Leaching of simulated heavy metal waste stabilized/solidified in different cement matrices

Abstract

Leaching experiments have been carried out on samples of ordinary portland cement (OPC), sulphate-resistant portland cement (SRPC) with fly ash (FA) added and an alumina cement (AC), with each receiving various concentrations (0.1–1.0 M of chromium (Cr 6+), vanadium (V 5+) and cadmium (Cd 2+. The samples were prepared and evaluated with a statistical experimental matrix corresponding to a Box-Behnken fractional factorial design. Leaching was done in PTFE vessels using the standard TCLP (toxicity characteristic leaching procedure of the U.S. Environmental protection Agency). Analyses of the leachates were carried out for elements Cr, Cd, V, Si, Al, Ca and Fe by inductively coupled plasma spectroscopy. Selected solid samples were investigated before and after leaching by scanning electron microscopy, in conjunction with energy dispersive X-ray spectrometry, and by scanning transmission electron microscopy on thinned specimens. The normalized mass losses of chromium varied between 3.3 10 3-tand 3.5 10 1-t kg/m 2. Calculated maximum release was found for a hypothetical cement with approximately 20% alumina, while minimum release was found for AC with 36.7% alumina. The normalized mass losses of vanadium varied between 110 −3 and 810 −2 kg/m 2. Calculated maximum release was found for a hypothetical cement with approximately 25% alumina and minimum release was found for OPC. All samples of the cadmium series except three showed solution concentrations below the detection limit (0.1 ppm), suggesting very effective retention of this element by all three cement matrices investigated.