A Peptide-Based Hydrogel for Adsorption of Dyes and Pharmaceuticals in Water Remediation

Abstract

The removal of dyes and pharmaceuticals from water has become a major issue in recent years due to the shortage of freshwater resources. The adsorption of these pollutants through nontoxic, easy-to-make, and environmentally friendly adsorbents has become a popular topic. In this work, a tetrapeptide-pyrene conjugate was rationally designed to form hydrogels under controlled acidic conditions. The hydrogels were thoroughly characterized, and their performance in the adsorption of various dyes and pharmaceuticals from water was investigated. The supramolecular hydrogel efficiently adsorbed methylene blue (MB) and diclofenac (DCF) from water. The effect of concentration in the adsorption efficiency was studied, and results indicated that while the adsorption of MB is governed by the availability of adsorption sites, in the case of DCF, concentration is the driving force of the process. In the case of MB, the nature of the dye-hydrogel interactions and the mechanism of the adsorption process were investigated through UV-Vis absorption spectroscopy. The studies proved how this dye is first adsorbed as a monomer, probably through electrostatic interactions; successively, at increasing concentrations as the electrostatic adsorption sites are depleted, dimerization on the hydrogel surface occurs.

Keywords: diclofenac; dye adsorption; hydrogel; methylene blue; peptide; pharmaceuticals adsorption; supramolecular gel; water remediation.

Figure 1

Structure of previously reported peptide 1, and structures of peptide 2, dyes, and PhACs employed in the present work.

Figure 2

(A) Photographs of vials containing from left to right: the peptide at 0.5% in solution, the GdL gel, and the HCl gel; (B) frequency sweep of the HCl gel (blue) and the GdL gel (red) at 0.5% concentration, where G’ values are indicated with filled circles and G’’ is indicated with open circles; (C) strain sweep of the GdL gel at 0.5% concentration; (D) dynamic strain amplitude cyclic test of the GdL gel at 0.5% concentration.

Figure 3

TEM micrographs of GdL xerogels at 0.5% concentration.

Figure 4

(A) UV–Vis absorption (solid line), emission spectra (dotted line), and (B) CD spectra of peptide 2 in solution and the GdL gel.

Figure 5

Adsorption experiment setup: (A) the gel is placed inside a syringe; (B) the solution of MB is allowed to flow through the gel; (C) upon elution, the gel maintains its mechanical stability and does not flow when the syringe is turned upside down.

Figure 6

UV–Vis absorption spectra of the initial MB solutions (dotted line) and MB adsorbed on the hydrogel (solid line) upon elution of 5 mL of a 25 mg/L (black), 50 mg/L (red), and 100 mg/L (blue) solution of MB, and upon elution of 10 mL of a 50 mg/L (green) solution of MB.