Adsorção de cromo no pseudofruto do cajueiro: Desenvolvimento de metodologia de tratamento de resíduos laboratoriais e otimização por planejamento fatorial

Abstract

The Higher Education Institutions are responsible for the production of scientific and technological knowledge, and opinion formation. However, they also generate a variety of wastes. The use of agro-industrial wastes in the removal of metals from wastewater is a promising alternative to existing technologies. In this work, the adsorptive properties of cashew pseudo-fruit (Anacardium occidentale L.) were studied and a method was developed for its use in the treatment of Mohr residue generated in teaching laboratories of the Department of Analytical Chemistry and Physical Chemistry of Universidade Federal do Ceará. The batch adsorption system was studied. The preliminary tests indicated the viability of cashew fiber on chromium adsorption. Four solutions were tested for desorption, and the 0.05 mol/L EDTA was the best desorbent through qualitative tests. The effects of varying pH (5.0 and 7.0), drying – S (drying oven and lyophilized), particle size – P (0.10 – 0.25 and 0.25 – 0.84 mm), adsorbent mass – m (1.0 and 1.5 g), chromium concentration – C (500 and 1000 mg/L), time – t (1 and 3 hours) and stirring speed – v (0 and 150 rpm), in the adsorption process were studied through a 2^(7-4) fractional factorial design. A second factorial design (2^(4-1)) was performed for optimization. In this design the variables were m (1.0, 2.0 and 3.0 g), C (500, 750 and 1000 mg L-1), t (1, 2 and 3 h) and v (0, 150 and 300 rpm), and the fixed parameters were pH (7.0), S (drying oven), and P (0.25 - 0.84 mm). The regions of maximum adsorption were mapped by contour plots. Adsorption achieved a maximum at intermediate m values, lower C values and higher v and t values. The adsorbent was characterized by infrared spectroscopy and X-ray fluorescence. Reaction of chromium with aqueous extract of adsorbent and pH monitoring during adsorption were performed to better understand the mechanism of adsorption. The proposed mechanism consists of two steps: reduction of Cr(VI) in solution or in contact with the surface, and subsequent adsorption of Cr(III) through ion-exchange or complexation. Isotherm and kinetic parameters were estimated from the data obtained with the 2^(4-1) factorial design. The data followed Langmuir model and pseudo-second order kinetics. The Mohr residue treatment achieved 89.18% of total chromium adsorption. The remainder of chromium in solution (10.82%) were present in Cr(III) less toxic form. 33.40% of total chromium were recovered with 0.05 mol/L EDTA in the desorption step. Proposals for reuse of the residue in teaching activities were made.