(166e) Anodic Oxidation of Wastewater Constituents; Investigation of Process Parameters

Authors: 
Zelenka, J., Graz University of Technology
Gangl, W., University of Technology Graz, Department of Chemical Engineering and Environmental Technology
Letonja, P., Graz University of Technology
Siebenhofer, M., Graz University of Technology
Marr, R., Graz University of Technology


Since the natural resources potable water is endangered to run short it is necessarily important to find optimised ways for wastewater treatment. EAOPs (Electrochemical Advanced Oxidation Processes), like anodic oxidation, are expected to contribute to improved water management. Innovative electrode materials such as several metal oxides and in special diamond doped titanium electrodes appear to be the right answer. Investigation of this project has been anodic oxidation of the wastewater constituent ethylenediaminetetraacetic acid (EDTA) in open cell electrolysis. Target of the project has been modelling of kinetics of degradation and TOC depletion. Modelling has been based on the mechanism of degradation and TOC depletion proposed for photochemical treatment [Sörensen et al., 1998]. Although electrocatalytic effects are well reported in general, detailed data for specific applications still need extended experimental investigation of operation parameters. Depending on the operation conditions cleavage of the molecule or TOC depletion is favoured. While cleavage may suffice the necessity of standard wastewater treatment, water reclaim may need highly efficient TOC depletion. As a consequence the clean water specification is an important boundary for process configuration and the operation conditions. Summarized qualitatively the investigation of operation parameters shows the following correlations and trends. For fixed start pH-value different current density may either lead to abstraction of carboxylic fragments in a first period of electrolysis or it may favour cleavage of the nitrogen-acetate bond. The start pH-value has a severe effect on the ratio of TOC depletion and cleavage of nitrogen-acetate bonds. TOC depletion passes optimum conditions in the region of dissociation step two of the basic molecule, while cleavage is favoured at lower pH-values. As expected from electrode specifications diamond coated electrodes enable efficient TOC depletion at much higher current density compared with any alternative anode material. Elevated temperature of operation will cause an unwanted break down of oxygen overvoltage, resulting in an acceptable degree of molecule cleavage but poor TOC depletion. Cleavage of EDTA has been analysed through the complexation properties of the synthetic effluent. Oxidation has been quantified through TOC-analysis.

References

Sörensen, M., Zurell, S., Frimmel, F. H.: Degradation Pathway of the Photochemical Oxidation of Ethylenediaminetetraacetic Acid in the UV/H2O2-process, Acta hydrochim. Hydrobiol. 26, 109-115, (1998)