Effect of Natural Organic Matter on the Reduction ... - ACS Publications

Aug 8, 2008 - University of Georgia, Athens, Georgia 30602-2556. Received ..... (1) Colón, D.; Weber, E. J.; Anderson, J. L. QSAR study of the reduct...
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Environ. Sci. Technol. 2008, 42, 6538–6543

Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species ´ N , * ,† E R I C J . W E B E R , † DALIZZA COLO AND JAMES L. ANDERSON‡ National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Road, Athens, Georgia 30605-2720, and Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556

Received February 11, 2008. Revised manuscript received June 6, 2008. Accepted June 6, 2008.

Uncertainty still exists regarding the role(s) of natural organic matter in the reduction of chemicals in anoxic environments. This work studied the effect of Suwannee river humic acid (SRHA) on the reduction of nitrobenzenes in goethite suspensions by Fe(II) species. The pseudo-first-order rate constant for the reduction of p-cyanonitrobenzene (kCNNB) was different for the first 3 half-lives in systems where Fe(II)aq and dissolved SRHA were equilibrated in reverse orders with goethite in suspensions. kCNNB and the reduction capacity of the system having SRHA added after Fe(II)aq was equilibrated with goethite was lower than that of the system for which the components were added in the reverse order. SRHA decreased the reduction capacity of the former system by oxidizing and/or complexing the surface-associated Fe(II), Fe(II)surf, and/or hindering the access of CNNB to Fe(II)surf. The log kCNNB increased linearly with increasing concentrations of Fe(II)aq, which decreased as a result of increasing concentrations of SRHA in the system. Different kCNNB’s were observed for systems in which Fe(II)aq was equilibrated with goethite/SRHA suspensions for 24 and 48 h, suggesting sorbed SRHA oxidized and/or complexed Fe(II)aq. Findings suggest the concentration of Fe(II)aq and accessible Fe(II)surf will influence the reduction rates of nitroaromatics in anoxic environments.

Introduction Reductive transformation is the most common degradation pathway for nitroaromatics in natural anoxic environments (1). Several studies indicate that mineral oxide surfaceassociated Fe(II), Fe(II)surf, plays a major role in the transformation of organic chemicals in natural reducing environments (2–4). Redox zones in nature (i.e., nitrate-, manganese-, iron-, sulfate-reducing, and methanogenic) develop as a result of the reduction of inorganic species coupled to the microbial oxidation of organic matter (5). Therefore, a prerequisite for the establishment of naturally occurring redox zones is the presence of organic matter. Consequently, it is reasonable to expect that Fe(II)surf reactive sites will coexist with organic matter in many natural scenarios. * Corresponding author phone: (706) 355-8223; fax: (706) 3558202; e-mail: [email protected]. † U.S. Environmental Protection Agency. ‡ University of Georgia. 6538

