Chemical and Biological Processes: the Need for Mixing

Except for spontaneous reactions such as radioactive decay, chemical transformations often require that two or more substances be brought together for the transformation to occur. Examples of particular interest in groundwater are oxidations of inorganic or organic species, which require the presence of some oxidant, such as diatomic oxygen (O2), nitrate, sulfate, or ferric iron (Fe(III)). In biological reactions, three entities generally are required, the compound being oxidized (electron donor), the oxidant (electron acceptor), and the microorganism carrying out the transformation. At times, the required entities are already present together, and then transformation occurs based simply on normal reaction kinetics. However, this is often not the case in groundwater remediation, and then the missing reactants must be supplied through some means and mixed with the substance or substances targeted for removal. The speed of the reaction is then likely to be governed primarily by the rate at which the required substances can be brought together. Natural attenuation for transformation of materials may require mixing brought about by the diffusion of oxygen into an aquifer from the vadose zone above, or from an adjacent groundwater flow stream. The process of adding and mixing needed substances for desired transformation is one of the most challenging and costly aspects of in situ remediation of contaminated groundwater and soil. This is a much more difficult process than with an aboveground reactor because of complex and often undefined hydrogeology and the general uncertainty of the exact location of the contaminants. Some form of mixing may also be required for processes other than chemical oxidations. Included are the addition of reducing compounds for chemical reductions; acids or bases for pH control; chemicals that promote precipitation for in-place stabilization; detergents, solvents, or other chemicals that promote solubilization of the compound of interest for easier removal; addition of a separate phase such as air; use of thermal treatment to enhance vaporization; as well as chemical changes resulting from groundwater-surface water interactions that are driven by variability in rates of precipitation, extraction, and aquifer recharge. All such processes involve mixing in one form or another. The emphasis in this chapter is not on the mixing processes themselves, but on the chemical and biological requirements for contaminant transformation, destruction, or removal. A few examples of field studies where mixing has been used to bring the reactants together are provided for illustration, and many others are provided in other chapters of this volume.