Concrete stabilization of arsenic-bearing iron sludge generated from an electrochemical arsenic remediation plant

Abstract

In this study, concrete stabilization is adopted to sustainably manage hazardous arsenic-iron sludge near the vicinity of a community-based arsenic water treatment plant for potential use as material for local construction. The strength and workability of the sludge mixed with fresh concrete were investigated to determine the suitability of the concrete mixture for building materials. We found that over 25% sludge (with respect to cement weight) can be incorporated safely into different grades of concrete (M15 and M20). Structural characterization of the concrete mixtures by Fe and As K-edge X-ray absorption spectroscopy indicated a structural transformation of Fe in the sludge from a hydrous ferric oxide to a less ordered phase consistent with Fe siliceous hydrogarnet. Differences in the As K-edge XAS data of samples before and after stabilization in concrete were interpreted as a decrease in As-Fe coordination after concrete stabilization in favor of As-Ca coordination. The leaching of arsenic in the stabilized concrete was examined by the Toxicity Characteristics Leaching Procedure (TCLP) and found to produce < 15 μg/L As, even at the highest sludge mixture fraction (40% sludge with respect to cement weight). The formation of calcite in concrete stabilized arsenic sludge, which was detected by X-ray diffraction (XRD), contributes to the low leachability of arsenic in the sludge for a variety of reasons, including decreasing pore size. In addition, the formation of poorly soluble calcium arsenates can also be responsible for the low mobility of arsenic. Overall concrete stabilization of arsenic-iron sludge can be an effective pre-treatment to safe landfill disposal and, when the arsenic-iron sludge is mixed in specific proportions to achieve desired strength, we propose this concrete can be used locally in nearby construction.