Modeling of Leaching Surface Evolution in Stressed Particulate Media

Abstract

Many reuse scenarios of waste as construction aggregates involve the application of load. The leachability of contaminants from aggregates is affected by the exposed surface area of constituent particles. Loading decreases interparticle porosity and permeability of bulk materials but enlarges particle-to-particle contacts. A model that accounts for the effects of particle deformation on exposed surface area of particles is developed herein and applied to fly ash. Brunauer, Emmett, and Teller (BET) measurements of the specific surface area of fly ash were compared with those of glass beads and theoretically generated data for spherical particles of equivalent diameter. The ratios of the BET-measured specific surface areas of the glass beads to the theoretically calculated values (TCV) in the same size ranges were found to range from 2 to 11, while those of wet-sieved and dry-sieved fly ash to the TCV ranged from 30 to over 2,825. Plausibly, internal porosities, surface asperities, and shape irregularities of real fly ash particles account for their higher specific surface area values relative to that of size-equivalent glass beads. The effects of surface area evolution on leachability of contaminants were demonstrated using experimental leaching data obtained for Cu, Se, and As. Using contaminant leaching coefficient data, it was found that stress level has minimal effects on the leachability of Cu. Leaching coefficients of Se and As were found to increase with an increase in load. Although the values for Se are comparable to column leaching data at stress levels smaller than 1,425 kPa, they are much higher at 5,710 kPa than column leaching data. In the case of As, the leaching coefficients are smaller under load than those of column leaching. Leaching coefficients measured on the basis of BET-measured specific surface areas are smaller than those based on calculated specific surface areas by a factor of about 10,000. Models that incorporate the effects of load, surface asperities, and internal porosities into surface area estimates are proposed and demonstrated for ash of known physicochemical characteristics and reuse scenarios.