cr(VI) molar ratio of 2:3,an FeS-to-Pb molar ratio of 1:1,and an FeS-to-Cd molar ratio of 1:1,respectively.For the kinetic tests to simultaneously remove Cr(VI),Cd,and Pb in an aqueous solution,the initial concentrations of Cr(VI),Pb,and Cd were 50,50,and 10 mg·L-1,respectively.The dosage of the FeS NPs was 85 mg·L-1with an FeS-to-metal molar ratio of 3:4.This was calculated using the initial concentrations of Cr(VI),Cd,and Pb and the molar ratios of FeS-to-Cr(VI),FeS-to-Cd,and FeS-to-Pb in aqueous solutions of the single heavy metals.The remaining conditions were consistent with the single heavy metal aqueous solutions.The suspensions were horizontally oscillated at 200 r·min-1and at(20±3)°C for 5-240 min.The suspensions were filtered using a 0.22-μm mixed cellulose membrane filter at the end of this predetermined time.The filtrates were preserved to determine the concentrations of Cd,Pb,Cr(VI),and the total Cr.

Next,a comparative study between the FeS NP suspension and the washed FeS NPs with an FeS dosage of 85 mg·L-1at pH 7.0 was conducted to test the effects of the preparation procedures of the FeS NPs on the simultaneous removal of Cr(VI),Cd,and Pb.Tests on the effects of the molar ratio of the FeS-to-metal were conducted at a molar ratio range of 0.6-1.5viaa FeS NP suspension at pH 7.0.Tests on the effects of pH were conducted at pH values ranging from 4.0 to 10.0 with a FeS NP suspension dosage of 85 mg·L-1.Tests on the effects of exposure to oxygen were conducted under anoxic and oxic conditions.The oxic removal experiments were conducted with exposure to the atmosphere.For the anoxic removal,the aqueous solution was purged with argon gas for approximately 15 min to remove oxygen followed by the addition of the FeS NPs.This sample was continuously purged with argon gas throughout the anoxic reaction process.All experiments were performed in duplicate.

2.3.Speciation analysis of the heavy metals

Geochemical modeling with Visual MINTEQ[27]was utilized to simulate the species of Cr,Cd,and Pb in the aqueous solution.The input molar concentration for each component was based on the initial concentration of the aqueous solution containing Cr(VI),Cd,and Pb(Table S2).The thermodynamic parameters for the solid phases are shown in Table S3.

2.4.Measurements

Aqueous Cr(VI) was measured using diphenyl hydrazine spectrophotometry (T6 New Century,China).The detection limit of Cr(VI)was 0.004 mg·L-1.The total chromium,Cd,and Pb in aqueous solution were measured using flame atomic adsorption spectrometry(AA-6300C,Shimadzu,Japan).The detection limits of the total chromium,Cd,and Pb were 0.05,0.005,and 0.01 mg·L-1,respectively.The X-ray diffraction (XRD) patterns of the FeS NPs were obtained on a Shimadzu XRD-7000 device.The surface composition and morphology were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis(JSM-7800F,JEOL,Japan).The chemical elements and species analysis of the surface of the FeS NPs were characterized using an X-ray photoelectron spectrometer (XPS,Thermo,ESCALAB250Xi) with a monochromatic Al-Kα radiation (hv=1486.6 eV).

3.Results and Discussion

3.1.Removal of Cr(VI),Cd,and Pb from aqueous solutions

The removal efficiencies of Pb,Cr(VI),and Cd by the FeS NP suspension from aqueous solutions under neutral conditions are shown in Fig.1.Fig.1(a)shows that the aqueous Pb concentration decreased from 50 to 2 mg·L-1with a removal efficiency of 96%after 60 min.The removal efficiency of Pb in the control group reached 75%.This may be explained by the formation of a Pb(OH)2precipitate (Eq.(1)) [28].The pseudo first-order model (Eq.(S1)) and pseudo second-order model(Eq.(S2)) were used to simulate the kinetic data.The rate constants(k)and regression coefficients (R2) for Pb removal by the FeS NPs are summarized in Table S4.Overall,the pseudo second-order model fits the experimental data better than the first order-model withk=0.0955 L·mg-1·min-1.Therefore,it is concluded that Pb removal involves a chemical reaction [29].The removal of Pb by the FeS NPs may be attributed to surface complexation and chemical precipitation(Eqs.(2) and (3)) [25]:

