Stefan B. Haderlein

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.

Anaerobic Degradation of Benzene, Toluene, Ethylbenzene, and o-Xylene in Sediment-Free Iron-Reducing Enrichment Cultures

ABSTRACT Monoaromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) are widespread contaminants in groundwater. We examined the anaerobic degradation of BTEX compounds with amorphous ferric oxide as electron acceptor. Successful enrichment cultures were obtained for all BTEX substrates both in the presence and absence of AQDS (9,10-anthraquinone-2,6-disulfonic acid). The electron balances showed a complete anaerobic oxidation of the aromatic compounds to CO2. This is the first report on the anaerobic degradation of o-xylene and ethylbenzene in sediment-free iron-reducing enrichment cultures.

Compound-specific chlorine isotope analysis: a comparison of gas chromatography/isotope ratio mass spectrometry and gas chromatography/quadrupole mass spectrometry methods in an interlaboratory study.

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of (37)Cl/(35)Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ(37)Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1‰), to GC/qMS (1σ ≈ 0.2-0.5‰ for Agilent GC/qMS and 1σ ≈ 0.2-0.9‰ for Thermo GC/qMS). Nonetheless, δ(37)Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.

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.

Environmental Processes Influencing the Rate of Abiotic Reduction of Nitroaromatic Compounds in the Subsurface

Numerous synthetic chemicals contain one or several nitro groups that are bound to an aromatic ring. Figure 1 shows the structures of some prominent representatives of such nitroaromatic compounds (NACs). The high toxicity of some NACs, particularly the mutagenic and carcinogenic potential of some nitrated polycyclic aromatic hydrocarbons (PAHs), has led to considerable interest in the fate of such compounds in the environment. Due to their widespread use, NACs are ubiquitous contaminants, especially in aqueous environments. In addition to contamination originating from agricultural use, from production facilities, and waste disposal sites, diffuse input into the pedosphere via the atmosphere has been documented (21, 27, 36, 37, 59, 65, 69, 80, 81). Atmospheric production of significant quantities of NACs by photochemical processes has been reported (19, 29, 42, 80). Table 1 lists some typical concentrations of NACs that have been measured in various compartments of the environment. Very high concentrations of nitroaromatic explosives (2,4,6-trinitrotoluene (TNT) and by-products) have been found especially in soil and subsurface systems. At those sites, significant concentrations of substituted aromatic amines that may have been formed from the reduction of NACs are frequently encountered.

Reductive Dechlorination of TCE by Chemical Model Systems in Comparison to Dehalogenating Bacteria: Insights from Dual Element Isotope Analysis (13C/12C, 37Cl/35Cl)

Chloroethenes like trichloroethene (TCE) are prevalent environmental contaminants, which may be degraded through reductive dechlorination. Chemical models such as cobalamine (vitamin B12) and its simplified analogue cobaloxime have served to mimic microbial reductive dechlorination. To test whether in vitro and in vivo mechanisms agree, we combined carbon and chlorine isotope measurements of TCE. Degradation-associated enrichment factors ε(carbon) and ε(chlorine) (i.e., molecular-average isotope effects) were -12.2‰ ± 0.5‰ and -3.6‰ ± 0.1‰ with Geobacter lovleyi strain SZ; -9.1‰ ± 0.6‰ and -2.7‰ ± 0.6‰ with Desulfitobacterium hafniense Y51; -16.1‰ ± 0.9‰ and -4.0‰ ± 0.2‰ with the enzymatic cofactor cobalamin; -21.3‰ ± 0.5‰ and -3.5‰ ± 0.1‰ with cobaloxime. Dual element isotope slopes m = Δδ(13)C/ Δδ(37)Cl ≈ ε(carbon)/ε(chlorine) of TCE showed strong agreement between biotransformations (3.4 to 3.8) and cobalamin (3.9), but differed markedly for cobaloxime (6.1). These results (i) suggest a similar biodegradation mechanism despite different microbial strains, (ii) indicate that transformation with isolated cobalamin resembles in vivo transformation and (iii) suggest a different mechanism with cobaloxime. This model reactant should therefore be used with caution. Our results demonstrate the power of two-dimensional isotope analyses to characterize and distinguish between reaction mechanisms in whole cell experiments and in vitro model systems.

Chlorine isotope analysis of organic contaminants using GC-qMS: method optimization and comparison of different evaluation schemes.

Compound-specific online chlorine isotope analysis of chlorinated hydrocarbons was evaluated and validated using gas chromatography coupled to a regular quadrupole mass spectrometer (GC-qMS). This technique avoids tedious off-line sample pretreatments, but requires mathematical data analysis to derive chlorine isotope ratios from mass spectra. We compared existing evaluation schemes to calculate chlorine isotope ratios with those that we modified or newly proposed. We also tested systematically important experimental procedures such as external vs. internal referencing schemes, and instrumental settings including split ratio, ionization energy, and dwell times. To this end, headspace samples of tetrachloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cDCE) at aqueous concentrations in the range of 20-500 μg/L (amount on-column range: 3.2-115 pmol) were analyzed using GC-qMS. The results (³⁷Cl/³⁵Cl ratios) showed satisfying to good precisions with relative standard deviations (n = 5) between 0.4‰ and 2.1‰. However, we found that the achievable precision considerably varies depending on the applied data evaluation scheme, the instrumental settings, and the analyte. A systematic evaluation of these factors allowed us to optimize the GC-qMS technique to determine chlorine isotope ratios of chlorinated organic contaminants.

