23
Jul
Enhanced Bioavailability and Contamination of Neonicotinoid Pesticides Linked to Microplastics in Soil
(Beyond Pesticides, July 23, 2024) In analyzing the interactions between neonicotinoid pesticides (NNPs) and microplastics (MPs), a recent study in The Science of The Total Environment finds that neonicotinoids such as thiacloprid (THI) become more bioavailable in soils containing traditional and biodegradable plastics. Increased bioavailability, which quantifies the extent to which organisms are exposed to chemicals in soil or sediment, puts soil microbiota at risk and leaves all consumers susceptible to adverse effects in contaminated food crops. This study raises a grave deficiency, among others, in the pesticide registration and regulatory review process, which currently ignores interactions of pesticides with other contaminants, like microplastics, in the environment when conducting health and ecological effects assessments. Â
The researchers, from China University of Geosciences, Beijing Academy of Agriculture & Forestry Sciences, Chinese Academy of Agricultural Sciences, and University of Swat in Pakistan, chose to study thiacloprid as an example of NNPs to explore “the adsorption-desorption process and mechanism of NNPs on MPs,†as well as the main factors affecting adsorption, since these are two contaminants of concern in agricultural environments.Â
Adsorption [clinging to the surface] and desorption [releasing after adsorption] of thiacloprid by both traditional and biodegradable MPs, and the impact of MPs aging on adsorption, is assessed. Prior to the study, the authors hypothesized that “(1) traditional and biodegradable MPs have different mechanisms of adsorption-desorption for NNPs, and due to the stronger hydrophilicity of biodegradable MPs, we assume that biodegradable MPs will adsorb more of the similarly hydrophilic NNPs; (2) Aging can alter the properties of MPs, thereby affecting their adsorption of NNPs; (3) The MPs in soil can alter the mode of existence of NNPs in the soil, thereby altering the bioavailability of neonicotinoid pesticides in the soil.â€Â
The researchers share that, “Polyamide 6 (PA6) and poly (butylene adipate co-terephthalate) (PBAT) were selected as representative MPs. Among them, PA6 is a non-degradable plastic, and PBAT is the most frequently used degradable plastic in degradable plastic films. Further, as a representative of NNP, thiacloprid (THI) was selected due to its high detection rate.†Adsorption kinetic studies were performed using “a slightly higher initial concentration than the actual environmental level,†which the authors say “reduces the impact of measurement errors on the experimental results, which is favorable for precisely investigating adsorption behavior and mechanism.â€Â
PA6 and PBAT age differently, with PA6 generating pores on the surface and PBAT with a roughened surface containing wrinkles and cracks. However, both of these changes over time allow for “solutions to penetrate the interior of the MPs easily and potentially offers more sites for adsorbing pollutants,†as the researchers state. This suggests that chemicals can not only adsorb on the surface but also absorb into the plastics.Â
In studying the surface morphology of these plastics, “the elemental analysis results of the MP surface showed a reduction in C [carbon] content and an increase in O [oxygen] content after aging. This suggests that oxygen-containing functional groups are produced through oxidation on the surface of MPs during aging. The increase of oxygen-containing functional groups also increases the hydrophilicity of MPs, making it easier to interact with water-soluble substances.†This study shows that aging of microplastics increases the adsorption capacity for pollutants, which leads the authors to postulate that, “Aged MPs have a higher ability to absorb THI and are more likely to become environmental carriers of THI.â€Â Â
The effects of pH, salinity, and dissolved organic matter (DOM) on the adsorption of MPs with THI are also analyzed. The researchers find that:Â Â
- “The pH value of the solution mainly affects adsorption in two aspects: one is the change in the form or solubility of pollutants, and the other is the change in the surface charge of MPs.â€Â
- “The potential for MPs to adsorb THI increases as the environment transitions from freshwater to marine [salt water].â€Â
- “The adsorption behavior of contaminants on MPs can be impacted by the diverse functional groups present in DOM, which is abundant in aquatic environments.â€Â
The authors conclude that the “addition of MPs alters the physical and chemical properties of soil, the presence of MPs increases the number of soil aggregates, which due to their anaerobic conditions, may promote the selective growth of denitrifying bacteria while inhibiting the activity of aerobic bacteria. These changes could ultimately affect the structure of the bacterial community and potentially reduce the degradation efficiency of THI in the soil.†They continue in saying, “The bioavailability of pollutants in soil may be altered by MPs for various reasons, including altering the distribution of THI in soil through a series of adsorption and desorption processes. Another cause may be that MPs modify the physicochemical characteristics of the soil and fill its adsorption sites, thereby increasing the instability of contaminants and their susceptibility to ingestion by plants and animals.â€Â
