Reactivity of Nanoscale Zero-Valent Iron in Unbuffered Systems Effect of pH and Fe(II) Dissolution
Reactivity of Nanoscale Zero-Valent Iron in Unbu?ered Systems: E?ect of pH and Fe(II)Dissolution
Sungjun Bae and Khalil Hanna*
E c ole Nationale Supe r ieure de Chimie de Rennes,UMR CNRS6226,11Alle e de Beaulieu,Rennes35708Cedex7,France *Supporting Information
loading may slow the4-NP reduction through acceleration of the
bu?ered pH systems con?rmed that high pH values(8?9)can
enhancing the reduction kinetics of4-NP.Furthermore,reduction tests
showed that surface-bound Fe(II)on oxide coatings can play an
These unexpected results highlight the importance of pH and
bu?ered systems,particularly at a low amount of NZVI(i.e.,
In the past two decades,zerovalent iron(ZVI)has attracted signi?cant attention as a promising reactant for reductive removal of various environmental contaminants from waste-water and groundwater due to its high reductive capacity,the environmental friendliness of iron,and the production of nontoxic iron oxides after the removal of the contaminants.1?8 Nanoscale ZVI(NZVI)especially has shown a remarkable reductive capacity toward various organic(chlorinated aliphatics and aromatics),3?10inorganic(chromate,arsenite (As(III)),and nitrate),11?13and radioactive(uranium)14,15 contaminants due mainly to its small size with a high surface area.
NZVI particles,spherical in shape,are typically covered by an iron(hydr)oxide layer with a thickness of several nanometers. Because of the coexistence of both an Fe(0)core and oxidized forms of iron layers in the NZVI structure,NZVI can act as a reductant(i.e.,via the Fe(0)core and surface-bound Fe(II))for environmental contaminants,a sorbent(i.e.,via the iron (hydr)oxide layer)for other metals,and a coagulant(i.e.,via dissolution of Fe(II)from the NZVI surface)for various anions in groundwater.16Therefore,the complex interfacial reactions (e.g.,dissolution,adsorption,redox reaction,and precipitation) can occur simultaneously or sequentially on the surface of NZVI during the decontamination process.Some environ-mental factors signi?cantly in?uence these reactions and can ultimately a?ect the fate and transport of the contaminants. Natural organic matter signi?cantly a?ects NZVI reactivity during the degradation of various organic and inorganic contaminants.17?21The presence of cations(Cu2+,Co Mn2+,Mg2+,Pb2+,Ni2+)enhances the extent of degradation due to the formation of zerovalent metals and/or depassiva-tion/dissolution of the oxide layer coating the aged Fe(0),17,22,23while the degradation can be inhibited by the presence of anions(HPO42?,HCO3?,SO42?,and Cl?)due mainly to the formation of chemical complexes with iron oxide coatings on the surface of the ZVI.21
One of the most important factors controlling the complex interfacial reactions is known to be pH.3,4,24?28Kinetic rate constants decreased as the pH increased during the degradation of chlorinated solvents by NZVI.3,25,26The enhancement in NZVI reactivity at low pH is generally accepted to be induced by the acceleration of iron corrosion and dissolution of the passivated layers on the NZVI surface,25,28while a high pH lowers NZVI reactivity due to the precipitation of ferrous hydroxide on the NZVI surfaces,leading to the inhibition of electron transfer from the Fe(0)core to the outermost surface.3,24,25,28However,lowering the pH to approximately 3.8also decreased the ZVI reactivity due to the fast loss of ZVI particles through Fe dissolution.24Although many studies have highlighted the e?ect of pH on NZVI reactivity toward a variety of environmental pollutants,3,4,24?26,29there is limited knowl-edge regarding the reactivity of NZVI in poorly bu?ered pH
Received:March13,2015
Revised:June26,2015
Accepted:July29,2015
Published:July29,2015
