Stoichiometric excesses of H2O2 as dosimetry strategy: proof of concept for UVC-H2O2, dark-Fenton, and UVC-Fenton

Abstract

Hydrochlorothiazide (HCT) is a pharmaceutical micropollutant highly toxic to the environment, being absolutely necessary to oxidize it completely to CO₂. Here, the variables stoichiometric H2O2 excess for (a) degradation and (b) mineralization are defined and used as metric to quantify the dosimetry of the H₂O₂. So that, dose of H₂O₂ qualifies being under- and over-dose respectively for values below and above such standards. In this work, these concepts have been elucidated across AOPs regarding the H₂O₂ degradation excess, whereas only UVC-Fenton was used regarding the H₂O₂ mineralization excess. At a H₂O₂ mineralization excess of 0.68 (equivalent to degradation excess of 36.74), oxidation via UVC-H₂O₂ enables absolute (100%) HCT degradation within 60 min; however, the mineralization of HCT demonstrated limited optimization for all AOPs employed in the beaker-like reactor of this work, being the underlying reasons investigated hereby. At best, 26.70% HCT mineralization was observed within 60 min of UVC photo-Fenton using an initial 2.00 H₂O₂ mineralization excess. Such mineralization of 26.7% is unexpectedly low considering that, in addition, the residual H₂O₂ concentration almost fully depletes within 30 min of UVC-Fenton oxidation. Taken all that together, the loss of H₂O₂ due its decomposition induced by the risen temperature from 28 to 70ºC very likely were the underlying reason preventing better mineralization performance. We successfully demonstrated 18% of mean efficiency of radical •OH consumption signals that the overheating is indeed a designer problem with the photo-reactor since a well-refrigerated photo-reactor shows a mean efficiency of 38% for the same H₂O₂ excess.

Keywords: Accelerated degradation; Beaker-assembled reactor; H2O2 dosing method; H2O2 excess; Hydrochlorothiazide; Time-resolved H2O2 concentrations.