Environmental fate of nanopesticides: durability, sorption and photodegradation of nanoformulated clothianidin

A lot of research efforts are currently dedicated to the development of nano-enabled agrochemicals. It is thus urgent to develop suitable strategies for their ecological assessment.


Table of Contents
concentrations were 190 and 75 mM, respectively. ζpotentials were also measured at 0.1 mM NaCl. Regarding fertilizer background, measurements could not be conducted at the exact experimental conditions due to the low concentrations of nanopesticide. Hence, measurements were conducted at a dilution of 1:400 as above (about 3.1, 8.4 and 76.9 fold more concentrated than in the photodegradation, centrifugation and batch sorption tests, respectively) but maintaining the experimental fertilizer-to-formulation ratios.
The hydrodynamic diameter was measured immediately upon mixing (crosses on Figure 1), and included 10 individual measurements to resolve aggregation (3 seconds acquisition time for each individual run), followed by 3-6 individual measurements (10 seconds acquisition time for each individual run). The last three runs were stacked and averaged to determine the final size (> 8 min after mixing, bars in Figure 1). After size measurements, sample aliquots were transferred into folded capillary cells (Malvern DTS1070) for ζ-potential measurements in triplicates.

Photodegradation: foliar spray application scenario
Clothianidin is most likely to be exposed to sunlight when applied as foliar spray, possibly together with some fertilisers. Concentrations of nutrients were calculated based on the maximum recommended application rate as foliar spray 40 L of fertiliser/ha (applied before full coverage on corn and sugar beet) diluted in a minimum of 300 L water/ha 3 . According to the label of the insecticide Belay (clothianidin content: 2.13 lbs/gal = 255.23 g/L) the maximum application rate for most crops is 6 fl.oz./A, diluted in a minimum of 100 gal of water/A, which gives a spray concentration of 120 mg/L. Note that for crops such as grapes and pome fruits, concentrations could be even higher (200-480 mg/L)

Sorption to soil: in-furrow application scenario
The high concentration of fertilizer and clothianidin in soil are most likely to occur when applied infurrow while sowing. Experimental concentrations were based on the maximum realistic usage rate. When the fertilizer PowerPhos is used for basic soil fertilization, the label recommends using 40 L/ha e.g. in corn, potatoes, sunflowers, oil-pumpkins and sugar beets 3 . Clothianidin is often used for seed treatment of corn and the scenario was thus developed for corn. Applying 40 L/ha in-furrow would lead to very high local salt concentrations, possibly causing salt injury of the seeds. However, the recommended application rate for 10-34-0 fertilizer in-furrow on corn is 5 gal/A (≈ 47 L/ha) in 30inch (≈ 76 cm) rows 4 , indicating that the scenario is realistic.
With an average number of 10 plants per m² and 75 cm row distance, planting distance within rows needs to be about 13 cm 5 . Each plant accounts for 13 cm of furrow, so the total furrow length is 10 plants × 13 cm = 130 cm/m². It was assumed that 3 × 3 cm of soil is influenced along the furrow length. The volume of influenced soil would then be 11.7 m³/ha. The fertiliser should be mixed with a minimum of 300 L water per ha 3 , giving a total of 340 L/ha. With a concentration of 138 g of N/L , the scenario results in 315 mg of N/kg in dry soil, assuming a soil bulk density of 1.5 kg/dm³. We used 320 mg of N/kg N and corresponding 1088 mg of P2O4/kg (NPK ratio: 10-34-0).
Similar considerations were made to calculate the concentration of clothianidin in soil: in-furrow application of Belay against corn wireworm is 12 fl.oz./A (= 0.8769 L/ha). The insecticide contains 2.13 lbs clothianidin per gallon (= 255.23 g/L) 6 , so the maximum application amounts to 223.81 g/ha.
Considering the influenced volume of soil 11.7 m³/ha (as calculated above), clothianidin concentration in soil would be 19.13 g/m³, equivalent to ~13 mg/kg considering a soil density of 1.5 g/cm³. Experimental concentrations of clothianidin and fertilizer in soil are summarised in the Table  above.  Table S5. Photodegradation half-lives (DT 50 in days ± standard deviation) for the series of nanoformulations (NFA, NFB and NFC), the commercial formulation (Com) and the pure AI (data presented in Figure 2 in the manuscript)  Figure S1. Examples of photodegradation curves (n=3) fitted with first order kinetics.
The DT 50 values derived are shown in Figure 2.  Figure S2. Comparisons of photodegradation rate constants (k, min -1 ) for the different formulations across concentration and background (corresponding colour-coded data are presented in Figure 2)  Figure S3. Comparisons of sorption over time as measured by centrifugation after 1 day (diagonal) and after 7 days (horizontal strips).
The significance level was set to α=0.05 and significant differences by * .  Figure S4. Comparisons of methods: batch vs centrifugation 7 days The significance level was set to α=0.05 and significant differences by * .  Figure S6. pH values measured after equilibration of the soil suspensions when measuring sorption by batch.
A two-way ANNOVA indicates that the effect of formulation on pH was overall not significant.