Lead Immobilization and Hydroxamate Ligand Promoted Chloropyromorphite Dissolution

The immobilization of lead, a major environmental contaminant, through phosphate amendments to form the sparingly soluble lead phosphate mineral chloropyromorphite [Pb5(PO4)3] (CPY) is an effective in situ strategy for soil remediation. An important question is the effect of microbial processes on this remediation. Here, we investigate the role of the microbial siderophore ligand desferrioxamine-D1 (DFO-D1) and its analog acetohydroxamic acid (aHA) in CPY lability using pH-dependent batch dissolution kinetics and model calculations. Both (0.01)？M aHA and (0.00024)？M DFO-D1 are similarly effective and enhance lead release from CPY by more than two orders of magnitude at pH > 6 compared to in the absence of ligands. This is consistent with model calculations of pH-dependent (aqueous) complexation of lead with hydroxamate ligands. More importantly, pH-dependent ligand sorption is predictive of its ligand promoted dissolution behavior. Our results suggest that organic ligands can significantly increase CPY lability at alkaline pHs in soils and sediments and that addition of P amendments to immobilize Pb as CPY may only be successful at acid pHs. 1. Introduction Lead is a major environmental contaminant in soils, sediments, and surface waters [1] and concentrations greater than 10？μg/dL are toxic to children. Exposure to lead contaminated soils is one of the main pathways of lead toxicity [2]. Lead immobilization through phosphate amendments to form the sparingly soluble lead phosphate mineral, chloropyromorphite [Pb5(PO4)3] (CPY), can be an effective in situ strategy to remediate lead in soils [3]. The effectiveness of this strategy depends on the long-term stability of CPY, which may be affected by pH and the presence of organic ligands. Microbial organic ligands such as trihydroxamate siderophores, secreted by bacteria and fungi, are commonly present in soil environments [4, 5]. Siderophores can chelate Fe(III) with high binding affinities and also trace metals such as Pb(II) [6]. Therefore, hydroxamate siderophores such as desferrioxamine-B (DFO-B) can potentially increase the lability of CPY. For example, trihydroxamate siderophores [7] and acetohydroxamic acid containing only one hydroxamate moiety [8] have been shown to promote dissolution of a Fe-oxide mineral, goethite. It is therefore important to characterize proton-promoted and organic ligands-promoted CPY dissolution commonly present in terrestrial environments for predicting the mobility of CPY particles in porous media such as soils. Proton-promoted CPY dissolution has been investigated as a