systems to date.Indeed,most of the reported studies used a bu ?ered pH system to maintain the pH,possibly missing the variations in pH and Fe dissolution during the reaction in poorly bu ?ered pH systems and their e ?ect on the kinetics of decontamination.However,some contaminated matrixes,such as industrial/domestic wastewaters,are poorly bu ?ered pH systems,29for which previous ?ndings on bu ?ered pH systems may not be applicable.In addition,the northern part of Fennoscandia and the Kola Peninsula are well-known to be poorly bu ?ered natural environments because glacier melt/snowmelt has removed the bu ?ered soil and most of the weathered materials,resulting in increased sensitivity to
acidi ?cation.30Consequently,a systematic study of NZVI
reactivity in unbu ?ered pH systems merits investigation from both fundamental and applied points of view.The objectives of this study are (a)to characterize the reduction of 4-nitrophenol (4-NP)with NZVI by monitoring the pH,oxidation ?reduction potential (ORP),and Fe(II)dissolution in unbu ?ered pH systems,(b)to investigate the e ?ect of the NZVI concentration on variations in pH and ORP and its sequential e ?ect on the dissolution of NZVI and transformation of the contaminant,and (c)to elucidate the degradation mechanism of the contaminant at a ?xed pH in bu ?ered systems.4-NP was selected as a target contaminant in this study because nitrophenols are one of the most commonly used chemicals
in the production of dyes,pesticides,and pharmaceuticals,31,32and 4-NP is a suspected carcinogen.4-NP has been restricted below 10ng/L in natural waters by the United
States Environmental Protection Agency.33■EXPERIMENTAL PROCEDURES
https://www.wendangku.net/doc/0a2756947.html,VI was synthesized by reducing FeCl 3·6H 2O with NaBH
4solution in an anaerobic chamber (JACOMEX),modifying our previously reported method.3After synthesis,the NZVI was washed three times with deaerated deionized water (DDW),prepared using ultrapure water (18M Ω·cm)
purged
Figure 1.Production of 4-AP and variations in pH,dissolved Fe(II),and ORP during reduction of 4-NP by NZVI (75mg/L,1.35mM)for (a)injection of 4-NP into a pre-equilibrated NZVI suspension and (b)injection of NZVI into a pre-equilibrated 4-NP solution.The initial concentration of 4-NP was 0.1mM,and error bars indicate the standard deviation of duplicate samples.
with N2for4h and then centrifuging for5min at4000rpm. Then the NZVI was dried and stored in the anaerobic chamber. Transmission electron microscopy(TEM)and X-ray di?raction (XRD)show a spherical shape for the NZVI and a di?ractogram of NZVI(Figure S1).Maghemite(γ-Fe III2O3) was purchased from Sigma-Aldrich.Other iron minerals (magnetite(Fe II1III2O4)and hematite(α-Fe III2O3))were synthesized in our previous study.34A complete list of chemicals used in this study is provided in the Supporting Information.
Reduction of4-NP by NZVI in Unbu?ered pH Systems.The reduction of4-NP by NZVI was carried out in a200mL glass?ask equipped with a magnetic stirrer under anaerobic conditions(glovebox).An exact amount of DDW (99.5mL)was transferred to the glass?ask and mixed at450 rpm.Then NZVI(75mg/L,1.35mM Fe equivalent)was transferred to the glass?ask containing DDW.Variations in pH,ORP,and aqueous Fe(II)concentration during the reaction were monitored by a pH/ORP meter(Hach, sensION+5045)and the ferrozine method.After a steady state was reached for both pH and ORP,0.5mL of4-NP stock solution(20mM)prepared in DDW was spiked into the glass ?ask(initial concentration0.1mM)to initiate the reduction of 4-NP by NZVI.A UV?vis spectrometer was used to investigate the change in UV?vis spectra during the reduction of4-NP by NZVI.We con?rmed the concentrations of4-NP and4-aminophenol(4-AP)by high-performance liquid chromatog-raphy(HPLC)using a UV detector.The e?ect of the injection order(for NZVI and4-NP)on the reduction of4-NP by NZVI was investigated by a reversed mode of injection order(i.e., NZVI into4-NP solution).To investigate the e?ect of the NZVI concentration on the reduction of4-NP,three di?erent concentrations(10,25,and50mg/L,which are equal to0.18, 0.45,and0.90mM Fe equivalent,respectively)were used. Finally,we evaluated the e?ect of the pH in unbu?ered systems using a stock solution of4-NP prepared at pH9.87±0.1. Unless stated otherwise,the experimental setup and procedures were followed as described above,and all tests in this study were duplicated.
