The effect of flow-through leaching on the diffusivity of heavy metals in stabilized/solidified wastes

The flow-through leaching test is a test method employed to study the leach ing behavior of monolithic stabilized/solidified (S/S) hazardous wastes und er the condition that the leachate hows through the sample. This method sim ulates the leaching process of the S/S hazardous waste disposed under a par ticular landfill condition when the S/S waste is more permeable than its su rrounding materials or when the deterioration of the solidified waste Form has reached a state that ground water can flow-through the waste via the po rosity system of the S/S waste matrix. This paper describes a study on the long-term performance of the cement-bas ed S/S heavy metal wastes using a flow-through leaching test method. Two se ries of leaching tests with different synthetic heavy metal waste samples w ere carried out. The S/S samples were made from five types of heavy metals with two kinds of binders. The metals ware Pb2+, Zn2+, Cu2+, Ni2+ (positive ions as nitrates), and Cr6+ (a negative ion as potassium dichromate), and the binders were type I ordinary portland cement (OPC) and pulverised fuel ash (PFA). The model developed by Godbee and Joy for simulating the leaching behavior was modified to estimate the diffusivity parameter in this study. The resul ts obtained indicate that since the matrix of the solidified waste in a flo w-through leaching tests is always being degraded, the values of diffusivit ies increase continuously during the leaching period. The diffusivity varia tion range was from 10(-13) to 10(-3) cm(2)/s, and were normally higher tha n those obtained from other test methods such as ANS 16.1 test and other dy namic leaching tests. (C) 2001 Elsevier Science B.V. All rights reserved.