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Environ. Sci. Technol. 2008, 42, 2092–2098

Chromium(VI) Reduction Kinetics by Zero-Valent Iron in Moderately Hard Water with Humic Acid: Iron Dissolution and Humic Acid Adsorption TONGZHOU LIU,† D A N I E L C . W . T S A N G , ‡,§ A N D I R E N E M . C . L O * ,† Department of Civil Engineering, The Hong Kong University of Science and Technology, Hong Kong, China, and Institute for the Environment, The Hong Kong University of Science and Technology, Hong Kong, China

Received August 17, 2007. Revised manuscript received December 1, 2007. Accepted December 3, 2007.

In zerovalent iron treatment systems, the presence of multiple solution components may impose combined effects that differ from corresponding individual effects. The copresence of humic acid and hardness (Ca2+/Mg2+) was found to influence Cr(VI) reduction by Fe0 and iron dissolution in a way different from their respective presence in batch kinetics experiments with synthetic groundwater at initial pH 6 and 9.5. Cr(VI) reduction rate constants (kobs) were slightly inhibited by humic acid adsorption on iron filings (decreases of 7–9% and 10–12% in the presence of humic acid alone and together with hardness, respectively). The total amount of dissolved Fe steadily increased to 25 mg L-1 in the presence of humic acid alone because the formation of soluble Fe-humate complexes appeared to suppress iron precipitation. Substantial amounts of soluble and colloidal Fe-humate complexes in groundwater may arouse aesthetic and safety concerns in groundwater use. In contrast, the coexistence of humic acid and Ca2+/Mg2+ significantly promoted aggregation of humic acid and metal hydrolyzed species, as indicated by XPS and TEM analyses, which remained nondissolved (>0.45 µm) in solution. These metal-humate aggregates may impose long-term impacts on PRBs in subsurface settings.

Introduction Permeable reactive barriers (PRBs) using zerovalent iron (Fe0) as a reactive medium have been proven to be a viable and cost-effective technology in a number of laboratory-, pilot-, and full-scale studies for removing inorganics (e.g., chromate, nitrate, bromate, and arsenate) (1–4) as well as chlorinated hydrocarbons and nitroaromatic compounds from groundwater (4–7). The majority of early studies investigated chemical reduction of contaminants under simplified solution conditions with a single contaminant (1, 5, 6). Although * Corresponding author phone: 852-23587157; fax: 852-23581534; e-mail: [email protected]. † Department of Civil Engineering. ‡ Institute for the Environment. § Current address: Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ. 2092

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recent studies started to take into account the effects of solution composition on contaminant reduction (7–10), most of the effects were singly studied. Two or more solution components that coexist together may interact with one another and exert combined effects that are different from their respective individual effects. To develop a better understanding of the performance of Fe0 PRBs under more complicated geochemical conditions, the present study investigates the Cr(VI) reduction by Fe0 in natural organic matter (NOM)-rich groundwater in the absence and presence of hardness. In Fe0 treatment systems, the removal mechanisms of Cr(VI) are believed to involve instantaneous adsorption of Cr(VI) on Fe0 surface where electron transfer takes place and Cr(VI) is reduced to Cr3+ with oxidation of Fe0 to Fe3+, and subsequently, Cr3+ precipitates as Cr3+ hydroxides and/ or mixed Fe3+/Cr3+ (oxy)hydroxides (11–14). The reactivity of Fe0 is highly dependent on its surface characteristics and aqueous chemistry of groundwater. NOM is ubiquitous in shallow aquifers where Fe0 PRBs are most applicable, and has a high tendency to be adsorbed on different mineral surfaces such as Fe oxides (15–18). Adsorption of NOM, even at low surface coverage, could significantly modify the properties of mineral surfaces. It has been found that NOM adsorption could adjust, or even reverse, the electrostatic charges of mineral surfaces (18) that would alter the interactions between the surface and ions in solution. The adsorbed NOM may also block the surface sites of Fe0 where the chemical reduction of contaminants takes place. Several studies have shown that the presence of NOM inhibited Fe0 reactivity for chlorinated hydrocarbon degradation, which was attributed to strong competition between organics and NOM for reactive sites (7–9, 19) and alteration of reduction potential of surface sites (7). Although Cr(VI) reduction by NOM was possible, it was found to occur at a very slow rate (20, 21). On the other hand, Dries et al. (10) suggested that the formation of metal-humate complexes in solution may be responsible for the delay of metal removal (Zn2+ and Ni2+) by Fe0, whereas the removal of chromate (CrO42-) was not significantly affected due largely to its little interaction with negatively charged humic acid. Nevertheless, a significant amount of dissolved Fe was released from Fe0 columns fed with humic acid, which was not further investigated. NOM in solution is known to have strong binding affinity for Fe3+ (22, 23) to form stable and soluble complexes. The formation of Fe-humate complexes may influence the precipitation of corrosion products on iron filings, which would lead to Fe0 surface passivation reducing PRB reactivity (7, 24). On the other hand, it has been found that hardness (e.g., Ca2+ or Mg2+) and carbonate in solution would influence the reactivity of Fe0 for Cr(VI) removal by the formation of passivated precipitates, such as CaCO3 and Mg(OH)2 (25, 26). With the presence of Ca2+ and Mg2+ (equivalent to 100–200 mg L-1 as CaCO3) in solution, Fe0 removal capacity toward Cr(VI) was reduced by 10–17% (26) to 45% (27) in different column experiments. However, the removal rate of trichloroethylene (TCE) by Fe0 was slightly lower in hard groundwater with high alkalinity than soft groundwater with low alkalinity (28). Previous studies have demonstrated that the presence of NOM or hardness in groundwater could, respectively, produce negative impacts on the removal of contaminants by Fe0; but, there is a lack of study to assess the effects of coexisting NOM and hardness, which is particularly important in shallow aquifers where water is often moderately hard to 10.1021/es072059c CCC: $40.75

