Innovative Solution for Invasive Species and Water Pollution: Hydrochar Synthesis from Pleco Fish Biomass

Abstract

In recent years, the invasive pleco fish has emerged as a global concern due to its adverse effects on ecosystems and economic activities, particularly in various water bodies in Mexico. This study introduces an innovative solution, employing microwave-assisted hydrothermal carbonization (MHTC) to synthesize hydrochar from pleco fish biomass. The research aimed to optimize synthesis conditions to enhance hydrochar yield, calorific value, and adsorption capacities for fluoride and cadmium in water. MHTC, characterized by low energy consumption, high reaction rates, and a simple design, was employed as a thermochemical process for hydrochar production. Key findings revealed that through response surface analysis, the study identified the optimal synthesis conditions for hydrochar production, maximizing yield and adsorption capacities while minimizing energy consumption. Physicochemical characterization demonstrated that hydrochars derived from pleco fish biomass exhibited mesoporous structures with fragmented surfaces, resembling hydroxyapatite, a major component of bone. Hydrochars derived from pleco fish biomass exhibited promising adsorption capacities for fluoride and cadmium in water, with hydrochar from Exp. 1 (90 min, 160 °C) showing the highest adsorption capacity for fluoride (4.16 mg/g), while Exp. 5 (90 min, 180 °C) demonstrated superior adsorption capacity for cadmium (98.5 mg/g). Furthermore, the utilization of pleco fish biomass for hydrochar production not only offers an eco-friendly disposal method for invasive species but also addresses fluoride and cadmium contamination issues, contributing to sustainable waste management and water treatment solutions. The resulting hydrochar, rich in solid fuel content with low pollutant emissions, presents a promising approach for waste management and carbon sequestration. Moreover, the optimized synthesis conditions pave the way for sustainable applications in energy production, addressing critical environmental and public health concerns. This research provides valuable insights into the potential of microwave-assisted hydrothermal carbonization for transforming invasive species into valuable resources, thereby mitigating environmental challenges and promoting sustainable development.

1. Introduction

An invasive species is one that has been moved outside its natural area and can colonize and persist. Its proliferation poses a threat to biodiversity, causing damage to the environment, economy, and human health in the place of settlement. Additionally, it displaces native species through various mechanisms such as direct competition, predation, disease transmission, and habitat modification [1].

In recent years, the presence of pleco fish in different water bodies in Mexico and elsewhere in the world has been reported with major concern [2]. In Mexico, these catfishes are mainly represented by two species: Pterygoplichthys pardalis and Pterygoplichthys disjunctivus [3]. Pleco fish, also known as devil fish, is an invasive species from the Amazon, belonging to the Loricariidae family. It poses a significant threat to ecosystems by feeding on the eggs of other fish in river and lake sediments, disrupting aquatic biota and impacting economic and recreational activities [4].

According to the Mexican Official Standard NOM-060-SAG/PESC-2016 and the Commission for Environmental Cooperation (CEC) for Mexico, USA, and Canada, all specimens of the Loricariidae family captured in these countries should not be returned to the environment since it is an invasive species [5]. In response to this situation, some research has been conducted to address the issue of plecos, directing the findings to its use as food for the general population or to obtain by-products as fertilizers, fish silage as a feed supplement for livestock, as well as fishmeal for fish feed. The most promoted option among fishing communities is fish consumption as part of the diet, although without much acceptance so far [6].

Invasive species like the pleco fish can have significant environmental impacts on local ecosystems, including the disruption of ecosystem balance, alteration of habitat structure, predation on native species, transmission of diseases, and genetic pollution through interbreeding with native populations. The proliferation of invasive species is often exacerbated by water pollution, which degrades habitats, disrupts natural barriers to their spread, increases stress on native species, and facilitates their introduction into new environments. Addressing the proliferation of invasive species requires holistic management strategies that target both the spread of invasive species and the underlying drivers, such as water pollution, through measures such as early detection and rapid response, habitat restoration, pollution prevention, and public education and outreach.

Currently, there is no specific use for this fish; therefore, when caught, they are sacrificed and left on the banks of rivers, lakes, and dams, becoming an environmental threat [7]. For that reason, it is necessary to develop technologies and promote new uses of biomass in order to control the proliferation of this species.

Microwave hydrothermal carbonization (MHTC) is a thermochemical process suitable for wet waste management that has the potential to revolutionize the conversion of food or organic waste, transforming it from a complicated process into a simple and fast one while converting heterogeneous biomass (water content between 50 and 95%) into sterile, transportable, and storable hydrochar. MHTC uses relatively low-temperature ranges (180–250 °C) under autogenous pressure conditions to convert organic biomass into a carbonaceous solid. The advantages of MHTC over other biomass conversion techniques are that the activation energy and energy consumption are low and that the configuration design is simple [8].

The hydrochar obtained using MHTC has a higher amount of solid fuel and lower emission of pollutants than biochar produced from low-temperature pyrolysis. In addition, it has been reported that hydrochars, which are solid products derived from MHTC, have desirable properties with various applications, including bioenergy production [9]. Microwave carbonization offers many advantages over other methods; microwave chemical reactions have higher reaction rates and lower energy consumption. On the other hand, microwave-assisted hydrothermal carbonization has been used in lignocellulosic and animal wastes; therefore, it could be a good option for rapid hydrochar production [10].

