Metal ion promoted degradation mechanism of chlorpyrifos and phoxim

1 Introduction

Organophosphorus pesticides (OPPs) are a large group of highly effective pesticides. The widespread use of OPPs has led to severe environmental pollution because these compounds are often transported from their target sites (Liang et al., 2011). Phoxim and chlorpyrifos which have been widely used to control insects are organophosphate pesticides that kill insects by inhibiting cholinesterase, an enzyme necessary for nerve function (Health Risk From the Insecticie Phoxim). The persistence of pesticides can generally be termed as its lasting power in the environment. Their persistence in the environment is controlled by a number of different factors, specifically the different routes by which a pesticide can degrade. Some of these often show higher toxicity than the parent compound. Therefore, toxicological evaluation of these decomposed products is important from the viewpoint of their effect on human health (Hirahara et al., 2003). Recent decades have witnessed increases in the levels of contamination of water with toxic organic compounds. Among those highly toxic compounds dissolved in water are pesticides, which, through their extensive use, have become increasingly present in water. Thus pesticides removal from the aquatic environment has become a high priority (Derbalbalah et al., 2004). Degradation of organophosphorothioate (OP) compounds occurs by either of the following pathways: hydrolysis (Benoit-Marqui et al., 2004), oxidation (Acero et al., 2000), ultrasonic radiation (Zhang et al., 2011), electrical field (Chen et al., 2009) and photolysis (Menager et al., 2007).

Homogeneous process applies when a nucleophile interacts with the OP compound in solution with or without the presence of dissolved metal ions or other catalysts. Dissolved metal ions are known to play a wide and varied role in enhancing the rate of many significant hydrolysis reactions. A number of studies have shown that dissolved metal ions can play an important role in catalysis of organophosphorus pesticides. In most cases the catalytic enhancement is attributed to metal co-ordination with the substrate. Understanding the degradation of OPPs (such as chlorpyrifos and phoxim) is important for removing this substance assessing its potential risk to non-targets. In this work we investigate the catalytic effect of Ag⁺ ions on the hydrolysis of two pesticides chlorpyrifos and phoxim. In addition, the degradation products were identified by ³¹P NMR technique in order to establish the possible mechanism for degradation.

2 Experimental

3 Instrumentation

4 Results and discussion

In our previous work, the degradation of chlorpyrifos and phoxim with Ag⁺ led to the formation of O,O-diethyl-O-methyl phosphorothionate at metal ion/pesticide ratio ⩽ stoichiometry and completely decomposed at higher ratio. Thus for kinetic investigation, pesticide to ion ratio of 4 or less was selected.

4.1

4.1 Degradation kinetics

4.1.1

4.1.1 First-order reactions

A first-order reaction depends on the concentration of only one reactant (a unimolecular reaction). Other reactants can be present, but each will be zero-order. The rate law for an elementary reaction that is first order with respect to a reactant A is: $$C_t=C_0{e}^{-\mathit{kt}}$$ where C_t represents the concentration of the chemical of interest at a particular time, and C₀ represents the initial concentration and k is the first order rate constant, which has units of time⁻¹. The integrated first-order rate law is: $$\ln \left(\frac{C_t}{C_0}\right)=-\mathit{kt}$$ A plot of $\ln \left(\frac{C_t}{C_0}\right)$ versus time t gives a straight line with a slope of −k. The half-life of a reaction describes the time needed for half of the reactant to be depleted (same as the half-life involved in nuclear decay). The half life of a first-order reaction is independent of the starting concentration and is given by: $$t_{\frac{1}{2}}=\frac{\ln (2)}{k}$$ The degradation of OPPs over time in the presence of Ag⁺ was monitored by using ³¹P NMR. As shown in Fig. 1, chlorpyrifos degraded with increasing of time and resulting new products formation at 25 °C. It is important to note that in lower pesticide to Ag⁺ ratio (<8), the product 2 was not formed and only product 1 was formed and its concentration increased versus time.

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Figure 1

³¹P NMR spectrum of chlorpyrifos as function of time in the presence of Ag⁺ ions (conditions: 0.2 mL of 0.05 mol L⁻¹ of chlorpyrifos, 0.05 mL of 0.2 mol L⁻¹ of phosphoric acid, 0.05 mL (upper) and 0.025 mL (lower) of 0.05 mol L⁻¹ of Ag⁺).

