| With the development of modern industry, a large amount of chromium-containing slag was annually discharged from chromate industries. The deposition of chromium-containing slag will result in a significant risk to the environment. Accordingly, chromium-contaminated soil was urgently required to be remediated. At present, the traditional remediation methods of chromium-contaminated soil were not practically used because of theirs disadvantages, and bio-remediation technology gradually became have a potential technology for remediating the polluted soil. Based on the previous technique establishment of chromium-contaminated soil remediation, the thesis investigate the characteristics of Cr(Ⅵ) microbial reduction and Cr(Ⅲ) stabilization. The results could provide important theoretical proofs for the microbial remediation of chromium-contaminated soils.In this thesis, the soils selected from a chromate-containing slag site at one Iron-Alloy Factory in Hunan province was used to investigate the ability of Cr(Ⅵ) reduction by indigenous microorganism in chromium contaminated soils, to determine the optimal condition of Cr(Ⅵ) reduction, to characterize Cr(Ⅵ) reduction by indigenous microorganism under both aerobic and anaerobic conditions, to discuss the oxidation-reduction sequence in redox systems in the contaminated soil. The thesis further investigated the Cr(Ⅲ) stabilization and the mechanism under different soil situations including the presence of indigerous microorganism, soil moisture and initial soil pH when Cr(Ⅵ) was reduced to Cr(Ⅲ).The results showed that the indigenous microorganism had a strong reduction ability of Cr(Ⅵ). The supplying nutrition into soils can stimulate microbial activity and fulfil Cr(Ⅵ) microbial remediation in chromium-contaminated soil. The water soluble hexavalent chromium was removed from initial 306 mg/kg to detection limit by indigenous microorganism within 48h under 30℃of temperature,8g/L of carbon source,13g/L of nitrogen source and pH 10.Under aerobic condition, The Cr(Ⅵ) reduction proved to be a two-step process:Cr(Ⅵ) concentration in soils exponetiallly declined within 36h and the kinetic equation of Cr(Ⅵ) reduction was:C=-5.56*exp(t/7.33)+15338.88(R2=0.995). Thereafter, Cr(Ⅵ) concentration in soils linearly decreased and the kinetic equation of Cr(Ⅵ) reduction was:C= 1157.65-14.43t (R2=0.999). Under anaerobic condition, the change of Cr(Ⅵ) concentration within 12h was minor, indicating there was no Cr(Ⅵ) reduction during this period. However, after 12h, Cr(Ⅵ) concentration declined and the kinetic equation of Cr(Ⅵ) reduction was: C= 1776.95-75.825*t+2.891*t2-0.0388*t3(R2=0.998).During the procedure of Cr(Ⅵ) reduction, NO3- concentration rapidly declined with time while Mn2+ concentration increased; SO42-concentration of ions showed slight change. However, Fe2+ concentration increased with time and then decreased; when Cr(Ⅵ) was reduced, Fe2+ concentration increased again. The change trend of Fe2+ implied that Cr(Ⅵ) reduction was closely associated with iron reduction. During Cr(Ⅵ) reduction, iron ion could involve electron transfer to reduce Cr(Ⅵ). The Cr(Ⅵ) reduction occurred after NO3-, Mn2+ and Fe2+ reduction, but before SO42- reduction. The reduction sequence was NO3--N2 > Mn4+-Mn2+>Fe3+-Fe2+>Cr(Ⅵ)-Cr(Ⅲ)>SO42--H2S.After microbial remediation, Cr(Ⅵ) in soils was converted into Cr(Ⅲ). The Cr(Ⅵ) concentration in the remediated soils was monitored for 240 days. There was no Cr(Ⅵ) presence in soils with pH range of 6-10, whatever the soil was autoclaved or non-autoclaved and whatever the soil was dry, alternately dry and wet and waterlogged. The presence of Fe2+ and Mn2+ in soil could contribute to Cr(Ⅲ) stabilization. In addition, the measured electric potential in soils was lower than the theoretical potential of Cr(Ⅲ) oxidation. Furthermore,99.86% of chromium in the remediated soils was residual fraction that could stably exist in the soil. Therefore, it is verified that Cr(Ⅲ) remains stabilization after remediated by indigeous microorganism. |
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