Effects of organic ligands on granite dissolution in batch experiments at pH 6

Yosemite granite from the Tuolumne River series was dissolved under oxic and anoxic conditions in the presence (0.001 or 0.01 M) and absence of organic ligands in batch experiments at pH 6. Release of all elements was enhanced considerably in the presence of organic ligands. Citrate (tridentate) and gallate (tetradentate) increased element release to the greatest extent among the aliphatic and aromatic ligands, respectively. The extent of element mobilization observed for the aliphatic ligands decreased in the order citrate > oxalate approximate to malonate and for the aromatic ligands gallate > salicylate approximate to phthalate. The effects of the ligands generally followed trends in cation-ligand stability constants, but aromatic ligands were less effective in element mobilization than aliphatic ligands. This was attributed to protonation of the functional groups of aromatic ligands at pH 6. Rock dissolution was non-stoichiometric regardless of the presence or absence of organic ligands. Release of P and Cu (% of total element content in the unweathered rock) in the presence of organic ligands was considerably greater than that of other elements. Findings of the present study were compared to the findings of our previous study on dissolution of basalt. Differences in the mobility patterns of Y, Fe, Cu, Ti, and Zr during dissolution of granite and basalt were observed. In basalt, release of Y was enhanced considerably in the presence of citrate. In contrast, in the present study, release of Y from granite with and without citrate was at the detection limit. This difference was attributed to the difference in Y-hosting parent minerals: apatite (basalt) and sphene (granite). With and without organic ligands, no significant differences were observed in the extent of Fe released from granite under oxic versus anoxic conditions, while the extent of Fe release from basalt was considerably greater under anoxic versus oxic conditions with and without organic ligands. These differences can be attributed to the difference in major Fe-hosting phases in the rocks: biotite (granite) versus glass and augite (basalt). In the granite, concentrations of Cu released increased considerably in the presence of organic ligands under both oxic and anoxic conditions. In contrast, concentrations of Cu released from basalt were increased considerably only under oxic conditions, especially in the presence of organic ligands. This difference was attributed to the different extent of Fe release from basalt and granite that affected dissolution of Cu/Fe-sulfides. Although both Ti and Zr can be mobilized in the presence of organic ligands, in the rock where zircons hosted Zr (granite), this element was the most immobile, while in the rock where Fe/Ti oxides hosted Zr (basalt), Ti was the most immobile.