Effect Of Charcoal Amendment On Adsorption, Leaching And Degradation Of Isoproturon In Soils

Herbicides in the soil are easy into surface water and groundwater with rain or irrigation through runoff and leaching, which will lead to the pollution of water bodies. The adsorption and desorption behaviors of a herbicide on soil are one of the important factors governing the fate of the herbicide in soil-water environment. Therefore, the adsorption and desorption of pesticides in soil have important impact on their environment behavior such as leaching, transformation, and are also important index for water body pollution evaluation. In this study, we have studied the effects of charcoal amendment on the adsorption-desorption, leaching and degradation of isoproturon in soils to explore the mechanisms of reducing isoproturon loss from soils by charcoal amendment. And these will provide a theoretical basis for pollution and control of herbicides in the soil-water environment .Adsorption-desorption of the herbicide isoproturon in five different particle sizes of charcoals and three different soils were studied using batch equilibration technique. The studies focused on the influences of charcoal amendment on the adsorption-desorption of isoproturon in soils. The experimental data were fitted by the Freundlich empirical equation. The results showed that charcoal had a great adsorption capacity for isoproturon, and the smaller the particle size of charcoal, the more the adsorption of isoproturon. The amendment with charcoal could improve the adsorption of isoproturon in soils. The adsorption of isoproturon in soils increased with the rate of charcoal amended (correlation coefficient r=0.9568**). Desorption of isoproturon from charcoals and soils showed significant hystersis which was shown by the higher adsorption slope (1/nads) compared to the desorption slope (1/ndes). The hysteresis effects of charcoal on desorption of isoproturon were closely correlated with the content of charcoal in the soils, and the hysteresis index (H) increased with the rate of charcoal amended. Observation of environment scan electron microscope (ESEM) exhibitted the fine pore structure and special surface characteristics of the tested charcoal. Analysis of Fourier transform infrared spectra (FTIR) of isoproturon adsorbed on charcoal suggested probable bonding interactions between isoproturon and charcoal.The factors affecting transfer ability of isoproturon in soils and the influences of charcoal amendment on the leaching were studied with the soil column leaching method. The results showed that: (1) under the same water volume, leaching of isoproturon in different soils varied greatly, and the amount of isoproturon leached in leachate followed the order: yellow brown soil > shajiang blacke soil > paddy soil. (2) The amount of isoproturon reclaimed in leachate increased with the water volume increasing and followed the order: 750 mm > 500 mm > 250 mm. The distribution of isoproturon in soils after leached significantly influenced by water volume. (3) the amount of isoproturon leached from soils varied with different pH values, and followed the order: pH 5>pH 9>pH 7. (4) The amount of isoproturon in leachate increases with the increasing of the amount of charcoal addition to soil. (5) The amount of isoproturon in leachate decreases with the increasing of the amount of charcoal addition to soil, while the residue of isoproturon in soils increases with increaing of charcoal content of soil samples. There is a highly significant positive correlation of the leachate of isoproturon with the content of charcoal in the soils.By incubation experiment, the degradation of isoproturon in non-sterile and sterile soils and effects of charcoal amendment on isoproturon degradation rate were studied. The differences of isoproturon degradation half life (t1/2) between non-sterile and sterile systems in the three soils(the t1/2 in the sterilized Yellow brown soil, Shajiang black soil and Paddy soil are 161.16 d, 87.72 d and 87.72 d,respectively, and in the non-sterilized soils are 16.00 d, 15.20 d and 54.57 d, respectively)indicated that biodegradation is the most significant mechanism for its dissipation in yellow brown soil and shajiang black soil. But chemical hydrolysis may be the main mechanism of isoproturon degradation in paddy soil. The half-lives of isoproturon degradation increased with the increasing of the amount of charcoal additon in the soil in which the degradation of isoproturon was dominated by biological effect: the t1/2 increased from 16.00 d to 135.88 d with charcoal additive increased from 0 g·kg-1 to 50 g·kg-1 for yellow brown soil, and the t1/2 increased from 15.2 d to 106.62 d for shajiang black soil. In paddy soil, the charcoal amendment has no significant effect on the degradation of isoproturon (the t1/2 increase from 54.57 d to 77.44 d with charcoal additive increased from 0 g·kg-1 to 10 g·kg-1, but when 50g·kg-1 charcoal was added, the t1/2 decreased to 53.31 d). This research suggests that charcoal amendment may be an effective management practice for controlling pesticide leaching in soils, and extended isoproturon's effective life-time due to reduce the bioavailability of isoproturon in soils.