Biogenic dissolution of a soil cerium-phosphate mineral

The productivity of many terrestrial ecosystems is controlled or limited by phosphorus bioavailability. Within these ecosystems, nearly all of the bioavailable phosphate is ultimately derived via the weathering of apatite [Ca₅(PO₄)₃ (OH,F,Cl)]. Highly insoluble lanthanide phosphate minerals form during apatite weathering and are important secondary phosphorus repositories in soils. Prior studies indicate that these phases can be dissolved via biologically mediated pathways. However, mechanistic understanding of biotically- and abiotically-mediated dissolution mechanisms is lacking. We tested the impact of biogenic substances ubiquitous in soils, namely oxalate, ascorbate, citrate, and humic acids, as well as the commercial chelating agent EDTA, on the dissolution of rhabdophane [CePO₄ · H₂O], a representative of a large class of soil phosphate minerals. We show that these compounds can facilitate the dissolution of rhabdophane at 3 < pH < 8 and result in the non-stoichiometric release of Ce³⁺ _(aq) and PO₄ ³ ₍a_q). Release of Ce³⁺ ₍a_q) and PO ₄ ³ _(aq) is a function of ligand type oxalate reacted at pH 3, the effectiveness of EDTA surpasses that of any other ligand in releasing Ce³⁺ _(aq) from the CePO₄ · H₂O surface. Speciation calculations are consistent with mineral dissolution through formation of aqueous Ce³⁺ -EDTA complexes. Mineral dissolution in the presence of oxalate at low pH likely involves concomitant proton and ligand attack of the mineral surface. In these experiments, a fast release of Ce³⁺ _(aq) is followed by a sharp decrease in solution Ce³⁺ _(aq) concentration, consistent with precipitation of a Ce phase. In the presence of ligands other than EDTA and oxalate, no accumulation of Ce³⁺ occurred in solution due to the precipitation of CeO_2(s) on the rhabdophane surface. CeO₂ may be responsible for the observed oxidation of ascorbate, as it has been reported for catechol (Cervini-Silva and Banfield, 2003). Our results indicate that rhabdophane dissolution is controlled either by strong ligand complexation of Ce³⁺ _(aq) or by sequestration of Ce⁴⁺ ions as CeO_{2 (s)}, effectively increasing the mineral solubility. This work shows that interactions between organics, CePO₄ · H₂O, and CeO_2(s) imply potentially important linkages among the cerium, phosphorus, and organic carbon cycles in soil.