Phytic Acid-Mediated Hydrothermal Valorization of Woody Biomass Containing Heavy Metals into Functional Hydrochar: Mechanistic Insights and Sustainable Resource Pathways

doi:10.21203/rs.3.rs-7517648/v1

Download PDF

Research Article

Phytic Acid-Mediated Hydrothermal Valorization of Woody Biomass Containing Heavy Metals into Functional Hydrochar: Mechanistic Insights and Sustainable Resource Pathways

https://doi.org/10.21203/rs.3.rs-7517648/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Phytoremediation-derived biomass poses risk of secondary pollution due to the retention of heavy metals (HMs) in conventional hydrothermal carbonization (HTC). In this study, a novel phytic acid (PA)-enhanced HTC system was developed to address the challenge using willow biomass harvested from HMs-contaminated soils. The results showed that PA significantly enhanced biomass decomposition and carbonization, yielding 37.04–54.42% hydrochar and modifying the surface structure of the resulting hydrochar. This process produced energy-dense hydrochar with high heating values ranging from 23.73 to 27.75 MJ/kg, demonstrating promising potential as a biofuel. Importantly, the addition of PA promoted the substantial migration of HMs from the solid phase into the liquid phase, with transfer rates of 99.95% for Cd and 99.65% for Zn at 5 wt% PA. The hydrochars exhibited excellent HMs adsorption capacities (1.57 ± 0.08–6.40 ± 0.47 mg/g for Cd²⁺; 4.80 ± 0.46–15.30 ± 0.06 mg/g for Cu²⁺), with PAHC-40 achieving the highest maximum adsorption capacities (Qₘₐₓ) of 22.82 mg/g for Cd²⁺ and 87.78mg/g for Cu²⁺. These adsorption behaviors fit well with Pseudo-second kinetics and the Langmuir isotherm model. The adsorption mechanisms were governed by synergistic interactions, including surface complexation, cation exchange, and cation–π interactions. Application of PAHCs at a 3% rate significantly improved soil properties, increasing porosity by 16.8% and enhancing cation exchange capacity to 97.61–102.57 cmol/kg in HMs-contaminated soils. Further, the addition of 3% PAHC can promote the transformation of HMs in the soil from active to stable states (Cd: 0.59%, Zn: 0.29%, 10th week). In terms of ecological risk reduction, PAHC is superior to WHC (PAHC: RI = 224.81 vs WHC: 250.44 vs control: 301.48). The PA-HTC system demonstrates potential for closing the remediation cycle by converting hazardous biomass into functional materials for soil HMs immobilization. It shed light on establishing a sustainable circular mode for phytoremediation biomass management, simultaneously enabling energy recovery and environmental decontamination through engineered hydrochar applications.

Dendroremediation

Safe utilization

Bioenergy

Biochar

Phytic acid (PA) facilitates Cd and Zn migration from solid to liquid in hydrothermal systems.
The hydrochar produced at 0.75 wt% PA exhibits optimal cost-effectiveness and safety.
The produced hydrochar meets the Grade I biochar standard at 5 wt% PA.
Phytic acid hydrochar (PAHC) is promising in metals immobilization and biofuel.
The PAHC amendment can reduce the ecological risks of soil metals.

Heavy metals (HMs) have been extensively released into the environment due to increasing anthropogenic activities such as mining, smelting, industrial emissions, agricultural fertilization, and waste disposal [1, 2]. These emissions have led to elevated concentrations of HMs that exceed the environment’s self-purification capacity, posing significant risks to both ecosystems and human health[3]. According to China’s 2014 National Soil Pollution Survey, 16.1% of the nation's soil was found to be contaminated, with HMs pollution accounting for 82.8% of these cases [4].

To address the persistent environmental challenge posed by HMs-characterized by their high toxicity, non-degradability, and bioaccumulation potential - multiple remediation strategies have been developed. Current approaches include chemical immobilization, electrokinetic separation, soil leaching, thermal treatment, and phytoremediation [5–7]. Among these, phytoremediation has emerged as a green and sustainable technology widely adopted for remediating HMs-contaminated soils, due to its low cost, minimal environmental impact, and suitability for in-situ applications. Particularly noteworthy are fast-growing tree species such as willows and poplars, which exhibit rapid growth rates, strong tolerance to poor soils, and high resistance to HMs[8] These woody plants play a crucial role in the ecological restoration of soils affected by HMs pollution and mining activities, owing to their high biomass productivity and substantial capacities for HMs accumulation and translocation [9, 10].

However, an emerging challenge for phytoremediation is the safe treatment and reuse of large amounts of HMs-rich plant biomass. If handled inappropriately, the accumulated HMs may return to the environment and cause secondary pollution. To achieve harmless treatment of plant biomass enriched by HMs, a variety of thermal conversion technologies are applied, including incineration, pyrolysis, gasification, and hydrothermal carbonization (HTC) etc. [11, 12]. Among them, HTC is useful for environmental management because it can process solid waste with high moisture content at low temperatures without drying, producing hydrochar with a good pore structure, oxygen-containing groups and high energy density [13, 14].

Due to uncertainties associated with environmental acceptability, questions remain regarding the reuse of hydrochars derived from the hydrothermal treatment of biomass enriched in HMs. Hence, numerous studies focus on the distribution of HMs and the potential environmental risks associated with the HTC process was conducted. This is largely dependent on factors such as reaction temperature, reaction time, and reaction medium during the HTC process. For example, Zhao et al.[15] demonstrated that HTC of Salix jiangsuensis '172' biomass effectively immobilized HMs in solid phase (Cd: 98.65%; Zn: 58.20%), while only 0.05% in mineral acids (HCl/H₂SO₄/H₃PO₄), which promoted more than 80% Cd/Zn transfer to liquid phase. Their subsequent study revealed temperature-dependent HMs retention, with minimal accumulation (Cd: 0.90–8.98%; Zn: 4.63–8.20%) in hydrochar produced at 180°C under identical acid concentration (0.05%) and temperature range (180–240°C) [16]. Additionally, Zhang et al. [17] applied HCl assisted the HTC of the hyperaccumulator Sedum alfredii, revealing that over 89.07% of Cd and 78.66% of Zn were transferred to the liquid phase; Wilk et al [18] added sulfuric acid as a catalyst during the HTC of sewage sludge and found that the catalyst caused the migration of HMs from solid products to liquid products. Furthermore, adding acid ions in HTC reaction systems has the potential to lower the reaction activation energy, facilitating the HTC reaction process and yielding higher-value hydrochars. Collectively, these findings indicate that constructing acid-assisted HTC systems can efficiently facilitate the separation of HMs from biomass into the liquid phase, offering a promising pathway for the safe valorization of phytoremediation-derived biomass.

Phytic acid (PA), or inositol hexaphosphate, contains 12 exchangeable protons and six phosphate groups. Naturally found in plant seeds, PA exhibits strong acidity and excellent HMs chelation ability, making it a promising agent for material modification [19]. Hu et al [20] demonstrated that the adding PA in the HTC of bamboo sawdust led to proton etching of the biomass, inducing surface granulation and increasing hydrochar surface area. Under conditions of 50 wt% PA and 24 h treatment, the resulting hydrochar achieved exceptional adsorption capacities for Pb(II) (185.9 mg/g) and Cd(II) (128.2 mg/g). Xia et al [21] prepared a PA-modified chitosan-graphene oxide composite (PCG) rich in hydroxyl and phosphate groups via HTC, which adsorbed U(VI) 325.56 mg/g at pH 5. These findings indicate that PA can serve as an effective modifier during the HTC process. However, existing studies have primarily focused on the role of PA in enhancing hydrochar surface properties and adsorption performance, while neglecting its broader effects on the hydrothermal system chemistry, including biomass conversion efficiency and HMs migration behavior during HTC. Furthermore, the optimal dosage of PA, its influence on the structure–function relationship of hydrochar, and its practical applicability for in-situ remediation of contaminated soils remain poorly understood.