Fig.1(b) and (c) show the kinetics for removal of chromium from the aqueous solutions.The Cr(VI) concentration decreased from 50 to 21 mg·L-1,with a removal efficiency of 58% after 120 min.The equilibrium concentrations of Cr(VI) and Crtotalwere nearly identical,which indicates that little Cr(III)was detectable in the aqueous solution in agreement with previous studies[1,23,30].The data of Cr(VI)fit a pseudo second-order model well withk=0.0011 L·mg-1·min-1(Table S4).The removal of Cr(VI) by the FeS NPs may be explained by the rapid adsorption of Cr(VI) on the FeS NPs,the reduction of adsorbed Cr(VI) or aqueous Cr(VI) to Cr(III),and the precipitation of Cr(III)-Fe(III) or Cr(III) (oxy)hydroxides (Eqs.(4) and (5)) [23]:

Fig.1(d) shows that the aqueous Cd concentration decreased from 10 to 0.38 mg·L-1with a removal efficiency of 62% after 240 min.The removal of Cd may be best described using a pseudo second-order model with k=0.0031 L·mg-1·min-1(Table S4)indicating that Cd removal involves a chemical reaction [29].The removal of Cd by the FeS NPs may be explained by the precipitation of CdS (Eq.(6)) [31-33,35],the complexation of Cd with reactive sites (Eq.(7)) [32-34],and the ion exchange between the surface protons and CdOH+(Eq.(8)) [35].These results confirmed that FeS NPs could effectively remove Cd,Pb,and Cr(VI) from aqueous solutions:

3.2.Simultaneous removal of Cr(VI),Cd,and Pb from aqueous solutions

Fig.1. Kinetics for the removal of(a)Pb,(b)Cr(VI),(c)Crtotal,and(d)Cd from aqueous solutions.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1,FeS NP suspension,FeS:Pb=1:1,FeS:Cr(VI)=2:3,FeS:Cd=1:1,pH=7.0,under oxic conditions,T=(20±3)°C).Data are plotted as the mean of the duplicates,and the error bars(the calculated standard deviation) indicate data reproducibility.

Fig.2. Kinetics for the simultaneous removal of(a)Pb,(b)Cr(VI),(c)Crtotal,and(d)Cd from aqueous solutions.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1;FeS NP suspension,FeS-to-metal molar ratio was 0.75,pH=7.0,under oxic conditions, T=(20 ± 3) °C).Data are plotted as the mean of the duplicates,and the error bars (the calculated standard deviation) indicate data reproducibility.

The simultaneous removal efficiencies of Pb,Cr,and Cd by the FeS NP suspension from an aqueous solution under neutral condition are shown in Fig.2.Fig.2(a) shows that the concentrations of Pb were all below 0.1 mg·L-1before and after reacting with the FeS NPs.This may have been caused by the formation of a PbCrO4precipitate (Eq.(9)) [20].The simultaneous removal efficiency of Pb was 99% greater than the removal efficiency of Pb in the aqueous Pb solution.The coexisting Cr(VI)and Cd increased the Pb removal because of the formation of PbCrO4and PbSO4precipitates [28]:

Fig.2(b)and(c)shows the simultaneous removal efficiencies of aqueous chromium.The Cr(VI)concentration decreased from 36 to 2.5 mg·L-1after 120 min,and the equilibrium concentration of Crtotalwas nearly identical to that of Cr(VI),which agrees with the results for aqueous Cr(VI)solution.The initial concentration of Cr(VI)was lower than 50 mg·L-1due to the formation of the PbCrO4precipitate.The simultaneous removal efficiency of Cr(VI)was 95%greater than the removal efficiency of Cr(VI)in the aqueous Cr(VI)solution.The pseudo second-order model could adequately fit the removal kinetic data withk=0.0057 L·mg-1·min-1(Table S4),which is higher thankin the aqueous Cr(VI)solution.The results confirmed that the coexistence of Cd and Pb increased the Cr(VI)removal due to the formation of a PbCrO4precipitate,the cationic bridge effect by Cd and Pb[36],and the enhanced adsorption of Cr(VI)due to Cd complexation and CdxFe1-x(OH)2[4,21].

