Melanoidin Removal Mechanism in An Aqueous Adsorption System: An Equilibrium, Kinetic and Thermodynamic Study.

Melanoidins are colored products that can be found in food and drinks, formed by Maillard reactions. Sometimes these compounds are considered undesirable in certain food products, because they impart a brownish color and must be removed. An overview of recent patents related to melanoidin removal indicates that it can be performed by chemical/biological degradation or by adsorption processes. Therefore, in the present study, the adsorption mechanism for synthetic melanoidin removal from aqueous solutions was studied using different Raphanus sativus press-cake sorbents, with the precursor material being carbonized in a microwave oven, either with direct heating or after a chemical activation process with phosphoric acid, nitric acid or potassium hydroxide. Physical and chemical modifications were evaluated by FTIR, pHPZC, thermogravimetry and BET. The adsorption kinetics was better described by a pseudo-second order model for all activated carbons (ACs). Evaluation of the diffusion process showed dependence on the initial melanoidin concentration due to the wide range of sizes of the adsorbed molecules. The equilibrium data were best fitted by the Langmuir model for the acid-treated AC and by the Freundlich model for the base-treated and non-chemically treated ACs. Melanoidin adsorption was characterized as a spontaneous, favorable and endothermic process involving hydrogen bonds and π-π interactions between the adsorbents surfaces and the adsorbed molecules.

Use of Raphanus sativus L. press cake, a solid residue from biodiesel processing, in the production of adsorbents by microwave activation.

The objective of this study was to evaluate the performance of an adsorbent, based on a solid residue arising from biodiesel production, for the removal of cationic dyes from wastewaters. The adsorbent was produced by microwave thermal activation. The Freundlich model provided the best fit for equilibrium data, indicating heterogeneous adsorption. The adsorption capacity increased in comparison to the adsorbent obtained by thermal processing of the same residue in a conventional oven, showing that microwave processing is an attractive alternative for adsorbent production, given the significant reduction in processing time (decreased from 60 min to 3 min). Both the removal efficiency and the removal capacity decreased with an increase in temperature, pointing towards the exothermic nature of the removal process.