E?ect of the pH on Reduction of4-NP and Fe(II) Dissolution in Bu?ered pH Systems.To investigate the reduction of4-NP by NZVI at each pH,two di?erent biological bu?ers(50mM)with a pH range of6.0?9.0(MOPS for pH6 and8and TRIS for pH7and9)were introduced into the glass ?ask with NZVI(75mg/L).
E?ect of Surface-Bound Fe(II)on Reduction of4-NP in Bu?ered pH Systems.The reduction of4-NP(0.1mM)by aqueous Fe(II)and surface-bound Fe(II)on three di?erent iron oxides(magnetite,maghemite,and hematite)was performed to investigate the e?ect on the reduction of4-NP by NZVI at each pH value.The pH values used for this experiment were6,7,and8,which showed a signi?cant amount of Fe(II)dissolution from the NZVI suspension.MOPS and TRIS bu?er solutions containing Fe(II)(0.5mM)and Fe(II) (0.5mM)with iron oxides(75mg/L)were prepared by adding exact amounts of ferrous sulfate and each iron oxide. Characterization of NZVI and Analytical Methods.The particle morphology and di?raction pattern of NZVI were investigated by TEM and XRD.More details on the instrumentation,sample preparation,and analytical procedure for TEM and XRD are provided in the Supporting Information. In addition,analytical methods for the UV?vis spectrometer,HPLC,and ferrozine assay are also provided in the Supporting Information.
■RESULTS AND DISCUSSION
Reduction of4-NP by NZVI in Unbu?ered pH Systems.Figure S2a shows the typical UV?vis spectra for4-NP before and after the reaction in the NZVI suspension.The original absorption peak of4-NP(p K a=7.15)was shifted from 317to400nm immediately after the reaction with NZVI due to the formation of4-nitrophenolate ions35in the basic NZVI suspension(i.e.,pH>8).The time-dependent UV?vis spectra clearly showed that the peak at400nm continuously decreased and completely disappeared with the formation of a new peak at298nm,assigned to4-AP after40min of reaction(Figure S2b).We did not observe peaks at388and302nm,which can correspond to4-benzoquinone monoxime and4-nitrosophe-nol.36In addition,the two isosbestic points at280and312nm during the reaction stress that no other byproducts were generated during the reduction of4-NP to4-AP by NZVI. HPLC measurements clearly con?rmed the degradation of4-NP by NZVI and its stoichiometric conversion to4-AP during the reaction(i.e.,mass balance was achieved)(Figure1a), indicating that possible losses of4-NP or4-AP by sorption on NZVI surfaces are negligible.The rate constant for the kinetics of4-NP degradation by NZVI can be described by the following pseudo-?rst-order kinetic model:
=?
C
t
k C
d
d
4NP
obsd,4NP4NP(1) where C4NP is the concentration of4-NP at the sampling time (t)and k obsd,4NP is the observed pseudo-?rst-order rate constant.
A linear regression of eq1allowed the determination of k obsd,4NP as0.135±0.009min?1with R2≈0.99.
Figure1a also shows the variation in pH,ORP,and dissolved Fe(II)during(1)the reaction of NZVI with DDW and(2)the transformation of4-NP in the NZVI suspension.After the addition of NZVI to DDW,the pH and dissolved Fe(II) concentration increased from6.4to8.8and from0to0.006 mM in30min due to the anaerobic corrosion of NZVI(Fe(0) +2H2O→Fe(II)+2OH?+H2).5,37According to the standard reduction potential(E°)of?440mV for the half-reaction between the Fe2+/Fe0couple,highly reducing conditions can be expected in the NZVI suspension.Indeed, ORP decreased rapidly from+4mV to approximately?750 mV in10min and seemed to be constant for30min.Even2?3 mg/L NZVI has been reported to be su?cient to achieve a negative ORP solution in1h due to the large reactive surface and rapid reaction with water molecules.27A rapid decrease in pH(from8.8to7.8)and an increase in ORP(from?750to ?465)were observed in4min due probably to the acidity of the4-NP solution,and the pH and ORP became almost constant during the reduction reaction.After the4-NP reduction was?nished(40min),the concomitant increase in pH and decrease in ORP was observed,indicating that NZVI re-reacts with water molecules to reach the equilibrium condition again.The concentration of dissolved Fe(II) increased to0.013mM in10min and seemed to?uctuate around this value for60min.The increase in dissolved Fe(II) upon adding4-NP may be caused by the pH decrease.The relationship with dissolved Fe(II)in the pH range of6?9will be discussed later.