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hard. This may be a challenging environment for the application of Fe0 PRB, because Ca2+ and Mg2+ are known to promote NOM aggregation in solution due to charge neutralization and conformation compaction of humic substances as a result of electrostatic binding of divalent cations (23). Additionally, Ca2+ and NOM may have synergistic effects that alter the charge and potential of the head end of diffuse double layer (d-plane) of mineral surfaces (29), enhancing the adsorption of Ca2+ and NOM on mineral surfaces. All these suggest that the effects of NOM or hardness alone on Cr(VI) reduction may not be similar to those of Fe0 treatment systems containing both NOM and hardness; therefore, the objectives of this study were to evaluate the kinetics of Cr(VI) reduction, Fe dissolution, and NOM adsorption in a batch setting of Fe0 treatment system in the presence of NOM (as humic acid) and/or hardness.

Experimental Section Zero-Valent Iron and Surface Charaterization. Iron filings (ETI-CC-1004) were obtained from Connelly GPM Inc. Their elemental iron content, grain size, specific surface area, and particle density were 96.28%, 0.25–2.0 mm, 1.8 m2 g-1, and 6.43 g cm-3, respectively (26). Sieved fractions of 18–35 mesh were used without chemical pretreatment. Chemical oxidation states and compositions analyses of iron filings surface using X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5600) revealed that the iron filings were covered by a passive film of Fe2O3, which was consistent with the findings of Ritter et al. (30). The depth profile scans of time-of-flight secondary ion mass spectroscopy (Tof-SIMS, Perkin-Elmer PHI 7200) showed that the thickness of this Fe2O3 film was at least 5 nm. The point of zero charge (pHpzc) of the iron filings was determined to be pH 7.6 using a zeta potential analyzer (Zetaplus). Chemicals and Chemical Analyses. Solutions containing Cr(VI) and various compositions were prepared by dissolving chemicals (K2Cr2O7, NaCl, CaCl2 · 2H2O, MgCl2.6H2O, Na2SO4, FeCl3 · 6H2O, NaOH, and HCl) into ultrapure water. The chemicals are reagent grade and obtained from RiedeldeHaën and BDH. Ultrapure water (Barnstead D11911) was deoxygenated by purging with nitrogen gas for an hour prior to usage. Commercially available sodium salt of humic acid (Aldrich) was used to represent NOM. Humic acid stock solution was prepared by dissolving certain amount of humic acid sodium salts into ultrapure water followed by filtering through 0.45-µm acetate cellulose membranes (ADVANTEC). Concentrations of humic acid were expressed as dissolved organic carbon (mg L-1 as DOC). Background electrolyte was 5 mM NaCl unless specified. Initial solution pH was adjusted to 6 and 9.5 by dropwise addition of 0.01N NaOH and 0.01N HCl, to cover the groundwater pH range from slightly acidic to alkaline conditions that have been widely used in many studies on PRBs (4). No buffer was used. Solution pH was measured using pH electrode (Orion model 420A). Concentrations of Cr(VI) and Fe2+ were measured by 1,5-diphenylcarbazide (with detection limit and reproducibility of 10 µg/L and (2%, respectively) (31) and 1,10-phenanthroline colorimetric methods (with detection limit of 10 µg/L) (32), respectively, using UV/visible spectrophotometer (Ultrospec 4300 Pro) at wavelengths of 540 and 510 nm. Concentrations of humic acid were determined by a TOC analyzer (Shimadzu TOC-5000A). Total concentrations of dissolved metals (Ca, Cr, Fe, and Mg) were determined using atomic absorbance spectrometer (AAS, Varian 220FS). Batch Experiments. Batch experiments were conducted using 40 mL glass vials containing 0.4 g of Fe0 and 38 mL of 10 mg L-1 Cr(VI) solutions with or without humic acid and/ or hardness. To investigate the effects of humic acid on Fe0 reactivity, concentrations of humic acid were varied from 0 to 20 mg L-1 as DOC, a typical range in groundwater (23).

