Removal of Cr (VI) from synthetic effluents using hydrochar from waste açaí (Euterpe precatoria Mart.) seeds as a low-cost biosorbent

Remoção de Cr (VI) de efluentes sintéticos usando hidrocarvão de sementes residuais de açaí (Euterpe precatoria Mart.) como biossorvente de baixo custo

ABSTRACT

Chromium (Cr) is a common contaminant in aquatic ecosystems, and in its hexavalent form (Cr (VI)) can cause serious risks to human health. This study utilized açaí waste to prepare a novel hydrochar (AWR) for Cr (VI) removal. AWR was characterized by Boehm titration, point of zero charge, infrared spectroscopy, thermogravimetric analysis, specific surface area measurement, scanning electron microscopy, and elemental analysis. A 2³ factorial design examined the influence of key factors (pH, hydrochar mass, and Cr (VI) concentration). Results showed that acidic functional groups predominate on the surface of AWR. Under optimal conditions (pH = 2, hydrochar mass = 0.150 mg, and solution concentration = 150 mg L⁻¹), Cr (VI) removal exceeded 96% with maximum adsorption capacity (Q_max) = 20.60 mg g⁻¹ at 35°C. Adsorption kinetics followed a pseudo-second-order model, and the Tenkin isotherm best described the data. Thermodynamic analysis indicated that Cr (VI) adsorption is spontaneous and endothermic. This study demonstrated that AWR is a novel, effective, and cost-effective adsorbent for removing Cr (VI) from water environments.

Keywords:
Residual biomass; heavy metals; absorbent; hydrothermal carbonization

RESUMO

Cromo (Cr) é um contaminante comum em ecossistemas aquáticos, e na sua forma hexavalente (Cr (VI)) pode causar sérios riscos à saúde humana. Este estudo utilizou resíduos de açaí para preparar um novo hidrochar (AWR) para remoção de Cr (VI). O AWR foi caracterizado por titulação de Boehm, ponto de carga zero, espectroscopia de infravermelho, análise termogravimétrica, medição de área de superfície específica, microscopia eletrônica de varredura e análise elementar. Um planejamento fatorial 2³ examinou a influência de fatores-chave (pH, massa de hidrochar e concentração de Cr (VI)). Os resultados mostraram que grupos funcionais ácidos predominam na superfície do AWR. Sob condições ótimas (pH = 2, massa de hidrochar = 0,150 mg e concentração da solução = 150 mg L⁻¹), a remoção de Cr (VI) excedeu 96% com capacidade máxima de adsorção (Qmax) = 20,60 mg g⁻¹ a 35°C. A cinética de adsorção seguiu um modelo de pseudo-segunda ordem, e a isoterma de Tenkin descreveu melhor os dados. A análise termodinâmica indicou que a adsorção de Cr (VI) é espontânea e endotérmica. Este estudo demonstrou que o AWR é um adsorvente novo, eficaz e econômico para a remoção de Cr (VI) de ambientes aquosos.

Palavras-chave:
Biomassa residual; metais pesados; adsorvente; carbonização hidrotérmica

INTRODUCTION

Industrial discharge of heavy metals, especially chromium (Cr), pollute water and threaten human health (Liu et al. 2024). This metal is discharged into water bodies through industrial activities such as mining, leather processing and finishing, refractory steel production, and electroplating (Zulfiqar et al. 2023). Hexavalent chromium (Cr (VI)) is highly toxic due to its solubility, mobility, and carcinogenicity (Nur-E-Alam et al. 2020; Koc et al. 2024). Bioaccumulation and chronic exposure to Cr (VI) cause several pathophysiological failures, including wounds, dermatitis, anemia, allergic reactions, and damage in the gastrointestinal tract and liver (Habib et al. 2024; Katsas et al. 2024). Therefore, developing efficient methods to remove Cr (VI) from the environment is urgent.

Removal of Cr (VI) includes precipitation (Yang et al. 2025), ion exchange (Tan et al. 2024), reverse osmosis (Nie et al. 2024), coagulation (Chen et al. 2024), and adsorption (Li et al. 2025). Of these, adsorption is the simplest, cheapest, and easiest to work with (Tee et al. 2022; Irshad et al. 2023). Ideal adsorbents combine large adsorption capacity, eco-friendliness, and sustainability (Tounsadi et al. 2025). Biomass-derived material, particularly agricultural waste, show promise as adsorbents for heavy metal removal (Rohman et al. 2024; Wibowo et al. 2024). One particularly useful way of using this biomass waste is to convert it into carbonaceous biosorbent material through thermochemical treatment (Bian et al. 2019, Maniscalco et al. 2020).

Hydrothermal carbonization (HTC) is a clean, low-energy method for biomass processing (Malool and Moraveji 2025). HTC operates at moderate temperatures (180 - 350°C) for 0.5h - 24h, under autogenous pressures (25 - 60 bar), using water as the reaction medium (Li et al. 2021; Ţurcanu et al. 2022). During HTC, the biomass undergoes hydrolysis, dehydration, decarboxylation, polymerization, and aromatization, forming a solid product of hydrothermal carbon with the same energy level as bituminous coal, known as hydrochar (HC) (Fang et al. 2018, Dang et al. 2023).

