Facile and economic Fe-modification of rice straw biochar for efficient removal of Cr(VI): Mechanistic insights and application in real wastewater

Abstract

To achieve higher removal of Cr(VI) from wastewater by biochar, iron modification have been often considered. However, previous reports suffered from complicated preparation procedures or large consumption of iron, and thus a simpler protocol with less consumption of chemical reagent should be developed. In this study, three different Fe-modified biochars were synthesized by pyrolysis of rice straw after pre-soaking it in three different iron salt solutions (i.e. FeCl3, FeSO4 and (NH4)2Fe(SO4)2), and the performances and mechanisms in Cr(VI) elimination was examined. The Fe-BC produced by FeCl3 with the lowest pH, highest surface charge and most Fe content had the maximum adsorption capacity for Cr(VI) (21.98 mg/g). The removal mechanism revealed by detailed characterizations indicated that the pH-sensitive adsorption was the crucial and primary step followed by the Cr(VI) reduction and Cr(III) immobilization. Reductive Fe species played the main influential role for Cr(VI) reduction in Fe-BC, which could be further facilitated by the graphitic structure of the carbon matrix. The synergistic effects of the carbon matrix and Fe strengthened the Cr(VI) removal, promising the objective of high Cr(VI) removal efficiency even at low Fe dosage. The high removal capacity by Fe-BC in the electroplating wastewater proved its potential application.