Assessing the transport potential of polymeric nanocapsules developed for crop protection
Graphical abstract
Introduction
Nanotechnology is increasingly applied in agriculture, with novel nanomaterials being developed and used in pesticide delivery, genetic plant transformation and the development of biopesticides and fertilizers (Ghormade et al., 2011). Polymer-based nanoformulations can improve pesticide efficiency by (i) decreasing the rate at which the active ingredient (AI) is released to the surrounding environment, (ii) protecting the AI against biodegradation and/or (iii) by increasing the transport potential of AIs with low water solubilities (Kah and Hofmann, 2014). Loha et al. examined the performance of a nanoformulation consisting of poly(ethylene glycol)-based (PEG-based) nanospheres encapsulating the pyrethroid insecticide β-cyfluthrin against the cowpea seed beetle. The presence of the PEG-based nanospheres resulted in prolonged AI activity and decreased average half maximal effective concentrations (EC50) versus a commercially available β-cyfluthrin formulation due to delayed insecticide release (Loha et al., 2012). In a recent study, Kah et al. found that nanoformulations had a significant impact on the fate of the pesticide bifenthrin, particularly in soil with low organic content. More specifically, the sorption and degradation of bifenthrin (as part of a nanoformulation) differed by up to a factor of 10 and 1.8, respectively, when compared to the pure active compound (Kah et al., 2016).
Pyrethroid insecticides such as cyfluthrin and bifenthrin are manufactured analogues of pyrethrins, compounds with insecticidal properties found in flowers of the genus Chrysanthemum or Tanacetum. Effective against a broad array of insects and mites, pyrethroids are employed to treat crops, in nurseries and on construction sites (termite control). Pyrethroids are also the primary insecticides utilized in urbanized areas, having largely replaced organophosphate pesticides such as diazinon and chlorpyrifos (Frank and Marshall, 2008, Weston et al., 2013). Consequently, American and European studies have identified various pyrethroids in municipal wastewater (Weston et al., 2013).
In the United States alone, the quantity of bifenthrin employed agriculturally has increased from an estimated 115,080 lbs between 1992 and 1995 to an estimated 844,000 lbs in 2009 (Pennington et al., 2014, Thelin and Gianessi, 2000). These figures do not account for urban use, which has been reported to surpass agricultural application in some areas (Moran, 2007). In a study investigating pesticide occurrence in urban wetland settings, bifenthrin exhibited the highest frequency of detection in wetland sediments, appearing in 33% of sites (Allinson et al., 2015).
The presence of bifenthrin in aquatic settings is worrisome as it has been found to be extremely toxic to fish and aquatic organisms. Among freshwater organisms, 96 h LC50 values of 1.5 × 10−4, 3.5 × 10−4 and 1.6 × 10−3 ppm bifenthrin have been reported for rainbow trout, bluegill sunfish and Daphnia magna, respectively (Fecko, 1999). Among estuarine species, Harper et al. reported LC50 values of 2.0 × 10−5, 1.3 × 10−5 and 2.0 × 10−2 ppm for adult grass shrimp, larval grass shrimp and sheepshead minnow, respectively (Harper et al., 2008). Improved delivery of pyrethroids such as bifenthrin would potentially reduce the quantities required to effectively protect crops, therefore curbing unwanted impacts on non-target organisms. Conversely, within a nanoformulation, pyrethroid interactions with nanocarrier components may result in enhanced AI transport potential and persistence. Thus, adequate risk assessments must be performed to better predict the hazards posed to non-target organisms, along with the potential for groundwater and surface water contamination. Overall, the impact of the nanoformulation on AI transport, relocation and bioavailability should be examined closely (Fecko, 1999).
Information regarding the mobility of nanocarriers (with and without associated AI) or the impact of nanoformulations on environmental fate processes remains limited. Herein, the transport potential of polymeric nanocapsules (nCAPs) destined to facilitate the transport of various pesticides, including pyrethroids, was investigated prior to their inclusion in agricultural nanoformulations. While the nCAPs examined have distinct compositions, they are all being developed in the aim of decreasing the need for costly and potentially harmful pesticides, thus mitigating their impact on the environment. Previous work examined the transport potential of a hollow nCAP consisting of partially cross-linked poly(acrylic acid), PAA (Petosa et al., 2013). While these polymeric carriers were found to be highly mobile in quartz sand, the large number of carboxyl functional groups on the nanocapsule surface was found to favor interaction with clays present in loamy sand, thus decreasing nanocapsule transport (Petosa et al., 2013).
In developing polymeric nanocapsules for pesticide delivery, it is essential to consider (i) nanocapsule transport potential and (ii) nanocapsule-pesticide interactions. The former can be achieved using laboratory-scale soil- or sand-packed columns saturated with natural or artificial porewater. Herein, hollow nCAP transport behavior in model saturated soil environments is investigated. Further studies are conducted with the aforementioned PAA nCAPs (Petosa et al., 2013), and a series of experiments is performed with three other capsule types (described below), allowing for comparison. Additionally, the transport potential of a nanoformulation containing active bifenthrin and an nCAP carrier consisting of poly(methacrylic acid-ran-butylmethacrylate) is considered and compared to that of the commercially available bifenthrin-containing formulation Capture® LFR. Finally, the impact of a commonly used fertilizer on the mobility of the two bifenthrin-containing formulations is also considered. It is noteworthy that these conditions do not directly mimic the exact application scenario in an agricultural field (e.g., where soils may not be fully saturated with porewater and/or nCAP application loads may not be as high as those used in this study). Nonetheless, the experiments described herein are essential in elucidating the fundamental interactions governing nCAP-soil interactions.
Section snippets
Synthetic porewater properties
Transport experiments were conducted using Collaborative International Pesticides Analytical Council (CIPAC) standard water D (i.e., CIPAC D), a synthetic porewater containing divalent salts (2.74 mM CaCl2 and 0.68 mM MgCl2 molar concentrations) and a total ionic strength (IS) of 10 mM. To verify the influence of the cation valence, selected transport experiments were also conducted using a monovalent salt solution of equivalent IS (10 mM NaNO3). Solution pH was adjusted with NaOH.
Granular collector characterization
Columns were
Results and discussion
Understanding the impact of a nanoformulation on processes including AI transport and bioavailability is essential in developing robust environmental risk assessments for nanopesticides (Kah and Hofmann, 2014, Kookana et al., 2014). Herein, the mobility of four nanocarriers and that of a novel bifenthrin nanoformulation (nCAP4-BIF) is investigated in sand and soil saturated with artificial porewater.
Conclusions
In this study, four types of polymeric nanocapsules were found to exhibit varying transport potentials in water saturated agricultural loamy sand. Overall, the poly(methacrylic acid)-ran-poly(ethyl acrylate) copolymer-based nCAP2 demonstrated the greatest transport potential, with 14 and 32% elution at pH 6 and 8 (at 6 PVs), respectively. While the poly(acrylic acid)-based nCAP1 also exhibited significant mobility at pH 8, it was virtually immobile at pH 6 due to enhanced aggregation and
Acknowledgements
This research was supported by NSERC, the Ministère du développement économique, innovation et exportation du Québec, the CRC Program, Environment Canada, Vive Crop Protection Inc., and the CFI. ARP was partially funded by NSERC (PGS) and a McGill Engineering Doctoral Award. FR and OS were partially supported by a McGill SURE award. The authors also thank K. J. Wilkinson (U. Montreal) for NTA assistance and D. Anderson, D. Norton, R. Fraser, M. Coulter, J. Dinglasan, and P. Thomson (Vive Crop
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