Tube-in-tube membrane photoreactor as a new technology to boost sulfate radical advanced oxidation processes
Graphical abstract
Introduction
Conventional municipal wastewater treatment plants (MWWTPs) are not able to fully degrade contaminants of emerging concern (CECs) resulting in the significant and continuous release of such compounds to the anthroposphere which constitutes a stress factor for water resources (Loos et al., 2013). Reducing CECs risks is globally urgent and new mitigation technologies are needed towards a water protection approach (One Health Initiative, 2020; US Water Alliance, 2020).
In recent years, sulfate radical-based Advanced Oxidation Processes (SR-AOPs) are gaining attention as a potential technology for removal of CECs from municipal wastewaters (Cvetnić et al., 2019; Ike et al., 2018; Wacławek et al., 2017). Sulfate radicals can be generated from peroxydisulfate (PDS), which has high stability, high solubility (730 g L−1, at 25 °C), high molar extinction coefficient (21.1 M−1 cm−1), high quantum efficiency (1.8 mol Einstein−1) at 254 nm, and comparable price (0.18 USD mol−1) when compared to other oxidants such as hydrogen peroxide (He et al., 2014; Ike et al., 2018; Wacławek et al., 2017). The activation of PDS involves the homolytic cleavage of the O−O bond that leads to the generation of SO4•−, which has a high redox potential (2.5–3.1 V vs. NHE) (Neta et al., 1988). The main activators of the PDS are: energy in different forms (e.g. heat, sonochemical, photochemical), transition metals, carbonaceous materials, alkaline conditions, electrochemical, and through its combination with other oxidants (e.g., ozone, hydrogen peroxide, and calcium peroxide) (Duan et al., 2020).
There are several limitations in SR-AOPs systems: i) the recovery of catalysts or metal ions, which requires more steps for separation; ii) catalytic performance is highly dependent on solution pH and iii) in real wastewater, the existence of a large number of organic compounds can bind to the catalyst, which also reduces the generation of radicals (Duan et al., 2020). A possible approach to overcome such barrier is the combination of two or more different activation methods (hybrid systems), such as photochemical and photocatalytic, able to produce different reactive species (synergistic effects), such as sulfate, hydroxyl and superoxide radicals, boosting the process performance of SR-AOPs. The combination of photochemical and photocatalytic processes involves: i) direct oxidation by S2O82−; ii) photolysis of S2O82− into SO4•− in the presence of UVC light; iii) photo-oxidation of H2O/S2O82− into •OH/S2O8•− by holes (hVB+) of semiconductor valence band (VB) under UV light; iv) photo-reduction of S2O82−/O2 into SO4•−/O2•− by electrons (eCD−) of semiconductor conduction band (CB) under UV light (Yang et al., 2019a). The addition of persulfate to the photocatalytic system i) minimizes e−/h+ recombination since eCD− are captured by S2O82− more easily than by H2O2 and O2, due to the higher electron affinity of S2O82− (2.1 against 1.77 and 0.44 eV, respectively) (Guerra-Rodríguez et al., 2018; Huling and Pivetz, 2006) and, ii) increases the ratio of “radicals product/electron-hole pairs consumption” for S2O82− pathway compared to that for O2 pathway (Monteagudo et al., 2019; Schneider et al., 2014). Another possible strategy is to develop new oxidant dosing methods able to reduce the dosage of PDS required and minimize the quenching reactions between radicals, decreasing the sulfate post-contamination. Vilar and co-workers proposed a low footprint tube-in-tube membrane reactor (Castellanos et al., 2020) to drive in a smart way the oxidant into the catalyst surface and water to be treated, through ”virtually” unlimited number of oxidant dosing points across the membrane and further dispersed to the annular reaction zone where the oxidant is activated by UVC light, reducing its consumption, improving the contact with the pollutants, and avoiding catalyst deactivation.
Therefore, this paper investigates, for the first time, the use of a tube-in-tube membrane reactor to boost the efficiency of UV/S2O82− and UV/S2O82−/TiO2 for the treatment of pharmaceutical compounds in municipal secondary effluent. Additional objectives are: (i) to identify the main pharmaceuticals transformation products (TPs) with proposed pathways and, (ii) to assess the potential toxicity and biodegradability of the TPs generated during the treatment.
Section snippets
Chemicals and materials
Analytical grade (purity > 98.99%) nimesulide-NMD, furosemide-FRS, paracetamol-PCT and diazepam-DZP were used in this work as target pollutants (see Table S.1). These pharmaceuticals are frequently detected in hospital and urban wastewaters (Becker et al., 2019; Papageorgiou et al., 2016). Sodium persulfate (Merck, 98%) was used as oxidant. AeroxideⓇ TiO2-P25 powder (>99.5% w/w purity, crystalline phases: 20% wt. rutile and 80% wt. anatase) was supplied by Evonik and used as catalyst. Ethanol
S2O82– radial addition tests and steady-state conditions
Steady-state conditions for S2O82– concentration (in the absence of UVC light and pharmaceuticals - Fig. S1a and b) and for concentrations of pharmaceuticals at the reactor outlet using the UVC/S2O82– (membrane without catalyst) (Fig. S2a) or UVC/S2O82–/TiO2 (membrane coated with catalyst) (Fig. S2b) systems in the water flow side at the reactor outlet are obtained after t/τ ≥ 100 (τ is the space-time inside the illuminated ARZ). In the next sections, the conversion of pharmaceuticals will be
Conclusions
A low footprint tube-in-tube membrane photoreactor, operated in a continuous-mode, was able to boost sulfate radical advanced oxidation processes and pharmaceuticals removal due to two main points: i) combination of photolysis and chemical electron transfer activation methods for ROS production and, ii) ability to drive in an efficient way the oxidant into the catalyst surface and water to be treated, through ”virtually” unlimited number of oxidant dosing points across the membrane, providing a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This study was financially supported by: i) Base Funding - UIDB/50020/2020 of the Associate Laboratory LSRE-LCM - funded by national funds through FCT/MCTES (PIDDAC); ii) Project NOR-WATER funded by INTERREG VA Spain-Portugal cooperation program, Cross-Border North Portugal/Galiza Spain Cooperation Program (POCTEP), iii) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and, iv) Brazilian authors thank CNPq (Process: 403051/2016-9). Elisabeth Cuervo
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