To investigate the effects of copresent humic acid and hardness, solutions contained 0 or 20 mg L-1 (as DOC) humic acid and 0.8 mM Ca2+ or Mg2+, equivalent to 80 mg L-1 as CaCO3 hardness representing a moderately hard water. Vials were sealed with screw caps containing Telfon-lined rubber septa and shaken end-over-end at 26 rpm under room temperature (23 ( 1 °C). Solutions were sampled at regular time intervals up to 30 min and filtered through 0.45 µm membranes, followed by immediate measurement of pH and subsequent chemical analyses. All batch experiments were run in duplicate. Cr(VI) reduction by Fe0 was described by a pseudo-first-order kinetics model, of which the observed first-order rate constant, kobs (min-1) was the slope of the plot of ln (C/C0) versus time (where C0 and C are initial Cr(VI) concentration and its concentration at any time during the reduction reaction, respectively). All reported kobs values were obtained with regression coefficient (R2) greater than 0.96. In addition, two sets of supplementary experiments were conducted to examine the competitive adsorption between an oxyanion (SO42-, which undergoes adsorption only and was applied in place of CrO42-) and humic acid on iron filings; and the aggregation of humic acid in the presence of Ca2+ and Fe3+ in solution (without iron filings). The concentrations of humic acid, SO42-, Ca2+, and Fe3+ were 20 mg L-1 as DOC, 18.46 mg L-1 (equivalent to 10 mg L-1 CrO42- in molarity), 0.8 mM, and 20 mg L-1, respectively. Other experimental procedures were the same as before. XPS, TEM, and SEM Analyses. The aggregates/precipitates that were observed to form in suspensions during the batch experiments were collected, and then freeze-dried for subsequent XPS and scanning/transmission electron microscopic (SEM, model JEOL-6300F; TEM, model JEOL2010F) analyses. For experiments conducted at initial pH 9.5 in the absence/presence of humic acid, samples were filtered and centrifuged at 20 000 rpm (HITACHI himac CR 21GII). The solids separated by centrifugation were collected and freeze-dried for subsequent TEM analyses. All freeze-dried samples were stored in N2-filled glass bottles.

Results and Discussions Cr(VI) Reduction Kinetics. Table 1 summarizes the observed pseudo-first-order kinetics rate constants (kobs) of Cr(VI) reduction. The kobs values decreased as initial pH increased whether humic acid was present or not. The dependence of kobs on pH has been widely reported (11, 33, 34). Since both Cr(VI) reduction and iron corrosion consume H+, low initial pH would favor the overall reaction (CrO42- + Fe0 + 8H+ S Cr3+ + Fe3+ + 4H2O) (11, 34) that solution pH was raised steadily to 9–10 (Figure S2 in the Supporting Information). Another reason is that at initial pH 9.5, the iron filings (with pHpzc 7.6) would develop negative surface charges and electrostatic repulsion with CrO42-, thereby decreasing Cr(VI) reduction rates. Addition of humic acid did not show any significant inhibitory effect on Fe0 reactivity toward Cr(VI) reduction at both initial pH 6 and 9.5: humic acid in solution reduced kobs values by about 7–9% (Table 1). In addition, under the same initial pH, the difference in kobs values was indistinguishable with humic acid concentration increasing from 5 to 20 mg L-1 as DOC. This is in agreement with the findings of Dries et al. (10), which concluded that chromate removal was not affected by humic acid. A comparison between the firstorder rate constants (Table 2 in ref 10) also revealed a decrease of about 14% of Cr(VI) reduction in the presence of humic acid despite relatively large standard deviation. By contrast, the inhibitory effect of humic acid was more crucial for trichloroethylene and carbon tetrachloride degradation, of which the reactivity was decreased by about 40% in batch experiments (8) or by 2- to 5-fold in column studies (7, 9). As mentioned in the introduction, this was attributed to VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Observed Pseudo-First Order Cr(VI) Reduction Rate Constantsa

electrolyte components 5 mM NaCl

5 mM NaCl and 0.8 mM CaCl2 5 mM NaCl and 0.8 mM MgCl2 7.4 mM NaCl

initial pH 6

initial pH 9.5

humic acid concentraion (mg L-1 as DOC)

kobs, min-1

t1/2 min

R2

kobs, min-1

t1/2 min

R2

0 5 10 15 20 0 20 0 20 0

0.0499 ( 0.0011 0.0456 ( 0.0007 0.0460 ( 0.0008 0.0452 ( 0.0005 0.0465 ( 0.0007 0.0494 ( 0.0005 0.0450 ( 0.0013 0.0491 ( 0.0007 0.0439 ( 0.0005 0.0504 ( 0.0013

