Sewage-sludge derived activated carbon impregnated with polysulfide-sulfidated nZVI:A promising material for Cr(VI) reductive stabilization

This study aims to examine the reductive performance of a sulfidated iron-carbon composite synthesized using a sewage sludge from a typical biological wastewater treatment process. Sewage sludge was first carbonized under optimal conditions to prepare activated carbon (AC), followed by sequential treatment with nZVI and polysulfide to synthesize the iron-carbon composite S-nZVI/AC. Polysulfide was used to sulfurize nZVI due to its unique properties in stability, persistence and safety. Boehm titration and BET test showed that AC carried both acidic and basic sites in concentrations of 1.04 and 0.81 mmol/g per gram of material. The unmodified AC served mainly as a carrier to disperse nZVI grains from aggregation, although it also possessed a limited capacity of 3.5 mg/g for Cr(VI) sorption at neutral pH. Functionalized AC with nZVI boosted its capacity to about 29 times, while further treatment with polysulfide could further expand the material's capacity by 36% to 136.0 mg/g. Sulfurizing the material with polysulfide also raised its reactivity as revealed by the kinetic experiments. Material characterizations suggested that multiple reductive species from S-nZVI/AC, including S(-II) and Fe(II) dissociated from FeS, S-x(2-), and Fe0, might have participated in immobilizing Cr(VI). Specifically, S(-II) and Sx2-were oxidized to SO42-, S2O32-, and S-0, in decreasing order of concentration as inferred from XPS analysis.