Hence, we hypothesize that adding PA to the hydrothermal system can create a new organic acid-based HTC environment. The system promotes proton ionization, enhancing biomass conversion and HMs leaching from the solid phase, while improving the hydrochar’s capacity for HM remediation via phosphate groups on the surface. This approach holds promise for the safe disposal of HMs-containing woody biomass and the production of reusable, functionalized hydrochar. The main objectives are: (1) to investigate the effects of different PA concentrations on the structure and physicochemical properties of hydrochar, as well as the distribution and migration behavior of HMs in hydrothermal products; (2) to evaluate the adsorption performance of PA-modified hydrochar (PAHC) for HMs, as well as its in-situ remediation potential in HMs-contaminated soils and its ecological safety. This study supports the efficient utilization of HMs-containing dendroremediation biomass, contributing to a closed-loop remediation strategy for contaminated soils through the application of fast-growing trees.

2.1 Experimental Materials

The willow biomass (WB) tested in this study originates from one-year-old branches of Salix jiangsuensis “172” cultivated in a greenhouse located in Hangzhou, Zhejiang Province, China. The plants were grown in soil with strictly controlled HMs contamination levels (Cd: 45.86 mg/kg; Zn: 1846.41 mg/kg). Upon collection, the WB was immediately washed with distilled water, then dried at 105°C for 30 minutes, followed by further drying at 60°C for 24 h. After being shredded, ground, and sieved through a 60-mesh sieve, the biomass was sealed and stored for subsequent use.

2.2 HTC Procedure

The HTC of WB followed the procedures as previously reported and described in the supporting information [15]. The obtained products are represented as WHC when using water as hydro medium. The PAHCs were obtained with the same procedures by replaced the distilled water (100 mL) with PA solutions of mass percentages 0.01%wt, 0.25%wt, 0.5%wt, 0.75%wt, 1%wt, 2.5%wt, 5%wt, 10%wt, 20%wt, and 40%wt, respectively. The resulting hydrochar is denoted as "PAHC-mass percentage", where for example, "PAHC-0.01" signifies hydrochar derived from willow biomass with a 0.01%wt mass fraction of PA as the hydrothermal medium.

2.3 Hydrochar Characterization

The yield of hydrochar is defined as the ratio of the obtained solid product to the WB feedstock. Elemental composition of hydrochar is conducted using an elemental analyzer (Elementar UNICUBE, Germany), and the oxygen content is determined through the subtraction method. The higher heating value (HHV) of hydrochar is calculated employing the specific formula known as the Dulong equation [22].

HHV (MJ/kg) = 0.3389×C + 1.422×(H-O/8)-Ash (1)

where the C, H, and O in the equation represent the mass percentages of carbon, hydrogen, and oxygen in both biomass and hydrochar, respectively. Combusting biomass or hydrochar at 800°C for 5 h in a muffle furnace yields a white ash powder, with the mass ratio to the original sample determining ash content.

The properties of WHC and PAHCs were analyzed via composition, texture, morphology, and surface chemistry (see Supporting Information). The concentration and distribution ratio of HMs in the solid and liquid phases are calculated using the Method S1.

2.4 Batch adsorption experiments

The batch adsorption experiments of hydrochar for Cd²⁺ and Cu²⁺ were conducted in polyethylene centrifuge tubes placed in a constant temperature shaker, at 160 rpm/min and 293.15 K. The hydrochar was evenly distributed in deionized water as the adsorbent solution. And prepare a solution containing the specified concentration of adsorbent (Cd²⁺ and Cu²⁺), mixing it with a background solution to achieve the desired concentration for each component. The solution pH was adjusted by using negligible volume of 0.001, 0.01 and 0.1 mol/L HCl and NaOH solutions. After reaching adsorption equilibrium, then promptly perform solid-liquid separation through a 0.45 µm filter membrane. Finally, the concentrations of Cd and Cu in the aqueous were determined by ICP-MS. The details for the adsorption capacity of the hydrochars are shown in Method S2. All experimental data are the average of three replications, and the relative error of the data is less than 5%.

Under the above conditions, adjust the pH of the adsorption solution (2–10). For the adsorption isotherm experiment, set the initial concentration to be 0.1–80 mg/L Cd²⁺ and 0.5–200 mg/L Cu²⁺. For the adsorption kinetics experiment, set the Cd²⁺ concentration to 20 mg/L and the Cu²⁺ concentration to 40 mg/L, and take samples at different time intervals (0.1–24 h). The adsorption amount of the hydrochars was calculated from the Cd²⁺ and Cu²⁺ concentration differences between the initial solution and the equilibrium solution. The adsorption isotherm and adsorption kinetics calculation formula are shown in Methods S2 and Method S3. All adsorption experiments were performed in triplicate.

2.5 Soil experiments and safety risk assessment

PAHC-0.75 was selected as the remediation material for HMs contaminated soil due to its low production cost, excellent adsorption amount and low environmental safety risk. WHC is the control group to observe the effects of HMs on soil and the release of HMs. Using a modified BCR sequential extraction method [23], the bioavailability content of HMs in soil was determined. This method categorizes HMs in soil into four forms: Acid-soluble (AS), Reducible (OX), Oxidizable (OM) and Residual (RES). The potential ecological risk index method is employed to evaluate the ecological risk index of the soil before and after remediation [24]. The specific testing procedures were conducted according to the guidelines outlined in Method S4, Method S5 and Method S6.

2.6 Statistical analysis

The figures are generated using Origin Pro 2022. The data is processed through the "Descriptive Statistics" option under the "Statistics" tab to obtain mean and variance for error bar plotting. The "Differentiation" option under the "Mathematics" tab is employed for obtaining DTG from thermal analysis data. The XPS data for C1S, O1S, and P2P peaks are processed using XPSPEAK4.1 for peak deconvolution.

3.1 Hydrochar production and distribution of HMs during HTC process

3.1.1 Hydrochar yield

As shown in Table 1, the yield of hydrochar produced in the water medium was 49.64%, while the introduction of PA resulted in a yield ranging from 37.04% (PAHC-10) to 54.42% (PAHC-40). Increasing the PA dosage from 0.01 wt% to 10 wt% significantly decreased the hydrochar yield from 50.23–37.04%. However, further increasing the dosage to 20 wt% and 40 wt% boosted the yield up to 54.42%, the highest observed. The variation can be attributed to the strong ionization capacity of PA, which releases H⁺ ions that accelerate the HTC process. This promotes biomass carbonization and dehydration, while enhancing organic matter breakdown [25, 26], thereby intensifying reactions but reducing solid-phase retention. At higher PA dosages, excess PA also acts as an additional organic substrate, thus increasing hydrochar yield significantly [19]. Furthermore, the ash content of hydrochars declined from 4.87–0.29% (Table 1), which can be explained by the enhanced dissolution of inorganic minerals into the acidified liquid phase [17, 27].

Table 1
The yield of hydrochars and their physicochemical properties
Samples	Yield	N	C	H	S	O	H/C	O/C	Ash	HHV	Total energy
Samples	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(MJ/kg)	MJ
WB	--	0.92	44.21	6.05	0.61	42.50	1.64	0.72	5.71	15.99	159.90
WHC	49.64	0.96	57.93	5.75	0.12	31.39	1.19	0.41	3.86	22.17	110.05
PAHC-0.01	50.23	0.98	60.99	5.68	0.09	27.87	1.12	0.34	4.87	23.73	119.20
PAHC-0.25	48.43	1.08	63.46	5.71	0.10	26.01	1.08	0.31	3.09	24.94	120.78
PAHC-0.5	41.66	1.15	66.33	5.27	0.11	26.17	0.95	0.30	0.87	25.25	105.19
PAHC-0.75	40.20	1.19	67.52	5.39	0.14	25.06	0.96	0.28	0.71	26.03	104.64
PAHC-1	40.17	1.13	69.32	5.65	0.11	22.89	0.98	0.25	0.43	27.40	110.07
PAHC-2.5	40.04	1.14	69.03	5.66	0.10	22.95	0.98	0.25	0.20	27.30	109.31
PAHC-5	38.20	0.84	69.08	5.75	0.11	22.71	1.00	0.25	0.66	27.48	104.97
PAHC-10	37.04	0.76	69.48	5.76	0.11	22.06	0.99	0.24	0.49	27.75	102.79
PAHC-20	38.80	0.65	69.44	5.67	0.11	22.91	0.98	0.25	0.23	27.47	106.58
PAHC-40	54.42	0.60	69.4	5.16	0.14	24.14	0.89	0.26	0.29	26.50	144.21
^a O%= 100% – (C + H + N + S) %

^b Higher heating value.

