Sorção de fósforo por biocarvão de bagaço de cana-de-açúcar sem ativação química e ativado com óxido de magnésio

Abstract

Phosphorus (P) is an essential element for living beings. However, it is a pollutant when overrun in watercourses as it causes eutrophication. The input into water resources arises from several sources, such as the phosphate rock weathering (natural origin) and the discharge of the wastewater (anthropogenic origin). Hence, it is essential to carry out studies aimed at removing this nutrient from effluents. Phosphorus sorption by activated carbon, prepared from mineral coal, is an efficient treatment method; nevertheless, the adsorbent's high cost may limit its use. Thus, the present work evaluated the sorption capacity of phosphorus by adsorbents produced from agricultural waste. The experiment produced two types of biochars from sugar cane bagasse, one without chemical activation (CN) and the other activated with magnesium oxide (CA). Both biochars were physically activated in a muffle furnace for 3 hours at 500ºC. The biochar characterization includes yield, particle density, pH, point of zero charge, adsorption of methylene blue, iodine number, and surface area estimation. In order to obtain the sorption isotherms, batch tests were performed in triplicate, in which 40 mL of water solution enriched with different concentrations of phosphorus were placed in falcon tubes containing 0.5 g of adsorbent each. For the CN, the concentrations of the solutions ranged between 10 and 400 mg.L-1; and for the CA, between 10 and 10,000 mg.L-1. The same mixtures were assembled in closed bottles, shaken for 24 h in a Wagner shaker at 50 rpm to obtain the remaining phosphorus concentration, and the results were fitted using Langmuir and Freundlich adsorption isotherm models. The kinetics assays were performed in triplicate by keeping 0.5 g of adsorbent in contact with 40 mL of phosphorus solution of 300 and 9000 mg L-1 for CN and CA, respectively. The sample mixing process occurred for 60, 120, 180, 300, 420, 540, 720, 900, and 1400 min for both, but for CN, there was still the test at 1200 min; then the P concentrations were determined. The results were fitted using the Elovich, Pseudo-first order and Pseudo-second order rates, and Weber and Morris models. The CA showed better phosphorus sorption capacity than CN, achieving 99.8% removal efficiency at the initial concentration of 400 mg.L-1 of P and maximum sorption capacity of 428.6 mg.g-1. In comparison, the CN reached 26.5% phosphorus removal and a maximum sorption capacity of 5.962 mg.g-1. The CA reached adsorption equilibrium before the CN in the kinetic tests, at 540 and 900 minutes, respectively. The biochar characterization validates adsorption tests, showing better CA results than CN, including higher yield results, particle density, adsorption of methylene blue, and surface area estimation. Therefore, the sugarcane bagasse activated with magnesium oxide is an excellent phosphorus adsorbent and can be used as a low-cost alternative to removing this nutrient.