Among the different wastewater treatment processes, adsorption is widely accepted due to its high efficiency and ability to remove pollutants in solution. Commercial activated carbon provides good adsorption capacity for water pollutants such as organic compounds and heavy metals [11]. Hydrochar generally has a smaller area and porosity than biochar produced via pyrolysis; however, due to functional groups containing oxygen such as carboxyl and hydroxyl groups that were formed on the surface of the hydrochar, it is expected that this material could have a considerably higher adsorption capacity than other carbonaceous materials [12].

MHTC produces hydrochar from fish waste, which has potential applications in the energy sector, as well as in carbon sequestration and agricultural applications. Moreover, it is possible to use the complete specimen to produce these materials, so that the optimization of the process parameters will be sought by implementing a design of experiment that later can be used to obtain a response surface. Tests were conducted to calculate the hydrochar yield, assess the physicochemical and energetic properties, and visualize the surface morphology of the material to evaluate its possible use as an adsorbent for removing pollutants (i.e., fluoride and heavy metals) from water and compare its adsorption capacity with other commercial materials.

Among the contaminants of concern are heavy metals such as cadmium (Cd) and fluoride (F), which pose serious health risks even at low concentrations. According to the World Health Organization (WHO), the maximum acceptable concentration of cadmium in drinking water is 0.003 mg/L, while the maximum acceptable concentration of fluoride is 1.5 mg/L. Similarly, in wastewater, concentrations of cadmium and fluoride must be reduced to levels compliant with WHO guidelines to ensure environmental safety and human health.

Cadmium is a toxic heavy metal that can contaminate water sources and pose serious health risks to humans and the environment. It is often found in industrial and agricultural activities, as well as in natural deposits in the earth’s crust. Cadmium can enter water bodies through various sources, such as wastewater discharges, mining operations, corrosion of galvanized pipes, and the use of certain fertilizers and pesticides [13].

Once cadmium enters the water supply, it can persist for a long time and accumulate in aquatic organisms and sediments. Human exposure to cadmium-contaminated water can occur through drinking, cooking, and bathing. Cadmium is particularly harmful because it can bioaccumulate in the body, meaning it gradually builds up in tissues over time. Chronic exposure to cadmium has been associated with a wide range of health effects, including kidney and bone damage, respiratory issues, developmental and reproductive effects, and carcinogenicity [13].

To mitigate cadmium contamination in water, several measures can be taken, including stricter industrial regulations to limit discharges, the proper treatment of wastewater, and the use of alternative materials in pipes and plumbing fixtures. Regular monitoring and testing of water sources for cadmium levels are essential to ensure safe drinking water for communities [14].

Excessive levels of fluoride in drinking water can become a water contaminant and lead to adverse health effects. Fluoride contamination typically occurs through natural geological sources, industrial discharges, and the improper use of fluoride-containing products. While fluoride is beneficial for dental health at appropriate levels, excessive fluoride intake over a prolonged period can result in a condition known as dental fluorosis. Dental fluorosis affects the enamel of teeth, causing discoloration and, in severe cases, pitting and weakening of the tooth structure. It is mostly a cosmetic issue but can be a concern for individuals with severe fluorosis [15].

In addition to dental fluorosis, excessive fluoride exposure can have systemic health effects. Some of the potential health risks associated with high levels of fluoride in drinking water include skeletal fluorosis, acute toxicity, and neurological diseases. To prevent fluoride contamination, water treatment facilities carefully regulate fluoride levels to maintain the optimal concentration for dental health (typically around 0.7 to 1.2 mg/L). However, in areas where natural fluoride levels in water sources are already high, additional treatment may be required to lower the fluoride content to safe levels [16].

The production of carbonaceous materials to be used as adsorbents of these pollutants using biomass of the invasive species devilfish is a promising alternative to simultaneously solve two environmental problems. The objective of this work was to synthesize hydrochar from pleco fish biomass using microwave-assisted hydrothermal carbonization. In addition, the synthesis conditions were optimized using a response surface methodology (RSM) to maximize the hydrochar yield, its calorific value (CV), and the adsorption capacities of fluoride and Cd(II) in aqueous solution.

The novelty of this work lies in its pioneering utilization of MHTC to convert invasive pleco fish biomass into hydrochar, a process that optimizes synthesis conditions to enhance yield, calorific value, and adsorption capacities for fluoride and cadmium in water. By employing MHTC, a thermochemical process known for its energy efficiency and rapid reaction rates, this study not only offers a sustainable solution for invasive species control but also addresses water pollution challenges by repurposing a problematic biological resource into a valuable material with promising applications in waste management, energy production, and water treatment. Through comprehensive physicochemical characterization and adsorption tests, the research provides valuable insights into the structural and functional properties of hydrochars derived from pleco fish biomass, paving the way for innovative approaches to environmental remediation and resource utilization.

2. Materials and Methods

2.1. Sample Preparation

The pleco-fish used in this study were collected, sacrificed, and eviscerated by local habitants in the city of Balancán, Tabasco, Mexico, from the Grijalva Usumacinta River. According to Mexican legislation NOM-060-SAG/PESC-2016, these fish are considered an invasive species and must be removed from the bodies of water where they are located; therefore, no special permits are required for their management. These fish were dehydrated at 80 °C in an oven for 48 h and transported to the facilities of the Department of Bioresource Engineering of McGill University. The dry biomass obtained included fish heads, tails, skin, meat, and fins and was ground to homogenize it and generate a composite sample.

