Article pubs.acs.org/est
Reduction and Reoxidation of Humic Acid: Influence on Speciation of Cadmium and Silver Felix Maurer, Iso Christl,* Martin Hoffmann, and Ruben Kretzschmar Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: Naturally occurring variations of redox conditions are considered to affect the interactions between trace metals and humic substances in a 2-fold manner. First, additional proton binding sites of humic substances formed under reducing conditions may also act as binding sites for trace metals. Second, reduced humic substances may transfer electrons to redox-active trace metals. In this study, we investigated the influence of electrochemical reduction of a purified soil humic acid on the binding of two chalcophile metal cations of environmental concern, Cd2+ and Ag+, with metal titrations conducted under monitored redox conditions. The binding of cadmium to reduced humic acid was slightly enhanced compared to humic acid reoxidized by O2 and quantitatively in excellent agreement with the increase in binding sites formed upon reduction. Competitive experiments with calcium indicated that sulfur-containing sites played a minor role in cadmium binding, although sulfur K-edge XANES revealed that 36% of humic sulfur was in a reduced oxidation state. In all experiments with silver, the formation of Ag(0) was detected with transmission electron microscopy. Free Ag+ activities under reducing conditions were controlled by Ag(0) formation rather than by binding to humic acid.
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INTRODUCTION Humic substances play an important role as sorbents for many essential and toxic trace elements in the environment, thereby controlling their mobility and bioavailability in soils, sediments, and aquatic systems.1 Additionally, their redox reactivity has important consequences for redox transformations occurring in environments with redox gradients or fluctuating redox conditions which comprise freshwater wetlands characterized by periodic flooding and variations in groundwater level. It was shown that humic substances can accept electrons during microbial2−5 or electrochemical reduction,6,7 and in reactions with metallic Zn8 or H2 in the presence of Pd.9,10 Likewise, electron transfers from reduced humic substances to O2,5,9−11 Fe,2,6,8,12,13 and other metals (e.g., Hg,13 Cr14) were reported. Apparent standard reduction potentials of redox active moieties in humic substances were found to be distributed between 0.15 and −0.3 V.15 Many studies attribute the observed redox reactivity of humic substances mainly to quinoid functional groups.3,9,10,15−18 The reduction of quinones to hydroquinones is also consistent with the observed increase of phenolic-like proton binding sites after electrochemical reduction.10 To date, most experiments that contributed to the large databases for modeling metal binding to humic substances19,20 were performed with humic material that was exposed to oxygen during extraction and storage. Therefore, existing studies mostly reflect cation binding properties of humic substances under oxic conditions, even when the actual experiments were performed under an inert gas atmosphere. © 2012 American Chemical Society
But in environments with variable redox conditions, redox transformations of humic substances may affect metal binding by humic substances21 including the binding of environmentally relevant contaminants. The effects of redox transformations of humic substances on their metal binding properties is likely to depend on the metal considered. For metal cations classified as hard, speciation is expected to be dominated by binding to hard, oxygencontaining ligand groups.22 For these cations, it is therefore expected that the effects of reduction directly corresponds to changes in carboxylic and phenolic groups which can be detected with acid−base titrations. For metals classified as soft, however, sulfur-containing functional groups may also become important because of their high affinity for soft cations, although sulfur is only a minor component of humic substances (0.1−2.6 wt %1). For instance, exclusive Hg−S coordination was found for an organic soil and a ratio of Hg to reduced sulfur smaller than 0.1.23 Reduced sulfur species, thiol groups in particular, are considered most important for the binding of soft cations. The bonding environments and redox states of sulfur in humic substances depend, as the structure and composition of humic substances in general, on precursor material and formation Received: Revised: Accepted: Published: 8808
