The Use of Hydrogen Release Compound for the Accelerated Bioremediation of Anaerobically Degradable Contaminants: The Advent of Time-Release Electron Donors

Hydrogen Release Compound (HRC®) is a simple, passive, low‐cost, and long‐term option for the anaerobic bioremediation of chlorinated hydrocarbons (CHs) via reductive dehalogenation. Applications to the remediation of other compounds, such as MTBE and perchlorate, that are anaerobically degradable by other reductive mechanisms, are in progress. HRC should be viewed as a tool for the acceleration of natural attenuation at sites that would otherwise require high levels of capital investment and operating expense. HRC is a proprietary, food‐quality, polylactate ester that, upon being deposited into the subsurface, slowly degrades to lactic acid. Lactic acid is then metabolized to hydrogen, which in turn drives the reductive dechlorination of CHs. This has been demonstrated effectively in the laboratory and in the field. HRC can be manufactured as a moderately flowable, injectable material, or as a thicker, implantable hard gel, to facilitate localized treatment and passive barrier designs. HRC is best utilized for the remediation of dissolved phase plumes and the associated hydrophobically sorbed contaminant. The use of HRC is not appropriate for use on free‐phase DNAPL unless the total mass to be remediated is within the scope of economic feasibility in comparison to alternative treatments. Evidence suggests there is competition between reductive dehalogenators and methanogens in which the methanogens compete for the use of hydrogen in the conversion of carbon dioxide to methane. Some researchers believe that a low concentration of hydrogen favors the reductive dehalogenators and starves out the methanogens. The objective, therefore, is to keep hydrogen concentrations low. The time‐release feature of HRC, which is based on the hydrolysis rate of lactic acid from the ester and the subsequent lag time to hydrogen conversion, facilitates this objective. HRC, therefore, becomes a passive form of accelerated natural attenuation, in contrast to the more capital‐and management‐intensive alternatives now available. Laboratory and field results are presented, the latter expanding on the first uses of HRC by various members of the engineering and consulting firm community. © 1999 John Wiley & Sons, Inc.