Leaching behaviors of arsenic from arsenic-iron hydroxide sludge during TCLP

The toxicity characteristic leaching procedure (TCLP) is normally used to evaluate if sludge should be managed as hazardous waste. This study examines immobilization mechanisms of arsenic onto arsenic-iron hydroxide sludge, the byproduct of arsenic removal by coagulation with ferric chloride. The leaching mechanism of arsenic from the sludge due to the TCLP is also investigated. Microscopic characterization techniques including scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) were employed to characterize the sludge samples with the As-to-Fe ratios of 0.07 to 0.15 before and after the TCLP. SEM-EDS and FT-IR results suggested that arsenic-iron hydroxide sludge be ferric hydroxide, whose surface is inner or outer spherically sorbed by arsenic, rather than the precipitate of insoluble iron-arsenic compounds such as Fe(AsO)4. This is also confirmed by XRD results, which revealed that none of such crystalline iron-arsenic compounds were detected in the arsenic-iron hydroxide sludge. Therefore, adsorption among other possible arsenic immobilization mechanisms, namely, precipitation, coprecipitation, and occlusion, is supposed to play the major role. Due to the TCLP, the arsenic concentrations ranging from 0.26 to 2.54 mgL were leached out of the sludge samples with the As-to-Fe ratios ranging from 0.07 to 0.15, respectively. The changes of FT-IR patterns of the sludge after the TCLP suggested that during the TCLP, desorption and resorption of arsenic occurs. The relationship between arsenic in TCLP leachate and that remaining in the leached sludge can be modeled by Langmuir isotherm, an adsorption isotherm. This indicates that desorption and resorption of arsenic onto the leached sludge is the main phenomenon controlling arsenic leachability due to the TCLP.