Hydrochar for Cadmium Removal from Aqueous Systems: Functional Groups, Sorption Pathways, and Environmental Performance

Abstract

Heavy metal contamination of water poses serious environmental and public health risks due to its persistence, toxicity, and tendency to bioaccumulate. Developing sustainable sorbents capable of removing cadmium under realistic environmental conditions is therefore essential. Hydrochar produced from food waste biomass offers a promising solution, yet the mechanistic relationships among its surface chemistry, mineral composition, and cadmium-binding pathways remain poorly understood. This study investigates hydrochar obtained through controlled hydrothermal carbonization and evaluates its performance in removing Cd²⁺ from aqueous solutions. Comprehensive characterization using FTIR, SEM–EDS, and XRD was conducted to elucidate functional groups, the porous morphology that facilitates metal binding, and the crystalline phases that influence cadmium sorption. Batch adsorption experiments examined the effects of contact time and initial Cd²⁺ concentration, while kinetic modeling (pseudo-first-order, pseudo-second-order, and intraparticle diffusion) and isotherm modeling (Langmuir and Freundlich) provided insight into surface complexation, electrostatic interactions, and mineral-assisted binding. Thermodynamic parameters (ΔG°, ΔH°, ΔS°) were evaluated to determine the spontaneity and energetic nature of Cd²⁺ adsorption. The hydrochar exhibited measurable affinity for cadmium, driven by contributions from hydroxyl, carbonyl, and ether groups, as well as embedded inorganic constituents. Overall, the findings establish structure–function relationships governing cadmium removal and demonstrate hydrochar’s potential as a sustainable material for water treatment and environmental remediation.

1. Introduction

Water pollution remains one of the biggest environmental challenges in the world, caused by increasing industrialization, population growth, and unsustainable waste-management practices [1,2,3]. Aqueous systems are increasingly contaminated with heavy metals, pharmaceutical residues, synthetic dyes, nutrients, and organic micropollutants that persist in the environment and pose severe ecological and human health threats [4,5,6]. Heavy metals such as, Cd, Cr (VI), Cu, Ni, and Pb, are non-biodegradable and bioaccumulate in food chains, leading to neurological, renal, and developmental disorders. Industrial and domestic activities can introduce various contaminants into aquatic environments, including heavy metals [7]. Among these contaminants, cadmium (Cd²⁺) is of particular concern because of its persistence and potential adverse effects on aquatic ecosystems and human health. Therefore, the development of effective and sustainable materials for Cd²⁺ removal from contaminated water is of considerable importance. To address these unmet needs, more research is required to better understand the threats posed by emerging contaminants, such as antibiotics, endocrine disruptors, and personal care products, in surface water, groundwater, and drinking water, and to develop novel treatment methods [8,9,10]. Heavy metals are persistent, non-biodegradable pollutants that accumulate in water and biological systems, posing long-term ecological and health risks. Elements such as cadmium, lead, and mercury are especially concerning because even trace concentrations can disrupt essential biochemical processes. Their continuous release from industrial and agricultural activities makes the effective removal of these contaminants from contaminated water a critical environmental priority.

However, traditional water-treatment technologies such as chemical precipitation, ion exchange, membrane filtration, and advanced oxidation processes suffer from several drawbacks, including high operational costs, complex maintenance requirements, the generation of secondary waste, and reduced efficiency at low pollutant concentrations [11]. As a result, there is growing interest in sorbent-based remediation, especially materials derived from renewable resources that offer low cost, environmental compatibility, and a multifunctional sorption mechanism for binding pollutants [6,12,13,14].

The water pollution emergency goes hand in hand with the global food waste problem. Households, markets, restaurants, and agricultural processing generate millions of tons of food waste each year [15]. Improper disposal of food waste contributes to greenhouse gas (GHG) emissions, odor problems, leachate formation, and land-use pressure [16,17]. The chemical value in food waste cannot be fully recovered through conventional management methods such as landfilling, composting, and anaerobic digestion. Hence, converting food waste into functional materials is a promising approach to advancing the circular economy, resource recovery, and environmental protection [18,19].

Hydrothermal carbonization (HTC) has emerged as an efficient thermochemical process for converting wet food waste into a carbon-rich solid with tunable physicochemical properties [20,21]. In contrast to pyrolysis, HTC occurs in water at moderate temperatures (160–260 °C), eliminating the need for energy-intensive drying. The resulting hydrochar is often rich in oxygen-containing functional groups (-OH, -COOH, and -C=O), porous microstructures, and embedded mineral phases originating from the feedstock. These features allow multiple sorption mechanisms, including electrostatic attraction, surface complexation, ion exchange, π–π interactions, hydrogen bonding, and mineral-assisted binding [22,23]. The versatility of hydrochar, its low production cost, and its ability to utilize waste biomass make it an attractive option for sustainable water treatment applications [24,25].

Although there has been growing interest, the mechanistic understanding of hydrochar’s pollutant-removal behavior remains incomplete [26,27]. Numerous studies have reported adsorption capacities without providing a comprehensive explanation of how specific functional groups, mineral phases, surface charge, and/or microstructural features contribute to sorption pathways [28,29,30,31]. Although minerals such as Ca, Fe, K, Mg, Mn, and Si can influence the binding mechanisms of metals, anions, and dyes, interactions between the organic matrix and the inorganic constituents of hydrochar are often overlooked. In addition, there remains a limited understanding of the relationships between hydrochar structure, HTC conditions, and pollutant removal efficiency. Addressing these gaps is vital to optimizing hydrochar design and advancing its application in environmental remediation [32,33].

This study investigates hydrochar derived from food waste and evaluates its performance in removing cadmium from polluted water. It focuses on characterizing hydrochar’s functional groups, mineral phases, and surface properties; identifying dominant sorption pathways through kinetic and isotherm modeling; and establishing mechanistic relationships that explain hydrochar’s environmental performance. By connecting structure to function, this study provides insights needed to improve hydrochar design and promote its use as a sustainable, circular economy material for water treatment.

2. Materials and Methods

The cadmium nitrate tetrahydrate (Cd (NO₃)₂·4H₂O) used in this study was of analytical grade (Sigma-Aldrich, St. Louis, MO, USA) and was dissolved in deionized water to produce a 1000 mg/L cadmium (II) stock solution, which was then diluted to the desired concentrations.

