Carbothermal reduction synthesis of sulfide-modified biochar for heavy metal removal

Abstract

Although sulfide is effective for heavy metal immobilization, it rarely exists in pristine pyrogenic biochar and easily undergoes intensive hydrolysis. In this work, carbothermal reduction of sulfate for on-site sulfide formation and simultaneous carbon layer encapsulation was developed to synthesize sulfide (CaS) modified biochar for enhancing heavy metal immobilization capacity. The initial reaction temperature for this carbothermal reduction was 700 °C. Increasing the pyrolysis temperature, modifier electron accepting ability and loading content could facilitate carbothermal reduction as further partly confirmed by on-site MS of pyrolysis gas and S 2p XPS of biochar analysis. The formed CaS on biochar could be greatly inhibited from hydrolysis due to the encapsulation effect of carbon layer and reached nearly 100% utilization efficiency in Cd2+ fast immobilization. TEM line scan and XRD of post-adsorbed biochar indicated that high heavy metal immobilization capacity was mainly attributed to the coprecipitation reaction governed by the formation of metal-sulfur bond. Compared with reported absorbents, CaS-modified biochar via carbothermal reduction with on-site encapsulation exhibited an excellent stability and outstanding immobilization capacity for various heavy metal ions (such as Cd2+, Pb2+, Cu2+, Zn2+, Ag+).Graphical