Preliminary study on the characterization of in natura malt residue from a brewery for potential use as an adsorbent in removing Reactive red 120 from aqueous solution

Abstract

Malt residue is the main by-product of the brewing process and representing around 85% of the by-products generated in this segment. Due to its abundance and considering the high development of the beer market in recent years in Brazil and worldwide, there is also great concern regarding environmental issues to its final disposal, making it necessary to develop alternatives that generate products of greater added value and that can reduce environmental impacts. Given the growing demand for alternative adsorbents, especially for agro-industrial residues, this study aimed to carry out a preliminary characterization of malt residue to assess its potential as in natura adsorbent, to add value to the residue as well and to contribute to the mitigation of impacts on the environment. The malt residue was dried, ground, and classified, and this material was given the name of fresh malt residue (RMIN). For the characterization of RMIN, pH stability test analysis, zero charge point (pHPCZ), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) were performed. After characterization, preliminary adsorption tests were carried out to remove a textile dye, in this case, Reactive red 120. As a result of the characterization analyses, the malt has a relatively stable pH when in aqueous suspension and the value pHPCZ found, was close to neutral, thus demonstrating the potential for removal of acidic substances. The result of the thermogravimetric analysis shows mass losses in temperature ranges characteristic of the presence of lignocellulosic compounds that make up the malt, which is corroborated by the results of the FTIR analysis, since the spectra revealed the presence of functional groups, such as group hydroxyl, carbonyl group, alkanes, and alkenes, related to the presence of cellulose, hemicellulose, and lignin. These functional groups can interact with the groups of the Reactive red dye 120 molecule, helping to remove them from an aqueous solution. Through SEM analysis it was possible to observe morphological changes when comparing the adsorbent particles before and after the adsorption process. The potential as an in natura adsorbent was evaluated through preliminary tests of adsorption, followed by filtration, whose system showed 30% removal. However, studies to determine the optimal adsorption conditions, kinetic and isotherm studies, and thermodynamic studies are still necessary to better understand the phenomenon. Studies to determine the useful life of the RMIN, as well as studies in pilot units or continuous systems, are also still needed to enable its use on a larger scale.