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Natural organic matter (NOM) encompasses humic substances (i.e., humic acid and fulvic acid) and other components such as protein-like materials, polysaccharides, fatty acids, and alkanes (6). In particular, humic substances are (i) ubiquitous in soils, sediments, surface water, and groundwater (7), (ii) usually regarded as recalcitrant under anaerobic conditions, and (iii) contain redox active functional groups, including but not limited to quinones (8, 9). Various NOMs have shown an electrochemical behavior similar to that of several quinone model compounds (10) and it has been reported that dissolved organic matter (DOM) may act as a redox buffer from -0.9 V to +1.0 V (11). NOM could influence the partitioning of organic chemicals in solid-water systems in several ways. For instance, (i) dissolved organic matter can increase the apparent water solubility of some organic solutes including pesticides and PCBs (12, 13) and (ii) humic substances can deposit onto oxide surfaces, potentially increasing the sorption of organic contaminants to the solid phase (14). In the case of reduction of contaminants by Fe(II)surf, a relatively high partitioning of the organic solute to dissolved NOM could result in less exposure to the surface reactive sites, potentially decreasing their rates of transformation. Meanwhile, an enhanced sorption of an organic contaminant to the organic phase could slow down the kinetics by isolating the reducible species from the Fe(II)surf. NOM can affect the reductant and/or the electron transfer process in several ways, which include but are not limited to the following. First, the NOM sorption to mineral surfaces has been suggested to occur through ligand-exchange mechanisms, where carboxylic/phenolic functional groups from the NOM replace surface-coordinated OH groups (15), which are thought to be the surface-association sites for Fe(II) (16). Meanwhile, it has been reported that the rate of mineralmediated reduction of nitroaromatics is limited by the rate of regeneration of Fe(II)surf species (4). Consequently, it would be reasonable to expect that if a significant decline occurs in the number of available surface-association sites for Fe(II), the rate of reduction would decrease. Second, Fe(II)aq and/ or Fe(II)surf could be complexed or oxidized by NOM; both of these Fe(II) species have been demonstrated to be necessary for the complete reduction of p-cyanonitrobenzene in an Fe(II)-treated goethite suspension (1). Third, NOM could accelerate the reduction of organic chemicals in natural environments by acting as an electron-transfer mediator (ETM) between the bulk reductant (i.e., Fe(II)surf) and the reducible chemical. It has been reported that NOM acted as an ETM in the reduction of nitro functional groups in aqueous solutions containing hydrogen sulfide (17, 18) and solution phase Fe(II) (19). Fourth, NOM could block the access of the reducible organic chemical to Fe(II)surf. The possible effects of the interaction of Fe(II), iron oxides, and humic substances on the reductive transformation of anthropogenic chemicals have not been investigated thoroughly. Thus, to improve the predictability of rates of reduction of pollutants in the environment, it is necessary to account for the effects of NOM on this transformation pathway. The purpose of this work was to survey the effect of (i) a humic substance isolate on the reductive transformation of nitroaromatics in goethite suspensions by Fe(II) species, (ii) the order of equilibration of Fe(II)aq and a humic substance isolate with goethite on the reduction of a probe nitroaromatic compound, (iii) the equilibration time of Fe(II)aq in goethite/SRHA suspensions on the rate of reduction of a probe nitroaromatic compound, and (iv) the hydro10.1021/es8004249 CCC: $40.75

 2008 American Chemical Society

Published on Web 08/08/2008

phobicity of nitroaromatics on their reduction in goethite/ SRHA/Fe(II) suspensions. p-Cyanonitrobenzene (CNNB) was selected to study the effect of the order of equilibration of Fe(II)aq versus NOM with goethite, the equilibration time of Fe(II)aq in a goethite/ NOM suspension, and the presence and concentration of NOM on the reduction of nitroaromatics because it sorbs insignificantly to goethite (1) or natural organic matter (5), log Kow ) 1.24 at 21 °C (20), simplifying the kinetic analysis. Furthermore, both p-cyanoaniline (CNAN) and p-N-hydroxylaminobenzonitrile (CNHA), the final reduction product and one of the reduction intermediates, respectively, are weak nucleophiles having pKas of 1.95 and -3.35 at 21 °C (21, 22), which should only negligibly bind through nucleophilic addition to quinones and carbonyl groups present in humic substances, improving the organic chemical mass balance (23). A series of p-alkylated nitrobenzenes (i.e., MeNB, EtNB, and PrNB) was selected to study the effect of the hydrophobicity of the reducible organic chemical and the concentration of sorbed organic matter in the system on their reduction kinetics, while keeping the possibility of steric hindrance of the nitro group and the variability of the electron affinity (EA) of the nitrobenzenes to a minimum. The SPARC (SPARC Performs Automatic Reasoning in Chemistry; http:// sparc.chem.uga.edu/sparc/) calculated log Kow and EA of MeNB, EtNB, and PrNB are 2.51, 3.08, and 3.45 (20), and 0.954, 0.980, and 1.00 eV (24), respectively. The outcome of this work should improve our understanding of the influence of various environmentally relevant factors in the reductive transformation of nitroaromatics, which could be applicable to natural anoxic systems and the building of fate and exposure models.

