Nanoscale Fe(0) (nFe(0)) can detoxify Cr(VI)-bearing wastewater and groundwater, but rapid passivation is a negative factor for large-scale remediation applications. In this study, a magnetic FeS@Fe(0) hybrid material was fabricated by immobilization of iron sulfide (FeS) onto Fe(0) particles to improve the Cr(VI) removal capacity. The solid characterization confirmed that Fe(0) particles were encapsulated by amorphous iron monosulfide. The Cr(VI) uptake by FeS@Fe(0) hybrid particles was found to follow pseudo-second-order rate kinetics, and the Langmuir isotherm was most appropriate to describe Cr(VI) sorption. Meanwhile, the FeS@Fe(0) hybrid particles showed a much higher efficiency towards Cr(VI) sequestration compared to individual nFe(0). Moreover, the results of batch experiments with various adsorbent doses indicated that the reactivity of FeS@Fe(0) varies with different FeS-to-Fe(0) molar ratios. The reaction rate constants for Cr(VI) removal first increased with an increasing FeS-to-Fe(0) ratio from 0/1 to 1/9, and then decreased for the FeS-to-Fe(0) ratio increased further 1/5 or 1/3. For environmental parameters, there was a negative effect of increasing the solution pH and dissolved oxygen on Cr(VI) removal. Furthermore, a mechanistic analysis revealed that Cr(VI) reduction occurred predominantly at the solid-liquid interface, and that Fe(II) regenerated from FeS@Fe(0) corrosion may account for 52% of the Cr(VI) reduction, while electrons from Fe(0) and FeS account for the rest. After treatment, Cr(VI) was completely transformed and immobilized as solid Fe-Cr hydroxide precipitates, thus avoiding secondary contamination. The FeS@Fe(0) hybrid material has a better potential for treating Cr(VI)-bearing wastewater than nano Fe(0).

Advanced oxidation processes (AOPs) based on peroxydisulfate (PDS) or peroxymonosulfate (PMS) activation have attracted much research attention in the last decade for the degradation of recalcitrant organic contaminants. Sulfate (SO₄^•−) and hydroxyl (^•OH) radicals are most frequently generated from catalytic PDS/PMS decomposition by thermal, base, irradiation, transition metals and carbon materials. In addition, increasingly more recent studies have reported the involvement of singlet oxygen (¹O₂) during PDS/PMS-based AOPs. Typically, ¹O₂ can be produced either along with SO₄^•− and ^•OH or discovered as the dominant reactive oxygen species (ROSs) for pollutants degradation. This paper reviews recent advances in ¹O₂ generation during PDS/PMS activation. First, it introduces the basic chemistry of ¹O₂, its oxidation properties and detection methodologies. Furthermore, it elaborates different activation strategies/techniques, including homogeneous and heterogeneous systems, and discusses the possible reaction mechanisms to give an overview of the principle of ¹O₂ production by activating PDS/PMS. Moreover, although ¹O₂ has shown promising features such as high degradation selectivity and anti-interference capability, its production pathways and mechanisms remain controversial in the present literatures. Therefore, this study identifies the research gaps and proposes future perspectives in the aspects of novel catalysts and related mechanisms.