Rate-controlled processes, dependence surface structure

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

The factors that control the availability of the mineral reductants to react with dissolved O2 are not well understood, and can be difficult to quantify (Malmstrom Banwart 1996). The extent of reaction depends on redox reactions that occur on mineral surfaces and on dissolution reactions that release the structural reductants such as Fe(II) to solution. The rate of reaction depends on the amount of surface area in contact with flowing groundwater and on slow dissolution and diffusion processes within the rock matrix. The reactions may be microbially mediated. In the absence of verified models for these processes, and lacking field methods to determine relevent parameters, empirical approaches to measuring RDC are useful. 

The CVD reactor must be designed and operated in such a manner that changes in film thickness, crystal structure, surface morphology, and interface composition can be accurately controlled. The overall process performance depends on the reactor design and process variables such as reactant concentrations, flow rates, energy input, pressure, and substrate conditions. 

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