pyromorphite and malic-acid

Understanding the effects of low molecular weight organic acids (LMWOAs) on the transformation of Pb(II) to geochemically stable pyromorphite (PY) by apatite materials (AMs), has considerable benefits for risk assessment and remediation strategies for contaminated water and soil. In this study, we systematically investigated the immobilization of Pb(II) from aqueous solution by natural phosphate rock (PR) and different crystallized hydroxyapatite (HAp) in the absence and presence of LMWOAs (oxalic, malic and citric acids). The results indicated that the effectiveness of PR and HAp in immobilizing Pb(II) followed in descending order by HAp2 (the poorly crystallized HAp), HAp1 (the well crystallized HAp) and PR, regardlessof the presence of LMWOAs. The presence of malic and citric acids significantly decreased the immobilizationefficiency of Pb(II) by HAp1 and PR, clarifying the lower adsorption affinities of Pb(II)-organic acid complexes on HAp1 and PR rather than Pb(II) ion. On thecontrary, oxalic acid could markedly enhance the removal of Pb(II) from aqueous solution by HAp1 and PR through the formation of lead oxalate, which was confirmed by FT-IR and XRDanalysis. Results also showed that LMWOAs had little promoting or inhibiting effect on the immobilization of Pb(II) by HAp2. This study suggested that the ubiquity of LMWOAs in natural environments could retard the transformation efficiency of Pb(II) to PY by AMs, especiallyin thepresenceof oxalic acid, and the poorly crystallized HAp2 had great potential to remediate Pb(II)-contaminated water and soil due to its insusceptibility to LMWOAs.

Topics: Adsorption; Citric Acid; Crystallization; Durapatite; Environmental Restoration and Remediation; Lead; Malates; Minerals; Molecular Weight; Oxalic Acid; Phosphates; Spectroscopy, Fourier Transform Infrared; Water Pollutants, Chemical; Water Purification; X-Ray Diffraction

The long-term stability of pyromorphite [Pb(5)(PO(4))(3)Cl] (PY) in root-soil interface (or rhizosphere) where production of organic acids from biological activities takes place is not fully understood. We conducted a 1-year long laboratory batch dissolution experiment to elucidate the release of Pb (and P) from PY by four commonly occurring low molecular weight organic acids (LMWOA) in rhizosphere: acetic, citric, malic and oxalic acid. Mean maximum amount of Pb in milliQ (mQ) water (1.8microM) was lower than in solutions from LMWOA alone or in combination with each other (i.e., mixed acid). However, there was no significant difference in the amount of Pb (and P) in solution in all treatments including mQ water after 6months. Among the 100microM LMWOA, mean of five highest soluble Pb (muM Pb in solution) followed the order: oxalic acid (17.6)>citric (6.2)>malic (5.6)>acetic acid (3.0microM Pb). Mixed acid solution had a maximum amount of 14.0microM Pb. We calculated a range of solubility product constant (K(sp)) of PY in this study from 8.6x10(-54) (mQ) to 7.0x10(-45)(oxalic acid); these values are within the range of PY K(sp) reported elsewhere. Despite the low K(sp) values, LMWOA-induced released Pb from PY are in concentrations higher than both Canadian and international drinking water and agricultural water use quality standards. This suggests that soil organic acids such as in rhizosphere can potentially liberate Pb from PY in contaminated soils.

Topics: Acetic Acid; Carboxylic Acids; Citric Acid; Lead; Malates; Minerals; Oxalic Acid; Phosphates; Soil Pollutants