Metal mobilization from metallurgical wastes by soil organic acids

- Organic acids enhance metallurgical wastes dissolution.
- Artificial root exudates have a higher impact on metal release than humic/fulvic acids.
- Slags susceptibility to weathering depends on their chemistry and mineralogy.

Three types of Cu-slags differing in chemical and mineralogical composition (historical, shaft furnace, and granulated slags) and a matte from a lead recovery process were studied with respect to their susceptibility to release Cu, Zn and Pb upon exposure to organic acids commonly encountered in soil environments. Leaching experiments (24-960 h) were conducted with: i) humic acid (20 mg/L) at pH t0 = 4.4, ii) fulvic acid (20 mg/L) at pH t0 = 4.4, iii) an artificial root exudates (ARE) (17.4 g/L) solution at pH t0 = 4.4, iv) ARE solution at pH t0 = 2.9 and v) ultrapure water (pH t0 = 5.6). The results demonstrated that the ARE contribute the most to the mobilization of metals from all the wastes analyzed, regardless of the initial pH of the solution. For example, up to 14%, 30%, 24% and 5% of Cu is released within 960 h from historical, shaft furnace, granulated slags and lead matte, respectively, when exposed to the artificial root exudates solution (pH 2.9). Humic and fulvic acids were found to have a higher impact on granulated and shaft furnace slags as compared to the ultrapure water control and increased the release of metals by a factor up to 37.5 (Pb) and 20.5 (Cu) for granulated and shaft furnace slags, respectively. Humic and fulvic acids amplified the mobilization of metals by a maximal factor of 13.6 (Pb) and 12.1 (Pb) for historical slag and lead matte, respectively. The studied organic compounds contributed to different release rates of metallic contaminants from individual metallurgical wastes under the conditions tested.