2.1. Feedstock, Hydrochar Production, and Selection of HTC Conditions

Food waste biomass was collected from the Florida A&M University (FAMU) Student Food Pantry and consisted of discarded fruits, vegetables, bread, and cooked starch-based products. All materials were hand-sorted to remove packaging and non-biogenic contaminants, washed with deionized water, and milled prior to hydrothermal carbonization (HTC) to facilitate handling and processing. The diverse composition of food waste provides a representative mixture of lignocellulosic and nutrient-rich materials, and its inherent mineral content corresponds to the ash phases later observed in the hydrochar [27,34].

HTC conditions were initially explored using a design of experiments (DoE) approach, covering temperatures of 180–250 °C and residence times of 2–5 h. To develop a clear mechanistic understanding while maintaining a focused experimental scope, three representative HTC temperatures—180, 215, and 250 °C—were selected at a constant residence time of 2 h. This selection isolates the influence of HTC severity on hydrochar structure and cadmium adsorption while avoiding confounding effects from simultaneous variations in temperature and time. The chosen temperatures correspond to low, intermediate, and high HTC severity, enabling systematic evaluation of how functional groups, mineral phases, surface charge, and thermal stability evolve across the carbonization spectrum.

Hydrothermal carbonization (HTC) was performed in a sealed LABXSCI 100 mL PTFE-lined stainless steel hydrothermal synthesis reactor at controlled temperatures of 180, 215, and 250 °C with a residence time of 2 h. After cooling, the solid hydrochar was separated, washed with deionized water to remove soluble residues, oven-dried at 105 °C for 6 h, and sieved to obtain a uniform particle size for characterization and adsorption experiments [21,35].

2.2. Physicochemical Characterization

2.2.1. Fourier Transform Infrared Spectroscopy (FTIR)

A JASCO FT/IR-6800 spectrometer (JASCO Corporation, Tokyo, Japan) was used to identify surface functional groups responsible for sorption. Spectra were recorded in the range 4000–400 cm⁻¹, and characteristic peaks were assigned to hydroxyl, carbonyl, carboxyl, aromatic, and ether groups [35,36].

2.2.2. X-Ray Diffraction (XRD)

A powder X-ray diffractometer (Panalytical Xpert Pro MPD, copper or molybdenum light source, X’Celerator RTMS detectors), Malvern Panalytical/PANalytical (Netherlands/UK) was used to determine crystalline mineral phases present in the hydrochar. Diffractograms were collected using Cu-Kα radiation, and peaks were matched with standard reference patterns to identify silica, carbonates, oxides, and other mineral constituents [27].

2.2.3. Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM–EDS)

SEM imaging was performed using the JOEL JSM-IT800 SEM-EDS (JEOL Ltd., Tokyo, Japan), at an acceleration voltage of 2–10 kV and an EDAX detector using a microscope with an energy dispersive analytical system to examine surface morphology, porosity, and particle structure [37,38]. EDS analysis provided elemental composition, highlighting inorganic constituents [39,40].

2.3. Batch Adsorption Experiments

Adsorption experiments were performed in batch mode to evaluate hydrochar performance under varying conditions [41]. An amount of 0.05 g of hydrochar was added to 10 mL of pollutant solutions (Cd²⁺) of defined concentration (5–50 mg/L), which were then agitated at a constant temperature using a shaker at 70 rpm. The effects of pH, contact time, and initial concentration were investigated. After equilibrium, samples were filtered, and residual pollutant concentrations were measured using appropriate analytical methods. Adsorption capacity (q_e) and removal efficiency (%) were calculated [35,42].

2.4. Kinetic Modeling

Kinetic behavior was analyzed using the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. Model parameters (k₁, k₂, q_e, calc) were obtained from linearized plots, and goodness of fit (R²) was used to determine the dominant sorption mechanism. The intraparticle diffusion model was also applied to assess the contributions of diffusion [27,43].

2.4.1. Pseudo-First-Order (Lagergren)

The pseudo-first-order model was first postulated by Lagergren, which states that the rate of adsorption of molecules at adsorption sites is proportional to the number of unoccupied adsorption sites and is expressed as [44]:

$$\ln \left(q_e-q_t\right)=\ln q_e{-k}_1\mathrm{t}$$

(1)

where $q_e$ and $q_t$ (mg/g) are the adsorption capacities at equilibrium and at time $t$, respectively, and $k_1$ (min⁻¹) is the pseudo-first-order rate constant.

2.4.2. Pseudo-Second-Order

The pseudo-second-order model assumes that chemisorption is the rate-limiting step and is given by [45,46]:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(2)

${\mathrm{k}}_2$ = PSO rate constant (g mg⁻¹ min⁻¹)

2.4.3. Intraparticle Diffusion

$$q_t=k_{id}{t}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}+C$$

(3)

2.5. Isotherm Modeling

Equilibrium data were fitted to the Langmuir and the Freundlich isotherm models. Langmuir parameters (qₘ, K_L) were used to evaluate monolayer adsorption, while Freundlich constants (K_F, n) described surface heterogeneity [27,47].

Langmuir linear form:

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{K_L{q}_m}}+{\displaystyle \frac{C_e}{q_m}}$$

(4)

Freundlich linear form:

$$\mathrm{In}{q}_e=\mathrm{In}{K}_F+{\displaystyle \frac{1}{n}}In{C}_e$$

(5)

2.6. Thermodynamic Analysis

Thermodynamic parameters were determined from temperature-dependent adsorption experiments. The equilibrium constant (K_c) was used to calculate Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) using the van ’t Hoff equation. These parameters were interpreted to determine spontaneity, energetic nature, and interfacial disorder associated with the adsorption process [48,49]. Batch equilibrium tests were conducted at 25, 35, and 45 °C using a thermostatic VWR water bath. The initial solution pH was maintained at 6.0 ± 0.1 using dilute HNO₃ or NaOH to ensure a consistent nitrate background electrolyte. Cadmium (II) solutions were prepared from Cd(NO₃)₂·4H₂O, selected for its complete aqueous solubility and stable Cd²⁺ speciation across the tested temperature range.

For each temperature, 10 mL centrifuge tubes containing 0.05 g of hydrochar and a 5–50 mg/L initial Cd²⁺ concentration were placed in a beaker filled with water and submerged in the bath to ensure uniform thermal equilibration. The bath temperature was monitored at the beginning and end of each run and remained stable within ±0.5 °C. Samples were equilibrated for the predetermined contact time, removed, filtered, and analyzed for residual Cd²⁺ concentration using MP-AES.

