Fenton and Photo-Fenton like at neutral pH for the removal of emerging contaminants in water and wastewater effluents

Abstract

In the last decades, the scientific community has been involved in the research of new kinds of contaminants generally known as of “emerging concern” (CECs). The harmfulness of CECs, even at small concentrations as well as, property of bioaccumulation and persistence, makes them extremely dangerous for the human health. The scientific community is constantly researching about novel treatments able to achieve the removal of these contaminants.

Advanced Oxidation Processes (AOPs) are considered one of the most useful treatments to achieve CECs degradation. Among the AOPs, Fenton and photo‐Fenton processes are particularly powerful, cheap and easily managed. Nevertheless, some setting requirements of Fenton processes have limited their application at industrial scale. One of the most important limits is the necessity to operate a tight control of the pH in order to avoid iron precipitation (optimum pH~2.8). Unfortunately, the optimum pH for Fenton reaction is essentially far from the normal values of the wastewater treatment plant (WWTP) effluents. Scientific community is then working on the improvement of the operating conditions of Fenton processes in order to improve the applicability in wastewater treatment. These modifications are essentially focused on the possibility to perform the treatment at circumneutral pH (Fenton and photo‐ Fenton like processes). Fenton like processes can be carried out in heterogeneous or homogeneous way according to the phase of the catalyst into the solution.

In this study was firstly confirmed the suitability of Fenton based processes in recalcitrant compounds removal. Fenton, UV‐A photo‐Fenton and UV‐C photo‐Fenton were, in fact, applied for atrazine removal from secondary effluent (SE) of municipal wastewater treatment plant (MWWTP). UV‐A and UV‐C photo‐Fenton allowed remove 50% and 100% of the initial atrazine content respectively.

The main objective of this thesis was then the assessment of photo‐Fenton’s suitability for recalcitrant contaminant at circumneutral pH. Thus, homogeneous photo‐Fenton like at neutral pH was applied for sulfamethoxazole (SMX) removal. In order to avoid iron precipitation, chelating agents were used to keep soluble the iron at circumneutral pH. The chelating ability of four chelating agents (ethylenediaminetetraacetic acid‐EDTA, nitrilotriacetic acid‐NTA, oxalic acid and tartaric acid) was tested. Then, once determined the optimum molar ratio L:Fe for iron chelation (1.5:1 for EDTA and NTA, 10:1 for tartaric acid and 20:1 for oxalic acid), their catalytic activity was evaluated when employed in photo‐Fenton like for SMX removal. The highest SMX percentage removal, together with the minimum chelating agents required and the better property of biodegradability and low toxicity, demonstrated the suitability of NTA for the purpose.

A further study on the stability of the chelates under reaction was carried out. The operating conditions adopted for the treatment significantly influence the stability of the chelate solution. Thus, in order to proper control the parameter set up the behavior of chelates has been study under thermal, oxidative and photochemical stress. It was demonstrated as the temperature control can represent an interesting tool to extend the chelates lifetime under oxidative and photochemical stress. By adopting different H2O2 doses, a linear correlation between doses and chelate decomposition could be identified. The better suitability of UV‐A irradiation, against UV‐C and Xe lamp, to preserve the iron chelate solution was demonstrated.

Moreover, the influence of the influent characteristics on the process efficiency needed to be also considered. Thus, different water matrices were used for the experiments. The efficiency of photo‐Fenton like catalyzed by Fe(III)‐NTA has been compared when applied to different aqueous matrixes (Milli‐Q water, tap water, secondary effluent wastewater and well water). It was demonstrated as the ions content, especially Ca2+ and Mg2+, significantly compromise the process of chelation. High alkalinities and organic matter, instead, mainly influenced the phase of process, when acting as radicals scavengers, reduced the amount available for SMX oxidation. Some strategies were then adopted to promote SMX removal. Between them, Mn2+ mediated photo‐Fenton like showed somehow possibility for improvement. Highest removal rate was in fact exhibited in the first minutes of reaction when adding Mn2+ to the solution in ratio molar Mn:Fe 0.5:1.

The conclusive study of the thesis regarded the assessment of the Br‐ presence on the efficiency achievable in recalcitrant compounds removal when applying UV/PS/Fe2+ for removal of benzophenone‐4 (BZ4), nitrobenzene (NB), nitrobenzoic acid (NBA), atrazine (ATZ) and ampicilline (AMP). Br‐ demostrated to be a strong inhibitor in the removal of all the considered contaminants except for NB when, the removal was instead enhanced in bromide containing water.