Hydrologic and Biochemical Processes Controlling Chromium Immobilization in a Low-Permeability Groundwater Zone

Experiments and modeling were performed to investigate the coupled hydrologic and biochemical processes that control chromium (Cr) immobilization in a low-permeability groundwater zone. Bench-top flow cells were packed with a water-saturated high-permeability zone (HPZ) overlying a low-permeability zone (LPZ). Cr(VI) was initially flushed into the LPZ to establish a reservoir of this contaminant. Next, the electron donor acetate and the bacterium Geobacter sulfurreducens were flushed into the HPZ; they mixed with Cr(VI) in the LPZ and promoted its reduction. Experimental depth profiles show that approximately 80% of Cr(VI) introduced to the flow cell was immobilized as Cr(III) over 180 h within a small region on either side of the HPZ–LPZ interface. Groundwater flow and reactive transport in the flow cell were simulated using MODFLOW and RT3D, respectively, with dual-Monod kinetics defined in the custom reaction module of RT3D. Modeling results adequately matched experimental data and were extended to simulate Cr(VI) fate in a numerical flow cell with the same dimensions but with the LPZ replaced by a diffusion-controlled lower permeability clay matrix. For this scenario, approximately 99% of Cr(VI) in the LPZ could eventually be immobilized as Cr(III); this primarily occurred in the LPZ and mitigated Cr(VI) back diffusion to the HPZ. Overall, results from this work support acetate amendment to HPZs as an effective strategy to trap Cr(III) in LPZs and mitigate back diffusion of Cr(VI) into adjacent HPZs of a groundwater aquifer

Influence of Phytoplankton on Fate and Effects of Modified Zerovalent Iron Nanoparticles

Nanoscale zerovalent iron (nZVI) and its derivatives hold promise for remediation of several pollutants but their environmental implications are not completely clear. In this study, the physicochemical properties and aggregation kinetics of sulfide/silica-modified nZVI (FeSSi) were compared in algal media in which <i>Chlamydomonas reinhardtii</i> had been cultured for 1, 2, or 11 days in order to elicit the effects of organic matter produced by the freshwater algae. Furthermore, transformation of FeSSi particles were investigated in <i>C. reinhardtii</i> cultures in exponential (1-d) and slowing growth (11-d) phases while monitoring the response of algae. We found evidence for steric stabilization of FeSSi by algal organic matter, which led to a decrease in the particles’ attachment efficiency. Transformation of FeSSi was slower in 11-d cultures as determined via inductively coupled plasma and X-ray analyses. High concentrations of FeSSi caused a lag in algal growth, and reduction in steady state population size, especially in cultures in exponential phase. The different outcomes are well described by a dynamic model describing algal growth, organic carbon production, and FeSSi transformations. This study shows that feedback from algae may play important roles in the environmental implications of engineered nanomaterials

Enhanced Oxidative and Adsorptive Removal of Diclofenac in Heterogeneous Fenton-like Reaction with Sulfide Modified Nanoscale Zerovalent Iron

Sulfidation of nanoscale zerovalent iron (nZVI) has shown some fundamental improvements on reactivity and selectivity toward pollutants in dissolved-oxygen (DO)-stimulated Fenton-like reaction systems (DO/S-nZVI system). However, the pristine microstructure of sulfide-modified nanoscale zerovalent iron (S-nZVI) remains uncovered. In addition, the relationship between pollutant removal and the oxidation of the S-nZVI is largely unknown. The present study confirms that sulfidation not only imparts sulfide and sulfate groups onto the surface of the nanoparticle (both on the oxide shell and on flake-like structures) but also introduces sulfur into the Fe(0) core region. Sulfidation greatly inhibits the four-electron transfer pathway between Fe(0) and oxygen but facilitates the electron transfer from Fe(0) to surface-bound Fe­(III) and consecutive single-electron transfer for the generation of H₂O₂ and hydroxyl radical. In the DO/S-nZVI system, slight sulfidation (S/Fe molar ratio = 0.1) is able to nearly double the oxidative removal efficacy of diclofenac (DCF) (from 17.8 to 34.2%), whereas moderate degree of sulfidation (S/Fe molar ratio = 0.3) significantly enhances both oxidation and adsorption of DCF. Furthermore, on the basis of the oxidation model of S-nZVI, the DCF removal process can be divided into two steps, which are well modeled by parabolic and logarithmic law separately. This study bridges the knowledge gap between pollutant removal and the oxidation process of chemically modified iron-based nanomaterials