Hydrochar provides economic and practical advantages, including no dehydration needs, higher yield, many functional groups, and a moderate pH (Guo et al. 2024). In addition, hydrochar is obtained at a relatively lower temperature compared to pyrolysis, resulting in lower energy consumption (Elhassan et al. 2024). Applications span energy production, soil amendment, and environmental remediation (Magdziarz et al. 2020, Thakur 2024). Biomass-derived hydrochar stands out for its low environmental impact and stability at low pH (Zhen et al. 2025). Hydrothermal carbonization also valorizes biomass waste into value-added products (Arevalo-Gallegos et al. 2017; de Freitas et al. 2024). Each kind of feedstock yields a hydrochar with distinct properties (e.g., functional groups, porosity, adsorption capacity) that influence their effectiveness in heavy metal removal (Zhang and Zhang 2022, Li et al. 2025).

Açaí fruits are derived from two palm species, Euterpe oleracea Mart. and Euterpe precatoria Mart. (de Freitas et al. 2024), They are known for their high energy and nutritional value (Melo et al. 2021). Pulping generates seed waste is often inappropriately discarded (Sato et al. 2019). Hydrochar from açaí waste is rarely reported and serves as a zero-cost carbon precursor. Its synthesis reduces costly sorbent expenses, reduces greenhouse gas (GHG) emissions, ensures carbon capture, offers a low-carbon economy, and enhances sustainability (Yadav et al. 2025).

This study used açaí waste to prepare novel hydrochar for Cr (VI) removal. Factorial design to test its effectiveness was applied to minimize time and costs. Hydrochar biosorption performance and cost assessment highlight açaí biomass valorization for toxic heavy metals removal and circular bioeconomy promotion.

MATERIALS AND METHODS

Hydrochar production

In Manaus, Amazonas, Brazil, açai seeds were collected, washed, dried (60°C), crushed, sieved (~ 2 mm), and dried again (103°C, 24 h). The product was mixed with distillate water (1:10 m/v ratio) in a 100 mL hydrothermal reactor and heated at 220°C for 6 h. The solid product was vacuum filtered, washed to neutral pH, dried (105°C overnight), and designated “hidrodrocarvão de Açaí” or Açaí Water Reactor (AWR).

Chemicals

A stock solution of potassium dichromate (K₂Cr₂O₇, 1 mol L⁻¹) was prepared in distilled water and adjusted to the desired pH. Working solutions with target Cr (VI) concentrations were obtained by appropriate dilution with double-distilled water. Hexavalent chromium (Cr (VI)) concentrations were determined using UV-Vis spectrophotometry at 540 nm with 1,5-diphenylcarbazide in acidic medium, according to ABNT (1996). For surface functional group analysis of the açaí seed-derived hydrochar, NaCl (≥ 98%), NaHCO₃ (≥ 99.7%), and Na₂CO₃ (≥ 99.0%) were used. Solution pH was adjusted using NaOH (98%) and HCl (37%). All reagents were of analytical grade and supplied by Merck.

AWR hydrochar characterization

Boehm titration (direct method) quantified the functional groups of hydrochar. The equilibrium batch method in triplicate determined the point of zero charge (pH_PCZ). The pHpcz was obtained from the initial pH vs. final graph. FTIR (Shimadzu spectrophotometer, IRAffinity-1) technique analyzed hydrochar functional groups. Thermogravimetry techniques (TG/DTG) tested the thermal stability of the hydrochars (SHIMADZU thermal analyzer, model TGA-51, under an N₂ atmosphere). Hydrochar morphology was observed via scanning electron microscope (SEM; TESCAN VEGA 3). Surface area was measured using a Micromeritics (TriStar II 3020 V1.03). The sample chemical elemental analysis (C, H, and N) was conducted in a PE 2400 Series II CHNS/O analyzer. The Oxygen content was determined by subtracting the CHN results from 100%.

Biosorption tests - Factorial experimental design

A 2³ factorial experimental design was carried out in duplicate, totaling 16 randomly performed tests. Analyzed variables were pH, hydrochar mass (m), and Cr (VI) concentration (Conc). The response variable was the percentage of Cr (VI) removal, using real and coded values (Table 1). Main effects, interactions between variables, and linear model regression coefficients were analyzed using Student’s t-test (α = 0.05). Analysis of variance (ANOVA) tested the model (α = 0.05). The coefficient of determination (R²) and the F-test were used as analysis criteria (α = 0.05). R software (version 4.3.1) was used to determine the set-up of factorial designs and statistical analysis.

Table 1
Parameters and coded values used in the factorial experimental design.

