Grafting of regenerated cellulose films with fibrous polymer and modified into phosphate and sulfate groups: Application for removal of a model azo-dye

Highlights

• Polymer grafted RCM containing phosphate and sulfate groups were prepared and used for removal of a benzidine based dye.

• RCF-g-p(GMA)-SO₃H, showed extraordinary adsorption capacity to Chlorazole Black E dye.

• High adsorption could resulted from the presence of large quantity of sulfate and hydroxyl groups on adsorbent.

• The maximum equilibrium adsorption capacity of RCF-g-p(GMA)-SO₃H for Chlorazole Black E dye was 268.6 mg/g.

Abstract

Regenerated cellulose films (RCF) were functionalized with bromoacetyl bromide as atom transfer radical polymerization (ATRP) initiator. Then, poly(glycidyl methacrylate), p(GMA), as a functional polymer was grafted via surface initiated ATRP (SI-ATRP) on the RCF. The polymer grafted RCF was modified into phosphate [i.e.,(RCF-g-p(GMA)-PO₃H₂)] and sulfate [i.e.,(RCF-g-p(GMA)-SO₃H)] groups using H₃PO₄ and H₂SO₄, respectively. The synthesized composite adsorbents were characterized using FTIR, SEM and analytical methods to evaluate the structural and morphological characteristics of the modified cellulose based films. The effectiveness of both cation exchange films [(RCF-g-p(GMA)-PO₃H₂) and (RCF-g-p(GMA)-SO₃H)] for the removal and recovery of Chlorazole Black E (CBE) dye was investigated from aqueous solutions. The adsorbent dosage, pH, ionic strength, initial concentration of dye, and contact time were studied to optimize the conditions for maximum adsorption. The adsorption process was pH dependent, shows maximum removal at pH 5.5 and 2.0 for RCF-g-p(GMA)-PO₃H₂ and RCF-g-p(GMA)-SO₃H, respectively. The maximum adsorption capacities were found to be 165.7 and 268.6 mg/g for the RCF-g-p(GMA)-PO₃H₂ and RCF-g-p(GMA)-SO₃H, respectively. Kinetic studies showed that pseudo-second-order kinetic model described well the kinetics of adsorption of the CBE dye. Adsorption equilibrium data were correlated with the Langmuir, and Freundlich isotherm models. Adsorption/desorption for 10 cycles showed the possibility of repeated use of modified RCF for removal and recovery of the dye from aqueous solutions. These presented both cationic adsorbents had several operational advantages for the dye removal from aqueous medium indicating that they are talented materials to be used in wastewater treatment. Finally, the RCF-g-p(GMA)-SO₃H adsorbent with strong acidic groups can be utilized for the removal of basic dyes from industrial wastewaters in sight of its favorable high adsorption capacity.

