Estudos de adsorção de inseticidas com carvões ativados derivados de tabaco

Abstract

Pesticides are essential inputs that drive agricultural production worldwide, but their incorrect use has caused an increase in their concentrations in water resources. As water treatment systems for human supply are generally not designed to remove pesticides, their accumulation in sites intended for human supply can be considered a public health problem. In this scenario, the adsorption process by activated carbons is one of the main advanced treatments applied to water contaminated by such hazardous substances. Such technology becomes even more attractive when considering waste materials for water remediation. Therefore, this master's dissertation aims to show that it can produce activated carbons with excellent physicochemical and morphological characteristics from tobacco residues. The tobacco studied comes from cigarettes seized by the Federal Revenue Service, which was used exclusively in the production of activated charcoal intended for laboratory studies of remediation of water contaminated with acephate and thiamethoxam through insecticides adsorption processes. Activated carbon production studies were optimized under different temperatures (800 and 900 ºC) and mass ratios of chemical agent (potassium hydroxide, or KOH) and tobacco (m/m ratio of 4/1 and 5/1), resulting in the following materials tested: CA800-4-KOH (example: activated carbon at 800 ºC, in the ratio 4/1 m/m KOH/tobacco), CA900-4-KOH and CA900-5- KOH. It should be noted that the definition of the conditions that resulted in the aforementioned activated carbons resulted from an exhaustive study summarized in ANNEX 1. After the activated carbons were developed, their characterization was carried out using chemical desorption in water, point of zero charge (pHPZC), infrared spectra (FTIR), surface area, pore diameter, and volume, in addition to adsorption tests, through studies of kinetics, equilibrium, and thermodynamics of adsorption of the pesticides acephate and thiamethoxam. Pseudo-first, pseudo-second order, Elovich, intraparticle diffusion, Langmuir, Freundlich, Sips, Temkin, Liu, Toth, Redlich- Peterson and Khan empirical models were used to investigate the nature of the adsorption process. Furthermore, the isotherms constructed at different temperatures allowed the estimation of thermodynamic parameters inherent to the pesticide adsorption process. According to the desorption results in water, the activated carbons developed do not release heavy metals or other substances into the liquid medium. In addition, hydroxyl, phenolic, aromatic, carboxylic, and carbonate surface functional groups are observed, which can act in the various adsorption processes of contaminants. The modifying agent (KOH) and the heat treatment resulted in pHPZC changes, with the following values being observed: 6.9 for CA800-4-KOH, 9.41 for CA900-4-KOH, and 9.41 for CA900- 5-KOH. The results indicated that CAs have a high surface area (> 2000 m2 g-1), with a maximum of 3294 m2 g-1 for CA900-4-KOH. The pseudo-first and pseudo-second order kinetic adsorption models fit the experimental data, with values of qe (exp.) and qe (calc.) very close. For both pesticides, the adsorption process was fast, with a high adsorption rate in the initial 5 minutes. The adsorption rate of acephate, estimated by k1 (min-1), assumes the following order CA 800-4-KOH (18.85) > CA 900- 5-KOH (11.35) > CA 900-4-KOH (10.70), with values of k2 (g mg-1 min-1) extrapolated, suggesting physisorption. The adsorption rate of thiamethoxam, estimated by k1 (min-1), followed the order CA900-5-KOH (20.500) > CA900-4-KOH (19.736) > CA800-4- KOH (18.436), with good fits (R2-adj), corroborating with values obtained in thermodynamics, indicating physical adsorption. As for k2 (g mg-1 min-1) CA900-4- KOH (0.679) > CA900-5-KOH (0.677) > CA800-4-KOH (0.644). The adsorption equilibrium studies showed qmax adsorption order (mg g-1) following: CA900-5-KOH (56.372) > CA800-4-KOH (40.355) > CA900-4-KOH (34.389). The KLangmuir constant indicated that all materials showed low acephate-CA interaction, a fact that corroborates bTemkin and Liu, indicating low heat of adsorption and preferred affinity of acephate for certain active sites of CA, which may be attributed to the hydrophilic character of the pesticide (KOW = -0.85), which may explain the low removal rate. The nSips parameter indicates homogeneous and monolayer adsorption by CA800-4-KOH, linked to the Langmuir model, while for CA900-4-KOH and CA900-5-KOH, the model returns to Freundlich, suggesting heterogeneity and multilayer formation. The maximum adsorption capacity of thiamethoxam followed the order: CA900-4-KOH (150.5 mg g- 1) > CA900-5-KOH (117.3 mg g-1) > CA800-4-KOH (99.2 mg g-1), with removal rate approaching 100 %: CA900-5-KOH (99.2 %) > CA900-4-KOH (99.0 %) > CA800-4- KOH (98.3 %). The Freundlich model was more accurate than the other models in describing the adsorption process due to the high R2 and low variance, suggesting a multilayer adsorption process on an energetically heterogeneous CA surface. There is a general tendency for the isotherms studies to reduce the constant values and the adsorbed amount due to temperature rise. For thermodynamics, CA800-4-KOH and CA900-4- KOH showed system order (∆Sº < 0), and CA900-5-KOH showed higher affinity between the material and the acephate solution (∆Sº = 48.8 J mol-1), which corroborated the highest rate of pesticide removal by the material (qmax). The thiamethoxam adsorption process was spontaneous (ΔGº < 0), exothermic with a decrease in adsorption capacity with increasing temperature, in addition to corroborating the kinetic results indicating physisorption (ΔHº < 0). The positive values of ΔSº are related to the sorption of thiamethoxam, compressing and disturbing the surface of the CAs, being the force responsible for the process. In this way, it can be said that the uses of different activations for the activated carbons produced from tobacco were efficient in the removal of pesticides from water, mainly thiamethoxam, reaching almost 100 % of removal, evidencing an excellent alternative in the remediation of contaminated water, in addition to enabling the environmentally appropriate use of tobacco from seized cigarettes.