RESUMEN
Thermal treatment can not only efficiently remove volatile pollutants but also distinctly alter the speciation of organic carbon (C) and the behaviors of residual pollutants in contaminated soils. Here we examined the distribution and bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in industrially contaminated site soils affected by thermal treatment (temperature ranging of 105-650 â) using synchrotron-based infrared microspectroscopy and n-butanol extraction (a mild solvent extractant). In the pristine soils, the sequestration and distribution of PAHs were simultaneously controlled by aromatic C, aliphatic C and clay minerals. Desorption efficiency of PAHs was substantially increased with increasing temperature, whereas the residual PAHs were strongly immobilized within their binding sites evidenced by their dramatically decreased bioaccessibility. Aliphatic and carboxylic C were gradually decomposed and/or carbonized with increasing temperature. In contrast, aromatic C remained relatively recalcitrant during the thermal treatment and was the key controlling factor for the desorption of residual PAHs in the soils with either thermal treatment or n-butanol extraction. This study is the first to visualize the changes in the binding sites and bioaccessibility of PAHs induced by thermal treatment, which have important implications for understanding the sequestration mechanisms of organic pollutants in soil and optimizing the remediation technique.
RESUMEN
Lowpermeability zone (LPZ) can play an important role as a sink or secondary source in contaminant transport in groundwater system. This study investigated the rate and end product of nitrate bioreduction in LPZ sediments. The sedimentswere fromthe U.S. Department of Energy's Hanford Site,where nitrate is a groundwater contaminant as a by-product of radionuclide waste discharges. The LPZ at the Hanford site consists of two layerswith an oxidized layer on top and reduced layer below. The oxidized layer is directly in contact with the overlying contaminated aquifer, while the reduced layer is in contact with an uncontaminated aquifer below. The experimental results showed that nitrate bioreduction rate and end-product differed significantly in the sediments. The bioreduction rate in the oxidized sediment was significantly faster than that in the reduced one. A significant amount of N2O was accumulated in the reduced sediment; while in the oxidized sediment, N2O was further reduced to N2. RT-PCR analysis revealed that nosZ, the gene that codes for N2O reductase, was below detection limit in the reduced sediment. Batch experiments and kinetic modeling were performed to provide insights into the role of organic carbon bioavailability, biomass growth, and competition between nitrate and its reducing products for electrons fromelectron donors. The results revealed that it is important to consider sediment redox conditions and functional genes in understanding and modeling nitrate bioreduction in subsurface sediments. The results also implied that LPZ sediments can be important sink of nitrate and a potential secondary source of N2O as a nitrate bioreduction product in groundwater.