Effects of Zwitterionic buffers on sorption of ferrous iron at goethite and its oxidation by CCl4.

A major factor which controls sorption and oxidation of Fe(II) at the mineral-water interface is pH, hence buffers are commonly used to control pH in experimental studies. Here, we examined the effects of widely used organic buffers (3-morpholinopropane-1-sulfonic acid (MOPS) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)) on Fe(II) uptake and oxidation by CCl(4) in aqueous suspensions of goethite. Significant sorption of these zwitterionic buffers occurred only at Fe(II)-loaded goethite but not at native goethite. The addition of MOPS and HEPES caused substantial release of Fe(II) from goethite, retarded the oxidation of surface-bound Fe(II) by CCl(4) and changed the reaction pathway as indicated by lower yields of CHCl(3). To explore electrostatic and steric contributions of MOPS and HEPES to the observed phenomena we studied sorption and competitive effects of model sorbates (Ca(2+), sulfonates) which suggest the formation of a complex between surface-bound Fe(II) and MOPS or HEPES. Our study shows for the first time that these frequently used zwitterionic organic buffers may interfere significantly with the surface chemistry and thus with redox reactions of Fe(II) at goethite. Hence, kinetic or mechanistic information obtained in such systems requires careful interpretation.

Effect of water content on solute transport in a porous medium containing reactive micro-aggregates

Abstract The water content of porous media may substantially affect the transport behaviour of conservative and sorbing solutes. Physical processes potentially involved include alterations of the flow velocities, flow patterns, or of accessible surface sites. We performed column experiments using a synthetic porous medium, in which a substantial part of the sorption sites was concentrated in regions within small grained aggregates that were accessible only by diffusion, a feature often found in natural soils and sediments. We investigated the transport of solutes exhibiting very different sorption characteristics under steady state conditions at different water contents of the porous medium. The tracers used were either nonreactive, partitioned into organic matter or sorbed specifically and nonlinearly to clay minerals. Hydrodynamic dispersion generally increased with decreasing water content, reflected by the breakthrough curves (BTCs) of conservative and only slightly sorbing tracers, which exhibited stronger spreading and early breakthrough of the fronts at lower water saturation. Nonlinear sorption and nonequilibrium mass transfer between the mobile region and the immobile water present within the aggregates dominated the BTCs of the strongly sorbing tracer at all degrees of water saturation, and, thus, rendered the effects of increased hydrodynamic dispersion negligible. Due to a relative increase in the ratio of sorption sites per water volume, the retardation of this tracer distinctly increased at low water contents of the porous medium. Solute transport of all tracers was successfully simulated with an advective–dispersive transport model that considered the respective sorption behaviour and retarded intra-aggregate diffusion as predominant processes. All parameter values of the model had been determined previously in independent experiments under completely saturated conditions. Our results demonstrate that a well parameterised transport model that was calibrated under completely saturated conditions was able to describe rate-limited advective-dispersive transport of reactive solutes also under unsaturated steady-state conditions. Enhanced relative retardation of strongly sorbing compounds under such conditions is likely to affect biological and chemical transformation processes of these compounds.

Simulation of nonlinear sorption of N-heterocyclic organic contaminates in soil columns.

The transport of organic contaminants in porous media is frequently influenced by nonequilibrium sorption and/or nonlinear sorption. In this study, sorption of coal tar related contaminants with different sorption properties, i.e., toluene, quinoline, quinaldine, and benzotriazole, was studied in column experiments using a European reference soil and compared with batch sorption results in order to quantify the governing sorption features. The breakthrough curves (BTCs) were simulated with a versatile 1-D reactive transport model using a one-site first-order sorption approach. Some differences in fitted parameters from batch and column experiments were found and discussed in terms of different sorption mechanisms in different aqueous concentration ranges, effects of solution properties (e.g., pH) and differences in solid-to-solution ratio and accessible sorption sites. The modeling results show that the fitting results were not sensitive to mass transfer coefficients and that a local equilibrium assumption provides excellent agreement with BTCs in our designed column when Damkohler numbers were greater than 20. Nonequilibrium sorption resulting from intraparticle diffusion thus was negligible in the column experiments. Tailing of BTCs nevertheless occurred and was primarily attributed to nonlinear sorption due to specific interactions in the sorption processes rather than to sorption nonequilibrium. Our study demonstrates how column experiments with different concentrations and flow velocities can be designed to obtain reliable sorption parameters for polar solutes with nonlinear sorption isotherms from modeling.