Maintaining neutral pH in deep soils and ground water utilizing insoluble colloidal buffers.
Optimal pH for most biodegradation processes falls between 6.5 and 8.5. Enhanced bioremediation in poorly buffered soils and ground water is likely to lower pH below 6.5 as the added electron donor produces carbon dioxide and organic acids. The common agricultural practice of liming soils adds alkalinity and adjusts soil pH by mixing finely divided insoluble calcium carbonate into the shallow subsurface. Conversely, delivery of an insoluble solid buffer into deeper soils and ground water aquifers presents a formidable technical challenge. The principal challenges include: 1) keeping particles in suspension during storage and transportation; 2) moving the buffer solids a useful distance through the soil or aquifer; 3) retaining sufficient buffer in the soils that will not be washed out by adjective ground water flow after injection; and 4) minimizing changes to soil permeability. Soluble buffers are commonly lost within relatively short periods of time as a result of advective ground water flow. Colloidal buffer systems can be transported significant distances in the subsurface, yet can also have substantial retention in the soil or aquifer after injection. Suspensions of micro- and nano-scale insoluble buffer particles are explored in soil columns. Subsurface mobility of the diluted suspensions is enhanced by careful control of particle size and treatment with selected additives to produce a negative surface charge. Stable suspensions of insoluble buffer solids containing more than 500,000 mg/Kg of total alkalinity (as CaCO3 equivalents) were produced through the use of optimal particle size and proprietary additives (U.S. patent pending). These suspensions were stored for more than six months without significant settling, agglomeration or cementing of buffer particles. The concentrated suspensions can be easily diluted with water in the field by gentle mixing for injection into deep soils or ground water. Column studies using sandy soils indicated the effective transport of more than five pore volumes of a suspension containing more than 20,000 mg/L total alkalinity (as CaCO3 equivalents), with significant alkalinity retained in the soils, and less than 10% loss in permeability. The commercial product developed from this research is being tested in field trials and will be marketed under the trademark Neutral Zone™.