Spatial dynamics of pH in the rhizosphere of Leersia hexandra Swartz at different chromium exposure.

The roots of hyperaccumulators can significantly alter soil pH and thus change the chromium (Cr) availability in the rhizosphere. The pH dynamics in the rhizosphere of Cr hyperaccumulator Leersia hexandra Swartz remains unknown. In this study, the spatial dynamics of pH in the rhizosphere of L. hexandra at different Cr exposure were examined using planar optode (PO). The effects of different Cr concentrations on the biomass, physiological parameters, and soil enzyme activity were investigated. The results showed that pH in the rhizosphere of L. hexandra was highly heterogeneous and followed the root shape. There were obvious soil acidification in all groups and the average pH values in the control, Cr50, and Cr100 groups decreased by 0.26, 0.27, and 0.35 pH unit, respectively. At a certain concentration (50 mg kg-1), Cr significantly increased the plant height and biomass of L. hexandra compared to the control (p < 0.05). The concentrations of chlorophyll a, chlorophyll b, and total chlorophyll in the leaves increased with increasing Cr concentrations. The acid phosphatase, urease, and catalase activities in the rhizosphere were higher than those in the bulk soil. These results provide new insights into elucidating the hyperaccumulating mechanism of Cr and improving the phytoremediation efficiency.

Feasibility study on applying the iron-activated persulfate system as a pre-treatment process for clofibric acid selective degradation in municipal wastewater.

Clofibric acid (CFA) was selected as an example of the widespread micropollutants in municipal wastewater to investigate the feasibility of the application of an iron-activated persulfate (Fe-PS) system for selective micropollutants removal prior to biological wastewater treatment. In pure CFA solution, the CFA degradation rate was accelerated with an increase in oxidant dosage and 2.15 mg·L-1 (0.01 mM) CFA could be completed removed within 30 min with 270 mg·L-1 (1 mM) potassium persulfate (PS) activated by 56 mg·L-1 iron powder (Fe). Although both sulfate radicals (SO4∙-) and hydroxyl radicals (HO∙) were generated in the Fe-PS system, SO4∙- was identified as the dominant oxidant for CFA degradation. To investigate the interference from model compounds in the municipal wastewater, CFA degradation in different concentrations of ammonia or/and glucose solutions, the synthetic municipal wastewater, and real municipal wastewater systems were investigated. A complete removal of CFA was achieved with ammonia or/and glucose interferences. Less than 3% ammonia was removed due to the formation of aminopropyl radicals. About 15% degradation of dissolved organic carbon (DOC) was mainly attributed to the oxidation of glucose by HO∙, Indicating the excellent selective oxidation ability of the Fe-PS system targeting at CFA over glucose. Even though the alkalinity significantly hindered the oxidation of CFA in both synthetic and real municipal wastewater system, the removal efficiency of CFA was significantly higher than that of DOC. The decrease of CFA removal efficiency in municipal wastewater system comparing to the other tests was due to the slow degradation of PS in the system and further hindered the SO4∙- generation. Therefore, the impacts of other impurities in municipal wastewater on the oxidation activities of Fe-PS system should be further investigated. In general, this study confirmed the feasibility of using the Fe-PS system for selective degrading resistant CFA in municipal wastewater.