Parameter	Level
	-1	+1
pH	2.0	10.0
Hydrochar mass (mg)	0.050	0.150
Concentration of K2Cr2O7 (mg/L)	50	150

In the factorial design experiments, adsorption tests were performed in 16 Erlenmeyer flasks under constant stirring for 24 h. After the contact period, the contents of the flasks were centrifuged, and the supernatant was collected to determine the residual Cr (VI) concentration. The analysis was performed by measuring the absorbance at 540 nm by the diphenylcarbazide colorimetric method (ABNT, 1996). The hydrochar adsorption capacity at equilibrium (Qe, mg/g) and the percentage of Cr (VI) removal (R%) were determined using Equation 1 for capacity and Equation 2 for removal.

$Q_e=\left(C_0–C_e\right).\frac{V}{m}$ (1)

$R\%=\frac{\left(C_0–C_t\right)}{C_0}.100$ (2)

where:

Q_e: Cr (VI) adsorbed per adsorbent mass of (g g^-1); 𝐶𝑖: initial Cr (VI) concentration (mg L⁻¹); 𝐶𝑒: equilibrium Cr (VI) concentration (mg L⁻¹); 𝑉: solution volume (L).

To study the adsorption kinetics of Cr (VI) on hydrochar, the material was placed in a shaker at 25°C, at the optimum dosage of the adsorbent, the pH and the optimum concentration of Cr (VI) obtained by the experimental design (150 mg L⁻¹, 0.150 mg AWR, at pH 2). Samples of the supernatant from the mixture (adsorbate/adsorbent) were evaluated in the range of 0 to 1440 min and then centrifuged for 20 min at 3000 rpm. Finally, the Cr (VI) concentration was measured by UV-visible spectrometry and applied pseudo-first order (PFO) and pseudo-second order (PSO) adsorption models (Eqs. S1-S2).

Isothermal experiments were conducted in duplicates using a 20 mL Cr (VI) solution concentration of 150 g L⁻¹, 150 mg AWR, at pH 2. The mixture was agitated in a shaker at a controlled temperatures (25, 35, and 45ºC) for 60 minutes. Different isotherms were tested by Langmuir, Freundlich, and Temkin models (Eqs. S3, S4, S5).

Thermodynamic parameters are calculated by Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) (Eqs. S6, S7, S8), and used to compare adsorption at different temperatures.

RESULTS

AWR hydrochar characterization

The hydrochar derived from açaí seed waste (AWR) had acidic functional groups (0.072 mmol g^-1) dominated by carboxylic (0.045 mmol g^-1) and lactonic groups (0.022 mmol g^-1), with minimal phenolic groups (0.005 mmol g^-1), while basic groups measured 0.004 mmol g^-1 (Table 2). Elemental analysis revealed a carbon content of 65.21%, hydrogen 6.60%, nitrogen 2.08%, and oxygen and other elements 26.11%, with ash content below 1% (Table 2). The surface area was 18.90 m² g (Table 2), and the pH at the point of zero charge of hydrochar was 4.94 (Figure 1A).

Table 2
Contents of the elemental composition, atomic ratio, ash, surface area, and surface groups on the açaí seed hydrochar (AWR).

Elemental composition (%)
N	2.08
C	65.21
H	6.60
O	26.08
Atomic ratio
H/C	0.01
O/C	0.53
Ash (%)	0.025
Surface area (m² g-1)	18.90
Surface groups (mmol g-1)
Carboxylic	0.045
Lactones	0.022
Phenolic	0.005
Total Acids	0.072
Total Basics	0.004

Figure 1
A) Point of zero charges (pH_pzc) measured by pH variation experiments for AWR hydrochar, B) FTIR spectrum of the AWR hydrochar.

The FTIR spectrum of AWR revealed several characteristic absorption bands associated with specific functional groups. A broad band at ~3400 cm⁻¹ was attributed to O-H stretching vibrations, indicating the presence of hydroxyl groups. Bands at 2928-2840 cm⁻¹ are attributed to C-H stretching vibrations of aliphatic chains. The absorption band at 1700 cm⁻¹ was attributed to C=O stretching of carbonyl groups, while the bands at 1610 cm⁻¹, 1510 cm⁻¹, and 1450 cm⁻¹ were attributed to C=C stretching vibrations from aromatic structures. Additional bands at 1235 cm⁻¹ and 1057 cm⁻¹ are attributed to C-O stretching of ether/alcohol groups, and C-N stretching vibrations. The band at 756 cm⁻¹ is attributed to the presence of aromatic C-H bending (Figure 1B).

Scanning electron microscopy (SEM) revealed irregularly shaped carbon microspheres on the hydrochar surface (Figure 2). Thermogravimetric analysis (TGA) showed three mass-loss stages: 2.67% below 190°C (moisture), 18% between 196 - 395°C (hemicellulose/cellulose degradation), and additional loss above 400ºC (lignin decomposition) (Figure 3).

Figure 2
Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) of the AWR hydrochar.

Figure 3
Micrographs of açaí hydrochar (AWR): A) 2008 times; and B) 10600 times.