Adding MPs to soil can also alter the distribution pattern of THI in the soil, and the different patterns found in the two soil types within the study also highlight the fact that “soil properties cannot be ignored when studying the bioavailability of pollutants,†according to the researchers. The addition of MPs to the soil leads to a greater distribution of THI in the soil solution and “increases the possibility of THI migration and potential passive entry into plants, which is also the reason for the increased bioavailability of THI after adding MPs to the soil.â€Â
Sorption processes, which include absorption, adsorption, and desorption, are all physical and chemical processes that involve one substance attaching to another. According to an article in Nature Education Knowledge, while absorption is the process of removing ions or molecules from a solution and accumulating them within solid constituents, adsorption collects the ions or molecules at the surface. Desorption is the opposite process where ions or molecules are released back into a solution. These processes are crucial in evaluating the effects of pesticides within the soil, as well as exposure through the skin.Â
Neonicotinoid pesticides “represent an emerging class of chemical insecticides derived from the natural toxin nicotine. Since their introduction in 2010, they have become the most widely used insecticides, comprising approximately a quarter of the pesticide market,†the authors comment. They continue, “70% of NNPs will persist in soil or water after application, causing potential hazards to the health of plants and animals and the ecological environment.†Beyond Pesticides has extensively covered the adverse effects of neonicotinoids, which greatly threaten pollinators and biodiversity. Â
“In addition to NNPs, the pollution problem of microplastics (MPs) in agricultural environments is also becoming increasingly prominent. After entering the soil, these MPs have the potential to alter the physicochemical properties of soils, impair soil fertility, and have detrimental effects on the soil microbiota,†the authors state. “Soil organisms, including plants and animals (e.g., earthworms), are susceptible to absorbing MPs, which can hinder their development and have a negative impact on the quality of the harvested crops.â€Â
MPs in agriculture occur through many avenues, such as with mulch film and fertilizer application. “Presently, biodegradable mulch film is [considered] a significant solution to the residual pollution caused by agricultural plastic film… Degradable plastics can degrade 85% to 99% within 18 weeks under appropriate composting conditions such as temperature, humidity, and microbial biomass. However, the degradation of biodegradable plastics in natural environments also takes a long time. Thus, degradable plastic film can also contribute to the pollution caused by MPs,†the authors share. Beyond Pesticides has written extensively on “biodegradable biobased mulch film,†and has raised concerns before the National Organic Standards Board (NOSB) while pointing to organic compatible mulching systems. See comments (scroll to mulch sections) to the NOSB here.Â
These MPs, both traditional and biodegradable, can adsorb pollutants and then desorb them back into the environment, “resulting in changes to the coexistence of pollutants’ levels, persistence, bioavailability, and ecological hazards.†Within this study, the MPs in soil altered the distribution of THI in the soil which led to an increase in the bioavailability of THI while inhibiting its degradation. These results showcase “the significance of examining the combined pollution caused by MPs and NNPs,†the authors highlight.Â
As previously covered by Beyond Pesticides, petrochemical pesticides, fertilizers, and plastics are linked to dire health effects and there is overwhelming regulatory failure regarding this that comes from a lack of data on the effects of pesticides prior to approved registration, as well as the ignorance of peer-reviewed scientific research that shows pesticides that pose unreasonable adverse effects. Â
This study adds to the growing body of science that supports the need to transition away from plastics and pesticides with a holistic strategy. Take action to tell USDA, EPA, and FDA to create strong restrictions on plastics in farming, water, and food to prevent health risks from plastics and adhered pesticides here and sign up to receive Action of the Week and Weekly News Updates.Â
The solution is organic. To protect human health and biodiversity, mitigate climate change, and preserve natural lands and wildlife, a national land management plan with organic agriculture as the basis is needed. Join Beyond Pesticides in our mission to protect healthy air, water, land, and food for ourselves and future generations by eliminating the use of petrochemical pesticides and fertilizers by 2032. Â
All unattributed positions and opinions in this piece are those of Beyond Pesticides. Â
Source:Â
Wang, K. et al. (2024) Interactions of traditional and biodegradable microplastics with neonicotinoid pesticides, The Science of The Total Environment. Available at: https://pubmed.ncbi.nlm.nih.gov/38972406/. Â
