Effect of phosphate on the dissolution of serpentine

Protons, weak acids, and anions present in the soil solution because of biotic activity or because of fertilizer additions may affect the dissolution of minerals, depending on their ability to weaken the metal-oxygen bonds and consequently detach the metal from the surface. Among the anions able to form inner-sphere complexes, phosphate has been shown to be effective in promoting dissolution of oxides and allophane clays, but no information is available about its effect on Mg-silicates, even though the presence of the dissolution products of serpentine in soils may be of concern for plant nutrition. The interaction of phosphate with serpentinite and its effect on the dissolution of the rock in the presence of different electrolytes was studied. A serpentinitic rock was shaken for 24 h with 0.01 M KCl or CaCl2 and increasing concentrations of KH2PO4 (P). The pH and the contents of P, Mg, and Si in the solutions were determined, and the dissolution data compared with those obtained by computer chemical simulations. At low P additions, adsorption predominated both in KCl and in CaCl2, and the ligand exchange reactions buffered the acidity caused by the weak acid dissociation. The release of Mg was not different from that obtained with the electrolyte alone, whereas Si in solution probably derived also from sorbed forms on the serpentine surface. In the presence of CaCl2, however, the Mg concentration was higher, probably because of the presence of exchangeable forms. When the initial P concentration increased, the situation was different with the two electrolytes; adsorption continued with KCl without reaching a plateau, whereas precipitation of Ca-phosphates occurred with CaCl2. In KCl, the ligand exchange reactions were not sufficient to neutralize the protons from acid dissociation, the pH decreased, and the mineral dissolution was enhanced. However, the elemental release in the presence of phosphate was lower than what was expected, probably because of some shielding effect of the active metal sites by the anion. In the presence of CaCl2, instead, the precipitation of insoluble salts enhanced the H+ concentration and consequently the dissolution of serpentine. At the highest P additions, the amounts of elements released into the solution were lower than the calculated ones and independent of P concentration, probably because of the precipitation of Ca phosphates on the mineral surface that might protect serpentine from further dissolution. A different situation was observed for KCl, with which the dissolution increased, probably because of the destabilization of active sites.