Establishing optimal biosorption conditions

The factorial design identified pH 2.0, hydrochar dose 150 mg, and initial Cr (VI) concentration 150 mg L⁻¹ as optimal conditions for Cr adsorption, achieving > 99% removal efficiency (Table 3). Statistical analysis indicated that pH had the greatest weight in the data set (-29.60, p < 0.05), while interactions between pH and dose (pHxm) and pH and concentration (pHxConc) were unimportant (Table S1). The Pareto chart (Figure S1) confirmed significance thresholds (α = 0.05). The derived model (Equation 3, R² = 0.99) predicted Cr (VI) removal as

$\mathrm{R}\%=9.2156\mathrm{}-\mathrm{}29.6094\mathrm{pH}-3.5806\mathrm{m}-1.5894\mathrm{Conc}-\mathrm{}4.5281\mathrm{pHm}-9.8356\mathrm{mConc}-10.7831\mathrm{pHmConc}$ (3)

Response surface method analysis of second-order interactions supported these ideal conditions to maximize Cr (VI) removal with hydrochar AWR (Figure 4).

Table 3
Real values of the 2³ factorial design and experimental results: m is the mass of hydrochar (mg) and Conc is the concentration of Cr (VI) (ppm).

Tests	Random Order	pH	m(mg)	Conc. (ppm)	Removal (%)
1	1.1	2	50	0.05	99.91
2	6.1	10	50	0.15	54.21
3	8.1	10	150	0.15	7.66
4	2.1	10	50	0.05	33.36
5	7.1	2	150	0.15	99.56
6	5.1	2	50	0.15	99.91
7	3.1	2	150	0.05	96.07
8	4.1	10	150	0.05	66.24
9	8.2	10	150	0.15	9.91
10	3.2	2	150	0.05	95.62
11	1.2	2	50	0.05	99.91
12	6.2	10	50	0.15	54.21
13	4.2	10	150	0.05	66.24
14	2.2	10	50	0.05	25.02
15	7.2	2	150	0.15	99.68
16	5.2	2	50	0.15	99.91

Figure 4
Three-dimensional surface-response plots for the interactive effect of (A) mass (m) x pH, and (B) Concentration (Conc) x mass (m).

Adsorption kinetics

Pseudo-second-order model provided the best fit (R² = 0.99) for Cr (VI) adsorption, with an equilibrium capacity (Qe) of 25.89 mg g⁻¹ and a rate constant (K₂) of 0.00093 (Table 4). Kinetic curves confirmed rapid adsorption within 120 min (Figure 5A-B).

Table 4
Kinetic and isotherm parameters for the adsorption of Cr (VI) on AWR.

Kinetic parameters
Pseudo-first order										Pseudo-second order
Qe (cal) (mg g-1)			23.63							Qe (cal)(mg g-1)				25.89
K1 (min-1)			0.01781							K2 (g mg-1 min-1)				0.00093
R2			0.93							R2				0.99
										Qe(exp)(mg g-1)				26.82
Isotherm Parameters
Langmuir						Freundlich							Temkin
TT (°C)	Qmax (mg g)	KL(L/m)		R²		K f( L/g)	1/nf			nf	R²		AT (L mg)	B(L/g)	bT	R²
25	17.33	0.65		0.59		8.49	0.2			5.04	0.95		137.93	1.97	1258.2	0.88
35	20.60	0.93		0.94		10.20	0.25			4.06	0.96		15.85	3.63	705.8	0.96
45	22.95	18.84		0.34		20.06	0.15			6.59	0.51		2218.23	2.68	986.9	0.50

• Kinetics parameters: solution volume 40 mL, adsorbent mass 0.150 mg and chromium solution concentration of 150mg L⁻¹. Isotherm parameters. Conditions: pH = 2, m = 0.150 mg and Conc. = 150 mg L⁻¹, 120 min.

Figure 5
A) Experimental results for adsorption kinetics of Cr (VI) on AWR at 25°C; B) adsorption kinetics pseudo-first-order reaction model, and pseudo-second-order reaction fit model for Cr (VI) adsorption on AWR.

Adsorption isotherm

The Temkin isotherm best describes equilibrium data (R² > 0.95), outperforming Langmuir (R² = 0.89) and Freundlich (R² = 0.91) models (Table 4, Figure 6). The Langmuir maximum monolayer capacity (Q_max) 20.60 mg g^-1, with an affinity constant (K_L) of 0.93 L.mg^-1 (Table 4). Isotherm plots and residuals are provided (Figures S2 and S3). AWR’s maximum monolayer adsorption capacity at pH 2 (Table 5) exhibits a satisfactory adsorption capacity compared to other adsorbents.

Figure 6
Experimental data of Cr (VI) adsorption on AWR adjusted to isotherm models at 35°C, and Model fit of adsorption isotherm of Cr (VI) adsorption onto AWR with adsorption isotherms (Conditions: pH = 2, m = 0.150 mg, and Conc. = 150 mg L⁻¹, 240 min).

Table 5
The maximum adsorption capacity (Q_max) of AWR compared to the other adsorbents of Cr (VI) adsorption at pH 2.

