Paramjeet Kaur Dhiraj Sud Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, India Research Article Adsorption Kinetics, Isotherms, and Desorption of Monocrotophos and Dichlorvos on Various Indian Soils Pesticide sorption on to the soil has a significant role in deciding the fate and behavior of pesticides in soil and aquatic environment. The present study investigates the adsorption of monocrotophos (MCP) and dichlorvos (DDVP) on the three soils of Malwa region of Punjab, India under different conditions. Batch adsorption exper- iments were preformed in replicates using 2 g of air-dried soil and varying concen- trations of pesticides and 20 mL of 0.01 M CaCl 2 as background electrolyte. The results revealed high adsorption of MCP and DDVP in soil B with k f -values 0.1261 and 0.0498 and n-values 2.7345 and 1.831, respectively. The adsorption isotherms obtained were analyzed and the data was subjected to classical Langmuir, Freundlich, and Temkin models. The experimental data best fitted to the logarithm form of Freundlich and Temkin model. Kinetics analyses were performed using pseudo-first order, pseudo- second order, and intraparticle diffusion models. The regression results showed that the experimental data fitted very well with the pseudo-second order kinetic model as correlation coefficient value is very closer to 1 and also followed the intraparticle diffusion model, whereas, diffusion is not only the rate controlling step. The percen- tages desorption with tap and distilled water is 32–64% for MCP and 25–48% for DDVP. Keywords: Adsorption; Adsorption isotherm; Kinetics; Pesticides; Toxicity Received: July 23, 2010; revised: October 6, 2010; accepted: November 6, 2010 DOI: 10.1002/clen.201000289 1 Introduction Pesticides contamination of soil and aquatic environment is an issue of major concern throughout the world. The widespread use of organophosphate pesticides (OP’s) as an alternative to organochlor- ine pesticides along with their resistance to natural decomposition and biodegradation has resulted in the emergence of these chemi- cals and their transformed products in natural waters [1]. The pes- ticide movement from the point of application to the different environmental segments involves the processes such as degradation, dissipation, transportation, sorption, and leaching from soil to pol- lute ground and surface, etc. Pesticide sorption on the soil directly or indirectly influences these processes and, thereby, plays the key role in deciding the fate of chemicals in soil and aquatic environment [2–4]. Desorption of pesticides from soil matrix determines the release rate and the potential mobility of pesticides in soil. Soils being complex heterogeneous mixture of organic and inorganic compounds, having varying composition and surface activity, show the variability in sorption capacity of pesticides [5]. Hence, knowledge of the pesticide adsorption–desorption characteristics of soils is of paramount importance for predicting their mobility and fate in soil environments. OP’s are relatively soluble in water and have significant potential for entering aquatic environments through surface runoff, sprays, and soil leachates. In order to understand the fate, behavior, and redistribution of these chemicals in water and desorption from the complex soil environment, it becomes imperative to study the sorp- tion of these chemicals on the soil. Monocrotophos (MCP) (dimethyl [(E)-4-(methylamino)-4-oxobut-2-en-2-yl] phosphate) is most popular and widely used OP owing to its low cost and high efficiency in controlling pests mainly on cotton crop, rice, and sugarcane and active against large variety of insects in India. The EPA classifies MCP as a class I toxicity – highly toxic compound. However, it is continu- ously used for the control of major pests in agriculture in developing countries like India primarily due to lack of alternative replace- ments [6]. The total reported national production of MCP in India was 5118 metric tonnes in 2007–2008 [7]. Dichlorvos (DDVP) (2,2- dichlorovinyl dimethyl phosphate) is an insecticide widely used for crop protection mainly in greenhouses and for controlling parasites and insects in houses, aircraft, and outdoor areas (as aerosols and liquid sprays) [8]. It is high soluble in water and the EPA has classified it in toxicity class I – highly toxic. The important physico-chemical properties and chemical structures of MCP and DDVP are given in Tab. 1 and Fig. 1. Some studies are reported regarding the degra- dation of MCP [9] in two Indian agricultural soils and fate and movement of DDVP in soils [10, 11]. Despite the indiscriminative Correspondence: Dr. D. Sud, Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal 148106, India E-mail: suddhiraj@yahoo.com Abbreviations: DDVP, dichlorvos (2,2-dichlorovinyl dimethyl phosphate); MCP, monocrotophos (dimethyl [(E)-4-(methylamino)-4-oxobut-2-en-2-yl] phosphate) 1060 Clean – Soil, Air, Water 2011, 39 (12), 1060–1067 ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com