Lactate Transport in Soil by DC Fields

Electrokinetic injection of lactate, a negatively charged biodegradable organic, in homogeneous soils is evaluated. Net lactate migration rate on the order of 5 cm^2/V day is measured in sand from cathode towards the anode. The ionic injection in sand was dependent on current density; however, the increase in electric current did not result in an equivalent increase in lactate transport due to development of an appreciable electroosmotic (EO) flow from the anode to the cathode. While high EO flow (ke on the order 10^-6 to 10^-5 cm^2/V s) occurred in clay samples, ion migration from cathode to anode is the dominant transport process under relatively high current density (5.3 A/m^2 in this study) and can be used as an effective transport mechanism for negatively charged additives. An effective lactate reactive transport rate of more than 3 cm/d (under 1 V/cm) can be achieved in clays, which is at least two orders of magnitude greater than hydraulic injection under unit hydraulic gradient. Even though lactate concentrations in the clay were below 10% of the boundary value due to biological transformation, these concentrations (few 100 s mg/L) are high enough to maintain microbial activities capable of degrading organic contaminants. At the same time, control experiments showed that, while lactate adsorption was negligible, hydraulic injection under a unit gradient was ineffective because of the low hydraulic conductivity of clay and the biodegradation of lactate.