Nano-TiO2 enhances the adsorption of Cd(II) on biological soil crusts under mildly acidic conditions

Biological soil crusts (BSCs), which are ubiquitous in paddy fields, are known to remove pollutants from paddy fields systems. The Nano-TiO2 enhanced the removal of Cd(II) by BSC under acidic irrigation water was found, and its mechanism was investigated. After the addition of nano-TiO2, the Cd(II) removal efficiency of BSCs increased by 26.70% than that of pure BSCs, and the Nano-TiO2 induced faster removal velocity as well. Zeta potential and potentiometric titration results revealed that BSCs generated more negative charges and sites concentration after addition of Nano-TiO2 at acidic environment. The carboxyl and amino/hydroxyl groups were the main functional groups on BSC and the BSC + TiO2. The higher concentration of amino/hydroxyl groups in BSC + TiO2 (0.33 +/- 0.08 mmol/g) was present than that of pristine BSCs (0.62 +/- 0.02 mmol/g), and they were with similar concentration of phosphate groups and carboxyl groups. This result was attributed to the Nano-TiO2 stimulated the BSCs to produce more extracellular polysaccharides and proteins. Our findings would provide novel strategy for the removal of cadmium from acidic irrigation water.