Ruben Kretzschmar

Biogeochemical Redox Processes and their Impact on Contaminant Dynamics

Life and element cycling on Earth is directly related to electron transfer (or redox) reactions. An understanding of biogeochemical redox processes is crucial for predicting and protecting environmental health and can provide new opportunities for engineered remediation strategies. Energy can be released and stored by means of redox reactions via the oxidation of labile organic carbon or inorganic compounds (electron donors) by microorganisms coupled to the reduction of electron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids, and actinides. Environmental redox processes play key roles in the formation and dissolution of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals often controls the release or sequestration of inorganic contaminants. Redox processes control the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements including Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox-active humic substances and mineral surfaces can catalyze the redox transformation and degradation of organic contaminants. In this review article, we highlight recent advances in our understanding of biogeochemical redox processes and their impact on contaminant fate and transport, including future research needs.

Mobile Subsurface Colloids and Their Role in Contaminant Transport

Publisher Summary It is noted that colloidal particles can be effectively transported through subsurface porous media under certain hydrogeochemical conditions. If present in large concentrations, mobile colloids can act as carriers for strongly sorbing contaminants and thereby provide an unretarded transport pathway for contaminants that are otherwise strongly retarded. This potential transport pathway can be considered in risk assessments of sites heavily contaminated with toxic chemicals, such as certain radionuclides, heavy metals, and hydrophobic organic compounds. The chapter reviews the current state of knowledge about the behavior of colloids in porous media and their role in contaminant transport. The chapter also identifies some important future research needs. Colloids are commonly defined as small particles or other entities with dimensions roughly between 1 nm and 1 μm. These size limits to dissolved molecules on one side and to larger suspended particles on the other side. Colloidal particles remain stable in suspension over long time periods, unless they coagulate to form larger aggregates or deposit onto surfaces of larger grains.

Experimental determination of colloid deposition rates and collision efficiencies in natural porous media

Mobile colloids in groundwater aquifers and soils can serve as carriers for strongly sorbing contaminants and thereby facilitate contaminant transport. Therefore mobile colloids may have to be considered in modeling the fate of strongly sorbing contaminants in subsurface environments. In this study we present a Chromatographic short-pulse technique for measuring colloid deposition rate coefficients and experimental collision efficiencies in natural porous media. The method was evaluated using four different experimental systems of increasing complexity. Short pulses (equivalent to 0.002 to 0.03 pore volumes) of latex or humic-coated hematite suspensions were injected under saturated flow conditions into laboratory columns packed with glass beads, soil, or aquifer materials. Colloid breakthrough curves were measured on-line using fluorescence and UVVIS spectrophotometers. Deposition rate coefficients determined with the short-pulse method were in excellent agreement with results from step-input experiments. Experiments with different flow rates and column dimensions showed that colloid deposition generally followed a first-order kinetic rate law. On the basis of experimental fast deposition rates, collision efficiencies for colloid deposition can be calculated. The results demonstrate that the short-pulse method can be used very efficiently for studying the effects of solution chemistry and flow velocity on the kinetics of colloid deposition in natural porous media. The short-pulse method has several advantages over the more traditionally used step-input experiment and allows running several experiments on a single column without significant blocking or filter ripening effects.

Relating Ion Binding by Fulvic and Humic Acids to Chemical Composition and Molecular Size. 2. Metal Binding

Binding of Cu(II) and Pb(II) to a soil fulvic acid, humic acid, and two different size fractions of the humic acid was investigated with metal titration experiments at pH 4, 6, and 8. Proton and free metal ion activities in solution were monitored after each titration step using pH and ion selective electrodes (ISE), respectively. The amounts of base required to maintain constant pH conditions were recorded and used to calculate stoichiometric proton-to-metal ion exchange ratios. Despite clear differences in chemical composition and protonation behavior, the fulvic acid and all humic acid fractions exhibited very similar metal binding behavior. Binding of Cu(II) and Pb(II) generally increased with increasing pH and total metal concentration. At low to moderate metal ion concentrations, Cu(II) was bound more strongly to the humic substances than Pb(II). Only at high free metal concentrations, the amounts of metal ions sorbed were higher for Pb(II) than for Cu(II). The molar proton-to-metal ion exchange ratios ranged from 1.0 to 1.8 for Cu(II) and from 0.6 to 1.2 for Pb(II), suggesting that Cu(II) was bound as monodentate and bidentate complexes, while Pb(II) was bound predominantly as monodentate complexes. The metal ion binding data were quantitatively described with the consistent NICA-Donnan model. The best description of an entire multicomponent data set consisting of proton titration, Cu(II), and Pb(II) binding data was achieved when the entire data set was fitted simultaneously. To reduce the number of fitting parameters, results from size exclusion chromatography and solid state 13C NMR spectroscopy were used to estimate two of the NICA-Donnan model parameters. The values of the remaining NICA-Donnan parameters for the humic substances are within a narrow range, suggesting that generalized model parameters may be useful in geochemical modeling involving humic substances.