Fig.2(d)shows that the Cd concentration decreased from 10 to 0.1 mg·L-1with a removal efficiency of 99%after 60 min.Table S4 shows that the pseudo second-order model could nicely interpret the kinetic data withk=1.41 L·mg-1·min-1.The simultaneous removal rate and efficiency of Cd were significantly higher than those of Cr(VI)because the reduction of Cr(VI)has a high activation energy and a small rate constant (k) [1].Additionally,the point of zero charge(PZC)of CMC-FeS was less than 2.5;therefore,the electrostatic adsorption was more effective for the positively charged Cd2+than for the negatively charged[37].The simultaneous removal efficiency of Cd was higher than that of Cd in the aqueous Cd solutions because of the anion bridging and shielding effect of Cd by Cr(VI) [21] and the enhanced adsorption by [Cr,Fe](OH)3[38].

3.3.Influencing factors in the simultaneous removal of Cr(VI),Cd,and Pb

3.3.1.Preparation procedures of the FeS NPs

Washed FeS NPs are convenient for storage and dosing.Therefore,a comparative experiment was conducted between washed FeS NPs and a FeS NP suspension.Fig.3 shows the kinetics for the simultaneous removal of Cr(VI),Pb,and Cd by the FeS NP suspension and washed FeS NPs.The removal efficiencies of Cr(VI)by the washed FeS NPs and FeS NP suspension were 60% and 93%;those of Cd were 64% and 99%;and those of Pb were 96% and 99%,respectively.The FeS NP suspension was more effective for the simultaneous removal of Cr(VI),Cd,and Pb than the washed FeS NPs.The FeS NP suspension contained more CMC,which can act as a space barrier to prevent particle agglomeration[39].Some small particles could not be centrifuged to the solid phase after washing and centrifugation resulting in a change in the particle size composition.In addition,CMC is an ionic polysaccharide that is rich in hydroxyl and carboxyl groups to remove Pb2+,Cd2+,and Cr3+through ion exchange and complexation [40-44].The FeS NP suspension was used in subsequent experiments due to the high efficiency of the FeS NP suspension.

Fig.3. Comparison of the FeS NP suspension and washed FeS NPs for the simultaneous removal of Cr(VI),Pb,and Cd from aqueous solutions.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1;FeS NP suspension,FeS-to-metal molar ratio was 0.75,pH=7.0,under an oxic condition,T=(20±3)°C).Data are plotted as the mean of the duplicates,and the error bars (the calculated standard deviation)indicate data reproducibility.

3.3.2.FeS-to-metal molar ratios

Fig.4 shows the simultaneous removal efficiencies of Cr(VI),Cd,and Pb with FeS-to-metal molar ratios of 0.6-1.5.The removal efficiencies of Cr(VI) and Cd increased from 90.3% to 99.9% and from 96.7%to 99.7%as the molar ratios increased from 0.6 to 1.5.However,there was no significant change in Pb removal.The increase in the Cr(VI) and Cd removal efficiencies was attributed to the increased driving force of mass transfer,adsorption sites and collisions between the FeS NPs and metals.The negligible change in the removal efficiency of Pb was because most of the Pb reduction was due to the formation of PbCrO4precipitates.

Additionally,with an increase in the FeS-to-metal molar ratio from 0.6 to 1.5,the simultaneous removal capacity of Pb rapidly decreased from 729 to 292 mg·g-1,Cd decreased from 118 to 49 mg·g-1,and Cr(VI) decreased from 504 to 223 mg·g-1.This may have been due to unoccupied binding sites and the aggregation of the FeS NPs at high dosages [1].Versus the heavy metal removal capacities of previous studies listed in Table S1,the FeS NP suspension demonstrated an apparently good capacity for the simultaneous removal of Cr(VI),Cd,and Pb.

