Adsorption of Hg(II) in solution by mercaptofunctionalized palygorskite

In the past 10 years, the treatment and restoration of soil and water bodies contaminated by mercury and other heavy metals have received unprecedented attention and support from China's environmental protection authorities. The search for low-cost and high-efficiency adsorbents has become one of the research hotspots in this field. In this paper, a simple and environment-friendly method was used to graft 3-mercaptopropyl trimethoxysilane on the surface of palygorskite. The synthesized mercaptofunctionalized palygorskite (M-PAL) was characterized by XRD, FT-IR, BET and SEM-EDS, respectively, and its adsorption conditions, adsorption models and thermodynamic parameters for Hg²⁺ were systematically investigated. The experimental results indicated that the saturated adsorption capacity of Hg²⁺ on the M-PAL could reach 203.4 mg·g^-1, within 120 min at pH 4 and 298 K. By analyzing the experimental data of adsorption kinetics and thermodynamics, it was found that the adsorption process of Hg²⁺ conformed to the pseudo-second-order kinetic model, which belonged to chemical adsorption of the rate-controlled step; the Langmuir model better described the adsorption isotherm. The thermodynamic parameters obtained (ΔH=29.95 kJ·mol^-1, ΔS=103.09 J·mol^-1·K^-1 and ΔG<0) show that the whole process is a spontaneous endothermic process. When the concentration of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, NO₃^-, H₂C₂O₄ and C₆H₈O₇ was 200 times that of Hg²⁺, although these organic acids had a slightly greater effect on the adsorption of Hg²⁺ on mercaptofunctionalized palygorskite than inorganic ions, the adsorption capacity remained above 185 mg·g^-1. The adsorption products could be still stable in simulated acid rain with pH 3, 4, 5, 6, 7 and oxalic acid solution with concentration of 1, 2, 3, 4 and 5 mmol·L^-1, and the desorption rates were about 3%. Through XPS analysis, the specific coordination of Hg²⁺ with the S atom on the surface of M-PAL was confirmed.