13.89 ( 0.30 15.20 ( 0.23 15.07 ( 0.26 15.34 ( 0.17 14.91 ( 0.22 14.03 ( 0.14 15.40 ( 0.43 14.12 ( 0.20 15.79 ( 0.18 13.75 ( 0.35

0.9977 0.9940 0.9971 0.9967 0.9986 0.9996 0.9990 0.9933 0.9711 0.9785

0.0427 ( 0.0008 0.0398 ( 0.0010 0.0391 ( 0.0011 0.0396 ( 0.0007 0.0389 ( 0.0009 0.0452 ( 0.0007 0.0433 ( 0.0011 0.0455 ( 0.0009 0.0413 ( 0.0006 0.0428 ( 0.0004

16.23 ( 0.30 17.42 ( 0.43 17.73 ( 0.49 17.50 ( 0.30 17.82 ( 0.40 15.34 ( 0.23 16.01 ( 0.40 15.23 ( 0.30 16.78 ( 0.24 16.20 ( 0.15

0.9963 0.9903 0.9901 0.9964 0.9959 0.9966 0.9992 0.9976 0.9914 0.9856

a The ionic strength of 7.4 mM NaCl is equivalent to 5 mM NaCl plus 0.8 mM CaCl2 or MgCl2, and was used as reference for comparison. Errors in table indicate standard deviation of duplicated batch experiments.

humic acid adsorption that out-competes the contaminants for the reactive sites on iron surfaces (7–9, 19) and alters the reduction potential of neighboring sites (7). In view of the marginal suppression of Cr(VI) reduction in this study, it appears that the extent to which humic acid was adsorbed (Supporting Information Table S1) did not effectively block the surface or out-compete Cr(VI), or the majority of humic acid and Cr(VI) were adsorbed on different types of reactive sites. The sole presence of hardness (0.8 mM Ca2+ or Mg2+) slightly enhanced Cr(VI) reduction with a 6% increase of kobs values at initial pH 9.5, whereas it had little effect at initial pH 6 (Table 1). The effect of ionic strength was found to be negligible that using 7.4 mM NaCl as background electrolyte did not affect Cr(VI) reduction; therefore, the enhancement was ascribed to the presence of Ca2+ or Mg2+. Decreases of Ca2+ and Mg2+ concentrations in solutions were observed during the course of kinetics experiments (Supporting Information Figure S1), which might reflect Ca2+ and Mg2+ adsorption or precipitation. Cation adsorption on iron surface through electrostatic attraction increases with increasing pH (35, 36). At high initial pH, adsorbed Ca2+ and Mg2+ could reduce the electrostatic repulsion between the negatively charged iron surface (pHpzc 7.6) and chromate, thus enhancing Cr(VI) adsorption and reduction. When humic acid was present together with hardness, a minor decrease of Cr(VI) reduction rates (10–12%) was observed at initial pH 6 (Table 1), probably because Ca2+ and Mg2+ could promote NOM adsorption on iron oxides due to synergistic effects (29) that resulted in greater blockage of reactive sites. But at initial pH 9.5, adsorbed Ca2+ and Mg2+ probably enhanced Cr(VI) reduction that obscured the inhibitory effect of NOM adsorption; consequently, noticeable difference in Cr(VI) reduction was not observed. A greater decrease of Ca2+ and Mg2+ concentrations (20–25%) in the presence of humic acid than in its absence (4–14%) (Supporting Information Figure S1) suggested the correlation of Ca2+ and Mg2+ with humic acid adsorption/aggregation. Humic Acid Adsorption/Aggregation. The fate of humic acid is of particular concern. Humic acid that is adsorbed could significantly modify the characteristics of iron surface and, in turn, influence the performance of Fe0 PRB; whereas humic acid that remains in aqueous phase could complex or aggregate with cations (e.g., Fe3+) and subsequently influence the metal precipitation on iron filings that leads to Fe0 surface passivation. Figure 1 shows the changes of the amounts of humic acid and dissolved iron in solutions along with time under different solution conditions. The decrease in humic acid concentration has been generally regarded as a reflection of humic acid adsorption on iron surfaces (8–10, 37). In the absence 2094

