Removal of heavy metals from aqueous solutions by precipitation with sodium di-(n-octyl) phosphinate

Two organophosphorus compounds: sodium di-(n-octyl) phosphinate and sodium di-(n-dodecyl) phosphinate, were synthesized and purified. Sodium di-(n-octyl) phosphinate was used both as a surfactant to form reverse micelles and as a precipitating agent to remove heavy metals from aqueous nitrate and chloride solutions.The heavy metals lead, cadmium, and zinc were precipitated from aqueous solutions with sodium di-(n-octyl) phosphinate (NaL) in the form of PbL 2(s), CdL2(s), and ZnL2(s). The mole ratio of NaL to lead in the feed was varied from 0.1 to 6.5, depending on the acidity of the feed. The effects of the feed pH, concentration of chloride, concentration of calcium, and of the chain length of the precipitating agent on the removal of lead, were investigated. Adding acid to the feed solution reduced the removal of lead as some of the phosphinate precipitated in the acid form as HL(s). The presence of chloride or calcium in the feed solution, up to mole ratios to lead of 250 and 2.75, respectively, had no effect on the removal of lead. The solubility of the precipitate PbL 2(s) was reduced by increasing the length of the alkyl group of the phosphinate. The removal of cadmium, zinc, and a mixture of lead, cadmium and zinc was investigated. Behavior similar to that of lead was observed. The selectivity of the precipitating agent for the three metals was in the order Zn > Pb > Cd.The precipitating agent was completely regenerated by adding NaOH to the precipitate, and then contacted with diethyl ether to extract the reagent. The lead was completely recovered from the PbL2(s). Pure lead-free precipitating agent, and an aqueous solution of lead at a concentration 100 times its concentration in the feed, were obtained.Using measured solubility products of the precipitates and literature values of the stability constants for the other expected reactions, the removal of metal, the loss of precipitating agent, and the equilibrium pH were predicted