Introduction

Benzidine based azo-dyes are used mostly to color textiles, leather and paper products and also in the petroleum based rubber plastics. These dyes have become a major environmental concern and retained color and structural stabilities under exposure to sunlight. They also exhibited a high resistance to microbial degradation in wastewater treatment systems. Benzidine based azo dyes have been found to be tumerogenic and carcinogenic due to their biotransformation to benzidine [1,2]. Therefore, these benzidine based azo dyes should be reduced to the limited levels within the effluents using available methods [3,4]. The removal of these dyes from wastewaters have to be realized due to their possible toxicity on human life and ecosystems as well as aesthetic reasons. For the removal of these pollutants many methods have been suggested such as adsorption, biological treatment, photocatalytic degradation, electrochemical oxidation, and membrane separation [[5], [6], [7], [8], [9]]. Among them, adsorption is a desirable method for removal of these dyes by using various adsorbents due to its simplicity, low cost, and effectiveness. The interaction between the pollutant dyes and the adsorbent are primarily by ion-exchange, hydrogen bonding, hydrophobic and/or the combining forces of van der Waals interactions [[10], [11], [12], [13], [14], [15]]. Many kinds of adsorbents including ion-exchange resins [16] composite adsorbents, silica particles [[17], [18], [19]], synthetic and natural polymers [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]] have been reported. Among them, composite materials can be talented materials. Especially, natural polymer such as synthetic polymer grafted cellulose and chitosan can be used as cheap and abundant materials with their high adsorption capacities [30,31]. These can be also improved by grafting with functional fibrous polymer such as p(GMA) [4,22]. The oxiran groups of the grafted p(GMA) polymer can be modified into different functional groups via single step reaction mechanism [32]. Many composite adsorbents have been prepared and used for removal of synthetic organic pollutants from textile effluents. In this regard, different composite materials were prepared as adsorbents such as tetramethoxysilane and [3-(2-aminoethylamino) propyl] trimethoxysilane were combined with polymeric particles as a result polymer-amino-functionalized silica systems were formed and the adsorption capacity of composite adsorbent to an azo dye was more than 100 mg/g [33]. Low-grade palygorskite was functionalized with inorganic and organic groups and used for removal of Congo Red dye and the adsorption capacity of the adsorbent for Congo Red was [34]. Kaolin/calcium alginate (Kaolin/CaAlg) composite hydrogel membranes with different thicknesses were prepared using Ca²⁺ cross-linking, and the adsorption kinetics and isotherms for Coomassie Brilliant Blue G250, Crystal Violet, and Neutral Red dyes on the composite membrane were studied [17]. Agricultural wastes such as rice husk were also combined with carboxymethyl cellulose/alginate/polyvinyl alcohol for preparation of composite adsorbents. Adsorption/desorption, kinetics Direct Orange-26, Direct Red-31, Direct Blue-67 and Ever direct Orange-3GL dyes were studied from aqueous solutions [20]. Silica based adsorbent was prepared via p-tert-butylcalix [6] arene modification, and the adsorption of the Direct Black-38 azo dye on the resin was studied [6]. Poly(N,N-dimethylacrylamide) and vinylated chitosan as a macro-cross-linker were used for preparation of composite hydrogels and used for removal of Acid Red dye from aqueous medium [35]. A highly hydrophilic polyvinyl alcohol/SBA-15 composites were synthesized by impregnation method and removal of Methylene Blue dye was studied from wastewaters [36]. Graphene oxide, N-isopropyl acrylamide (as a temperature sensitive unit) and acrylic acid (as a pH-sensitive unit) composite adsorbent was prepared by in situ radical copolymerization and used for removal of Rhodamine B dye [37]. Additionally, as a biodegradable material, regenerated cellulose can be also used for preparation of composite materials. For example, fibrous polymer with functional groups can be grafted on to cellulose films via atom transfer radical polymerization. In our earlier studies, the p(GMA) with reactive epoxy groups have been grafted via SI-ATRP method on the various polymer materials, and converted into versatile functional groups such as strong or weak ion-exchanges groups for adsorption of organic and inorganic pollutants [4,16,21]. The advantages of using fibrous polymer grafted composite material in water treatment are as follows; (i) they can be operative in absorbing a large amount of water containing organic pollutants under certain conditions; (ii) reusability of the composite adsorbents with repeated adsorption–desorption cycles; (iii) composite materials do not release any toxic residues or products; (iv) they can be used as degradable adsorbent materials [16,21].

In this work, the regenerated cellulose films were grafted with p(GMA) via ATRP method and the epoxy groups were converted into phosphate (RCF-g-p(GMA)-PO₃H₂) and sulfate (RCF-g-p(GMA)-SO₃H) groups with the reaction of H₃PO₄, and H₂SO₄, respectively. They were used for removal of a model benzidine based azo dye (i.e., Chlorazole Black E). The effect of various adsorption parameter was studied to attain optimal environments for the adsorption of the dye showing to be a suitable and effective alternative to eliminate it from wastewater. Finally, the FT-IR studies showed that the probable interaction with the dye could comprise of surface complexation, ion exchange and hydrogen bonding. The results of isotherm, kinetic and thermodynamic studies suggested the functional phosphate and sulfate groups on the modified RCF played important roles for the adsorption of the dye. Also, different adsorption kinetics and isotherm models for the dye were studied. Besides, both acidic groups could be applied for removal of various pollutants through ion-exchange and hydrogen binding and p-p interactions.