Experimental Section Anoxic Conditions. See Supporting Information (SI). Chemicals. Nanopure water (g18 MΩ, Barnstead) was used to make all solutions. The nanopure water was degassed by boiling 900 mL under argon, followed by 4 h of argon degassing. The oxygen level in degassed nanopure water was not different from zero as determined by the Indigo Carmine method, using Hach Accu Vac Ampules. p-Cyanonitrobenzene (CNNB), p-methylnitrobenzene (MeNB), p-ethylnitrobenzene (EtNB, from Avocado research Chemicals Ltd.), p-i-propylnitrobenzene (PrNB), p-cyanoaniline (CNAN), p-methylaniline (MeAN), p-ethylaniline (EtAN), and p-i-propylaniline (PrAN) were obtained from Aldrich, unless otherwise noted, and used as received. p-Cyano-Nhydroxylaniline (CNHA) was synthesized by the addition of zinc and ammonium chloride to a stirred solution of p-cyanonitrobenzene in acetone (25). The purification of CNHA included its extraction with ether and two crystallizations to yield yellow crystals. CNHA was kept in the dark under nitrogen. Stock solutions of organic chemicals were prepared in deoxygenated methanol (g99.9%, Fisher). Goethite was selected for this study because it is a common and abundant ferric oxide in nature and because of its thermodynamic stability (26). Goethite (R-FeOOH) was made by the slow addition of 2.2 L of 0.26 M Fe(NO3)3 · 9H2O (in 0.1 N HNO3) to 5.0 L of 0.73 M KOH at 10 °C. The reaction product was aged at 70 °C for 24 h (27, 28). The XRD information for goethite is shown in the Supporting Information Figure S1 and Table S1. Suwannee River Humic Acid (SRHA), a natural organic matter isolate, was selected for the study because it has been well-characterized (29–31) and broadly studied. Stock solutions of SRHA (standard grade, International Humic Substance Society) were prepared in nanopure water. Deoxygenated perchloric acid (60%, J. T. Baker) was used to acidify the reaction aliquots. FeCl2 · 4H2O (Fluka) was used to prepare stock solutions of Fe(II) by mixing 0.15 mL of 1.0

M HCl (diluted and deoxygenated trace metal grade, Fisher) and 1.50 mL of 0.02 µm Anotop-filtered Fe(II) aqueous solution. Batch Experiments. To study the effect of SRHA on the reduction of nitroaromatics, individual degradation experiments, including the appropriate control studies, were conducted inside an anoxic glovebox. The experiments were contained in 59-mL borosilicate glass serum bottles closed with Teflon-lined gray butyl septa and crimped. In general, reaction components were goethite (28 m2/L or 0.55 g/L goethite, surface area ) 51 m2/g), SRHA, 387 µM Fe(II), 25 mM MES buffer (4-morpholineethanesulfonic acid, Sigma) pH 6.6, 0.1 M NaCl (Aldrich), and 15 µM nitroaromatic chemical. The total volume of the reaction mixtures was 50 mL. SRHA-containing reactors were protected from light. Reactions were agitated at 49 rpm in an end-over-end rotor. Aliquots were removed from the reactors and filtered through 0.2 µm PTFE syringe filter units. The filtered aliquots (1 mL) were acidified with 35 µL of 60% perchloric acid. The kinetic time was defined as the time elapsed between the spike of the nitroaromatic and the end of the filtration step. The advancement of the reaction was monitored by quantifying the consumption and production of the various species involved in the reaction. The effect of order of addition of NOM and Fe(II)aq to goethite suspensions (pH 6.6) on the rate of reduction of CNNB was evaluated. The total concentration of SRHA was 10 ppm organic carbon (OC). SRHA or Fe(II)aq were equilibrated in the suspension for 24 h prior to the addition of Fe(II)aq or SRHA, respectively. In another set of experiments, the reduction kinetics of CNNB were studied as a function of the concentration of SRHA, i.e., 0 -50 ppm OC (32), in goethite suspensions (28 m2/L or 0.55 g/L goethite, pH 6.6, and 0.1 M NaCl). SRHA was equilibrated for 24 h with goethite in suspension, followed by the addition and equilibration of Fe(II)aq for 48 h. The effect of the hydrophobicity of the reducible organic chemical on its rate of reduction was evaluated for MeNB, EtNB, and PrNB in goethite suspensions (28 m2/L or 0.55 g/L goethite, pH 6.6, and 0.1 M NaCl). SRHA (0 to 10 ppm OC) was equilibrated for 24 h with goethite in suspension, followed by the addition and equilibration of Fe(II)aq for 48 h. Analysis of Organic Chemicals, Fe(II), and Humic Acid. See Supporting Information.

