Daniel Leigh, P.G., CH.G.

Biogeochemically Enhanced Treatment of Chlorinated Organics and Metals

Daniel Leigh, P.G., CH.G. Technical Applications Manager. Evonik Active Oxygens

Recently, biogeochemical reduction (BGCR), a process which combines biological and chemical processes, has been combined with ERD and ISCR to provide an additional mechanism to more aggressively degrade CVOCs and to sequester toxic metals. These processes recently have been applied for treatment of CVOCs at sites in California and for treatment of arsenic at a site in Florida.  During ERD and ISCR, highly reducing conditions are established which are favorable to the reduction of ferric iron (Fe[III]) to ferrous (Fe[II]) and sulfate (SO4) to sulfide (HS-), both of which are substantially more soluble than their oxidized form. Ferrous and sulfide in solution rapidly combine to produce iron-sulfide minerals such as mackinawite (FeS), and pyrite (FeS2). These biologically generated minerals have been demonstrated to abiotically degrade CVOCs on contact, primarily by the β elimination pathway. This additional biogeochemical degradation pathway minimizes the generation of toxic degradation products such as vinyl chloride, thereby substantially reducing the clean-up time.   In addition, iron and sulfide remove toxic metals from solution as iron-sulfide. Also the generated sulfide will precipitate on zero valent iron (ZVI) if present. This sulfidation of zero valent iron (ZVI) has been demonstrated to substantially enhance reactivity and reduce passivation of the ZVI.  Bench tests, and full-scale treatment were conducted at multiple sites affected by CVOCs and metals to evaluate the effectiveness a BGCR enhancing reagent (Geoform® Extended Release; Geoform® ER) for in situ remediation of CVOCs including chlorinated -ethenes, -ethanes and -methanes and metals. At each site, full-scale treatment was conducted by distribution of these reagents into the affected aquifer by high-pressure injection or through injection wells. Following distribution, groundwater monitoring was conducted to confirm and quantify treatment.   The bench tests demonstrated that BGCR significantly increased the reactivity of the ZVI containing ISCR reagents for treatment of CVOCs. The field tests demonstrated that enhanced the biological reduction of the supplied sulfate to sulfide. Simultaneously, the CVOCS plume were rapidly degraded by both biotic and abiotic processes. Bench tests demonstrated that arsenic was rapidly removed from solution. A description of the BGCR processes applied, and the methods and results of the bench tests and full-scale application of biogeochemical reduction for treatment of various CVOCs, and for sequestration of arsenic at the Florida site will be presented.   

Dan Leigh is a Professional Geologist with over 40 years of professional experience, of which, 38 years have been in the area if biological, chemical and biogeochemical treatment of chlorinated organics and metals in soil and groundwater. He has applied these technologies at sites around the world. As Technology Application Manager for In Situ Reductive Technologies, he continues to work on the development and application of innovative remedial technologies.