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of Cr(VI), steady humic acid adsorption (up to 25%) on iron filings was observed (curves 1 and 2, Figure 1a) regardless of pH values (Supporting Information Figure S2a), which suggests the specific adsorption of humic acid (e.g., ligand exchange) should have occurred despite electrostatic repulsion between the iron filings surface and humic acid at high pH (15, 16). Recent FTIR study also confirmed the formation of inner-sphere surface complex between nanoscale Fe0 and humic acid (37). The amounts of adsorbed humic acid showed disproportional increase with increasing initial humic acid concentrations (Supporting Information Table S1), indicating that humic acid adsorption sites on iron filings of small specific surface area (1.8 m2 g-1) were limited and easily saturated. For this reason, the majority of humic acid remained in solution in this study. It should be noted that humic acid obtained from different sources may exhibit different extents of adsorption (7, 8), which was speculated to reflect the differences in the characters of humic acid. In chromate/humic acid binary solutions, apparently less humic acid adsorption (curves 3 and 4, Figure 1a) signified that chromate could compete with humic acid for adsorption sites to some degree despite specific interactions between humic acid and iron filings surface. Supplementary experiments with solutions containing sulfate and humic acid (Supporting Information Figure S3) corroborated the competition between oxyanion and humic acid for a fraction of adsorption sites on the iron surface. However, distinct kinetics of humic acid adsorption was revealed when Ca2+ or Mg2+ was also present in solution (curves 5–8, Figure 1c). Binding with multivalent cations (such as Fe3+, Ca2+, and Mg2+) can lead to a more compact conformation of NOM molecules by reducing intramolecular electrostatic repulsions and by forming multidentate complexes with neighboring functional groups on NOM molecules (23); or can reduce the electrostatic repulsion between NOM molecules and negatively charged surfaces (17, 29). Both effects are favorable for NOM adsorption and aggregation. To evaluate whether the decreases in humic acid concentration derived from aggregation regardless the presence of Fe0 (because aggregates with size >0.45 µm in solution were filtered out), supplementary experiments were conducted using 20 mg L-1 (as DOC) humic acid, 0.8 mM Ca2+, and/or 20 mg L-1 Fe3+ (prepared by dissolving FeCl3 · 6H2O into ultrapure water) without the addition of iron filings (Supporting Information Figure S4). In the solution containing Ca2+ only, humic acid aggregation was negligible on this time scale. Nevertheless, the copresence of Ca2+ and Fe3+ in solution resulted in spontaneous humic acid aggregation, that is, a 27% decrease of humic acid concentration in solution shortly after Fe3+ was added. The concentrations of humic

FIGURE 1. Amounts of humic acid (a) and (c), and dissolved iron (b) and (d) in solutions along with time in batch kinetic experiments of Cr(VI) reduction by Fe0 under various conditions. Solutions contained: (b) humic acid alone at initial pH 6 (line 1); (O) humic acid alone at initial pH 9.5 (line 2); (9) both humic acid and Cr(VI) at initial pH 6 (line 3); (0) both humic acid and Cr(VI) at initial pH 9.5 (line 4); (1) Cr(VI) alone at initial pH 6 (line 9); (3) Cr(VI) alone at initial pH 9.5 (line 10). (2)humic acid and Cr(VI) with Ca2+ at initial pH 6 (line 5); (4) humic acid and Cr(VI) with Ca2+ at initial pH 9.5 (line 6); (() humic acid and Cr(VI) with Mg2+ at initial pH 6 (line 7); ()) humic acid and Cr(VI) with Mg2+ at initial pH 9.5 (line 8). Initial concentrations of Cr(VI), humic acid, Ca2+/ Mg2+ were 10 mg l-1, 20 mg l-1 as DOC, and 0.8 mM respectively. Background electrolyte was 5 mM NaCl. Lines are for guidance only. Error bars indicate standard deviation of duplicate batch experiments. acid and Fe3+ in solution remained unchanged thereafter. Dissolved Fe3+ was also shown to form settleable aggregates with NOM via charge neutralization or bridging of stretched humic macromolecules (38). Therefore, it is possible that the substantial decrease (about 85%) of dissolved humic acid in Ca2+ or Mg2+ solutions (Figure 1c) resulted from both adsorption on iron filings and aggregation in solution. The latter was particularly important in the copresence of dissolved iron and Ca/Mg, because the presence of these divalent cations can enhance aggregation through double layer compression or chemical association (39). Nevertheless, the molecular size and composition of humic acid fractions that are adsorbed on the iron surface or that remain as aggregates in solution are unclear and require detailed characterization in future. Comparisons between curves 5–8 in Figure 1c revealed that significant humic acid adsorption/aggregation took place earlier in solutions with Ca2+ than Mg2+, and at low initial pH than high initial pH, respectively. It has been shown that, compared with Mg2+, Ca2+ exhibits higher binding affinity with humic acid (40) and leads to greater humic acid aggregation due to its smaller size of hydrated ion (41). Thus, the enhancement of humic acid adsorption on Fe0 by Ca2+ to a greater degree was in agreement with Giasuddin et al. (37). In addition, since humic acid adsorption/aggregation increases substantially with decreasing pH due to greater proton neutralization, electrostatic attraction, and specific interactions (17, 18, 38), the decrease of humic acid concentration was observed earlier at low initial pH than high initial pH. It should be noted that, despite different starting time, humic acid adsorption/aggregation in solutions with Ca2+ or Mg2+, at low initial pH or high initial pH almost reached the same extent at the end of experiments (Figure 1c). Iron Dissolution. Dissolved iron was released as a result of iron corrosion. The predominant form should be Fe3+

