Transformation of Chlorinated Hydrocarbons On Synthetic Green Rusts.

Green rusts (GRs) are layered double hydroxides that contain both ferrous and ferric ions in their structure. GRs can potentially serve as a chemical reductant for degradation of chlorinated hydrocarbons. GRs are found in zerovalent iron based permeable reactive barriers and in certain soil and sediments. Some previous laboratory studies show substantial chemical reduction of chlorinated hydrocarbons such as trichloroethene (TCE) by GRs; other studies show slow chemical reduction of TCE by GRs. Additional studies are needed to understand the extent, kinetics, and mechanisms of green rust interactions with chlorinated solvents.

 In the present microcosm study, synthetic carbonate GR (GR(CO₃^2-)) or sulfate GR(GR(SO₄^2-) (0.25 g) was reacted with 60 mL of TCE (2 to 10 mg L^-1) without headspace in serum bottles at 23 ± 1 ^oC on a shaker at 100 rpm in the dark for up to 31 days. At preset time intervals, serum bottles were sacrificially sampled for determination of TCE and its degradation products (dichloroethene isomers, vinyl chloride, chloride, acetylene, ethene, and ethane). Dissolved iron, boron, and silicon were measured by Inductively Coupled Plasma – Optical Emission Spectrometry. In some batches, ferrous sulfate was added to GR(CO₃^2-) suspensions before TCE addition to bring about an initial total dissolved Fe(II) concentration at 35 mM to test the effect of surface sorbed Fe(II) on TCE degradation. In other batches, CuSO₄ was added at 0.5 to 10 mM to GR suspensions with or without a Trizma buffer (pH 8) before TCE addition to test if there is a catalytical effect of added copper ions.

 TCE was not reduced substantially by either GR(CO₃^2-) or GR(SO₄^2-). The addition of Fe(II) did not enhance TCE degradation. The addition of 0.5 mM Cu(II) increased TCE degradation rate; whereas the addition of 0.5, 1, 2, 5 and 10 mM Cu(II) increased carbon tetrachloride degradation as compared to the no copper treatment.

 Characterization of solid phases was performed by X-ray diffraction and scanning electron microscopy. When SGR suspension was not buffered, added Cu(II) was reduced to Cu(I) to form Cu₂O and CuFe(III)O₂ at pH 4-5 in the presence of TCE; whereas in Trizma buffer (pH 8), SGR reduced Cu(II) to Cu(0) (final pH > 5.4). The results show that Cu(II) ions are reduced much faster than the chlorinated solvents by GRs.