Characterization of sorption processes for the development of low-cost pesticide decontamination techniques.

The adsorption/desorption behavior of four pesticides (atrazine, alachlor, endosulfan sulfate and trifluralin) in aqueous solutions onto four adsorbents (sunflower seed shells, rice husk, composted sewage sludge and soil) was investigated. Pesticide determination was carried out using stir bar sorptive extraction and gas chromatography coupled with mass spectroscopy. Maximum removal efficiency (73.9%) was reached using 1 g of rice husk and 50 mL of pesticide solution (200 µg L(-1)). The pseudo adsorption equilibrium was reached with 0.6 g organic residue, which was used in subsequent experiments. The pseudo-first-order, pseudo-second-order kinetics and the intra-particle diffusion models were used to describe the kinetic data and rate constants were evaluated. The first model was more suitable for the sorption of atrazine and alachlor while the pseudo-second-order best described endosulfan sulfate and trifluralin adsorption, which showed the fastest sorption rates. 4h was considered as the equilibrium time for determining adsorption isotherms. Experimental data were modeled by Langmuir and Freundlich models. In most of the studied cases both models can describe the adsorption process, although the Freundlich model was applicable in all cases. The sorption capacity increased with the hydrophobic character of the pesticides and decreased with their water solubility. Rice husk was revealed as the best adsorbent for three of the four studied pesticides (atrazine, alachlor and endosulfan sulfate), while better results were obtained with composted sewage sludge and sunflower seed shell for the removal of trifluralin. Although desorption percentages were not high (with the exception of alachlor, which reached a desorption rate of 57%), the Kfd values were lower than the Kf values for adsorption and all H values were below 100, indicating that the adsorption was weak.

Effectiveness of acid-treated agricultural stones used in biopurification systems to avoid pesticide contamination of water resources caused by direct losses: part I. Equilibrium experiments and kinetics.

Several recent publications report the use of low-cost and locally available adsorbents in biopurification systems to eliminate pesticides from water. Nonetheless, the literature is insufficient to cover this problem, and more investigations are needed in this field to evaluate the possible application of economical adsorbents to prevent pesticide water resources contamination caused by direct losses. This study focuses on the fate of endosulfan metabolites, in small-scale biopurification systems, using as adsorbents treated and untreated organic agricultural stones (date, olive and avocado). The effects of sorbent particle size, adsorbent dose, contact time, concentration of pesticide solution and temperature on the adsorption processes were systematically studied in batch experiments. Pesticide determination was carried out using stir bar sorptive extraction and gas chromatography coupled with mass spectroscopy. Maximum removal efficiency (94.8%) was reached for endosulfan sulfate (0.1 mg L(-1)) using the acid-treated date stones (ATOS) fraction <125 microm (solid/liquid ratio: 1 g L(-1)). According to the general behavior of the adsorption mechanism, date stones show the higher removal efficiency followed by olive and avocado stones, respectively. The analyses of the results reveal that the thermo chemical treatment improves notably the pesticides adsorption efficiency of the studied sorbents and that the adsorption decreases when the pesticide solubility rises. Experimental adsorption data were analyzed according to various kinetic models. Lagergren and Morris-Weber equations were applied to fit the kinetic results. The second order model was the most suitable and intra-particle diffusion was found to be the rate controlling the adsorption process.