René P. Schwarzenbach

The Challenge of Micropollutants in Aquatic Systems

The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity. Although most of these compounds are present at low concentrations, many of them raise considerable toxicological concerns, particularly when present as components of complex mixtures. Here we review three scientific challenges in addressing water-quality problems caused by such micropollutants. First, tools to assess the impact of these pollutants on aquatic life and human health must be further developed and refined. Second, cost-effective and appropriate remediation and water-treatment technologies must be explored and implemented. Third, usage and disposal strategies, coupled with the search for environmentally more benign products and processes, should aim to minimize introduction of critical pollutants into the aquatic environment.

Reduction of substituted nitrobenzenes by Fe(II) in aqueous mineral suspensions

The kinetics of the reduction of 10 monosubstituted nitrobenzenes (NBs) by Fe(II) has been investigated under various experimental conditions in aqueous suspensions of minerals commonly present in soils and sediments. Aqueous solutions of Fe(II) were unreactive. In suspensions of Fe(III)-containing minerals (magnetite, goethite, and lepidocrocite), Fe(II) readily reduced the NBs to the corresponding anilines in a strongly pH-dependent reaction. Our results suggest that on other mineral surfaces (γ-aluminum oxide, amorphous silica, titanium dioxide, and kaolinite) iron (hydr)oxide coatings are indispensable to promote the reduction of NBs by adsorbed Fe(ll). Apparent pseudo-first-order rate constants, k obs , were used to describe the initial kinetics of the NB reduction, covering several half-lives of the compounds. The distinct effect of substituents on k obs and the observed pronounced competition between different NBs indicate that precursor complex formation as well as the (re)generation of reactive surface sites are rate-determining steps in the overall reduction of the NBs. The results of this study demonstrate that Fe(ll) adsorbed on iron (hydr)oxide surfaces or surface coatings may play an important role in the reductive transformation of organic pollutants in subsurface environments. Our findings may also contribute to a better understanding of the various redox processes involved in groundwater remediation techniques based on Fe(0) as the bulk reductant.

Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances

Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential E(h). Leonardite Humic Acid (LHA) showed an H(+)/e(-) ratio of unity and a decrease in potential from E(h) = +0.18 to -0.23 V upon transfer of 822 mumol(e-) g(LHA)(-1) at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O(2)-reoxidation revealed that electron transfer to LHA was largely reversible. However, LHA contained a small pool of moieties that were not reoxidized, likely due to endergonic first electron transfer to O(2). Electron accepting capacities of 13 different HS, determined by MER, strongly correlated with their C/H ratios and aromaticities and with previously published values, which, however, were a factor of 3 smaller due to methodological limitations.

Mechanistic studies on baseline toxicity and uncoupling of organic compounds as a basis for modeling effective membrane concentrations in aquatic organisms

Abstract. Mechanistic studies on membrane toxicity are reviewed and linked to effects observed in-vivo. Time-resolved spectroscopy on energy-transducing membranes is an in-vitro test method that provides information on the toxicodynamics of membrane toxicity, namely baseline toxicity and uncoupling. Without blurring effects of the toxicokinetic phase, the intrinsic potency of membrane toxicants can directly be determined thus allowing the development of a classification system to distinguish between baseline toxicity and uncoupling. Toxicity data of known baseline toxicants and of substituted phenols from literature are reevaluated in light of the information obtained from the mechanistic test system. Exposure-based effect data (aqueous effect concentrations) of substituted phenols (baseline toxicants and uncouplers) from various aquatic organisms (bacteria, protozoa, algae, daphnids, fish) are compared to the corresponding effect concentrations expected from baseline toxicity. The results of the comparison support the view that the membrane is the primary target site for acute toxicity of substituted phenols in aquatic organisms. The classification into baseline toxicants and uncouplers based upon the criteria derived from the mechanistic test system is correct. Nevertheless, the acute toxicity in-vivo cannot be correlated quantitatively to the mechanistic data presumably because bioaccumulation is not directly proportional to intrinsic toxicity and the metabolic transformation is variable within the test set of substituted phenols. The pH-dependence of acute toxicity is mainly determined by the pH-dependence of bioccumulation, the internal effect concentrations are virtually independent on external pH. The internal effect concentrations, also called lethal body burdens or critical body residues, which are taken from the literature or calculated from aqueous effect concentrations and bioconcentration factors, are related to membrane concentrations using a simple three-compartment equilibrium partitioning model. The modeled membrane concentrations of non-polar and polar narcotics turn out to be statistically indistinguishable, which is consistent with the findings from the mechanistic test system. The toxic ratios, i.e., the excess toxicity of uncouplers in relation to their baseline toxicity, agree for most compounds upon comparison of the mechanistic test system with the aquatic organisms thus confirming that uncoupling is the dominant mode of action responsible for lethality.

Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids

Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric E(h) measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials E(h) of HA with increasing pH. This was confirmed by potentiometric E(h)-pH titrations of HA at different redox states. E(h) measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from E(h)(0)* = +0.15 to -0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing E(h). The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions.

Influence of solution composition and column aging on the reduction of nitroaromatic compounds by zero-valent iron.

Granular iron is used in reactive permeable barriers for the reductive treatment of organic and inorganic groundwater contaminants. The technology is well established, however, its long-term performance and the importance of the groundwater composition are not yet well understood. Here, the influence of chloride, nitrate, silicate, and Aldrich humic acid on the reactivity of Master Builder iron was studied under anoxic conditions using small packed columns and 2-nitrotoluene (2-NT) as a model contaminant. After initially complete reduction of 2-NT to 2-aminotoluene (2-AT) in the column, possibly under mass-transfer controlled conditions, the reactivity of the iron was found to decrease substantially. In the presence of chloride, this decrease was slowed while exposure to silicate resulted in a very quick loss of iron reactivity. Nitrate was found to interfere strongly with the effect of chloride. These observations are interpreted in terms of corrosion inhibition/promotion and competition. Our results suggest that reactive barrier performance may be strongly affected by the composition of the treated groundwater.

A tiered procedure for assessing the formation of biotransformation products of pharmaceuticals and biocides during activated sludge treatment.

Upon partial degradation of polar organic micropollutants during activated sludge treatment, transformation products (TPs) may be formed that enter the aquatic environment in the treated effluent. However, TPs are rarely considered in prospective environmental risk assessments of wastewater-relevant compound classes such as pharmaceuticals and biocides. Here, we suggest and evaluate a tiered procedure, which includes a fast initial screening step based on high resolution tandem mass spectrometry (HR-MS/MS) and a subsequent confirmatory quantitative analysis, that should facilitate consideration of TPs formed during activated sludge treatment in the exposure assessment of micropollutants. At the first tier, potential biotransformation product structures of seven pharmaceuticals (atenolol, bezafibrate, ketoprofen, metoprolol, ranitidine, valsartan, and venlafaxine) and one biocide (carbendazim) were assembled using computer-based biotransformation pathway prediction and known human metabolites. These target structures were screened for in sludge-seeded batch reactors using HR-MS/MS. The 12 TPs found to form in the batch experiments were then searched for in the effluents of two full-scale, municipal wastewater treatment plants (WWTPs) to confirm the environmental representativeness of this first tier. At the second tier, experiments with the same sludge-seeded batch reactors were carried out to acquire kinetic data for major TPs that were then used as input parameters into a cascaded steady-state completely-stirred tank reactor (CSTR) model for predicting TP effluent concentrations. Predicted effluent concentrations of four parent compounds and their three major TPs were corroborated by comparison to 3-day average influent and secondary effluent mass flows from one municipal WWTP. CSTR model-predicted secondary effluent mass flows agreed within a factor of two with measured mass flows and confidence intervals of predicted and measured mass flows overlapped in all cases. The observed agreement suggests that the combination of batch-determined transformation kinetics with a simple WWTP model may be suitable for estimating aquatic exposure to TPs formed during activated sludge treatment. Overall, we recommend the tiered procedure as a realistic and cost-effective approach to include consideration of TPs of wastewater-relevant compounds into exposure assessment in the context of prospective chemical risk assessment.

