Dissolution of Granitic Sand by pH-Buffered Carboxylic Acids: A Flow-Through Experimental Study at 100 degrees C and 345 Bars (1)

Abstract

Experiments were performed to investigate the effects of monofunctional and difunctional carboxylic acids on feldspar dissolution and aluminum mobility under conditions which approximate diagenesis, including low temperature, high mineral surface area/fluid volume, and open-system conditions. Granitic sand (63% feldspar) was reacted at 100 degrees C and 345 bars with distilled water, pH-buffered oxalic acid, and pH-buffered acetic acid in a flow-through system at various flow rates. At essentially constant pH, the concentrations of Al, SiO[2], and other components are enhanced in acetate-bearing and oxalate-bearing pore fluids. For example, steady-state concentrations of Al reached 80 ppm in buffered oxalic acid (approximately 1000 ppm oxalate) and 14 ppm buffered acetic acid (approximately 4000 ppm acetate). These concentrations are higher by factors of 800 and 130, respectively, than previously calculated values for kaolinite solubility at the same pH in the absence of organics. In general, Al and SiO[2] in the pore fluid vary directly with oxalate and inversely with flow rate. Elevated metal concentrations are apparently due to formation of organic anion complexes and/or increased reaction kinetics. Al/Si, Al/K, and Al/Na molar values in solution exceed stoichiometric ratios for feldspar. Al-favored ratios seem to be due to incongruent dissolution of feldspars by preferential formation of Al-oxalate complexes. Geologically reasonable concentrations of both monofunctional and difunctional carboxylic acids can enhance substantially the solubility of Al under moderately acidic conditions, although oxalate has a much greater effect than acetate. The acetate system may buffer the pH in formation waters and allow rapid dissolution of aluminosilicates and creation of secondary porosity through reaction with carboxylic acid anions. The effectiveness of the buffering and dissolution processes depends on many factors, including type and availability of organic acids, pH, temperature, flow rate, mineralogic composition and associated reaction kinetics.