Hydrolysis And Absorption Of Chlorpyrifos And Phoxim In Humic Acid

With the increasing use of pesticides, the pollution caused by pesticides is worsening. The pesticide residue can be found in many agricultural products. Wide spread pesticide use causes environmental pollution, consequently, land degradation, pollution of soil and other associated problems.Research results demonstrate that the time duration for half life of many pesticides are closely hydrolysis related to the absorption of soil. Humic acid (HA) can have a number of chemical reactions with pesticides. These reactions can affect hugely to the residue levels of pesticides and make significant contributions to the removal of such residues. Soil HA affects pesticides through absorption. However, there are few scientific reports on either the compassion of the absorption capability of various HA or the absorption characteristics of HA. Research on the effect of pesticides to soil can provide scientific guidance in both safe application of pesticides and maintenance of soil fertility. Therefore, research into the interaction between pesticides and soil HA is of vital importance to agriculture.This dissertation presents research results on the functions of pesticides and HA; their effects to the environment; pesticide decay ratios in soil; residue levels and various other factors. The main method utilized for this research is apparatus analysis, using experimental data under static conditions. The investigation on the interaction of pesticides and HA provides deeper understanding on the effects of HA to the environment; insight on potential approaches to reduce the harmful effects of HA to the environment and guidance on the appropriate use of HA. The research undertaken examines the absorption characteristics of pesticide Chlorpyrifos and Phoxim in the soil HA in Chongqing region, the hydrolysis effect of HA to pesticides and the associated factors. The analysis of the IR spectrum of 5 types of soil HA and HA-pesticide show: similar peak absorptions appear in all spectrums. It indicates that HA structures are similar and HA contains benzene derivative and hydroxybenzene.There exist some differences in these spectrums. The absorption band of Purple Alluvial HA,Acid Purple Soil HA,Forest Soil HA are in descending order 2900cm^-1, 1720cm^-1, 1520cm^-1 and 1366cm^-1. The absorption band weakens and that of Forest Soil HA is 1720cm" which has almost vanished. The variation of the absorption band indicates the decrease of benzene in HA and the extent of shrinkage in the order of Purple Alluvial HA,Yellow Soil HA,Neutral Purple Soil HA,Acid Purple Soil HA, Forest Soil HA. It also indicates the level of polymerize of Forest Soil HA molecules on the C framework, benzene derivative in HA and the molecule complexity are all rather small. The IR spectrum analysis results of the level of humification are consistent with those obtained from ultra-violet spectrum analysis.From the IR spectrums of Chlorpyrifos-HA, it is clear that after the reaction of HA and Chlorpyrifos, the C=O vibration absorption peak at the original value of 1720cm^-1 decreases. However, at 1550—1640cm^-1,1380—1425 cm^-1 the COO^- symmetric and asymmetric vibration absorption peaks increase and broaden. This shows that the COOH in the HA, after its reaction with the Chlorpyrifos, partially ionization and forms COO^- and form electrovalent bond for each other. At 3400^-1 cm^-1, 1550—1640cm^-1, and 1380—1425 cm^-1, COO^- vibration absorption peaks exhibit no obvious changes.The results from analysing the ultra-violet spectrums of 5 type of soil HA show: the ultra-violet absorptive curves are similar. The rate of the absorption increases as the decrease of the wavelengths. Peaks appear at wavelengths 202—204nm for all 5 type of soil HA.This result is consistent with those reported in the literature. However, the maximum values of the absorption differ greatly and are in the descending order of Purple Alluvial HA>Neutral Purple Soil HA>Forest Soil HA>Acid Purple Soil HA,Yellow Soil HA. It appears that HA with large absorptive peaks have plenty of chromophore (such as carbonyl,aryl, et al.) and auxochrome (such as phenolic hydroxide) or have plenty of such groups on the surface.The 5 type of HA have different values of E465/E665, ranging from 3 to 5. They belong to the category of HA with a large number of molecules. Among them, the Forest Soil HA from Jingyun Mountain has the maximum amount of E465/E665, which means that it has the minimum extent of humification, benzene derivative in HA and molecule complexity. The extent humification of HA have the descending order of: Purple Alluvial HA> Yellow Soil HA> Neutral Purple Soil HA> Acid Purple Soil HA> Forest Soil HA. This indicates that these HA have similar compositions, but different structures and properties.Results from analysing E465/E665 are the same as those obtained by analysing molecule structure using colour index coefficientΔ^log K.Compassion study of HA in NaOH and NaHCO₃ solutions show that the absorption peaks shift towards the direction of longer wavelength. The absorption peaks are lower with decreased brightness. The extents of shift are similar, but the level of decrease in peak values differ significantly, which indicates different solution have different effects to different HA.Experimental study on 5 type of HA and environment factors has been undertaken to ascertain whether these enhance or reduce pesticide hydrolysis. Results show: the logarithm of Chlorpyrifos residue percentage is linear to the residue time. This indicates that hydrolysis of Chlorpyrifos satisfies the first order reaction equation in the soil HA solution. At the same concentration of HA, their effects to pesticide hydrolysis are in descending order: Purple Alluvial