Iron dissolution from fayalitic slag with 3,4-dihydroxybenzoic acid under different redox and alkaline conditions

Various inorganic industrial by-products, such as fayalite slag (FS), have potential to be used as secondary materials, such as cementitious binders in construction or in metal recovery. However, the dissolution processes and chemical stability of these by-products often inhibit their utilization. In particular, their behavior under diverse chemical conditions is complex and not well understood. This study investigates the impact of 3,4-dihydroxybenzoic acid (DHBA) with 1 mM concentration on the redox chemistry and dissolution of FS within the alkaline pH range of 12-14 under anaerobic and aerobic conditions. Experimental results show that DHBA provided ∼4x enhancement for dissolution of iron under low oxygen conditions at pH 14. The redox potential measurements reveal that FS + DHBA at pH 14 exhibits very low and negative potentials at anaerobic conditions, indicating the presence of Fe(II) in solution. Liquid-state ¹H NMR spectroscopy shows formation of soluble complexes with Fe(II)/Fe(III) and DHBA ligand at pH values 13-14. The XPS data demonstrate that DHBA plays a vital role in increasing dissolution by preventing cations from accumulating on the FS surface. These results, therefore, provide valuable information about the complex interactions between FS, DHBA, and environmental conditions, and highlight new potential approaches that could improve the possibilities to valorize industrial by-products.