**1. Introduction**

Pollutants released by industrial liquid waste affect the quality of water in water bodies. They can engender serious health effects in plants, animals, and humans. Wastewater pollutants include dyes, surfactants, oils, lubricants, organic solvents, petroleum derivatives, and pharmaceuticals such as antibiotics, anti-allergy, and hormones [1,2]. Artificial dyes, which are largely used in various industries such as textiles, food, cosmetics, leather, paper, and pharmaceuticals, are highly dangerous organic pollutants [3,4]. Dyes are mutagenic agents even at low concentrations and render an undesirable color to water bodies [5]. The presence of dyes in wastewater that drains into water bodies makes it difficult for light to penetrate natural water bodies and negatively impacts photosynthetic activity [6]. A representative artificial dye is methyl orange (MO, dimethylaminoazobenzenesulfonate), which is non-biodegradable in nature; besides, it is a water-soluble carcinogen, azo dye that is widely used in textile industries, printing paper manufacturing, textile laboratories, chemical research, pharmaceuticals and research laboratories [7]. It pollutes water at low concentrations; large volumes of MO are produced as waste.

The MO molecule has a bright orange color when dissolved in water, stable chemical structure due to the presence of azo (–N=N–) and aromatic groups (which are highly toxic, carcinogenic and teratogenic), and is harmful to the environment and organisms since it shows low biodegradability [8]. MO can lead to critical health issues, like cyanosis, vomiting, tachycardia, tissue necrosis, and jaundice, and has been declared carcinogenic and

**Citation:** Roa, K.; Tapiero, Y.; Thotiyl, M.O.; Sánchez, J. Hydrogels Based on Poly([2-(acryloxy)ethyl] Trimethylammonium Chloride) and Nanocellulose Applied to Remove Methyl Orange Dye from Water. *Polymers* **2021**, *13*, 2265. https:// doi.org/10.3390/polym13142265

Academic Editors: Andrea Lazzeri, Maria Beatrice Coltelli and Patrizia Cinelli

Received: 2 June 2021 Accepted: 7 July 2021 Published: 10 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

tumorigenic by the International Agency for Research on Cancer (IARC) and the National Institute for Occupational Safety and Health [9]. Additionally, it is an allergenic substance that can cause eczema upon contact with the skin. Its presence in living organisms is considered harmful and can lead to a significant increase in the activity of the azo-nitro-reductase enzymes, producing aromatic amines that may cause intestinal cancer [10,11].

It is difficult to eliminate and control the dye concentration efficiently through traditional treatment methods such as coagulation, sedimentation, chemical oxidation, and biological digestion [12]. In general, conventional water treatment methods generate large volumes of residual sludge, use excessive process times, and consume large amounts of energy [13]. Adsorption technology is considered one of the most effective methods because of its simplicity, low energy consumption, short treatment times, low generation of sludge, high efficiency, flexibility, and insensitivity to toxic substances.

Various materials have been used to adsorb MO, for example, activated carbon from natural sources [14], algae [15], hybrid materials with metal oxides [16], chitosan and hydrogen peroxide–treated anthracite sheets [17], and hypercrosslinked cyclodextrin networks in the form of nanofibrous membranes [18]. For example, Borsagli et al. designed and developed novel three-dimensional porous scaffolds made of *N*-acyl thiolated chitosan using 11-mercaptoundecanoic acid, with high adsorption capacities for the anionic MO dye in an aqueous medium [19]. Liu et al. prepared hydrogel particles of methacrylateethyltrimethylammonium chloride and acrylamide copolymer, with the ability to eliminate anionic dyes such as amaranth red, orange G, and MO reaching 94% efficiency [20]. Onder et al. prepared copolymer hydrogels of poly([2-(acryloyloxy)ethyl] trimethylammonium chloride-co-1 vinyl-2-pyrrolidinone) (p(AETAC-co-NVP)) which showed the ability to retain the MO and alizarin red S dyes through electrostatic interactions when the test pH values were 7.0 and 5.0, respectively [21], and Dalalibera et al. prepared hydrogels based on polyacrylic acid with the ability to absorb and selectively separate cationic and anionic dyes at a pH of 8.0 to 10.0 [22].

Recently, nanocomposites based on cationic polymers and nanocellulose have been prepared for application in water treatment. These materials have shown remarkable capabilities to remove oxyanions such as chromates and have improved mechanical properties [23], for example, Szekely et al. prepared nanocomposite hydrogels based on cellulose acetate, modified with the addition of small amounts of polymers of intrinsic microporosity and graphene oxide (GO), which demonstrated the ability to absorb neonicotinoid insecticidal pollutants in an aqueous medium [24]. Khan et al. prepared nanocomposite hydrogels with a porous 3D network structure based on cellulose-aluminum oxide nanoparticles-graphene oxide (GO) (Al2O3/GO), with application in the removal of fluoride ions from drinking water [25]. Hameed et al. prepared carboxymethyl cellulose/potato starch/amylum starch hydrogels where aluminum sulfate octahydrate was used as a crosslinking agent, which reached high capacity in the retention of heavy metals (cadmium, lead, and iron) from municipal drinking water [26].

In addition, nanocellulose has been used for the synthesis of hydrogels with applications in biomedicine, as in the case of the research work by Chen et al. They prepared fluorescent compound hydrogels of nitrogen-doped carbon points/cellulose nanofibrils (NCD/CNF-gel), where the mechanical properties were highlighted [27].

Several synthetic resins have been studied (such as Amberlite [28]), and the challenge now is to manufacture biomass-derived materials for removal of anionic dyes such as MO. There are not many papers on biomass-derived hydrogels that remove this dye, and that is why in this study hydrogels with ammonium groups reinforced with CNF were developed.

The aim of this study was to synthesize poly([2-(acryloyloxy)ethyl] trimethylammonium chloride) and poly(ClAETA) hydrogels containing fibrillated nanocellulose (CNF). The amount of crosslinkers, CNF, and initiators was optimized by studying the effects on the percentage yield of synthesis, degree of crosslinking, and water adsorption. Developing applicable hydrogels for adsorption and treatment of aqueous MO-containing wastes.