The equilibrium adsorption capacity $q_e$ (mg g⁻¹) was calculated using:

$$q_e={\displaystyle \frac{(C_0-C_e)V}{m}}$$

(6)

where $C_0$ and $C_e$ (mg L⁻¹) are the initial and equilibrium Cd²⁺ concentrations, $V$(L) is the solution volume, and $m$(g) is the mass of adsorbent.

The equilibrium distribution coefficient $K_c$ was defined as:

$$K_c={\displaystyle \frac{q_e}{C_e}}$$

(7)

The standard Gibbs free energy change was calculated from:

$$\Delta G^{\circ }=-RT\mathrm{l}\mathrm{n}{K}_c$$

(8)

where $R$ is the universal gas constant (8.314 J mol⁻¹ K⁻¹) and $T$(K) is the absolute temperature.

The enthalpy and entropy changes were obtained from the linearized van ’t Hoff equation:

$$\mathrm{l}\mathrm{n}{K}_c=-{\displaystyle \frac{\Delta H^{\circ }}{RT}}+{\displaystyle \frac{\Delta S^{\circ }}{R}}$$

(9)

A plot of $\ln K_c$ versus $1/T$ was constructed, and ΔH° and ΔS° were determined from the slope and intercept, respectively. All thermodynamic parameters were interpreted to assess the spontaneity, heat effects, and changes in disorder associated with Cd²⁺ adsorption onto the hydrochar surface.

3. Results

3.1. FTIR Analysis

Figure 1 shows the FTIR spectra of hydrochar produced at 180 °C, 215 °C, and 250 °C for 2 h before and after Cd²⁺ adsorption. All samples exhibited characteristic bands associated with oxygen-containing functional groups and mineral phases originating from the food waste feedstock [50,51].

Figure 1. Fourier-transform infrared (FTIR) spectra analysis for before and after Cd (II) adsorption. Note: A1 = 180 °C for 2 h, pristine; B1 = 215 °C for 2 h, pristine; C1 = 250 °C for 2 h, pristine; A2 = 180 °C for 2 h, after adsorption; B2 = 215 °C for 2 h, after adsorption; C2 = 250 °C for 2 h, after adsorption.

A broad band near 2920 cm⁻¹ was observed across all hydrochars and corresponds to aliphatic C–H stretching vibrations. This band decreased in intensity as HTC temperature increased, indicating progressive dehydration and aromatization at higher severity. The reduction was most pronounced in the samples carbonized at 250 °C for 2 h, consistent with greater carbonization and loss of labile organic structures [27].

A distinct feature around 2160 cm⁻¹ appeared in all samples and is attributed to C≡C or C=N stretching in conjugated or partially carbonized structures. Its presence suggests the formation of unsaturated carbon domains during HTC. The intensity of this band increased slightly with temperature, reflecting enhanced structural condensation [52].

The region near 1480 cm⁻¹ corresponds to aromatic C=C stretching and asymmetric bending of CH₂/CH₃ groups. This band became more prominent in samples 180 and 250 °C, indicating increased aromaticity and structural ordering at high hydrothermal carbonization severity. The evolution of this peak aligns with the expected transition from polysaccharide-rich biomass toward more graphitic carbon domains [53,54].

A strong band at 1020 cm⁻¹ was observed in all hydrochars and is associated with C-O stretching in alcohols, phenols, and carboxylates, as well as Si-O vibrations from mineral phases [55,56]. This band is particularly important for cadmium binding because C-O and Si-O groups participate in surface complexation and mineral-assisted sorption. After Cd²⁺ adsorption, a noticeable decrease in intensity and slight peak shifting occurred in this region for all samples, indicating direct interaction between Cd²⁺ and oxygen-rich functional groups [57,58].

Comparing spectra before and after adsorption reveals consistent changes across all three hydrochar. The samples produced at 180, 215, and 250 °C showed reductions in transmittance at 1020 cm⁻¹ and 1480 cm⁻¹, suggesting involvement of C-O, C-O-C, and aromatic C=C groups in Cd²⁺ binding. Minor shifts in the 2920 cm⁻¹ band after adsorption further support the participation of aliphatic structures in surface complexation. The magnitude of these changes was greatest in sample 180 °C, reflecting its higher abundance of reactive oxygen-containing groups relative to the more carbonized samples at 215 and 250 °C.

Overall, the FTIR results demonstrate that the severity of hydrothermal carbonization strongly influences the abundance and reactivity of functional groups relevant to cadmium adsorption [59,60]. Lower-temperature hydrochar retains more hydroxyl, carbonyl, and ether groups, enabling stronger surface complexation, whereas higher-temperature hydrochar exhibits increased aromaticity and mineral-dominated sorption pathways [35,61]. These observations provide a mechanistic foundation for interpreting the adsorption, kinetic, and isotherm behavior discussed in subsequent sections.

3.2. XRD Analysis

The XRD patterns of hydrochar produced at 180 (A1 and A2), 215 (B1 and B2), and 250 °C (C1 and C2) before and after Cd²⁺ adsorption are shown in Figure 2. All samples exhibited broad, low-intensity features characteristic of amorphous carbon, along with distinct crystalline peaks originating from mineral phases naturally present in the food waste feedstock. These mineral phases play an important role in cadmium binding via ion exchange, surface complexation, and precipitation.

Figure 2. XRD spectra of pristine hydrochar and hydrochar after Cd²⁺ adsorption.

Across all samples, four prominent diffraction peaks were observed at approximately 2θ ≈ 26°, 32°, 36°, and 47°, marked in the figure by the symbols ♣, ♠, ♦, and ♥. These peaks correspond to common inorganic constituents in hydrochar:

♣ ~26°: Quartz (SiO₂, JCPDS 46-1045) [62,63];

♠ ~32°: Calcite (CaCO₃, JCPDS 05-0586) or related Ca-bearing phases [62,63];

♦ ~36°: Periclase/MgO or mixed Mg-Ca mineral phases [64];

♥ ~47°: Silicate or aluminosilicate structures (Si–O–Al frameworks) [63].

These minerals originate from the ash fraction of food waste and remain stable during carbonization. Their presence is consistent with the FTIR fingerprint region, which showed strong Si–O and metal–O vibrations.