Adsorbent	Qmax (mg g-1)	References
AWR	20.60	This work
Magnetic-modified biochar from raw corncob	25.94	(Hoang et al. 2019)
ZnCl2 modified hydrochar	14.0	(Li et al. 2020)
AlCl3 modified hydrochar	12.3	(Li et al. (2020)
Pristine-modified hydrochar	9.22	(Nguyen et al. 2021)
Iron-modified hydrochar	14.33	(Nguyen et al. 2021)
Acid-modified poultry litter-derived hydrochar	26.2	(Ghanim et al. 2022)
Hydrochar from biomass of Eupatorium adenophorum spreng	7.76	(Wang et al. 2023c)
Hydrochar derived from coking sludge	19.92	(Zhong et al. 2023)

Thermodynamic parameters and adsorption mechanisms

Negative ΔG⁰ values (-3623.16 to -11614.41 J mol⁻¹), positive ΔH⁰ (115.51 kJ mol⁻¹), and positive ΔS⁰ (399.56 J mol⁻¹) indicated spontaneous, endothermic adsorption with increased entropy (Table S2). The van’t Hoff plot (Figure S4) confirmed temperature-dependent behavior. The Cr (VI) adsorption mechanisms can include pore filling, surface complexation, electrostatic interaction, and oxidation/reduction (Figure 7).

Figure 7
Possible mechanism of Cr (VI) adsorption by AWR.

DISCUSSION

Characteristics of AWR hydrochar

Synthesized hydrochar from açaí seeds (AWR) using the hydrothermal carbonization (HTC) is efficient for Cr (VI) removal. Hydrochar structural and chemical properties and the interaction of oxygenated functional groups contribute to its adsorption effectiveness, while low ash content, high carbon content, and specific surface distinguish it from conventional adsorbents. The oxygenated groups, such as carboxylic and phenolic, derived from the aromatization and polymerization during HTC (Knežević et al. 2010), strengthen the interactions between the AWR and Cr (VI). At acidic pH (< pH_PCZ), protonation of carboxyl groups (pKa ≅ 5) enhances electrostatic attraction of anionic Cr (VI) species (HCrO₄ ⁻ and Cr₂O₇ ²⁻), while phenolic groups (pKa = 7-10) can mediate the reduction to Cr (III) via electron transfer (Huang et al. 2023; Yu et al. 2024). Concurrently, basic sites (carbonyl, ether, pyrone, and chromene) contribute π-electron density, potentially stabilizing reduced Cr (III) through complexation (Lobo et al. 2024). This dual acid-base functionality allows hydrochar to adsorb anionic and cationic metal species (Shafeeyan et al. 2010).

The pH_PCZ of 4.94 indicates a protonated surface below this threshold, favoring Cr (VI) anion adsorption (Navas-Cárdenas et al. 2023; Hamad et al. 2024). At higher pH, deprotonation and OH^- competition diminish removal efficiency, aligning with trends observed in poultry manure and bamboo-derived hydrochars (Ghanim et al. 2022; Li et al. 2020).

The functional groups identified in this study reflect the complex structure of AWR and provide an understanding of its physical and chemical characteristics. The prominent O-H bending and stretching vibration probably arises from chemically adsorbed water and groups such as alcohols and phenols, a feature consistent with biomass-derived carbonaceous materials (Chen et al. 2022; Wu et al. 2023; Mutabazi et al. 2024). Aliphatic C-H stretching and aromatic C=C vibrations highlight that hydrothermal carbonization can preserve alkyl fragments of the precursor biomass while simultaneously promoting aromatization (Khan et al. 2021; Zhang et al. 2024), suggesting a carbon matrix rich in π electrons and hydrophobic regions (Zhang et al. 2022; Lestari et al. 2022). The carbonyl and carboxyl/alkoxy groups suggest a highly oxygenated surface of AWR, which may act as primary ligands for Cr (VI) through ion exchange and electrostatic interaction (Uzun, 2023; Yu et al. 2022). The detection of C-N and C-O interactions enhances Lewis basicity and stabilizes adsorbed Cr (VI) through covalent bonding (Escobar et al. 2021; Huang et al. 2022).

The low H/C and O/C ratios indicate advanced carbonization through dehydration and aromatization during HTC (Wang et al. 2022; Ali Babeker et al. 2024), improving hydrochar stability and aromaticity (Hejna et al. 2023; Guo et al. 2025b). AWR ash content rises with temperature (Tomczyk et al. 2020), but remains lower than typical feedstock (Barros et al. 2021), indicating good energy efficiency. High carbon content aligns with its elevated calorific value (Ye et al. 2023). In addition, the high C content confirms carbonization through dehydration and deoxygenation, while low H and O content reflect H and O loss as H₂O and CO₂ during HTC (González-Fernández et al. 2024; Kousar et al. 2024). Low N is advantageous because it avoids nitrogen oxide formation at high temperatures (Shi et al. 2023). However, N-containing groups are important in the adsorption (Zhang et al. 2022; Chen et al. 2023). Furthermore, AWR has a competitive surface area, outperforming oyster mushroom (10 m² g^-1) and corn stalk (5.8 m² g^-1) hydrochars (Romero et al. 2025; Lan et al. 2024), which highlights the role of specific HTC conditions and unique feedstock characteristics (Netto et al. 2022).