Results and Discussion Effect of Order of Addition. The effect of the order of addition of Fe(II)aq and dissolved SRHA to an aqueous suspension of goethite (pH 6.6) on the reduction kinetics of p-cyanonitrobenzene (CNNB) and the reduction capacity of the system was evaluated by adding (i) Fe(II)aq after SRHA was equilibrated in the system (G/SRHA/Fe(II)) and (ii) SRHA after Fe(II)aq was equilibrated in the system (G/Fe(II)/SRHA). The total equilibration time having together Fe(II) and SRHA in suspension was 24 h for both systems, i.e., G/Fe(II)/SRHA and G/SRHA/Fe(II). The pseudo-first-order approximation was used to assess the kinetic behavior of CNNB and other target chemicals throughout this work. The pseudo-firstorder rate constants were calculated from the linear regression of the concentration data (Ln [reducible organic chemical]/[reducible organic chemical]t ) 0) vs time (h) for replicate experiments and reported as average values. The rate constants are reported with an associated error, which represents the confidence limits at the 95% probability level. The average kCNNB for the first 3 half-lives (t1/2, i.e., the time it takes for 50% of the chemical to disappear) of triplicate experiments were 0.746 ((0.063) and 0.819 ((0.056) h-1 in the G/Fe(II)/SRHA and G/SRHA/Fe(II) systems, respectively (see Figure 1). The variances for these two experiments were found to be statistically equal according to the F-test at a VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effect of the order of addition of Suwannee River Humic Acid and Fe(II) on the rate of reduction of CNNB in goethite suspensions. Initial conditions: 15 µM CNNB, 387 µM Fe(II), 10 ppm OC SRHA, 28 m2/L goethite, pH ) 6.6, and 0.1 M NaCl. 0.05 significance level and the average rates of reduction were statistically different using the t test (33). For the sake of clarity, only one set of data from the triplicate experiments is shown for each system in Figure 1. Based on the work by others, it could be expected that the individual equilibrium times of Fe(II)aq and SRHA in goethite suspensions would influence the outcome. It has been reported (34) that the normalized reduction rate constants for two halogenated alkanes increased with longer Fe(II)aq equilibrium times with goethite. The rate constants remained fairly constant after about 20 h of exposure time. In a study on the degradation of aromatic amines, Klausen, et al. (35) reported that the dissolved concentration of a humic acid in a MnO2 suspension decreased and leveled off after about 10 h of equilibration. Torrents and Stone (36) equilibrated a Suwannee River humic substance in goethite suspensions for g10 h to assess the inhibitory effect of this NOM isolate on the hydrolysis of picolinate ester. Assuming the previously mentioned findings and experimental conditions used by other research groups are translatable to our systems under study, our experimental designs allowed for enough exposure time for Fe(II) and SRHA to reach equilibrium with goethite in the G/Fe(II)/SRHA and G/SRHA/Fe(II) experiments, respectively. The reaction did not proceed to completion in the G/Fe(II)/SRHA system; there was some unreacted CNNB left in its aqueous phase (see plateau in Figure 1), thus, the reduction capacity of G/Fe(II)/SRHA, assessed as the consumption of CNNB, was less than that observed for G/SRHA/ Fe(II). Some possible explanations for the observed decrease in the reduction capacity and kCNNB of the G/Fe(II)/SRHA system relative to that of G/SRHA/Fe(II) are that SRHA could have acted to (i) hinder the access of CNNB to Fe(II)surf or (ii) complex and/or oxidize the Fe(II)surf. It could be speculated that a change in the concentration of Fe(II) and/or SRHA in the G/Fe(II)/SRHA system should shift the observed plateau. Effect of Concentration of Organic Matter. The effect of the presence and concentration of natural organic matter on the reduction kinetics of CNNB was studied in batch systems in which Fe(II)aq was added to a goethite suspension that had been equilibrated with SRHA in a concentration range of 0-50 ppm OC (G/SRHA/Fe(II), pH 6.6). This particular experimental design (G/SRHA/Fe(II)), versus (G/ Fe(II)/SRHA), was chosen to maximize the number of observable half-lives as evidenced in the previous section. No loss of CNNB, and/or production of CNHA and CNAN, was detected in control studies conducted in systems consisting of (i) goethite/SRHA (0-50 ppm OC) or (ii) SRHA (0-50 ppm OC)/Fe(II) at pH 6.6. The latter control study showed (i) no decrease in Fe(II)aq, which rules out the possibility of Fe(II)aq oxidation and/or complexation by SRHA 6540

