Heavy metals in cement and cement kiln dust from kilns co-fired with hazardous waste-derived fuel: application of EPA leaching and acid-digestion procedures

Abstract

Substantial leaching from cement and cement kiln dust (CKD) of specific metals, especially Cr and Ba, resulted from application of the standard Toxicity Characteristic Leaching Procedure (TCLP). However, all TCLP concentrations are below limits for hazardous waste defined in the Resource Conservation and Recovery Act (RCRA). Four variations on the TCLP were assessed. The primary discernible enhancement of Tier-III metals release, relative to the standard TCLP, is increased leaching of Cr by chlorinated-water solutions. This likely results from enhancement of metal dissolution by chloride complexation. The final pH of each extract remained high. Thus, the capacities of these cement and CKD samples to buffer the acidity of the leaching solutions were not exceeded, even after two sequential exposures. This factor calls into question whether these high-pH solutions could represent any `real-world' condition and implies that the TCLP is not suitable for assessing the leaching behavior of alkaline systems such as cement and CKD. The EPA SW-846 microwave-assisted, acid-soluble procedure produces only partial recovery of any metal relative to dissolution-independent concentrations obtained by X-ray fluorescence spectrometry. Our results contradict previously published statements that TCLP and acid-soluble metal concentrations showed no consistent relationship. Both current results and re-evaluation of previously published data indicate correlations between TCLP and SW-846 acid-soluble concentrations, for several metals, that are significantly >0 at high levels of confidence.