Abstract
The dechlorination of chlorinated aliphatic hydrocarbons including carbon tetrachloride (CT), tetrachloroethane (PCE) and trichloroethene (TCE) by different types of Fe(II)/Fe(III) systems and the synergistic effect of Cu(II) ions on the dechlorination were investigated. The well-crystalline goethite (a-FeOOH), hematite (a-Fe2O3), magnetite (Fe3O4) and amorphous ferrihydrite (Fe(OH)3) were associated with dissolved Fe(II) to form surface-bound iron species those were found to be reactive under anoxic conditions. X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRPD) were used to characterize the chemical states and crystal phases of solid phases, respectively. Also, scanning electron microscopy (SEM) was employed to identify the surface morphology of the solid phases. CT was not dechlorinated by dissolved Fe(II) or iron oxides at neutral pH. However, significant dechlorination of CT was observed at pH 7.2 when Fe(II) was associated with iron oxides. The dechlorination followed pseudo first-order kinetics and the rate constants (kobs) for CT dechlorination ranged between 0.0144 and 0.836 h-1 in the Fe(II)-iron oxide suspensions. Chloroform (CF) was identified as the major product during the dechlorination processes, depicting that reductive dechlorination is the dominant degradation pathway of CT by surface-bound iron species. Also, the kobs for CT dechlorination mainly depended on pH and surface-bound Fe(II) concentration. A linear relationship between surface-bound Fe(II) concentration and the kobs was established. The pH of the surface-bound Fe(II) system strongly influenced the rate and efficiency of dechlorination processes and the kobs was exponentially increased with increasing pH ranging from 4 to 8.5. A similar relationsip between pH and surface –bound Fe(II) concentration also found indicating that the pH effect on the dechlorination efficiency is mainly due to the variation in the surface-bound Fe(II) concentration. The efficiency and rate of CT dechlorinated was significantly enhanced by the amendment of Cu(II) into the suspension of iron oxide and Fe(II). The kobs values for CT dechlorination were 119, 100, 30 and 3 times greater than those without Cu(II) in the magnetite, goethite, hematite and ferrihydrite suspensions, respectively at pH 7.2. A linear relationship between kobs for CT dechlorination and the concentration of Cu(II) was observed when the amended Cu(II) concentration was lower than 0.5 mM. Moreover, the kobs for CT dechlorination was dependent on the initial Fe(II) concentration in the 0.5 mM Cu(II)-amended goethite system and followed a Langmuir-Hinshelwood relationship. The synergistic effect of Cu(II) in the Fe(II)/Fe(III) systems is primarily attributed to formation of secondary mineral phases was observed. When Cu(II) was added into Fe(II) solution in the absence of other iron oxide minerals, the oxidation of Fe(II) into Fe(III) coupling with Cu(II) reduction to form a new mineral phase. XRD and XPS analysis suggested that this solid phase contained amorphous ferrihydrite and Cu2O. When the initial Fe(II)/Cu(II) ratios in the solution varied from 1 to 10, the kobs for CT dechlorination increased 250-fold and the produced secondary minerals also changed from ferrihydrite to goethite and hematite and then again ferrihydrite. Cu(II) ion also has the synergistic effect on the dechlorination of chlorinated hydrocarbons in the presence of green rust and biogenic iron oxides. The addition of Cu(II) in to the green rust suspension effectively dechlorinated chlorinated methanes (CT, CF) and chlorinated ethenes (PCE, TCE). The kobs for dechlorination increased by 84 times for CT, 4.7 times for PCE and 7 times for TCE. XRD and XPS results showed that the oxidation of green rust chloride(GR(Cl)) to magnetite resulted in the reduction of Cu(II) to Cu(0) and Cu(I). In the presence of Geobacter sulfurreducens, ferrihydrite can be biologically dissoluted to produce Fe(II) and magnetite, resulting in the formation of biogenic Fe(II)-Fe(III) system for the dechlorination of chlorinated hydrocarbons. Addition of Cu(II) also enhanced the dechlorination of CT by biogenic Fe(II) under microbial Fe(III) reducing condition. Results obtained in this study give impetus that Cu(II) can increase the reductive dechlorination process led by natural Fe(II) systems in the subsurface conditions for the natural attenuation of highly chlorinated compounds or engineered systems that facilitate the in-situ cleanup of chlorinated hydrocarbons using Fe(II)/Fe(III) systems.