Reutilization of pyrite-rich alkaline leaching tailings as sorbent must consider the interplay of sorption and desorption

Sustainable schemes favor the reutilization of waste over direct disposal. In the case of mine waste from mineral processing (tailings), the reutilization of such waste as sorbents of contaminants in water can be advantageous, considering that tailings have already a plan for disposal. In this article, a methodology to evaluate the convenience of using residual minerals as sorbents is presented. The sorption ability of pyrite-based tailings using synthetic solutions containing copper ions (Cu⁺²) is followed and contrasted to the sorption ability of minerals before processing (feed). In the case of as-received samples, tailings show a higher capacity for copper removal in batch tests (1.3 vs. 0.4 mg Cu/g for tailings and feed, respectively). This large difference is associated with (a) the smaller particle size distribution for tailings, and (b) the ability of tailings to increase the pH and favor precipitation of metals. Interestingly, tailings were found also to release smaller amounts of undesired elements such as arsenic and cadmium. Tests using defined particle sizes for feed and tailings demonstrate that copper removal was around 0.45 mg Cu/g in batch experiments and 0.68 mg Cu/g in recirculation experiments. In all cases, copper sorption follows pseudo-second-order kinetics, suggesting a mechanism where copper is chemisorbed, forming copper sulfides according to X-ray photoelectron spectroscopy (XPS) measurements. More importantly, sorption experiments were complemented with simultaneous measurements of metal ion release from the minerals into solution. The metals released during batch sorption are mainly Pb and Zn, in the respective amounts of 0.79 and 0.32 mg/g (feed) and 0.75 and 2.19 mg/g (tailing). Thus, even though the use of mineral residues as sorbents is possible, the metal release should be considered to determine the overall convenience of the process.