A tannin-polymer adsorbent created from the freezing-thawing method for removal of metal-complex acid black 172 and methylene blue from aqueous solutions

This study presents an innovative adsorbent material obtained from the dialysis of an aqueous tannin derivative solution, followed by one freezing-thawing cycle at -4 degrees C. The freezing precipitated the tannin (459 +/- 97 kDa) in water, yielding the solid adsorbent (d-TN) with aqueous insolubility and stability. The tannin derivative is mainly composed of amino-functionalized polyphenolic chains, but it comprises hydrolyzable tannins as well. The d-TN adsorbent removed the metal-complex acid black 172 (AcB) and methylene blue (MB) from water. Kinetic studies suggested that the nonlinear pseudo-first-order and pseudo-second-order models adjusted well to the experimental AcB and MB kinetic curves, respectively. Equilibrium studies indicated that the Sips model the experimental isotherms well, supporting maximum removal capacities (q(m)) of 119.1 mg/g for AcB and 310.2 mg/g for MB. The free Gibbs energy (Delta G(0)) was -3.33 kJ/mol for AcB adsorption and -4.16 kJ/mol for MB adsorption. The dye adsorption was spontaneous and endothermic, being governed by physisorption. Hydrophobic interactions are primarily responsible for dye removal. Therefore, dye desorption quickly occurs in ethanol. High-resolution XPS spectra characterized the d-TN surface after dye adsorption, confirming the adsorption of AcB composed of chromium ions (Cr3+). The eco-friendly d-TN can replace conventional adsorbents used to remove dyes from water. (C) 2022 Elsevier B.V. All rights reserved.