Cationic Polystyrene-Based Hydrogels as Efficient Adsorbents to Remove Methyl Orange and Fluorescein Dye Pollutants from Industrial Wastewater

Abstract

Water pollution from dyes is harmful to the environment, plants, animals, and humans and is one of the most widespread problems afflicting people throughout the world. Adsorption is a widely used method to remove contaminants derived from the textile industry, food colorants, printing, and cosmetic manufacturing from water. Here, aiming to develop new low-cost and up-scalable adsorbent materials for anionic dye remediation and water decontamination by electrostatic interactions, two cationic resins (R1 and R2) were prepared. In particular, they were obtained by copolymerizing 4-ammonium methyl and ethyl styrene monomers (M1 and M2) with dimethylacrylamide (DMAA), using N-(2-acryloylamino-ethyl)-acrylamide (AAEA) as cross-linker. Once characterized by several analytical techniques, upon their dispersion in an excess of water, R1 and R2 provided the R1- and R2-based hydrogels (namely R1HG and R2HG) with equilibrium degrees of swelling (EDS) of 900% and 1000% and equilibrium water contents (EWC) of 90 and 91%, respectively. By applying Cross’ rheology equation to the data of R1HG and R2HG’s viscosity vs. shear rate, it was established that both hydrogels are shear thinning fluids with pseudoplastic/Bingham plastic behavior depending on share rate. The equivalents of -NH3+ groups, essential for the electrostatic-based absorbent activity, were estimated by the method of Gaur and Gupta on R1 and R2 and by potentiometric titrations on R1HG and R2HG. In absorption experiments in bulk, R1HG and R2HG showed high removal efficiency (97–100%) towards methyl orange (MO) azo dye, fluorescein (F), and their mixture (MOF). Using F or MO solutions (pH = 7.5, room temperature), the maximum absorption was 47.8 mg/g in 90′ (F) and 47.7 mg/g in 120′ (MO) for R1, while that of R2 was 49.0 mg/g in 20′ (F) and 48.5 mg/g in 30′ (MO). Additionally, R1HG and R2HG-based columns, mimicking decontamination systems by filtration, were capable of removing MO, F, and MOF from water with a 100% removal efficiency, in different conditions of use. R1HG and R2HG represent low-cost and up-scalable column packing materials that are promising for application in industrial wastewater treatment.

Cationic Polystyrene-Based Hydrogels: Low-Cost and Regenerable Adsorbents to Electrostatically Remove Nitrites from Water

“…In this regard, the adsorbent efficiency of water-insoluble polymers and copolymers (resins) in the form ammonium salts in adsorbing anionic compounds has long been studied [26,27]. These cationic macromolecules, if properly structured, may also supply hydrogels when dispersed in water.…”

“…These cationic macromolecules, if properly structured, may also supply hydrogels when dispersed in water. As recently reported, hydrogels are characterized by well-defined, three-dimensional (3D) porous structures, and the presence of hydrophilic functional groups, such as protonated amine groups, confers to them a high level of hydrophilicity [27]. As adsorbents, cationic hydrogels represent an important tool to remove anionic pollutants from water because of their flexible network, which allows solutes to quickly penetrate water and form stable complexes with them [27].…”

“…Additionally, a recent paper reported the immobilization of hybrid gold-cesium nanoparticles (Au-Cs NPs) onto a magnesium ferrite (MgFe 2 O 4 ) surface and detailed the behavior of the prepared composites in N 2 adsorption-desorption experiments [32]. Recently, we reported on the synthesis and characterization of polystyrenebased cationic resins (R1 and R2) [27,33], obtained in the form of hydrochloride salts by reverse suspension copolymerization (see Scheme S1, Section S1 in the Supplementary Materials (SM)) of monomers M1 and M2 (see Figure S1, Section S1 in Supplementary Materials) [34]. Upon dispersion into an excess of water, R1 and R2 yielded cationic hydrogels (R1HG and R2HG) without the use of any other additive or gelling agent.…”

“…Upon dispersion into an excess of water, R1 and R2 yielded cationic hydrogels (R1HG and R2HG) without the use of any other additive or gelling agent. These were believed to possess adsorbent capacities to act against aniconic pollutants due to electrostatic interactions [27].…”

“…When tested in adsorption experiments in batches over time monitored with UV-Vis spectrometry, R1 and R2 showed rapid and high adsorption efficiency towards the methyl orange (MO) azo dye and fluorescein sodium salt (F) selected as models of anionic dyes based on electrostatic interactions (97-100%) [27]. Moreover, R1HG-and R2HGbased short columns, mimicking possible decontamination systems, were capable of totally removing (100%) MO, F, and a mixture of the two by filtration under pressure in different conditions [27].…”