XRD analyses of NZVI before and after the4-NP reduction showed an increase in the peaks of magnetite versus a decrease
in the peaks ofα-Fe(Figure S1c,d).This prompt formation of magnetite suggests that4-NP can be converted to4-AP through the following reaction:9Fe0+4H2O+4C6H5NO3→3Fe3O4+4C6H7NO.Although oxidation of Fe(0)to Fe(II)is most often assumed in the literature,38,39the oxidation of NZVI to magnetite(Fe3O4)seems to be more thermodynamically favorable during the decontamination at pH above6.1.26,40
E?ect of the Injection Order on the Reduction of4-NP by NZVI in Unbu?ered pH Systems.The reduction of4-NP by NZVI was investigated as previously explained but with a change in the injection order,i.e.,NZVI into a pre-equilibrated 4-NP solution instead of4-NP into an NZVI suspension (Figure1b).After addition of the desired volume of4-NP stock solution into DDW,the pH decreased rapidly from6.0to4.8in 1min with an increase in ORP from+7to+236mV because of the acidic character of the4-NP solution,and those values seemed to become constant in10min.The reduction of4-NP to4-AP was initiated by the addition of NZVI into a pre-equilibrated4-NP solution.The k obsd,4NP was0.041±0.004 min?1,which is3.3times lower than the value obtained by injecting4-NP into an NZVI suspension.The injection order of NZVI into a pre-equilibrated4-NP solution can apparently and signi?cantly slow the reduction rate of4-NP by NZVI.This inhibition e?ect may be induced by a competitive electron transfer from NZVI to water(i.e.,anaerobic corrosion of NZVI)and4-NP(i.e.,reduction of4-NP).In the case of4-NP injection into an NZVI suspension,the electron transfer from NZVI to a water molecule?rst occurred when the equilibrium state was reached.Therefore,an electron transfer process from NZVI to4-NP could be a dominant reaction when4-NP is injected into an NZVI suspension.However,both the equilibrium reaction with water and the4-NP reduction can simultaneously consume the electron from NZVI when NZVI is injected into a pre-equilibrated4-NP solution.This phenomenon may kinetically decrease the reduction rate of 4-NP.
The pH increased to7.8in10min and seemed to be constant in60min,while a decrease of ORP to?410mV was observed.The equilibrium values(pH and ORP)during the4-NP reduction were very similar for both injection modes (Figure1),except that a di?erence of55mV in the ORP value was observed.The concentration of dissolved Fe(II)increased to0.036mM,which may result from the low initial pH value (i.e.,4.8)(Figure1b).However,the concentration of dissolved Fe(II)continued to decrease to0.020mM in60min due probably to(1)precipitation of Fe as a form of hydroxide at pH 7.8and/or(2)readsorption of Fe(II)on the surface oxide coating.Although the precipitation of iron hydroxide under anaerobic conditions has normally been reported in the pH range of8.5?10,41,42a lower pH(approximately8)can also precipitate iron hydroxides.43Adsorption of aqueous Fe(II)on iron oxides(e.g.,magnetite and hematite)has been reported to be easily observed at high pH values.44,45Therefore,both reactions may be occurring until the concentration of aqueous Fe(II)reaches a steady state(i.e.,approximately0.013mM in Figure1a).The results obtained from this study revealed that the injection order can signi?cantly in?uence the key reaction parameters,such as pH,ORP,and dissolved Fe(II),and,?nally, the reduction rate constant of4-NP.
