Unraveling biochar surface area on structure and heavy metal removal performances of carbothermal reduced nanoscale zero-valent iron

Carbothermal reduction using biochar (BC) is a green and effective method of synthesizing BC-supported nanoscale zero-valent iron (nanoFe 0 ) composites. However, the effect of BC surface area on the structure, distribution, and performance such as the heavy metal uptake capacity of nanoFe 0 particles remains unclear. Soybean stover-based BCs with different surface areas (1.7 − 1 472 m 2 /g) were prepared in this study. They have been used for in-situ synthesis BCs-supported nanoFe 0 particles through carbothermal reduction of ferrous chloride. The BCs-supported nanoFe 0 particles were found to be covered with graphene shells and dispersed onto BC surfaces, forming the BC-supported graphene-encapsulated nanoFe 0 (BC-G@Fe 0 ) composite. These graphene shells covering the nanoFe 0 particles were formed because of gaseous carbon evolved from biomass carbonization reacting with iron oxides/iron salts. Increasing BC surface area decreased the average diameters of nanoFe 0 particles, indicating a higher BC surface area alleviated the aggregation of nanoFe 0 particles, which resulted in higher heavy metal uptake capacity. At the optimized condition, BC-G@Fe 0 composite exhibited uptake capacities of 124.4, 121.8, 254.5, and 48.0 mg/g for Cu 2+, Pb 2+, Ag +, and As 3+, respectively (pH 5, 25 °C). Moreover, the BC-G@Fe 0 composite also demonstrated high stability for Cu 2+ removal from the fixed-bed continuous flow, in which 1 g of BC-G@Fe 0 can work for 120 h in a 4 mg/L Cu 2+ flow continually and clean 28.6 L Cu 2+ contaminated water. Furthermore, the BC-G@Fe 0 composite can effectively immobilize the bioavailable As 3+ from the contaminated soil, i.e., 5% ( w ) of BC-G@Fe 0 composite addition can immobilize up to 92.2% bioavailable As 3+ from the contaminated soil.