Degradation of diatrizoate in a pin-to-liquid plasma reactor, its transformation products and their residual toxicity

Abstract

Diatrizoate, a recalcitrant iodinated contrast medium, has been detected in wastewaters and water bodies worldwide, with concentrations in hospital wastewaters in the μg/L range. Considering that conventional wastewater treatment technologies and ozonation fail to adequately remove diatrizoate, there is a need for researching alternative treatment technologies. In the present work, aqueous solutions of diatrizoate were treated in a compact (7.5 mL liquid volume) non-thermal plasma–liquid reactor. Oxygen and air plasmas were used for diatrizoate removal, with oxygen plasma performing better. After 20 min of treatment of 200 μg/L diatrizoate in pure water, oxygen led to 90% degradation whereas air achieved only 20% degradation. Two diatrizoate concentrations were studied: 10 mg/L to facilitate the identification of transformation products when using an oxygen plasma and 200 μg/L, a concentration in the range expected in hospital wastewater. For the oxygen plasma treatment, the energy efficiency for removing diatrizoate was 0.140 g/kWh for C0 = 10 mg/L and 0.006 g/kWh for C0 = 200 μg/L. The following transformation products of diatrizoate were identified: m/z 486A, 504A, 504B, 628, 630A, 630B (previously described in literature) and m/z 486B, 550, 555, 565, 578 (novel). Acute toxicity tests on Aliivibrio fischeri revealed no residual toxicity associated to transformation products. Since diatrizoate is a particularly recalcitrant aqueous organic pollutant, its successful removal and the lack of formation of toxic transformation products indicates that the developed plasma-based technology is promising for treating challenging wastewaters, such as hospital wastewaters.