The thermal decomposition of AWR is a consequence of the structural components of the biomass. Although the initial mass loss at ~190°C is due to moisture evaporation (Solanki et al. 2025), the release of adsorbed water at relatively low temperatures may reflect strong hydrogen bonding between hydroxyl groups and the lignocellulosic matrix, which is a characteristic of biomass hydrophilicity (Zhang et al. 2022). Hemicellulose and cellulose degradation ranges (196-395°C), mass loss may be due to dehydration, decarboxylation, and decarbonylation reactions (Mashkoor and Nasar 2020). Above 400°C, additional loss occurs due to lignin degradation (Lobo et al. 2024), reinforcing its role as a thermal stabilizer. Furthermore, water extraction during HTC can recover unreacted cellulose, hemicellulose, and lignin from açaí seeds, which can be incorporated into the hydrochar through condensation and polymerization (Dhaouadi et al. 2021; Güleç et al. 2021).

The formation of carbon microspheres on the surface of AWR may have occurred due to the hydrolysis of cellulose, hemicellulose and lignin fragments in biomass microfibers, in addition to the polymerization and condensation in the soluble phase, increasing the aromatization of the hydrochar (Song et al. 2023; Wang et al. 2023a). The formation of nano to micro-sized carbon spheres on the surfaces of hydrochars is an advantage of the HTC method, which has a wide variety of surface functional groups, such as -OH, -C=O, and -C-OOH (Donar et al. 2016). Similar microsphere formation via HTC is reported (Taher et al. 2023; Lan et al. 2024).

Influence of adsorption parameters

The pH-dependent adsorption of Cr (VI) on AWR illustrates the relationship between the changes in surface charge and the forms of chromium present. Below a pH of 6, Cr (VI) mainly occurs as hydrogen chromate (HCrO₄ ^-) and dichromate (Cr₂O₇ ^2-) (Emara et al. 2023, Suručić et al. 2023). Under these conditions, AWR efficiently removes Cr (VI), due to the protonation of the surface, attracting the HCrO₄ ^- and Cr₂O₇ ^2-. As nH increases, hydrochar becomes less positive (more negative) and is progressively less attractive to the anionic Cr (VI) (Wu et al. 2023). At pH > 6, the amount of OH⁻ in the solution increases, and HCrO₄ ⁻ and Cr₂O₇ ²⁻ undergo deprotonation to form CrO₄ ²⁻. Excessive OH⁻ in the solution can compete with CrO₄ ²⁻ for adsorption sites on the AWR surface (Mutabazi et al. 2024), thereby reducing the probability of CrO₄ ²⁻ ions forming bonds with the adsorbent surface (Zhang et al. 2022, Mutabazi et al. 2024, Zhen et al. 2025). Additionally, in alkaline conditions, the redox potential of chromium ions decreases, leading to a reduced transformation from Cr (VI) into Cr (III), which could serve as the electron donor (Bandara et al. 2020, Juturu et al. 2024). Experimental data indicate reduced removal at higher pH. The effect of initial pH on Cr (VI) removal aligns with previous studies, such as poultry manure hydrochar (Ghanim et al. 2022) and modified bamboo sawdust hydrochar (Li et al. 2020).

The optimal hydrochar dose (150 mg) reflects a balance between active site availability and particle aggregation. The accentuated increase in sorption capacity is a consequence of higher surface area, more exchangeable sites becoming available for Cr (VI), and the quantity of available functional groups as the adsorbent dose increased (Khalil et al. 2021, Mutabazi et al. 2024). When the amount of hydrochar is high, biomass particles readily encounter each other, leading to physical interactions, primarily hydrogen bonds, between superficial functional groups (Guo et al. 2025a). The increase in the initial Cr (VI) concentration led to an increase in the metal removal efficiency. This was attributed to an increased driving force resulting from higher Cr (VI) concentrations (Liu et al. 2020, Rind et al. 2024).

Kinetic, isotherms, and thermodynamic investigation

The study of adsorption kinetics focuses on the rate at which a solute molecule binds to a solid adsorbent surface (Solanki et al. 2025). The pseudo-second-order best described Cr (VI) adsorption onto AWR hydrochar (R² > 0.95), which indicated that the chemisorption was the rate-limiting step. Thus, the adsorption of Cr (VI) on hydrochar may involve multiple mechanisms such as electrostatic attraction, ion exchange, and surface complexation (Li et al. 2020; Ghanim et al. 2022; Wang et al. 2023b; Rohman et al. 2024). Similar kinetics have been reported for hydrochar derived from corn straw and corncob for Cd²⁺ and Cr (VI) adsorption (Li et al. 2019).

