Fe(o)-based bioremediation of aquifers contaminated with...

Details Fe(o)-based bioremediation of aquifers contaminated with... Fe(o)-based bioremediation of aquifers contaminated with...

2001-08-27

2004-04-13

Prince, Fred G. (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S617000, C210S631000, C210S170050, C210S903000, C210S909000, C210S913000, C210S914000, C435S176000, C435S177000, C405S128450

Reexamination Certificate

active

06719902

ABSTRACT:

BACKGROUND OF THE INVENTION
1.1 Field of the Invention
The present invention relates generally to the field of bioremediation. More particularly, it concerns compositions, methods and apparatus for the removal of nitrogenous and halocarbon pollutants from environmental sources including agricultural areas, soils, ground and surface water, sewage, sludges, landfill leachates, and wastewater. In particular embodiments, the invention discloses and claims compositions comprising zero-valent iron and hydrogenotrophic bacteria for use in removing target contaminants by a synergistic combination of abiotic and biological reductive mechanisms.
1.2 Description of Related Art
1.2.1 Abiotic Processes
Various abiotic processes have developed in recent years for the remediation of hazardous environmental pollutants (National Research Council, 1994). One process for abiotic remediation of organic and inorganic pollutants has been developed using zero-valent iron-[Fe(0)] mediated processes.
In this process, as elemental iron is oxidized (corrodes), electrons are released according to the following equations:
Fe
0
→Fe
2+
+2
e
  (Equation 1)
These electrons are available for a variety of reduction-oxidation (redox) reactions. Water is reduced to produce hydrogen gas and alkalinity in the form of OH
−
, with the net reaction resulting in a pH increase:
2H
2
O+2
e
−
→H
2
+2OH
−
  (Equation 2)
Fe
0
+2H
2
O→Fe
2+
+H
2
+2OH
−
  (Equation 3)
Encouraging results in both laboratory and field experiments have stimulated a rapid increase in the use of Fe(0) as a reactive material to treat reducible contaminants (so-called reactive Fe(0) barriers). This approach has been used to degrade waste chlorinated solvents (e.g., Gillham and O'Hannesin, 1994; Johnson et al., 1996; Sweeny, 1980) and nitrate (Till et al., 1998). Reducible heavy metals such as Cr(VI) can also be removed from aqueous solution by reduction to less toxic forms (e.g., Cr(III)) and subsequent precipitation and immobilization, using Fe(0) as the sacrificial metal (Gould, 1982; Khudenko, 1987; Powell et al., 1995; Rickard and Fuerstran, 1968).
Semipermeable reactive Fe(0) barriers have been attractive for groundwater remediation in that they conserve energy and water, and through long-term low operating and maintenance costs, are considerably less costly than conventional cleanup methods. Fe(0) can be placed in the path of a contaminant plume, either on a trench (O'Hannesin and Gillham, 1992), buried as a broad continuous curtain (Blowes et al., 1995), or injected as colloids (Kaplan et al., 1994), to name a few options. However, the efficacy of Fe(0) systems can be limited by (site-specific) slow rates of reaction and by the potential accumulation of products of equal or greater toxicity (Matheson and Tratnyek, 1994; NRC, 1994; Roberts et al., 1996).
1.2.1.1 Chromium Remediation Using Abiotic Processes
Depending upon solution chemistry and pH, Cr(VI) can be present in the form of CrO
4
2−
, HCrO
4
−
, H
2
CrO
4
, and Cr
2
O
7
2−
. All of this hexavalent chromium species could be reduced to the less toxic, less mobile trivalent form, which is removed from solution as the hydroxide (i.e., Cr(OH)
3
) under most conditions, using Fe(0) (Gould, 1982; Khudenko, 1987; Powell et al., 1995; Rickard and Fuerstran, 1968):
 2Cr
6+
+6
e
−
→2Cr
3+
  (Equation 4)
3Fe
0
→3Fe
2+
+6
e
−
  (Equation 5)
2Cr
3+
+6OH
−
→2Cr(OH)
3
  (Equation 6)
2Cr
6+
+3Fe
0
+6OH
−
→2Cr(OH)
3
+3Fe
2+
  (Equation 7)
The increase in pH caused by iron corrosion (Equation 3) is thus beneficial in removing Cr(III) from solution.