2.2. Enzymatic Hydrolysis

Previous studies have reported that it is necessary to treat raw fish waste using enzymatic hydrolysis prior to hydrothermal carbonization to obtain hydrochar [17]. In this study, three commercial enzymes Viscozyme, Lipase, and Protease (provided by Sigma Aldrich, Toluca, State of Mexico, Mexico) were used. According to previous studies [9], no significant differences in extraction results were observed when samples larger than 20 g were used; therefore, 20 g of homogenized fish waste was treated with the enzyme cocktail (12.5%, w/w of each enzyme) in the ratio 1:1:1 (viscozyme:protease:protease:lipase; w/w/w). Digestion was carried out in an incubator with a rotary shaker at approximately 40 °C and 120 rpm for 4 h.

2.3. Microwave Hydrothermal Carbonization

MHTC was performed using the Mini-WAVE Digestion Module (SCP Science, Baie D’Urfé, QC, Canada) operating at a frequency of 2.45 GHz. The products of the MHTC process were subjected to vacuum filtration to separate the solid fraction from the liquid fraction. The hydrochar obtained was dried in an oven at 105 °C for 24 h, and the yield was calculated on a dry basis as shown in Equation (1).

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=\frac{\left.\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} (\mathrm{d}\mathrm{r}\mathrm{y} \mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{s}\right)}{\left.\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{w}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{e} \mathrm{b}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{e} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{i}\mathrm{s} (\mathrm{d}\mathrm{r}\mathrm{y} \mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{s}\right)}\times 100$$

(1)

2.4. MHTC Optimization Protocol

A design of experiment–response surface methodology (DoE-RSM) approach was employed using a face-centered Central Composite Design (CCD) to optimize the production of hydrochar from biomass. This approach allowed for the simultaneous investigation of multiple factors affecting hydrochar yield, carbonization value (CV), and adsorption capacity for fluorides and Cd(II).

The two main factors considered in the experimental design were time (t, min) and temperature (T, °C), maintaining a constant water-to-biomass ratio of 1. Three levels were chosen for each factor: for Factor A (time), levels were coded as 90, 120, and 150 min, and for Factor B (temperature), levels were coded as 160, 180, and 200 °C.

The relationship between the factors and the responses was modeled using a second-order linear regression model, as represented by Equation (2):

y = β₀ + β₁ A + β₂ B + β₃ AB + β₄ A² + β₅ B²

(2)

where y represents the response variable (hydrochar yield, CV, or adsorption capacity); A and B represent the coded levels of Factors A and B, respectively; and β₀, β₁, β₂, β₃, β₄, and β₅ are the regression coefficients.

The experimental design layout, depicted in Figure 1, was generated based on the CCD. The design matrix, shown in

Table 1. Design of experiment proposed for this study and yield (%) of the hydrochars.

Experiment	Synthesis Conditions		Yield (%)
	Time (min)	Temperature (°C)
1	90	160	60.3
2	150	160	34.8
3	90	200	51.0
4	150	200	30.8
5	90	180	53.5
6	150	180	32.4
7	120	160	45.8
8	120	200	37.0
9	120	180	40.7
10	120	180	40.3
11	120	180	40.3
12	120	180	40.2
13	120	180	40.4

, illustrates the combination of factor levels used in the experiments. Model fitting and analysis of significant effects were conducted using Design Expert 11.0.0 software. Analysis of Variance (ANOVA) was employed to evaluate the statistical significance of the model terms and identify the most influential factors on hydrochar properties.

2.5. Hydrochar Characterization

The samples were dried in an oven at 110 °C for 24 h before the analysis to eliminate the residual moisture. The surface morphology and the microstructure of the raw sample and the hydrochar were observed with a Quanta FEG 250 FEI Scanning Electron Microscope (SEM) equipped with a system of energy-dispersive microanalysis (EDS) to carry out a qualitative and semi-quantitative elemental analysis of the surface.

Infrared spectroscopy (IR) was used to identify the functional groups present on the surface of the dried and pulverized samples. The characterization was performed in a Thermo Scientific IR spectrophotometer with ATR, model Nicolet iS10 (Walthman, MA, USA), sweeping between 510 and 4000 cm⁻¹.

To determine the textural properties (specific area, pore volume, and size and distribution of pores) of the hydrochar samples, N₂ physisorption was carried out using a Micromeritics physisorption equipment model ASAP 2020 (Norcross, GA, USA). The volume data of adsorbed N₂ referred to normal conditions and were plotted against the N₂ pressure to obtain the adsorption isotherm. The calculation of the specific area was made based on the Brunauer, Emmett, and Teller (BET) isotherm.

A thermogravimetric analysis of the raw material and the synthesized hydrochars was performed using a TA Instruments thermogravimetric analyzer model TGA Q500 (New Castle, DE, USA) to study the thermal behavior of the material. The analysis was carried out in an inert atmosphere using an N₂ flow of 100 cm³ min⁻¹ and a temperature ramp of 5 °C min⁻¹ between 30 and 700 °C.