Adsorption isotherms illustrate interaction between an adsorbent and an adsorbate molecule at a constant temperature (Rajendran et al. 2022; Nassar et al. 2023). The experimental data showed high R² values ​​(> 0.95) for both the Temkin and Langmuir isotherm models. However, the Freundlich model had a poor fit (~0.51). The Temkin model describes the multilayer adsorption process where the heat of adsorption decreases linearly with increasing surface coverage (Wang and Guo 2023), and the high binding strength (bT > 8) indicates strong interactions between AWR and Cr (VI) (Choudhary and Paul 2018; Yu et al. 2024; Anwar et al. 2009). The maximum adsorption capacity (Q_max) calculated using the Langmuir model is in close agreement with the testing data, and its adsorption ability is higher than that of other sorbents, suggesting that AWR is an efficient adsorbent. Additionally, an increase in the calculated Q_max with an increase in solution temperature suggests that sorption is endothermic (Bilal et al. 2022). This may be attributed to the enhanced mobility of the adsorbate ions within the pores of the adsorbent due to a decrease in the viscosity of the solution (Li et al. 2010).

Thermodynamic data confirmed the spontaneity and feasibility of Cr (VI) adsorption on AWR (ΔG° < 0) (Emara et al. 2023). Furthermore, the adsorption process is endothermic (ΔH° > 0), thus suggesting the predominance of chemical adsorption (Mutabazi et al. 2024; Wang et al. 2023c). The increased randomness at the hydrochar/solution interface during adsorption (ΔS° > 0) further favors the process (Qu et al. 2024).

Possible sorption mechanism and cost analysis of AWR production

Based on the discussions and AWR’s complex structure, Cr (VI) adsorption at low pH likely occurs through four mechanisms. Mechanism I involves electrostatic attraction: at low pH, protonation of oxygenated groups imparts positive charges to AWR, attracting anions (HCrO₄⁻, Cr₂O₇²⁻) (Mei et al. 2025). Mechanism II involves Cr (VI) reduction to Cr(III) via electron-donating groups (-OH, C-O, C=O) (Wang et al. 2023a). Mechanism III involves surface complexation between Cr(III) and functional groups (Hoang et al. 2019). Lastly, pore filling, favored by AWR’s high surface area, also contributes to Cr (VI) removal. Thus, AWR adsorption combines physical and chemical interactions.

AWR was produced from abundant açaí seed waste in the Amazon, mitigating environmental harm. The raw material was collected without cost, and transport was ~7.97 USD/ton. Cleaning with deionized water added ~0.06 USD/kg. Producing 1 kg of AWR consumed 0.6 kg of waste and ~3 kWh of electricity (~0.33 USD/kg), with a 30-40% mass loss during HTC. Including 10% for labor and overhead, the total production cost was 9.20 USD/kg, considerably lower than commercial activated carbon (~16.97 USD/kg). Scaling up could further reduce costs. Environmentally, using agro-industrial waste aligns with sustainable waste management and promotes a circular bioeconomy (Yaashikaa et al. 2021). However, further studies are needed to comprehensively evaluate economic and environmental impacts.

CONCLUSIONS

The present work highlights the potential of hydrochar derived from açaí biomass as a sustainable and low-cost biosorbent for environmental remediation. The results underscore the relevance of surface oxygenated functional groups and process conditions in enabling effective Cr (VI) adsorption. Beyond confirming the viability of this material, the findings contribute to advancing the understanding of biomass-derived hydrochars in water treatment applications. Future investigations should focus on applying this material to real effluents, exploring adsorption of other pollutants, regeneration and reusability strategies, and optimizing the hydrothermal process through analysis of its liquid-phase by-products. Such efforts can strengthen the role of Amazonian biomass residues in circular economy strategies and sustainable environmental technologies.

ACKNOWLEDGEMENTS

The authors would like to thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES/Brazil and Programa de Pós-graduação em Química - PPGQ/UFAM for the scholarships to Railane Santos (Ph.D.), to Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq and Instituto Federal de Educação Ciência e Tecnologia - IFRO for the scholarships to Ana Clara Mendes (graduation student), Naelly Araújo (graduation student) and Nathiely Moraes (high school student), to Centro de Bionégócios da Amazônia - CBA for allowing the development of work by some students from the PPGQ-UFAM, to Wyvirlany Lobo, William Pinheiro and Rosangela Duarte for the some caracterization analysis.

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• CITE AS:
  Santos, R.I.L.; Souto, L.F.L.; Moraes, N.A.; Mendes, A.C.S.; Araújo, N.N.A.; Freitas, F.A. 2025. Removal of Cr (VI) from synthetic effluents using hydrochar from waste açaí (Euterpe precatoria Mart.) seeds as a low-cost biosorbent. Acta Amazonica 55: e55mt24314.

Data availability

The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

SUPPLEMENTARY MATERIAL

Santos et al. Removal of Cr (VI) from synthetic effluents using hydrochar from waste açaí (Euterpe precatoria Mart.) seeds as low-cost biosorbent.