The concentrations of Pb,Cr(VI),and Cd were reduced to approximately 0.1,2.5,and 0.1 mg·L-1,respectively,with a FeSto-metal molar ratio of 0.75.The addition of more FeS NPs did not cause a significant decrease in the Pb and Cd concentrations.Therefore,a molar ratio of 0.75 was used in the subsequent experiments.

3.3.3.The effect of pH

Fig.5 shows the simultaneous removal efficiencies of Cr(VI),Pb,and Cd at pHs ranging from 4.0 to 10.0.The removal efficiencies of Cr(VI) decreased from 98% to 53%,and those of Cd increased from 32%to 99%as the pH increased from 4.0 to 10.0.These agreed well with previous results for the removal of Cr(VI) and Cd by iron sulfide [23,35].The Crtotalremoval efficiency increased with an increase in pH from 4.0 to 7.0,but decreased as the pH increased from 7.0 to 10.0.The aqueous Pb concentration was as high as 8.2 mg·L-1at pH 4.0.

The pH affects the surface charge of remediation materials,the degree of ionization,and the speciation of heavy metals [45].The PZC value of the FeS NPs was approximately 2.5[37],which means that the surfaces of the FeS NPs were negatively charged at pH ＞2.5.Cr(VI) existed predominately asat a pH range of 2.0-6.0,and asat pH ＞7.0.Cd2+was the major species at pH ＜8.5,and the equilibrium shifted to Cd(OH)2(aq) as the pH increased to 9.0.The negative charge on the surface of the FeS NPs increased with an increase in pH;thus the electrostatic repulsion to Cr(VI)and the electrostatic attraction to Cd were enhanced.However,at a lower pH,the H+would compete with Cd2+for adsorption sites [45],and the adsorption sites would be lost with dissolution of adsorbents [46].In addition,the reduction of Cr(VI)was much lower in neutral solutions than in acidic solutions [9].The Crtotalremoval efficiencies first increased and then decreased with an increase in pH because the lower pH was favorable for Cr(VI) reduction;a higher pH could dramatically enhance the formation of Cr(III)-Fe(III) hydroxides.Moreover,the increase in the Pb concentration at pH 4.0 was attributed to the dissolution of the PbCrO4precipitates.These results suggest that the pH of the aqueous solution is an important influential factor,and the removal efficiencies of the three heavy metals could reach more than 70% at pH values of 6.0 and 8.0.

3.3.4.Exposure to oxygen

Fig.6 shows the simultaneous removal efficiencies of Cr(VI),Cd,and Pb under anoxic and oxic conditions.Within 120 min,the anoxic Cr(VI) removal efficiency reached nearly 100%,while only 95%of the Cr(VI)was removed under oxic conditions.Additionally,the removal efficiencies of Cd and Pb reached 99% under anoxic and oxic conditions.It was concluded that oxygen inhibited the removal of Cr(VI)but did not significantly affect Cd and Pb removal under neutral conditions.

Fig.4. Effect of the FeS-to-metal molar ratio on the simultaneous removal of Cr(VI),Cd,and Pb from aqueous solutions:(a)C/C0 and(b)removal capacity.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1;FeS NP suspension,pH=7.0,under an oxic condition,T=(20±3)°C).Data are plotted as the mean of the duplicates,and the error bars(the calculated standard deviation) indicate data reproducibility.

Fig.5. Effect of pH on the simultaneous removal of Cr(VI),Pb,and Cd from aqueous solutions.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1;FeS NP suspension,the FeS-to-metal molar ratio was 0.75, T=(20 ± 3) °C,under an oxic condition,t=2 h).Data are plotted as the mean of the duplicates,and the error bars(the calculated standard deviation) indicate data reproducibility.

