SOLIDIFICATION STABILIZATION OF ARSENIC SALTS - EFFECTS OF LONG CURE TIMES/

Leachability of As-III and As-V from various solidification-stabilizat ion (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 additive s, results in toxicity characteristic leaching procedure (TCLP) leacha bilities of less than or equal to 3 mg l(-1) for arsenite and less tha n or equal to 2 mg l(-1) for arsenate. There is no appreciable leachab ility after 3 years of cure, compared with 28 days of cure. The combin ation of OPC and Class F fly ash as a binder results in substantially degraded performance, as measured by TCLP leachability. Furthermore, t he OPC-FA-As mixtures show increasing leachability with time. These so lidified products have been studied sing powder X-ray diffraction (XRD ), derivative thermal gravimetry (DTG) and solid-state magic angle spi nning 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 wh en sodium arsenite is the original form of As. The OPC-FA mixtures sho w substantial respeciation during long curing times. There is evidence for formation of stratlingite from XRD, and there is substantial conv ersion 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 incre ased leachability, although direct cause and effect cannot be establis hed. These results emphasize the importance of long-term testing to id entify specific combinations of S/S binders and wastes that are prone to undergo respeciation, and consequent leachability changes, after lo ng cure times. (C) 1997 Elsevier Science B.V.