HA, Yellow Soil HA, Neutral Purple Soil HA, Acid Purple Soil HA. That is, the effect increases as the extent of humification decreases. The effect of Forest Soil HA to the hydrolysis of Chlorpyrifos is rather significant when the concentration is low. However, the effect decreases as the concentration increases.Experimental results show that the logarithm of the Phoxim residue percentage and the residue time have linear relationship. Soil HA can slow down the process of Phoxim hydrolysis. Within the concentration range of the experiments, soil HA from various sources all slow down the Phoxim hydrolysis process, increase the concentration of HA and reduce the rate of Phoxim hydrolysis. However, the extent of the effects differs greatly. Under the same conditions, except Forest Soil HA, the effects to the rate of Phoxim hydrolysis are in the descending order: Purple Alluvial HA, Yellow Soil HA, Neutral Purple Soil HA, Acid Purple Soil HA. This is exactly opposite to the order of E465/E665 in HA and the order ofΔ^log K. It shows that the larger the extent of humification, the larger the effect for reducing the rate of Phoxim hydrolysis, because the extent of HA humification is closely related to the extent of benzene in HA and the amount of carbonyl, aryl, phenolic hydroxide. Therefore, the extent of HA humification reflects the effect HA to the pesticide hydrolysis better than only using the amount of carbon contained in the HA.The experimental results on the absorption-adsorptions effect of HA to Chlorpyrifos show: the absorption increases as the balanced concentration of Chlorpyrifos increases. HA has strong compatibility to Chlorpyrifos. The absorption constant temperature contours are nearly straight lines. The absorption capabilities of different soil HA have significant differences.Linear and Freundlich equations both describe the absorption behaviours of HA to Chlorpyrifos well. Fitting effects have reached significantly levels. The absorption constants show the absorption capabilities of various soils HA to Chlorpyrifos. They are in the descending order: Purple Alluvial HA,Yellow Soil HA,Neutral Purple Soil HA,Acid Purple Soil HA,Forest Soil HA. This order is consistent with the extent of HA humification using ultra-violet spectrum E465/E665. It indicates that, under experimental conditions, the absorption capability increases as the extent of HA humification increases. The absorption capability also reflects its extent of benzene derivative in HA and molecule complexity.When pH=2, except yellow soil HA, the absorption coefficients of HA to Chlorpyrifos increase significantly. Of these, the variation of absorption coefficients of Neutral Purple Soil HA is the largest (approximately 3 times). The absorption of ionic strength to chlorpyrifos is negligible. The total amount of absorption of Chlorpyrifos on HA increases as the increase of the amount of added HA. However, the unit absorption decreases.Analysis results show that, except Purple Alluvial HA, the correlation between the amount of absorption and adsorptions in the remaining 4 samples all reach significant levels. As the increase of absorption, the amount of Chlorpyrifos adsorptions in Purple Alluvial HA increases first and then slowly decreases. The rate of Chlorpyrifos adsorptions in various soil HAs are all below 26.70%. The descending order under average values: Yellow Soil>Acid Purple Soil> Neutral Purple Soil >Forest Soil>Purple Alluvial. The absorption of Chlorpyrifos on HA is generally rather strong. It cannot be easily adsorptions. The absorption of Purple Alluvial to Chlorpyrifosis the strongest followed by Forest Soil. The absorption of Yellow Soil and Acid Purple Soil are weakest. Hence, they may cause secondary pollution and may pose larger risks to environment.The experimental results of the absorption-desorption of HA to Phoxim show: the amount of absorption of Phoxim on the soil HA increases with the increase of the Phoxim concentration. Except that the fitting using Langmuir equation to the absorption data is less than desirable (R²=0.8415), the other 3 constant temperature absorption to various samples are all pretty good. R² are all above 0.930. The constant temperature contours are L-shaped. At low concentration, Phoxim molecules are rapidly absorbed. The variation of the absorbed amount decreases after the majority of the absorption positions on HA are saturated by Phoxim molecules. This suggests that HA has stronger compatibility to Chlorpyrifos. It has rather strong cleansing capability. The descending order of the amount of absorption of the Phoxim in 5 type of soil HA: Purple Alluvial HA,Yellow Soil HA,Neutral Purple Soil HA,Acid Purple Soil HA,Forest Soil HA。The desorption amount of Phoximin all 5 type of soil HA increase with the increase of the absorbed amount. The desorption amount in all soil HA are small compared to the amount of absorption, in particular, Yellow Soil HA andAcid Purple Soil HA。Through the research into the behaviours of the absorption and desorption in various soil types, it is possible to predict their effect to Phoxim mobility and estimate their environmental risks.The effects of pH to the Phoxim absorption in various soils HA are relatively small. As the increase of temperature, the absorption capabilities of all 5 soil HA to Phoxim all increase. Temperature affects the Phoxim absorption differently to different soil HA, most significantly to Purple Alluvial HA, followed by Acid Purple Soil and least to Forest Soil. The total amount of absorption increases as the increase of the amount of added HA. As the increase of the amount of the added HA, the unit Phoxim absorption (Q_e) decreases in Forest Soil HA and Acid Purple Soil HA. However, Q_e increases first and then decreases in the remaining 3 soil HAs.