Carbon sequestration through the formation of carbonates is a potential means to reduce CO2 emissions. Alkaline industrial solid wastes typically have high mass fractions of reactive oxides that may not require preprocessing, making them an attractive source material for mineral carbonation The degree of mineral carbonation achievable in cement kiln dust (CKD) underambienttemperatures and pressures was examined through a series of batch and column experiments. The overall extent and potential mechanisms and rate behavior of the carbonation process were assessed through a complementary set of analytical and empirical methods, including mass change, thermal analysis, and X-ray diffraction. The carbonation reactions were carried out primarily through the reaction of CO2 with Ca(OH)2, and CaCO3 was observed as the predominant carbonation product. A sequestration extent of over 60% was observed within 8 h of reaction without any modifications to the waste. Sequestration appears to follow unreacted core model theory where reaction kinetics are controlled by a first-order rate constant at early times; however, as carbonation progresses, the kinetics of the reaction are attenuated by the extent of the reaction due to diffusion control, with the extent of conversion never reaching completion.

A mathematical model was derived to examine the impact of gas advection, gas diffusion, gas‐water mass transfer, gas‐water partitioning, sorption, and intraaggregate diffusion on subsurface movement of organic vapors. Laboratory experiments were performed to determine the validity of the model and to investigate the impact of the various mechanisms on vapor transport. Columns were packed with a uniform Ottawa sand and an aggregated porous soil material (APSM) to compare transport in different soil structures. Toluene vapor transport was observed in the sand under dry and wet (27% water saturation) conditions. The experiments with the APSM were performed dry and at 67% water saturation. In all the sand and the dry APSM experiments, gas advection and diffusion had the greatest impact. In a wet APSM experiment, intraaggregate (liquid) diffusion was also important to consider for gas velocities greater than approximately 0.05 cm s−1. For both soil materials, sorption of toluene vapors occurred for dry conditions, while vapor sorption was negligible when liquid water was present. These findings imply that vapor extraction performance in moist, aggregated soils will be affected by nonequilibrium transport. Therefore models that are developed for predicting the complete removal of contaminants by vapor extraction must account for nonequilibrium.

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Measured breakthrough and elution curves for trichloroethene, bromoform, and chloride in columnsof a sandy loam soil were compared to various models describing one-dimensional chemical transport through saturated soil columns. The local equilibrium model and the segregated flow model approximate the retardation of organic chemicals but do not account for the amount of spreading seen in the breakthrough and elution data. A dispersed flow, local equilibrium model (DFLEM) could simulate the breakthrough of the organic chemicals and tracer but only if the axial dispersion coefficient were adjusted to match the breakthrough data. Existing correlations for axial dispersion based on soil and fluid properties could not predict the apparent dispersion seen in the miscible displacement experiments. A dispersed flow, pore and surface diffusion model (DFPSDM) could also simulate the chemical breakthrough if an aggregate radius were adjusted to fit the data. Neither the adjusted radii nor the apparent dispersivities could be related to the hydraulic characteristics of the column or to soil properties. Both models were able to reasonably predict the elution of chemical from the column when either an aggregate radius or an apparent dispersivity were estimated from the breakthrough data. Neither the DFLEM nor the DFPSDM were able to predict the increased asymmetry or the leftward shift of the breakthrough data when the average pore water velocity was increased from 12.0 to 36.6 cm/h. While the DFPSDM appears to be more phenomenologically correct, this work suggests that an additional kinetic mechanism should be included in the model. INTRODUCTION Many groundwater supplies have become contaminated with volatile, weakly hydrophobic, slightly degradable toxic organic chemicals such as carbon tetrachloride and trichloroethene (TCE) [Environmental Protection Agency (EPA), 1984].Many of these pollutants originate from improperly designed hazardous waste disposal facilities, accidental chemical spills, and leaky storage tanks. A number of mechanisms, such as dispersion, diffusion, and adsorption, influence the movement of organic chemicals with saturated groundwater flow, and it is necessary to study these mechanisms separately as well as in combination to adequately predict the movement of organic chemicals with groundwater flow.The development of mathematical models describing the transport and attenutation of these pollutants through soil and with saturated groundwater flow can greatly aid in the proper management of drinking water supplies. A conceptually correct and predictive model can be used to estimate the time required for a pollutant to travel through a groundwater aquifer and the dilution it undergoes en route. Engineers and managers need to assess the relative importance of each mechanism that transports toxic organic chemicals through soil in order to predict the extent of pollution, design groundwater clean-up systems, and locate groundwater protection devices.It is very difficult to model a groundwater system and even m...