Combining Plants With The Bioelectrochemical Systems In Chromium-Contaminated Soil Remediation And Electricity Generation

Soil is an irreplaceable resource for the survival and development of human being. However, with the development of heavy industries, the soil contamination caused by heavy metals has become a serious problem facing human beings. Among them, the soil pollution caused by hexavalent chromium is becoming an increasingly serious environmental problem. Traditional remediation technologies of soil polluted by hexavalent chromium are mainly physical and chemical methods, which have many disadvantages that are hard to be overcome, such as the expensive capital consumption and residues of chemical medicament. Because of its low cost and zero secondary pollution, the phytoremediation has received more and more attention in recent years. This study combined the plants with microbial fuel cell(MFC), constructing a chromium-contaminated soil restoring system through a plant microbial fuel cell(PMFC). The chromium removal efficiency and the performance of electricity generation were investigated.In this research, a new single-chamber reactor was designed, and Spathiphyllums and Isoetes sinensis were used to launch the reactors. Compared with reactors without plants, the effect of plants on the removal efficiency of hexavalent chromium was investigated. And we controlled the difference of hydraulic retention times(HRT), open or closed circuit conditions, and the initial concentrations of chromium in the soil, expecting to analyze the effects of these factors on the removal efficiency of hexavalent chromium in the PMFC system.Compared with reactors without plants, the existent of Spathiphyllums and Isoetes sinensis contributed to accelerated PMFCs’ start-up. The performance of electricity generation was the best when the HRT was 10 hours. In this condition, the maximum voltage outputs of non-plant control groups were around 300 mV and the maximum power density reached to 18.35 W/m~2, while the maximum voltage outputs of Spathiphyllums reactors were approximately 403 mV and the maximum power density reached to 24.68 W/m~2, which increased by 34% compared with reactors without plants. After planted 45 days in the reactors, roots of Isoetes sinensis festered gradually, indicating Isoetes sinensis couldn’t survive for a long time in the soil of high concentrations chromium. The analysis of concentration of total chromium and Cr(Ⅵ) in the effluent showed that the concentrations of both total chromium and Cr(Ⅵ) in the Spathiphyllums-PMFC were significantly less than those of the control reactors. By analyzing the plant tissues, we found that Spathiphyllums could enrich chromium, and the concentrations in the roots were highest, which reached to 102.41 mg/kg.The initial chromium concentration of the soil were set to 20 mg/kg, 100 mg/kg and 500 mg/kg to plant Spathiphyllums. The result indicated that the start-up time of reactors under 20 mg/kg and 100 mg/kg were 190 h and 200 h, respectively, which were obviously faster than the reactors under 100 mg/kg. The performance of electricity generation was best when the initial chromium concentration was 100 mg/kg. The maximum voltage outputs were around 677 mV and the maximum power density reached to 25.81 W/m~2. The analysis of pH showed that the pH was around 7 during almost the whole study, and this provided a comfortable environment for the growth of plants and microorganisms in the soil. The analysis of DO showed that the oxygen secretion ability of Spathiphyllums would increase the dissolved oxygen of PMFC. The effluent TN was lower in the reactors under initial chromium concentration of 100 mg/kg, and the final TN concentration was 5.06 mg/L.The reactors were operated under closed circuit and open circuit conditions, and the concentrations of total chromium and Cr(Ⅵ) of closed circuit reactors were significantly less than those of the open circuit control reactors. Take the reactors under initial chromium concentration of 100 mg/kg as example, the final concentrations of total chromium and Cr(Ⅵ) of the open circuit control reactors were 17.36 mg/L and 13.96 mg/L, respectively, while the concentrations of total chromium and Cr(Ⅵ) of the closed circuit control reactors were 0.886 mg/L and 0.49 mg/L, respectively. Therefore the biological electricity production has a remarkable impact on the chromium remove. By analyzing the plant tissues, we found that Spathiphyllums planted in reactors under 100 mg/kg initial chromium and closed circuit concentration had the heaviest fresh weight and dry weight, meanwhile their chromium concentrations in the tissues were highest. The chromium concentrations of their roots were 554.61 mg/kg, 154.30 mg/kg in stems, and 29.61 mg/kg in leaves. The chromium concentrations of the reactors under the same initial chromium but open circuit concentration were relatively lower, where the roots were 181.02 mg/kg, 65.91 mg/kg in stems, and 16.52 mg/kg in leaves. The ability to accumulate chromium of the PMFC under the closed circuit were remarkable higher than those under the open circuit, thus proving PMFC could promote the chromium absorption of plants.The experiment results indicated Spathiphyllums could enrich chromium in the soil, and they were beneficial to improve the soil quality. The present of roots exudates could enhance the electricity generation and accelerate the reduction of Cr(Ⅵ) to Cr(III), besides a stable energy supply was obtained at the same time.