The effect of the metal-amendments on the reduction of CT by GRSO4 is shown in Figure . CT was reduced by GRSO4 alone, with only 7% remaining after 670 h. However, the reduction of CT was greatly enhanced by amendment of the GRSO4 suspension with Hg(II); CT was below detection limits (< 10 nmol) within 1 h. By comparison, CT reduction was moderately enhanced in the Cr(III)-, Mn(II)-, Mo(VI)-, Pb(II)-, and V(III)-amended systems; amendment with Co(II), Ni(II), or Zn(II) had essentially minimal or no effect. Reductive dechlorination of CT by sequential hydrogenolysis can lead to the formation of methane (Figure ); however, in all systems, CF was the primary transformation product (Table ). CF concentrations tracked with CT consumption (Figure ) and remained stable after CT was depleted. Furthermore, DCM and CM were not detected (Table ), even though reductive dechlorination for these compounds is less thermodynamically favorable than for CT and CF (and therefore likely to accumulate as end products), suggesting that methane was not formed as a result of sequential hydrogenolysis. However, the accumulation of detectable concentrations of methane and traces of C2 coupling products (ethane and ethene) suggests the potential for methane formation via cryptic pathways involving radicals and carbenes/carbenoids (Figure ). The rate of CT reduction in the GRSO4 system amended with Hg (HgGRSO4) was >1000 faster than in the unamended system, and comparable to that previously reported for Cu(II)-amended GRSO4 (Table ). However, unlike with CuGRSO4 (and AgGRSO4 and AuGRSO4), where 57-64% CT was transformed to non-chlorinated products, CF was the primary product (88%) of CT reduction by HgGRSO4 (Table and Figure ). The terminal products of CT transformation observed with AgGRSO4, AuGRSO4, or CuGRSO4 were primarily non-chlorinated hydrocarbons (principally methane, with lesser amounts of ethane, ethane, acetylene, and a suite of C3 and C4 alkanes and alkenes) and CO; with minor amounts of DCM, CM, and PCE (Figure ). Enhanced reduction of chlorinated hydrocarbons has been reported for Ag(I)-, Au(III)-, Cu(II)-, and Pt(IV)-amended GR, and in each system the metals are reduced by GR to their zero-valent forms (i.e., Ag(0), Au(0), Cu(0), and Pt(0)) . We have speculated that the association of these zero valent metals with GR forms galvanic cells , with GR as the anode and the zero valent metal as the cathode (where the chlorinated hydrocarbons are reduced). Since Hg(II) is reduced to Hg(0) by GRSO4 , the enhanced reduction of CT we observed in the HgGRSO4 system may be the result of the same process. Furthermore, Liu et al. reported that the kinetics and product distribution of the electrolytic reduction of carbon tetrachloride varies with the metal used for the cathode, consistent with the differences in the rates/extents and product distributions observed among zero-valent metal GR systems. The rates of CT reduction were moderately higher in the Cr(III)-, Mn(II)-, Mo(VI)-, Pb(II)-, and V(III)-amended systems relative to the rate for unamended GRSO4, and no increase in the rate of CT reduction was observed in the Co(II)-, Ni(II)-, or Zn(II)-amended systems (Table ). However, unlike the HgGRSO4 system, XANES analysis of GRSO4 suspensions amended with Mo(VI), Ni(II), Pb(II), or Zn(II) showed no change in valence of these metals (Figure ). The XANES spectrum of the MoGRSO4 system has the same edge position as the Mo(VI) standard, indicating that no reduction of the added molybdate occurred. In addition, the spectrum has the pre-edge feature characteristic of the MoO4 entity, indicating that Mo(VI) remained in the molybdate form while adsorbing to the GRSO4 solids. The XANES spectrum of the Ni-GRSO4 system has the same edge position as the Ni(II) standard, indicating that no reduction of the added Ni(II) occurred. The spectrum also has a large white line (an intense absorption the near-edge region) similar to the Ni(II)-carbonate standard, suggesting that Ni(II) remained coordinated to O atoms while being adsorbed to the GRSO4 solids, or perhaps incorporated in the octahedral layers of GRSO4 . The XANES spectrum of the Pb GRSO4 system has the same edge position as the Pb(II) standard, indicating that no reduction of the added Pb(II) occurred. The spectrum also has a white line similar to the aqueous Pb(II) standard, suggesting that Pb(II) remained coordinated to O atoms while being adsorbed to the GRSO4 solids. The XANES spectrum of the Zn GRSO4 system has the same edge position as the Zn(II) standard, indicating that no reduction of the added Zn(II) occurred. The spectrum also has a large white line similar to the aqueous Zn(II) standard, suggesting that Zn(II) remained coordinated to O atoms while being adsorbed to the GRSO4 solids.