A novel manganese sulfide encapsulating biochar-dispersed zero-valent iron composite for high removal ability of Cr(VI) in water and its mechanism

The nanoscale Zero-Valent Iron (nZVI) is vulnerable to being oxidized and aggregated, which weakens its reactivity and hampers application potential, manganese sulfide (MnS) modification and biochar supporter could provide a new possibility for this issue. In this work, the nZVI was firstly anchored onto bagasse biochar and then vulcanized by MnS (MnS-nZVI@BC) to achieve low aggregation and preeminent removal capacity. The flaky nZVI was uniformly in-situ growth on the biochar's surface to construct flower-like gullies, of which the surface was covered with some amorphous MnS layer. The monolayer chemical reaction dominated the Cr(VI) removal process, and higher temperature benefited the redox reaction to enhance the prominent removal capacity at 405.10 mg/g at 318.15 K, and it was slightly promoted with the coexisting ions. The quadruplicity of redox, electrostatic attraction, adsorption, and co-deposition should be taken into consideration in the mechanism, in which MnS could not only enhance the reducibility from dominant contributor nZVI, but also directly participate in the redox reaction; and Fe(II) also played an important role in the process; whereafter, Fe(III) and Cr(III) form Cr(III)/Fe(III) hydroxide deposited on the surface of the material. Overall, the findings exhibited the great potential of MnS-nZVI@BC in wastewater treatment while also providing an alternative basis to reveal mechanisms involving versatile aspects.