The identification of the crystalline structures in the raw material and the hydrochars was performed with a Bruker X-ray Diffractometer model Da Vinci (Billerica, MA, USA). The samples were pulverized, and the analysis was performed in a range of 2θ from 2° to 60°, at a scanning speed of 1.8 degrees min⁻¹, 20 mA, 30 kVA, and CuKα radiation ($\lambda =0.15405 \mathrm{n}\mathrm{m}$). The different crystalline structures present in the samples were identified by comparing the samples’ diffractograms with the diffraction patterns of different compounds found in the JCPD database.

2.6. Chemical and Energy Properties of Hydrochar

Using the protocols of the ASTM [18], the hydrochar samples were analyzed to determine moisture content, ash content, and volatile solids. Elemental analyses were performed to characterize the chemical composition of the hydrochars produced using different MHTC process conditions. Carbon (C), hydrogen (H), and nitrogen (N) content were determined using an elemental combustion system from Costech Instruments. The oxygen (O) content was obtained using the difference shown in Equation (3).

$$\mathrm{\%}\mathrm{O}=100\mathrm{\%}-(\mathrm{\%}\mathrm{C}+\mathrm{\%}\mathrm{H}+\mathrm{\%}\mathrm{N}+\mathrm{\%}\mathrm{A}\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{s})$$

(3)

The energy value was determined using an adiabatic bomb calorimeter Model 1341EB, Parr (Moline, IL, USA).

2.7. Adsorption Experiments

Experimental data on the adsorption of fluorides and Cd(II) on the synthesized materials were obtained using a batch adsorber. Batch adsorbers consist of 50 mL conical tubes containing a known concentration of each of the contaminants and a selected mass of hydrochar. These tubes were placed in a thermostatic bath, and the pH was regularly evaluated using a potentiometric pH meter to keep it constant by pouring a few drops of 0.01 and/or 0.1 N NaOH and/or HNO₃ as needed.

Once equilibrium was reached, a sample was taken from each tube and the final concentration of the solution was measured. The mass of fluoride and Cd(II) adsorbed was estimated by performing a mass balance, which is represented in Equation (4).

$${\mathit{q}}_{\mathit{X}}=\frac{\mathbf{V}({\mathit{C}}_0-{\mathit{C}}_{\mathit{e}})}{\mathit{m}}$$

(4)

where C₀ is the initial concentration of fluoride or Cd(II) (mg/L); C_e is the fluoride or Cd(II) concentration at equilibrium (mg/L); m is the amount of anhydrous hydrochar added (g); q is the mass of fluoride or Cd(II) adsorbed on the material per gram of adsorbent (mg/g); and V is the solution volume (L).

3. Results and Discussion

A total of thirteen hydrochars were obtained from the proposed CCD. The evaluation of the independent variables (time and temperature) was carried out using an RSM to determine the effect of the synthesis variables on the experimental responses and to elucidate the optimal parameters for each of them. Based on the adsorption experiments, which will be discussed below, some of the following characterizations were performed only for the feedstock and for two selected hydrocarbons with the best adsorption capacities, Exp 1 for fluorides and Exp 5 for Cd(II), before and after adsorption.

3.1. Hydrochar Yield

The yield (%) values obtained using Equation (1) are presented in Table 1, where the maximum value achieved was 60.3%, corresponding to the factorial point of 90 min and 160 °C, demonstrating that, at high temperatures, the yield (%) decreases.

The yields (%) obtained during the synthesis experiments are plotted in the response surface diagram in Figure 2.

The experimental yield data were subjected to ANOVA which showed that time (A), temperature (B), the interaction between time and temperature (AB), the quadratic term of time (A²) and temperature (B²) significantly affected the hydrochar yield (Supplementary Table S1). The equation of the quadratic model for the yield was optimized using the Microsoft Excel 2023 Solver add-in program using the upper and lower levels of the variables as constraints. The optimized quadratic model is the one represented in Equation (5) (R² = 99.6%) and the optimization conditions obtained are 90 min and 160 °C.

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=300.28-1.48\mathrm{A}-1.47\mathrm{B}+0.0022\mathrm{A}\mathrm{B}+0.0030{\mathrm{A}}^2+0.0028{\mathrm{B}}^2$$

(5)

As can be seen in this equation, the variables synthesis temperature and time have a negative effect on the yield of hydrochar; therefore, with an increase in the variables, the amount of hydrochar produced decreases. Similar results have been reported in [2,3].

3.2. Hydrochar Characterization

SEM allows the analysis of microstructural changes on the surface of the hydrochars produced using the MHTC process at different carbonization conditions. Figure 3 shows the SEM micrographs of the raw material, as well as some of the hydrochars synthesized, revealing highly distinctive surface morphologies. The raw fish waste sample exhibits a flatter and smoother surface morphology compared to the surface of the synthesized hydrochars, which is more fragmented, rough, porous, and has an irregular fibrous texture.

This fragmented appearance in hydrochars is due to the release of gases in the thermolytic decomposition and devolatilization reactions characteristic of the MHTC process [19]. Figure 4a,b shows no apparent morphological variations on the surface of the hydrochars synthesized at different temperatures: Exp 1 (90 min; 160 °C) and Exp 5 (90 min; 180 °C).

