The Environmental Behaviors Of Chlorpyrifos As Influenced By Humic Acids Isolated From Different Soils

The environmental problems caused by pesticides are currently of great concern, and many studies on their environmental behavior have been reported, which focused mainly on their adsorption and degradation in the soil. It is indicated that components of the soil, especially the organic matter, strongly influence the bioavailability of pesticides. Among the soil organic-matter constituents, humic acids(HA) are the most important, because of their content in the organic matter and their most active role on the pollutants. In addition, chloipyrifos is one of the most widely used organophosphate pesticides today, with obvious toxicity in natural systems. So far, many of studies focused on the environmental behavior of chlorpyrifos, while little is known about the environmental behaviors of chlorpyrifos as influenced by humic acids isolated from different soils. Consequently, in this paper,the compostition and structure of different HA were compared simply; on these base, the influences of HA on hydrolysis of chlorpyrifos and the adsorption/desorption characteristics of chlorpyrifos on HA were studied furthermore.Results showed that the amount of organic carbon in all kinds of HA was high, ranged in (369.20—448.32)mg/g .The amount of acidic group was in the range of (4.66～7.09)mmol/g, the amount of carboxy group in (2.29～3.64)mmol/g, the amount of phenolic hydroxyl group in(2.38～3.78)mmol/g. Their UV-VIS absorption spectra were similar, with a peak near 210nm. However, the intensity of absorbance peak and the values of their E4/E6(3.74～6.06) were different. IR absorption spectra showed that there were characteristic absorbance peaks at 3500-3000cm^-1, 2400-2200cm^-1, 1640-1630cm^-1, 1420-1330cm^-1 1200-1040cm^-1, 830-820cm^-1and 680cm^-1, which indicated that all of them contained benzene, phenol group, carboxyl group, polysaccharides and proteins, et al.. However, their IR absorption spectra also had some differences in the intensity of absorbance peaks, with the order of purple alluvium HA>limy yellow soil HA> neutral purple soil HA>acid purple soil HA> humus soil HA at 2900cm^-1 , 1720cm^-1, 1520cm^-1 and 1366cm^-1. In conclusion, the compositions of humic acids in various soils were common in nature, but they had significant differences in aromatic structures, function groups, molecular weight, and their level, et al.. It was showed that the hydrolysis effect differed for HA from different soils. For humus soil with the lowest level of HA, when the concentration was 120mg/L, it boosted hydrolysis of chlorpyrifos most significantly; However, the larger the concentration of HA, the smaller the hydrolysis effect. When the concentrations of HA were Omg/L, 120mg/L, 240mg/L, 360mg/L, 480mg/L, the hydrolysis rate constants (k) of chlorpyrifos were 0.0019, 0.0048, 0.0039, 0.0034, 0.0029 relatively. With the concentration of the other four types of HA increasing, hydrolysis of chlorpyrifos increased gradually, and the effect by acid purple soil with the lower level of HA was more distinct as the concentration of HA less than 240mg/L. For purple alluvium, acid purple soil, neutral purple soil and limy yellow soil, the half-life of chlorpyrifos decreased from 365min to 162min, 165min, 151min and 187min relatively when the concentration of HA increasing from 0mg/L to 480mg/L. Therefore, it may be conferred that the hydrolysis rate of chlorpyrifos was related to the concentration of HA and the level of HA; and the later was concerned with the acid level of HA and the role of adsorbed-catalysed by HA, the mechanism of which must be tested further.The adsorption rate and desorption rate of chlorpyrifos were both corelated to the sorpt of HA. For Purple alluvium HA and acid purple soil HA, the adsorption amount could be up to 90% of the equilibrium adsorption amount within 30min, and the other samples within 2h. Compared with the adsorption process, desorption processes of chlorpyrifos by HA realized less slowly. For purple alluvium HA, acid purple soi HA, neutral purple soil HA, the desorption amount could be up to 90% of the equilibrium desorption amount