Sustainable removal of Hg(II) by sulfur-modified pine-needle biochar

Abstract Sulfur-modified pine-needle biochar (BC–S) was produced for the removal of Hg(II) in aqueous media via post-pyrolysis S stream exposure. Fourier-transform infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy confirmed the addition of S(0) groups on the surface of BC–S. Hg(II) adsorption on BC–S was best described by the Freundlich isotherm with a KF of 21.0 mg L g−1 and a pseudo-second-order adsorption kinetics model with a rate of 0.35 g mg−1 min−1. Hg(II) removal on BC–S was found to be an endothermic process that relied on C-Hg and S-Hg interactions rather than reduction by S(0) groups. The adsorption increased with increasing solution pH and decreased with increasing dissolved organic matter concentration, but was unaffected by increasing salt concentrations. BC–S showed a maximum of 3 % S leaching in aqueous media after 28-d exposure time, and exposure to aqueous media did not convert Hg(II) to elemental Hg. Overall, BC–S exhibited superior Hg(II) removal performance over unmodified BC, thus having potential applications in natural water and wastewater treatment with no significant threat of secondary pollution.