As a strong electron acceptor,oxygen may compete with Cr(VI)for FeS NPs,and FeS would preferentially react with oxygen according to the calculated electrode potentials in Eqs.(10) and(11).Therefore,oxygen would consume surface electrons and the Fe(II) species of FeS NPs to inhibit the reduction of Cr(VI) [11,47].In terms of the effect on Cd removal,oxygen would oxidize S(-II)and subsequently inhibit the formation of CdS.Oxygen could further promote the hydrolysis of Fe2+and produce γ-FeOOH and hydrous ferric oxides to advance the adsorption of Cd (Eq.(12))[48] so that exposure to oxygen would not significantly influence the removal of Cd.In addition,the exposure to oxygen did not significantly influence the removal of Pb because Pb primarily exists as a PbCrO4precipitate.Oxygen inhibits the reduction of Cr(VI),and the concentration ofincreases.Hence,the Pb2+concentration decreases to maintain the equilibrium of PbCrO4precipitation and dissolution,while oxygen will oxidize S2-to inhibit the formation of PbS.Therefore,there is ultimately no significant influence on Pb removal from aqueous solutions.

Fig.6. Effect of exposure to oxygen on the simultaneous removal of Cr(VI),Pb,and Cd from aqueous solutions.([Pb]=50 mg·L-1,[Cr(VI)]=50 mg·L-1,[Cd]=10 mg·L-1;FeS NP suspension,FeS-to-metal molar ratio was 0.75,T=(20±3)°C,pH=7.0).Data are plotted as the mean of the duplicates,and the error bars (the calculated standard deviation) indicate data reproducibility.

Overall,the oxygen would slightly inhibit the removal of Cr(VI),but there was no significant influence on Cd and Pb removal.Therefore,the FeS NPs effectively removed Cr(VI),Cd,and Pb from aqueous solutions under oxic conditions.

3.4.Mechanisms of the simultaneous removal of heavy metals

3.4.1.Speciation analysis of heavy metals

The interactions among heavy metals may affect their species in aqueous solutions.Table 1 shows the species of Cr(VI),Cd,and Pb in the aqueous solutions containing the three metals.Cd existed in the liquid phase as Cd2+,and CdOH+.Approximately 99.9%of Pb and 25%of Cr(VI)existed in the solid phase as PbCrO4precipitate.Therefore,the initial concentrations of Pb and Cr(VI) were lower than the dosage,which is consistent with the measured data in Section 3.2.

TheKspvalues of PbCrO4,PbS,and Cr(OH)3were 2.8 × 10-13,8.0 × 10-28,and 7.0 × 10-31,respectively.TheKspvalues of Cr(OH)3and PbS were smaller than that of PbCrO4,suggesting that the solid phases of Cr(OH)3and PbS were more stable than that of PbCrO4.Furthermore,environmental conditions,including pH and organic matter affect the formation and stabilization of PbCrO4precipitates.Fulvic acid (FA) was selected as a model of dissolved organic matter[49,50].Fig.7 shows the effect of pH and FA concentration on the PbCrO4precipitates.The ratio of PbCrO4to total Pb was close to 100% at a pH of 4.0 to 8.5.The ratio dramatically decreased at pH ＜4.0 and ＞8.5 [51-53].This agreed with the simultaneous removal of Pb shown in Fig.5.In addition,the ratio of PbCrO4precipitate gradually decreased with increasing FA concentrations due to the reduction in Cr(VI)and the complexation of Pb by organic matter [54,55].Overall,the reduction of Cr(VI) to Cr(III)and the precipitation of Pb to PbS produced more stable states for Cr(VI) and Pb than PbCrO4.