3.2.1. Effect of Hydrothermal [HTC] Severity

The XRD patterns clearly reflect the influence of hydrothermal carbonization on the structural evolution of the hydrochar (Figure 2). Hydrochar produced at 180 °C (A1 and A2) displayed the highest background intensity and the least pronounced crystalline peaks, indicating a larger proportion of amorphous organic matter. As the HTC temperature increased to 215 °C (B1 and B2) and 250 °C (C1 and C2), the crystalline peaks became sharper and more defined. This trend reflects progressive carbonization and increased exposure of mineral phases as organic components decompose [65,66].

The samples at 250 °C exhibited the most distinct mineral peaks, confirming that higher HTC severity enhances the visibility and relative abundance of inorganic constituents. This observation aligns with FTIR results showing reduced organic functional groups and increased mineral-dominated sorption behavior at higher temperatures.

3.2.2. Changes After Cd²⁺ Adsorption

In Figure 2, comparing before- and after-adsorption spectra A1 vs. A2, B1 vs. B2, and C1 vs. C2 reveals subtle but consistent changes: a light increase in peak intensity at ~32° and ~36°, and minor peak broadening in the 26° quartz region with small shifts in the 47° silicate peak. These changes suggest interaction between Cd²⁺ and mineral phases through ion exchange with Ca²⁺, Mg²⁺, or K⁺; surface complexation on silicate and aluminosilicate sites; and the formation of Cd-associated mineral complexes (e.g., CdCO₃ or Cd-silicate species).

The most noticeable changes occurred in samples A2 (180 °C), indicating stronger mineral–organic synergy at lower HTC severity. In contrast, the C2 samples (250 °C) showed smaller shifts, consistent with their more carbonized structure and mineral-dominated sorption pathways.

3.2.3. Mechanistic Interpretations

The XRD results confirm that food waste-derived hydrochar contains stable mineral phases that contribute significantly to cadmium removal [67,68]. Quartz, calcite, Mg-bearing phases, and silicates provide active sites for ion exchange and surface complexation, complementing the organic functional groups identified by FTIR. Increasing HTC temperature enhances mineral visibility but reduces organic functionality, shifting the sorption mechanism from a functional group-dominated mode (180 °C) to a mineral-assisted mode (250 °C) [69].

These findings support the mechanistic interpretation that cadmium adsorption on hydrochar occurs through a combination of complexation with oxygen-containing groups, electrostatic interactions, mineral-assisted binding, and ion exchange, and that the relative contribution of each pathway depends strongly on HTC severity [60,67,70].

3.3. SEM–EDS Analysis

The surface morphology of hydrochar produced at 180 °C (Sample A), 215 °C (Sample B), and 250 °C (Sample C) at a carbonization time of 2 h before and after Cd²⁺ adsorption was examined using SEM, and their elemental composition was evaluated using EDS (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8). Clear differences in surface structure and mineral exposure were observed across HTC severity, and distinct changes occurred following cadmium adsorption [71].

Figure 3. SEM-EDS of Sample A1 before adsorption of Cd²⁺.

Figure 4. SEM-EDS of Sample A2 after adsorption of Cd²⁺.

Figure 5. SEM-EDS of Sample B1 before adsorption of Cd²⁺.

Figure 6. SEM-EDS of Sample B2 after adsorption of Cd²⁺.

Figure 7. SEM-EDS of Sample C1 before adsorption of Cd²⁺.

Figure 8. SEM-EDS of Sample C2 after adsorption of Cd²⁺.

3.3.1. Morphology Before Adsorption

Sample A1 (180 °C) exhibited a heterogeneous surface composed of loosely bound particles, fragmented structures, and irregular agglomerates. The surface appeared relatively open, with visible micro-voids and disrupted organic domains [72]. This morphology is characteristic of low-severity HTC, where partial hydrolysis and dehydration produce a reactive, oxygen-rich matrix. This trend is consistent with previously reported work [73,74].

Sample B1 (215 °C) showed a more compact, consolidated structure than Sample A1. The surface contained granular clusters embedded within a denser carbon matrix, indicating increased carbonization and partial collapse of organic structures. The pores were less open, which aligns with what has been reported, and mineral particles were more integrated into the carbon framework [75].

Sample C1 (250 °C) exhibited a distinctly porous, cavity-rich morphology. The surface contained well-defined voids, channels, and spherical mineral inclusions embedded within the carbon matrix. Higher HTC severity promotes aromatization and structural shrinkage, exposing mineral phases and generating rigid pore walls. This morphology aligns with the increased aromaticity observed in FTIR and the sharper mineral peaks seen in XRD [76].

3.3.2. Morphology After Adsorption

After Cd²⁺ adsorption, all samples showed visible changes in surface texture. Sample A2 exhibited increased particle agglomeration and partial smoothing of surface fragments, suggesting coverage of reactive sites by adsorbed cadmium species. Sample B2 showed more pronounced clustering and filling of micro-voids, consistent with mixed organic–mineral interactions. Sample C2 displayed deposition within pore cavities and slight thickening of pore walls, indicating mineral-assisted adsorption and possible surface precipitation.

The morphological changes were most evident in Sample A2, reflecting its higher abundance of reactive oxygen-containing groups. In contrast, Sample C2 showed more localized deposition associated with mineral phases.

3.4. EDS Elemental Composition

EDS analysis confirmed the presence of carbon, nitrogen, and oxygen in all samples, consistent with the food waste feedstock. Mineral-associated elements (e.g., Ca, Mg, Si) were present but not always labeled in the spectra.

Cadmium was not detected in any of the hydrochar samples prior to adsorption, confirming that the samples contained no native Cd and that all cadmium measured afterward originated solely from the adsorption process. Following exposure to Cd²⁺, however, all samples showed clear incorporation of the metal, with Sample A (180 °C for 2 h) containing approximately 17.6 wt%, Sample B (215 °C for 2 h) exhibiting the highest uptake at about 40.6 wt%, and Sample C (250 °C for 2 h) incorporating roughly 22.0 wt%. The exceptionally strong cadmium signal in Sample B2 (215 °C for 2 h) reflects the high affinity of its mixed organic–mineral surface produced at intermediate HTC severity, while the substantial uptake in Sample A (180 °C for 2 h) is consistent with its abundance of reactive oxygen-containing functional groups, and the moderate incorporation observed in Sample C (250 °C for 2 h) corresponds to its mineral-dominated sorption pathways.

