Natural organic matter and colloid-facilitated arsenic transport and transformation in porous soil media

Prediction of arsenic transport and transformation in soil environment requires understanding the transport mechanisms and proper estimation of arsenic partitioning tong all three phases in soil/aquifer systems: mobile colloids, mobile soil solution, and immobile soil solids. The primary purpose of this research is to study natural dissolved organic matter (DOM)/colloid-facilitated transport of arsenic and understand the role of soil derived carriers in the transport and transformation of both inorganic and organoarsenicals in soils. ^ DOM/colloid facilitated arsenic transport and transformation in porous soil media were investigated using a set of experimental approaches including batch experiment, equilibrium membrane dialysis experiment and column experiment. Soil batch experiment was applied to investigate arsenic adsorption on a variety of soils with different characteristics; Equilibrium membrane dialysis was employed to determine the 'free' and 'colloid-bound/complexed' arsenic in water extracts of chosen soils; Column experiments were also set up in the laboratory to simulate arsenic transport and transformation through golf course soils in the presence and absence of soil-derived dissolved substances. ^ The experimental results revealed that organic matter amendments effectively reduced soil arsenic adsorption. The majority of arsenic present in the soil extracts was associated with small substances of molecular weight (MW) between 500 and 3,500 Da, Only a small fraction of arsenic was associated with higher MW substances (MW > 3,500 Da), which was operationally defined as colloidal part in this study. The association of arsenic and DOM in the soil extracts strongly affected arsenic bioavailability, arsenic transport and transformation in soils. The results of column experiments revealed arsenic complicated behavior with various processes occurring in soils studied, including: soil arsenic' adsorption, facilitated arsenic transportation by dissolved substances presented in soil extracts and microorganisms involved arsenic species transformation. ^ Soil organic matter amendments effectively reduce soil arsenic adsorption capability either by scavenging 'soil arsenic adsorption sites or by interactions between arsenic species and dissolved organic chemicals in soil solution. Close attention must be paid for facilitated arsenic transport by dissolved substances presented in soil solution and microorganisms involved arsenic species transformation in arsenic-contaminated soils.^

Movement Effects on the Flow Physics and Nutrient Delivery in Engineered Valvular Tissues

Mechanical conditioning has been shown to promote tissue formation in a wide variety of tissue engineering efforts. However the underlying mechanisms by which external mechanical stimuli regulate cells and tissues are not known. This is particularly relevant in the area of heart valve tissue engineering (HVTE) owing to the intense hemodynamic environments that surround native valves. Some studies suggest that oscillatory shear stress (OSS) caused by steady flow and scaffold flexure play a critical role in engineered tissue formation derived from bone marrow derived stem cells (BMSCs). In addition, scaffold flexure may enhance nutrient (e.g. oxygen, glucose) transport. In this study, we computationally quantified the i) magnitude of fluid-induced shear stresses; ii) the extent of temporal fluid oscillations in the flow field using the oscillatory shear index (OSI) parameter, and iii) glucose and oxygen mass transport profiles. Noting that sample cyclic flexure induces a high degree of oscillatory shear stress (OSS), we incorporated moving boundary computational fluid dynamic simulations of samples housed within a bioreactor to consider the effects of: 1) no flow, no flexure (control group), 2) steady flow-alone, 3) cyclic flexure-alone and 4) combined steady flow and cyclic flexure environments. We also coupled a diffusion and convention mass transport equation to the simulated system. We found that the coexistence of both OSS and appreciable shear stress magnitudes, described by the newly introduced parameter OSI-t , explained the high levels of engineered collagen previously observed from combining cyclic flexure and steady flow states. On the other hand, each of these metrics on its own showed no association. This finding suggests that cyclic flexure and steady flow synergistically promote engineered heart valve tissue production via OSS, so long as the oscillations are accompanied by a critical magnitude of shear stress. In addition, our simulations showed that mass transport of glucose and oxygen is enhanced by sample movement at low sample porosities, but did not play a role in highly porous scaffolds. Preliminary in-house in vitro experiments showed that cell proliferation and phenotype is enhanced in OSI-t environments.