Control of pH during denitrification in subsurface sediment microcosms using encapsulated phosphate buffer

Denitrification, a process in which nitrate (NO−3) is used as an electron acceptor during microbial respiration, often occurs in saturated sediment where oxygen has been depleted. During denitrification, pH may increase several units as nitrate is consumed and HCO−3 and OH− are produced. A novel approach to control pH is the use of encapsulated buffers. Previous research has shown that KH2PO4 microcapsules were effective at maintaining a neutral pH during denitrification in suspended bacterial cultures. Our objective was to test the ability of encapsulated buffers to control pH in subsurface sediment microcosms amended with NO−3. Sediment samples from three depths (37, 40, and 43 m) in the saturated zone beneath the Savannah River Site, near Aiken, SC, were amended with 1 mg NO−3/g. Nitrous oxide (N2O) production was measured by gas chromatography (GC) using the acetylene block technique. NO−3, ammonium (NH+4), total alkalinity, and sediment particle grain size also were measured as part of a nitrogen mass balance and to assess base neutralizing capacity. In two of the three samples, all of the NO−3 added was converted to N2O at a rate between 0.7–2.3% h−1. pH levels above 8.0 were measured in vials to which no microcapsules had been added while the pH in vials containing microcapsules remained within 0.2 pH units of 7.0. The results indicated that over time periods as short as five days, denitrification increased the pH beyond the range considered to be optimal for microbial metabolism. Because these encapsulated buffers were effective at maintaining a neutral pH during denitrification in sediment microcosms, they may facilitate remediation of subsurface contaminants through the mediation of pH-limited biological or chemical processes.