657 J. Indian Chem. Soc., Vol. 91, April 2014, pp. 657-662 Extractive spectrophotometric study of adsorption and leaching of malathion on four Indian soils D. K. Sharma* a , Aditi Gupta a , Tilak Dharmani a and Rajinder Kashyap b a Department of Chemistry, Himachal Pradesh University, Shimla-171 005, Himachal Pradesh, India b Department of Chemistry, Govt. P. G. College, Solan-173 212, Himachal Pradesh, India Manuscript received online 14 May 2013, revised 11 September 2013, accepted 12 September 2013 Abstract : Pesticide adsorption by soil affects its toxicity, mobility, persistence and volatilization which in turn influ- ence the ground water contamination through leaching. The leaching potential of the pesticide is evaluated in terms of ground water ubiquity score. An extractive spectrophotometric study of the adsorption of malathion on four soils of different soil characteristics has been carried out with a view to evaluate its leaching potential. The proposed study is based on the microwave assisted alkaline hydrolysis of the insecticide to dimethyl dithiophosphate which form yellow coloured chloroform extractable complex with copper( II) acetate showing maximum absorbance at 419 nm. The ground water ubiquity score (GUS) of malathion with all the soil types has been found in the range 1.38–1.60, which classify it as a non leacher insecticide and hence it does not represent hazard to ground water contamination. Keywords : Malathion, extractive spectrophotometric study, soil adsorption, leaching. Introduction The environmental fate of pesticides is strongly influ- enced by their interaction with soil as the latter is the ultimate reservoir for these chemicals irrespective of their application target. Pesticide adsorption by the soil is a naturally occurring phenomenon which affects the vari- ous processes like toxicity, mobility, persistence and vola- tilization which in turn affect ground and surface water contamination 1 . The frequent detection of pesticides in surface water and groundwater has led to a number of experimental studies on adsorption of malathion and re- lated insecticides for evaluation of their leaching poten- tial 2–8 . Hence, thorough understanding of pesticide ad- sorption on soil is of great importance for the prediction of its movement in soils and water bodies. In continua- tion to our earlier work on the adsorption study of thiophanate methyl on some soils 9 , the present study deal- ing with the adsorption of malathion on some soils has been undertaken. Malathion is a nonsystemic organophosphorous insec- ticide widely used to control insects that attack fruits, vegetables, cereals and stored grains 10,11 . Like other pes- ticides, malathion is also toxic and its adverse effects on human health and ecosystem are also reported in literature 10–14 . In the present work an extractive spectro- photometric study of the adsorption of malathion on four Indian soils of different soil characteristics has been car- ried out. Though malathion has been determined by vari- ous analytical methods 15–22 , spectrophotometric methods being simple and easily accessible in all laboratories are extensively used. The spectrophotometric study is based on colour reaction of dimethyl dithiophosphate (formed from microwave assisted alkaline hydrolysis of the malathion) with copper(II) acetate forming chloroform extractable yellow product showing maximum absorbance at 419 nm. The various adsorption parameters viz. soil- adsorption coefficient (K d ), soil organic carbon partition coefficient (K oc ) and Gibb’s free energy ( Gº) and Groundwater Ubiquity Score (GUS) for predicting leach- ing behaviour of the insecticide in terms of groundwater contamination has been calculated. Results and discussion Concern about the environmental impact of excessive use of malathion insecticide has prompted research into the environmental fate of this insecticide 2–4 . The behaviour of malathion in soil is strongly influenced by soil charac- teristics which is an important consideration for the evalu-