[Preparation of Modified Watermelon Biochar and Its Adsorption Properties for Pb(Ⅱ)]

In this study, watermelon rind was used as a raw material to modify watermelon rind biochar (MBC) with ammonium sulphate[(NH₄)₂S] for adsorption of Pb(Ⅱ) ions. The effects of solution pH, adsorption time, adsorbent addition amount, initial mass concentration of Pb(Ⅱ) ions, and ionic strength on the adsorption of Pb(Ⅱ) ions were investigated. The results show that the saturated adsorption time was 5 h, the optimum pH of the adsorption reaction was 6, and when the initial mass concentration of Pb(Ⅱ) ions were 1000 mg·L^-1, and the amount of adsorbent was 2.0 g·L^-1. The maximum adsorption amount of MBC to Pb(Ⅱ) ions can reach 97.63 mg·g^-1, which is significantly higher than unmodified watermelon husk biochar (BC). The adsorption of Pb(Ⅱ) ions by modified watermelon biochar was in accordance with the Langmuir isotherm adsorption model and the pseudo second-order kinetic model, which proves that adsorption is dominated by monolayer chemical adsorption. The desorption of MBC after adsorption of Pb(Ⅱ) ions was carried out using a sodium hydroxide solution to study the reusability of MBC, and the adsorption amount was still 64.74 mg·g^-1 in the sixth cycle. Characterization and analysis of adsorbents by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption, scanning electron microscopy-energy spectroscopy, zeta potential analysis, and X-ray diffraction (XRD) were carried out, which showed that the adsorption mechanism is mainly that MBC oxygen- and MBC sulfur-containing groups adsorb Pb(Ⅱ) through complexation and precipitation. Therefore, ammonium sulfide modified watermelon rind biochar can be used as a highly efficient lead adsorbent.