Promoted iron corrosion and subsequent hexavalent chromium removal in zero-valent iron systems by oxidant activation

Zero-valent iron (ZVI), as an effective medium, is widely used to eliminate heavy metal ions in filter tanks. However, it will react with Cr(VI) to generate Fe-Cr precipitates with low conductivity on its surface, resulting in slow iron corrosion and low Cr(VI) removal efficiency. In this study, three oxidants (KMnO4, NaClO, and Na2S2O8) were employed to promote iron corrosion in ZVI systems for enhanced Cr(VI) removal at a concentration of 5 mg/L through batch tests and column experiments. The ZVI/KMnO4, ZVI/NaClO, and ZVI/Na2S2O8 systems achieved significantly higher Cr(VI) removal rates of 31.5%, 52.8%, and 65.9% than the ZVI system (9.8%). Solid phase characterization confirmed that these improvements were attributed to promoted iron corrosion and secondary mineral formation (e.g., lepidocrocite, ferrihydrite, and magnetite) by oxidants. Those minerals offered more reaction sites for Cr(VI) reduction, adsorption, and sequestration. Cycle experiments indicated that ZVI/oxidant systems could stably remove Cr(VI). In long-term column experiment, the ZVI/NaClO column showed a much longer life-span and exhibited a 34.8 times higher Cr(VI) removal capacity than that of the ZVI column. These findings demonstrated that ZVI in combination with a reasonable amount of oxidants was a promising method for removing Cr(VI) in practical filter tanks and provided a new insight to enhance Cr(VI) removal.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.