Comparison and Evaluation of Experimental Studies on Dissolution of Minerals by Organic Acids

Summary

Numerous experiments have been conducted to study the interactions of organic acids and minerals in weathering and diagenetic processes. Most work has focused on feldspar dissolution by water-soluble organic acids. It has been proposed that simple carboxylic acids (e.g., acetic and oxalic) may complex with aluminum and increase the solubility and dissolution kinetics of feldspar. This would provide a mechanism for mobilizing aluminum and creating secondary porosity. In addition, dissolved organic species may act as proton donors and pore-fluid pH buffers.

Feldspar and clay dissolution experiments conducted at elevated temperatures (70–100°C) are reviewed in this chapter. The data indicate that the aluminum content of carboxylic acid solutions is a complex function of temperature, pH, and concentration and type of organic acid anion. There is considerable scatter in the data reflecting different initial solids, fluids, experimental systems, and procedures. However, most of the data indicate that carboxylic acids do enhance the solubility of feldspar, especially at moderately acidic conditions (pH ≈ 4–5). Solutions containing difunctional acids (e.g., oxalic) are more effective at dissolving aluminum than solutions with monofunctional acids (e.g., acetic) and, in general, the aluminum content of the reacted solutions varies inversely with pH and directly with the concentration of the organic acid anion. For example, at 100°C with buffered pH ≈ 4.5–4.7, aluminum reached steady-state concentrations as high as 80ppm in oxalic acid (≈1000 ppm oxalate) and 14ppm in acetic acid (≈4000ppm acetate).

Experimental data also indicate that feldspar dissolution rates increase significantly in oxalate and acetate solutions over that observed for water or CO₂-water solutions. In general, dissolution rates vary inversely with pH and directly with oxalate concentration.

The combination of enhanced solubility and increased dissolution rates indicates that acetate and oxalate may play major roles in mineral dissolution and secondary porosity development in sandstones. In addition, the acetate-acetic acid system appears capable of effectively buffering fluid pH over a time scale of months, even at high mineral surface area/fluid volume. The effectiveness of the buffering and dissolution processes will depend on many factors including type and availability of organic acids, pH, temperature, flow rate, mineralogic composition, and associated reaction kinetics.