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in solution, and (ii) that SRHA did not play the role of an electron transfer mediator, which would have reduced CNNB. The rate of reduction of CNNB (kCNNB) reached a maximum in the absence of SRHA and decreased as the total amount of SRHA increased in the system (Figure 2a and Supporting Information Table S2). Also, kCNNB decreased as the concentration of SRHA associated with the surface of goethite, following equilibration and before the addition of the reducible organic chemical, SRHAsurf,o, increased in the system (see insert in Figure 2a). The R2 values for all reaction rates were g0.98. Figure 2b shows the dependence of the concentration of SRHAsurf,o on the dissolved concentration of SRHA. In particular, it shows that for the low values of total SRHA concentration, most of the organic matter is associated with the surface of goethite, and for the high values of total SRHA concentration, the surface appears to become saturated (Supporting Information Table S2). The concentration of Fe(II)aq remaining in the aqueous phase after equilibration with the goethite/SRHA system and before the addition of the reducible organic chemical, Fe(II)aq,o, decreased and correlated well with increasing concentrations of SRHAsurf,o, (Figure 2c, R2 ) 0.925, and Supporting Information Table S2). kCNNB was observed to be faster in goethite suspensions with higher concentrations of Fe(II)aq,o. Log kCNNB correlated well with the concentration of Fe(II)aq,o (Figure 2d, R2 ) 0.959, and Supporting Information Table S2). Therefore, the relationship previously observed for kCNNB in independent Fe(II)-treated hematite, goethite, lepidocrocite, and ferrihydrite suspensions, where greater losses of Fe(II)aq during its equilibration period of time with the oxides led to faster rates of reduction of the nitroaromatic (1), is no longer valid when SRHA is in suspension with Fe(II) and goethite. Even though the highest concentration studied of SRHA (i.e., 50 ppm OC) caused a substantial decrease in the Fe(II)aq concentration, it did not impede the total conversion of CNNB to p-cyanoaniline (its final reduction product). That is, a diminished reduction capacity, which would have been indicated by the observation of unreacted CNNB, was not observed over the range of concentrations of SRHA studied. If this type of study had been conducted using the G/Fe(II)/ SRHA experimental design instead, it would have been reasonable to expect a similar correlation between log kCNNB and the concentration of Fe(II)aq,o for the decay in the concentration of CNNB. One plausible explanation for the observed kinetic behavior of CNNB is depletion of Fe(II)aq through complexation and/or oxidation by SRHAsurf,o (Figure 2c), as evidenced by Ferrozine analysis (see Fe(II) analysis in the Supporting Information). kCNNB was 0.698 ( 0.044 h-1 for the G/SRHA (10 ppm OC)/Fe(II) experiment after 48 h equilibration time with Fe(II), while kCNNB was 0.819 ( 0.056 h-1 for the G/SRHA/ Fe(II) experiment after 24 h equilibration time with Fe(II) (see Order of Addition section). The variances for these two experiments were found to be statistically equal according to the F-test at a 0.05 significance level and the average rates of reduction were statistically different according to the t test (33). The reduction in kCNNB due to 24 h longer equilibration time with Fe(II) was ca. 15%, which was considerably smaller than the magnitude of variation of kCNNB with varying NOM loadings. Thus, longer equilibration times should be considered for future work, but expected effects are likely to be small. The additional 24 h of equilibration time for Fe(II)aq in the G/SRHA suspension led to a slower kCNNB for the experiment at 10 ppm OC SRHA of this section, possibly due to additional complexation and/or oxidization of Fe(II)aq by SRHAsurf,o. It has been shown that Fe(II)aq was consumed throughout the progress of the reduction of CNNB in an Fe(II)-treated goethite suspension (1), thus, it could be expected that the concentration of Fe(II)aq, which was

