Leaching of heavy metals from solidified wastes
Academic Article
Overview
Additional Document Info
View All
Overview
abstract
The goal of solidification/stabilization is to limit the ability of disposed hazardous wastes to migrate in the environment via leaching mechanisms. The pore water components of the solidified wastes constitute the leachable fraction of the wasteform. Previous experiments provided information about the fraction of cadmium, chromium, lead, and mercury in the pore water. Subsequent experiments were carried out to determine the leaching characteristics of the wasteform in an acidic leaching environment. The wasteform subjected to leaching was a cement-solidified heavy metal waste containing 1000 mg/1 cadmium, chromium, lead, and mercury. A water/cement ratio of 0.7 was used. The solid samples leached were thin cylindrical samples of uniform size, 50 mm in diameter and 8 mm thick. Only the top and bottom cylinder surfaces were exposed to the leach bath, which contained a constant volume of 250 ml of acid. Experiments were carried out in 0.1 M acetic acid and 0.02 M acetic acid. Results for cadmium and lead show a similar trend of initially rapid leaching that results in a rise in the mass of metal in the bath that peaks at relatively short leaching times and low pH (pH 4 to pH 5) followed by a gradual decrease in the cumulative metal concentration in the bath. The cumulative mass levels off and becomes fairly constant at around pH 6 to 7. Chromium and mercury exhibit a similar rapid initial leaching up to about pH 5 to 7 followed by a relatively constant cumulative bath concentration between pH 7 and pH 11 followed by an another increase in metal concentration in the bath above pH 11. The results in general show that metal leaching is a function of the pH of the leaching fluid. It appears that in the case of cadmium and lead, which show a decreasing metal concentration in the bath over a certain pH range, some of the metal is reprecipitating in the solid.