Equilibrium Mercury Isotope Fractionation between Dissolved Hg(II) Species and Thiol-Bound Hg

Stable Hg isotope ratios provide a new tool to trace environmental Hg cycling. Thiols (-SH) are the dominant Hg-binding groups in natural organic matter. Here, we report experimental and computational results on equilibrium Hg isotope fractionation between dissolved Hg(II) species and thiol-bound Hg. Hg(II) chloride and nitrate solutions were equilibrated in parallel batches with varying amounts of thiol resin resulting in different fractions of thiol-bound and free Hg. Mercury isotope ratios in both fractions were analyzed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Theoretical equilibrium Hg isotope effects by mass-dependent fractionation (MDF) and nuclear volume fractionation (NVF) were calculated for 14 relevant Hg(II) species. The experimental data revealed that thiol-bound Hg was enriched in light Hg isotopes by 0.53 per thousand and 0.62 per thousand (delta(202)Hg) relative to HgCl(2) and Hg(OH)(2), respectively. The computational results were in excellent agreement with the experimental data indicating that a combination of MDF and NVF was responsible for the observed Hg isotope fractionation. Small mass-independent fractionation (MIF) effects (<0.1 per thousand) were observed representing one of the first experimental evidences for MIF of Hg isotopes by NVF. Our results indicate that significant equilibrium Hg isotope fractionation can occur without redox transition, and that NVF must be considered in addition to MDF to explain Hg isotope variations.

Spectroscopic Evidence for Ternary Complex Formation between Arsenate and Ferric Iron Complexes of Humic Substances

Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ~10). The local (<5 Å) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5-12.5 μmol As(V)/g HS (25-70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binuclear ((2)C) and monodentate mononuclear ((1)V) complex stabilized by H-bonds (R(As-Fe) = 3.30 Å). The analysis of Fe K-edge EXAFS spectra revealed that Fe in Fe(III)-HS complexes was predominantly present as oligomeric Fe(III) clusters at neutral pH. Shell-fit results complied with a structural motif in which three corner-sharing Fe(O,OH)(6) octahedra linked by a single μ(3)-O bridge form a planar Fe trimer. In these complexes, the average Fe-C and Fe-Fe bond distances were 2.95 Å and 3.47 Å, respectively. Our study provides the first spectroscopic evidence for ternary complex formation between As(V) and Fe(III)-HS complexes, suggesting that this binding mechanism is of fundamental importance for the cycling of oxyanions such as As(V) in organic-rich, oxic soils and sediments.

COMPETITIVE SORPTION OF COPPER AND LEAD AT THE OXIDE-WATER INTERFACE : IMPLICATIONS FOR SURFACE SITE DENSITY

Abstract The competitive sorption of Cu(II) and Pb(II) to colloidal hematite was investigated as a function of pH and total metal concentration. Acid–base titrations of the hematite and single-metal sorption experiments for Cu and Pb at low to medium surface coverages were used to calibrate two surface complexation models, the triple layer model, and a 2-pK basic Stern model with ion-pair formation. The surface site density was systematically varied from 2 to 20 sites/nm2. Three different metal surface complexes were considered: (1) an inner-sphere metal complex; (2) an outer-sphere metal complex; and (3) an outer-sphere complex of singly hydrolyzed metal cations. Both models provided excellent fits to acid–base titration and single-metal sorption data, regardless of the surface site density used. With increasing site density, ΔpK of the stability constants for protonation reactions increased and metal surface complexes decreased steadily. The calibrated models based on different site densities were used to predict competitive sorption effects between Cu and Pb and single-metal sorption at higher total metal concentrations. Precipitation of oversaturated solid phases was included in the calculations. Best predictions of competitive sorption effects were obtained with surface site densities between 5 and 10 sites/nm2. The results demonstrate that surface site density is a key parameter if surface complexation models are exposed to more complex, multicomponent environments. We conclude that competitive metal sorption experiments can be used to obtain additional information about the relevant surface site density of oxide mineral surfaces.