E?ect of the NZVI Concentration on the Reduction of 4-NP,pH,ORP,and Dissolved Fe(II)in Unbu?ered pH Systems.To investigate the e?ect of the NZVI concentration on k obsd,4NP,the suspension pH,ORP,and dissolved Fe(II),the reduction of4-NP with three NZVI concentrations(0.18,0.45, and0.90mM)was conducted and compared with the data obtained at1.35mM(Figure2and Figure S3).To compare the
initial rates of4-NP reduction,the kinetic rate constants were determined by the pseudo-?rst-order kinetic model using the data collected in the?rst20min.Inevitably,the reduction kinetics of4-NP was expedited as the NZVI concentration increased.Previous studies using a bu?ered pH system have shown a linear increase in the degradation rate constant with respect to the NZVI concentration due to the proportional increase in the reactive surface area of NZVI.3,25However,the rate constant in this study increased exponentially with respect to the NZVI concentration(Figure2b),indicating that not only the NZVI concentration but also additional factors may contribute to the improvement in the degradation kinetics of 4-NP.The latter were investigated by monitoring the variation in pH and Fe(II)dissolution during each NZVI loading(Figure S3).We observed that the variation in pH after the injection of 4-NP was signi?cantly di?erent under each set of conditions.At 0.18and0.45mM NZVI loading,the pH?rst dropped, following by a slight increase to reach a steady-state
value Figure2.(a)E?ect of the NZVI concentration on the reduction of4-NP.The circles,squares,triangles,and tilted squares indicate the experimental data.The solid lines show the pseudo-?rst-order?ts over the?rst20min of reaction.(b)Change in the kinetic rate constants for reduction of4-NP with respect to the NZVI concentration.The initial concentration of4-NP was0.1mM,and error bars indicate the standard deviation of duplicate samples.
(Figure S3).However,at0.9mM NZVI loading,the pH dropped from8.7to8.0after the injection of4-NP and continued to decrease to7.4during the4-NP reduction.In contrast to the highest NZVI loading(1.35mM)(Figure1a), the rebound of the pH during the4-NP reduction was not observed in the range of0.18?0.90mM NZVI,probably because(1)the anaerobic corrosion could not be restarted by exhaustion of the NZVI reactivity with the limited amount of NZVI(i.e.,at0.18and0.45mM)or(2)the reduction of4-NP was still in progress(i.e.,at0.90mM).The concentration of dissolved Fe(II)at0.18and0.45mM was2.27and2.43times higher than that at1.35mM after the4-NP reduction(60min) (Figure S4).In particular,the ratio of dissolved Fe(II)to initial NZVI after60min of reaction with4-NP was the highest (15%)at0.18mM NZVI,followed by0.45mM(6.6%),0.90 mM(2.2%),and1.35mM(1.0%)(Figure S5a).We also observed that these values were proportionally related to the suspension pH after the injection of4-NP(Figure S5b).These ?ndings imply that a lower pH at a low NZVI loading(0.18and 0.45mM)resulted in the acceleration of Fe(II)dissolution from the NZVI surface,which may explain the exponential increase in the4-NP reduction as the NZVI concentration increased.In addition,we observed that ORP reached its equilibrium condition in30min(0.45and0.90mM)and60 min(0.18mM).The minimum value of ORP at the equilibrium condition was the lowest at 1.35mM NZVI (?750mV),followed by0.90mM(?732mV),0.45mM (?677mV),and0.18mM(?670mV),which is consistent with previous?ndings in which the minimum value of ORP increased as the NZVI concentration increased.46
E?ect of the pH on the Reduction of4-NP by NZVI in Unbu?ered pH Systems.To investigate the e?ect of the pH on the kinetics of4-NP reduction by NZVI in unbu?ered pH systems,a similar experiment was conducted by the injection of 4-nitrophenolate ions prepared at pH9.87instead of4-nitrophenol(pH4.40)(Figure S6).First,we observed that0.1 mM4-NP was completely reduced to4-AP by the NZVI suspension in30min,and the k obsd,4NP was0.203±0.016 min?1,1.5times higher than in the previous experiment(Figure 1a).The pH dropped to8.2,seemed to be constant until the reduction of4-NP was?nished,and then increased gradually to approximately8.6(Figure S6).The ORP showed a lower equilibrium value at approximately?485mV compared to the previous data(?465mM)(Figure S6vs Figure1a).The addition of4-nitrophenolate ions to the NZVI suspension showed a lower pH drop from8.8to8.2compared to4-NP (pH drop to7.8),which may signi?cantly in?uence the4-NP reduction by NZVI.Because the sorption of4-NP and4-nitrophenolate on the NZVI surface was found to be negligible, the protonation/deprotonation of4-NP may not signi?cantly a?ect the interactions with NZVI surfaces.However,no published data are available regarding the e?ect of the protonation/deprotonation of4-NP on its reduction potential. The results obtained in this study suggest that a high pH may improve the reduction of4-NP in the range of pH6?9.This ?nding is somewhat unexpected on the basis of the previous studies of NZVI applications,in which high pH lowers the NZVI reactivity due to the surface passivation.3,25,28This unusual trend cannot be explained by a pH dependence of compound sorption to iron oxide coatings because the sorption was found to be negligible regardless of the pH value.One possible explanation for the enhancement in NZVI reactivity at high pH may be the preservation of ZVI particles against aqueous dissolution.Many past studies using a high dosage of ZVI(>0.4g/L)have shown enhanced decontamination as the suspension pH decreased regardless of the size of the ZVI particles(10nm to1mm)and type of contaminant (trichloroethylene,1,1,1-trichloroethane,Cr(VI),nitroben-zene)(Table1).3,4,24,26,28Because our pH range(6?9)was
similar to the pH range previously reported,the knowledge to date cannot properly explain our?ndings at a low dosage of NZVI(<0.075g/L).Therefore,additional experiments using bu?ers are needed to investigate the reaction mechanism for enhanced4-NP reduction by NZVI at high pH values.
