(247d) Electrochemical and Photochemical Oxidation of Ethylendiaminetetraacetic Acid

Advanced oxidation processes will increasingly contribute to wastewater treatment in the near future. Advanced oxidation processes focus on persistent, poorly biodegradable substances and substances with mutagenic or carcinogenic properties. Ethylendiamintetraacetic acid (EDTA) is widely used in both industrial as well as consumer application. It has well known complexing properties and it is poorly biodegradable. On this account EDTA was chosen as a representative persistent organic pollutant for investigation of degradation and mineralization by chemical, electrochemical and photochemical techniques. On the other hand the chemical composition and properties of the substance EDTA itself have been major reason for its choice.

Chemical oxidation treatment with H2O2 and H2O2/Fe2+ was investigated for comparison purposes with photochemical and electrochemical treatment.

The effect of pH, heavy metal ion load and photocatalysts on degradation under UV-irradiation and air saturation of the effluent has been reported elsewhere [Gangl et al., 2004]. Extinction is pH-dependent. The impact and interaction of heavy metal ions depends on the element properties according to the periodic system of elements. As a consequence heavy metal ions may undergo oxidation or precipitation or even retard or accelerate degradation under air saturated oxygen load and varying pH. Photocatalysts may contribute to degradation (e.g. anatas) but also undergo chemical reaction (e.g. zinc sulphide).

Rate and mechanism of electrochemical degradation differs from photochemical treatment [Letonja, 2004; Kracker-Semler, 2004]. In this research project the effect of the electrolyte composition (e.g. waste water) and the operation conditions on rate and mechanism of degradation and mineralization has been investigated. Subject of present investigation has been the optimization of treatment and the correlation of degradation, mineralization and specific power consumption. Photochemical treatment has been carried out with a Hg-medium pressure UV-lamp.

Figure 1 shows a representative comparison of degradation and mineralization under UV-irradiation of air saturated synthetic effluent. While degradation undergoes fast 1st order reaction mineralization will stop after several minutes. Comparison indicates that mineralization is concurrent to degradation by cleavage. Heavy metal ion load tremendously affects the extent and rate of mineralization.

Experimental investigation of electrochemical treatment was carried out in a membrane cell with the basic electrolyte sodium sulphate and a DSA (IrO2-doped titanium grid). While cleavage and mineralization are favoured at low current density oxygen evolution controls the mid current density range without significant improvement of both degradation and mineralization. Finally at elevated current density both processes will improve. For application reasons the specific energy consumption and as a consequence the current density are important design boundaries.

Figure 1: EDTA and TOC decrease for: (A) UV only; (B) UV and EDTA : Heavy metal ion = 1:1

References:

Gangl W., Marr R., Siebenhofer M.: Effect of Oxygen Partial Pressure and Catalysts on the Rate of UV irradiated Oxidation of EDTA, AIChE Annual Meeting, 2004

Letonja P.: Elektrochemische Redoxreaktionen in der Umweltechnik, Dissertation TU-Graz, 2004

Kracker-Semler G.: Katalytische und photokatalytische Beschleunigung der Reaktivabsorption von Sauerstoff - Experimentelle Untersuchung und Modellierung, Dissertation TU-Graz, 2004