Efficient stabilization of arsenic in the arsenic-bearing lime-ferrate sludge by zero valent iron-enhanced hydrothermal treatment

Arsenic-bearing lime-ferrate sludge (ABLFS) is a common hazardous waste from nonferrous metallurgical industry. The leaching of arsenic from ABLFS during waste storage and disposal causes serious environmental pollution. In this study, a new method was developed to effectively stabilize arsenic in ABLFS by treating ABLFS with combined zero valent iron (ZVI) and hydrothermal treatment. The treatment converted the unstable arsenic adsorbed or complexed on ferrihydrite into well-crystallized lattice form in a highly stable natural mineral, Johnbaumite. When treated under the optimum conditions (ZVI dosage = 0.1 g/g-sludge, hydrothermal treatment at 150 degrees C for 12 h), the leachable arsenic concentration was decreased from 22.4 mg/L for untreated ABLFS to 0.015 mg/L, which is far below the Chinese regulatory level of 2.5 mg/L. XRD, FTIR, and TEM analyses were carried out to understand the phase evolution and the underlying arsenic stabilization mechanism. The results revealed three key steps: (i) ZVI-mediated reaction with arsenic-laden 2-line ferrihydrite (Fh) leading to the release of arsenate ions, (ii) hydrothermally facilitated phase transformation of gypsum causing the release of calcium ions, and (iii) hydrothermally enabled formation and crystal growth of Johnbaumite. This work provides a promising treatment strategy for the safe disposal of ABLFS.