Thermal activation of persulfates for wastewater depollution on pilot scale solar equipment
Graphical abstract
Introduction
Advanced oxidation processes (AOPs) are based upon the formation of highly reactive free radicals species from different routes. Owing to the high redox potentials, the way free radicals react and degrade soluble organics is highly relevant regarding environmental remediation (Brienza et al., 2016, Marjanovic et al., 2018, Shawaqfeh and Al Momani, 2010, Vilar et al., 2011). Typically, free radicals catalyzed chain reactions readily make possible degradation and/or mineralization of a wide range of hardly biodegradable organic molecules. Combined with the direct use of solar energy for radicals production, they offer promising opportunities for designing oxidative treatment processes, at almost no additional energy cost, in agreement with the concepts of green chemistry and sustainable development (Malato et al., 2016, Strazzabosco et al., 2019). For many years since the discovery of free radicals potential for water decontamination, hydroxyl radicals (OH˚) have been considered as the most attractive (Brienza et al., 2014, Durán et al., 2010, Liu et al., 2018). In the late 1990s, OH˚ is successfully used to run in situ chemical oxidation reaction, based upon utilization of oxidant reactants such as ozone, potassium permanganate and hydrogen peroxide (Cai et al., 2018). Since then, sulfate radicals (SO4−°), generated from a primary oxidant like persulfate (PS) salt (S2O82−) have also been progressively studied regarding major advantages such as the high redox potential (E0 = 2.6 V/SHE) and the longer lifetime of corresponding sulfate free radicals, i.e. 1000 fold more than OH˚ radicals (Cai et al., 2018, Matzek and Carter, 2016). For any of the aforementioned options, running the process with solar energy upon the well-known photo-Fenton (Fe2+- H2O2, Fe2+-- S2O82−), has been so far the most popular and sustainable way to achieve radicals production (Cuervo Lumbaque et al., 2019, Malato et al., 2016, Soriano-Molina et al., 2018). However, the acidic pH conditions required to avoid iron ions precipitation for achieving Fenton reaction, now appear as a serious bottleneck for process dissemination (Clarizia et al., 2017, Johnson et al., 2008).
A number of publications have then addressed on the possibility to run AOPS under less acidic or mild pH conditions. In the event of using Fe2+ chelates under neutral pH conditions, the efficiency to achieve free radical chain reactions is still yet to be attractive (Clarizia et al., 2017, Ike et al., 2018, Matzek and Carter, 2016, Wu et al., 2014). Iron free direct generation of OH° from hydrogen peroxide has also been addressed very recently and seems promising, as far as solar driven AOPs are concerned (Aguas et al., 2019, Ferro et al., 2015). Regarding sulfate radicals (SO4−°), “iron free” heat activation route of persulfate (schematically represented with Eq. (1) is also reported (Cai et al., 2018, Ghauch et al., 2015, Olmez-Hanci et al., 2013) but few research studies addressed solar heat activation of PDS and the real asset it could provide as compared with more conventional existing AOPs.
The few recent papers dealing with the issue showed how successful is the activation of persulfate (single used reactant) towards a wide range of biorefractory organic pollutants in water (Cai et al., 2018, Ghauch et al., 2012, Johnson et al., 2008, Yang et al., 2017). The results obtained on lab scale experiments are quite promising but the demonstration of the technique on real pilot equipment needs further investigation, especially in the event of using solar irradiation as the only source of energy. Indeed, regarding heat energy needed to achieve relevant target temperature, energy efficiency added value of what could be the future real processes is definitely relevant. Unlike conventional “in direct sunlight” photocatalysis scheme, thermal activation based process appears more “robust” since the heat energy collected (from sunlight) is stored over time for optimal usage, including during less fair weather episodes.
This paper investigates on the capacity of a pilot equipment to run solar driven heat activation of PS for water depollution. The system efficiency is investigated regarding emerging contaminants found downstream wastewater (WW) plants. To the best of the author knowledge, this is the first time a pilot scale solar demonstration is used to run PS thermal activation for the removal of micropollutant in real WW stream. In the south west of Europe, some areas face overexploitation of water resources together with lack of precipitations. In the perspective of achieving the new circular economic paradigm of water reuse, emerging and biorefractory microcontaminants need to be discarded from WW effluents. However, most of these microcontaminants are biorefractories molecules like pharmaceuticals and pesticides constituents, which are hardly removed by existing biological treatments (Devi et al., 2016, Faraldos and Bahamonde, 2017, Oh et al., 2016). At the meantime, their high toxicity level and bioaccumulation fate represent a real threat to public health and environmental protection as well (Brienza and Chiron, 2017, Miralles-Cuevas et al., 2018).
The main objective of this study is to achieve PS thermal activation and then removal of emerging pollutants from WW, on a pilot scale newly designed solar equipment. Prior to real pilot scale demonstrations, degradation of ten target molecules was lab scale investigated on small wastewater samples. This was necessary to collect information on the influence of relevant experimental parameters on the depollution process. The ultimate aim was then to set favorable conditions for running the pilot equipment under solar irradiated and optimal basis.
Section snippets
Preliminary experiments
This section deals with indoor experiments, carried out in the laboratory prior to real solar pilot scale experimentations. Working first on low WW volumes (500 mL) was necessary to understand how impactful experimental parameters on degradation process are.
Pilot experimental set up
A picture of the pilot is presented in Fig. 7, together with a descriptive diagram. It is mainly composed of two tanks (upper and lower) of 1 m3 capacity, three flat solar panels (3 × 2 m2 light harnessing surface) set in series and a heat exchanger (plate type model). Irradiation conditions were monitored with a radiometer (Kimo CR-110) and temperatures of WW in the upper tank, at the outlet of the panels and the inlets/outlets of the heat exchanger were measured with thermocouples (K type)
Conclusion
Persulfate thermal activation can be effective in removing emerging contaminants in water upon advanced oxidation reactions. In this paper, persulfate is thermally activated on a solar pilot to drive degradation of ten target pollutants typically found in wastewater streams. Preliminary studies were first carried out prior to outdoor experimentations on real solar pilot equipment of 1000 L capacity. Indoor preliminary experiments highlighted how increasing temperature and oxidant concentration
Declaration of Competing Interest
The author declare that there is no conflict of interest.
Acknowledgements
The results published in this research paper are part of the findings of the INTERREG FEDER project called 4Kets4Reuse. The authors are thankful to the European project funding.
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