Comparative analysis of the sorption behaviors and mechanisms of amide herbicides on biodegradable and nondegradable microplastics derived from agricultural plastic products

Coexisting of microplastics (MPs) and residual herbicides has received substantial attention due to concerns about the pollutant vector effect. Here, the widely used amide herbicides were examined for their sorption behaviors on the priority biodegradable and nondegradable MPs identified in intensive agriculture. The fitting results indicated that the interactions between napropamide (Nap)/acetochlor (Ace) and the MPs, i.e., poly (butyleneadipate-co-terephthalate) microplastic (PBATM), polyethylene microplastic (PEM), and polypropylene microplastic (PPM), may be dominated by hydrophobic absorptive partitioning on the heterogeneous surfaces. Additionally, chemisorption cannot be ignored for the sorption of Nap/Ace on the biodegradable MPs. The sorption capacities of Nap/Ace on the MPs followed the order of PBATM > PEM > PPM. The differences in sorption capacity which varied by the MP colors were not significant. The hydrophobicity of the herbicides and the MPs, the rubber regions, surface O-functional groups, benzene ring structures and large specific surface area of the biodegradable MPs played key roles in the better performance in sorbing amide herbicides. Moreover, MPs, especially biodegradable MPs, might lead to a higher vector effect for residual amide herbicides than some other common environmental media. This study may provide baseline insights into the great potential of biodegradable MPs to serve as carriers of residual amide herbicides in intensive agrosystems.

Developmental assays using invasive cane toads, Rhinella marina, reveal safety concerns of a common formulation of the rice herbicide, butachlor

Identifying the adverse impacts of pesticide exposure is essential to guide regulations that are protective of wildlife and human health. Within rice ecosystems, amphibians are valuable indicators because pesticide applications coincide with sensitive reproductive and developmental life stages. We conducted two experiments using wild cane toads (Rhinella marina) to test 1) whether environmentally relevant exposure to a commercial formulation of butachlor, an acetanilide herbicide used extensively in rice, affects amphibian development and 2) whether cane toad tadpoles are capable of acclimatizing to sub-lethal exposure. First, we exposed wild cane toads to 0.002, 0.02, or 0.2 mg/L of butachlor (Machete EC), during distinct development stages (as eggs and hatchlings, as tadpoles, or continuously) for 12 days. Next, we exposed a subset of animals from the first experiment to a second, lethal concentration and examined survivorship. We found that cane toads exposed to butachlor developed slower and weighed less than controls, and that development of the thyroid gland was affected: exposed individuals had smaller thyroid glands and thyrocyte cells, and more individual follicles. Analyses of the transcriptome revealed that butachlor exposure resulted in downregulation of transcripts related to metabolic processes, anatomic structure development, immune system function, and response to stress. Last, we observed evidence of acclimatization, where animals exposed to butachlor early in life performed better than naïve animals during a second exposure. Our findings indicate that the commercial formulation of butachlor, Machete EC, causes thyroid endocrine disruption in vertebrates, and suggest that exposure in lowland irrigated rice fields presents a concern for wildlife and human health. Furthermore, we establish that developmental assays with cane toads can be used to screen for adverse effects of pesticides in rice fields.

Adsorption of epoxiconazole and tebuconazole in twenty different agricultural soils in relation to their properties

Conazole fungicides are currently used pesticides with considerable chronic toxicity and ecotoxicity that are also on EU list for substitution. They enter the soil forming short- or long-term residues. In this study two of their representatives, epoxiconazole (EPC) and tebuconazole (TBC), have been tested with 20 soils from the Czech Republic for their adsorption. Adsorption, by means of Kd coefficients, was compared to "basic" (TOC, pH, clay …) and "advanced" (surface area, minerals ..) soil properties. After doing multivariate analysis of the variables it was apparent that adsorption of both pesticides was highly associated with pH (negatively correlated), and less associated with soil organo-mineral complex (TOC, clay and surface area) and C and N in soil organic matter (OM). Particle sizes or cation exchange capacity (CEC) did not show correlation with adsorption, but showed an association in multidimensional space in factor analysis (FA). Some correlations were revealed between EPC adsorption and soil organic matter parameters. Recalculating Kd to K_oc and to Gibb's free energy (ΔG) and its values indicated that the adsorption of EPC and TBC is mainly weak physical adsorption - partitioning. Also, ΔG values gave better correlation with pH_(H2O) than Kd. Surface area impacted EPC adsorption. From the several soil minerals, kaolinite showed EPC and TBC adsorption. EPC adsorption was not highly influenced with pH changes compared to TBC. The number and types of H-bonds with molecular geometry govern the sorption, which might crucially affect leachibility in soil, and this may indicate that TBC is more leachable than EPC for the same soil.

