Ecotoxicity of Diazinon and Atrazine Mixtures after Ozonation Catalyzed by Na+ and Fe2+ Exchanged Montmorillonites on Lemna minor

Abstract

The toxicity of two pesticides, diazinon (DAZ) and atrazine (ATR), before and after montmorillonite-catalyzed ozonation was comparatively investigated on the duckweed Lemna minor. The results allowed demonstrating the role of clay-containing media in the evolution in time of pesticide negative impact on L. minor plants. Pesticides conversion exceeded 94% after 30 min of ozonation in the presence of both Na⁺ and Fe²⁺ exchanged montmorillonites. Toxicity testing using L. minor permitted us to evaluate the change in pesticide ecotoxicity. The plant growth inhibition involved excessive oxidative stress depending on the pesticide concentration, molecular structure, and degradation degree. Pesticide adsorption and/or conversion by ozonation on clay surfaces significantly reduced the toxicity towards L. minor plants, more particularly in the presence of Fe(II)-exchanged montmorillonite. The results showed a strong correlation between the pesticide toxicity towards L. minor and the level of reactive oxygen species, which was found to depend on the catalytic activity of the clay minerals, pesticide exposure time to ozone, and formation of harmful derivatives. These findings open promising prospects for developing a method to monitor pesticide ecotoxicity according to clay-containing host-media and exposure time to ambient factors.

Keywords: Lemna minor; adsorption; atrazine; diazinon; ozonation; toxicity.

Figure 1

Effect of pesticide concentration on growth inhibition of Lemna minor based on fronds number (a) and fresh weight (b) after 7 days of exposure. Triplicates measurements were achieved for all pesticides concentrations including the pesticide-free control sample (0 mg L⁻¹). The control has an original value of fresh weight of 99.5 mg and an average number of 63.5 fronds. Different letters used as symbols account for different data significances at p < 0.05. Data with similar significance are marked with similar symbols (Characters).

Figure 2

Effect of DAZ (a) and ATR (b) concentration on (Chl a/b) ratio of Lemna minor after 7 days exposure. Triplicates measurements were achieved for all pesticides concentrations including the pesticide-free control sample (0 mg L⁻¹). Different letters used as symbols account for different data significances at p < 0.05. Data with similar significance are marked with similar symbols (Characters).

Figure 3

Effect of DAZ (a) and ATR (b) concentration on Lemna minor ROS level after 7 days exposure. Triplicates measurements were achieved for all pesticides concentrations including the pesticide-free control sample (0 mg L⁻¹). The control has a relative fluorescence intensity value of 435. Different letters used as symbols account for different data significances at p < 0.05. Data with similar significance are marked with similar symbols (Characters).

Figure 4

Removal efficiency of DAZ (a) and ATR (b) by adsorption and ozonation in the presence of 2 g L⁻¹ NaMt (upper panel) and Fe(II)Mt (down panel). Sample volume = 25 mL. The relative peak area (A/A0) was calculated as the instant/initial HPLC-UV peak area ratio of DAZ or ATR. This ratio allows expressing the adsorption efficiency as: (1 − A/A0) × 100%. Triplicates measurements were achieved including the non-adsorbed and non-ozonized pesticide solution taken as the starting control sample (10 mg L⁻¹).

Figure 5

Effect of the retention efficiency (%) of DAZ (a) and ATR (b) with 2 g L⁻¹ of Fe(II)Mt and NaMt on the inhibition grade (%) of Lemna minor. Triplicates measurements were achieved including the non-adsorbed and non-ozonized pesticide solution taken as the starting control sample (red symbol, 10 mg L⁻¹).

Figure 6

Effect of the conversion efficiency by ozonation (%) of DAZ (a) and ATR (b) with 2 g L⁻¹ of Fe(II)Mt and NaMt on the inhibition grade (%) of Lemna minor. Triplicates measurements were achieved including the non-adsorbed and non-ozonized pesticide solution taken as the starting control sample (red symbol, 10 mg L⁻¹).

Figure 7

Change in ROS level of L. minor in DAZ ozonized mixture in the presence of NaMt (a) and Fe(II)Mt (b) catalysts (1, 2 and 3 g L⁻¹). The data represent triplicate measurements after plant exposure for 7 days including the control sample (Ctrl: L. minor in SIS without DAZ ozonized).

Figure 8

Change in ROS level of L. minor exposed to ATR ozonized mixture in the presence of NaMt (a) and Fe(II)Mt (b) catalysts (1, 2 and 3 g L⁻¹). The data represent triplicate measurements after plant exposure for 7 days including the control sample (Ctrl: L. minor in SIS without ATR ozonized).

Figure 9

Schematic procedure for investigating the effects of pesticide adsorption and ozonation in the presence of two montmorillonites (NaMt and Fe(II)Mt) on the toxicity towards Lemna minor.