Colloids and Surfaces A: Physicochem. Eng. Aspects 240 (2004) 55–61 Phorate and Terbufos adsorption onto four tropical soils J. Paul Chen ∗ , Simo O. Pehkonen, Chia-Chiun Lau Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 21 August 2003; accepted 10 March 2004 Available online 10 May 2004 Abstract Adsorption of Phorate and Terbufos onto four tropical soils was investigated in this study. It was found that the adsorption kinetics was fast and that the equilibrium was established within 6 h. Adsorption isothermal data could be well described by the Freundlich equation. It was demonstrated that the soils were more favorable for the adsorption of Terbufos than Phorate, which was due to the higher hydrophobicity of Terbufos (and its lower water solubility). The presence of organic compounds in soils played an important role. A higher organic content caused higher adsorption. A new term of K ′ oc , the Freundlich based organic content-normalized partition coefficient K ′ oc = K/f oc was defined. In the above equation, K and f oc are the Freundlich constant and the fraction by weight of organic content in the soils, respectively. It was demonstrated that the K ′ oc was independent of the types of soils (or organic content). The pH effect was found to be insignificant for the adsorption of both pesticides. Finally, a competitive study demonstrated that the presence of the more strongly adsorbed Terbufos played a more important role than that of the more weakly adsorbed Phorate. © 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Phorate; Terbufos; Organophosphorus pesticide; Soil; Organic content; pH 1. Introduction Pesticides are the main means of controlling insects, dis- ease, and weeds in crops and pastures. Although pesticides are indispensable in modern agriculture, their use and mis- use can lead to serious water quality problems. Once a pes- ticide is applied to crops, several processes may occur. It may be taken up by plants and/or ingested by animals, in- sects, worms, or microorganisms in the soil; it may move downward in the soil and adhere to soil particles, or it may dissolve; it may volatilize; it may be broken down into less toxic compounds; it may be leached or moved out of the plants’ root zone by rain or irrigation water; or it may be carried away by runoff water or erosion. Increased pesticide use in agriculture has led to a general concern regarding their potential threat as contaminants in the soil and the aquatic environment. Fish deaths, reproduc- tive failure of birds, and acute illnesses in people have all been attributed to the ingestion of pesticides or exposure to ∗ Corresponding author. Tel.: +65-874-8092; fax: +65-779-1936, +1-831-303-8636. E-mail addresses: jchen.enve97@gtalumni.org, checjp@nus.edu.sg (J.P. Chen). pesticides. These problems are due to many factors, includ- ing misapplication, careless storage, or careless disposal of pesticides and containers. Groundwater contamination, how- ever, is a potential problem following normal pesticide use. Pesticide adsorption processes are undoubtedly one of the major factors affecting the persistence, and movement in the soil. Soil–pesticide interactions have been examined previ- ously [1,2]. Once a pesticide is introduced into the environment, whether by application, disposal, or a spill, its fate can be influenced by many processes. These processes determined the ultimate fate of the pesticide by affecting its persis- tence and the movement in the multiphase environment. These fate processes, can have both positive and negative influences on the pesticide’s effectiveness toward pests or its impact on the environment. On the positive side, they can bring a pesticide into contact with the target pest for effective control, or they can reduce biologically significant concentrations of the active ingredient to relatively harmless levels. Sometimes, however, these same mechanisms can be detrimental, leading to injury of non-target plants and animals or reduced control of the target pest. Adsorption of pesticides in soils plays an important role in the fate of most pesticides. Therefore, a better understanding on their 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.03.008