Dissipation of S-metolachlor and butachlor in agricultural soils and responses of bacterial communities: Insights from compound-specific isotope and biomolecular analyses

The soil dissipation of the widely used herbicides S-metolachlor (SM) and butachlor (BUT) was evaluated in laboratory microcosms at two environmentally relevant doses (15 and 150 μg/g) and for two agricultural soils (crop and paddy). Over 80% of SM and BUT were dissipated within 60 and 30 days, respectively, except in experiments with crop soil at 150 μg/g. Based on compound-specific isotope analysis (CSIA) and observed dissipation, biodegradation was the main process responsible for the observed decrease of SM and BUT in the paddy soil. For SM, biodegradation dominated over other dissipation processes, with changes of carbon isotope ratios (Δδ¹³C) of up to 6.5‰ after 60 days, and concomitant production of ethane sulfonic acid (ESA) and oxanilic acid (OXA) transformation products. In crop soil experiments, biodegradation of SM occurred to a lesser extent than in paddy soil, and sorption was the main driver of apparent BUT dissipation. Sequencing of the 16S rRNA gene showed that soil type and duration of herbicide exposure were the main determinants of bacterial community variation. In contrast, herbicide identity and spiking dose had no significant effect. In paddy soil experiments, a high (4:1, V/V) ESA to OXA ratio for SM was observed, and phylotypes assigned to anaerobic Clostridiales and sulfur reducers such as Desulfuromonadales and Syntrophobacterales were dominant for both herbicides. Crop soil microcosms, in contrast, were associated with a reverse, low (1:3, V/V) ratio of ESA to OXA for SM, and Alphaproteobacteria, Actinobacteria, and Bacillales dominated regardless of the herbicide. Our results emphasize the variability in the extent and modes of SM and BUT dissipation in agricultural soils, and in associated changes in bacterial communities.

Carbon and nitrogen stable isotope fractionation during abiotic hydrolysis of pesticides

Compound-specific Stable Isotope Analysis (CSIA) has been recently established as a tool to study pesticide degradation in the environment. Among degradative processes, hydrolysis is environmentally relevant as it can be chemically or enzymatically mediated. Here, CSIA was used to examine stable carbon and nitrogen isotope fractionation during abiotic hydrolysis of legacy or currently used pesticides (chloroacetanilide herbicides: Acetochlor, Alachlor, S-Metolachlor and Butachlor, acylalanine fungicide: Metalaxyl, and triazine herbicide: Atrazine). Degradation products analysis and CN dual-CSIA allowed to infer hydrolytic degradation pathways from carbon and nitrogen isotopic fractionation. Carbon isotopic fractionation for alkaline hydrolysis revealed similar apparent kinetic isotope effects (AKIE_C = 1.03-1.07) for the 6 pesticides, which were consistent with S_N2 type nucleophilic substitutions. Neither enantio-selectivity (EF ≈ 0.5) nor enantio-specific isotope fractionation occurred during hydrolysis of R (AKIE_C = 1.04 ± 0.01) and S (AKIE_C = 1.04 ± 0.02) enantiomers of a racemic mixture of Metalaxyl. Dual element isotope plots enabled to tease apart CCl bond breaking of alkane (Λ ≈ ε_N/ε_C ≈ 0, Acetochlor, Butachlor) and aromatic π-system (Λ ≈ 0.2, Atrazine) from CO bond breaking by dealkylation (Λ ≈ 0.9, Metalaxyl). Reference values for abiotic versus biotic S_N2 reactions derived from carbon and nitrogen CSIA may be used to untangle pesticide degradation pathways and evaluate in situ degradation during natural and engineered remediation.