3.1.2 Distribution of HMs during the HTC process

The migration and distribution patterns of HMs contained in WB are extremely important for the safety risk assessment of hydrochar. As shown in Fig. 1, 98.65% of Cd and 41.80% of Zn were retained in the solid-phase WHC under water medium condition. However, with the addition of PA to the hydrothermal system, most Cd and Zn were distributed into the liquid phase, and this migration intensified with increasing PA dosage. At a PA concentration of 0.75 wt%, 99.61% of Cd and 92.45% of Zn were removed from the solid phase, and when the concentration increased to 5 wt%, only 0.05% of Cd and 0.35% of Zn remained in the hydrochar, suggesting a near-complete transfer of these metals to the liquid phase. This effect results from the acidification of the solution by proton ionization from PA, which promotes both the dissociation of HMs from solid substrates and the hydrolysis of biomass, generating organic acids [28]. These organic acids further form soluble complexes with HMs, facilitating their migration into the liquid phase [17]. It was observed that 0.75 wt% PA can dissolve more than 99% of Cd from the solid phase, and 5 wt% PA can make Zn reach the same level (Fig. 1). It is due to the relatively high initial content of than that of Cd in WB which need more PA to release Zn. Additionally, Zn often combines with structural components such as cell walls to form a relatively stable complex or deposition form, which also inhibits its migration. The studies show most Cd accumulates in plant cells as organic acid complexes and is unstable [29]). Therefore, under the catalysis of PA, Cd is more easily transferred to the liquid phase than Zn [30].

Figure S1 shows the concentrations of HMs in WB and the resulting WHC. The initial contents of Cd and Zn in WB were 71.89 mg/kg and 736.26 mg/kg, respectively, indicating a high ecological risk. After HTC, the concentrations of Cd and Zn in WHC increased to 129.65 mg/kg and 798.34 mg/kg, respectively. This increase is mainly attributed to the low volatility and high thermal stability of most HMs under HTC conditions, which retain them in the solid phase. The decomposition of organic matter (primarily hemicellulose and cellulose) during HTC results in a carbon concentration effect, further elevating the HMs content. Thus, although HTC effectively reduces biomass volume and may stabilize certain metal species, the elevated concentrations of HMs in WHC still pose potential ecological risks. However, the addition of PA led to a reduction in Cd and Zn concentrations in all PAHCs, except PAHC-0.01. According to the NY/T4159-2022 standard, the use of biochar in agriculture is categorized as Level I or II, based on the degree of carbonization and pollutant content. As shown in Table S1, when the PA concentration exceeds 0.5 wt%, the resulting PAHC meets the Grade II biochar standard, allowing its use in fields or as fertilizer under compliance requirements. PAHC produced at 5 wt% PA meets the Grade I standard, enabling its application directly in agriculture. In summary, the addition of PA during HTC of HMs-contaminated woody biomass facilitates metal migration, reduces the metal content in hydrochars, and mitigates associated environmental risks.

3.2 Hydrochars Structure and surface properties

3.2.1 Hydrochars composition

Figure 2(a) shows the digital photographs of WB and hydrochars. With increasing PA concentration, the color of PAHC changes from brown to black, accompanied by an increase in carbon content from 44.21% (RM) to 57.93% (WHC) and 69.48% (PAHC-40). (Table 1). These changes in color and carbon content are attributed to the catalytic effect of PA on the hydrothermal polymerization and carbonization processes of cellulose, hemicellulose, and lignin, thereby enhancing the material's aromaticity and carbon retention [31]. When the PA concentration exceeds 10 wt%, the carbon content stabilizes at approximately 69.4%, indicating that the reaction has reached equilibrium and a stable carbon structure has formed [32].

The O content in PAHC initially decreases (from 27.87–22.06%) and then slightly increases (to 24.14%) (Table 1). At low levels of PA ranged from 0.1 to 10 wt%, protons (H⁺) facilitate decarboxylation and dehydration reactions, thereby reducing oxygen content [33]. At higher PA levels (20–40 wt%), more phosphate groups may be incorporated into the solid phase, accounting for the slight increase in oxygen content. The nitrogen content exhibits an initial increase followed by a decrease, which is likely due to the Maillard reaction under hydrothermal conditions that produces nitrogenous heterocycles retained in the solid phase. However, excessive PA can promote the transformation of organic nitrogen into soluble NH₄⁺-N, thereby reducing nitrogen retention in PAHC [34, 35]. The H/C and O/C ratios (Table 1) decrease with increasing PA content, indicating a higher degree of carbonization, increased aromaticity, and improved structural stability [36]. The Van Krevelen diagram (Fig. 2(b)) further confirms that PA facilitates both dehydration and decarboxylation, which are key pathways in HTC [37]. At 40 wt% PA, a shift in the reaction mechanism is observed, potentially due to the abundance of phosphate groups, leading to a relative increase in oxygen content [38].

3.2.2 Surface properties

The crystalline and chemical structures of PAHC were analyzed using XRD, FTIR, and XPS. As shown in Fig. 2c, the XRD pattern of WHC exhibits distinct diffraction peaks at 15°, 22.4°, and 24.4°, corresponding to cellulose I, cellulose II, and calcium carbonate, respectively [25]. With increasing PA concentration, the intensities of these characteristic cellulose peaks progressively diminish and eventually vanish at 0.5–0.75 wt% PA, indicating the destruction of the crystalline cellulose structure. Simultaneously, broad peaks emerge in the 19°–21° range, indicative of amorphous carbon formation, likely due to the isomerization, depolymerization, and polymerization of cellulose-derived monosaccharides into carbonaceous clusters [38]. Additionally, the 24.4° peak corresponding to calcium carbonate disappears, and new peaks at 28°, 30°, and 34° emerge in PAHC-0.01 and PAHC-0.25, which are identified as calcium phosphate salts (ICSD 98-041-0782) using High-Score Plus software. However, further increases in PA result in the disappearance of calcium phosphate peaks, which can be attributed to their dissolution by excess protons, consistent with SEM observations. Notably, at 0.01 wt% PA, PAHC's ash content briefly surpassed WHC's (Table 2), potentially from calcium phosphate precipitation between Ca²⁺ and phosphate ions [19, 39]. Beyond 0.25 wt%, excessive proton release leads to further phosphate salt dissolution and reduced ash content.

Table 2
The specific surface area and pore structure of WB and hydrochars in different media
Samples	SSA	Total pore volume	Mean pore diameter
Samples	[m²/g]	[cm³/g]	[nm]
WB	2.78	0.01	10.38
WHC	8.65	0.03	13.23
PAHC-0.01	7.12	0.04	19.70
PAHC-0.25	8.86	0.05	23.63
PAHC-0.5	9.94	0.08	33.86
PAHC-0.75	9.79	0.09	35.90
PAHC-1	11.12	0.09	30.68
PAHC-2.5	11.31	0.09	30.91
PAHC-5	12.14	0.11	36.28
PAHC-10	10.18	0.09	35.21
PAHC-20	13.53	0.08	24.00
PAHC-40	13.59	0.07	21.85

XPS analysis (Fig. S5-S7) reveals that the surface elemental composition of PAHC varies with PA dosage: C (73.3–81.92%), O (0.65–1.09%), and P (17.11–22.72%). Increasing PA enhances the C1s peak while decreasing O1s, reflecting greater dehydration, decarboxylation, and carbonization. Deconvolution of the C1s spectrum shows variations in C–C (284.8 eV), C–O (286.0 eV), and C = O (289.0 eV) [40]. Initially, C–C increases and then declines, while C–O and C = O exhibit inverse trend, attributed to hydroxyl group removal and polymerization-induced double bond formation. Phosphorus content shows a non-linear trend: initially present (0.16%) in PAHC-0.01 due to intrinsic phytate phosphorus, then decreasing as PA hydrolyzes it to inositol and orthophosphoric acid [41], and finally rising again at higher PA dosages (e.g., PAHC-40 at 0.2%) due to phosphate adsorption and incorporation [42, 43]. This observation verifies phosphorus' dual transformation pathway: biomass-derived release coupled with PA-mediated phosphate re-association.

