Stabilization of Arsenic in Iron-Rich Residuals by Crystallization to a Stable Phase of Arsenic Mineral

Abstract

Many water treatment technologies for arsenic removal that are used today produce arsenic-bearing solid residuals (ABSR), which are disposed in mixed solid waste landfills. It is now well established that many of these residuals will release arsenic into the environment to a much greater extent than predicted by standard regulatory leaching tests and, consequently, require stabilization to ensure benign behaviour after disposal. Conventional waste stabilization technologies, such as cement encapsulation and vitrification, are not suitable for ABSR applications due to their lack of effectiveness or high cost, thus creating a need for a more effective and low-cost treatment technology for ABSR. Arsenic Crystallization Technology (ACT) is a proposed arsenic stabilization method that involves in converting the ABSR into arsenic-bearing minerals that resemble natural materials and have high arsenic capacity, long term stability, and low solubility compared to untreated ABSR. Three arsenic minerals, scorodite, arsenate apatite and ferrous arsenate, have been investigated in this research for their potential application as ACT for ABSR stabilization. Among the three minerals, ferrous arsenate is demonstrated to be the most suitable arsenate mineral for safe arsenic disposal due to its low arsenic solubility and ease of synthesis. An innovative treatment procedure has been developed in this research for stabilization of ABSR to a stable phase of ferrous arsenate using zero-valent iron (ZVI) as the reducing agent. The procedure works at ambient temperature and pressure, and neutral pH. In addition, a modified four-step sequential extraction method has been developed as a means to determine the proportions of various arsenic phases in the stabilized as well as untreated ABSR matrices. This extraction method, as well as traditional leach and solubility tests, show that arsenic stability in the solid phase is dramatically increased after formation of crystalline ferrous arsenate.