Surface Decontamination by Photocatalysis
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Surface Decontamination by Photocatalysis Richard J. Wilbraham1, Colin Boxall1 and Robin J. Taylor2 1 Engineering Department, Lancaster University, Lancashire, LA1 4YW, UK 2 National Nuclear Laboratory, B170 Central Laboratory, Seascale, Cumbria CA20 1PG, UK ABSTRACT Currently in the nuclear industry, surface contamination in the form of radioactive metal or metal oxide deposits is most commonly removed by chemical decontamination, electrochemical decontamination or physical attrition. Physical attrition techniques are generally used on structural materials (concrete, plaster), with (electro)chemical methods being used to decontaminate metallic or painted surfaces. The most common types of (electro)chemical decontamination are the use of simple mineral acids such as nitric acid or cerium (IV) oxidation (MEDOC). Use of both of these reagents frequently results in the dissolution of a layer of the substrate surface increasing the amount of secondary waste which leads to greater burden on downstream effluent treatment and waste management plants. In this context, both mineral acids and MEDOC can be indiscriminate in the surfaces attacked during deployment, e.g. attacking in transit through a pipe system to the site of contamination resulting in both diminished effect of the decontaminating reagent upon arrival at its target site and an increased secondary waste management requirement. This provides two main requirements for a more ideal decontamination reagent: Improved area specificity and a dissolution power equal to or greater than the previously mentioned current decontaminants. Photochemically promoted processes may provide such a decontamination technique. Photochemical reduction of metal ion valence states to aid in heavy metal deposition has already been extensively studied, with reductive manipulation also being achieved with uranium and plutonium simulants (Ce). Importantly photooxidation of a variety of solution phase metals, including neptunium, has also been achieved. Here we briefly review existing decontamination techniques and report on the potential application of photo promoted oxidation technologies to metal dissolution (including process steels) and to the dissolution of adsorbed actinide contaminants. INTRODUCTION Decontamination of both metallic and non-metallic surfaces presents a number of challenges to the chemist and engineer. No ‘one for all’ technique exists and therefore decontamination processes must be applied on a case by case basis. In general, techniques may be categorised as either physical or chemical depending on their nature. While physical techniques are more useful in surface bound contamination removal, chemical techniques are often deployed to deal with nonporous surfaces with particularly deep contamination levels [1]. Thus, it is important to understand chemical decontamination technologies currently employed by the nuclear industry. Chemical decontamination techniques can be defined as either single step or multi-step processes. Generally single step chemical processes are v
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