Oxalate-enhanced solubility of lead (Pb) in the presence of phosphate: pH control on mineral precipitation

Here we study the precipitation of lead ( Pb)- phosphate minerals over the pH range of 4.0 to 8.0 with and without oxalate, a ubiquitous and abundant low- molecular- weight organic acid derived from plants and microorganisms in environmental matrices. In the aqueous Pb- phosphate systems, phosphate precipitated Pb efficiently, reducing the dissolved Pb concentration below 1 mM at all the tested pH values, with the minimum solubility of about 0.1 mM measured at the intermediate pH of 6.0. The measured dissolved Pb and free Pb2+ ion activity were not in agreement with predictions from generallyaccepted solubility products of the Pb phosphate minerals, particularly hydroxypyromorphite [ Pb5( PO4) 3OH]. Discrepancies between our measured Pb phosphate solubility products and older reported values are attributed to non- ideal behavior of these minerals ( incongruent dissolution) as well as uncertainties in stability constants for soluble Pb- phosphate ion pairs. The presence of equimolar levels of oxalate and phosphate resulted in up to 250- fold increase in Pb solubility at acidic pH and about a 4- fold increase at pH 7.0, due to the strong suppression of Pb phosphate precipitation by oxalate and formation of soluble Pb- oxalate complexes. At pH 4.0 and 5.0, Fourier transform infrared spectroscopy ( FTIR) and X- ray diffraction ( XRD) identified a Pb- oxalate mineral phase as the only precipitate despite the presence of phosphate; in the absence of oxalate, Pb hydrogen phosphate, PbHPO4, stably formed under these acidic conditions. At pH 6.0 and greater, FTIR and XRD data revealed that Pb- phosphate [ Pb3( PO4) 2], and hydroxypyromorphite [ Pb5( PO4) 3OH] to a lesser extent, were the predominant precipitates both in the absence and presence of oxalate. Therefore, oxalate did not strongly interfere with Pb- phosphate mineral formation at aqueous pH greater than 6.0 but oxalate controlled Pb solubility at acidic pH values.