FIGURE 2. Effect of the presence of Suwannee River Humic Acid (SRHA) on the rate of reduction of CNNB and the distribution of Fe(II) in goethite suspensions. (a) Reduction rate constant of CNNB against the total concentration added and the initially sorbed concentration of SRHA (see insert). (b) Initially sorbed versus dissolved concentration of SRHA. (c) Concentration of Fe(II)aq,o against the initially sorbed concentration of SRHA. (d) Log of the reduction rate constants of CNNB (b) against the concentration of Fe(II)aq,o. (e) Log of the reduction rate constant of CNHA (O) against the concentration of Fe(II)aq,o. The data point labels indicate the total concentration of SRHA (ppm OC) added. Initial conditions: 387 µM Fe(II), 28 m2/L goethite, pH ) 6.6, and 0.1 M NaCl. significantly affected by the presence of organic matter in this work, would have an effect on the reduction kinetics of CNNB as it occurred herein. The requirement of Fe(II)aq for the reduction of chemicals species to take place in Fe(II)-ferric oxide systems has been highlighted in the works of (i) Williams and Scherer (37), who reported that the reduction of nitrobenzene would not occur when only Fe(II) sorbed to the bulk oxide was present in the system, and (ii) Park and Dempsey (38), who reported that Fe(II)surf would not be oxidized by oxygen after the consumption of detectable Fe(II)aq in Fe(II)-treated ferric oxide systems. Alternatively, SRHA could block goethite surface hydroxyl sites either by direct complexation or by steric exclusion. It has been suggested that NOM carboxylic and phenolic groups can replace mineral surface-coordinated hydroxyl groups through a ligand exchange mechanism (15). SRHA contains 0.642 µeq/ppm OC of carboxylic and phenolic groups (i.e., 32.1 µeq for 50 ppm OC, see calculations and Table S3 in Supporting Information). For OC levels studied here, SRHA

carboxylic and phenolic groups outnumber goethite surface hydroxyl sites (estimated as 3.25 µeq in 50 mL of goethite suspension, see calculations in Supporting Information), which could potentially be the surface association sites for Fe(II). The logarithm of the rate of reduction of CNHA (generated in situ), log kCNHA, decreased as the total concentration of SRHA increased and the concentration of Fe(II)aq,o decreased in the system (Figure 2e, R2 ) 0.851, and Supporting Information Table S2), but less noticeably. The differences in the rate-determining steps of CNNB (i.e., gain of the first electron) and CNHA (i.e., cleavage of the N-O bond) could account for the lower dependence of the rate (assessed as the slope of log kreducible chemical versus Fe(II)aq,o) of the intermediate compound (slope ) 0.0159 in Figure 2e and Supporting Information Table S2) compared to that of the parent compound (slope ) 0.0441 in Figure 2d and Supporting Information Table S2) in Fe(II)-treated goethite/SRHA suspensions. Specifically, the effect of the electron-transfer VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of hydrophobicity on the rate of reduction of p-alkylated nitrobenzenes in goethite suspensions as a function of the concentration of Suwannee River Humic Acid (SRHA). Initial conditions: 387 µM Fe(II), 28 m2/L goethite, pH ) 6.6, 0.1 M NaCl, and number of replicates ) 2. kinetics on kCNHA could be masked by the kinetics of N-O bond cleavage. A similar rationale was offered as a possible explanation for the lower response, assessed as the slope of kreducible chemical versus Fe(II)surf,o, of CNHA compared to that of CNNB in Fe(II)-treated oxide suspensions as a function of the oxide surface area loading (22). The results of this work suggest that under conditions of a high demand for electrons (i.e., high concentration of the reducible organic chemical), the presence of a sufficient concentration of a humic substance with behavior similar to that of SRHA shown herein, could impose an upper limit on the reduction capacity of the system. The presence of such a humic substance could result, in the absence of other factors, in a lower available concentration of Fe(II)aq,o and Fe(II)surf, which could translate into a decreased supply of electrons available for the reduction process. Effect of Hydrophobicity. A series of p-alkylated nitrobenzenes (i.e., p-methylnitrobenzene (MeNB), p-ethylnitrobenzene (EtNB), and p-propylnitrobenzene (PrNB)) was selected to study the effect of hydrophobicity on the reduction kinetics in systems in which Fe(II)aq was added to a goethite suspension that had been equilibrated with SRHA in concentrations of 0, 2.5, 5.0, and 10 ppm OC (G/SRHA/Fe(II), pH 6.6). The hydrophobicity of the chemicals was assessed by their log octanol-water partition constants (log Kow) of 2.51, 3.08, and 3.45 for MeNB, EtNB, and PrNB, respectively, using the SPARC calculator (20). Through the selection of these p-monosubstituted nitrobenzenes, the steric hindrance of the nitro group and the variability of the electron affinity (EA) of the reducible chemicals were kept to a minimum. The SPARC-calculated EAs of MeNB, EtNB, and PrNB are 0.954, 0.980, and 1.00 eV, respectively (24). SRHA was selected to study the effect of hydrophobicity on the reduction kinetics of alkylated nitrobenzenes because it showed no electron-transfer capability for transforming CNNB in Fe(II)/SRHA control study, as demonstrated in the previous section, minimizing the convolution of relevant variables in the observed results. To study the possible role of SRHA as a sorptive sink for the reducible chemical, the range of total concentration of SRHA was restricted to those in which its distribution into the aqueous phase was at a low level (see Figure 2b and Supporting Information Table S4), minimizing any effect it could have as a cosolvent on the reducible organic chemicals. The concentration of dissolved SRHA in the system containing the highest total concentration of SRHA (10 ppm OC) was 0.18 ppm OC. The hydrophobicity of the p-alkylated nitrobenzenes did not play a significant effect in their relative rate of reduction, that is, the order of reactivity was MeNB > PrNB > EtNB (Figure 3 and Supporting Information Table S4), while their order of increasing hydrophobicity is MeNB < EtNB < PrNB. 6542

