Removal of lead, cadmium and zinc from aqueous solutions by precipitation with sodium di-(n-octyl) phosphinate

Sodium di-(n-octyl) phosphinate (NaL) was used as a precipitating agent to remove heavy metals from aqueous nitrate solutions. Cadmium, zinc and mixtu res of lead, cadmium and zinc were precipitated in the form of PbL2(s), CdL 2(s), and ZnL2(s). Lowering the pH of the feed solution reduced the removal of the metals as some of the phosphinate precipitated in the acid form as HL(s). The removal of lead, cadmium, and zinc, from a solution containing t he three metals gave a selectivity in the order Zn > Pb > Cd. Predictions o f an equilibrium-constant model, using measured solubility products of the precipitates and literature values of stability constants, gave metal remov als, loss of precipitating agent, and equilibrium pH in good agreement with measured values.