Effects of electrooxidation vs photoelectrooxidation on bentazone ecotoxicity for different supporting electrolytes

Introduction

Directive (EU) 2024/3019 on the treatment of urban wastewater mandates using quaternary treatments for reducing both the concentration and toxicity of micropollutants. In this work the effectiveness of electrooxidation and photoelectrooxidation processes to eliminate bentazone (BTZ) was studied. BTZ is a micropollutant detected in the Albufera lake (Spain).

Methods

A new ceramic anode made of Sb-SnO₂ and coated with a Bi₂WO₆ photocatalyst was used. Three supporting electrolytes were tested: 1.65 g L^-1 of NaCl; 2 g L^-1 of Na₂SO₄; and a mixture of 0.46 g L^-1 of NaCl and 1.3 g L^-1 of Na₂SO₄ (this last being similar to Albufera lake conditions). The initial concentration of BTZ was 100 mg L^-1. Two current intensities (0.2 and 0.6 A) were applied, both in the absence and presence of light provided by a xenon lamp. After 4 h, the degradation and mineralization percentages were determined. Ecotoxicity was determined using Lactuca sativa seeds according to US EPA OPPTS 850.4200 standard, after adjusting the pH between 5.5-8, if necessary.

Results

The degradation and mineralization degrees achieved using the mixed electrolyte show intermediate values between those achieved with pure electrolytes. Applying 0.6 A, they are very close to the maximum values achieved with pure NaCl. Moreover, the final effluents toxicity is significantly lower, especially when light is applied. Therefore, the photoelectrooxidation process applying 0.6 A with the mixed electrolyte is the most effective technique from the combined point of view of final degradation (90.9%), mineralization (62.4%) and toxicity (elongation of 47.4% compared to control, p-value < 0.05). Furthermore, the degradation achieved is greater than 80%, minimum value required by Directive (EU) 2024/3019 (Table 3, Annex I) to reduce the micropollutants concentration.

Conclusions

Photoelectrooxidation is a good option as quaternary treatment in wastewater treatment plants with the following additional advantages. It does not need the addition of chemicals, and energy consumption could be supplied by alternative sources, especially in small communities.