Study on stabilization of mercury in wastewater and soil by modified coal-based humic acid residue

Coal-based humic acid residue (HAS), as a waste generated during humic acid production, has been gaining attention in recent years due to its adsorption capacity and containing nutrients. In this study, to improve the adsorption capacity of HAS for and Hg, phosphate-modified materials (N-HAS) were prepared by a hydrothermal method and HAS and N-HAS were characterized by scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), X-ray fluorescence (XRF); batch adsorption experiments investigated the adsorption capacity of N-HAS on Hg²⁺ under different pH, and isothermal adsorption model and kinetic model fitted the adsorption process to explore the adsorption mechanism; the effects of various amounts of N-HAS on the Hg content in maize and the effective Hg in the soil under Hg-contaminated soil were investigated by field trial. The results showed that the pseudo-second-order kinetic model (R² = 0.9641) and Langmuir isothermal (R² = 0.95) adsorption model could better describe the adsorption behavior of N-HAR on Hg²⁺, the maximum adsorption amount was 124.20 mg/g, and the time to reach adsorption equilibrium was shorter for N-HAS compared to HAS; after 5 times of adsorption-desorption, the removal rate of Hg²⁺ by N-HAS was still higher than 80%, with stable cyclic adsorption performance; the adsorption mechanism of Hg²⁺ by N-HAS included physical adsorption, precipitation, and surface complexation, and compared with HAS, the percentage of complexation for Hg²⁺ adsorption by N-HAS increased by 77.81%, and the percentage of precipitation increased by 7.68%; compared to the control group, it was shown that the addition of N-HAS significantly decreased (p < 0.05) the Hg content of maize kernels by 27.44%-72.09%, increased biomass by 4.34%-11.26%, and decreased the available Hg content in soil by 50.00%-82.80%. In addition, the addition of N-HAS at 0.4 kg/m² was optimal for the field trial. The study showed that N-HAS not only achieved resource utilization but also demonstrated great potential for treating Hg²⁺ in water and soil.