A Study on the Soil Passivation of Nano-Manganese Dioxide-Modified Biochar under High-Arsenic Water Irrigation

[Objective] The irrigation area in northern Henan is an important grain producing area in China. Native high arsenic groundwater exists in the area and has long been used for agricultural irrigation. Increased soil arsenic (As) content under long-term irrigation threatens the quality and safety of crop products. Soil passivation is the use of adding passivators to the soil to fix pollutants to achieve the purpose of limiting their migration. Therefore, the preparation of an efficient and clean passivator and its arsenic fixation effect in soil are important research areas to reduce the risk of high arsenic groundwater. [Method] Firstly, nano-manganese dioxide (MnO2)-modified biochar was prepared via the pyrolysis of sawdust biochar, potassium permanganate and manganese sulfate monohydrate at a mass ratio of 1:0.18:0.29. Secondly, the adsorption characteristics were explored using adsorption kinetics and adsorption isothermal experiments. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) feature mapping and other characterization methods were used to study its physical properties and adsorption mechanism. Finally, a potting experiment was designed to explore the changes in arsenic content in soil when the passivator dosages were 0%, 1% and 5%. [Results] (1) The nano-MnO2 modified biochar could reach the adsorption dynamic equilibrium after 180 min, and its maximum adsorption capacity was 58.12 μg/g. (2) When the dosing ratio was 1%, the fixed efficiency of soil effective As content in potted crops of unplanted crops and planted crops was 4.18–5.51% and 1.99–3.83%. When the dosing ratio was 5%, it was 7.48–8.75% and 5.58–9.58%. [Conclusions] The results show that when the addition ratio is 0–5%, the passivation effect of soil effective As is positively correlated with the passivator dosage.