**1. Introduction**

Methylene blue is a cationic dye with high water solubility that has many diverse applications which includes the dyeing of paper, cotton, wool, and hair [1]. The occurrence of synthetic dyes in industrial effluent has led to global environmental concern due to the inadvertent release into aquatic environments and the impacts of such contaminants on ecosystems and human health. Exposure to lethal doses of cationic dyes such as methylene blue may lead to vomiting, cyanosis, jaundice, shock, and tissue necrosis in humans [2]. To address the removal of synthetic dyes from industrial wastewater, conventional methods such as electrochemical, coagulation, flocculation, chemical oxidation, solvent extraction, and adsorption have been reported [3–5]. Among these methods, adsorption is a popular choice for contaminant removal due to its simplicity of operation, cost-effectiveness, and availability of commercial adsorbents such as zeolites and activated carbon. Whereas the efficiency of adsorption processes is often limited by the physicochemical properties of the adsorbent and its regeneration capability, there is continued interest that exists in the development of biomaterial adsorbents derived from renewable sources such as cellulose and chitosan. Chitosan is a natural product derived from chitin upon deacetylation via alkaline hydrolysis, where the resulting copolymer contains glucosamine and acetylated glucosamine co-monomer units. The solubility of chitosan and its chemical reactivity scale as the degree of deacetylation reaches 60% or more [1]. The synthetic versatility of chitosan is evidenced by its various modified forms upon surface functionalization, cross-linking, and composite formation. As well, the physical modification of native chitosan through the alteration of its morphology in the form of nanomaterials, beads, and fibers can also lead to changes in the textural and adsorption properties toward ionic species. Raw chitosan and its derivatives are promising biopolymers for cation–anion adsorbate binding due to its unique adsorption properties [1,6,7].

The continued interest in the development of biopolymer-based sorbents provides an opportunity to develop sustainable adsorbent technology [8]. A previous study by Sabzevari et al. on the preparation of chitosan composites that contain graphene oxide (GO) displayed unique adsorption with methylene blue, as compared with pristine chitosan. Whereas GO is an arene base fragment with polar functional groups (-OH, -COOH) due to the controlled oxidation of graphite, pectin contains α-(1–4) linked D-galacturonic acid units and α-(1–2) linked L-rhamnopyranose residues. In comparison to GO, pectin is a suitable precursor for the preparation of cross-linked chitosan-based composites [9,10] due to its relatively low *pKa* (*pKa* = 2.9–3.2). The galacturonic acid (GalA) groups of pectin can react with methanol in an acidic environment to form methyl esters, where the majority of these GalA units are present as methyl esters in their native form. The degree of substitution (DS) of methyl ester formation is used to classify pectin polymers, where such biopolymers with a high methyl ester content (DS > 50%) are referred to as HM pectins [11].

Since chitosan contains glucosamine and N-acetyl glucosamine units linked through a (1–4) linkage, the biopolymer can exist in its cationic form upon protonation at acidic pH below its pKa. The protonated amine groups of chitosan are considered as the active sites to attract anion species through electrostatic interactions. As well, the amine groups can also undergo reaction with carboxylic acids to form hybrid composite materials [9,12]. Chitosan-based composites that contain pectin may undergo covalent or ionic bonding, as shown by the formation of a polyelectrolyte complex (PEC) or amide linkages between chitosan and pectin to yield a covalent biopolymer framework (CBF), as conceptually illustrated in Scheme 1.

In this study, various pectin–chitosan composite adsorbents were prepared and their physicochemical properties were characterized using infrared (IR) spectroscopy and other complementary methods. The dye adsorption properties of the composites were studied using methylene blue (MB), which is a model cationic dye that can provide insight on the nature of composites formed between pectin and chitosan (cf. Scheme 1). The overall goal of this study was to synthesize and characterize novel hybrid biopolymer adsorbents derived from chitosan and pectin, where the following objectives were addressed: (1) to synthesize pectin–chitosan composites at variable composition ratios using two different solvents (DMSO versus water), (2) to characterize the structure and physicochemical properties of the composites using complementary methods, and (3) to characterize the equilibrium adsorption properties of the biopolymer composites using methylene blue as a dye probe. This research addresses the knowledge gap concerning the structure–adsorption properties of pectin and chitosan composites according to the mode of preparation.

**Scheme 1.** Formation of pectin–chitosan composites through a polyelectrolyte complex (PEC) or a covalent biopolymer framework (CBF).