over the course of Cr(VI) reduction in the absence of humic acid because Fe2+, which may result from anaerobic iron corrosion of water and autoreduction of Fe2O3 film, could be completely oxidized by Cr(VI) instantaneously (14). Colorimetric tests indicated the absence of Fe2+ in solutions without humic acid. However, it should be noted that only Fetotal in solution was measured in the presence of humic acid due to the interference of humic acid with the colorimetric measurement of Fe2+. For the solutions containing Cr(VI) alone, the amount of dissolved iron was far less at initial pH 6 than 9.5 (curves 9 and 10, Figure 1b) despite greater Cr(VI) reduction at lower pH. It is likely that most dissolved Fe3+ expeditiously formed precipitates such as Fe3+ hydroxides and/or mixed Fe3+/Cr3+ (oxy)hydroxides (11–14), which would coagulate fast near pHpzc (pH 7–8.5 for Fe (oxy)hydroxides) but remain stable at higher pH (42). It is, therefore, speculated that at high solution pH there was substantial amount of colloidal precipitates (0.45 µm), resulting in significant decreases of humic acid and dissolved Fe concentrations. The presence of large aggregates was clearly evidenced by TEM and SEM images (Figure 2c and Supporting Information Figure S5). Additionally, Fe-, Cr- and Ca-hydrolyzed species and carboxylic groups in humic acid were incorporated in these aggregates. XPS spectra (Figure 3 and Supporting Information Figure S6) showed that the binding energy of O(1s) centered at 531.6 eV and a small peak of C(1s) appeared at 288.5 eV, indicating hydroxyl oxygen and carboxyl carbon, respectively (Supporting Information Table S2). Thus, these aggregates might comprise Fe-, Ca-, and Cr-hydrolyzed species and adsorbed/aggregated humic acid. Charge neutralization, coprecipitation, and/or adsorption of humic acid and hydrolyzed metal species are probable mechanisms (38, 39). The less substantial pH increase compared with solutions without hardness (curves 5–8, Supporting Information Figure S2b) also signified the consumption of OH- by aggregation. It appears that large aggregates were formed by dissolved iron and humic acid in the presence of Ca/Mg, whereas soluble complexes and colloids of Fe-humate were formed in the absence of Ca/ Mg (Supporting Information Figure S7), indicating that Fe-humate aggregation was significantly enhanced by the copresence of hardness ions. Implications for Fe0 PRB Application. The inhibitory effects of humic acid on Fe0 reactivity toward Cr(VI) reduction were not significant at both low and high initial pH; kobs values were reduced by 7–9% and 10–12% in the absence and presence of hardness, respectively. However, in contrast to previous studies that mainly focused on the reduction 2096

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FIGURE 3. XPS spectra of freeze-dried aggregates formed in the solution with Cr(VI), humic acid, and Ca2+ at initial pH 6: (a) O1s and (b) C1s. efficiency, this study raised important concerns about the behaviors of humic acid and dissolved iron during Cr(VI) reduction, which were shown to be different when humic acid was present alone and together with hardness. In the absence of hardness, a large fraction of humic acid remaining in solutions appears to be an issue since it significantly increased the amount of dissolved iron in the form of soluble and colloidal Fe-humate complexes. Although less iron precipitates formed on the surface of iron filings may prolong the longevity of Fe0 PRBs (7, 24), elevated dissolved iron concentration (up to 25 mg L-1) in groundwater may arouse aesthetic and safety concerns (43, 44). On the contrary, the coexistence of humic acid and Ca2+/Mg2+ significantly promoted the formation of large aggregates that comprised metal-hydrolyzed species and humic acid. The deposition of aggregates may result in precipitation on the iron surface and thereby block the pores, decrease the permeability, and

develop preferential flow path of PRB. The long-term impacts of the aggregates on Fe0 PRB remain uncertain. Therefore, the major concern of Fe0 applications would shift depending on whether or not NOM is present in hard water.

Acknowledgments This work was supported by the Hong Kong Research Grants Council under grant HKUST RGC 617006.

