Solar photocatalytic reclamation of agro-waste water polluted with twelve pesticides for agricultural reuse
Graphical abstract
Introduction
The geographical conditions of a country influence largely its economic and social development. Spain, in addition to a smaller rainfall average volume compared to other European countries, has big temporal and territorial imbalances in water distribution and availability (Fanlo, 2016). Despite this fact, the use of reclaimed water alongside high quality of soils and warm winter weather offer excellent suitability for irrigation and have led to a strongly competitive agriculture, largely export-oriented (Claver, 2016). Thus, Spain is a pioneer country in production and exportation of fruits and vegetables and the Region of Murcia (SE) is a national powerhouse in this regard (Pérez Hernández et al., 2015). However, this modern agriculture depends to a large degree on the use of pesticides with the consequent risks for humans and the environment in greater or lesser degree (Pimentel, 2009; Aktar et al., 2009). During 2014, 100,373 tonnes of pesticides were applied in Spain, of which 9272 tonnes were applied in Murcia. A total of 250,055 ha was cultivated in 2014 in Murcia, and more than 60,000 cubic metres of agro-waste water were produced (average 240 L/ha), in particular from the cultivation of crops to which pesticides are intensively applied (AEPLA, 2015).
To solve this problem, apart from reducing discharges, the main water strategy is the chemical treatment of wastewaters containing biocides and bio-recalcitrant pollutants such as pesticides. Chemical treatments of polluted wastewater pertains to a long term strategy to improve the quality of water by removing toxic compounds of anthropogenic origin before returning the water to its natural cycle. In this context, the Directive 2013/39/EU (EC, 2013) promotes the development of innovative wastewater treatment technologies, avoiding expensive solutions. In this context, the development of Solar Chemistry Applications is of crucial relevance, especially photochemical processes where solar photons are absorbed by reactants and/or a catalyst causing a chemical reaction. Consequently, in last decades there has been growing interest in the use of Advanced Oxidation Processes (AOPs) to remove pesticide residues as alternative to conventional methods (adsorption or coagulation) because they allow the abatement of them by mineralization instead of moving them from location to location (Quiroz et al., 2011; Ghatak, 2014; Oturan and Aaron, 2014; Tsydenova et al., 2015; Riberiro et al., 2015).
Among AOPs (commonly defined as near ambient temperature treatment processes based on highly reactive radicals, especially the hydroxyl radical, OH), heterogeneous photocatalysis using different semiconductor oxides has been extensively reviewed in the recent literature (Malato et al., 2009; Ahmed et al., 2011; Fechete et al., 2012; Qu and Duan, 2013; Djurišić et al., 2014). This fact is due to its efficiency in the removal/transformation of pollutants at ambient temperatures and pressures by mineralization and/or generation of less toxic by-products than the parent molecules in most cases. The use under irradiation of a stable solid semiconductor for stimulating a reaction at the solid/solution interface is a technique of environmental interest for the treatment of pesticide-polluted water combining low cost, mild conditions and the possibility of using natural sunlight as the source of irradiation (Vela et al., 2017). Among the different semiconductor materials tested as potential photocatalysts, titanium dioxide (TiO2) is the most popular because of its photochemical stability, commercial availability, non-toxic nature and low cost, high photoactivity and ease of preparation in the laboratory (Herrmann, 2012; Lazar et al., 2012). On the other hand, the mode of TiO2 application (suspended, immobilized or doped) is fundamental to rate the photocatalytic activity. Several studies have compared the decomposition of substrates using suspended and immobilized catalysts (Silva et al., 2012). However, these studies reached different conclusions. Slurry systems were variously reported to be more efficient, less efficient, and as efficient as the immobilized systems. The morphology and pores size of the selected support materials play an important role in enhancing the heterogeneous catalyst's stability and performance. Generally, the use of TiO2 slurries has been demonstrated to have higher photocatalytic activity as compared to the same immobilized catalyst. This is due to changes on the surface of the catalyst by blocking pores and the appearance of by-products causing the loss of active sites on its surface. In addition, suspended TiO2 is quite effective in utilizing sunlight, because suspended TiO2 particles have a high specific surface area in the range from 50 to 300 m2 g−1 (Gumy et al., 2006). Therefore, mass transfer limitations are avoided and high photocatalytic activity is obtained although a small transport limitation appeared on high catalyst loading. However, the use of the bare TiO2 phases presents an important condition (i.e., necessity of irradiation with UV light due to the small amount of photons absorbed in the VIS region) and disadvantage because it is difficult to separate small TiO2 particles from water after treatment. For this, doping, loading and sensitization of TiO2 are methods aimed to modify the light absorption towards visible light and/or to increase the lifetime of the photo-produced e−/h+ pairs (Vela et al., 2017).
With this aim, we have assessed a technique to degrade pesticide residues in wastewater produced in farms by remnants in containers and tanks of plant protection products from treatment equipment, to ensure that the operations of the professionals do not endanger human health or the environment. For this, we have developed a pilot facility for wastewater decontamination using natural sunlight and titanium dioxide (TiO2) as photocatalyst in tandem with an oxidant (Na2S2O8) used as electron acceptor. The study was aimed to remove residues of 12 pesticides in agro-waste water commonly used on different vegetables and fruit crops in the Region of Murcia (SE of Spain), where solar irradiation is highly available (more than 3000 h of sunshine per year on average) making this process quite attractive.
Section snippets
Chemicals
The analytical standards of the pesticides used in the optimization experiments were purchased from Dr. Ehrenstorfer GmBh (Augsburg, Germany) with >95% purity. FitoDolores S.L. (Murcia, Spain) supplied the commercial formulations of the pesticides. Table 1 shows the commercial products used and the main physical-chemical properties of all pesticides. Titanium dioxide, TiO2 P25 (99.5% purity, 55 m2 g−1 BET surface area, <21 nm particle size) was provided by Nippon Aerosil Co Ltd. (Osaka, Japan).
Preliminary experiments at laboratory scale under artificial irradiation
As result of the laboratory scale experiments, Fig. 2 shows the mean remaining percentage of the pesticides as a function of irradiation time for different concentrations using only Na2S2O8 (top graphic) and TiO2 (bottom graphic) at a fixed concentration of Na2S2O8 (150 mg L−1). As can be observed, the degradation rate significantly increases by adding the electron acceptor as compared with the test carried out in absence of Na2S2O8 where minimal degradation was observed after 90 min of
Conclusions
The necessary use of pesticides results in a certain volume of agro-waste water containing pesticide residues mainly coming from: i) remnants in empty containers, ii) remnants in spray tanks after the treatments, iii) rinse of the treatment tanks, and iv) water from machine and equipment cleaning. Therefore, a development of an effective treatment technology to decontaminate water is highly desirable and convenient before returning the water to its natural cycle.
Herein, we have demonstrated
Acknowledgments
The authors acknowledge financial support received from European Commission through the LIFE + Program (LIFE-AQUEMFREE 13 ENV/ES/000488).
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