3.4.2.Characterization of the solid phase

To further investigate the removal mechanism,the surface morphology,element distribution,and mineral compositions were analyzed using SEM and XRD (Fig.8).The FeS particles prior to the reaction had a shuttle-like morphology with an average length of 400 nm and a diameter of approximately 100 nm in the middle portion [10].Fig.8(a) shows that the particles evolved into larger cuboid ones after the simultaneous removal of Cd,Pb,and Cr(VI).This suggested that FeS particles were oxidized during the reaction.The XRD pattern shown in Fig.8(w) reveals that the oxides were Fe2O3and FeOOH[1,56,57].There were fine floccus on the surface of the cuboid particles due to the formation of PbCrO4,Cr(OH)3,and FeCrO4[58];these structures are consistent with the XRD results (Fig.8(w)) [59].According to the elemental mapping results,Cd and Pb were positively correlated with S confirming the precipitation of CdS and PbS.In addition,Cr was positively correlated with O,Fe,and Pb,confirming the precipitation of Cr(OH)3,FeCrO4,and PbCrO4(Fig.8(b)-(g)).Fig.8(h),(m),and(r)show that the FeS particles became larger cuboid ones after the removal of Cd,Pb,and Cr(VI)similar to Fig.8(a).The fine particles on the surface of the cuboid particles may be due to the formation of CdS,PbS,Pb(OH)2,and Cr(OH)3[60].These observations are also consistent with the elemental mapping (Fig.8(i)-8(v)) and XRD results(Fig.8(w)).

XPS examined the surface elemental compositions and oxidation states of the FeS NPs after the removal of heavy metals from aqueous solutions(Fig.9).The Fe 2p and S 2p spectra of FeS before the reaction demonstrate that Fe(II)-S and S2-anions were themain species in FeS NPs[10].Fig.9(a)shows apparent peaks in the binding energy of approximately 400,140,and 580 eV represented Cd 3d,Pb 4f,and Cr 2p.These data indicate the immobilization of Cd,Pb,and Cr on the surface of the FeS NPs.

Table 1 The emission values of visual MINTEQ

Fig.7. Effect of pH and FA on the PbCrO4 precipitates.

Fig.9(b)shows that most of the Cr(VI)was reduced to Cr(III)by the FeS NPs,and a low level of Cr(VI)was still detected on the surface of the FeS NPs.The narrow scan of Cr 2p exhibits two Cr 2p3/2peaks at 577.1 and 579.1 eV corresponding to Cr(OH)3and,respectively,and a peak at 587.1 eV corresponding to Cr 2p1/2of Cr(III) in the form of FexCr1-x(OH)3or Fe(OH)3-Cr(OH)3[61].Cr(III) became the main species on the surface of the FeS NPs confirming the reduction of Cr(VI) to Cr(III).The presence ofmay have been due to the PbCrO4precipitates,which agrees with the Visual MINTEQ simulation,SEM,and XRD results (Table 1).

Fig.9(c)shows that Cd was precipitated to CdS and Cd-Fe-(OH)2by FeS NPs,and a small amount of Cd existed as a complex of Cd and[Cr,Fe](OH)3.Two intense peaks for Cd 3d3/2and Cd 3d5/2were observed at 412 and 405 eV,respectively,indicating the presence of CdS and Cd-Fe-hydroxides [4].In addition,due to the coexistence of Cr(VI) and Pb,a peak appears at 414 eV corresponding to the complex of Cd and [Cr,Fe](OH)3[38].This indicates that FeS NPs removed Cd primarily via precipitation,and Cd could also be complexed by [Cr,Fe](OH)3.

The narrow scan of Pb 4f shown in Fig.9(d) indicates that Pb was precipitated to PbCrO4,PbSO4,PbS,and Pb(OH)2.The peaks of Pb (4f7/2) and Pb (4f5/2) at 138.8 and 143.5 eV confirm the appearance of PbS and Pb(OH)2.The two peaks at 137.7 and 142.2 eV represent PbSO4and PbCrO4,respectively,and appear with the coexistence of Cr(VI) and Cd.The result agrees with the Visual MINTEQ simulation results,SEM images,XRD patterns and XPS spectra of Cr 2p and S 2p.Therefore,Pb may be precipitated to PbS and Pb(OH)2by FeS NPs and could be precipitated to PbSO4and PbCrO4with the coexistence of Cr(VI) and Cd.

