Mechanistic insights into adsorptive and oxidative removal of monochlorobenzene in biochar-supported nanoscale zero-valent iron/persulfate system

Nanoscale zero-valent iron (nZVI) supported on rice stalk (RS) derived biochar composite (nZVI/RS) was synthesized as persulfate (PS) activator, achieving efficient adsorptive and catalytically oxidative removal for monochlorobenzene (MCB). Development of porous structures and enhanced aromaticity of RS with increased pyrolysis temperatures promoted MCB adsorption, with removal efficiencies of MCB rising from 11.2% for nZVI/RS300 to 72.3% for nZVI/RS700 after the adsorption for 14 h. The best PS activation performance was achieved for nZVI/RS500, with MCB removal efficiency being further increased to 98.8% after the oxidation for 3 h and reaction stoichiometric efficiency (RSE) reaching a maximum of 4.1% under conditions of 0.2 g·L−1 nZVI/RS500, 50.0 μM MCB, 2.0 mM PS and pH0 6.5. The pre-adsorption of MCB was a rate-promoting step in the oxidative degradation processes within the nZVI/RS-PS systems under low RS pyrolysis temperatures (<500 °C). However, excessive adsorptive capacity of nZVI/RS600 and nZVI/RS700 induced blocking of the active sites and low PS activation efficiencies. In addition to transformation of nZVI and surface functional groups on RS500, electron transfer from sp2 hybridized carbon enhanced by internal electron migration of sp3-sp2 hybridized carbon was also responsible for PS activation, producing SO4•−, ·OH and 1O2 for MCB degradation through both radical and nonradical pathways. Six degradation intermediates were identified with possible degradation pathways of MCB in the nZVI/RS500-PS system being proposed. Additionally, 80.2% and 67.4% of MCB removal rates were achieved in tap water and groundwater contaminated by MCB respectively. These findings have significant implications for the synthesis and application of metal-carbon composite based multifunctional materials with strong adsorption and catalytic potential for activating PS to completely remove organic contaminants in groundwater.