Author Topic: Compost Adsorption Desdorption of Picloram in the Presence of Surfactant on Six  (Read 1717 times)

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Author :  Rounak M. Shariff
International Journal of Scientific & Engineering Research, Volume 2, Issue 5, May-2011
ISSN 2229-5518
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Abstract— To investigate the effect of different types of surfactants on adsorption behavior of pesticide, picloram [4-amino-3, 5, 6-trichloropicolinic acid] which is an ionic herbicide on six agricultural soil samples, picloram soil –water system-surfactant. The Freundlich adsorption coefficients Ks values for picloram adsorption in the presences of surfactant in three concentration of critical micelles concentration (cmc), batch equilibrium experiments performed of cationic surfactant Hexadecyltrimethylammonium bromide (HDTMA). The Ks values of picloram range between 0.940-1.344, 0.943-1.407, and 0.952-1.434 ml/g, for cmc/10, cmc  and cmc*20 respectively. Freundlich adsorption coefficients of picloram in the presence of anionic surfactant sodium dodecyl sulphate (SDS) was determined, the values of Ks obtained were in the range 0.761-1.151, 0.654-1.141, and 0.631-1.099ml/g, for cmc/10, cmc, and cmc*20 respectively. The Ks values for polyoxyethylene sorbitanmonooleate ( tween-80) were in the range 0.971 -1.229, 1.104-1.303, and 1.189-1.404  ml/g,  for cmc/10, cmc, and cmc*20 respectively. The values of Freundlich desorption coefficients (Ksdes), linearity factor for desorption (nsdes) and regression factor for desorption (R2) ranged from 0.839-1.286, 0.267-0.619, and 0.889-0.993 respectively with HDTMA. The values of Ksdes, nsdes and R2 values ranged from 0.895-1.289, 0.494-0.818, and 0.889-0.999 for desorption process respectively by anionic surfactant SDS, the values of Ksdes, nsdes and R2 ranged from 0.953-1.270, 0.429-0.650, and 0.888-0.995 respectively for non-ionic surfactant Tween-80.
Index Terms — Adsorption - desorption kinetics, Adsorption isotherms, HPLC, Picloram, Surfactant. 

1   INTRODUCTION                                                                     
THe use of kinetic models in the study of adsorption and desorption processes in heterogenous system is important. Three reasons for the use of kinetic or time-dependent models in soils have been suggested. First, many reactions in soil are slow; yet, they proceed at measurable rates. Second, non-equilibrium conditions can exist as a result of the physical transport of gases and solutes. Third, information about reaction mechanisms and processes occurring may be obtained from such data [1], [2].  The sorption pattern indicates an initial fast sorption that occurs within the first 24h tail it attainted the equilibrium within 48h. This was followed by slow reactions that appear to be the dominant processes i.e desorption of picloram if compared to the amount of picloram still sorbed on the soil. Picloram; is anionic herbicide is used to control unwanted woody plants and to prepare sites for planting trees and used to control broad-leaf plants and trees [3], [4]. Its adsorption involved ionic interaction with positive charges in soil and also the less energetic Van der Waals forces and charge transfer [5], [6]. A two step adsorption-desorption mechanism was used to model the observed behavior that can be described in terms of external and internal sorption sites. Desorption from external sites is relatively fast, taking place in about 5h and is characterized by a first-order rate constant. Many studies have indicated that the sorption and desorption of the organic chemicals in soils are not rapid, reversible process, despite past assumption to the contrary [7], [8].   A study of picloram desorption isotherms show positive hysteresis coefficients H in the six selected soil samples [9], [10]. Hysteresis coefficients H1, where Na¬ /Ndes¬ ratio for Ferundlich adsorption and desorption constants, respectively, indicating the greater or lesser irreversibility of adsorption in all samples, the highest values corresponding for which the highest adsorption constant was obtained. The coefficient H1 is a simple one and easy to use, indicating an increase in the irreversibility of the adsorption of herbicid as the clay content increases [8]. 
2.1 Soils
Fresh soil samples were taken from plough layer (0-15 cm depth), after removal of stones and debris, air dried under shade, ground then sieved through 2mm sieve and stored in black plastic container in dark[11], [12]. The six soil samples were collected from six main agricultural, representing a range of physico-chemical properties. Subsamples of homogenized soils were analyzed for moisture content, organic matter content, particle size distribution, texture, pH, loss on ignition and exchangeable basic cations were listed in Table 1 a & b.
2.2 Pesticide and Surfactant
 Analytical grad substituted picloram herbicide was purchased from Riedal-de Haen, Sigma-Aldrich com-pany ltd. With following purities expressed in weight percent picloram >97.4% [CAS-No.1918-02-1] respectively. The three different surfactants employed in this study comprised; the cationic Hexadecyltrimethylammonium bromide (HDTMA), the anionic sodium dodecyl sulphate (SDS) and nonionic polyoxyethylene sorbitanmonooleate (Tween-80). These compounds were purchased at reagent grade purity from BDH, and were used without further treatments. The critical micelles concentration cmc of the three surfactants are shown in Table 2. The three surfactants were studied at three concentrations: the critical micelles concentration (cmc), twenty-fold higher than cmc (20cmc), and 10-fold lower than cmc (cmc/10). All chemicals used were of analytical grade reagents and used without pre-treatments. Standard stock solutions of the pesticides were prepared in deionised water.

