Microwave assisted remediation of organic waste in aqueous effluent streams

Abstract

The nuclear industry in line with many other industries has a problem with contaminated organic waste in aqueous effluent.
During the PUIREX process operated by British nuclear Fuels Ltd, the recovery of radioisotopes using 30% tributyl phosphate in odourless kerosene, the organic phase is degraded due to the effects of radiolysis. This results in a very complex mixture with dibutyl phosphate, monobutyl phosphate and phosphoric acid amongst its components; the odourless kerosene is also degraded forming long chain carboxylic acids. Another consequence of this process is the production of finely divided solids such as zirconium phosphate, these solids together with the complex organic mixture form stable emulsions, known as cruds. These tend to form at the interface between the organic and aqueous layers. These cruds cause problems with further recovery of metal and also physical problems such as mass transfer.
This project was initially aimed to ascertain whether microwaves could be used to assist the total oxidation of these cruds. However, due to the complexity of these cruds it was decided that a model system should be studied. Tributyl phosphate was
chosen as this model, this was deemed to be appropriate as it is the single most stable component of the "cruds".
The theory behind the technique is if a suitable sensitiser was used tributyl phosphate would absorb onto its surface. When exposed to microwave energy a localised plasma would be generated around the surface of the particles. It is in this very highenergy field that oxidation of the tributyl phosphate would occur.
Work performed at the University by John Rawcliffe for his MSc thesis, had demonstrated the concept that tributyl phosphate could be converted to its degradation products by the use of microwave radiation.
The aim of this project was initially to establish sound analytical methods for measuring the degradation products of tributyl phosphate and other chemicals that were to be investigated during the work, and to optimise the reaction conditions to
greatly improve the conversion of tributyl phosphate to its degradation products. It was hoped this could be achieved by altering a number of parameters including air and liquid flow and the way in which the microwaves are applied to the type of
sensitiser used.
When the optimum conditions had been established then further work was undertaken to scale-up the size of the reaction vessels to ascertain the difficulties that would be faced in scaling up to pilot plant as this was the ultimate aim of the project sponsor.
In order for scale-up to go ahead new reaction vessels and microwave cavities had to be designed and commissioned, and the initial microwave equipment adapted to accommodate the larger reaction vessels. It was envisaged that this technique could have much wider application. To demonstrate this hypothesis the system was tested on other chemicals that were
potentially problematic for the nuclear industry, in terms of their disposal.