Revalorization of agro-industrial effluents based on gallic acid recovery through a novel anionic resin

Gallic acid is a natural polyphenol with important biological implications. Nevertheless, gallic acid present in agro-industrial wastewaters is considered like a micropollutant, due to its toxicity above certain levels. Therefore, recovery of gallic acid from these effluents is interesting both from industrial and environmental points of view. Industrially, it is attractive the recovery of polyphenols and the obtainment of added value products. On the other hand, from the environmental point of view it is always welcome the decrease of the contaminant charge of an effluent, which could be, for instance, further reused for irrigation. In this work, the adsorption of gallic acid on a novel anionic resin has been investigated. In first place, thermodynamic studies have been carried out. The adjustment of experimental data to Langmuir, Freundlich and Temkin isotherms has been checked. Results evidence that the Langmuir model offers the best fitting (coefficient of determination, R2 = 0.9961). Secondly, in an attempt of understanding the kinetic behaviour of the system, the fitting of experimental data to three different kinetic models has been carried out. Namely, pseudo-first order, pseudo-second order and Weber-Morris intraparticle diffusion models have been considered. The highest coefficients of determination (R2) are obtained when a pseudo-second order model is assumed (R2 in the range 0.9840-0.9997). Lately, the evaluation of Gibbs free energy revealed that gallic acid uptake is a spontaneous process for the considered Dowex 21K XLT resin (ΔG° = −11.30 kJ mol−1). Desorption studies have been also conducted and it has been found that after 120 min recovery efficiencies of gallic acid close to 100% are obtained. It has been proven that the resin performance is not altered at least for ten complete adsorption/desorption cycles.