Evaluation of arsenic pollution in field-contaminated soil at the soil's actual pH

The pollution and ecological risks posed by arsenic (As) entering the soil are the major environmental challenges faced by human beings. Soil phosphatase can serve as a useful indicator for assessing As contamination under specific soil pH conditions. However, the study of phosphatase kinetics in long-term field As-contaminated soil remains unclear, presenting a significant obstacle to the monitoring and evaluation of As pollution and toxicity. The purpose of this study was to determine phosphatase activity and explore enzyme kinetics in soils subjected to long-term field As contamination. Results revealed that the soil phosphatase activity varied among the tested soil samples, depending on the concentrations of As. The relationship between total As, As fractions and phosphatase activity was found to be significant through negative exponential function fitting. Kinetic parameters, including maximum reaction velocity (V_max), Michaelis constant (K_m) and catalytic efficiency (V_max/K_m), ranged from 3.14 × 10^-2-53.88 × 10^-2 mmol/(L·hr), 0.61-7.92 mmol/L, and 0.46 × 10^-2-11.20 × 10^-2 hr^-1, respectively. V_max and V_max/K_m of phosphatase decreased with increasing As pollution, while K_m was less affected. Interestingly, V_max/K_m showed a significant negative correlation with all As fractions and total As. The ecological doses (ED₁₀) for the complete inhibition and partial inhibition models ranged from 0.22-70.33 mg/kg and 0.001-55.27 mg/kg, respectively, indicating that V_max/K_m can be used as an index for assessing As pollution in field-contaminated soil. This study demonstrated that the phosphatase kinetics parameters in the soil's pH system were better indicators than the optimal pH for evaluating the field ecotoxicity of As.