BEHAVIOUR OF PESTICIDES IN AGRICULTURAL SOILS: ASSESSMENT AND BIOCHAR-BASED CONTROL APPROACH

Abstract

The World Health Organization defines pesticides as chemical substances used to prevent, destroy, or mitigate pests, including vectors of disease and unwanted species harmful to food production and other commodities. While pesticides are crucial in modern agriculture for protecting crops and enhancing food production, their overuse can harm human health, the environment, and non-target species. Pesticide use raises environmental risks, especially regarding groundwater and surface water contamination. This necessitates a thorough understanding of pesticide behaviour in agricultural lands to address human health and environmental sustainability. Sorption characteristics of organic chemicals vary with location, influenced by factors like clay content, organic matter, and microbial presence. Understanding these characteristics is vital for developing effective remediation measures and pesticide management strategies. In the specific context of Telangana state, where pesticide usage has increased dramatically in recent years, there is a heightened risk of groundwater contamination with persistent organic pollutants. Imidacloprid and Atrazine are the two majorly used pesticides in this region. Therefore, this research aims to investigate the sorption, desorption, and leachability of widely used pesticides in the black cotton soils and red soil of the Telangana region. The objective is to provide a comprehensive assessment of the potential risks associated with groundwater pollution from these pesticides, offering valuable insights for tailored environmental management. Additionally, the study seeks to explore the efficacy of biochar derived from locally available cotton stalks in enhancing pesticide sorption. This aspect of the research is essential for developing targeted remedial measures against pesticide pollution and implementing effective pesticide management practices specifically tailored to the unique characteristics of the Telangana region. By shedding light on the sorption, desorption, and leaching behaviour of Imidacloprid and Atrazine, this research aims to empower policymakers with the knowledge necessary to formulate targeted strategies for environmental conservation and sustainable agricultural practices, thereby ensuring a harmonious balance between agricultural productivity and environmental preservation. vii The first phase of the study assesses the sorption desorption behaviour of insecticide Imidacloprid and herbicide Atrazine in four different agricultural soils (BC-1, BC-2, BC-3, RS) of Telangana region. The study reported that both the pesticides were weakly sorbed on the investigated soils. The sorption kinetics for both the pesticides were observed to occur in two stages- the quick initial adsorption stage, where 90% of the pesticide’s adsorption capacity reached within first few hours due to the rapid filling of empty spaces in soil particles, and the slow equilibrium stage. Pseudo second order kinetic model demonstrated the best fit to the experimental data. Analysis of isotherm models indicated that the Freundlich isotherm model provided the best fit compared to Langmuir or Temkin isotherm models. Among the soil samples, BC-2 soil exhibited the highest Freundlich coefficient value (Kf), followed by BC-3, BC-1, and red soil. A strong positive correlation (r>0.9) was observed between clay content and the Kf value, indicating that clay content significantly influences Imidacloprid adsorption to the soil. Interestingly, despite having a higher organic carbon content (0.57%), red soil showed a lower affinity for Imidacloprid compared to other soils, possibly due to the relatively low quantity of organic carbon present. Both temperature and pH exert significant influences on the sorption behaviour of Imidacloprid in the examined soils. As temperature rises, the sorption of Imidacloprid decreases, possibly due to increased solubility at higher temperatures. Additionally, lower pH levels demonstrate favourable conditions for sorption, as they facilitate the protonation of the solution, thereby increasing the cationic charge of the pesticide molecules. The second phase of the study focused on the degradation of Imidacloprid and Atrazine in soil to assess their persistence. The study reported that the half-life of Imidacloprid ranged from 67 to 83 days in the investigated soils, whereas the half-life of Atrazine ranged from 44 to 57 days. Imidacloprid showed greater persistence in the investigated soils compared to Atrazine. The Groundwater Ubiquity Score (GUS) index is employed to evaluate the leachability of pesticides in the soils under investigation. This screening method utilizes the KOC value and half-life (T1/2, days) of pesticides to assess the probability of groundwater contamination. The GUS values for Imidacloprid ranged from 2.25 to 2.62 in black cotton soils, while in red soil, it was reported as 3.72. For Atrazine, GUS values ranged from 2.01 to 2.41 in black cotton soils, and in red soil, it was 3.16. GUS values falling between 1.8 and 2.8 in all black cotton soils suggest that both pesticides fall into the potentially mobile category. However, in red soil, where the values exceeded 2.8 for both pesticides, they are considered highly mobile. The results of the soil column leaching study corroborated these findings, as both Imidacloprid and Atrazine were viii detected in the leachate sample and migrated to the deeper sections of the column. The presence of pesticides in the deeper layer of the soil matrix could make the pesticides more persistent due to the absence of microorganisms, ultimately leading to groundwater contamination. The final phase of the study investigated the efficacy of cotton stalk biochar as soil amendments to improve pesticide sorption capacity in soils and reduce the mobility. The results indicate that the introduction of 0.5% and 1% cotton biochar (CBC) into the soil significantly enhanced the Kf values of Imidacloprid and Atrazine sorption across all the examined soils. The improved sorption capacity is a direct outcome of the augmented surface area and additional sorption sites resulting from the addition of CBC to the soil. Pesticide degradation study in biochar amended soils revealed that the addition of biochar to the soil had a slight effect on the half-life of Imidacloprid and Atrazine, with all cases showing a modest increase. With the incorporation of biochar, GUS values notably declined, falling below the threshold of 1.8 suggests a significant reduction in the mobility of both Imidacloprid and Atrazine within the investigated soils. The soil column leaching study revealed that biochar amended soil can retain pesticides in the top layers of the soil matrix for both Imidacloprid and Atrazine. Overall, it can be concluded from the findings of the present study that the sorption capacity of both Imidacloprid and Atrazine was notably low in all three black cotton soils and red soil. Clay content, temperature and pH was identified as the primary factor influencing pesticide sorption, while soil organic matter showed a negative correlation with the sorption coefficient due to its limited availability in the soil. Imidacloprid's prolonged half-life compared to Atrazine indicates its greater persistence in all investigated soils. The calculated GUS value categorized both Atrazine and Imidacloprid as falling within the highly mobile to mobile category, highlighting the region’s vulnerability to groundwater contamination from pesticides. The study underscored the high susceptibility of groundwater in this specific area of the Telangana region to contamination, emphasizing the urgent need for proactive measures to prevent potential catastrophes. The incorporation of cotton stalk biochar into the soils emerged as a promising strategy to mitigate the leaching potential of both pesticides, offering a potential solution to reduce environmental risks associated with pesticide use.