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Chlorpyrifos itself shows a change in the chemical shifts of the phosphorus nuclei with addition of Ag⁺, establishing an interaction between Ag⁺ and chlorpyrifos (product 1) and then methanolize to O,O-diethyl-O-methyl phosphorothionate (product 2). Product 1 and product 2 accumulate with time and the obtained product profile is indicative of the role Ag⁺ plays in catalyzing the degradation of chlorpyrifos. The observed loss of chlorpyrifos in the presence of Ag⁺ was initially assumed to be first order with respect to chlorpyrifos concentration. If linearity is observed when plotting $\ln \left(\frac{C_t}{C_0}\right)$ versus time (t), then this assumption would be valid.

Fig. 2 shows that the concentration (C) of chlorpyrifos and phoxim versus time (t), over 230 min and the first order kinetics equation (linear model) was fitted with changes in concentrations of chlorpyrifos and phoxim (Fig. 3). Degradation equations and deduced parameters for different treatments are shown in Table 1. All regression coefficients (R²) are greater than 0.94, demonstrating that the degradation of pesticides could follow first-order reaction.

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Figure 2

Concentration of remaining chlorpyrifos as function of time in the pesticide to Ag⁺ ratio of 4 (conditions: 0.2 mL of 0.05 mol L⁻¹ of chlorpyrifos, 0.025 mL of 0.05 mol L⁻¹ of Ag⁺ and 0.05 ml of 0.2 mol L⁻¹ of phosphoric acid) (upper) and concentration of remaining phoxim as function of time in the pesticide to Ag⁺ ratio of 4 (conditions: 0.05 mol L⁻¹ of phoxim, 0.05 mL of 0.05 mol L⁻¹ of Ag⁺ and 0.2 mol L⁻¹ of phosphoric acid) (lower).

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Figure 3

Observed first order reaction degradation of chlorpyrifos and obtained products (1,2) in the pesticide to Ag⁺ ratio of 4 (conditions: 0.2 mL of 0.05 mol L⁻¹ of chlorpyrifos 0.05 mL of 0.05 mol L⁻¹ of Ag⁺ and 0.05 ml of 0.2 mol L⁻¹ of phosphoric acid).

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Table 1 First-order reaction degradation rate constant and correlation coefficient of chlorpyrifos and phoxim (conditions: 0.05 mol L⁻¹ of chlorpyrifos and 0.2 mol L⁻¹ of phosphoric acid).

Pesticide	Pesticide/Ag+	Chemical shift (ppm)	Kinetic equation	k (min−1)	t1/2 (min)	R2
Chlorpyrifos	4	59.5	$\ln \left(\frac{C_0}{C_t}\right)=0.0009t-0.0172$	0.0009	693	0.9972
	8	59.8	$\ln \left(\frac{C_0}{C_t}\right)=0.0008t+0.0066$	0.0008	866	0.9863
Chlorpyrifos product 1	4	66.4	$\ln (C_t)=0.0061t-7.5837$	0.0061	139	0.9483
	8	66.4	$\ln (C_t)=0.0063t-6.7103C$	0.0063	110	0.9774
Chlorpyrifos product 2	4	40.4	$\ln (C_t)=0.0079t-7.037$	0.0079	88	0.9929
	8	–	–	–	–	–
Phoxim	4	67.5	$\ln \left(\frac{C_t}{C_0}\right)=0.0006t+0.076$	0.0006	1155	0.9883
Phoxim product	4	66.9	$\ln (C_t)=0.0046t+-6.4855$	0.00002	34657	0.9994

The rate of degradation reaction increased with an increase of Ag⁺ concentration but rate of formation for product 1 is independent of Ag⁺ concentration. From the kinetic and analytical studies, we can propose Ag⁺ catalytic degradation of chlorpyrifos process as below:

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In this mechanism, the metal ion acts as an electrophile and co-ordinates the S and Cl atoms and enhances the electrophilicity of the phosphorus electrophilic center making it more prone to attack by methanol.

5 Conclusion

This study indicates that chlorpyrifos and phoxim can be effectively degraded in the presence of Ag⁺ and chlorpyrifos is degraded at a faster rate than phoxim and the rate of degradation was increased with concentration of Ag⁺ which means chlorpyrifos is much more labile to Ag⁺ than phoxim. Chlorpyrifos and its two degradation products and phoxim ant its product were determined by ³¹P NMR. The values of the first order rate constant for chlorpyrifos and phoxim at 25 °C were determined to be 9 × 10⁻⁴ min⁻¹ and 6 × 10⁻⁴, respectively.