Constant current has the advantage that the rate of electrolysis can be increased, but at the expense of allowing
the potential to reach more extreme values, and therefore permit more competing reactions to occur. High current
efficiency has generally been achieved in experiments run at constant potential.
One of the inefficiencies of electrolysis is that the reaction of interest being carded out at one electrode
must inevitably be accompanied by a sacrificial reaction at the counter electrode. One possible method of
minimizing this problem may be to develop two-compartment cells in which the waste is pumped first through
(say) the cathode compartment, and the partly-reduced catholyte is then subjected to oxidation in the anode
compartment. In the specific case of chlorinated aromatic compounds, this possibility has the attraction that the
most highly chlorinated compounds are dechlorinated at the least negative potentials, and the partly dechlorinated
products are then more susceptible to oxidation at less positive potentials.
3.2 Electrode materials
There are many technological challenges in the choice and fabrication of electrode materials, especially
with regard to the requirement of (i) high overvoltages for hydrogen evolution at the cathode and oxygen evolution
at the anode; (ii) high resistance to corrosion, particularly for electrodes working at high anodic potentials; (iii)
high catalytic activity, especially if reactive intermediates such as hydroxyl radicals are to be produced at the
surface; (iv) ruggedness and freedom from inactivation through poisoning; (v) manufacturing of electrodes of large
surface area and of large dimensions; (vi) cost efficiency (e.g., avoiding high loadings of noble metals).
3.3 Conclusions and unexplored research needs
Electrochemical methods of dechlorinating chlorinated aromatic compounds offer promise because of their
promise of high energy efficiency and relative simplicity of the equipment. These technologies are well suited
to leachate remediation at abandoned, often remote, landfill sites, or to the pretreatment of specific toxic process
streams ahead of a conventional biological reactor, because their cost-effectiveness does not depend on a large
throughput in order to achieve economies of scale. 42
Among chlorinated aromatics, most work has so far been done wih chlorinated benzenes and PCBs.
Although dipolar aprotic solvents have been used for most of these studies, sufficient work has been done in
methanol to demonstrate the feasibility of reduction in methanolic and aqueous methanolic solvents (oxidation in
methanol may be impractical because of the ease of oxidation of the solvent). Moreover, methanol is an
inexpensive co-solvent for water, and is non-toxic to the microorganisms in subsequent biological treatment. Work
must still be done to identify all the products of electrochemical conversion of a given contaminant, to check for
material balance, and to determine whether any electrolysis products are toxic to the microorganisms used in
subsequent biological treatment, or are persistent, bioaccumulable substances, such as dioxins. Mediated
electrolysis may offer a viable alternative to direct electrolysis, although the range of mediators that is compatible
with aqueous solution is presently restricted (ZnPc for reduction, and Ag + and Co 2+ for oxidation), and research
is needed to develop efficient mediators that are inexpensive and non-toxic. DSAs ale promising anode materials;
oxidation in this case will likely occur through the reaction of OH" with the chlorinated substrate. However, the
products of these reactions and the efficiency of formation of OH" at the DSA have so far been little explored,
as indeed is the case for electrooxidations of chloro compounds in general.