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The log kp-alkylated nitrobenzenes increased and correlated well with the concentration of Fe(II)aq,o, (R2 ) 0.963, 0.990, and 0.997 for MeNB, EtNB, and PrNB, respectively) as shown in Figure 3. For most of the evaluated conditions, the rates of reduction of MeNB, EtNB, and PrNB were not statistically different (95% level of probability, Supporting Information Table S4) from one another. The R2 values for all rates of reactions were g0.953. The sorption of MeNB, EtNB, and PrNB was approximately up to 5%, 9%, and 17%, respectively. The recovery, assessed as the concentration of the respective aniline, was approximately 93-95%, 99-103%, and 87-96% for p-methylaniline, p-ethylaniline, and p-propylaniline, respectively. Under similar conditions, MeNB, EtNB, and PrNB were reduced at a slower rate (Figure 3 and Supporting Information Table S4) than CNNB (Figure 2d and Supporting Information Table S2). This observation can be rationalized based on what is accepted as the rate-determining step for the reduction of nitroaromatics, i.e., the gain of the first electron (1). A p-monosubstituted nitrobenzene with an electrondonating functional group (e.g., methyl, ethyl, and propyl) is expected to have an increased electron density at the reaction center relative to that of nitrobenzene or a pmonosubstituted nitrobenzene bearing an electron-withdrawing functional group, consequently, the former should be reduced at a slower rate than any of the latter ones. Environmental Significance. This work demonstrated that the presence of SRHA can work to decrease the enhanced reactivity, compared to that of an anoxic solution of Fe(II)aq or a goethite suspension (1), of an anoxic Fe(II)/goethite suspension, toward the reduction of p-substituted nitrobenzenes. In particular, this work presents experimental results showing that the presence of sorbed SRHA caused a decrease in the concentration of Fe(II)aq in Fe(II)-treated goethite/ SRHA suspensions, and consequently, a decrease in the rate of reduction of selected p-substituted nitroaromatics. The reactivity of the system toward the reduction of CNNB was observed to be sensitive to the Fe(II)aq equilibration time in goethite/SRHA suspensions; a longer equilibration time led to lower reactivity. In a particular experimental design, SRHA was able to hinder the access of CNNB to, complex and/or oxidize a fraction of the concentration of, Fe(II)surf, which has been proposed as the dominant reductant in natural systems (2, 39, 40), leading to a decreased reduction capacity of the system. The results presented here suggest that (i) the presence of an ample concentration of a humic substance with behavior similar to that of SRHA shown herein, in the absence of other factors, could slow down the reduction kinetics of nitroaromatics and impose an upper limit on the reduction capacity of the system for the transformation of these chemicals and (ii) the concentrations of Fe(II)aq and accessible Fe(II)surf will influence the reactivity of natural anoxic environments toward the reduction of nitroaromatics. Consequently, it should be worth exploring the utility of Fe(II)aq and accessible Fe(II)surf in attempting to predict such transformation in natural systems. The quantification of the role of these chemical factors in the transformation of nitroaromatics in anoxic environments should prove valuable in the advancement of the development of environmental fate models.

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Supporting Information Available (i) Description of the anoxic conditions used in the conduction of kinetic experiments, (ii) data for the X-ray analysis of goethite, (iii) description of the analysis of organic compounds, Fe(II), and humic acid, (iv) experimental data for all kinetic experiments described herein, (v) calculation of the number of surface hydroxyl sites on goethite, and (vi) calculation of the number of carboxylic and phenolic groups of SRHA. This information is available free of charge via the Internet at http://pubs.acs.org.

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