Application of mine biological soil crust for cadmium removal in weakly acidic irrigation water: Mechanisms and field performance

Cadmium (Cd) contamination in weakly acidic irrigation water threatens rice safety across southern China’s agricultural ecosystems. Conventional bioremediation exhibits limited efficacy under such acidic conditions. This study comparatively evaluated the Cd removal efficiency of mine biological soil crust (MC) and paddy biological soil crust (PC). Key results demonstrated that at pH 5.5, MC achieved a Cd adsorption capacity of 149.93 mg/g and an adsorption rate constant of 0.17 min⁻¹, exceeding PC values (90.91 mg/g and 0.11 min⁻¹) by 65% and 54%, respectively. These advantages originated from MC’s 10.4-fold higher functional group concentration (4.09 × 10⁻¹ mmol/g vs. PC’s 3.94 × 10⁻² mmol/g) and 4.38-times greater extracellular protein content (6.35 mg/g vs. 1.45 mg/g). The sequence of functional groups binding to Cd on the MC surface differs from that of PC, and MC exhibits superior stress resistance. During chemisorption, proteins dominated Cd adsorption in MC (89% contribution), whereas polysaccharides prevailed in PC (55%). Field validation confirmed that MC reduced Cd concentrations in irrigation water from 15.32 μg/L to 8.53 μg/L and in rice from 0.76 μg/g to 0.47 μg/g over 30 days, while maintaining biological stability. This work establishes MC as an efficient, protein-driven bioremediation strategy for acidic Cd-contaminated paddy systems.