Solar photocatalytic reclamation of agro-waste water polluted with twelve pesticides for agricultural reuse

Highlights

• Rapid photoremoval of 12 pesticides from agro-waste water was achieved in SE of Spain.

• Degradation was conducted via solar photocatalysis using TiO₂ at pilot plant scale.

• Addition of persulfate significantly enhanced the degradation rate.

• Half-lives were below 1.5 h in all cases evincing rapid degradation.

• Seasonal sunlight fluctuations and constant rates were directly correlated.

Abstract

This study aims to demonstrate a technically feasible alternative to remove pesticide residues from agro-waste water produced in farms from remnants in containers and treatment tanks, rinse in tanks after treatments, and cleaning of machines and equipment. For this, the photocatalyzed degradation of 12 pesticides commonly used on vegetables, vines, citrus and fruit crops was investigated in aqueous suspensions of TiO₂ in tandem with Na₂S₂O₈ at pilot plant scale under natural sunlight in Murcia (SE of Spain) during summer and winter seasons. Previously, preliminary experiments were carried out at laboratory scale using a photoreactor to optimize the photocatalyst (200 mg L⁻¹) and oxidant (250 mg L⁻¹) concentrations on the rate constants of the studied pesticides. The photodegradation of all pesticides can be modelled assuming a pseudo-first-order kinetics. The time needed for disappearance of 90% (DT₉₀) of the studied pesticides, was lower than 4 h in summer in all cases with the exception of cyproconazole (4.9 h), while, cyproconazole (8.9 h), metalaxil (6.1 h) and propyzamide (7.9 h) showed DT₉₀ higher than 6 h in winter. The reaction rate was enhanced 3-fold in summer season, which is directly correlated to the higher accumulated fluence per time received during this season (about a factor of 2.9 higher than in winter). In both cases, the higher and lower degradation rates were obtained for cyprodinil and cyproconazole, respectively. The total fluence to get a 90% reduction (H₉₀) ranged from 4.6 to 5.2 J cm⁻² (cyprodinil) to 71.5–76 J cm⁻² (cyproconazole).

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.