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 (nanoFe⁰) composites. However, the effect of BC surface area on the structure, distribution, and performance such as the heavy metal uptake capacity of nanoFe⁰ particles remains unclear. Soybean stover-based BCs with different surface areas (1.7 − 1 472 m²/g) were prepared in this study. They have been used for in-situ synthesis BCs-supported nanoFe⁰ particles through carbothermal reduction of ferrous chloride. The BCs-supported nanoFe⁰ particles were found to be covered with graphene shells and dispersed onto BC surfaces, forming the BC-supported graphene-encapsulated nanoFe⁰ (BC-G@Fe⁰) composite. These graphene shells covering the nanoFe⁰ 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 nanoFe⁰ particles, indicating a higher BC surface area alleviated the aggregation of nanoFe⁰ particles, which resulted in higher heavy metal uptake capacity. At the optimized condition, BC-G@Fe⁰ composite exhibited uptake capacities of 124.4, 121.8, 254.5, and 48.0 mg/g for Cu²⁺, Pb²⁺, Ag⁺, and As³⁺, respectively (pH 5, 25 °C). Moreover, the BC-G@Fe⁰ composite also demonstrated high stability for Cu²⁺ removal from the fixed-bed continuous flow, in which 1 g of BC-G@Fe⁰ can work for 120 h in a 4 mg/L Cu²⁺ flow continually and clean 28.6 L Cu²⁺ contaminated water. Furthermore, the BC-G@Fe⁰ composite can effectively immobilize the bioavailable As³⁺ from the contaminated soil, i.e., 5% (w) of BC-G@Fe⁰ composite addition can immobilize up to 92.2% bioavailable As³⁺ from the contaminated soil.