*3.3. FTIR Spectral Results*

In Figure 3a, pectin reveals a strong intensity stretching band (C=O) from non-ionized carboxy groups (-COOH and -COOCH3) of galacturonic acid at 1750 cm−1, and lower intensity bands for the symmetric and anti-symmetric carboxylate (-COO−) vibration at 1442 and 1673 cm−1, which concur with spectral results for pectin from other reports [28,29]. The broad IR band at 1600 cm−<sup>1</sup> for the chitosan spectrum relates to the N–H bending of a primary amine group of the glucosamine units. The shift of this N–H band to 1660 cm−<sup>1</sup> for the pectin–chitosan composite indicates a change in the chemical environment of this group upon interaction with pectin. The IR shift results are consistent with the reported results of such pectin–chitosan PECs [12,26,28]. Various reported studies of pectin–chitosan composites indicate the formation of PECs in water [18,30], which are supported by the IR results for composites prepared in water in Figure 3b. In Figure 3b, the IR bands for the pectin polymer at 2924 cm−<sup>1</sup> indicate C–H stretching vibrations, and IR signatures between 950 cm−<sup>1</sup> and 1200 cm−<sup>1</sup> relate to the IR absorption of the pyranose ring of pectin [31]. The C=O stretching band of galacturonic acid from the pectin–chitosan composites (prepared in DMSO) appear at 1750 cm−<sup>1</sup> and relate to non-ionized carboxy groups (-COOH and -COOCH3). The bands at 1442 cm<sup>−</sup><sup>1</sup> and 1637 cm−<sup>1</sup> are assigned to the symmetric and anti-symmetric vibration of carboxylate (-COO−) groups [31]. By comparison, the carboxylate band intensity increased, whereas the band intensity of non-ionized carboxy groups decreased, according to the formation of PECs between chitosan and pectin [31] A comparison of the IR signatures of pectin–chitosan systems in water (PECs) with composites prepared in DMSO through sonication reveal an increased intensity in the C=O stretching from a secondary amide that provide support for amide bond formation between pectin and chitosan [24,26,27,29,32]. Notwithstanding the difference in solvent properties, a rationale for the product distribution between the water and DMSO synthesis can be attributed to the modes of energy employed. Sonication-assisted synthesis differs from conventional heating and stirring, since ultrasonic waves can create vapor cavities around the surface of dispersed solids due to heating and subsequent pressure gradients due to cavitation effects. The resulting temperature and pressure gradients adjacent to the reactant surface can facilitate the amide bond formation [33]. Udoetok et al. [27] reported enhanced cross-linking effects at ambient temperature conditions in the case of epichlorohydrin cross-linked cellulose. The formation of amide linkages between pectin and chitosan is further supported by the increased signature of amide II band (N–H) bending of NH2 from chitosan at 1595 cm−<sup>1</sup> for DMSO-based composites. The IR results provided herein are also supported by other reported studies of amide bond formation for related chitosan composite materials [9,24,26,27,29].

#### *3.4. Sorption Isotherm Results*

Dye adsorption isotherm results have been shown to provide insight on structurally similar systems due to the sensitivity of dye probe to its chemical environment, especially dyes with large molar absorptivity values. The change in dye adsorption reveals the variable surface accessibility of active sites on the adsorbent material due to differences in morphology and the number of active adsorption sites [34]. The trend in dye adsorption for MB with the various composites prepared in water and DMSO are shown in Figure 4, along with a comparison with results for pristine pectin. In all cases, the isotherm profiles show a nonlinear increase in dye uptake with increasing MB concentration. In the case of composites, the dye adsorption capacity increases as the pectin content increases, where the composites prepared in DMSO show notably greater uptake versus the composites prepared in water. The observed trend parallels the greater negative surface charge of composites prepared in DMSO versus the products prepared in water, which are in agreement with the offset in PZC values for each synthetic preparation. The uptake of MB by pectin and pectin–chitosan composites in aqueous solution were analyzed by several adsorption isotherms. According to Figure 4, the best-fit isotherm results for the adsorption profiles of pectin and pectin–chitosan composites with MB dye were obtained using the Sips model. Table 1 shows the Sips isotherm parameters, where Ks is the Sips adsorption constant that relates to the adsorption energy, *Qm* is the monolayer adsorption capacity of MB, and ns indicates the

adsorbent surface heterogeneity [6]. The *Qm* values for the composites reveal an incremental uptake of MB as the weight fraction of pectin increased. The values of *Qm* (mmol/g) listed in Table 1 reveal that pectin has the greatest MB uptake capacity, which concurs with its abundant –OH and –COOH active sites. As well, pectin is very soluble in water with highly accessible carboxylate groups since pH > *pKa*, which is in contrast to heterogeneous adsorbents that are water-insoluble with lesser surface accessibility [35]. The formation of pectin–chitosan composites with covalent amide bonding show promising dye uptake performance such as PC51 S DMSO, since such CBF-based systems are more amenable to phase separation and recovery after the dye adsorption process. Insoluble composites are contrasted with pristine pectin, in spite of the relatively high adsorption capacity of pectin. In the case of PECs prepared in water such as PC51 W, lower dye uptake is observed relative to PC51 S DMSO. The enhanced adsorption of MB by the PC51 S DMSO system can be attributed to its relatively high pectin content and the branched structure of this CBF-based adsorbent. The covalent framework likely contributes to potential cooperative effects between the biopolymer subunits to afford secondary adsorption sites for MB along the chitosan backbone. The primary adsorption sites are attributed to the carboxylate groups of pectin due to the key role of electrostatic interactions with MB. The prominent role of the carboxylate sites is evidenced by the unitary exponential term (*ns* ≈ 1) for the composites in Table 1, irrespective of the composition of the biopolymer composite. Hence, the use of MB as a dye probe enables elucidation of the active adsorption sites (–COOH, –OH, and –NHR) for pectin and the pectin–chitosan composites.

**Figure 4.** Methylene blue (MB) dye uptake isotherms for pectin and pectin–chitosan composites, where the solid lines represent best-fit results by the Sips isotherm model.

**Table 1.** Sips model fitting parameters for methylene blue (MB) dye uptake by pectin and pectin–chitosan composite adsorbents.


<sup>1</sup> The adsorbent surface area (SA) was estimated using an equilibrium dye adsorption method, as further described in [1].