In the EDS microanalysis of the fluoride-saturated hydrochar shown in Figure 5a, no particles containing fluoride were found. The EDS microanalysis performed on a particle of the Cd(II) saturated hydrochar is presented in Figure 5b, where the presence of Cd(II) on the surface was confirmed, being found at 6.62% w/w. The remaining elements found were C, O, Na, Al, Mg, P, and Ca. This is due to the fact that the main components of bone and fish waste samples are apatite group minerals [Ca₁₀(PO₄)₆(F,Cl,OH)₂], among them, hydroxyapatite (HAP) [Ca₁₀(PO₄)₆(OH)₂] [20].

Infrared spectroscopy characterization was performed to identify the functional groups present on the surface of the hydrochars synthesized in this study before and after the adsorption of fluoride and Cd(II). The IR spectra of the samples are shown in Figure 6. The peaks between 3430 and 3264 cm⁻¹ correspond to organic O-H axial deformation, with contributions from bounded water molecules that may have been retained in the samples [21]. The peaks around 2950–2850 cm⁻¹ are associated with the aliphatic C-H stretching [22]. As shown in Figure 6, the raw material did not show the peaks (C-H) corresponding to this bond, while on Exp 5 spectra, they became less intense than those observed in Exp 1. These peaks appear in the hydrochars because of the decarboxylation reactions that occur during the MHTC process.

The peaks at 1650 and 1550 cm⁻¹ are associated with the C=O stretching vibration of the ketone and amide groups and the carboxylic group, respectively [22]. The intensity of the peak at 1550 cm⁻¹ became stronger after Cd(II) adsorption. These changes suggest that the C=O group played an important role in the removal of Cd(II) from the aqueous solution through chemical interactions [23].

The peaks in the 1000–1040 cm⁻¹ and 840–880 cm⁻¹ regions are assigned to aromatic C−H vibrations [22]. The presence of aromatic structures and the variations in the spectra are evidence of the multiple alterations that fish waste undergoes during MHTC to produce hydrochar [8,9].

In the case of the fluoride-saturated and unsaturated hydrochar spectra, it can be observed that, in the peaks related to the phosphate present in the HAP in the band around 600 cm⁻¹, there is a decrease in their intensities, which indicates that there is an interaction between the fluoride and the HAP present in the analyzed material [24].

Figure 7a,b displays the adsorption–desorption isotherms of N₂ at 77 K for the hydrochars obtained in Exp 1 and Exp 5, while textural parameters, as evaluated from these isotherms, are reported in

Table 2. Textural properties of the hydrochars.

Sample	BET Surface (m2 g−1)	Pore Volume (cm3 g−1)	Pore Diameter (nm)
Raw material	ND	ND	ND
Exp 1 (90 min 160 °C)	47	0.21	17.7
Exp 5 (90 min 180 °C)	46	0.23	19.9

ND: non-detected.

. The adsorption isotherms of both samples are type IV(a), based on the IUPAC classification [25], and are, therefore, characteristic of mesoporous solids where the mesopore filling process is governed by the capillary condensation phenomenon and by the percolative properties of the solid. The hysteresis loop for these hydrochars is type H3, characteristic of slit-shaped pores of non-uniform size [26].

The BET surface of the Exp 1 and Exp 5 hydrochars was 46 and 47 m²/g, respectively. Similar areas have been reported for other hydrochars with no prior treatments [27]. Therefore, hydrothermal carbonization as a stand-alone process led to underdeveloped porosity in the absence of further processing. However, when analyzing the raw fish waste sample, no detectable surface was found due to the lack of porosity in the raw material, denoting that the slight porosity presented on the hydrochars was caused by the MHTC. The pore sizes of 17.7 and 19.9 nm also indicate that both materials are mesoporous. No differences attributable to the synthesis conditions were found.

Figure 8 shows the diffractograms of the samples of the raw material, the synthesized hydrochars (Exp 1 and Exp 5), and saturated with Cd(II) and fluoride, as well as the identification of the peaks corresponding to fluorapatite, HAP, and cadmium chloride.

The characteristic maximum peaks corresponding to calcium hydroxyapatite were found in all the samples; however, in the raw material, most of the peaks are fainter compared to the hydrochar diffractograms. This has been reported after carbonization processes like MHTC, with higher crystallinity as the loss of organic matter increases [9].

According to the results, the maximum characteristic peaks of the hydrochar samples match those present in the HAP and are attributed to the devilfish bones [2]. In the diffractogram of the fluoride-saturated sample, peaks corresponding to fluorapatite were identified at 2θ values of 25.67°, 28.69°, 31.61°, 33.98°, 38.88°, 46.46°, 52.95°, and 63.64° [28].

For the Cd-saturated hydrochar sample (Exp 5/Cd(II)), peaks with 2θ values of 28.38°, 29.34°, 31.46°, 32.22°, 33.22°, 39.00°, 39.82°, 40.00°, 46.96°, and 63.83° identified the presence of cadmium chloride, which is associated with surface interactions of cadmium with the hydrochar constituents [29].

