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Issue 4, 2019
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Oxalate-enhanced solubility of lead (Pb) in the presence of phosphate: pH control on mineral precipitation

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Abstract

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 μM at all the tested pH values, with the minimum solubility of about 0.1 μM measured at the intermediate pH of 6.0. The measured dissolved Pb and free Pb2+ ion activity were not in agreement with predictions from generally-accepted 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.

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

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Publication details

The article was received on 30 Nov 2018, accepted on 17 Mar 2019 and first published on 18 Mar 2019


Article type: Paper
DOI: 10.1039/C8EM00553B
Citation: Environ. Sci.: Processes Impacts, 2019,21, 738-747

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

    M. B. McBride, S. E. Kelch, M. P. Schmidt, S. Sherpa, C. E. Martinez and L. Aristilde, Environ. Sci.: Processes Impacts, 2019, 21, 738
    DOI: 10.1039/C8EM00553B

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