2.3 Adsorption Experiments
Adsorption of picloram from aqueous solution was de-termined at laboratory temperature (25±1 C˚) employing a standard batch equilibrium method [13], [14]. Duplicate air-dried soil samples were equilibrated with different pesticide concentrations (2, 5, 10, and 15 µg ml-1) were for the pesticide at the soil solution ratios 4:8, in 16 ml glass tube fitted with Teflon-lined screw caps. The samples plus blanks (no pesticide) and control (no soil) were thermostated and placed in shaker for 0.5, 1, 3, 6, 9, 12, 24, 48h. The tubes were centrifuged for 20 min. at 3500 rpm. One ml of the clear supernatant was removed and analyzed for the pesticide concentration [15]. Pesticide identification was done by PerkinElmer series 200 USA family high performance liquid chromatography (HPLC) equipped with a changed loop (20µl), C18 reversed phase column, flow rate 1.0  ml min-1, and a variable wave length UV detector at wavelength 220 nm . Separation of picloram in aqueous phase was achieved with a mobile phase of 40% acetonitrile and 60% water (acidified with 0.1% phosphoric acid). Each sample was injected twice to determine the pesticide content by integrating the obtained peak with the respective standard pesticides. The pesticide content was average of two measurements, with no more than 5% deviation between the measurements. The same procesure is repeted in precence of surfactants.

2.4 Desorption Experiments
Desorption processes were done as each test tube was placed in a thermostated shaker at 25şC after equilibra-tion for 48 h with different pesticide concentrations (2, 5, 10 and 15 µg ml-1) the samples were centrifuged, 5ml of supernatant was removed from the adsorption equili-brium solution and immediately replaced by 5ml of water and was this repeated for four times. The resuspended samples were shaken for 0.5, 1, 3, 6, 9, 12, 24, and 48h for the kinetic study. Desorption of picloram was studied in the six selected soil samples, initially treated with different concentrations alone (2, 5, 10 and 15) μgml-1 in presence of surfactant, after equilibrium had been reached for 24h, 5ml were removed from the solution and immediately replaced by 5ml of the surfactant suspension used in the study. The resuspended samples were shaken for 24h, after sufficient time, were centrifuged and the desorbed picloram was measured as reported previously, this desorption procedure was repeated two times for each soil.  The amount of picloram at each desorption stage was calculated as the difference between the initial amount adsorbed and the amount desorbed, all determinations were carried out in duplicate. Competitive picloram adsorption-desorption between soil and surfactant in the soil-picloram-water-surfactant system, in the presence HDTMA, SDS, and Tween-80 at concentrations of cmc/10, cmc, and cmc*20 were conducted adsorption-desorption isotherms [16].

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