Supporting Information Available Additional information on adsorbed amounts of humic acid (Table S1), identification of XPS spectral lines (Table S2); amounts of Ca2+ and Mg2+ in solutions (Figure S1); pH changes along with Cr(VI) reduction kinetics experiments (Figure S2); results of two sets of supplementary experiments (Figure S3 and S4); SEM images, and XPS spectra of freezedried aggregates (Figure S5 and S6); and particle size distribution of precipitates/aggregates (Figure S7). This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Cantrell, K. J.; Kaplan, D. I.; Wietsma, T. W. Zero-valent iron for the in situ remediation of selected metals in groundwater. J. Hazard. Mater. 1995, 42, 201–212. (2) Blowes, D. W.; Ptacek, C. J.; Benner, S. G.; McRae, C. W. T.; Bennett, T. A.; Puls, R. W. Treatment of inorganic contaminants using permeable reactive barriers. J. Contam. Hydrol. 2000, 45, 123–137. (3) Xie, L.; Shang, C. Role of humic acid and quinone model compounds in bromate reduction by zerovalent iron. Environ. Sci. Technol. 2005, 39, 1092–1100. (4) Lo, I. M. C.; Surampalli, R. Y.; Lai, K. C. K., Eds. Zero-Valent Iron Reactive Materials for Hazardous Waste and Inorganics Removal; American Society of Civil Engineers: Reston, VA, 2006. (5) Matheson, L. J.; Tratnyek, P. G. Reductive dehalogenation of chlorinated methanes by iron metal. Environ. Sci. Technol. 1994, 28, 2045–2053. (6) Orth, W. S.; Gillham, R. W. Dechlorination of trichloroethene in aqueous solution using Fe0. Environ. Sci. Technol. 1996, 30, 66–71. (7) Klausen, J.; Vikesland, P. J.; Kohn, T.; Burris, D. R.; Ball, W. P.; Roberts, A. L. Longevity of granular iron in groundwater treatment processes: solution composition effects on reduction of organohalides and nitroaromatic compounds. Environ. Sci. Technol. 2003, 37, 1208–1218. (8) Tratnyek, P. G.; Scherer, M. M.; Deng, B.; Hu, S. Effects of natural organic matter, anthropogenic surfactants, and model quinones on the reduction of contaminants by zero-valent iron. Water Res. 2001, 35, 4435–4443. (9) Marconetto, S.; Gui, L.; Gillham, R. Adsorption of natural organic matter and its effects on TCE degradation by iron PRBs. IAHS Publ. 2005, 297, 389–397. Bringing Groundwater Quality Researh to the Watershed Scale, proceedings of GQ 2004, the 4th International Groundwater Quality Conference. Waterloo, Canada, July, 2004. (10) Dries, J.; Bastiaens, L.; Springael, D.; Kuypers, S.; Agathos, S. N.; Diels, L. Effect of humic acids on heavy metal removal by zerovalent iron in batch and continuous flow column systems. Water Res. 2005, 39, 3531–3540. (11) Powell, R. M.; Puls, R. W.; Hightower, S. K.; Sabatini, D. A. Coupled iron corrosion and chromate reduction: mechanisms for subsurface remediation. Environ. Sci. Technol. 1995, 29, 1913– 1922. (12) Pratt, A. R.; Blowes, D. W.; Ptacek, C. J. Products of chromate reduction on proposed subsurface remediation material. Environ. Sci. Technol. 1997, 31, 2492–2498. (13) Blowes, D. W.; Ptacek, C. J.; Jambor, J. L. In-situ remediation of Cr(VI)-contaminated groundwater using permeable reactive walls: laboratory studies. Environ. Sci. Technol. 1997, 31, 3348– 3357. (14) Eary, L. E.; Rai, D. Chromate removal from aqueous wastes by reduction with ferrous ion. Environ. Sci. Technol. 1988, 22, 972– 977. (15) Tipping, E. The adsorption of aquatic humic substances by iron oxides. Geochim. Cosmochim. Acta 1981, 45, 191–199.

