Sulfur-functional group tunning on biochar through sodium thiosulfate modified molten salt process for efficient heavy metal adsorption

Sodium thiosulfate (Na2S2O3) was firstly adopted as a green and low toxic additive in molten salt medium to produce S enriched biochar (STBC) for the effectively heavy metal adsorption from aqueous system. The decomposition of Na2S2O3 could create abundant porous structures, introducing rich S surface motifs and persevering amounts of mineral species in the as-produced biochar. Batch adsorption tests with sorption kinetics and isotherms analyses prove that the STBC demonstrates the maximum adsorption capacity (Qm) of 421.8 and 185.0 mg/g for Pb(II) and Cu(II), which are much enhanced than the unmodified biochar and the biochars prepared with other sulfur-containing additives (Na2S, Na2SO3 or Na2SO4). In addition, the column tests indicates that STBC also shows great efficiency to remove multiple heavy metals from the simulated wastewater. Detailed mechanism measurements and DFT calculation indicate the enhanced sorption performance mainly derived from the chemical adsorption due to the strong binding effects of the sulfurous functional groups, and joint effects of physical adsorption and ion exchange. Thanks to the green and facile producing procedure, this method could potentially serve as a versatile strategy to yield biochars with high adsorption performances in heavy metal ion removal.