Chlorine, oxidation-reduction behavior

Two conceptual models are provided for environments in which Type 3 behavior occurs. For sources with PCE and TCE, the major natural attenuation processes are dilution and dispersion alone (no biodegradation). As shown in 23.1.7, the PCE and TCE plumes extend from the source zone and concentrations are slowly reduced by abiotic processes. Chloride concentrations and oxidation-reduction potential will not change as groundwater passes through the source zone and forms die chlorinated ethene plume. If TCA is the solvent of interest, significant abiotic hydrolysis may occur, resulting in a more rapid decrease in TCA concentrations and an increase in chloride concentrations. 

As mentioned above, a single chlorinated solvent plume can exhibit different types of behavior in different portions of the plume. This can be beneficial for natural biodegradation of chlorinated solvent plumes. For natural attenuation, this may be the best scenario. PCE, TCE, and DCE are reductively dechlorinated with accumulation of VC near the source area (Type 1) then, VC is oxidized (Type 3) to carbon dioxide, either aerobically or via Fe(III) reduction further downgradient and does not accumulate. Vinyl chloride is removed from the system much faster under diese conditions than under reducing conditions. 

Processes in which the oxidation state of an eiement decreases are reduction processes. Consider the behavior of chlorine in its reaction with sodium. Each chlorine atom accepts an electron and becomes a chloride ion. The oxidation state of chlorine decreases from 0 to —1 for the chloride ion (Rules 1 and 2, Figure 1.l). 

Usanovich attempted to explain the relationship by including oxidation as a special case of acidic behavior. Reduction was considered a special case of basic behavior. Chlorine was listed as an acid and sodium as a base. This classification has more experimental justification than would appear at first glance. Sodium, when reacting with water, increases the concentration of solvent anions as do bases when dissolved in many amphoteric solvents ... 

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