E?ect of the pH and Surface-Bound Fe(II)on the Reduction of4-NP in Bu?ered pH Systems.Figure3a shows the kinetics of4-NP reduction by NZVI at di?erent suspension pH values(6,7,8,and9).As we expected on the basis of the above-mentioned results,the extent of4-NP reduction at120min increased as the pH increased in the range of pH6?9,con?rming that the high pH can enhance the reduction of4-NP by NZVI in the range of pH6?9.Almost 53.5%of the initial Fe(i.e.,0.72mM)was dissolved after the addition of NZVI into DDW in the form of aqueous Fe(II)at pH6,and this value increased to66.1%(i.e.,0.89mM)after 120min of reaction with4-NP(Figure S7and Figure3b?d). The concentration of dissolved Fe(II)after addition of NZVI (0.62mM at pH7and0.27mM at pH8)and4-NP(0.68mM at pH7and0.21mM at pH8)decreased as the pH increased, while no signi?cant Fe(II)dissolution was detected at pH9. These results showed that more than half of the NZVI added was lost at pH6and7,which may slow the overall reduction rate.Furthermore,the highest reduction rate of4-NP at pH9 may be due mainly to the preservation of NZVI particles in spite of the possible formation of surface layer passivation.
At pH8,both Fe(II)dissolution and preservation of NZVI particles can e?ectively occur during the reduction of4-NP by NZVI.The dissolved Fe(II)can be readsorbed on the surfaces of the oxide layer coatings of NZVI and then act as a reducing agent through the formation of inner-sphere bonds,which increases the electron density of the adsorbed Fe(II).45,47,48As previously reported,44,48,49the type of iron oxide plays a signi?cant role in the rate of the reduction reaction,which has been attributed to di?erences in the surface speciation,site density,and amount of Fe(II)adsorbed.To investigate the e?ect of surface-bound Fe(II),magnetite,maghemite,and hematite were used as representative iron oxide coatings found on the NZVI surfaces(Figure4).The concentration of aqueous Fe(II)added was0.5mM,close to the average value of the aqueous Fe(II)concentration produced during the NZVI-induced reduction of4-NP in the pH range of6?8.We did not perform the experiment at pH9due to the very low solubility of Fe(II)and the rapid precipitation of all dissolved Fe(II)as solid ferrous hydroxide(Fe(OH)2)at this high pH.The Table1.ZVI Studies Showing the Enhanced Decontamination as the Suspension pH Decreased
ref target contaminant ZVI size
dosage
(g/L)pH range 31,1,1-trichloroethane10?40nm0.46?9
4Cr(VI)25nm26?9
24trichloroethylene?ner than100mesh
(～150μm)
2.5 4.9?8 26trichloroethylene～100nm0.5 6.5?8.9 28nitrobenzene1mm1
3.33?9
reduction of 4-NP by aqueous Fe(II)was not signi ?cant (Figure 4),indicating that the aqueous Fe(II)in the NZVI suspension did not in ?uence the reduction kinetics of 4-NP.In addition,the reduction of 4-NP was not observed in all mineral suspensions at pH 6and 7(Figure 4),suggesting that the surface-bound Fe(II)at pH 6and 7is not able to reduce 4-NP under our experimental conditions.The suspensions at pH 8,however,showed 50%and 65%reduction,thereby under-scoring the great reactivity of the surface-bound Fe(II)of magnetite and maghemite.No signi ?cant reduction of
4-NP
Figure 3.(a)Kinetics of 4-NP reduction by NZVI (75mg/L,1.35mM)at di ?erent bu ?ered pH values (6,7,8,and 9)and variation in Fe speciation after injection of NZVI (b,30min)and injection of 4-NP (c,60min;d,120min)at each bu ?ered
pH.Figure 4.Reduction of 4-NP (0.1mM)by aqueous Fe(II)(0.5mM)and Fe(II)(0.5mM)+iron oxides (75mg/L)at three pH values (6,7,and 8):magnetite (MT),maghemite (MM),and hematite (HT).