FTIR analysis (Fig. 2d) further elucidated the evolution of surface functional groups. Notable absorption bands at 3343 and 3372 cm⁻¹ (–OH stretching), and at 2853, 2922, and 1456 cm⁻¹ (–CH₂ stretching) were observed [44, 45], whose intensities diminished with increasing PA concentration. This phenomenon indicates the degradation of hydroxyl groups and aliphatic chains during HTC, facilitating the formation of aromatic structures [46, 47]. Aromaticity was supported by increased intensities at 1513 cm⁻¹ and 1610 cm⁻¹, corresponding to C = O/C = C bonds, due to enhanced H⁺-catalyzed aromatization [20, 48]. Simultaneously, the C–O stretching peak at 1032 cm⁻¹ weakened, suggesting ketone–enol tautomerism and molecular rearrangement [49]. Rising PA concentrations diminish P = O signals from phosphate bonds while enhancing phosphate group signals, indicating dual mechanisms: (1) PA-catalyzed biomass decomposition releases intrinsic phosphates into the liquid phase, and (2) hydrothermal incorporation of phosphate groups onto PAHC surfaces at elevated PA levels. In addition, XRD shows that phosphate groups in the liquid phase may produce metal precipitation, which increases P = O of the phosphate group [50, 51].

3.2.3 Structure properties

As shown in Fig. S2, the raw material primarily consists of elongated strips and rough particles, with embedded white material identified as calcium carbonate [15]. Figure 2(e–n) demonstrate that PA catalysis induces significant fragmentation of the hydrochar, forming deep ravines and large pores as a result of calcium carbonate dissolution, thereby increasing surface roughness. Carbon microspheres and clusters are also observed, with both their size and abundance increasing at higher PA concentrations. These features are attributed to the acid-catalyzed hydrolysis of biomass components into oligosaccharides, hexoses, and lignin fragments, followed by subsequent dehydration and polymerization reactions [26, 52].

Such surface morphological transformations directly influence adsorption performance, as verified by N₂ adsorption–desorption analysis. As shown in Table 2, PAHC-40 exhibits the highest specific surface area (SSA_BET) of 13.59 m²/g, representing a 90.87% increase compared to PAHC-0.01. The enhancement is attributed to ash removal, carbon microsphere formation, and biomass decomposition [20]. However, the total pore volume and average pore diameter initially increase and then decrease, likely due to microsphere aggregation and pore blockage under high PA loading, as shown in Fig. 2(e–n) [31, 38]. BJH pore size analysis (Fig. S3–S4) shows H₃-type hysteresis loops, indicating dominant microporosity [53]. As PA increases, smaller pores (< 10 nm) diminish, and the dominant pore size shifts from mesopores (16.97–42.02 nm at 0.25–2.5 wt%) to macropores (52.64–55.29 nm at 5–20 wt%). Notably, PAHC-0.01 has a peak diameter of 56.51 nm due to calcium carbonate dissolution, while PAHC-40’s 31.33 nm peak suggests pore blockage by carbon microspheres and the presence of additional micropores during the averaging calculation process. Thus, moderate PA (0.5–10 wt%) promotes HTC, enhancing porosity and SSA, whereas excessive PA (20–40 wt%) leads to pore obstruction and structural collapse due to over-catalysis [20].

3.2.4 Thermal stability and fuel traits

TG-DTG analysis revealed three distinct combustion stages for WB and the hydrochars obtained at different PA concentrations: (1) moisture evaporation, (2) volatile combustion associated with cellulose and hemicellulose degradation, and (3) oxidation of fixed carbon [54]. As illustrated in Fig. S8, WB exhibits weight loss peaks at 77°C, 330°C, and 460°C, corresponding to the evaporation of moisture, the decomposition of cellulose and hemicellulose, and the formation of stable carbonaceous structures, respectively. Following HTC, the thermal behavior of PAHC underwent substantial alterations. Dehydration and carbonization reactions during HTC, facilitated by the catalytic role of PA, reduced the moisture content in PAHC, resulting in lower initial-stage weight loss. In the second stage, sharper peaks were observed for PAHC-0.01 and PAHC-0.25, indicating a notable reduction in cellulose and hemicellulose components. At higher PA concentrations (PAHC-0.5 and PAHC-0.75), the peaks in the second combustion stage became broader and more complex, shifting from 320°C to 380°C, suggesting the generation of new intermediate structures and improved thermal resistance. These trends were corroborated by elemental analysis, XRD, and FTIR results, which demonstrate progressive aromatization with increasing PA concentration [54, 55].

Such structural transformation not only enhances the thermal stability of PAHC but also significantly improves its fuel characteristics. It has been confirmed that increasing PA concentration facilitates the transfer of most HMs into the liquid phase, thereby rendering PAHC produced under PA catalysis a promising candidate for high-quality, clean fuel. The calorific value of PAHC increases from 22.17 MJ/kg to 27.47 MJ/kg with rising PA concentrations (Table 1), comparable to that of lignite (23.95 MJ/kg) [56]. Additionally, the sulfur content of PAHC ranges from 0.09–0.14% (Table 1), qualifying it as an S-L1–L3 grade clean fuel (Std = 0–0.75%) according to the "Classification of Coal Product Variety" standard in China (GB17608-2022) [57].

3.3 Adsorption capacity of hydrochars to solution HMs

3.3.1 Batch adsorption experiments

To evaluate the environmental applicability of hydrochar, a series of batch adsorption experiments were performed to assess the HMs removal capacities of WHC and PAHC. As shown in Fig. 3(a–b), PAHC exhibited superior adsorption performance for both Cd²⁺ and Cu²⁺ among those biochars. Specifically, as the PA level increased from 0.01 wt% to 40 wt%, the adsorption amount of Cd²⁺ and Cu²⁺ by PAHC ranged from 1.88 ± 0.05 mg/g to 6.40 ± 0.47 mg/g and 5.55 ± 0.31 mg/g to 15.30 ± 0.06 mg/g, respectively, representing 1.20–4.08 times and 1.16–3.18 times higher adsorption than WHC. This is mainly attributed to the more abundant surface functional groups (e.g., C, O, and P groups) in PAHC, which play a dominant role in the adsorption of HMs. In addition, the relatively high SSA of PAHC may enhance the overall adsorption capacity by providing additional contact sites. Interestingly, as the PA dosage increased, the adsorption of Cd²⁺ and Cu²⁺ on PAHC initially decreased and then increased, a trend that did not correspond directly with the observed variations in pore structure, mineral composition, and active functional groups of PAHC. Previous studies have indicated that at low PA concentrations (0.01–0.25 wt%), phosphate groups derived from biomass can chelate with HMs to form polymeric precipitates through multi-element interactions, thereby enhancing adsorption [26, 48]. However, as the PA concentration rises to 0.5–1 wt%, the increased concentration of ionized protons may inhibit the formation of HMs precipitates [17] Additionally, H⁺ ions may accelerate the dissolution of inorganic salts in the biomass, thereby reducing the ion exchange capacity of the hydrochar [58]. Nevertheless, when the PA addition increases to 2.5–40 wt%, the significantly enlarged SSA and abundant phosphate functional groups of PAHC become the dominant factors contributing to HMs adsorption [21, 59].

