Removal of Cr(VI) ions by sewage sludge compost biomass from aqueous solutions: Reduction to Cr(III) and biosorption

•A novel method of Cr(VI) removal using natural biomaterials compost was put forward.•99% of Cr(VI) was removed with compost biomass under neutral and alkaline conditions.•Hydroxyl, carboxyl, and amino groups contribute to Cr(VI) adsorption and reduction.•Cr(VI) and Cr(Ⅲ) adsorption fitted the Langmuir and Freundlich models, respectively.•The adsorption process fitted to adsorption-coupled reduction kinetic model.