Arsenic immobilization by calcium–arsenic precipitates in lime treated soils

Lime-based stabilization/solidification (S/S) can be an effective remediation alternative for the immobilization of arsenic (As) in contaminated soils and sludges. However, the exact immobilization mechanism has not been well established. Based on previous research, As immobilization could be attributed to sorption and/or inclusion in pozzolanic reaction products and/or the formation of calcium–arsenic (Ca–As) precipitates. In this study, suspensions of lime–As and lime–As–kaolinite were studied in an attempt to elucidate the controlling mechanism of As immobilization in lime treated soils. Aqueous lime–As suspensions (slurries) with varying Ca/As molar ratios (1:1, 1.5:1, 2:1, 2.5:1 and 4:1) were prepared and soluble As concentrations were determined. X-Ray diffraction (XRD) analyses were used to establish the resulting mineralogy of crystalline precipitate formation. Depending on the redox state of the As source, different As precipitates were identified. When As (III) was used, the main precipitate formation was Ca–As–O. With As(V) as the source, Ca4(OH)2(AsO4)2√4H2O formed at Ca/As molar ratios greater than 1:1. A significant increase in As (III) immobilization was observed at Ca/As molar ratios greater than 1:1. Similarly, a substantial increase in As (V) immobilization was noted at Ca/As molar ratios greater than or equal to 2.5:1. This observation was also confirmed by XRD. Lime–As–kaolinite slurries were also prepared at different Ca/As molar ratios. These slurries were used to specifically investigate the possibility of forming pozzolanic reaction products. Such products would immobilize As by sorption and/or inclusion along with the formations of different As precipitates. Toxicity Characteristic Leaching Procedure (TCLP) tests were used to evaluate As leachability in these slurries. XRD analyses revealed no pozzolanic reaction product formation. Instead, As immobilization was found to be precipitation controlled. The same Ca–As precipitate, Ca–As–O, identified in the lime–As slurries, was also identified when As (III) was used as the As source, at Ca/As molar ratios greater than or equal to 2.5:1. When As (V) was used as the contamination source in the lime–As–kaolinite slurries, the formation of NaCaAsO4√7.5H2O was observed. The effectiveness of both As (III) and As (V) immobilization in these slurries appeared to increase with increasing Ca/As molar ratios.