ADSORPTION KINETICS EQUATIONS

Pseudo-first order (PFO) adsorption:

$q_t=q_e\left(1-e^{k_{1t}}\right)$ (S1)

Pseudo-second order (PSO):

$q_t=\frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}$ (S2)

where:

qt and qe are the amounts of Cr (VI) adsorbed in the specific time (t), and when the equilibrium is achieved (mg g⁻¹), respectively, K₁ is the rate constant of the pseudo-first-order adsorption process (1 min⁻¹), and K₂ is the rate constant of adsorption (g mg⁻¹ min⁻¹) of the pseudo-second-order model.

ISOTHERMAL MATHEMATICAL EQUATIONS

Langmuir model:

$Q_e=\frac{Q_{max}*K_L{C}_e}{{1+Q}_{max}*{K_{L}C}_e}$ (S3)

Freundlich model:

$Q_e=K_F*C_e^{\frac{1}{n}}$ (S4)

Temkin model:

$Q_e=\frac{RT}{b_T}lnln{A}_T*C_e$ (S5)

where:

Ce = equilibrium concentration of the Cr VI (mg L^-1), Qe = amount of Cr (VI) adsorbed at equilibrium per unit weight of AWR (mg g^-1), Qmax = maximum monolayer coverage capacity (mg g^-1), K_L = Langmuir isotherm constant (L mg^-1) related to the binding energy of adsorption, and R_L = dimensionless separation factor indicating the nature and favorability of adsorption process. K_F = Freundlich indicator of adsorption capacity, and 1/nF = Intensity of adsorption, indicating surface heterogeneity and the favorability of the adsorption process. b_T = the Temkin isotherm constant related to the heat of adsorption and A_T = the Temkin isotherm equilibrium binding constant (L g^-1), R = universal gas constant (8.314 J mol^-1 K^-1), T = absolute temperature in Kelvin, and B = RT/b_T = constant related to heat of sorption (J mol^-1) obtained either from intercept or slope.

THERMODYNAMIC PARAMETERS

Gibbs free energy (ΔG°)

$K_e=\frac{C_{Ads}}{C_e}$ (S6)

${ln}_{ke}=\frac{{∆S}^0}{R}-\frac{{∆H}^0}{RT}$ (S7)

Gibbs free energy (ΔG°)

${∆G}^0=∆H°-T∆S°$ (S8)

enthalpy (ΔH°)

and entropy (ΔS°)

where:

Ke is the equilibrium constant, CAds is the amount of adsorbent (mg g⁻¹), Ce is the equilibrium concentration of the dye in the solution (mg L⁻¹), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and T is the temperature (K). ΔH° (J mol⁻¹) and ΔS° (J mol⁻¹ K⁻¹) were calculated from the slope and intercept of the plot ln_Ke vs 1/t.

ANOVA FACTORIAL DESIGN

Table S1
Effects for Cr (VI) removal efficiency by adsorption on AWR.

	Effects	Standard Error	t	p-value
Constanta	69.2156	0.5406	128.024	1.55e-14
pH	-29.6094	0.5406	-54.767	1.37e-11
m	-3.5806	0.5406	-6.623	0.000165
Conc.	-1.5894	0.5406	-2.940	0.018716
pH x m	-4.5281	0.5406	-8.375	3.13e-05
pH x Conc.	-0.5044	0.5406	-0.933	0.378163b
m x Conc.	-9.8356	0.5406	-18.192	8.57e-08
pH x m x Conc.	-10.7831	0.5406	-19.945	4.16e-08

• ^a
  m = mass of hydrochar (mg), Conc. = concentration of Cr (VI) (ppm), and ^b = p > 0,05.

Figure S1
Pareto diagram for Cr (VI) removal efficiency by adsorption on AWR. Where: m is the mass of hydrochar (mg), and Conc is the concentration of Cr (VI) (mg.L^-1).

ISOTHERMS

Figure S2
Model fit of adsorption isotherm of Cr VI adsorption onto AWR at 25 °C. (Conditions: pH = 2, m = 0.150 mg, and Conc. = 150 mg L⁻¹, 240 min).

Figure S3
Model fit of adsorption isotherm of Cr VI adsorption onto AWR at 45 °C. (Conditions: pH = 2, m = 0.150 mg, and Conc. = 150 mg L⁻¹, 240 min).

THERMODYNAMIC PARAMETERS

Figure S4
Van’t Hoff plot for adsorption of Cr VI on AWR. (Conditions: pH = 2, m = 0.150 mg, and Conc. = 150 mg L⁻¹, 240 min

Table S2
Thermodynamic parameters for the adsorption of Cr (VI) ions onto AWR.

Temp. (K)	ΔG0 (J mol-1)	ΔH0 (kJ mol-1)	ΔS0 (J mol-1)	R2
298.15	-3623.165	115.506402	399.562526	0.9576
308.15	-7618.790
318.15	-11614.416

Edited by

• ASSOCIATE EDITOR:
  André Willerding https://orcid.org/0000-0002-0517-2835

Publication Dates

• Publication in this collection
  04 Aug 2025
• Date of issue
  2025

History

• Received
  26 Sept 2024
• Accepted
  02 May 2025