3.3.2 The effect of solution pH on adsorption

The pH significantly affects both the adsorbent's surface charge and the speciation of metal ions in solution. As illustrated in Fig. 3(c–d), the adsorption capacities of Cd²⁺ and Cu²⁺ onto hydrochars gradually increase as the pH rises from 2.0 to 5.0. During this range, WHC exhibits a higher adsorption capacity than PAHC. As the pH increases from 5.0 to 7.0, the adsorption capacities for both metals rise more sharply, with PAHC outperforming WHC. At pH ≥ 8, adsorption reaches equilibrium. This trend corresponds to changes in the zero point of charge (ZPC) of PAHC (Fig. S9). Increasing the amount of PA during hydrochar preparation shifts the ZPC to the left, resulting in a more negatively charged surface. The greater surface negativity enhances electrostatic attraction between PAHC and the positively charged Cd²⁺ and Cu²⁺, thereby increasing metal adsorption [59, 60]. As shown in Fig. S10, the speciation of Cd(II) and Cu(II) in aqueous solution was simulated and calculated using Visual MINTEQ 3.1 software [61]. At pH < 5, the dominant species are Cd²⁺ and Cu²⁺. Under these conditions, protonation of hydrochar surfaces leads to positively charged sites, which generate electrostatic repulsion with metal cations, thereby inhibiting adsorption. Between pH 5 and 7, the deprotonation of surface functional groups enhances their complexation capabilities, improving adsorption efficiency. At higher pH, metal ions begin to form insoluble hydroxide precipitates (e.g., Cd(OH)₂ and Cu(OH)₂), which are removed from the aqueous phase via precipitation rather than by adsorption [62, 63]. Notably, PAHC-40 exhibits enhanced adsorption even at low pH due to its lower ZPC and increased deprotonation capacity, which intensify the surface's negative charge and thus strengthen the electrostatic attraction to metal ions.

3.3.3 Adsorption isotherm and kinetics

Figure 4(a–b) shows that the adsorption isotherms of Cd²⁺ and Cu²⁺ on hydrochars exhibit typical L-shaped curves, indicating favorable and homogeneous adsorption processes [64, 65]. The Langmuir and Freundlich isotherm models were applied to analyze the adsorption data, with corresponding curves and fitting parameters presented in Fig. 4(a–b) and Table 3. For Cd²⁺ adsorption onto WHC, PAHC-0.75, PAHC-5, PAHC-20, and PAHC-40, the correlation coefficients (R²) for the Langmuir model are 0.9501, 0.9642, 0.9672, 0.9672, and 0.9715, respectively, all higher than those of the Freundlich model (Table 3). The results suggest that Cd²⁺ adsorption by hydrochars follows a monolayer adsorption mechanism, dominated by chemisorption [66, 67]. In contrast, the adsorption isotherm data for Cu²⁺ on PAHCs (excluding WHC) fit well with both the Langmuir and Freundlich models, suggesting a combination of chemical and physical adsorption mechanisms. The WHC, with relatively undeveloped pore structure and lower SSA compared to PAHCs, relies more on chemical adsorption processes, such as complexation and ion exchange, which enhance HMs affinity despite limited SSA [15]. Furthermore, Langmuir model fitting reveals that PAHC-40 exhibits the highest adsorption capacities for Cd²⁺ and Cu²⁺, with Qₘₐₓ values of 13.59 mg/g and 63.59 mg/g, respectively.

Table 3
The isotherm parameters of Cd²⁺ and Cu²⁺ adsorption by hydrochars
Samples	HMs	Langmuir model parameters			Freundlich model parameters
Samples	HMs	K_L	q_max	R²	K_F	n	R²
WHC	Cd²⁺	0.139	6.30	0.9501	1.41	0.34	0.8100
WHC	Cu²⁺	0.026	19.13	0.9509	1.02	0.58	0.9011
PAHC-0.75	Cd²⁺	0.070	10.84	0.9642	1.56	0.41	0.8705
PAHC-0.75	Cu²⁺	0.028	23.88	0.9679	1.40	0.58	0.9676
PAHC-5	Cd²⁺	0.059	12.67	0.9672	1.57	0.44	0.8870
PAHC-5	Cu²⁺	0.024	27.72	0.9850	1.36	0.60	0.9750
PAHC-20	Cd²⁺	0.059	12.67	0.9672	1.57	0.44	0.8870
PAHC-20	Cu²⁺	0.012	57.13	0.9914	1.09	0.75	0.9817
PAHC-40	Cd²⁺	0.058	13.59	0.9715	1.65	0.44	0.8890
PAHC-40	Cu²⁺	0.140	63.59	0.9936	1.57	0.72	0.9892

Adsorption kinetics were further investigated to elucidate the adsorption mechanisms, reaction pathways, and rates [7]. The adsorption behavior of Cd²⁺ and Cu²⁺ was modeled using Pseudo -first-order and Pseudo-second kinetic equations, with fitting curves shown in Fig. 4(c–f) and parameters listed in Table S2. As depicted in Fig. 4(c–f), WHC and PAHC both exhibit rapid initial adsorption of Cd²⁺ and Cu²⁺, followed by a gradual approach to equilibrium. The rapid uptake at the early stage is attributed to the abundant active sites on PAHC and the high initial concentration of metal ions, which promote fast surface binding. In the later stages, the adsorption rate slows down, likely due to intraparticle diffusion limitations [68]. The Cd²⁺ and Cu²⁺ reach equilibrium on WHC after 800 min and 240 min, respectively, while PAHC achieves equilibrium much faster—within 240 min for Cd²⁺ and 60 min for Cu²⁺. This acceleration is mainly due to the presence of abundant functional groups on PAHC, which show a strong affinity for HMs and enable faster complexation [20]. The Pseudo-second kinetic model provides the best fit for both metals (R² >0.99, Table S2), indicating that chemisorption involving electron sharing or exchange is the rate-limiting step [61]. Nonetheless, the Pseudo -first-order model also fits reasonably well (R² = 0.929–0.995), implying that physical adsorption mechanisms such as pore-filling and electrostatic interactions also contribute to the overall adsorption process [3]. The Pseudo-second kinetic model (R²>0.99, Table S2) better fitted HM adsorption data than the first-order model, suggesting rate-limiting chemical adsorption involving electron exchange [61]. The adsorption of Cd²⁺ and Cu²⁺ on hydrochars is mainly controlled by chemical adsorption.

3.3.4 Adsorption mechanisms

As shown in Fig. S11, SEM-mapping images demonstrate that, after adsorption, Cd²⁺ and Cu²⁺ are uniformly distributed across the hydrochar surface, as indicated by the red mapping regions. The distribution intensity follows the order: PAHC-40 > PAHC-0.75 > WHC, indicating a significantly enhanced adsorption capacity in PA-modified hydrochars.

FTIR spectra further support the involvement of functional groups in the adsorption process (Fig. 5a–b). After adsorption, the broad hydroxyl band shifts to 3201–3346 cm⁻¹, suggesting interactions with metal ions. Additional shifts and reductions in intensity were observed for aromatic C = C (1610 cm⁻¹), C = O (1703 cm⁻¹), and –COO (1459 cm⁻¹) groups, implying that complexation occurs between the metal ions and surface functional groups, including hydroxyl, carboxyl, and π-conjugated aromatic systems. Notably, PAHC-40 exhibits pronounced changes in phosphate-related peaks (1215–1283 cm⁻¹), suggesting that P = O and P–O–C bonds play a crucial role in Cd²⁺ and Cu²⁺ binding [69]. Additional peak shifts at 800 and 858 cm⁻¹ (P–OH and P–O–C) further support the role of phosphate groups in HMs removal [26].

XRD analysis (Fig. 5c–d) revealed no distinct crystalline peaks corresponding to Cd²⁺ or Cu²⁺ precipitates, suggesting that HMs removal was primarily driven by chemical adsorption and ion exchange rather than precipitation. A slight shift and reduced intensity at 22.3° after adsorption indicate changes in the hydrochar microstructure, likely due to interactions between the hydrochar matrix and metal ions. XPS analysis (Fig. 6) provides additional confirmation of successful metal adsorption. Characteristic peaks of Cd 3d (405.10 eV) and Cu 2p (935.24 eV) were detected in PAHC-0.75 and PAHC-40 [70], confirming the presence of Cd²⁺ and Cu²⁺ on the hydrochar surface. High-resolution C 1s and O 1s spectra revealed changes in the proportions of C = O, C–O, and O–C/O–P bonds after adsorption. In PAHC-40, for example, the peak area of O–C/O–P bonds decreased by 7.32–15.54%, suggesting the participation of these oxygen-containing groups in metal binding [43].

Collectively, the primary adsorption mechanisms for HMs on hydrochar include physical adsorption, ion exchange, and chemical complexation etc. (Fig. 7). Additionally, phosphate groups in PAHC-40 may facilitate co-precipitation with HMs, further enhancing removal efficiency.

