Gibbsite and goethite solubility: the influence of 2-ketogluconate and citrate

2-Ketogluconate (kG) is a microbial byproduct that has been isolated from the rhizosphere of several plants and has been found to accumulate around bacteria adhering to rock surfaces. It has long been suspected that kG may play a role in enhancing the bioavailability of soil nutrients. However, quantitative information relevant to its behavior in soils, its reactions with mineral surfaces, and its aqueous complexation of metal cations is unavailable. The objectives of this research were to examine the influence of kG on gibbsite and goethite solubility and to describe these effects by considering the aqueous complexation of Al(3+) and Fe(3+) by kG. Secondary to this was to compare the impact of kG on mineral solubility to that of citrate. The equilibrium solubility of gibbsite and goethite was examined at ambient (20-22 degrees C) or controlled (25 degrees C) temperatures as a function of pH, ionic strength, and kG or citrate concentration. Both ligands were observed to significantly enhance mineral solubility (as determined by the total soluble concentrations of Al and Fe), with kG and citrate having similar impacts on gibbsite solubility, but with kG having a lesser impact than citrate on goethite solubility. The solubility data were employed to characterize the aqueous complexation chemistry of Al- and Fe(III)-kG and citrate. In the 4 < pH < 8.5 range, the mononuclear species AlkG(2+)(aq), Al(OH)2kG0(aq), and Al(OH)3kG(-)(aq) describe Al-kG speciation, with the latter two species mechanistically described by the bidentate complexation of AlOH(2+)(aq) and Al(OH)2(+)(aq) [also written as AlOH(H(-1)kG)0(aq) and Al(OH)2(H(-1)kG)-(aq)]. Iron(III)-kG complexation is described by the FekG(2+)(aq) and Fe(OH)3 kG2(2-)(aq) [or FeOH[(H(-1))kG]2(2-)(aq)] species. For the citrate systems, the chemical models required to describe metal speciation differed from those reported in the literature: Alcit0(aq), Al(OH)2cit(2-)(aq), and Al(OH)3 cit(3-)(aq) for Al-citrate speciation and Fecit0(aq), Fe(OH)2 cit(2-)(aq), and Fe(OH)3cit(3-)(aq) for Fe(III)-citrate.