2 The 4 Technology Solutions
NEW APPLICATIONS OF REDOX REACTIONS
FOR IN-SITU GRONDWATER REMEDIATION
An On-Line Version of a Column First Published in:
Environmental Technology Nov./Dec. 1998 Vol. 8 No. 6
by: David B. Vance
Recent years have seen the acceptance by regulators, industry, and environmental service providers of Risk Based Clean-Up Standards and natural attenuation as alternatives to remediate soil and groundwater impacted by Chemicals of Concern (COC). These approaches have had significant impact on the level of effort required to remediate sites, particularly UST and other sites impacted petroleum hydrocarbons. However, there are still classes of sites that will and do require aggressive clean-up. These tend to have COC’s such as chlorinated hydrocarbons, wood treating chemicals, and metals. Recent developments have produced technologies that have the potential to significantly improve the remediation process for sites of those type. This column will briefly review three of them: chemical oxidation with Fenton’s Reagent; bio-reduction using molasses injection; and catalytic reduction using bi-metallic colloids.
Chemical Oxidation with Fenton’s Reagent
The Fenton’s reaction was first described in 1894, during the 1930’s the reaction mechanisms were fully defined, over the last 15 years commercial reactors have been available for waste water treatment, and in 90’s applications for in-situ groundwater treatment have been developed. Fenton’s Reagent generates hydroxyl radicals through the reaction of ferrous iron and hydrogen peroxide:
Fe+2 + H2O2 .OH + Fe+3 + OH-
The hydroxyl radical is a powerful oxidation agent, second only to fluorine. Now, the process is self replicating since the reaction of ferric iron with hydrogen peroxide to generate the perhydroxyl radical also occurs:
Fe+3 + H2O2 Fe+2 + .OOH + H+
The perhydroxyl radical is a weaker oxidizer (between hydrogen peroxide and permangenate). But, more importantly the process generates further ferrous ions that in turn stimulate further reaction with hydrogen peroxide to produce more hydroxyl radials. The hydroxyl radical can react with almost any hydrocarbon to produce carbon dioxide as a final product (an chlorides if a chlorinated hydrocarbon is treated). The problem is delivery of the reagents in-situ and pH control. Optimum pH for the reaction, in the range of 3.0 to 4.5, is driven by the iron chemistry Improper control of the reaction will only generate oxygen and water, not high intensity oxidation. The technology is ideal for application in source zone areas where there are high concentrations of adsorbed or even interstitial free product. Cost is mostly dependent upon the geochemistry of the geologic matrix and control of the iron chemistry, a typical cost range for feasible application is $50 to $200 per cubic yard of COC impacted saturated zone matrix. Better control of the iron chemistry may offer lower costs, that may be the subject of a future column.
In-Situ Reactive Zone Control Through Molasses Injection
Molasses injection is designed to function at the opposite extreme of the redox environment. The advantages of molasses are that it is readily available, contains many important trace nutrients, it is easily metabolized by a wide range of organisms, and it is a food grade material that can be injected into the groundwater with little potential for harm.
The injection of molasses into a COC impacted saturated zone induces anaerobic microbial activity that drives the system into increasingly lower redox (Eh) conditions and associated degradation mechanisms as follows (using vinyl chloride as an example):
C2H3C1 + 2NO3- + H+ 2CO2 + 2H2O + N2 + C1-
Denitrification Eh Range +250mv to 100mv
10Fe3+ +3(OH)- + C2H3C1 + 4H2O 2CO2 + 11H+ + C1- + 10Fe2+
Iron reduction Eh Range +100mv to 0mv
C2H3Cl + 5MnO2 + 9H+ 2CO2 + 6H2O + 5Mn2+ + Cl-
Manganese reduction Eh Range +100mv to 0mv
C2H3Cl + 1.5H+ + 1.25SO4= 2CO2 + 1.25H2S + H2O + Cl-
Sulfate reduction Eh Range 0mv to -200mv
C2H3Cl + 1.5H2O 0.75CO2 + 1.25CH4 + H+ + Cl-
Methanogenesis Eh Range -200mv and lower
Methanogenesis is a particularly valuable process for the reductive dehalogenation of a wide range of chlorinated hydrocarbons. It also performs well for the immobilization of metals through reduction. The figure illustrates an application of the technology to a site impacted with TCE and chromate that had become asymptotic under a conventional pump and treat remediation.
Implementation of a redox control program requires understanding of the biogeochemical setting of the site, groundwater hydrodynamics, and the ability to interpret the efficacy of the redox couple interactions with the indigenous microbial population and the impacting COC. Due to the high degree of geographic heterogeneity exhibited by anaerobic microbial consortia in groundwater systems, treatment regimes require engineering and operational process control.
Bi-Metallic Colloidal Iron
Reactive metallic iron walls (1) (2) have over the last few years become an accepted method of remediation for groundwater impacted with dissolved chlorinated hydrocarbons. In essence complete reductive dehalogenation to ethene or ethane gas takes place in these walls. Their disadvantages include the cost of installation and hydrogeologic requirements (the zone to be treated must be relatively shallow). In response to this there have been attempts to inject colloidal iron particles directly into water bearing zones. Aside from transport issues, the problem with that approach is the rapidity with which the colloidal iron metal will react under oxic groundwater conditions to form ferric hydroxide, which will not dehalogenate chlorinated solvents.
An answer to that problem is literally finding its way from the bench to the field as this column is being prepared. Bi-metallic colloids offer greater dehalogenation activity as well as geochemical stability under in-situ conditions. The trick then becomes control of injection procedures to maximize penetration of the bi-metallic colloidal suspension into the formation. The beauty of the process is first, that the degradation products are ethene and ethane gas and chlorides. Secondly, the reaction is almost instantaneous. The high surface area to volume ratio of a colloid makes them extremely chemically reactive.
These three technologies exploiting the extremes of redox conditions offer remediation choices for the treatment of chlorinated hydrocarbons and metals that are cost effective in application and most importantly rapid, especially compared to pump and treat.
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