Pyrolysis of different biomass feedstocks impregnated with Mohr’s salt to prepare ferrous sulfide-loaded nitrogen-doped biochar composites for sequestration of aqueous Cr(VI) ions

In this study, nitrogen-doped biochar-loaded FeS nanoparticles (FeS/NBCs) were prepared by directly pyrolyzing biomass feedstocks pretreated by Mohr’s salt ((Fe(NH4)2(SO4)2∙6H2O). The effects of major experimental conditions such as pyrolysis temperature, impregnation ratio, and biomass types on pyrolysis products were investigated. Under the conditions of a pyrolysis temperature of 700 °C and a mass ratio of 1:1, the three biomass feedstockss of coffee shells (CS), jute fibers (JF), and birch sawdust (BS) can be pyrolyzed to obtain FeS/NBCs. The spectral and textural properties of FeS/NBC were characterized by X-ray powder diffraction, Raman scattering, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy scan electron microscopy, transmission electron microscopy and N2 adsorption-desorption technologies. FeS/NBCs were assessed to remove aqueous Cr(VI) ions in a batch model. The results showed that the adsorption of Cr(VI) onto FeS/NBCs obeyed pseudo second-order kinetics and Langmuir isotherm. The theoretical adsorption capacities were estimated to be 204.5, 191.9 and 169.2 mg∙g−1 for FeS/NBC(CS), FeS/NBC(JF) and FeS/NBC(BS) at the intrinsic pH of Cr(VI) solution, respectively. The synergy between surface adsorption and FeS reduction under different pH gave FeS/NBCs outstanding activity to eliminate Cr(VI). This work provides a facile and low-cost strategy to fabricate nitrogen-doped biochar-loaded FeS composites for the treatment of Cr(VI)-contaminated water.