A framework for estimating soil water characteristic curve and hydraulic conductivity function of permeable reactive media.

The unsaturated behavior of permeable reactive barriers (PRB) is a critical component in predicting the removal efficiency through the adsorption of contaminants. This study investigates the framework to estimate the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF) for iron oxide-coated sand (IOCS) and zeolite, which are common materials used in PRBs. A multistep outflow (MSO) experiment was performed and the results of the MSO experiment were used to optimize associated parameters in Kosugi's SWCC and HCF. In addition, three scenarios of optimization analysis were investigated to evaluate the best-fitting model for estimating SWCC and HCF. The low root mean square error (RMSE) of fitted parameters indicates the Kosugi model well described the observed suction profiles in MSO experiments. In addition, the lowest RMSE and coefficient of variation suggested the inclusion of the additional parameter ß provided the best estimation of the three materials (clean sand, IOCS, and zeolite). The physically reasonable estimation of SWCC and HCF of the three materials from the optimized parameters suggests the proposed framework is a reasonable model for the unsaturated behavior of PRBs.

Thermal conductivity of dry fly ashes with various carbon and biomass contents.

Recently, sustainable energy portfolios have added biomass combustion and coal/biomass co-combustion as alternative fuel sources for generation of electricity. Fly ashes that result from combustion of biomass or its co-combustion with coal contain relatively high contents of unburned carbon, while increasingly stringent air quality regulations have also increased the residual carbon content in fly ash produced by coal combustion alone. While previous studies documented the mechanical and chemical behavior of fly ash relatively well, the thermal characteristics of those fly ashes have not been well studied. Therefore, this study evaluated the thermal conductivity of fly ashes with varied carbon and initial biomass contents to quantify the impact of unburned carbon particles and biomass-fired fly ash on thermal conductivity. Observed results demonstrated that the thermal conductivity of fly ashes almost linearly decreased as biomass content increased while the variation of thermal conductivity of fly ashes caused by unburned carbon content was relatively low. In addition, the thermal conductivity of fly ashes was lower than that of natural soils mainly because of the microporous structures of fly ash particles. The trend of thermal conductivity of fly ashes as a function of dry density was consistent with that of natural soils, due to the similar mineralogy of fly ash with that of natural soils. The developed stepwise regression model indicated that the porosity and the specific gravity was the most critical factor in predicting the thermal conductivity of fly ash.

Evolution of Small Strain Soil Stiffness during Freeze-Thaw Cycle: Transition from Capillarity to Cementation Examined Using Magnetic and Piezo Crystal Sensors.

Freeze-thaw cycles caused by seasonal temperature fluctuations significantly affect the geotechnical engineering properties. This study investigated the crucial role of water distribution patterns in the characterization of elastic wave properties for the fine F-110 sand during a freeze-thaw cycle. Sand specimens with four different water distribution patterns were prepared, namely homogeneously-mixed, evaporation-driven, vertically-, and horizontally-layered specimens. The P- and S-wave signatures of the specimens were monitored using piezo crystal sensors. Results indicated the criticality of water distribution patterns in the determination of small-strain soil properties even though the specimens had identical global water saturation. The nuclear magnetic resonance-based water volume depth profiles indicated that the evaporation-driven specimens had more heterogeneous pore-invasive ice-bonding layers at a high water saturation region; by contrast, the drying process facilitated uniform meniscuses around the particle contacts near the air percolation threshold. Elastic wave measurements for laboratory-prepared specimens might over/underestimate the small-strain soil stiffness of sediments in nature, wherein the drying processes prevailed to control the water saturation. This study highlighted a clear transition from capillary-controlled to cementation-controlled elastic wave properties during temperature oscillations.

An experimental study of cotransport of heavy metals with kaolinite colloids.

Cotransport of heavy metals, Pb, Cu and Zn (multi-metal system), and transport of those metals (single-metal system) were investigated by performing laboratory soil column experiment under the presence of kaolinite colloids. Preequilibrated kaolinite colloids with heavy metal solution was injected to the column until 10 pore volumes under two different flow rates and three different concentration of kaolinite colloids. Heavy metal concentration in effluent showed that the mobility of Pb was facilitated as kaolinite colloids concentration (Cc0) increases under high flow rate while the mobility of Pb and Cu were retarded as Cc0 increases under low flow rate. In addition, optimized first order rate coefficient related to sand-heavy metal interaction and estimated bed efficiency of experimental breakthrough curves demonstrated that the presence of mobile kaolinite colloids delayed the adsorption of heavy metals to the sand and facilitated the transport. Colloid associated contaminant transport model used in this study was found to be well fitted to the experimental breakthrough curves with the parameters associated with observed heavy metal transport without kaolinite colloids and adsorption/desorption between the heavy metals and the mobile kaolinite colloids.

Role of Immobile Kaolinite Colloids in the Transport of Heavy Metals.

Predicting the transport of contaminants in porous media is crucial to protecting public health and remediating contaminated soil and groundwater. However, the prediction of contaminant transport is challenging due to the presence of mobile and immobile colloids. The work performed in this experimental investigation quantified the role of immobile clay colloids on metal transport through sets of column breakthrough experiments under varying solution chemistry, clay content, and flow rate. Georgia kaolinite was chosen as the colloidal material, and Pb(II) was chosen as the dissolved contaminant. The silica sand used as the bed material was sized to ensure that the kaolinite colloids remained stationary during the column experiments. Results indicated that retardation of the Pb(II) breakthrough curve was observed as ionic strength decreased and kaolinite content and pH increased, while no significant variation of Pb(II) breakthrough was observed at any kaolinite content as flow rate decreased. This work demonstrated that, in the presence of immobile kaolinite colloids, Pb(II) breakthrough curves strongly depended on the pH and ionic strength, which controlled the charge on the surface functional groups and the surface availability of metal adsorption sites on immobile kaolinite colloids. In addition, the evaluation of unknown first-order coefficients in the continuum governing equation, bed efficiency, and Pb(II) saturation provided a quantitative description of Pb(II) breakthrough curves.