3.4. Hydrochars capacity in HMs immobilization in soil

3.4.1 The physio-chemical properties of soil

The hydrochar at application rates of 1–3% significantly reduced soil bulk density (Fig S12b). Notably, PAHC-treated soils exhibited slightly lower bulk densities (0.79–0.85 g·cm⁻³) compared to WHC-treated soils (0.82–0.85 g·cm⁻³). The result is likely due to the well-developed pore structure of PAHC, which enhances soil looseness and reduces compaction. Correspondingly, soil porosity was notably improved with the addition of PAHC, especially at the 3% level, where porosity increased to 68.03–70.10% (Fig. S12(c)). The higher porosity observed in PAHC-amended soils is mainly attributed to the enhanced internal pore structure of PAHC, which improves soil aeration and water permeability. The cation exchange capacity (CEC) decreased slightly with increasing WHC dosage (95.70–100.28 cmol·kg⁻¹), whereas it increased significantly with PAHC (97.61–102.57 cmol·kg⁻¹) (Fig. S12(d)). It is likely due to the higher abundance of oxygen-containing functional groups (e.g., carboxyl and phenolic groups), larger SSA and porous structure in PAHC, which facilitate cation binding or ion exchange compared with WHC.

3.4.2 The chemical speciation of HMs in soil

Hydrochars favorable pore structure and high oxygen concentration enable effective adsorption and immobilization of HMs [43]. As shown in Fig. 8(a) and (c), the percentage of Cd and Zn in CK soil increased during the first week of the WHC-1% and PAHC-1% treatments. The Cd proportion rose in weak AS and OX, from 80.8–82.92% and 81.17%, while the Zn proportion rose from 30.25–34.07% and 31.90%, respectively. The result may be attributed to the release of enriched Cd and Zn from WHC and PAHC-0.75 into the soil, which was not effectively stabilized by lower hydrochar addition. The WHC-3% and PAHC-3% reduced the proportions of AS and OX Cd and Zn in the soil, where Cd were reduced to 78.93% and 78.13%, Zn 28.22% and 27.04%, respectively. The findings suggest that an increase the hydrochar addition results in more effective reduction of the AS and OX of Cd and Zn in soil, and the PAHC has greater stabilization ability than WHC due to its bigger SSA and more oxygen/phosphorus -containing groups.

Figure 8(b) and (d) reveal that by 10th week, the AS fractions of Cd and Zn in CK displayed a significant increase, whereas soils amended with 3% WHC and 3% PAHC showed reductions of 21.24–21.84% and 7.62–8.14%, respectively. Concurrently, the RES fractions of Cd and Zn rose to 10.72–11.80% and 63.91–67.29%, respectively. This observation suggests that hydrochar effectively facilitates the transformation of HMs from labile, weakly acid-soluble/exchangeable forms to stabilized oxidizable and residual phases with prolonged cultivation duration.

3.4.3 Bioavailability of HMs

Figure 8(e–f) illustrates the impact of WHC and PAHC on the bioavailable concentrations of Cd and Zn in soil. In CK soil, the bioavailable contents of Cd and Zn remained relatively stable during the 10-week period, indicating only minor fluctuations without significant changes. In contrast, bioavailable Cd and Zn in WHC-amended soil showed an initial decline before rebounding during the experimental period. By the 10th week, the bioavailable contents of Cd and Zn had increased to 18.34–18.42 mg/kg and 127.58–127.82 mg/kg, respectively. This can be attributed to the initial immobilization of Cd and Zn on the surface of the WHC, which later dissolved and released into the soil. Additionally, PAHC-amended soil exhibited a sustained reduction in Cd/Zn bioavailability throughout cultivation, indicating superior HM stabilization compared to WHC.

3.5 Risk assessment of hydrochars in soil remediation

The potential ecological risk index method was used to evaluate the ecological and environmental effects of HMs in soil following the addition of hydrochar [71]. As shown in Table 4, the concentrations of Cd and Zn in the E_r from all soil types fell below the standard minimum values (≤ 40) after 10th weeks of soil cultivation, indicating the presence of a minor environmental hazard. The C_f index indicates that the overall level of Cd contamination in CK soil is classified as moderate pollution, with a significant ecological risk. The addition of WHC and PAHC resulted in a decrease in the C_f of Cd in the contaminated soil to a range of 7.48–8.33. As demonstrated in Table S4, apart from the 3% WHC sample, the remaining soil samples exhibited indications of mild contamination. The results manifested that the incorporation of hydrochar can effectively mitigate the pollution levels in HMs soil. After adding hydrochar, the E_r of Cd in the soil diminished from 300.86 to a range of 224.28–249.87, indicative of a 16.95–25.45% reduction. In particular, adding PAHC to soil has been demonstrated to be an effective method for reducing the E_r in contaminated soil. This phenomenon is attributed to the rich pore structure of PAHC and the highly immobilization efficiency of HMs by the oxygen/phosphate-containing groups. The WHC-3% has a higher Er than the WHC-1% due to the release of HMs during soil remediation. The E_r range for all soils is between 160 and 320, which is still a significant ecological hazard. The RI shows significant ecological risk in CK soil, 224.81–250.44 in hydrochar-amended soil, indicating moderate risk. Therefore, hydrochar can effectively reduce the ecological pollution index in HMs-contaminated soil, especially assisting with PAHC is an effective strategy for mitigating potential ecological risks.

Table 4
Ecological risk index of soil amended with different hydrochar
Ecological risk index	CK	WHC-1%	WHC-3%	PAHC-1%	PAHC-3%
C_{f (}Cd)	10.03	7.87	8.33	7.48	7.48
E_r(Cd)	300.86	236.18	249.87	224.45	224.28
C_f(Zn)	0.62	0.52	0.56	0.49	0.52
E_r(Zn)	0.62	0.52	0.56	0.49	0.52
RI	301.48	236.70	250.44	224.94	224.81

This study developed a novel hydrothermal system by introducing PA into the treatment of HMs-contaminated WB. The addition of PA significantly enhanced the HTC process, promoting carbon fixation, pore development, and the formation of functional groups and aromatic structures in the resulting PAHC. The PAHC exhibited a HHV of 23.73–27.35 MJ/kg, comparable to lignite, and superior adsorption amount compared to conventional WHC. The addition of PA improved the HMs adsorption capacities by hydrochars, with PAHC-40 achieving the highest maximum adsorption capacities (Qₘₐₓ) of 22.82 mg/g for Cd²⁺ and 87.78mg/g for Cu²⁺. The adsorption mechanism was dominated by physical adsorption, ion exchange, and surface complexation. The PAHC improved soil physical properties and outperformed WHC in stabilizing Cd/Zn (reducing acid-soluble fractions, increasing residuals) and lowering ecological risks. Taken together, integrating PA into the HTC process offers a sustainable strategy for converting dendroremediation biomass into multifunctional hydrochars with excellent properties for use as a biofuel, adsorbent, and soil amendment. This approach provides a promising pathway for the resourceful and safe utilization of contaminated biomass in ecosystems.

Willow biomass

Hydrothermal carbonization

HTC

Heavy metals

HMs

Phytic acid

Hydrochar (water as the hydrothermal medium)

WHC

Hydrochar (phytic acid as the hydrothermal medium)

PAHC

Higher heating value

HHV

Brunauer-Emmett-Teller

BET

Specific surface area

SSA

Barrett-Joyner-Halenda

BJH

Humic acid

CRediT authorship contribution statement

Bo Zhao: Investigation, Writing-original draft, Writing - review& editing; Haihua Li: Writing - review & editing; Yan Chen: Investigation, Xu Gai: Writing - review& editing; Xiaoli Yang: Writing - review & editing; Dongliu Di: Investigation, Jiang Xiao: Conceptualization, Investigation, Writing - review& editing, Funding acquisition; Guangcai Chen: Methodology, Supervision, Writing - review & editing, Funding acquisition. All authors read and approved the final manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

(1) Fundamental Research Funds of CAF (CAFYBB2022SY012)

(2) Zhejiang Provincial Department of Science and Technology (2023SDXHDX0006).

Author Contribution

B.Z., Y.C., and D.D. conducted the investigation. J.X. contributed to conceptualization, investigation, writing—review & editing, and funding acquisition. B.Z. prepared the original draft, while H.L., X.G., X.Y., J.X., and G.C. revised and edited the manuscript. G.C. was responsible for methodology, supervision, writing—review & editing, and funding acquisition. All authors read and approved the final manuscript.