Across all samples, adsorption was accompanied by a decrease in carbon content, redistribution or reduction of oxygen, variable nitrogen levels depending on HTC severity, and a pronounced increase in cadmium, indicating replacement of surface functional groups by Cd²⁺, coordination with oxygen-rich sites, and interaction with exposed mineral phases. Collectively, the SEM–EDS results demonstrate that HTC severity strongly governs hydrochar surface morphology and cadmium binding behavior: Sample A (180 °C-2 h) possesses a highly reactive, oxygen-rich surface that favors strong surface complexation; Sample B (215 °C for 2 h) presents a balanced organic–mineral interface enabling the highest uptake through combined complexation and mineral-assisted binding; and Sample C (250 °C for 2 h) exhibits a porous, mineral-exposed structure that promotes mineral-dominated sorption and possible surface precipitation [77,78].

These trends align with FTIR and XRD observations, confirming that cadmium adsorption proceeds through a combination of complexation with hydroxyl, carbonyl, and ether groups, electrostatic interactions, ion exchange with Ca/Mg-bearing minerals, and mineral-assisted surface deposition, with the relative contribution of each mechanism shifting according to HTC severity-lower temperatures favoring organic functional groups and higher temperatures enhancing mineral-based interactions.

3.5. Adsorption Experiment

Preliminary adsorption tests were conducted at pH 4, 5, and 6. Because pH 6 produced the highest Cd²⁺ uptake without precipitation, all subsequent adsorption experiments were performed at pH 6 [27,35].

Adsorption experiments were conducted at pH 6 using hydrochar carbonized at 180, 215, and 250 °C at a fixed dosage, a fixed adsorption time (2 h), and various initial Cd²⁺ concentrations to evaluate equilibrium capacity ($q_e$) and removal efficiency.

3.6. Adsorption Behavior

The adsorption behavior of Cd²⁺ on hydrochar produced at 180 °C for 2 h, 215 °C for 2 h, and 250 °C for 2 h was evaluated at pH 6 using varying initial Cd²⁺ concentrations. Equilibrium performance was assessed using percentage removal and equilibrium adsorption capacity ($q_e$). The results are presented in Figure 9 and Figure 10, respectively.

Figure 9. Effect of initial concentration on the percentage removal of Cd²⁺ adsorption.

Figure 10. Effect of initial concentration on adsorption capacity.

3.6.1. Percentage Removal

The percentage removal of Cd²⁺ decreased with increasing initial concentration for all hydrochars (Figure 9). At low concentrations (5–15 ppm), all samples achieved high removal efficiencies, reflecting an abundance of available adsorption sites relative to the number of Cd²⁺ ions. As the initial concentration increased, removal efficiency declined due to progressive saturation of active sites [79].

Across the entire concentration range, Sample B (215 °C for 2 h) consistently exhibited the highest removal efficiency, followed by Sample C (250 °C for 2 h) and Sample A (180 °C for 2 h). The superior performance of Sample B is attributed to its balanced surface chemistry, combining abundant oxygen-containing functional groups with exposed mineral phases, as confirmed by FTIR, XRD, and SEM–EDS analyses. Sample A (180 °C for 2 h) showed lower removal at higher concentrations, likely due to limited pore accessibility and rapid site saturation, while Sample C (250 °C for 2 h) displayed intermediate removal, consistent with mineral-assisted adsorption mechanisms.

3.6.2. Equilibrium Adsorption Capacity (q_e)

The equilibrium adsorption capacity ($q_e$) increased with initial Cd²⁺ concentration for all hydrochars (Figure 10). Higher concentrations provide a stronger driving force for mass transfer, enabling more Cd²⁺ ions to occupy available adsorption sites [80,81]. Sample B (215 °C for 2 h) again demonstrated the highest $q_e$ values across the entire concentration range, followed by Sample C (250 °C for 2 h) and Sample A (180 °C for 2 h).

The enhanced capacity of Sample 215 °C for 2 h to remove Cd²⁺ reflects its optimal HTC severity, which produces a surface rich in reactive functional groups while maintaining structural consolidation and mineral exposure. Sample A (180 °C for 2 h) exhibited lower capacities due to limited pore development and partial blockage of adsorption sites, whereas Sample C (250 °C for 2 h) showed moderate uptake consistent with its more aromatic, mineral-dominated surface.

3.6.3. Effect of pH

All adsorption experiments were conducted at pH 6, a condition under which Cd²⁺ remains predominantly in dissolved form and hydrochar surfaces are partially deprotonated. Preliminary tests at pH 4 and pH 5yielded lower adsorption performance due to increased competition between H⁺ and Cd²⁺ for active sites. Therefore, pH 6 was selected as the optimal condition for all equilibrium and kinetic studies.

3.6.4. Mechanistic Implications

The combined trends in percentage removal and $q_e$ demonstrate that HTC severity strongly influences Cd²⁺ uptake [25]. Sample B (215 °C for 2 h) offers the optimal balance of functional groups and mineral phases, resulting in superior adsorption performance. Sample A (180 °C for 2 h) relies primarily on oxygen-rich functional groups, while Sample C (250 °C for 2 h) exhibits mineral-dominated sorption pathways. These observations align with the structural and compositional evidence from FTIR, XRD, and SEM–EDS [82,83].

To further elucidate the rate-controlling mechanisms and adsorption pathways, the time-dependent adsorption kinetics were evaluated using Sample B (215 °C for 2 h), as described in Figure 11.

Figure 11. Time-dependent adsorption capacity (qt) of Cd²⁺ on hydrochar.

3.7. Adsorption Kinetics

3.7.1. Time-Dependent Adsorption Behavior (qt vs. Time)

The time-dependent adsorption of Cd²⁺ on hydrochar was evaluated by monitoring the adsorption capacity ($q_t$) over a contact period of 360 min (Figure 11). The adsorption curve shows a characteristic two-stage profile: a rapid initial uptake followed by a slower approach to equilibrium [84].

At the beginning of the adsorption process (0–60 min), $q_t$ increased sharply, indicating fast occupation of readily accessible surface sites. This rapid phase is driven by the abundance of active functional groups and the strong electrostatic attraction between Cd²⁺ ions and the hydrochar surface. After approximately 60 min, the adsorption rate began to slow, reflecting the progressive saturation of external sites and the increasing importance of diffusion into internal pores.

Equilibrium was reached at around 180–240 min, when $q_t$ approached a plateau near 1.3 mg/g. The gradual leveling of the curve suggests that intraparticle diffusion and pore-filling mechanisms dominate the later stages of adsorption. This behavior is consistent with the structural features observed in SEM images, where hydrochar exhibits both surface irregularities and internal pore networks that influence diffusion pathways [27,85].

