Effects of Intermittent Wetting on Concentration Profiles and Release from a Cement-Based Waste Matrix

The objective of the research presented here was to assess the impact on constituent release from cement-based waste materials caused by intermittent leaching with storage (i.e., nonleaching period) carried out under 100% relative humidity and an inert atmosphere to prevent carbonation processes. The test matrix of concern was a synthetic Portland cement matrix containing a mixture of metal oxides (i.e., arsenic, cadmium, copper, lead, and zinc). The impact of intermittent wetting was studied by comparison of (1) continuous tank leaching with (2) leaching having interspersed periods of storage under 100% relative humidity and an inert atmosphere. Interpretations of constituent release were conducted by simulation of leaching and nonleaching periods using adjustment of the (1) diffusion model for sodium and chloride (i.e., highly soluble species), and (2) coupled dissolution-diffusion model for arsenic, cadmium, and lead whose solubilities exhibit a strong dependence on pore water pH. Results showed that the duration of the storage interval influences the release of highly soluble species (i.e., sodium and chloride) as well as calcium and hydroxide (i.e., pH), and consequently, the release of pH dependent species (arsenic, cadmium and lead). Storage resulted in relaxation of the concentration gradient within the matrix for the species of concern. Although intervals of storage reduced the release of species when compared to the same time interval of continuous leaching without storage, more rapid release and depletion of species of concern can occur when only the same leaching period is considered. This was especially observed for sodium and chloride. Overall, experimental results and simulated results of leaching and nonleaching periods were in good agreement.