Preparation of a FeS/FeSn carbon cryogel for efficient removal of Cr(vi): adsorption performance and mechanism

Abstract

In the present study, sodium alginate (SA) was employed as the precursor and cross-linked with Fe³⁺ ions to form an SA–Fe³⁺ hydrogel. In an alkaline Na₂S solution, the prepared hydrogel gradually released Fe³⁺ ions, which subsequently reacted with S²⁻ ions to produce a uniformly dispersed alginate-FeS/S hybrid sol (denoted as SA-FeS/S). The SA-FeS/S sol was then converted into an FeS/FeS_n carbon cryogel (BC@FeS/FeS_n-T, where BC stands for biochar) through sequential freeze-drying and pyrolysis. The microstructure and composition of the resulting BC@FeS/FeS_n-T cryogel were systematically characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The performance for the adsorption of Cr(VI) and the corresponding reaction mechanism of BC@FeS/FeS_n-700 were thoroughly investigated. Moreover, BC@FeS/FeS_n-700 exhibited superior Cr(VI) removal efficiency over a wide pH range, making it a highly promising candidate for remediating Cr(VI)-contaminated soils. The adsorption process proceeded via five distinct pathways: (i) Cr(VI)'s reduction by Fe²⁺ and HS⁻ species released from FeS; (ii) direct electron transfer from FeS; (iii) redox reactions with various iron sulfides (FeS₂, Fe₃S₄, and Fe_1−xS); (iv) reduction by sulfite ions; and (v) surface adsorption onto biochar or metal oxides.