Multifunctional iron-biochar composites for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater

This paper evaluates a novel sorbent for the removal of potentially toxic elements, inherent cations, and heterochloride from hydraulic fracturing wastewater (FWW). A series of iron-biochar (Fe-BC) composites with different Fe/BC impregnation mass ratios (0.5: 1, 1: 1, and 2: 1) were prepared by mixing forestry wood waste-derived BC powder with an aqueous FeCl3 solution and subsequently pyrolyzing them at 1000 degrees C in a N-2-purged tubular furnace. The porosity, surface morphology, crystalline structure, and interfacial chemical behavior of the Fe-BC composites were characterized, revealing that Fe chelated with C-O bonds as C-O-Fe moieties on the BC surface, which were subsequently reduced to a C=C bond and nanoscale zerovalent Fe (nZVI) during pyrolysis. The performance of the Fe-BC composites was evaluated for simultaneous removal of potentially toxic elements (Cu(II), Cr(VI), Zn(II), and As(V)), inherent cations (K, Na, Ca, Mg, Ba, and Sr), hetero-chloride (1,1,2-trichlorethane (1,1,2-TCA)), and total organic carbon (TOC) from high-salinity (233 g L-1 total dissolved solids (TDS)) model FWW. By elucidating the removal mechanisms of different contaminants, we demonstrated that Fe-BC (1: 1) had an optimal reducing/charge-transfer reactivity owing to the homogenous distribution of nZVI with the highest Fe-0/Fe2+ ratio. A lower Fe content in Fe-BC (0.5: 1) resulted in a rapid exhaustion of Fe-0, while a higher Fe content in Fe-BC (2: 1) caused severe aggregation and oxidization of Fe-0, contributing to its complexation/(co-) precipitation with Fe2+/Fe3+. All of the synthesized Fe-BC composites exhibited a high removal capacity for inherent cations (3.2-7.2 g g(-1)) in FWW through bridging with the C-O bonds and cation-p interactions. Overall, this study illustrated the potential efficacy and mechanistic roles of Fe-BC composites for (pre-) treatment of high-salinity and complex FWW.