Fig.8(e)shows a detailed XPS spectra for the region of S 2p.The peak at 168.4 eV corresponds toafter the simultaneous removal of Cr(VI),Cd and Pb.The high-resolution spectrum of Fe 2p is shown in Fig.9(f).The two Fe (2p3/2) peaks observed at(724.8 ± 0.4) and (710 ± 0.1) eV represent Fe(III)-O,indicating the oxidation of Fe(II) to Fe(III).The SEM and XRD analyses also confirmed this result.

3.4.3.Theoretical calculations for the chemical precipitation

The precipitation order of Cd and Pb by the FeS NPs and the precipitation conversion of PbCrO4were determined using a theoretical calculation.Section 3.2 shows that the initial concentrations of Cr(VI),Pb,and Cd in the aqueous solution containing the three metals were 0.72,0.001,and 0.45 mmol·L-1,respectively.The theoretical calculations are as follows:

The concentration of S2-required for PbS was 8.0 × 10-25-mmol·L-1based on the calculation ofKsp(PbS)/c0(Pb2+).The concentration of S2-required for CdS was 8.89 × 10-26mmol·L-1based on the calculation ofKsp(CdS)/c0(Cd2+).The concentration of S2-required for CdS was smaller than that for PbS;hence,CdS was precipitated first.

The concentration of Cd2+decreases upon precipitation of CdS,and the concentration of S2-increases.Whenc(Pb2+)c(S2-) is greater thanKsp(PbS),the Pb2+starts to precipitate as the concentration of Cd2+(Ksp(CdS)/c(S2-))and is 0.01 mmol·L-1.In short,FeS NPs will first precipitate Cd2+to 0.01 mmol·L-1,and then precipitate Pb2+.

TheKvalue of Eq.(15)is 3.5×1014based on the calculation ofKsp(PbCrO4)/Ksp(PbS).The K value is high indicating that the conversion of PbCrO4to PbS is relatively complete,while the conversion of PbS to PbCrO4is difficult.

3.4.4.Potential mechanism

Cr(VI) and Pb could react to form a PbCrO4precipitate,which was further confirmed by Visual MINTEQ simulations showing that 99.9%of Pb and 25%of Cr(VI)existed in the solid phase as PbCrO4.The PbCrO4is less stable than Cr(OH)3and PbS due to theirKspvalues.Additionally,pH and organic matter would affect the stabilization of PbCrO4precipitates according to Visual MINTEQ.Hence,the reduction of Cr(VI) to Cr(III) and precipitation of Pb to PbS could make Cr(VI) and Pb more stable.

The removal efficiencies of Cr(VI),Cd,and Pb in compound contaminated aqueous solutions were higher than those in the single metal aqueous solutions.In the analysis of the simultaneous removal efficiency of Cr(VI),Cd,and Pb by FeS NPs,Cd was absorbed and precipitated by FeS NPs,and the coexisting Cr(VI)enhanced Cd removal via complexation of Cd by[Cr,Fe](OH)3.This conclusion was supported by the peaks of CdS,Cd-Fe-hydroxides,and Cd-[Cr,Fe](OH)3in the XPS spectra of Cd 3d.Cr(VI) was adsorbed by FeS NPs and reduced to Cr(III),and then precipitated to Cr(III)-Fe(III) or Cr(III) (oxy)hydroxides.This is supported by the Cr(III) peak in the Cr 2p XPS spectra,the Fe(III)-O peak in the Fe 2p spectra,and the SEM and XRD analyses.The coexisting Cd enhanced the adsorption of Cr(VI) via a cationic bridging effect,which was confirmed by the PZC of the FeS NPs and the simulation results of the existing forms of Cr(VI)and Cd by Visual MINTEQ.Pb existed as PbCrO4before the reaction according to the simulation results of Visual MINTEQ,and Pb existed as PbS,Pb(OH)2,PbSO4and PbCrO4after reacting with FeS NPs,as shown in the Pb 4f XPS spectra and XRD pattern.This indicates that PbCrO4underwent a precipitation transformation with FeS NPs,which was verified by theoretical calculations of the chemical precipitation.In addition,FeS NPs first precipitated Cd to 0.01 mmol·L-1,and then precipitated Pb.