Interaction of copper and fulvic acid at the hematite-water interface

The influence of surface-bound fulvic acid on the sorption of Cu(II) to colloidal hematite particles was studied experimentally and the results were compared with model calculations based on the linear additivity assumption. In the first step, proton and Cu binding to colloidal hematite particles and to purified fulvic acid was studied by batch equilibration and ion-selective electrode titration experiments, respectively. The sorption data for these binary systems were modeled with a basic Stern surface complexation model for hematite and the NICA-Donnan model for fulvic acid. In the second step, pH-dependent sorption of Cu and fulvic acid in ternary systems containing Cu, hematite, and fulvic acid in NaNO3 electrolyte solutions was investigated in batch sorption experiments. Sorption of fulvic acid to the hematite decreased with increasing pH (pH 3–10) and decreasing ionic strength (0.01–0.1 M NaNO3), while the presence of 22 μM Cu had a small effect on fulvic acid sorption, only detectable at low ionic strength (0.01 M). Sorption of Cu to the solid phase separated by centrifugation was strongly affected by the presence of fulvic acid. Below pH 6, sorption of Cu to the solid phase increased by up to 40% compared with the pure hematite. Above pH 6, the presence of fulvic acid resulted in a decrease in Cu sorption due to increasing concentrations of dissolved metal-organic complexes. At low ionic strength (0.01 M), the effects of fulvic acid on Cu sorption to the solid phase were more pronounced than at higher ionic strength (0.1 M). Comparison of the experimental data with model calculations shows that Cu sorption in ternary hematite-fulvic acid systems is systematically underestimated by up to 30% using the linear additivity assumption. Therefore, specific interactions between organic matter and trace metal cations at mineral surfaces must be taken into account when applying surface complexation models to soils or sediments which contain oxides and natural organic matter.

Temperature Dependence and Coupling of Iron and Arsenic Reduction and Release during Flooding of a Contaminated Soil

Arsenic (As) in soils and sediments is commonly mobilized when anoxic conditions promote microbial iron (Fe) and As reduction. Recent laboratory studies and field observations have suggested a decoupling between Fe and As reduction and release, but the links between these processes are still not well understood. In microcosm experiments, we monitored the formation of Fe(II) and As(III) in the porewater and in the soil solid-phase during flooding of a contaminated floodplain soil at temperatures of 23, 14, and 5 degrees C. At all temperatures, flooding induced the development of anoxic conditions and caused increasing concentrations of dissolved Fe(II) and As(III). Decreasing the temperature from 23 to 14 and 5 degrees C strongly slowed down soil reduction and Fe and As release. Speciation of As in the soil solid-phase by X-ray absorption spectroscopy (XAS) and extraction of the Fe(II) that has formed by reductive Fe(III) (hydr)oxide dissolution revealed that less than 3.9% of all As(III) and less than 3.2% of all Fe(II) formed during 52 days of flooding at 23 degrees C were released into the porewater, although 91% of the initially ascorbate-extractable Fe and 66% of the total As were reduced. The amount of total As(III) formed during soil reduction was linearly correlated to the amount of total Fe(II) formed, indicating that the rate of As(V) reduction was controlled by the rate of microbial Fe(III) (hydr)oxide reduction.

Contaminant mobilization by metallic copper and metal sulphide colloids in flooded soil

Colloids, such as submicrometre mineral particles or bacterial cells, can act as carriers enhancing the mobility of poorly soluble contaminants in subsurface environments. Spectroscopic and microscopic analysis of flooded soils suggests that copper colloids and metal sulphide colloids increase the concentration of contaminants in waterlogged soils.