Assessing the transport potential of polymeric nanocapsules developed for crop protection

Highlights

• Examine the mobility of 4 polymeric nanocapsules (nCAPs) in agricultural soil.

• Compare the transport potential of two bifenthrin delivery systems.

• Poly(methacrylic acid)-poly(ethyl acrylate) nCAPs are highly mobile.

• Poly(methacrylic acid-ran-butylmethacrylate) nCAPs may enhance bifenthrin delivery.

• Ammonium polyphosphate fertilizer hinders bifenthrin formulation mobility.

Abstract

Nanotechnology is increasingly important in the agricultural sector, with novel products being developed to heighten crop yields and increase pesticide efficacy. Herein, the transport potential of different polymeric nanocapsules (nCAPs) developed as pesticide delivery vehicles was assessed in model soil systems. The nCAPs examined are (i) poly(acrylic acid)-based (nCAP1), (ii) poly(methacrylic acid)-ran-poly(ethyl acrylate) copolymer-based (nCAP2), (iii) poly(methacrylic acid-ran-styrene) copolymer-based (nCAP3), and (iv) poly(methacrylic acid-ran-butylmethacrylate)-based (nCAP4). nCAP mobility was examined in columns packed with agricultural loamy sand saturated with artificial porewater containing Ca²⁺ and Mg²⁺ cations (10 mM ionic strength, pH 6 and 8). Furthermore, the impact of (i) cation species, (ii) sand type, and (iii) ammonium polyphosphate fertilizer on the transport potential of a nanoformulation combining nCAP4 capsules and the pyrethroid bifenthrin (nCAP4-BIF) was examined and compared to a commercial bifenthrin formulation (Capture^® LFR). Although nCAP4-BIF and Capture^® LFR formulations were highly mobile in quartz sand saturated with 10 mM NaNO₃ (≥95% elution), they were virtually immobile in the presence of 10% ammonium polyphosphate fertilizer. The presence of Ca²⁺ and Mg²⁺ did not hinder nCAP4-BIF elution in quartz sand saturated with 10 mM standard CIPAC D synthetic porewater; however, limited Capture^® LFR transport (<10% elution) was observed under the same conditions. Capture^® LFR also exhibited limited mobility in the presence or absence of fertilizer in loamy sand saturated with divalent salt solutions, whereas nCAP4-BIF exhibited increased elution with time and enhanced transport upon the addition of fertilizer. Overall, nCAP4 is a promising delivery vehicle in pyrethroid nanoformulations such as nCAP4-BIF.

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.