Adsorption Isotherms

Adsorption isotherms were obtained by plotting *Qe* versus Ce (*cf*. Equation (1)) and were analyzed by fitting to a suitable isotherm model (*cf*. Equations (2)–(4)). The Langmuir model (Equation (2)) accounts for monolayer adsorption onto a homogeneous surface.

$$Q\_{\mathfrak{c}} = \frac{Q\_m K\_L \mathbb{C}\_{\mathfrak{c}}}{1 + K\_L \mathbb{C}\_{\mathfrak{c}}} \tag{2}$$

*Qe* and *Ce* are defined as in Equation (1), whereas *Qm* is the maximum monolayer adsorption capacity of the dye per unit mass of adsorbent (mmol/g), and *KL* (L/mmol) is the Langmuir equilibrium adsorption constant. By comparison, the Freundlich model (Equation (3)) describes the possibility of multilayer adsorption onto a heterogeneous adsorbent surface.

$$Q\_{\mathfrak{c}} = K\_f \mathbb{C}\_{\mathfrak{c}}^{\frac{1}{\mathfrak{n}\_f}} \tag{3}$$

Kf is the Freundlich adsorption capacity constant and nf denotes the intensity of adsorption. The Sips adsorption model (Equation (4)) accounts for both Langmuir (when *ns* = 1) and Freundlich behavior (when *ns* - 1) under certain limiting conditions. The maximum monolayer adsorption capacity (*Qm*, mmol/g) of the adsorbent can also be estimated. *Ks* (L/mmol) is the Sips equilibrium adsorption constant, and ns denotes the Sips heterogeneity parameter

$$Q\_{\mathfrak{c}} = \frac{Q\_{\mathfrak{m}} K\_{\mathfrak{s}} \mathbb{C}\_{\mathfrak{c}}^{\eta\_{\mathfrak{s}}}}{1 + K\_{L} \mathbb{C}\_{\mathfrak{c}}^{\eta\_{\mathfrak{s}}}} \tag{4}$$

Surface Area Estimated from MB Adsorption

The dye sorption method provides an independent estimate of the adsorbent surface area (SA; m2/g), according to the following equation [15]:

$$SA = \frac{A\_{\text{nr}} Q\_{\text{m}} L}{\Upsilon} \tag{5}$$

where *Am* represents the cross-sectional area occupied by MB (*Am*, for a "coplanar" orientation is 8.72 <sup>×</sup> 10−<sup>19</sup> m2/mol, where the dimensions of the dye are 1.43 nm <sup>×</sup> 0.61 nm), *Qm* is the monolayer adsorption capacity per unit mass of sorbent, *L* is Avogadro's number (mol<sup>−</sup>1), and Y is the coverage factor (*Y* = 2.0 for MB) [16].

## **3. Results and Discussion**

As noted above, several pectin–chitosan adsorbent materials were prepared herein according to variable synthetic conditions using adapted methods reported by other groups [17,18]. The characterization of the materials and selected physicochemical properties rely on various complementary methods: pH analysis, TGA, IR spectroscopy, and dye adsorption properties in aqueous media using methylene blue (MB). The results for the structural and physicochemical characterization of the composite materials are outlined in the sections below.

## *3.1. PZC Analysis*

The point-of-zero charge (PZC) is the pH where the net surface charge of the adsorbent is zero [13]. The PZC value becomes an important parameter for interpreting interactions that occur at material surfaces, especially for charged adsorbate species when the dominant adsorption mechanism is driven by electrostatic interactions. At pH > PZC, the surface of the adsorbent shows a negative surface charge due to the adsorption of OH− ions or deprotonation of hydrogen ions. For conditions where pH < PZC, the adsorbent surface shows a positive surface charge due to the adsorption of hydrogen ions from solution [19]. In Figure 1, the PZC results show the pectin–chitosan composite that was prepared in water with a net charge of zero near pH 4.7. Since pectin is soluble in water at all pH values [20], an estimate of its pKa can be inferred according to the reported value for galacturonic acid (*pKa* = 3.24). An estimate of the PZC value for chitosan (ca. 6.5) has been reported [21], where changes in the PZC value upon the formation of pectin–chitosan composites reveals a unique material that differs relative to the biopolymer precursors. The reduced PZC value of the pectin–chitosan composite is within the range of an independent estimate (PZC = 4.4) [22]. On the other hand, the pectin–chitosan composites prepared in DMSO with sonication show a net charge of zero near pH 3.8. The lower PZC value for composites prepared in DMSO may reflect the greater contribution of the pectin fraction, according to the lower *pKa* value estimated for galacturonic acid. This implies that the level of pectin grafting onto chitosan is higher and/or there are fewer available amine groups of chitosan to buffer the hydrogen ions dissociated from pectin. In the case of a dominant electrostatic interaction, the adsorption mechanism for pectin–chitosan CBF composites with a lower PZC value have greater Coulombic attraction to cation species (MB). This is in contrast to composites with a higher PZC value that possess a reduced surface charge.

**Figure 1.** Point-of-zero charge (PZC) results of pectin and chitosan composites: (**a**) water-based synthesis; (**b**) dimethyl sulfoxide-based synthesis.