within 2.5h, and the other samples within 5h.Results showed that the adsorption rates of all the five HA samples were high, more than 63%. With the initial concentration of chlorpyrifos increasing, the adsorption rates changed very little, but the adsorption amount increased obviously. For humus soil HA, acid purple soil HA, neutral purple soil HA, limy yellow soil HA and purple alluvium HA, the adsorption amount were 855.38mg/kg, 921.28mg/kg, 974.63mg/kg, 976.61mg/kg, 1009.11mg/kg when the concentration of chlorpyrifos was 2.25mg/L. Adsorption isotherms of chlorpyrifos on HA could be explained by Freundlich Equation better; they were almost linear in the test. Adsorbability of all HA were strong, with K_f larger than 2125.14, and the order of K_f was: purple alluvium HA>limy yellow soil HA>neutral purple soil HA>acid purple soil HA>humus soil HA. Except for neutral purple soil HA, the values of n of all samples were larger than 1. However, the desorption rates of chlorpyrifos by HA were small relatively (<26.7%); the order of fixing ability was: purple alluvium HA> humus soil HA> neutral purple soil HA> acid purple soil HA> limy yellow soil HA. In conclusion, due to the distinct differences in the composition and structure of HA from different soils, their adsorptive / desorptive behavior displayed differently, which must be tested further.The mean partial molar free energy(F) of chlorpyrifos adsorbed on five HA indicated that the order of adsorbability was: neutral purple soil HA> purple alluvium HA>limy yellow soil HA>acid purple soil HA> humus soil HA. The changes of the free energy (△G) were all less than 40kJ/mol, suggesting that the adsorption mechanisms involved H-bonding, partitioning by water repellency, dipolar interactions and van der Waals forces, while chemical bonding did not take effect. The amount of chlorpyrifos adsorbed on the studied HA was related to pH, temperature, ionic strength, and the amout of HA, et al.. Results showed that the influences of pH on 5 samples were different, which could be due to an H-bonding process. When pH lower than 4.0, its influence on the adsorpition of chlorpyrifos by neutral purple soil HA was more significant than other samples. When pH was 5.0, the adsorption amount were lager for purple alluvium HA and limy yellow soil HA, of which the K_f were 6943.14 and 6043.67 relatively, but it had no significant influence when pH less than 4.0. For neutral purple soil HA, acid purple soil HA, humus soil HA, the adsorption amount were lager when pH was 2.0, but they took little change when pH increased from 4.0 to 5.0. Temperature also affected the adsorption of chlorpyrifos on the studied HA. In the test, it was largest at 15°C, and least at 25°C. Additionally, the adsorption of chlorpyrifos was positively correlated with the total content of HA, while ionic strength showed no significant influence.UV-VIS absorption spectra of humic acid and chlarpyrifos complex showed that the sites and intensity of major characteristic absorbance peak changed, that is to say, the wave got longer and the intensity got stronger, demonstrating that they interacted with each other. For humus soil HA, acid purple soil HA, limy yellow soil HA, neutral purple soil HA, purple alluvium HA, the values of AX were 7nm, 7nm, 6nm, 4nm and 7nm, and their intensity increased 0.21, 0.12, 0.21, 0.14, 0.22 relatively. In conclusion, the sites and intensity of major characteristic absorbance peaks changed differently for different soil HA, suggesting the different mechanisms between chlorpyrifos and HA.IR absorption spectra of humic acid and chlorpyrifos complex showed that, for acid purple soil HA and humus soil HA, the sites, shapes and intensity of major characteristic absorbance peaks changed extensively. For example, the vibration-absorption peaks at 1720cm^-1 got weaker, and peaks at 1550-1640cm^-1, 1380-1425cm^-1 got stronger and wider, which could be due to the -COOH in HA ionizing partially and the ionic bond forming. Also, it probably because of hydrogen bond of C = O in HA. However, for the other three samples, the vibration-absorption peaks at 3400cm^-1, 1550-1640cm^-1, and 1380-1425cm^-1 took no significant change.