The thermograms of the raw material and the hydrochars are shown in Figure 9 and Figure 10, respectively. For all the samples, the first stage corresponds to dehydration, which occurs at temperatures below 200 °C. In the case of the raw material, the moisture loss is approximately 6.0%, with a maximum peak at 69.17 °C, while, in the hydrochars (Exp 1 and Exp 5), it occurred at 57.34 °C and 66.18 °C, with weight losses of 0.9 and 1.0%, respectively.

After dehydration, the raw material exhibited only one peak at 309.21 °C, which can be attributed to the degradation of proteins, collagen, fatty tissues, and the release of volatile matter, losing 44.27% of weight [30].

For the hydrochar synthesized in Exp 1, the thermogram shows no difference between the unsaturated and saturated samples with fluoride, presenting three peaks. The first one belongs to the dehydration, the second and the third peaks correspond to the devolatilization stage [31], peaking at 207.4 °C for the unsaturated and 209.61 °C for the saturated with fluoride hydrochar, summiting the last peak at 307.02 °C for both samples. The temperature difference found in the first peak is due to the stabilization of the material by the presence of fluoride [32].

Figure 10b displays the thermograms for the hydrochar synthesized in Exp 5, which shows pronounced variations when the material is saturated with Cd(II). For the unsaturated sample, the devolatilization peaks occurred at 204.07 and 345.29 °C, with the most significant weight loss occurring between 253.41 and 422.86 °C, whereas, for the saturated sample, the major weight loss was 12.07% and the devolatilization peaks occurred up to 258.35 and 393.49 °C, also having a fourth peak at 648.71 °C, attributed to the char combustion or the decomposition of the remaining calcium phosphates and carbonates [33].

The discrepancies found between the thermograms of Exp 1 and 5 cannot be assumed to be a consequence of variations in the synthesis conditions; therefore, further study is required.

Figure 11 shows the percentages of ash, moisture, volatile material, and fixed carbon for each sample according to the CCD. In this diagram, it can be observed that the moisture values ranged from 0.26 to 3.49%, which is a result of the drying treatment applied to the hydrochars after synthesis. As for the ash content, values ranged from 37.34 to 50.1%, including the remaining minerals and materials that did not reach carbonization. However, lower ash contents (21 to 29%) have been reported for other hydrochars synthesized from fish waste, while ash contents similar to those in this study, up to 30.4% and 46.7%, respectively, have been reported for poultry litter and sewage sludge hydrochars [17].

The percentage of the volatile matter was similar to the ash content, ranging from 38.59 to 55%. It can be observed that the samples synthesized at 160 °C have a higher volatile matter content. The fixed carbon content was calculated by adding the above percentages and subtracting this amount from the 100% content of the sample. These values were the most variable (3.24–16.96%) and presented a significant increase based on the two synthesis variables of the design (time and temperature).

Elemental analysis of the hydrochar samples and the raw material was carried out to obtain its composition. A Van Krevelen diagram is presented in Figure 12. The oxygen content (between 59 and 72%) was calculated by difference. In the case of hydrogen, the smallest differences were found among the samples with values ranging between 2 and 3% of the total composition of the studied materials.

In this study, the carbon content ranges from 26 to 73%, which is consistent with that reported by (Afolabi and Sohail, 2017; Huezo et al., 2021) [34]. The carbon content found in the samples is generally lower than that reported for hydrochars from lignocellulosic materials, which have carbon contents up to 50% [35]. The nitrogen content present in the analyzed samples ranged from 0.4 to 1.5%, values also lower than those reported for the hydrochars of lignocellulosic origin [35].

A slight decrease in oxygen content was observed as the synthesis temperature increased, and the highest carbon content was found in the materials synthesized at the central points, with a temperature of 180 °C for 120 min. No alterations in hydrogen content were observed concerning the experimental conditions. Similarly, carbon does not seem to present a trend with respect to temperature or time.

The $H/C$ and $O/C$ ratios were calculated to discuss the possible reactions taking place in the conversion of the raw material to hydrochar. In accordance with the aforementioned diagram, the reactions that occurred from the raw material were dehydration, decarboxylation, and demethanization. These ratios are important because they allow the evaluation of the deoxygenation degree. A high $H/C$ ratio indicates that the aromatic content in the samples is low, while a decrease in the $O/C$ ratio implies that the material is undergoing decarboxylation reactions and chemical dehydration [36]. Figure 12 shows that with increasing temperature and/or hydrocarbonization time of the samples there is a marked tendency towards dehydration and decarboxylation, which coincides with that reported by [6,9].

The hydrochars synthesized in Exp 1 and Exp 5, for example, showed a decrease in the $O/C$ and $H/C$ ratio, respectively, compared to the feedstock, which allows us to ensure that decarboxylation and chemical dehydration reactions occurred in these cases as already discussed in this section.

3.3. Calorific Value

Figure 13 shows the response surface for the CV obtained, ranging from 8.58 to 14.81 MJ/kg, and illustrates the impact of synthesis time (A) and temperature (B). The surface plot presented curvature for both factors and a symmetrical behavior in terms of time. The highest values were found at low-temperature conditions (160 °C) and the lowest at 120 min and high temperatures (190–200 °C).

