Enhanced flocculation and sedimentation of trace cadmium from irrigation water using phosphoric fertilizer

Suspended substrate (SS) in natural waters controls the interaction, transportation, and biological effects of heavy metals in water bodies. The large amount of cadmium (Cd) carried by SS is an important source of Cd pollution in irrigation water. In this study, a novel procedure to remove trace Cd coexisting with SS from irrigation water by fertilizer flocculation was investigated. Four common fertilizers (K2HPO4, (NH4)2HPO4, KH2PO4, and K2SO4) were used as flocculants. Batch experiments with various fertilizers revealed that the removal efficiency followed the order: K2HPO4>(NH4)2HPO4>KH2PO4>K2SO4. When levels of K2HPO4 higher than 0.75g/L were applied, the total Cd decreased from 20 to 3.8μg/L after 12h of flocculation, i.e., the removal efficiency reached 80%. The mechanism analysis(Zeta potential, multi-element analysis) results demonstrated that the aggregation of SS and its sedimentation by K2HPO4 were due to the combined effect of the oxidation of Fe2+ to Fe3+ and cation binding behavior towards SS, as a result of charge neutralization and electric double layer compression. This method of Cd removal from irrigation water is simple and has the potential to be applied in agricultural production.