Decreased bioavailability of aminomethylphosphonic acid (AMPA) in genetically modified corn with activated carbon or calcium montmorillonite clay inclusion in soil

Abstract

The widespread use of pesticides has resulted in detectable residues throughout the environment, sometimes at concentrations well above regulatory limits. Therefore, the development of safe, effective, field-practical, and economically feasible strategies to mitigate the effects of pesticides is warranted. Glyphosate is an organophosphorus herbicide that is degraded to aminomethylphosphonic acid (AMPA), a toxic and persistent metabolite that can accumulate in soil and sediment and translocate to plants. In this study, we investigated the binding efficacy of activated carbon (AC) and calcium montmorillonite (CM) clay to decrease AMPA bioavailability from soil and AMPA translocation to plants. Adsorption isotherms and thermodynamic studies on AC and CM were conducted and showed tight binding (enthalpy values >-20 kJ/mol) for AMPA with high capacities (0.25 mol/kg and 0.38 mol/kg, respectively), based on derivations from the Langmuir model. A hydra assay was utilized to indicate toxicity of AMPA and the inclusion of 1% AC and CM both resulted in 90% protection of the hydra (**p ≤ 0.01). Further studies in glyphosate-contaminated soil showed that AC and CM significantly reduced AMPA bioavailability by 53% and 44%, respectively. Results in genetically modified (GM) corn showed a conversion of glyphosate to AMPA in roots and sprouts over a 10-day exposure duration. Inclusion of AC and CM reduced AMPA residues in roots and sprouts by 47%-61%. These studies collectively indicate that AC and CM are effective sorbents for AMPA and could be used to reduce AMPA bioavailability from soil and AMPA residues in GM corn plants.

Keywords: AMPA; Activated carbon; Calcium montmorillonite; Glyphosate; Sorbent.

Fig. 1 –

Plant uptake experimental design. These studies were used to determine sorbent ability to reduce plant uptake of chemicals from soil. Our hydroponic system simulates a flood scenario, during which plant roots would be exposed to high levels of chemical contaminants in floodwaters.

Fig. 2 –

(a) Langmuir plots of AMPA on AC at 25 °C indicating binding of AMPA. (b) Langmuir plots of AMPA binding to CM and collapsed CM at 25 °C. Q_max value was significantly decreased with collapsed CM clay, indicating interlayer surfaces are important in AMPA binding to CM clay.

Fig. 3 –

Langmuir plots of AMPA on CM and AC at 37 °C (CM: K_d = 3.10E+05; AC: K_d = 2.70E+05). K_d values for these plots were compared to those conducted at 25 °C to determine the enthalpy (ΔH) of the binding reactions. ΔH values for CM and AC were −48.2 and −26.6 kJ/mol, respectively.

Fig. 4 –

(a) Hydra assay showing AMPA toxicity at 20 μg/mL and the protection of hydra with inclusion of 1.0% W/V AC, CM, and 50:50 AC:CM. Control groups with no AMPA (hydra media and sorbent controls) are included for comparison and show scores of 10 throughout the test period. Sorbent inclusion at 1.0% was able to significantly protect hydra from AMPA toxicity (90% protection). (**p ≤ 0.01). (b) Toxicity of 20 μg/mL PMG resulting in moderate toxicity to the hydra.

Fig. 5 –

Soil studies showing significant reduction of AMPA bioavailability with inclusion of 1% AC, CM, and 50:50 AC:CM at 52.7%, 43.5%, and 56.6%, respectively. (**p ≤ 0.01)

Fig. 6 –

Time course plant study showing uptake of PMG and conversion to the metabolite AMPA. Root dry weight (a), sprout dry weight (b) and total sprout length (c) were measured over the duration of the 10-day exposure period. PMG and AMPA concentrations were measured in both the roots (d, f) and sprouts (e, g).

Fig. 7 –

Plant uptake study showing AMPA residues in corn roots and sprouts for sprouts grown in both compost and soil. No significant differences were seen between groups for root dry weight (a), sprout dry weight (b), and total sprout length (c). 1% AC and CM were both able to significantly reduce AMPA residues in corn roots (d) and sprouts (e) in both the compost and soil studies (**p ≤ 0.01)

Fig. 8 –

Molecular model of geometrically optimized CM clay layers and AMPA molecules showing predicted binding of AMPA onto CM surfaces. Binding sites may include interlayer surfaces, basal surfaces, and edge sites.