Acknowledgements

This work was supported by the Fundamental Research Funds of CAF (CAFYBB2022SY012) and Zhejiang Provincial Department of Science and Technology (2023SDXHDX0006).

Data availability

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

Li X, Cao Y, Xiao J, Salam MMA, Chen G (2022) Bamboo biochar greater enhanced Cd/Zn accumulation in Salix psammophila under non-flooded soil compared with flooded. Biochar 4(1),7. https://doiorg/101007/s42773-022-00139-0
Xiao J, Li X, Cao Y, Chen G (2023) Does micro/nano biochar always good to phytoremediation? A case study from multiple metals contaminated acidic soil using Salix jiangsuensis '172'. Carbon Research 2(21). https://doiorg/101007/s44246-023-00053-5
Di D, Xiao J, Zhao B, Chen Y, Song Z, Chen G (2025) Immobilization of Cd(II) by phosphorus-modified bamboo biochar from solution: mechanistic study from qualitative to quantitative analysis. Carbon Research 4 (1). https://doiorg/101007/s44246-025-00196-7
Ministry of Environmental Protection of the People's Republic of China and Ministryof Land and Resources of the People's Republic of China, National bulletin on soilpollution status (2014) https://wwwmeegovcn/gkml/sthjbgw/qt/201404/t20140417_270670htm
Fan J, Yan J, Zhou M, Xu Y, Lu Y, Duan P, Zhu Y, Zhang Z, Li W, Wang A, Sun D (2023) Heavy metals immobilization of ternary geopolymer based on nickel slag, lithium slag and metakaolin. J Hazard Mater 453,131380. https://doiorg/101016/jjhazmat2023131380
Sun Z, Zhao M, Chen L, Gong Z, Hu J, Ma D (2023) Electrokinetic remediation for the removal of heavy metals in soil: Limitations, solutions and prospection Sci Total Environ. 903,165970. https://doiorg/101016/jscitotenv2023165970
Xiao J, Hu R, Chen G, Xing B (2020) Facile synthesis of multifunctional bone biochar composites decorated with Fe/Mn oxide micro-nanoparticles: Physicochemical properties, heavy metals sorption behavior and mechanism. J Hazard Mater 399,123067. https://doiorg/101016/jjhazmat2020123067
Cui X, Zhang J, Wang X, Pan M, Lin Q, Khan K, Yan B, Li T, He Z, Yang X, Chen G (2020) A review on the thermal treatment of heavy metal hyperaccumulator: Fates of heavy metals and generation of products. J Hazard Mater 405,123832. https://doiorg/101016/jjhazmat2020123832
Cao Y, Tan Q, Zhang F, Ma C, Xiao J, Chen G (2022) Phytoremediation potential evaluation of multiple Salix clones for heavy metals (Cd, Zn and Pb) in flooded soils. Sci Total Environ 813,152482. https://doiorg/101016/jscitotenv2021152482
Li X, Xiao J, Salam MMA, Chen G (2022) Evaluation of dendroremediation potential of ten Quercus spp for heavy metals contaminated soil: A three-year field trial. Sci Total Environ 851,158232. https://doiorg/101016/jscitotenv2022158232
Chew K, Chia S, Chia W, Cheah W, Munawaroh HSH, Ong W (2021) Abatement of hazardous materials and biomass waste via pyrolysis and co-pyrolysis for environmental sustainability and circular economy. Environ Pollut 278,116836. https://doiorg/101016/jenvpol2021116836
Liu Z, Tran K (2021) A review on disposal and utilization of phytoremediation plants containing heavy metals. Ecotoxicol Environ Saf 226,112821. https://doiorg/101016/jecoenv2021112821
Lee J, Park K (2021) Conversion of heavy metal-containing biowaste from phytoremediation site to value-added solid fuel through hydrothermal carbonization. Environ Pollut 269,116127. https://doiorg/101016/jenvpol2020116127
Raheem A, He Q, Ding L, Dastyar W, Yu G (2022) Evaluating performance of pyrolysis and gasification processes of agriculture residues-derived hydrochar: Effect of hydrothermal carbonization. J Clean Prod 338,130578. https://doiorg/101016/jjclepro2022130578
Zhao B, Li H, Yang X, Xiao J, Chen G (2024) Hydrochar derived from willow using in phytoremediation: Hydrochar properties, metals behavior and potential applications. Sep Purif Technol 339,126697. https://doiorg/101016/jseppur2024126697
Zhao B, Li H, Yang X, Zhao W, Chen Y, Di D, Xiao J, Chen G (2024) Optimization conversion of willow biomass derived from phytoremediation into value-added hydrochars: Effects of temperature and medium on Cd/Zn distribution and application potentials. Energy Convers Manage:X 24,100698. https://doiorg/101016/jecmx2024100698
Zhang J, Wang Y, Wang X, Wu W, Cui X, Cheng Z, Yan B, Yang X, He Z, Chen G (2022) Hydrothermal conversion of Cd/Zn hyperaccumulator (Sedum alfredii) for heavy metal separation and hydrochar production. J Hazard Mater 423,127122. https://doiorg/101016/jjhazmat2021127122
Małgorzata W, Macie S, Klaudia C, Małgorzata śled (2023) The effect of an acid catalyst on the hydrothermal carbonization of sewage sludge. J Enviro Manag 345118820. https://doiorg101016/jjenvman2023118820
Xue X, Han Y, Wu X, Wang H, Wang S, Zheng J, Ran R, Zhang C (2023) Review: Phytate modification serves as a novel adsorption strategy for the removal of heavy metal pollution in aqueous environments. J Environ Chem Eng 11(6), 111440. https://doiorg/101016/jjece2023111440
Hu R, Xiao J, Wang T, Chen G, Chen L, Tian X (2020) Engineering of phosphate-functionalized biochars with highly developed surface area and porosity for efficient and selective extraction of uranium. Chem Eng J 379122388. https://doiorg/101016/jcej2019122388
Xia H, Ren Q, Lv J, Wang Y, Feng Z, Li Y, Wang F, Liu Y, Wang Y (2023) Hydrothermal fabrication of phytic acid decorated chitosan-graphene oxide composites for efficient and selective adsorption of uranium (VI). J Environ Chem Eng 1(5),110760. https://doiorg/101016/jjece2023110760
Peng C, Zhai Y, Zhu Y, Xu B, Wang T (2016) Production of char from sewage sludge employing hydrothermal carbonization: Char properties, combustion behavior and thermal characteristics. Fuel 76, 110–118. https://doiorg/101016/jfuel201602068
Moghaddam MH, Karimian N, Johnston SG, Choppala G, Rastegari M, Burton ED (2024) Antimony(V) sorption and coprecipitation with ferrihydrite: An examination of retention mechanisms and the selectivity of commonly-applied extraction procedures. J Hazard Mater 480,136297. https://doiorg/101016/jjhazmat2024136297
Yi Y, Yang Z, Zhang S (2011) Ecological risk assessment of heavy metals in sediment and human health risk assessment of heavy metals in fishes in the middle and lower reaches of the Yangtze River basin. Environ Pollut 159(10). https://doiorg/101016/jenvpol201106011
Zhang S, Sheng K, Yan W, Liu J, Shuang E, Yang M, Zhang X (2021) Bamboo derived hydrochar microspheres fabricated by acid-assisted hydrothermal carbonization. Chemosphere 263,128093. https://doiorg/101016/jchemosphere2020128093
Zhao X, Li M, Zhai F, Hou Y, Hu R (2021) Phosphate modified hydrochars produced via phytic acid-assisted hydrothermal carbonization for efficient removal of U(VI), Pb(II) and Cd(II). J Environ Manage 298,113487. https://doiorg/101016/jjenvman2021113487
Liu J, Meng C, Liu Q, Li N, Yu R, Zeng M (2022) Fire-resistant iron-based phosphates/phosphorus-doped carbon composites derived from phytic acid-treated metal organic frameworks as high-efficiency microwave absorbers. Carbon 200,472–482. https://doiorg/101016/jcarbon202208037
