Enhanced Adsorption and Biomineralization of Cadmium and Arsenic in Irrigation Water by Biological Soil Crusts: The Key Roles of Iron/Manganese and Urea

Heavy metal pollution has become increasingly severe, with distinctive physiological characteristics of rice leading to significant accumulation of arsenic (As) and cadmium (Cd) in grains, posing serious health risks. Biological soil crusts (BSC) are common in paddy soils and exhibit a strong capacity to bind trace heavy metals. This study investigated the effects of exogenous iron (Fe)/manganese (Mn) and urea on the effectiveness of BSC (20 mg L−1) in removing As (2 mg L−1) and Cd (100 μg L−1) and analyzed the heavy metal distribution. Fe/Mn addition increased As adsorption by BSC from 51.2% to 83.0% but reduced Cd adsorption from 73.2% to 50.3%, whereas urea inhibited As uptake but enhanced Cd capture. Under co-contamination, the As removal ability of the BSC remained unchanged, but Cd removal improved. As was primarily present in the non-EDTA exchangeable fraction (79.0%), which increased to 96.4% and 85.8% in the presence of Fe/Mn, and urea, respectively. Cd was mainly in the sorbed fraction (51.6%), which increased to 61.0% with urea. These results confirm that BSC exhibits a strong ability to adsorb As and Cd under irrigated water with combined As and Cd contamination, iron/manganese and urea can also enhance this ability. The application of exogenous Fe/Mn providing the raw material for the mineralization process and the presence of urea enhancing the biological activity of the colonies. This study provides an eco-friendly strategy for remediating As and Cd in paddy fields.