Dissolution Kinetics of Hornblende in Organic Acid Solutions

Abstract

The dissolution rates of most primary and secondary minerals are promoted by the presence of proton‐ and complex‐forming ligands. Effects on the rates of dissolution of hornblende by naturally occurring organic acids at concentrations commonly encountered in soil solutions and surface waters was studied in a pH 4.0, 0.01 M HOAc–LiOAc buffer. Crushed hornblende samples were purified by magnetic and density separation, followed by an initial period of weathering in a buffered aqueous solution for 39 d. After the initial weathering the rate of dissolution was measured at different concentrations of citric, oxalic, polygalacturonic, tannic, and fulvic acids at pH 4.0 for another 32 d in a batch‐type reactor. Although nearly linear kinetics were observed for the release of Si, the release of Al, Fe, and Mg decreased with time. The dissolution was nonstoichiometric during the study period with preferential release of Al, Fe, and Mg relative to Si. All ligands studied were found to accelerate the dissolution process and the rates increased with solution concentrations of organic acids. The reaction orders with respect to dissolved organic C were generally low, ranging from near zero to 0.37 for different organic acids. At a concentration of 2.5 mM of dissolved C the relative effectiveness of these ligands on promoting dissolution was oxalic > citric > tannic > polygalacturonic > fulvic acid. The strengths of organic acids to promote dissolution is related to the quantity of phenolic and carboxylic functional groups present on the organic compounds, and it is related to the strength of the metal–ligand complex formed in solution or on the mineral surface.