When present in the environment, it is possible for the various species of Cr(VI), and Cr(III) to be sorbed to soils and sediments. It has been shown that Cr(VI) can be reduced to Cr(III) spontaneously by soil organic matter and/or by microorganisms under reducing conditions (Wang et al., 1989; Ishibashi et al., 1990; Yamamoto et al., 1993). Similarly, it has been shown that U(VI) can be reduced to U(IV) by microorganisms (Lovely and Phillips, 1992a and 1992b; Gorby and Lovely, 1992; Thomas and Macaskie, 1996). Once Cr(VI) is reduced to Cr(III) whether by soil organic matter or zero-valent iron, it is highly unlikely (due to kinetic constraints) for it to be oxidized once again. Only in the presence of freshly precipitated manganese oxides (MnO
2
) or a strong oxidant like Fenton's reagent (hydroxyl radicals) can Cr(III) be reoxidized.
Fe(0) has shown significant promise in reducing, and thus removing from solution, Cr(VI) (e.g., Blowes et al., 1995; Powell et al., 1995; Gould, 1982). Presently, however, only one field site (Elizabeth City, N.C., Coast Guard site) exists where a reactive Fe(0) barrier is being evaluated to contain and remediate a groundwater plume contaminated with both Cr(VI) and TCE. (Morrison and Spangler, Roy E. West Geotech, Grand Junction, Colo.).
1.2.1.2 Uranium Removal Using Abiotic Processes
Uranium generally exists as the uranyl cation (UO
2
2+
) in soils and groundwaters. It is tightly bound to soil and aquifer media at pH values greater than 6.0. However, it can be complexed by sulfate and organic ligands as well. Longmire et al. (1990) found that the predominant species in acidic uranium mill tailings deposits of New Mexico and Colorado was uranyl disulfate, UO
2
(SO
4
)
2
2−
; uranyl sulfate aqueous complex, UO
2
(SO
4
)
0
, uranyl divalent cations, and uranyl biphosphate, UO
2
(HPO
4
)
2
2−
; in that order. Once U(VI) is reduced to U(IV), it becomes much less mobile, similar to chromium. Immobilization is caused by the precipitation of uranium dioxide, and by strong sorption of U(IV) species to soils and sediments. Similar to Cr(III), once uranium has been reduced, it is not likely to become mobilized again unless the pH is reduced or a strong oxidizing agent is encountered. Treatment with zero-valent iron removes dissolved oxygen and increases the pH; both conditions which aid the chemical reduction of chromium and uranium and which keeps them immobilized.
Hexavalent uranium can also be reduced to the less mobile U(IV) form which is removed from solution as the oxide under most conditions:
U
6+
+2
e
−
→U
4+
  (Equation 8)
U
4+
+4OH
−
→UO
2
+2H
2
O  (Equation 9)
Here again, the increase in pH caused by Fe(0) corrosion (Equation 3) is conducive to U(IV) precipitation from solution (Lovely and Phillips, 1992a; Morrison et al., 1995; Thomas and Macaskie, 1996).
Unfortunately, no reports in the literature describe the use of reactive Fe(0) barriers to reduce and remove U(VI), although a pilot facility in Durango, Colo., is presently being tested (S. Morrison of Roy E. West Geotech, Grand Junction, Colo.).
1.2.1.3 Removal of Polychlorinated Organics Using Abiotic Processes
Polychlorinated organics can also be reduced using the e
−
generated by iron corrosion by replacing Cl atoms with hydrogen atoms. This form of reductive dechlorination is termed hydrogenolysis (Vogel et al., 1987). Using carbon tetrachloride (CT) as an example, first chloroform (CF) and then dichloromethane (DCM) are formed:
CCl
4
+2
e
−
+H
+
→CHCl
3
+Cl
−
  (Equation 10)
CHCl
3
+2
e
−
+H
+
→CH
2
Cl
2
+Cl
−
  (Equation 11)
Several researches have shown that DCM is a “dead-end” product of abiotic treatment of CT or CF with Fe(0) (Helland et al., 1995; Matheson and Tratnyek, 1994). Thus, while abiotic processes are able to reduce some organic compounds, the process often results in endproducts which are toxic, themselves. As such, there are limitations to the use of abiotic processes alone in the remediation of organic compounds from the environment.
TCE is reportedly converted to e

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