Correlation of 2-chlorobiphenyl dechlorination by Fe/Pd with iron corrosion at different pH.

The rate of 2-chlorobiphenyl dechlorination by palladized iron (Fe/ Pd) decreased with increasing pH until pH > 12.5. Iron corrosion potential (Ec) and current (jc), obtained from polarization curves of a rotating disk electrode of iron, followed the Tafel equation at pH < or = 5.5 and pH > or = 9.5. The pH dependence of the dechlorination rate constant (k1) suggests four pH regimes. In the low pH regime (3-5.5), /Ec/ and je decreased with increasing pH and k1 was linearly correlated to /Ec/ and jc0.5. The correlation between k1 and jc0.5 indicates direct involvement of active hydrogen species (on the Pd surface) in PCB dechlorination. In the mid pH regime (5.5-9.5), no significant effect of pH was evident on the values of k1, je, and Ec, a combined result of limiting anodic oxidation of iron to an intermediate product (iron hydroxide) and a proton-independent overall reaction. Both /Ec/ and jc increased significantly as pH increased from 9.5 to 14. A cleartrough of the k1 values in solutions of pH between 12 and 13 and the mismatch between the kinetic and corrosion data suggest two pH regimes (9.5-12.5 and 12.5-14) of different corrosion mechanisms.