Utilização do carvão ativado de ossos bovinos como agente adsorvente para remoção do herbicida 2,4-D (ácido diclorofenoxiacético)

Abstract

The advancement of agriculture and the need for high food productivity has brought constant concern about the wide use of herbicides. These compounds have a relatively affordable cost and are very effective in controlling weed crops. Among these herbicides we can highlight dichlorophenoxyacetic acid (2,4-D), the second most used among herbicides in Brazil and Alagoas, which has among its applications sugar cane, a strong crop of agriculture in Alagoas. In this work, a feasibility study was carried out on the use of activated carbon from bovine bones as an adsorbent for the removal of this herbicide. Adsorption is a very versatile method and has a relatively low cost, while activated carbon is a material widely used in this technique because it mainly has a high surface area and a well-developed pore structure. The adsorption study was carried out using two particle sizes, 0.1 mm and 0.297 mm. The adsorption process was favored using a pH value equal to 2, obtaining an efficiency approximately 25% higher compared to the second best condition with pH 3. The kinetic test showed a similar removal efficiency for both particle sizes. Activated carbon with a diameter of 0.1 mm reached equilibrium in 30 minutes, while activated carbon with a diameter of 0.297 mm took about 120 minutes. Regarding the kinetic models, pseudo-first order and pseudo-second order models were used and both obtained an excellent fit with high R² values and low mean relative error values. The isothermal tests indicated that the increase in temperature impairs the efficiency of 2,4-D removal, with 30° C being the best working temperature among the values used. The Langmuir model showed the best fit to the experimental data. At 30° C and initial concentration of 30 mg/L of 2,4-D, the maximum adsorption capacity of 12.38 mg/g for 0.1 mm charcoal and 11.95 mg/g for charcoal was obtained of 0.297 mm, achieving a removal greater than 66% and 85% for the two particle diameters, respectively. The calculation of thermodynamic parameters confirmed that this adsorption process is exothermic and that the adsorption mechanism consists of physisorption together with chemisorption, in agreement with the result of kinetic experiments. The simulation of the process in a fixed bed using the Aspen Adsorption V11 computational package showed that activated carbon from bovine bones can be used to achieve a 2,4-D removal treatment within current legislation standards, with activated carbon of 0.297 mm superiority in the continuous process compared to 0.1 mm activated carbon.