Electrochemical Destruction of Toxic Organic Industrial Waste
Electrochemical Destruction of Toxic Organic Industrial Waste
A recent report by the House of Commons Environment Committee on toxic waste drew attention to the fact that no new technology for the treatment of industrial waste had emerged in the United Kingdom during the last decade (1). The majority of wastes continued to be disposed of by burial in landfill sites, sometimes after chemical treatment—for example, neutralisation—or solidification. A further, smaller proportion was incinerated, with the most toxic and persistent materials, such as chlorinated solvents and polychlorinated biphenyls, being dealt with by a few specialist operators in plants subject to stringent controls on combustion conditions and on the levels of emissions of substances like 2,3,7,8-tetrachlorodibenzo-p -dioxin.
Public and media concern about the siting and operation of such plants has led to some being closed down, and to planning consent for new plants being the subject of an almost inevitable protracted and expensive Public Enquiry.
This article describes a novel electrochemical process, developed by AEA Technology at their Dounreay site, for the safe destruction of a wide variety of organic waste types.
The new process involves the electrolytic production of highly oxidising species in an electrolyte consisting of a silver salt, usually silver nitrate, in nitric acid. These oxidising species in turn attack the organic waste material, ultimately converting it to carbon dioxide, carbon monoxide and water, along with inorganic compounds arising from any hetero-atoms, such as phosphorus or sulphur, which are converted to phosphate and sulphate, respectively.
This process is made possible by the unique characteristics of the platinum group metals as electrode materials. At the heart of the system is an electrochemical cell, consisting of an anode compartment, a cathode compartment and a membrane separator. Cells of this type are widely used in the chemical industry for the production of chlorine by the electrolysis of brine, and a typical cell, manufactured by Imperial Chemical Industries, is shown in Figure 1. The anode of the cell consists of a titanium substrate coated with a material which is appropriate for the particular cell application, this being ruthenium dioxide in the case of chlorine manufacture. However, for the electrochemical oxidation of wastes, the coating material of choice to date has been platinum, on account of its useful overvoltage behaviour with respect to oxygen evolution. This property of platinum, allied with its resistance to very highly oxidising conditions, allows the efficient production of highly oxidising silver(II) from silver(I) (E° of Ag(II)/Ag(I) couple = + 1.98 volts versus S.H.E.) at electrode potentials considerably higher than the reversible oxygen evolution potential (E° of O 2/H 2 O couple = + 1.23 volts versus S.H.E.).
Silver(II) is one of the strongest oxidants known, being exceeded in its oxidising power only by such species as persulphate, ozone and fluorine. Solutions of silver(II) in nitric acid are an intense brown colour, due to the presence of the complex ion AgNO3 + (2), but the colour fades at a rate dependant upon the temperature of the solution, due to the destruction of the silver(II), and is accompanied by oxygen from the oxidation of water:
It is this reaction of the silver(II) with water which is the key to the electrochemical oxidation of organic wastes. The water oxidation is not a single step reaction but proceeds via a number of intermediate processes, some of which involve very highly reactive species, including OH radicals (3). These radicals tend to react indiscriminately with organic compounds and ultimately oxidise the waste to carbon monoxide, carbon dioxide and water.
There is no net consumption of silver in the anolyte. The silver functions as a “coupler” between the electric power being fed into the cell and the organic wastes being destroyed. The concentration of silver(II) in the anolyte when the process is operating at steady state is actually very low, as silver(II) has very rapid kinetics and reacts as fast as it is formed. This fast reaction in the solution, coupled with the facile production of silver(II) on a platinum electrode (the activation energy for the Ag(I) ♦ Ag(II) oxidation on platinum is approximately zero, and the rate of silver(II) production is controlled almost entirely by the rate of transport of silver(I) from the bulk solution to the anode surface (4)) results in the process running at almost the theoretical electrochemical efficiency, even at high current density.
