Sulfate sorption on rape (Brassica campestris L.) straw biochar, loess soil and a biochar-soil mixture

The effects of biochar amendment on sulfur behavior in soils are unknown. In this paper, sulfate (SO₄^2-) sorption on rape (Brassica campestris L.) straw biochar produced at 600 °C (BC), loess soil (soil) and a 1:9 (w/w) biochar-soil mixture (BC-soil) was investigated by batch experiments. The effects of contact time, initial SO₄^2- concentration, temperature and solution pH value on sorption were tested. Kinetics, isotherms and thermodynamics for sorption were investigated. Pre- and post-sorption characterizations of BC and soil were respectively studied using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectroscopy, respectively. It has been shown that SO₄^2- sorption on three sorbents was well described by pseudo-second-order kinetic model. The sorption isotherms could be fitted using Langmuir and Freundlich models. BC amendment did not increase the sorption capacity of soil for SO₄^2-. The values of ΔG⁰, ΔH⁰ and ΔS⁰ indicated that the nature of sorption was spontaneous, endothermic and feasible. Increasing solution pH value led to a slight reduction in the sorption amount of SO₄^2-. Sulfate was mainly sorbed onto BC through electrostatic interaction, whereas onto the soil via electrostatic interaction and formation of poorly soluble CaSO₄.