Biogeochemistry of dimethylsulfide in a seasonally stratified coastal salt pond

Abstract Dimethylsulfide (DMS) is the major volatile reduced organic sulfur compound in the water column of coastal Salt Pond, Cape Cod, MA. DMS concentration and vertical distributions vary seasonally in response to changing biogeochemical processes in the pond. When the pond is thermally stratified in summer, maximum DMS concentrations of up to 60 nmol/1 were found in the oxygen-deficient metalimnion. DMS concentrations in the epilimnion (typically 5–10 nmol/1) were always an order of magnitude higher than in the hypolimnion (

Nonlinear sorption and nonequilibrium solute transport in aggregated porous media: Experiments, process identification and modeling

The combined effects of nonlinear sorption, nonequilibrium mass transfer and the distribution of sorption sites on transport of organic contaminants has been examined in porous media containing aggregates of clay minerals and organic matter as sorbents. The major goal was to develop general concepts for describing, deterministically, the transport processes of solutes with different adsorption characteristics in such systems. Various sets of batch adsorption and miscible displacement experiments were performed covering a wide range of time scales and other experimental conditions. Using a multiple reactive tracer approach, independent information was obtained on the hydrodynamic properties of the columns, on the relative importance of the two different sorbents present, and on the accessibility and the distribution of these sorbents at the pore scale. The breakthrough curves (BTCs) of the nonlinearly sorbing tracer generally exhibited sharp fronts and excessive tailing, consistent with the Langmuir–Freundlich type adsorption at clays. The effect of nonequilibrium mass transfer was most evident from the tailing of the self-sharpened fronts of the BTCs and from the results of interrupted flow experiments. A two-region model, which incorporated nonlinear sorption and retarded intra-aggregate diffusion, successfully described the results of our entire set of miscible displacement data using a single set of parameter values. Our study demonstrates that although nonlinear sorption and nonequilibrium mass transfer may have very similar effects on solute BTCs, these processes can be distinguished from experimental data if experiments with different solutes, different flow rates and different input concentrations are evaluated simultaneously. It is shown that a very small volume fraction of immobile regions (<0.1% of total porosity), which is insignificant for the transport of conservative solutes, may strongly affect the transport of sorbing solutes if sorption sites are concentrated within these regions. In soils and aquifers, clay minerals and other reactive surfaces are often present in aggregates. Thus, the transport of solutes that strongly interact with such sites generally is very susceptible to rate-limited mass transfer processes while the transport of conservative tracers is poorly affected.

pH-Dependent Sorption of Acidic Organic Chemicals to Soil Organic Matter

Due to their increased polarity, many contemporary biologically active chemicals exhibit acid functions and may thus dissociate to their anionic conjugated base at pH values typically present in the environment. Despite its negative charge, soil organic matter (SOM) has been demonstrated to be the main sorbent in soils, even for the anionic species of organic acids. Nevertheless, few data exist that allow for a systematic interpretation of the sorption of organic acids into SOM. Therefore, in this study, the sorption of the neutral and anionic species of 32 diverse organic acids belonging to nine different chemical groups to SOM was investigated. Partition coefficients were determined from HPLC retention volumes on a column packed with peat, at three Ca(2+)-concentrations and over a pH range of 4.5-7.5. The influence of Ca(2+)-concentrations on anion sorption was small (factor 2 in the usual environmental Ca(2+)-concentration range) and independent of molecular structure. Generally, the organic carbon-water partition coefficients, K(oc), of both the neutral and anionic species increased with increasing molecular size and decreased with increasing polarity. At an environmentally relevant Ca(2+)-concentration of 10 mM, the investigated anions sorbed between a factor of 7-60 less than the corresponding neutral acid. This factor was more homogeneous within a group of structurally related compounds. These results indicate that while similar nonionic interactions seem to govern the partitioning of both the neutral and anionic species into SOM, the electrostatic interactions of the anionic species with SOM are a complex and currently not well understood function of the type of acidic functional group. The HPLC-based, flow through method presented in this study was shown to yield consistent results for a wide range of organic acids in a high-throughput manner. It should therefore prove highly useful in further investigating how different acidic functional groups affect anion sorption to SOM.