was observed in the hematite suspension even at pH 8.Previous studies have reported that an increase in pH can improve the Fe(II)adsorption on iron (oxyhydr)oxide surfaces,resulting in the enhanced reduction rate of target contami-nants.45,48Here,the residual aqueous Fe(II)concentration was very low and did not signi ?cantly di ?er among the oxides tested (data not shown).Calculations of the pH dependence of the adsorption of Fe(II)using a surface complexation model (developed in our previous work 50by assuming two surface complexes with and without electron transfer)con ?rm that all Fe(II)is adsorbed at a pH higher than 7.These observations are consistent with the literature,which found that the Fe(II)adsorption edge was almost the same for all iron oxyhydroxide phases,and at pH >7.5,the adsorption of Fe(II)reaches 100%regardless of the oxide tested.51,52Therefore,these results suggest that the enhanced reduction of 4-NP may be partially caused by the increase in electron density of the Fe(II)adsorbed on the oxide layer coating 53of the NZVI such as magnetite and/or maghemite.Environmental Signi ?cance.Our ?ndings imply that both the pH and Fe dissolution in NZVI suspensions are of fundamental importance to investigating the kinetics of decontamination by NZVI and its reaction mechanism in the pH range of 6?9.In this study,we have notably demonstrated that the reduction rate of 4-NP can be improved at high pH values in both unbu ?ered and bu ?ered suspensions in the presence of low amounts of NZVI (0.010?0.075g/L).These surprising ?ndings are in contrast with numerous previous reports using bu ?ered pH systems and a high NZVI dosage (0.4?20g/L),which may underestimate the loss of NZVI by Fe dissolution at neutral pH values.3,4,26These unexpected results may be explained through one and/or a combination of the following processes:(1)relatively less Fe(II)dissolution than at pH 6and 7,leading to the preservation of solid Fe phases,and/or (2)an increase in the amount of surface Fe(II)complexed with oxide coatings and then in the electron density of the surface-bound Fe(II).The inhibition e ?ect observed at pH 6and 7may result from the loss of NZVI particles through rapid iron dissolution.The results obtained from this study can help in the understanding of the removal mechanism of contaminants in the presence of low amounts of NZVI and in poorly bu ?ered systems.Poorly bu ?ered conditions prevail in some contaminated media, e.g.,industrial/domestic wastewaters 29and poorly bu ?ered natural environments.30Our ?ndings suggest that the treatment of such systems using a reasonable amount of NZVI should be carefully reconsidered to economically and e ?ectively remove the target contaminants.In addition,an e ?ort has been started to develop the NZVI-based treatment system as a batch reactor for industrial/domestic wastewaters,54
and we expect that this practical application will be continued as a ?eld study.Therefore,our experimental results can provide a useful guideline to design the NZVI technology e ?ectively in poorly bu ?ered systems.■ASSOCIATED CONTENT *Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.est.5b01298.Details of the chemicals used in this study,material characterization,analytical methods,and additional results for reduction of 4-NP by NZVI (PDF )
AUTHOR INFORMATION Corresponding
Author *
Phone:+33223238027;fax:+33223238120;e-mail:
khalil.hanna@ensc-rennes.fr .Notes
The authors declare no competing ?nancial interest.■ACKNOWLEDGMENTS We thank the “Re g ion Bretagne ”for ?nancial support (Contract SAD-ReSolEau (8256)).We thank Dr.M.Pasturel and Dr.V.Dorcet for XRD and TEM analyses,respectively.■REFERENCES
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