Enhancing the potential use of microparticulate insecticides through removal of particles from raw grain

Highlights

• Microparticulate nano-engineered alumina (NAIP) insecticide powders could be an alternative to manage stored grain insect pests.

• We tested a method to remove these powders from treated grain, as Insecticide residues in food pose a public health risk.

• The device combed mechanical motion of grain, an air stream in a pneumatic conveyor system, and an electrostatic filter.

• The NAIP particles detach from the grain and bind to the electrodes, and the removal efficiency was 98.0% (±1.4).

• The present study provides an innovative strategy to remove NAIP particles from grains after silage.

Abstract

The current study presents the design and evaluation of a laboratory device combining mechanical motion of wheat grain and turbulent air streaming inside a positive pneumatic conveyor system. The device recovers microparticulate nano-engineered alumina insecticide powders (NAIP) from treated grain. The particle removal efficiency of the conveying system was experimentally quantified by using a laboratory prototype assembled by attaching an electrostatic filter (EF) to the conveyors exhaust. Then, the NAIP particles detached from the grain inside the conveyor were drawn by the conveyors’ exhaust air stream into the EF, where particles bound to the electrodes due to electric charge differences. The NAIP particle load bound to the EF electrodes was removed and weighed to determine the efficiency of the wheat grain cleaning process. Our experimental results, under laboratory conditions, show that the recovery efficiency of the prototype averaged 98.0% (±1.4). Thus, the present study provides an innovative strategy to remove NAIP insecticide particles after storage, once their role as insecticide in stored grain has been fulfilled. This technology provides advancement in grain technology allowing the possibility to provide insecticide-free grain to the food market.

Introduction

Insects cause significant post-harvest food losses which have been estimated to be around 9% in developed countries, and up to 20% or more in developing countries (Phillips and Throne, 2010). Grains are frequently stored for several months in bulk, at ambient temperatures, so insecticides are typically applied at this stage to reduce losses from storage pests (Holland et al., 1994). Conventional synthetic contact insecticides can be very effective to manage insect infestation on stored grain when there is no insecticide resistance in pest populations, but pesticide residues in treated stored grain are a concern, as these residues may have an impact on grain quality (Proctor, 1994). These residues may pose a risk for public health and have the potential to be a major source of synthetic organic pesticides in the diet given the key role of cereals in human nutrition (Grewal et al., 2017).

Consumer perception of risks associated with pesticides, the rise in insecticide resistance and regulatory pressures for low-risk pest control strategies, have led to the need for alternatives to conventional chemical pesticides in many agricultural ecosystems, including post-harvest protection of crops (Phillips and Throne, 2010). Topics that have received increased attention and support by researchers include biological control, insect growth regulators, botanical insecticides and inert dusts (Arthur, 1996; Subramanyam and Roesli, 2000). Insecticide dusts or powders, that are added directly to the grain mass for protection against insect and mite attack are defined as “grain protectants” and known as admixture treatments (Morallo-Rejesus and Rejesus, 2019). Inert dusts act by disrupting the insect cuticle, which leads to dehydration and death. Among inert dusts, diatomaceous earth, silica and volcanic ash are well-known insecticides and have been shown to be effective against many stored grain pests (Morallo-Rejesus and Rejesus, 2019; Athanassiou et al., 2003; Buteler et al., 2014). Additionally, the nano-engineered microparticulate insecticide powders (NAIP), based on alumina (Stadler et al., 2010, 2012) or silica (Debnath et al., 2011), have been proposed as potential tools to be incorporated into an integrated pest management strategy against stored product insect pests. The NAIP insecticide powders are more effective than bulk silica or DE, because of their increased exposed surfaces which interact with the insect cuticle and thus have great potential in sustainable agriculture as an alternative to conventional synthetic pesticides (Buteler et al., 2015; Stadler et al., 2017). Compared to bulk insecticide powders, the greatly reduced particle size of NAIPs and the associated surface enlargement, result in a fundamental change in the physical and chemical properties of these nano-engineered pesticides, making them more effective.

Unlike synthetic organic agrochemicals, when applied to stored grain, NAIP particles do not penetrate the seed epidermis (Evers and Bechtel, 1988; Niemann, 2013). In wheat treated with NAIP, the insecticide particles adhere to grains based on the size of the NAIP powder particles which shows a bi-modal size distribution varying from nano up to the micrometer scale (Stadler et al., 2012). Thus, gravitation and mass inertia are the main forces determining whether the particles on the micrometric scale adhere to the grain surface after the admixture process (Doss et al., 1993; Mercure et al., 1994; Stadler et al., 2018a).

Previous research shows, the use of inorganic insecticide powders is a highly promising alternative to organic synthetic pesticides, “which are toxic by design” (Forget, 1991). However, even though insecticide powders such as diatomaceous earth (DE) are of natural origin and have low mammalian toxicity (Athanassiou et al., 2007), the admixture of these to cereals increases the friction forces, which affects bulk density and flow properties of grain (Athanassiou et al., 2011). These drawbacks constitute the main limiting factor for the use of DE dusts in crop protection (Korunić, 2016). By contrast, microparticulate insecticide powders as NAIPs also have a distinctive physical mechanism of insecticide action, are of low toxicity and given that they are highly effective at low doses (125 ppm) (Stadler et al., 2010), their effect on bulk density is reduced.

In the case of NAIP particles, they have been shown to have a low toxicity in acute tests (Stadler et al., 2018a) while the dust is not expected to penetrate the seed (Niemann, 2013). However, the removal of NAIP particles from wheat grain after storage is a topic that needs to be studied. Electrostatic air filtration has been used previously to seize particulate matter suspended in a gas (air) by using an electrostatic force, together with the turbulent transport of charged particles by an air stream. It has been used widely in the industry for air cleaning (Liu et al., 2017), as collection efficiency reaches generally >99%, including sub-micrometric particles (Wen et al., 2015; Kim et al., 2014).

In the electrostatic filter, small particles move inside the collector towards electrodes, where electrostatic effects influence the particle motion. Thus, loaded particles attach to the electrodes through Coulomb forces. Therefore, the objective of this study was to develop and validate the effectiveness of a specific technology involving an electrostatic filter, for the removal of nano-engineered alumina insecticide particles from treated wheat grain surface after storage, focusing on the slight adherence of NAIP particles on wheat seeds as a result of their low electric charge affinity. The particle removal and recovery prototype was developed on the fundamentals of particle electro-filtration (Katatani et al., 2016) and the results from previous experiments using high-pressure airstream (Stadler et al., 2018b). It consists of an assembly of a positive pneumatic conveyor (Kroulík et al., 2016) and an electrostatic filter collector (EF) (Katatani et al., 2016). The positive pneumatic conveyor laboratory prototype was developed to achieve particle segregation by airflow and friction, as particles with distinct size and mass have different dynamics when moving in a fluid, which in this case is air (Wang, 1975), and finally for delivering the particles into the electro-filters’ collector.