Removal of lead 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. Lead was precipitated from aqueous nitrate and chloride solutions with sodium di-(n-octyl) phosphinate (NaL) in the form of PbL2(s). 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 for mole ratios of NaL to lead between 0.1 and 6.5. 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 removal of lead was not affected by the presence of chloride or calcium in the feed solution, up to mole ratios to lead of 250 and 2.75, respectively. The residual lead in solution was reduced by increasing the length of the alkyl group of the phosphinate to dodecyl. Over 99% of the precipitating agent was recovered by adding NaOH to the precipitate, and then contacting the solution with diethyl ether to extract the reagent. The lead was completely recovered from the PbL2(s). 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 expected reactions, the removal of lead, the loss of precipitating agent, and the equilibrium pH were calculated. (C) 2000 Elsevier Science B.V. All rights reserved.