Fig.8. SEM images of the FeS NPs after removal of (a-g) Cd,Pb,and Cr(VI),(h-l) Cd,(m-q) Pb,and (r-v) Cr(VI),and (w) XRD pattern of the FeS NPs after removal of heavy metals.([Pb]=50 mg·L-1,[Cd]=10 mg·L-1,[Cr(VI)]=50 mg·L-1.FeS NP suspension,FeS-to-metal molar ratio was 0.75, T=(20 ± 3) °C,under an oxic condition, t=2 h).

Based on the aforementioned analysis,a potential mechanism was proposed for the simultaneous removal of Cr(VI),Cd,and Pb by FeS NPs(Fig.10).The Cr(VI)is adsorbed by FeS NPs and reduced to Cr(III),and then precipitated to Cr(III)-Fe(III) or Cr(III) (oxy)hydroxides.Cd reacts with FeS NPs to form CdS and Cd-Fehydroxides.Pb is released from PbCrO4and precipitated to PbS by the FeS NPs.Interactions of the three heavy metals include the following:(1) Cd2+adsorbed on the surface of FeS NPs could act as a cationic bridge and promote the adsorption of Cr(VI) by FeS NPs;(2) [Cr,Fe](OH)3,as the reduction product of Cr(VI),may enhance the adsorption of Cd;(3),as the oxidation product of the FeS NPs by Cr(VI),will precipitate Pb2+to PbSO4;and (4)and Pb2+will form PbCrO4precipitates,and PbCrO4is then converted to PbS.

4.Conclusions

Fig.9. XPS spectra of the FeS NPs after the removal of Cd,Pb,and Cr(VI):(a) wide-scan survey,(b-f) narrow scan of Cd 3d,Cr 2p,Pb 4f,Fe 2p and S 2p.([Pb]=50 mg·L-1,[Cd]=10 mg·L-1,[Cr(VI)]=50 mg·L-1.FeS NP suspension,FeS-to-metal molar ratio was 0.75, T=(20 ± 3) °C,under an oxic condition, t=2 h).

The FeS NPs suspension can simultaneously remove Cr(VI),Cd,and Pb from an aqueous solution.The concentrations of Cr(VI),Cd,and Pb decreased from 50,10,and 50 mg·L-1to 2.5,0.1,and 0.1 mg·L-1,respectively.The removal capacities were up to 418,96,and 585 mg per gram of stabilized FeS NPs,respectively.The acidic conditions significantly favored the removal of aqueous Cr(VI) while the alkaline conditions favored the removal of Cd and Pb.Oxygen slightly inhibited the removal of Cr(VI),but it had no significant influence on the removal of Cd and Pb.A potential mechanism was proposed for the simultaneous removal of Cr(VI),Cd,and Pb using FeS NPs.Cr(VI) may be absorbed on the FeS NPs and reduced to Cr(III),and then precipitated to Cr(III)-Fe(III) or Cr(III) (oxy)hydroxides.Cd reacts with FeS NPs to form CdS and Cd-Fe-hydroxide solids.Pb is released from PbCrO4and precipitated to PbS by the FeS NPs.The interactions of the three heavy metals involved a cationic bridging effect on Cr(VI) by Cd,an enhanced adsorption effect on Cd by [Cr,Fe](OH)3,precipitation of PbCrO4,and transformation of PbCrO4to PbS.

Fig.10. Schematic of the simultaneous removal of Cr(VI),Cd,and Pb from aqueous solutions by FeS NPs.

These findings show that FeS NPs have great potential for use in the simultaneous removal of Cr(VI),Cd,and Pb from contaminated aqueous solutions.This method can act as a guide for the further development of approaches to tackle multiple heavy metal contamination problems.

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

This work was supported by the National Natural Science Foundation of China(51778084),the National key Research&Development program of China(2018YFC1800305),the Chongqing Ecology and Environment Bureau (2019-128),the Sichuan Science and Technology Program (2019YFSY0005) and the Large Instruments Open Foundation of Chongqing University (201903150051).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.10.021.