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 BCsupported 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(1.7−1472 m^(2)/g)were prepared in this study.They have been used for in-situ synthesis BCs-supported nanoFe^(0)particlesthrough 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℃).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.