1. Introduction
Carbonaceous materials constitute an attractive group of adsorbents due to their properties, such as large surface area and thermal and chemical stability. In addition, activated carbons with different pore sizes can be obtained, thus adapting them to the functions they are designed for. On the other hand, functionalizing the surface of activated carbons is possible, giving them specific chemical properties adapted to a particular purpose. It is also of great importance that carbonaceous materials are considered safe, accessible, and affordable [
1]. Various types of biomass residues, due to their low cost and abundance, have been evaluated to obtain functional carbonaceous materials. Many methods provide materials whose physicochemical properties can be tuned at will, thus obtaining materials that have shown promising results in different fields, such as adsorption or energy storage [
2]. For this reason, they are considered suitable candidates for obtaining sustainable materials. These materials have been successfully applied in catalysis, water purification, and soil remediation since they can remove heavy metals, agrochemicals, antibiotics, and other organic compounds. Excellent reviews on these aspects can be found in Soffian et al. [
3], Adegoke et al. [
4], Gęca et al. [
5], Borchardt et al. [
6], and Ma et al. [
2]. One such type of carbonaceous material is biochar, which can be produced from biomass waste through hydrothermal carbonization, chemical carbonization, gasification, torrefaction, and pyrolysis [
3,
7,
8,
9].
Sweet chestnut (
Castanea sativa) is a deciduous tree of the Fagaceae family, native to humid temperate regions of the Northern Hemisphere and widely distributed throughout Europe. It can reach 35 m in height and 2 m (exceptionally more) in diameter. Sweet chestnut trees live to an age of 600 years or even more. By autumn, the female flowers develop into spiny cupules containing 3–7 brownish nuts, which are shed in October [
10,
11,
12]. After the chestnuts are harvested, the spiny dome is disposed of as waste and used as fuel for fireplaces.
On the other hand, per- and poly-fluoroalkyl substances (PFASs) are a group of synthetic chemicals that have been used in a wide range of industrial and consumer products, such as firefighting foams, non-stick coatings, and water-repellent fabrics [
13,
14]. Due to their persistence, bioaccumulation, and toxicity, PFASs are considered Contaminants of Emerging Concern (CECs), with potentially adverse effects on human health and the environment [
15,
16,
17,
18,
19,
20,
21]. These substances have been detected in various environmental media, including water, soil, air, and biota, and have attracted increasing attention from regulators, scientists, and the public worldwide [
22]. Among the different methods for treating PFAS-contaminated water, adsorption has been recognized as a promising technology, given its effectiveness, versatility, and ease of implementation [
23,
24]. Carbonaceous materials are widely used as adsorbents for PFAS removal due to their high surface area, microporous structure, and affinity for hydrophobic compounds. One of the classic and best-known carbonaceous materials is activated carbon (AC) [
25,
26,
27]. It is usually obtained at high temperatures in a double carbonization and activation process. This material is characterized by a highly developed porous structure, a generally alkaline character, and a predominantly hydrophobic surface [
28,
29]. It is well known that these properties can be modified by adjusting the conditions of the activation processes or employing suitable chemical or thermal treatment [
30,
31].
In recent years, numerous studies have investigated using these adsorbents for PFAS adsorption in water, aiming to optimize the adsorbent properties and conditions, understand the adsorption mechanisms, and evaluate the performance and feasibility of the technology. The effectiveness of AC as an adsorbent for PFAS removal depends on its physicochemical properties, which can be tailored by adjusting the activation process and conditions. The surface area, pore size, surface chemistry, and surface charge of AC determine its adsorption capacity, selectivity, and kinetics for PFAS removal. The larger the surface area and the more mesopores present, the higher the adsorption capacity; this is due to the increased availability of adsorption sites and the enhanced accessibility of PFAS molecules to the internal surface of the carbonaceous material. The surface chemistry can also affect PFAS adsorption, as hydrophobic interactions between PFASs and the nonpolar surface are favored over electrostatic interactions with the polar surface of water molecules. Therefore, adsorbents with a higher content of surface functional groups, such as oxygen-containing or nitrogen-containing groups, may have a lower adsorption capacity for PFASs than non-functionalized ones due to the reduced hydrophobicity and the increased competition for adsorption sites.
In the last decade or so, a new type of carbonaceous material, hydrothermal carbons (HC), has attracted considerable interest from researchers in this field [
32,
33]. It is obtained by treating carbon-rich materials with water at a moderate temperature in a closed vessel so that endogenous water vapor pressure acts on this material [
34]. Generally, some biomass is used as raw material. This type of material is characterized by low pore development, acidity, and a hydrophilic surface, with many surface groups. As in the previous case, these properties can be modified by thermal or chemical treatments. This has the advantage of being a more environmentally friendly manufacturing process, as it is carried out in hermetically sealed containers and at a lower temperature. This allows for easier management of possible waste and lower energy consumption. Additionally, despite its limited specific surface area, its high proportion of functional groups can give rise to adequate adsorption capacity. We have been working on the obtention and use of hydrothermal carbons from chestnut cupules for the last few years. Our preliminary results (unpublished) show that these carbons are suitable adsorbents in the decontamination of heavy metals, such as thorium, from water. Given these results, we thought it would be apt to check for the adsorption capacity of apolar organic molecules such as PFAS. Adsorption onto carbonaceous materials has been established as a promising technology for PFAS removal from water [
20,
35,
36,
37,
38,
39,
40,
41,
42].
The objective of this work is to investigate the use of chestnut cupule waste as sustainable biomass raw material to obtain hydrothermal carbons—hence, adding value to it—and its use as adsorbent material for the removal from water of sodium perfluorooctanoate (PFONa), the anion of perfluorooctanoic acid, one of the most significant PFASs.
4. Conclusions
The preparation conditions of the hydrothermal carbons greatly influence the result; this allows for obtention of coals with tuned properties by optimizing the obtention process.
Carbonaceous material obtained by hydrothermal carbonization of sweet chestnut (Castanea sativa) cupule residues is a cost-effective, environmentally benign adsorbent, which effectively adds value to this biomass waste.
The carbons thus obtained show a limited surface area and a low microporosity while having a large volume of mesopores and macropores. All of them are acidic, with a low graphitization index, minimal moisture, and slight residual ash content; they are carbonaceous materials with a low mineral matter content.
It is possible to purify water contaminated with perfluorooctanoate by adsorption on this type of hydrothermal carbon. The adsorption process follows pseudo-second-order kinetics. Additionally, it fits the Freundlich model, showing that more than one layer can be adsorbed, which is expected in such an amorphous adsorbent.