April 17, 2012 July 10, 2012 July 18, 2012 July 18, 2012 dx.doi.org/10.1021/es301520s | Environ. Sci. Technol. 2012, 46, 8808−8816
Environmental Science & Technology
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M, titrating to pH 7 with 0.5 M NaOH and diluting with water to a concentration of 4 g L−1. Reduced HA solutions were prepared from untreated HA solutions (4 g L−1) by electrochemical reduction7 at a potential of −0.59 V and a pH of 7 using a glassy carbon electrode as described previously in Maurer et al.10 Reduction time and final Eh were 138 h and −0.2 V, respectively. The Ag/AgCl reference electrode was shielded from the HA solution by an electrolyte bridge which was continuously flushed with background electrolyte to prevent diffusion of silver into the HA solution. Uptake of protons in HA solution and release of protons in the anode compartment were counterbalanced in regular time intervals by the manual addition of 0.1 M HCl and 0.1 M NaOH, respectively, corresponding to the number electrons transferred which amounted to ∼0.55 mol kg−1 HA.10 Reoxidized HA solutions were prepared by purging reduced HA solutions with air for 5 min and leaving them for at least 24 h under oxic conditions before purging them with N2 and transferring them back into the glovebox. Sulfur K-Edge XANES. Untreated, reduced, and reoxidized HA solutions were prepared at pH 7 and 0.1 M NaCl (VWR, p.a.) and dried under N2 atmosphere in a vacuum chamber. Then, 50 mg of HA was homogenously mixed with 30 mg boron nitride (Merck) and 20 mg sulfur-free wax (Licowax C, Lonay, Switzerland), and pressed to 10 mm pellets under N2 atmosphere. Samples with reduced HA were sealed in aluminum for transport. All spectra were recorded at beamline 4−3 at the Stanford Synchrotron Radiation Lightsource (SSRL), Stanford, CA. To account for small energy shifts, calibration of the Si(111) double crystal monochromator was done using a Na2S2O3 standard (E0: 2471.5 eV), which was measured after each sample run. The measurements were performed in fluorescence mode with a Stern−Heald ionization detector filled with N2 gas.37 To prevent oxidation of the samples, all spectra were recorded under He atmosphere at ∼25 K using a He cryostat. Five spectra of each sample were taken. The spectra were merged and normalized to the absorption at 2490 eV using the software code Athena.38 The oxidation states of sulfur were identified by comparing the energy of absorption features in the merged spectrum with the white-line energy of reference compounds (SI Table S1, Figure S1A). Five PseudoVoigt-shaped (50% Gaussian) resonance peaks39 and two additional arctangent functions for the edge-steps at 2473 eV (reduced sulfur) and 2481 eV (oxidized sulfur) were used for deconvolution of the spectra in the software package WINXAS.40 The width of the arctangent functions was fixed to 1 eV. The areas of the fitted peaks were calculated and corrected for their specific oxidation state-dependent absorption cross section28 (SI Figure S1B). The contribution of each sulfur oxidation state to total sulfur was calculated by dividing the peak area by the sum of peak areas of all sulfur oxidation states present in the sample. Cadmium Titrations. All titrations were performed under N2 atmosphere in the glovebox at 24 ± 2 °C. The 180 mL glass vessel used was sealed by a plastic top (Metrohm 6.1414.010), equipped with a magnetic stirrer and thermo-insulated from the stirrer by a piece of styrofoam. Cadmium binding to reduced and reoxidized HA was investigated at pH 7 and 9 at an ionic strength of 0.1 M. A 0.1 M cadmium stock solution was prepared from CdCl2 (Merck, p.a.). Solutions for cadmium additions were prepared from the stock solution diluted to concentrations of 40 μM and 20 mM cadmium in 0.1 M NaCl. A 50 mM NaOH and a 50
conditions. While some sulfur-containing functional groups may be leftovers from larger biomolecules, thiols may also form by incorporation of sulfide formed under reducing conditions.24,25 Studies focusing on the investigation of sulfur speciation by sulfur K-edge XANES found that the fraction of reduced sulfur in humic substances (oxidation state
