Insight into zinc(II) biosorption on alginate extraction residue: Kinetics, isotherm and thermodynamics

Abstract

In this study, the by-product generated in the alginate extraction process (termed as RES) was recuperated and employed as biosorbent, without modifications, for Zn bioremoval from synthetic effluent. Different zinc concentrations (1, 1.5, and 2 mmol/L) and temperatures (293.15, 303.15, 313.15, and 323.15 K) were evaluated through kinetic and equilibrium studies. The involvement of ion exchange mechanism was observed during the kinetic assays with predominance of external diffusion. Within the evaluated zinc concentration range, there was no strong influence of concentration increase on experimental sorption capacity and the best results were achieved at 1.5 mmol/L. The temperature rise favored the process, as well as the affinity between RES and zinc. Langmuir best fitted the equilibrium experimental data and the highest sorption capacity was found at 323.15 K as 0.781 mmol/g. The thermodynamic parameters revealed that Zn bioremoval using RES occurred by physisorption, besides being endothermic and spontaneous. The selectivity study revealed that the alkali and alkaline earths metals ions naturally found in seaweeds (Ca, Na, Mg and K) may compete with Zn ions for the same binding sites, being that Ca has the greater potential for affecting the process. Mercury porosimetry and XRD analyzes revealed a macroporous and semi-crystalline structure. Similar contents of C, N, H and S were found before and after zinc removal by CNHS analysis, while thermal analysis indicated that RES remains stable up to 150 °C. The results indicated that RES has a great potential for being used as biosorbent, providing information that contribute to its valorization.