ASIAN JOURNAL OF CHEMISTRY ASIAN JOURNAL OF CHEMISTRY http://dx.doi.org/10.14233/ajchem.2016.19690 INTRODUCTION Paraquat dichloride belongs to bipyridylium group is a colourless, white, or pale yellow crystalline solids, hygroscopic and odourless [1] in nature and other properties are given in Table-1. Paraquat is extremely persistent in the soil environ- ment and has the field half-lives of greater than 1000 days [1,2]. It is degraded by ultraviolet light, sunlight and soil micro- organisms to products which are less toxic than the parent compound. The strong affinity of paraquat to soil particles and organic matter could limit its bioavailability to plants, earthworms and microorganisms [1,2]. However, the bound residues may persist indefinitely and can be transported in runoff with the sediment. Paraquat is not significantly mobile in most soils and that which does not become associated with soil particles can be decomposed to a non-toxic end product by soil bacteria [3]. Although paraquat is highly immobile in soil, limited movement of the chemical may occur if the adsor- ption capacity of a soil is exceeded, or if paraquat is weakly adsorbed [4]. Increase in soil pH increases its sorption to soil particles and vice versa and the sorption is weaker in highly organic soils and remains active for long time and present upto 29 days in soil with more than 98 % organic matter [5]. Thus Field Dissiation Kinetics of Paraquat in Acid Soil as Function of Concentration and its Residues in Tea Leaves P. JANAKI 1,* and C. CHINNUSAMY 2 1 Department of Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore-641 003, India 2 Weed Science Lab, AICRP-WM, Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore-641 003, India *Corresponding author: Fax: +91 422 6611563; Tel: +91 422 2441514; E-mail: janakibalamurugan@rediffmail.com Received: 26 October 2015; Accepted: 7 April 2016; Published online: 30 April 2016; AJC-17865 The increasing concern on ecological and public health confined the use of pesticides in agriculture. However the herbicide use is increasing alarmingly to address the issues of labour scarcity and have timely weed management in crop production. Paraquat is toxicity class I herbicide as per EPA classification and is continuously used throughout world as well as in India. Thus, to monitor residues in tea leaves, paraquat dissipation studies were conducted in acid soil under tea in tropical hilly zone. A tea grown field was applied with paraquat dichloride at 2 and 4 kg a.i. ha -1 as post-emergent herbicide. The paraquat dissipation in soil followed first-order reaction kinetics and its persistence increased with increase in the application rate. The half-life values calculated was ranged from 25.0 and 27.4 days. Detector response was linear within 0.05-1.0 μg mL -1 concentration range at per cent relative standard deviation of 3.14 %. The limit of quantification for soil and tea leaves were 0.08 and 0.05 mg kg -1 , respectively. The average recoveries of paraquat from soil and tea leaves were found between 77.8-84.0 % and 82.0-84.0 %, respectively. At both the application rates, residue of paraquat was BDL in soil and tea leaves on 100 day after application. Keywords: Acid soil, Degradation, Paraquat residue, Spectrophotometer, Tea leaves. Asian Journal of Chemistry; Vol. 28, No. 8 (2016), 1639-1642 the risk of ground water contamination by paraquat is not high until directly applied to the water bodies for aquatic weed control [6]. The major problem in tea cultivation is the weed control and could be achieved through chemical methods using herbicides. Many herbicides are registered for weed control in tea and the commonly used one is paraquat. Paraquat dichloride 24 % SL is the only registered formulation in India with the Central Insecticide Board and Registration Committee (CIBRC) and has been categorized as highly toxic. Although CIBRC has not provided any recommendations, it has approved the use of this herbicide in nine crops including tea. This chemical is also called quaternary ammonium salts and generally known as quats. It destroys plant tissue by disrupting photosynthesis and rup-turing cell membranes, which allows water to escape leading to rapid desiccation of foliage. Paraquat is not metabolized to any degree in plants. Summers [4] found that on plant surfaces, paraquat is degraded by sunlight to the extent of 66 % when exposed to 21 days of sunlight. Though it is tightly bound by the soil, continuous and indiscriminate use may cause bioaccumulation in soil and crops. Hence the paraquat products are classified under restricted use category and registered to control weeds and grasses in many