An ANOVA was performed with the calculated data of the CV of the samples, finding that the quadratic model based on the proposed CCD to evaluate the effects of the synthesis variables best described the experimental data (Supplementary Table S2). Temperature as well as the quadratic terms for time and temperature were found to be significant (p ˂ 0.05). The significance of the quadratic terms of both factors is correlated to the curvature observed in the response surface plot. The equation for this CCD is presented in Equation (6) (R² = 99.5).

$$\mathrm{C}\mathrm{V}=246.23-1.97\mathrm{B}+0.003{\mathrm{A}}^2+0.005{\mathrm{B}}^2$$

(6)

This equation provides information to understand the effects of the variables in the CV, where the negative coefficients indicate that bigger temperature values reduce the CV. The quadratic model was optimized using the Microsoft Excel 2023 Solver add-in program, maximizing the CV of the quadratic model with the upper and lower levels of the variables as constraints. The optimization conditions obtained were 160 °C and 90 min, obtaining a maximum CV of 15.05 MJ/kg and a %D of 1.6% between the experimental and the calculated data, which was considered excellent for this model.

3.4. Adsorption Experiments

The experimental data of the adsorption assays of fluorides and Cd(II) on hydrochars were determined individually, at T = 25 °C and pH = 7. Once equilibrium was reached, the final concentration was measured, and the corresponding adsorption capacities of the hydrochars were calculated.

Table 3. Adsorption capacity of hydrochars for fluorides and Cd(II).

Experiment	Synthesis Conditions		qexp (mg/g)
	Time (min)	Temperature (°C)	Fluorides	Cd(II)
1	90	160	4.16	96.55
2	150	160	1.74	46.40
3	90	200	2.79	62.14
4	150	200	2.05	54.90
5	90	180	3.95	98.49
6	150	180	2.00	80.40
7	120	160	2.66	79.26
8	120	200	2.31	39.17
9	120	180	3.14	82.03
10	120	180	2.98	85.91
11	120	180	2.91	82.39
12	120	180	2.98	79.75
13	120	180	3.04	92.26

shows the adsorption capacity obtained for the hydrochars, ranging from 1.74 to 4.16 mg/g for fluorides and from 39.17 to 98.49 mg/g for Cd(II), where the experiments with the highest adsorption capacity were Exp 1 and Exp 5, respectively, which were used for the characterization analysis.

It is evident in Figure 14 that the adsorption capacity increases considerably when hydrochars are synthesized at lower time and temperature values. This may be due to the reduction in surface functional groups at high temperatures and carbonization times as these groups decompose, resulting in lower ion exchange and low chances of precipitation of electrostatic interactions [37].

ANOVA was used to evaluate the effect of the factors on the adsorption capacity for both contaminants studied (Supplementary Table S3). The linear factors (A and B) present significant effects on the adsorption capacity of both pollutants, as does the interaction (AB) and the quadratic term of temperature (B²). The analyses of the quadratic models reveal their good fit as well as the significance of their terms and are represented by Equations (7) and (8), respectively, for fluoride (R² = 99.6%) and Cd(II) (R² = 99.5%).

$$\mathrm{q} \mathrm{F}\mathrm{l}\mathrm{u}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{d}\mathrm{e}=-5.19-0.22\mathrm{A}+0.27\mathrm{B}+0.001\mathrm{A}\mathrm{B}-0.001{\mathrm{B}}^2$$

(7)

$$\mathrm{q} \mathrm{C}\mathrm{d}\left(\mathrm{I}\mathrm{I}\right)=-1402.62-5.39\mathrm{A}+20.91\mathrm{B}+0.02\mathrm{A}\mathrm{B}-0.07{\mathrm{B}}^2$$

(8)

These equations helped to understand the effects of the factors on the measured responses. For the case of both contaminants, an increase in the synthesis time resulted in a decrease in the adsorption capacity of the synthesized materials, the opposite case of the synthesis temperature, which, when increased, produced an increase in the adsorption capacity of the hydrochars.

Table 4. Optimized values using the maximization of the quadratic models.

Contaminant	Time (min)	Temperature (°C)	qmax Exp (mg/g)	qmax Pred (mg/g)	%D
Fluoride	90	160	4.16	3.87	7.0
Cd(II)	90	171.5	98.5	108.3	9.9

shows the optimization of the quadratic models, which was performed using the Microsoft Excel 2023 Solver add-in program, maximizing the equations of the quadratic models for each pollutant with the upper and lower levels of the variables as restrictions. The %D between the experimental data and those calculated with the model was calculated. As can be seen, these %D were below 10% and were considered adequate, ensuring that the quadratic model adequately describes the experimental data of fluoride and cadmium adsorption capacities.

The adsorption capacities exhibited by the hydrochars for fluoride are comparable to other materials already reported, as shown in Table 5. There are reports on fluoride adsorption using hydrochars; however, most of them are plant-based with activation processes that functionalize the adsorbent to enhance its properties [38].

Other novel adsorbents, like bone char and hydroxyapatite, commonly used nowadays, have comparable adsorption capacities to those found in this study [39,40]. This is easily explained by the fact that around 70% of the hydrochar raw material used in this work is bone; nevertheless, it is interesting that hydrothermal carbonization processes can generate materials similar to those produced by pyrolysis but at lower costs [17].

Table 5. Comparative adsorption capacities of fluorides on other adsorbents.

