Tetrabromobisphenol A transformation by biochar supported post-sulfidated nanoscale zero-valent iron: Mechanistic insights from shell control and solvent kinetic isotope effects.

Post-sulfidated nanoscale zero-valent iron with a controlled FeS shell thickness deposited on biochar (S-nZVI/BC) was synthesized to degrade tetrabromobisphenol A (TBBPA). Detailed characterizations revealed that the increasing sulfidation degree altered shell thickness/morphology, S content/speciation/distribution, hydrophobicity, and electron transfer capacity. Meanwhile, the BC improved electron transfer capacity and hydrophobicity and inhibited the surface oxidation of S-nZVI. These properties endowed S-nZVI/BC with highly reactive (∼8.9-13.2 times) and selective (∼58.4-228.9 times) over nZVI/BC in TBBPA transformation. BC modification improved the reactivity and selectivity of S-nZVI by 1.77 and 1.96 times, respectively. The difference of S-nZVI/BC in reactivity was related to hydrophobicity and electron transfer, particularly FeS shell thickness and morphology. Optimal shell thickness of ∼32 nm allowed the maximum association between Fe core and exterior FeS, resulting in superior reactivity. A thicker shell with abundant networks increased the roughness but decreased the surface area and electron transfer. The higher [S/Fe] and [S/Fe] were conducive to the selectivity, and [S/Fe] was more influential than [S/Fe] on selectivity upon similar hydrophobicity. The solvent kinetic isotope effects (SKIEs) exhibited that increasing [S/Fe] tuned the relative contributions of atomic H and electron in TBBPA debromination but failed to alter the dominant debromination pathway (i.e., direct electron transfer) in (S)-nZVI/BC systems. Mechanism of electron transfer rather than atomic H contributed to higher selectivity. This work demonstrated that S-nZVI/BC was a prospective material for the remediation of TBBPA-contaminated groundwater.