1. Introduction
Finding effective methods to remove heavy metals has become essential and research with functionalized materials in this area is progressing [
1]. Various material composites have been used as methods to manage heavy metals in wastewater; however, adsorption [
2] has established itself as a systematic method that is particularly interesting because of its efficiency, selectivity [
3], and easy operation. Historically, the high cost of the adsorption material has limited its use, but recently bio-adsorbents that are suitable for this technique have been proposed, [
4]. Aerogels are promising advanced materials, well known for their ultra-light and highly porous properties [
5,
6], that are commonly made from gel through multiple drying methods [
7]. When the aerogel is a crosslinked material from a gel precursor, the solvent extraction ensures a large specific surface area, as this feature is essential for adsorption potential [
8]. However, the definition of aerogel is controversial: they are solids, with meso- and macro-pores, with nanometer scale porosity but 95% of the phase is gas [
5]. Complex processing and drying methods have limited the use of aerogel [
9]. In recent years, with the increasing concern for the environment, eco-friendly or natural materials have been used as precursors of many functional materials, such as aerogels [
10].
Since the beginning, aerogels have been manufactured with a large range of materials, such as inorganic materials, with carbon [
11,
12,
13], and polymer-based materials [
14]; now, a sustainable and cost-effective approach is to use chitosan, a natural polysaccharide that is attractive for aerogel formulation [
5,
9,
15]. The properties of chitosan (CS) are well known and include biodegradability, crystallinity, biocompatibility, and abundance of the polymer. Furthermore, on the surface, the hydroxyl and amine groups are free to uptake anions, which is a highlight for heavy metal applications [
16,
17]. The general pathway for chitosan aerogel formation includes a sol–gel reaction, where the aqueous solution of chitosan transitions to gel; this determines the formation of a three-dimensional (3D) porous material which is occasionally used as a template for composite materials [
18,
19]. It is critical that factors such as pH, and temperature are controlled. Then the aging treatment is applied, where the gels are immersed under controlled conditions to enhance mechanical strength, and to ensure the internal network becomes structured. The ranges of times and solvents used are large [
20,
21]. The formation mechanism of the aging process remain unclear and limited to speculation. Further research needs to be conducted on the physical and chemical changes in the gel [
6]. Another crucial factor is that the type of final aerogel structure also depends on the mold where the gelation process takes place, that being an essential determinant of ice crystal growth direction [
20]. The rate of freezing and the drying method to achieve the removal of the internal solvent, i.e., supercritical drying or freeze-drying, also impact the outcome of the aerogel, which further complicates the processing. The response surface methodology (RSM) is a combination of mathematical and statistical methods for improving the process variables or evaluating the significance of the parameters of complex interactions. It has the advantage of analyzing multiple factors with minimal take-up of experiment runs, which is important because, typically, optimizing a process requires several experiments, multiple runs, long intervals of time, and costs a lot. The design of experiments with a statistical base can drastically reduce the number of experiment runs and provides a model that can describe the process with a polynomial equation and optimize the process involved [
22,
23,
24]. The RSM is an effective tool for determining and understanding the desired effects. These results can be optimized using the model obtained with a polynomial equation describing the response surface.
Seyed et al. [
2] proposed a hollow chitosan fiber and used the RSM to analyze the influences of the main operating parameters; Santos-López et al. [
25] also obtained CS aerogels that maintained the chemical identity of the CS after processing, and Guo et al. [
26] accomplished polydopamine-modified chitosan aerogel beads with a sorption performance of 374.4 mg/g of Cr (VI).
In the processing of CS aerogels, this study encompassed CS-R aerogels with resole (R): these liquid resins are obtained from a condensation polymerization synthesis based on phenol with an excess of formaldehyde [
14,
15]. A facile strategy for sorbent preparation to obtain functional CS/R aerogel with a xylem-like structure involves dispersal of these resins on a CS solution to promote a sol–gel reaction, followed by a freeze-drying process and a then a thermal treatment for the curing of the dispersed thermosetting polymer, as [
15]. Enlarging the size of the used adsorbent, as a strategy to propose an industrial application, is essential for this study. Owing to the variability of the formulations of different volume combinations, an RSM design analysis was used to verify the model, and further optimize Cr (VI) adsorption, based on CS/R aerogel percentage concentration, initial concentration of Cr (VI), and adsorption time, using a three-level three-factor full-factorial Box–Behnken design (BBD). The removal of Cr (VI) was proposed due to its known carcinogenicity to humans, it is considered one of the significant chemical contaminants by the World Health Organization (WHO) [
26], and due to the ease of its measurement by spectroscopic techniques.
3. Conclusions
In this work, a CS/R aerogel was fabricated using a sol–gel reaction of a CS solution with a resole solution via phenol-formaldehyde polymerization; this accomplished a simultaneous self-polymerization of the CS gel dispersed with liquid resole. The aerogel shows a highly porous structure and large surface area, facilitating the removal of Cr (VI) ions. The morphological analysis also showed that the proposed processing includes a thermal treatment as the final step in order to broaden its application by enhancing acid resistance and giving mechanical strength to the internal network. FTIR analysis shows that the main chemical species, chitosan and resole, were present in the different formulations of the CS/R aerogel; it also established a chemical bond with NH and NH2 species after adsorption. Furthermore, the method is easy and cheap, representing a simple and reproducible method for manufacturing aerogels of any size. The experimental design established an empirical relationship between the adsorption and independent variables, which was further expressed by the polynomial equation and RSM graphs. The variables of CS/R percentage concentration and adsorption time were related, with the best experimental adsorption capacity being a 95% Cr (VI) uptake. The adsorption kinetics indicated a pseudo-second-order model, suggesting a chemisorption mechanism. The model proposed was effective for material optimization and processing, and the optimal parameters of operation were 87/13 of CS/R aerogel percentage concentration; this is of potential interest as a lower-cost adsorptive material that could efficiently remove other heavy metals. Further research should be carried out, such as measuring the effectiveness of the CS/R aerogel with continuous effluent, or testing its application with other heavy metals, so that we can determine the type of chemical adsorption that occurs and produce a more detailed surface analysis.