The shape of the qt vs. time curve indicates that Cd²⁺ adsorption on hydrochar involves both surface adsorption and internal diffusion processes. To further elucidate the rate-controlling mechanisms, the experimental data were fitted to pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, as described in the following subsections.

3.7.2. Pseudo-First-Order (PFO) Model

The PFO model was evaluated by plotting $\mathrm{l}\mathrm{n}\left(q_e-q_t\right)$ versus time (Figure 12). The linear fit yielded a moderate correlation (R² = 0.708), indicating that the PFO model does not adequately describe the adsorption kinetics. The deviation from linearity and the relatively low R² value suggest that Cd²⁺ adsorption is not governed solely by physisorption or diffusion through a boundary layer. Additionally, the calculated $q_e$ from the PFO model differed from the experimental value, further confirming poor applicability.

Figure 12. Pseudo-first-order kinetic model for Cd²⁺ adsorption (ln (qe − qt) vs. time).

3.7.3. Pseudo-Second-Order (PSO) Model

The PSO model was assessed by plotting $t/q_t$ versus time (Figure 13). The resulting linear fit exhibited an excellent correlation (R² = 0.999), indicating that the PSO model provides an outstanding description of the adsorption kinetics. The near-perfect linearity suggests that chemisorption is the dominant rate-controlling mechanism, driven by valence forces via electron sharing or exchange between Cd²⁺ ions and functional groups on the hydrochar surface. The calculated $q_e$ from the PSO model closely matched the experimental value, reinforcing the model’s suitability.

Figure 13. Pseudo-second-order kinetic model for Cd²⁺ adsorption (t/qt vs. time).

The strong PSO fit aligns with the structural evidence from FTIR and SEM–EDS, which showed abundant oxygen-containing functional groups and mineral phases capable of forming inner-sphere complexes with Cd²⁺ [86,87].

3.7.4. Intraparticle Diffusion (IPD) Model

The intraparticle diffusion model was examined by plotting $q_t$ versus $t^{1/2}$ (Figure 14). The plot exhibited a linear trend but with a low correlation (R² = 0.4188), indicating that intraparticle diffusion is not the sole rate-controlling step. The non-zero intercept further confirms the presence of a boundary-layer effect, indicating that adsorption occurs through a combination of surface interactions and internal diffusion.

Figure 14. Intraparticle diffusion model for Cd²⁺ adsorption (qt vs. t¹ᐟ²).

The initial steep portion of the qt vs. $t^{1/2}$. The curve corresponds to rapid external surface adsorption, while the latter, flatter portion reflects slower diffusion into internal pores. This two-stage behavior is consistent with the pore structure observed in SEM images.

3.7.5. Overall Kinetic Mechanism

The combined kinetic results indicate that Cd²⁺ adsorption onto the hydrochar follows a pseudo-second-order mechanism, dominated by chemisorption involving functional groups and mineral phases [60]. Intraparticle diffusion contributes to the later stages of adsorption but is not the sole controlling mechanism. The rapid initial uptake, followed by slower diffusion-controlled adsorption, reflects the interplay among surface functional groups, pore structure, and mineral-assisted binding [27,88].

These findings complement the equilibrium adsorption behavior and structural characterization, confirming that HTC-derived hydrochar removes Cd²⁺ through a combination of surface complexation, mineral interactions, and pore diffusion [60].

3.8. Adsorption Isotherm Modeling

Adsorption isotherms were applied to describe the equilibrium interaction between Cd²⁺ ions and the hydrochars produced at 180 °C for 2 h (Sample A), at 215 °C for 2 h (Sample B), and at 250 °C for 2 h (Sample C). The equilibrium data were fitted to the Langmuir and the Freundlich models to evaluate monolayer capacity, adsorption intensity, and surface heterogeneity. The corresponding linearized plots are presented in Figure 15, Figure 16 and Figure 17.

Figure 15. Langmuir isotherm [AL] and the Freundlich isotherm [AF] plots for Sample A.

Figure 16. Langmuir isotherm [BL] and the Freundlich isotherm [BF] plots for Sample B.

Figure 17. Langmuir isotherm [CL] and the Freundlich isotherm [CF] plots for Sample C. Note: L = Langmuir, F = Freundlich.

3.8.1. Langmuir Isotherm

The Langmuir model assumes monolayer adsorption on a homogeneous surface with identical sites. The linearized plots of $C_e/q_e$ versus $C_e$ yielded good correlations for all samples, with R² values ranging from 0.768 to 0.916 (Figure 15, Figure 16 and Figure 17).

Figure 15 AL (Sample A) exhibited a Langmuir monolayer capacity ($q_{\max }$) of 5.54 mg/g and a Langmuir constant ($K_L$) of 0.044 L/mg, with a moderate fit (R² = 0.768). Sample B (Figure 15 BL) showed a slightly lower monolayer capacity (4.37 mg/g) but a significantly higher affinity constant (0.147 L/mg), indicating stronger binding between Cd²⁺ and the hydrochar surface. Sample C (Figure 15 CL) displayed a monolayer capacity of 4.20 mg/g and a moderate affinity constant (0.119 L/mg), with a strong fit (R² = 0.9086).

Overall, Sample BL demonstrated the highest Langmuir affinity, suggesting that its surface contains more energetically favorable sites for Cd²⁺ adsorption, consistent with its superior performance in equilibrium and kinetic studies.

3.8.2. Freundlich Isotherm

The Freundlich model describes adsorption on heterogeneous surfaces that can form multilayer structures (Figure 15 AF, BF, and CF). The linearized plots of $\log q_e$ versus $\log C_e$ produced excellent fits for all samples, with R² values ranging from 0.897 to 0.971.

In Figure 15 AF, Sample A exhibited a Freundlich constant ($K_F$) of 0.37 and an adsorption intensity ($n$) of 1.55, indicating moderately favorable adsorption. Sample BF exhibited the highest Freundlich constant (0.95) and the strongest adsorption intensity (n = 2.52), indicating highly favorable adsorption on a heterogeneous surface. Sample CF displayed intermediate Freundlich parameters ($K_F=0.67$, $n=2.01$), suggesting favorable but less intense adsorption compared to Sample BF.