(16) Gu, B.; Schmitt, J.; Chen, Z.; Liang, L.; MaCarthy, J. F. Adsorption and desorption of natural organic matter on iron oxide: mechanisms and models. Environ. Sci. Technol. 1994, 28, 38–46. (17) Vermeer, A. W. P.; van Riemsdijk, W. H.; Koopal, L. K. Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interaction. Langmuir 1998, 14, 2810–2819. (18) Saito, T.; Koopal, L. K.; van Riemsdijk, W. H.; Nagasaki, S.; Tanaka, S. Adsorption of humic acid on goethite: isotherms, charge adjustments, and potential profiles. Langmuir 2004, 20, 689– 700. (19) Doong, R.; Lai, Y. Effect of metal ions and humic acid on the dechlorination of tetrachloroethylene by zerovalent iron. Chemosphere 2006, 64, 371–378. (20) Wittbrodt, P. R.; Palmer, C. D. Reduction of Cr(VI) in the presence of excess soil fulvic acid. Environ. Sci. Technol. 1995, 29, 255– 263. (21) Jardine, P. M.; Fendorf, S. E.; Mayes, M. A.; Larsen, I. L.; Brooks, S. C.; Bailey, W. B. Fate and transport of hexavalent chromium in undisturbed heterogeneous soil. Environ. Sci. Technol. 1999, 33, 2939–2944. (22) Senesi, N.; Griffith, S. M.; Schnitzer, M.; Townsend, M. G. Binding of Fe3+ by humic materials. Geochim. Cosmochim. Acta 1977, 41, 969–976. (23) Tipping, E. Cation Binding by Humic Substances; Cambridge University Press: Cambridge, 2002. (24) Kohn, T.; Livi, K. J. T.; Roberts, A. L.; Vikesland, P. J. Longevity of granular iron in groundwater treatment processes: corrsion product development. Environ. Sci. Technol. 2005, 39, 2867– 2879. (25) Lo, I. M. C.; Lam, C. S. C.; Lai, K. C. K. Competitive effects of trichloroethylene on Cr(VI) removal by zero-valent iron. J. Environ. Eng. 2005, 131, 1598–1606. (26) Lo, I. M. C.; Lam, C. S. C.; Lai, K. C. K. Hardness and carbonate effects on the reactivity of zero-valent iron for Cr(VI) removal. Water Res. 2006, 40, 595–605. (27) Karvonen, A. Cation effects on chromium removal in permeable reactive walls. J. Environ. Eng. 2004, 130, 863–866. (28) D’Andrea, P.; Lai, K. C. K.; Kjeldsen, P.; Lo, I. M. C. Effects of groundwater inorganics on the reductive dechlorination of TCE by zero-valent iron. Water, Air, Soil Pollut. 2005, 162, 401–420. (29) Weng, L. P.; Koopal, L. K.; Hiemstra, T.; Meeussen, J. C. L.; van Riemsdijk, W. H. Interactions of calcium and fulvic acid at the goethite-water interface. Geochim. Cosmochim. Acta 2005, 69, 325–339. (30) Ritter, K.; Odziemkowski, M. S.; Gillham, R. W. An in situ study of the role of surface filims on granular iron in the permeable iron wall technology. J. Contam. Hydrol. 2002, 55, 87–111. (31) Milacˇicˇ, R.; Štupar, J.; Kožuh, N.; Korošin, J. Critical evaluation of three analytical techniques for the determination of chromium(VI) in soil extracts. Analyst 1992, 117, 125–130. (32) Eaton, A. D.; Clesceri, L. S.; Greenberg, A. E. Standard Methods for the Examination of Water and Wastewater, 19 ed.; American Public Health Association: Washington, DC, 1995. (33) Gould, J. P. The kinetics of hexavalent chromium reduction by metallic iron. Water Res. 1982, 16, 871–877. (34) Alowitz, M. J.; Scherer, M. M. Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal. Environ. Sci. Technol. 2002, 36, 299–306. (35) Ali, M. A.; Dzombak, D. A. Effects of simple organic acids on sorption of Cu2+ and Ca2+ on goethite. Geochim. Cosmochim. Acta 1996, 60, 291–304. (36) Rietra, R. P. J. J.; Hiemstra, T.; van Riemsdijk, W. H. Interaction between calcium and phosphate adsorption on goethite. Environ. Sci. Technol. 2001, 35, 3369–3374. (37) Giasuddin, A. B. M.; Kanel, S. R.; Choi, H. Adsorption of humic acid onto nanoscale zerovalent iron and its effect on arsenic removal. Environ. Sci. Technol. 2007, 41, 2022–2027. (38) Jung, A. V.; Chanudet, V.; Ghanbaja, J.; Lartiges, B. S.; Bersillon, J. L. Coagulation of humic substances and dissolved organic with a ferric salt: an electron energy loss spectroscopy investigation. Water Res. 2005, 3849–3862. (39) Duan, J.; Wang, J.; Graham, N.; Wilson, F. Coagulation of humic acid by aluminium sulphate in saline water conditions. Desalination 2002, 150, 1–14. VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2097

(40) Nakahara, O.; Wada, S. I. Ca2+ and Mg2+ adsorption by an allophanic and a humic Andisol. Geodema 1994, 61, 203–212. (41) Wall, N. A.; Choppin, G. R. Humic acids coagulation: influence of divalent cations. Appl. Geochem. 2003, 1573–1582. (42) Cornell, R. M.; Schwertmann, U. Cation The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. Binding by Humic Substances; Wiley-VCH: Weinheim, 2002.

2098

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 6, 2008

(43) Munter, R.; Ojaste, H.; Sutt, J. Complexed iron removal from groundwater. J. Environ. Eng. 2005, 131, 1014–1020. (44) Pacini, V. A.; Ingallinella, A. M.; Sanguinetti, G. Removal of iron and manganese using biological roughing up flow filtration technology. Water Res. 2005, 39, 4463–4475.

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