The reaction in the anolyte overall is the same as if the organics had been oxidised directly at the anode. Using benzene as an example:
In fact this reaction can be carried out directly at a platinum anode, but only at very low current density. The H3O+ ions cross the membrane to the catholyte under the influence of the voltage being applied to the cell, typically 2-3 volts, and thereby carry the current from anode to cathode. At the cathode, nitric acid is reduced to (primarily) nitrous acid:
Some further reduction to nitric oxide and nitrogen dioxide also occurs if the nitrous acid concentration is allowed to rise.
The material of choice for the cathode is also platinum as it shows the lowest overvoltage for the nitric acid reduction reaction. This results in a saving of approximately 0.15 volts in the cell voltage, which is about 7 per cent. However, this saving must be offset against the greater cost of a platinised cathode compared to alternative materials such as stainless steel which perform satisfactorily, although at the expense of a higher cell voltage.
The catholyte is circulated between the cathode compartment and a regeneration system where air or oxygen is used to re-oxidise the nitrous acid and nitrogen oxides to nitric acid:
Apart from minor unavoidable losses, there is no net consumption of nitric acid in the catholyte. The reaction across the cathode/regeneration part of the process simplifies to:
which is, of course, the cathode reaction in a fuel cell. In both systems the oxidising power of oxygen is beneficially coupled into the process, although the oxidation of organics is being driven by electric power in one case and is driving electric power in the other.
Again using benzene oxidation as an example, the reaction across the whole process may be considered as:
(some carbon monoxide is also formed) which is stoichiometrically identical to combustion.
The initial interest in the work arose from the need to destroy the organic component of some types of combustible radioactive waste before it was conditioned for final disposal. Early trials demonstrated that materials like cellulose tissue, rubber gloves, some plastics, ion exchange resins, lubricating oils, hydraulic fluids and waste process solvent (tributyl phosphate/odourless kerosene mixture) could all be converted to carbon dioxide, carbon monoxide and inorganic products. There was an additional bonus in that the activity associated with the waste ended up in nitric acid solution, the usual form in which the waste streams from reprocessing operations arise. The acid can be removed by distillation and the residues solidified into glass or concrete before final disposal into a repository
The wide range of materials which could be destroyed led to the extension of the process to deal with organic wastes of industrial origin. To date trials have shown that phenols, chlorinated phenols, chlorinated aliphatic and aromatic solvents, organophosphorus compounds, organosulphur compounds and polychlorinated biphenyls can be destroyed, and the list is continually growing. A pilot scale rig, shown in Figure 2, has been built at Dounreay to derive the chemical engineering parameters required to build a full scale plant.
The United Kingdom Department of the Environment has recently published a provisional “Red List” of chemicals, for which it is intended to set strict environmental standards for the quality of water into which they have been discharged. This “Red List” is shown in the Table, and the 26 chemicals are accompanied by a further 23 candidates which are under investigation pending their possible inclusion. The United Kingdom list falls well short of the European Economic Community list which contains 129 chemicals. Note that all but two of the entries in the Table are organic chemicals, many of which would require well controlled incineration to effect their safe disposal. It is proposed to require processes responsible for significant point source discharges of “Red List” substances, and this would probably include toxic waste incinerators, to be authorised by Her Majesty’s Inspectorate of Pollution using standards based on the “batneec” (best available technology not entailing excessive costs) concept.
This article has described a novel process for the safe low-temperature destruction of organic waste materials. Current understanding of the process is such that it can be stated with some confidence that all of the entries in the Table, apart from mercury and cadmium, would be amenable to destruction. Although it has not yet been developed beyond pilot rig scale, appraisal of the process suggests that it may be both “bat” and “neec” for categories of both radioactive and non-radioactive organic waste which are troublesome to dispose of safely by any other acceptable means.
House of Commons Environment Clommittee on Toxic Waste, Volume 1 (Report with Appendices), H.M.S.O., March 1989
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