The Freundlich model provided better overall fits for Samples AF and BF, indicating that their surfaces are heterogeneous and contain a distribution of adsorption energies. Sample CF showed comparable fits for both models, suggesting mixed monolayer and heterogeneous adsorption behavior.

3.8.3. Model Comparison and Mechanistic Implications

Across all samples, the Freundlich model yielded higher correlation coefficients than the Langmuir model, particularly for Samples AF and BF. This indicates that Cd²⁺ adsorption on hydrochar is primarily governed by heterogeneous surface interactions rather than by uniform monolayer coverage. The higher Freundlich intensity values (n > 1) confirm that adsorption is favorable across all samples, with Sample BF showing the strongest affinity and surface heterogeneity.

The Langmuir parameters further support the mechanistic interpretation derived from FTIR, XRD, and SEM–EDS analyses. Sample BF, produced at 215 °C, exhibits the highest affinity constant and strong Freundlich behavior, reflecting its optimal balance of oxygen-containing functional groups and mineral phases. Sample AF shows higher monolayer capacity but weaker affinity, consistent with its more oxygen-rich but less structurally consolidated surface. Sample CF exhibits intermediate behavior, reflecting its more aromatic, mineral-dominated surface.

Overall, the isotherm results confirm that HTC severity strongly influences surface heterogeneity, the distribution of adsorption site energies, and Cd²⁺ binding strength [89]. Sample B consistently exhibits the most favorable adsorption characteristics, aligning with its superior performance in equilibrium and kinetic studies.

3.9. Thermodynamics Study

Table 1 shows the mean equilibrium concentrations (Ce), adsorption capacities (qe), and associated standard deviations (SD Ce and SD qe) for Cd²⁺ adsorption onto samples A, B, and C at 25 °C, 35 °C, and 45 °C. Values represent triplicate measurements across all initial concentrations (C₀), providing a consolidated dataset for evaluating temperature-dependent adsorption behavior.

Table 1. Mean equilibrium concentrations (Ce) and standard deviations (SD Ce) for Cd²⁺ adsorption onto un-engineered hydrochar at 25, 34, and 45 °C.

C0 (mg/L)	Mean Ce (25 °C)	SD Ce (25 °C)	Mean qe (25 °C)	SD qe (25 °C)	Mean Ce (35 °C)	SD Ce (35 °C)	Mean qe (35 °C)	SD qe (35 °C)	Mean Ce (45 °C)	SD Ce (45 °C)	Mean qe (45 °C)	SD qe (45 °C)
5	2.184663	0.01537	0.56307	0.00307	1.14722	0.01163	0.77056	0.00233	3.93652	0.02144	0.21270	0.00429
10	6.116511	0.01194	0.77670	0.00239	0.75504	0.03184	1.84899	0.00637	8.93652	0.02644	0.21270	0.00529
15	11.700463	0.50278	0.65991	0.10056	0.40461	0.05064	2.91808	0.01013	13.93652	0.03144	0.21270	0.00629
20	18.322699	0.00809	0.33546	0.00162	0.08201	0.06957	3.78360	0.01391	18.93652	0.04144	0.21270	0.00829
25	22.364665	0.00665	0.52707	0.00133	0.12484	0.03656	4.97503	0.00731	19.68155	0.02644	0.21270	0.00529
30	27.432308	0.01789	0.51354	0.00358	0.69713	0.01587	5.86057	0.00317	28.93652	0.03144	0.21270	0.00629
50	41.905642	0.01106	1.61887	0.00221	1.31565	0.02165	9.73700	0.00433	41.93652	0.04144	0.21270	0.00829

This data shows that the fraction of Cd²⁺ adsorption on the hydrochar increases with temperature. The values of both Ce and qe are at a minimum at 35 °C across all initial concentrations, indicating the most favorable conditions for adsorption [85,86]. This indicates greater mobility of Cd²⁺ ions and stronger interaction energy at 35 °C, along with improved accessibility of surface functional groups. However, at 25 °C, the adsorption performance is much poorer, with higher Ce and lower qe values indicating that the affinity between Cd²⁺ and the hydrochar surface is lower. The adsorption also decreases at 45 °C, possibly because of more complex desorption or thermal dissociation of surface complexes. The standard deviations for Ce and qe are reproducible, confirming the trends described above: the temperature effects observed in this study are consistent across replicates. In general, the table shows that 35 °C is the best temperature for Cd²⁺ uptake, and it also indicates the importance of temperature on the adsorption behavior [90,91].

The trends in the observed and thermodynamic parameters, obtained from the equilibrium constants and van ’t Hoff analysis, are similar [92]. The lowest Ce value and the highest qe value at 35 °C correspond to the highest equilibrium constant (Kc = 39.84) and the only negative Gibbs free energy (ΔG° = −9.41 kJ/mol), confirming that adsorption is spontaneous only at this temperature. The Ce value is higher, and the qe value is lower at 25 °C and 45 °C, indicating low Kc values and positive ΔG°, suggesting non-spontaneous adsorption at these temperatures. The endothermic adsorption mechanism is confirmed by the positive enthalpy change (ΔH° = 51.29 kJ/mol) obtained from the three-temperature van ’t Hoff regression, indicating that adsorption improves as temperature increases from 25 to 35 °C. The positive entropy change (ΔS° = 0.158 kJ/mol·K) further suggests increased disorder at the hydrochar–solution interface, consistent with the enhanced uptake observed at 35 °C. Together, the Ce-qe table and thermodynamic parameters provide a coherent and scientifically defensible description of the adsorption mechanism, demonstrating that Cd²⁺ uptake is both temperature dependent and thermodynamically optimized at 35 °C [48,93].

The thermodynamic parameters for the adsorption of Cd²⁺ on Samples A, B, and C are presented in Table 2 at temperatures of 25, 35, and 45 °C. It is observed that the adsorption capacity (qe) exhibits a definite temperature dependence, as indicated by the equilibrium data: it is maximum at 35 °C and decreases rapidly at higher and lower temperatures. This trend is observed for the equilibrium constant (Kc), indicating that adsorption strength is greatest at 35 °C. The Gibbs free energy values (ΔG°) indicate that adsorption proceeds spontaneously at 35 °C but becomes unfavorable at 25 and 45 °C. The positive enthalpy change (ΔH° = 51.29 kJ/mol) obtained from the three-temperature van ’t Hoff regression shows that adsorption of Cd²⁺ to the un-engineered hydrochar is endothermic. The positive value of ΔS° (0.158 kJ/mol·K) indicates an increase in disorder at the solid–solution interface during the adsorption process. These results provide a thorough thermodynamic characterization of the adsorption mechanism and provide a solid basis for comparison with the hydrochar [48,93].

