Solidification/stabilization of arsenic salts: Effects of long cure times

Leachability of As-III and As-V from various solidification-stabilization (S/S) binders has been studied over a period of four years. Type I portland cement (OPC), both alone and mixed with a number of additives, results in toxicity characteristic leaching procedure (TCLP) leachabilities of less than or equal to 3 mg l(-1) for arsenite and less than or equal to 2 mg l(-1) for arsenate. There is no appreciable leachability after 3 years of cure, compared with 28 days of cure. The combination of OPC and Class F fly ash as a binder results in substantially degraded performance, as measured by TCLP leachability. Furthermore, the OPC-FA-As mixtures show increasing leachability with time. These solidified products have been studied sing powder X-ray diffraction (XRD), derivative thermal gravimetry (DTG) and solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). The As-V salt, NaCaAsO4 . 7.5H(2)O, is identifiable by XRD in many of these samples, particularly when sodium arsenate is the model As waste, but even when sodium arsenite is the original form of As. The OPC-FA mixtures show substantial respeciation during long curing times. There is evidence for formation of stratlingite from XRD, and there is substantial conversion of octahedrally coordinated aluminum, which is the predominant form at 28 days, to tetrahedrally coordinated aluminum at longer cure times, as shown by NMR. These matrix changes are correlated with increased leachability, although direct cause and effect cannot be established. These results emphasize the importance of long-term testing to identify specific combinations of S/S binders and wastes that are prone to undergo respeciation, and consequent leachability changes, after long cure times. (C) 1997 Elsevier Science B.V.