Suppression of the release of arsenic from arsenopyrite by carrier-microencapsulation using Ti-catechol complex.

Arsenopyrite is the most common arsenic-bearing sulfide mineral in nature, and its weathering contributes to acid mine drainage (AMD) formation and the release of toxic arsenic (As). To mitigate this problem, carrier-microencapsulation (CME) using titanium (Ti)-catechol complex (i.e., Ti-based CME) was investigated to passivate arsenopyrite by forming a protective coating. Ti4+ ion dissolved in sulfuric acid and catechol were used to successfully synthesize Ti(IV) tris-catecholate complex, [Ti(Cat)3]2-, which was stable in the pH range of 5-12. Electrochemical studies on the redox properties of this complex indicate that its oxidative decomposition was a one-step, irreversible process. The leaching of As from arsenopyrite was suppressed by CME treatment using the synthesized Ti-catechol complex. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicate that this suppression was primarily due to the formation of an anatase (β-TiO2)-containing coating. Based on these results, a detailed 4-step mechanism to explain the decomposition of [Ti(Cat)3]2- and formation of TiO2 coating in Ti-based CME is proposed: (1) adsorption, (2) partial oxidation-intermediate formation, (3) non electrochemical dissociation, and (4) hydrolysis-precipitation.