Adsorção e dessorção do corante preto reativo 5 em solução aquosa utilizando adsorvente alternativo de lodo de esgoto sanitário (biocarvão)

Abstract

Sewage sludge is considered an environmental liability due to the amount generated and the problem in the destination. An alternative destination for this waste would be its transformation into biochar to be used to remove other pollutants, such as synthetic dyes. Most of these dyes are difficult to degrade and when present in effluents that are discarded in water bodies, they can cause damage to aquatic fauna and flora. In this sense, the objective of this research was to investigate the adsorptive and desorption capacity of previously pyrolyzed (LP) and functionalized (LF) sewage sludge in the removal of Reactive Black 5 dye. The adsorbents were characterized using physical, chemical, and textural analyzes. Subsequently, kinetic tests were performed according to statistical planning using the factorial central rotational composite design (DCCR) 23, varying the pH of the solution, test temperature, and mass of adsorbent. The adsorption equilibrium tests employing isotherms and determination of thermodynamic properties were performed based on the optimal condition verified by the desirability in statistical planning. In the desorption, regenerating solutions were tested, the best being used in the desorption, as well as in the adsorption/desorption cycles. For continuous flow adsorption, DCCR 23 was performed, varying the input flow, RB 5 concentration and adsorbent mass. In the optimum condition obtained in the planning, textile effluent and textile effluent fortified with RB 5 were treated. The same was applied for the optimal condition obtained in batch adsorption kinetics. With the realization of the physical, chemical, and textural characterizations it was found that the adsorbents showed characteristics of mesoporosity and that the pores present in the material are larger than the dye molecule, which shows the adsorption capacity of the proposed alternative adsorbents. The adsorption efficiencies obtained for the adsorbents under the optimal conditions obtained by the desirability were 72% for LP and 99% for LF. In adsorption, the best fit of the experimental data was obtained for the pseudosecond order kinetic model (R2> 0.98) for both adsorbents. As for the investigation of adsorptive capacity, the Sigmoid and Wu models were adjusted for LP and LF, respectively. The adsorption process occurred exothermic (-327.86 kJ mol-1) for LP and endothermic (23.03 kJ mol-1) for LF, and for both adsorption was spontaneous and thermodynamically favorable. In the desorption process, the best fit was obtained for the pseudo-second order model (0.326 and 1.233 mg g-1 for LP and LF, respectively) and the Langmuir model was the one that best fit the data in the isotherm, both for LP (0.348 mg g-1 and R2 0.75), and for LF (0.660 mg g-1 and R2 0.63). Desorption occurred exothermically (-278.26 kJ mol-1 for LP and – 104.21 kJ mol-1 for LF), with spontaneous and thermodynamically favorable affinity between adsorb and adsorbent. Also, an adsorptive mechanism by hydrogen bonds for LP and electrostatic interaction for LF was proposed. The desorption mechanism was prevented mainly by the present chemosorption. Finally, with the completion of this research, it can be concluded that sewage sludge can be used as an alternative adsorbent, as the analyzes performed indicate its good performance in the process.