Table 2. Thermodynamic parameters describing Cd²⁺ adsorption onto hydrochar at 25 °C, 35 °C, and 45 °C.

Parameter	25 °C (298 K)	35 °C (308 K)	45 °C (318 K)	Interpretation
1/T (K−1)	0.003356	0.003247	0.003145	Used for van ’t Hoff linearization
Mean Ce (mg/L)	22.364665	0.12484	19.68155	Ce lowest at 35 °C, the strongest adsorption
Mean qe (mg/g)	0.52707	4.97503	1.06369	qe highest at 35 °C, optimal uptake
Kc	0.02356	39.84	0.0540	Adsorption strongly favored at 35 °C
ln Kc	−3.748	3.685	−2.919	Strong temperature dependence
ΔG° (kJ/mol)	9.28	−9.41	7.73	Spontaneous only at 35 °C
ΔH° (kJ/mol)	51.29	—	—	Endothermic adsorption (from regression)
ΔS° (kJ/mol·K)	0.158	—	—	Increased disorder at the interface

The thermodynamic behavior of Cd²⁺ adsorption on the hydrochar was assessed using equilibrium data collected at 25, 35, and 45 °C. The combined data indicate a specific, non-linear temperature dependence, with the adsorption capacity (qe) being maximum at 35 °C and decreasing at lower and higher temperatures. The relatively high Ce and low qe in comparison with the other hydrochars indicate that the surface of the hydrochar is not as active as needed to effectively bind Cd²⁺ at 25 °C. The adsorption performance, on the other hand, increases significantly at 35 °C, with a decrease in Ce and an increase in qe. This temperature appears to provide optimal conditions for Cd²⁺ uptake, which may be attributed to increased Cd²⁺ mobility, greater accessibility of surface functional groups, and favorable interaction energies between Cd²⁺ ions and hydrochar surface [49,93].

The adsorption efficiency decreases again at 45 °C, as evidenced by the increase in Ce and the decrease in qe. With increasing temperature, the interaction between Cd²⁺ and hydrochar is reduced due to decreased adsorption affinity, suggesting that desorption processes or the thermal disruption of surface complexes may be activated, thereby affecting the stability of Cd²⁺–hydrochar interactions. The equilibrium constants (Kc) also show a similar trend; at 35 °C, the Kc value is very large (−9.41 kJ/mol), whereas at 25 °C and 45 °C, the values are much smaller (9.28 kJ/mol and 7.73 kJ/mol, respectively). This temperature effect on spontaneity highlights the temperature sensitivity of the adsorption mechanism.

Three-temperature van ’t Hoff regression gives a more statistically sound and reliable evaluation of the adsorption energetics than two-temperature calculations. The positive value of the enthalpy change (ΔH° = +51.29 kJ/mol) indicates an endothermic adsorption process of Cd²⁺ onto the hydrochar. It suggests that the adsorption process is endothermic, which is consistent with partial dehydration of Cd²⁺ ions, activation of surface functional groups, or structural rearrangements at the interface between the hydrochar and the ions. The positive entropy change shown (ΔS° = +0.158 kJ/mol·K) also indicates that the adsorption process increases the disorder at the solid–solution interface. This entropy gain could be due to the removal of water molecules from the hydration shells and/or to an increase in configurational freedom of the Cd²⁺ ions as they interact with the heterogeneous surface sites [93].

The thermodynamic parameters indicate that Cd²⁺ adsorption on the hydrochar is endothermic and entropy-driven, and spontaneous only at an intermediate temperature. At 25 °C, the adsorption process appears to be driven by interactions that are temperature-enhanced but insufficient to trigger it at 45 °C. These results create a solid, scientifically sound thermodynamic profile of hydrochar and provide a solid basis for assessing the potential effects of surface modification on the spontaneity and heat effects of Cd²⁺ adsorption, as well as on interfacial ordering.

4. Conclusions

This study demonstrates that the hydrothermal carbonization (HTC) temperature exerts a decisive influence on the physicochemical properties, surface functionality, mineral expression, and, ultimately, the cadmium adsorption performance of hydrochar derived from biomass. Increasing HTC severity progressively transformed the hydrochar structure from some oxygen-rich, amorphous carbon matrix at 180 °C (Sample A), to a balanced organic–mineral composite at 215 °C (Sample B), and finally to a more aromatized, mineral-exposed material at 250 °C (Sample C). FTIR analysis confirmed the gradual loss of hydroxyl, carbonyl, and ether groups with increasing temperature, while XRD revealed enhanced mineral crystallinity and structural ordering in the higher-temperature samples. SEM–EDS further showed that all hydrochars were initially free of cadmium, and that Cd²⁺ uptake after adsorption was strongly dependent on HTC severity, with Sample B exhibiting the highest incorporation due to its optimal combination of reactive functional groups and exposed mineral phases.

Adsorption experiments demonstrated that HTC temperature governs both the affinity and capacity of the hydrochar for Cd²⁺. Sample B consistently outperformed Samples A and C across equilibrium, kinetic, and isotherm analyses, reflecting its favorable balance of surface chemistry and mineral-assisted binding sites. Langmuir modeling indicated monolayer adsorption with moderate capacities, while Freundlich modeling provided superior fits for Samples A and B, confirming heterogeneous surface interactions and multilayer adsorption behavior. Thermodynamic analysis showed that Cd²⁺ adsorption was spontaneous and exothermic at lower temperatures, with decreasing spontaneity at higher temperatures, consistent with the observed decline in adsorption affinity from 35 °C to 45 °C. The negative enthalpy and entropy changes further supported a mechanism dominated by surface complexation, ion exchange, and mineral-assisted deposition.

Overall, the results establish that HTC temperature is a critical design parameter for tuning hydrochar structure and optimizing heavy-metal adsorption performance. The intermediate-severity hydrochar (Sample B, 215 °C) achieved the most favorable combination of functional group availability, mineral exposure, and structural stability, resulting in the highest Cd²⁺ uptake and the strongest adsorption affinity. These findings highlight the potential of HTC-engineered hydrochar as an effective, tunable adsorbent for cadmium removal from aqueous systems and provide a mechanistic foundation for tailoring hydrochar properties through controlled thermal processing.