Adsorbent	Adsorbate	pH	Adsorption Capacity (mg/g)	References
Pine wood biochar	Fluoride	NR	7.66	[41]
Activated alumina	Fluoride	7.0	2.41	[42]
Activated carbon	Fluoride	2.0	4.71	[43]
Bone char	Fluoride	7.0	4.51	[44]
		7.0	7.53	[45]
		5.0	13.49	[39]
		7.0	10.0	[46]
		5.0	11.2	[47]
	Cd(II)	3.0	85.5	[48]
		5.0	64.07	[49]
		7.0	213	[50]
Charcoal	Fluoride	7.0	13.64	[51]
Hydroxyapatite	Fluoride	NR	4.54	[52]
Waste carbon slurry	Fluoride	7.58	4.31	[53]
Sawdust hydrochar	Cd(II)	7.0	20.18	[54]
Chicken manure hydrochar	Cd(II)	6.0	30.0	[55]
Hydrochar/glucose	Cd(II)	4.0	88.8	[56]
Bamboo powder hydrochar	Cd(II)	4.0	90.37	[57]

NR: Not reported by the authors.

Table 5. Comparative adsorption capacities of fluorides on other adsorbents.

Adsorbent	Adsorbate	pH	Adsorption Capacity (mg/g)	References
Pine wood biochar	Fluoride	NR	7.66	[41]
Activated alumina	Fluoride	7.0	2.41	[42]
Activated carbon	Fluoride	2.0	4.71	[43]
Bone char	Fluoride	7.0	4.51	[44]
		7.0	7.53	[45]
		5.0	13.49	[39]
		7.0	10.0	[46]
		5.0	11.2	[47]
	Cd(II)	3.0	85.5	[48]
		5.0	64.07	[49]
		7.0	213	[50]
Charcoal	Fluoride	7.0	13.64	[51]
Hydroxyapatite	Fluoride	NR	4.54	[52]
Waste carbon slurry	Fluoride	7.58	4.31	[53]
Sawdust hydrochar	Cd(II)	7.0	20.18	[54]
Chicken manure hydrochar	Cd(II)	6.0	30.0	[55]
Hydrochar/glucose	Cd(II)	4.0	88.8	[56]
Bamboo powder hydrochar	Cd(II)	4.0	90.37	[57]

NR: Not reported by the authors.

As can be seen in Table 5, some Cd(II) removal studies have been conducted using hydrochars, which have reported adsorption capacities between 4.19 and 20.18 mg/g for sawdust hydrochar, 30 mg/g for chicken manure hydrochar, 88.8 mg/g for a hydrochar based on glucose functionalized with acrylic acid, and 90.37 mg/g for a bamboo powder hydrochar [54,55,57]. These results are similar to those found in this work for devilfish waste hydrochars, with Cd(II) adsorption capacities ranging from 39.17 to 98.49 mg/g.

4. Conclusions

Based on a CCD, 13 hydrochars were synthesized using MHTC. The materials were subjected to fluoride and Cd(II) adsorption tests on batch adsorbents at a constant pH and temperature. The effect of the synthesis variables (time and temperature) on the yield, CV, and adsorption capacity was evaluated using response surface analysis.

Based on the obtained results, the hydrochars with the best adsorption capacities were Exp 1 for fluoride and Exp 5 for Cd(II). These materials were used for the physicochemical characterization using SEM-EDS, infrared spectroscopy, N₂ physisorption, X-ray diffraction, and thermogravimetric analysis.

The characterization of the materials with the best adsorption properties for Cd(II) and fluorides led to the conclusion that the MHTC process using devilfish biomass develops areas in the hydrochars which are essentially mesoporous. Additionally, the materials have fragmented surfaces that are generated by the reactions that occur during hydrothermal carbonization. The functional groups of the materials correspond to those present in hydroxyapatite, an essential component of bone (a major component of the raw material).

The presence of the contaminants in the samples subjected to the adsorption experiments could be verified using analytical techniques that demonstrated the presence of new mineral phases associated with their presence and allowed predictions about the binding mechanisms of the adsorbed chemical species to the surface of the adsorbents.

The synthesis temperature and time have a negative effect on the yield of hydrochar, so that, with an increase in the variables, the amount of hydrochar produced decreases. In the case of the calorific value, an increase in temperature produces a decrease in this response; meanwhile, for the adsorption capacities of Cd(II) and fluoride, an increase in the synthesis time of the materials has a negative effect on their adsorption; however, an increase in temperature has a significant effect. These results will allow the selection of the best experimental conditions for the synthesis of hydrochars using MHTC in order to obtain materials with the desired properties for adsorption and energy production.

The findings align with previous research by Fu et al., 2019 [58], Maletić et al., 2022 [59], and Dhull et al., 2024 [60] highlighting the mesoporous nature and fragmented surface morphology of hydrochars, reminiscent of hydroxyapatite, a major component of bone. Furthermore, the superior adsorption capacities of the hydrochars synthesized in this study for fluoride and cadmium, as demonstrated by experiments outlined in Table 5, underscore their potential utility in wastewater treatment applications. By elucidating the effects of the synthesis parameters on hydrochar properties and adsorption performance, this study lays the groundwork for future investigations aiming to optimize hydrochar synthesis and enhance its efficacy as a sustainable solution for environmental remediation.