Ncube P, Pidou M, Stephenson T, Jefferson B, Jarvis P (2018) Consequences of pH change on wastewater depth filtration using a multimedia filter. Water Res 128,111–119. https://doiorg/101016/jwatres201710040
Fu H, Yu H, Li T, Zhang X (2017) Influence of cadmium stress on root exudates of high cadmium accumulating rice line (Oryza sativa L). Ecotox Environ Saf 150(15),168–175. https://doiorg/101016/jecoenv201712014
Tian S, Xie R, Wang H, Hu Y, Hou D, Liao X, Brown PH, Yang H, Lin X, Labavitch J, Lu L (2017) Uptake, sequestration and tolerance of cadmium at cellular levels in the hyperaccumulator plant species Sedum alfredii. J Exp Bot 68(9), 2387–2398. https://doiorg/101093/jxb/erx112
Zhang J, Feng Y, Liu H, Mu Y, Guo Z (2014) Preparation and evaluation of activated carbon with different polycondensed phosphorus oxyacids (H₃PO₄, H₄P₂O₇, H₆P₄O₁₃ and C₆H₁₈O₂₄P₆) activation employing mushroom roots as precursor. Journal of Analytical & Applied Pyrolysis. https://doiorg/101016/jjaap201405019
Gwóźdź M, Brzęczek-Szafran (2022) A Carbon-Based Electrocatalyst Design with Phytic Acid—A Versatile Biomass-Derived Modifier of Functional Materials. Int J Mol Sci 23(19), 11282. https://doiorg/103390/ijms231911282
Borghei M, Lehtonen J, Liu L, Rojas OJ (2018) Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction. Reaction Adv Mater 30(24), e1703691. https://doiorg/101002/adma201703691
Chen H, Ye Q, Wang X, Sheng J, Yu X, Zhao S, Zou X, Zhang W, Xue G (2024) Applying sludge hydrolysate as a carbon source for biological denitrification after composition optimization via red soil filtration. Water Res 249,120909–120909. https://doiorg/101016/jwatres2023120909
Khosravi A, Zheng H, Liu Q (2022) Production and characterization of hydrochars and their application in soil improvement and environmental remediation. Chem Eng J 430(4),133142. https://doiorg/101016/jcej2021133142
Han L, Ro K, Sun K, Sun H, Wang Z, Libra J, Xing B (2016) New Evidence for High Sorption Capacity of Hydrochar for Hydrophobic Organic. Pollutants Environ Sci Technol 50(24), 13274–13282. https://doiorg/101021/acsest6b02401
Li J, Zhao P, Li T, Lei M, Yan W, Ge S (2020) Pyrolysis behavior of hydrochar from hydrothermal carbonization of pinewood sawdust. Anal Appl Pyrolysis 146,104771. https://doiorg/101016/jjaap2020104771
Wu Z, Chen Z, Tang J, Zhou Z, Chen L, Fang Y, Hu X, Lv J (2023) Efficient adsorption and reduction of Cr(VI) in water using one-step H₃PO₄-assisted prepared Leersia hexandra Swartz hydrochar. Mater Today Sustain 21,100260. https://doiorg/101016/jmtsust2022100260
Yukhajon P, Somboon T, Seebunrueng K, Nishiyama N, Sansuk S (2023) Facile surface-controlled synthesis of phytic acid-facilitated calcium carbonate composites for highly efficient ferric ion adsorption and recovery from rust. Mater Today Sustain 24,100520. https://doiorg/101016/jmtsust2023100520
Puziy A, Poddubnaya O, Socha R, Gurgul J, Wisniewski M (2008) XPS and NMR studies of phosphoric acid activated carbons. Carbon 46:2113–2123. https://doiorg/https://doiorg/101016/jcarbon200809010
Teng Z, Zhu J, Shao W, Zhang K, Li M, Whelan M (2019) Increasing plant availability of legacy phosphorus in calcareous soils using some phosphorus activators. J Environ Manage 256,109952. https://doiorg/ 101016/jjenvman2019109952
Cai K, Shen W, Ren B, He J, Wu S, Wang W (2017) A phytic acid modified CoFe₂O₄ magnetic adsorbent with controllable morphology, excellent selective adsorption for dyes and ultra-strong adsorption ability for metal ions. Chem Eng J 330,936–946. https://doiorg/101016/jcej201708009
Heidarinejad ZMHM (2020) Methods for preparation and activation of activated carbon: a review. Environ Chem Lett 18(2). https://doiorg/ 101007/s10311-019-00955-0
Cui X, Shen Y, Yang Q, Kawi S, He Z, Yang X, Wang C (2018) Simultaneous syngas and biochar production during heavy metal separation from Cd/Zn hyperaccumulator (Sedum alfredii) by gasification. Chem Eng J 347,543–551. https://doiorg/101016/jcej201804133
Li C, Zhong F, Liang X, Xu W, Yuan Q, Niu W, Meng H (2023) Microwave-assisted hydrothermal conversion of crop straw: Enhancing the properties of liquid product and hydrochar by varying temperature and medium. Energy Convers Manag 290,117192. https://doiorg/101016/jenconman2023117192
Jin J, Li Y, Zhang J, Wu S, Cao Y, Liang P, Zhang J, Wong M, Wang M, Shan S, Christie P (2016) Influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochars derived from municipal sewage sludge. J Hazard Mater 320417-426. https://doiorg/101016/jjhazmat201608050
Huang X, Zhuang R, Muhammad F, Yu L, Shiau Y, Li D (2017) Solidification/stabilization of chromite ore processing residue using alkali-activated composite cementitious materials. Chemosphere, 168300-308. https://doiorg/101016/jchemosphere201610067
Zhou Y, Xiao J, Hu R, Wang T, Shao X, Chen G (2020) Engineered phosphorous-functionalized biochar with enhanced porosity using phytic acid-assisted ball milling for efficient and selective uptake of aquatic uranium. J Mol Liq 303(000), 11. https://doiorg/101016/jmolliq2020112659
Bacon R, Tang M (1964) Carbonization of cellulose fibers—II Physical property study. Carbon 2(3), 221, IN3, 223 – 222, IN4,225. https://doiorg/101016/0008-6223(64)90036-3
Liang L, Zhang H, Lin X, Yan K, Li M, Pan X, Hu Y, Chen Y, Luo X, Shang R (2021) Phytic acid-decorated porous organic polymer for uranium extraction under highly acidic conditions. Colloids Surf A Physicochem Eng Aspects 625, 126981. https://doiorg/101016/jcolsurfa2021126981
Wang Y, Xiao J, Wang H, Zhang T, Yuan S (2022) Binary doping of nitrogen and phosphorus into porous carbon: A novel di-functional material for enhancing CO₂ capture and super-capacitance. J Mater Sci Technol 99,73–81. https://doiorg/101016/jjmst202105035
Yu S, Dong X, Zhao P, Luo Z, Sun Z, Yang X, Li Q, Wang L, Zhang Y, Zhou H (2022) Decoupled temperature and pressure hydrothermal synthesis of carbon sub-micron spheres from cellulose. Nat Commun 13(1), 3616–3616. https://doiorg/101038/s41467-022-31352-x
Matthias T, Katsumi K, Alexander V, James P, Francisco R, Jean R, Kenneth S (2015) Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl Chem 87(7),1051–1069. https://doiorg/101515/pac-2014-1117
Chen H, Wang X, Lu X, Xu L, Wang J, Lu X (2018) Hydrothermal Conversion of Cd-Enriched Rice Straw and Cu-Enriched Elsholtzia splendens with the Aims of Harmless Treatment and Resource Reuse. Ind Eng Chem Res 57(46), 15683-15689https://doiorg/101021/acsiecr8b04378
Sabio E, Alvarez-Murillo A, Roman S, Ledesma B (2016) Conversion of tomato-peel waste into solid fuel by hydrothermal carbonization: Influence of the processing variables. Waste Manag 47(PartA), 122–132. https://doiorg/101016/jwasman201504016
Parikh J (2005) ChanniwalaSA, GhosalKG A correlation for calculating HHV from proximate analysis of solid fuels. Fuel 84(5), 487–494. https://doiorg/101016/jfuel200410010
China MOAA (2022) Biochar (NY/T4159-2022) [S] https://stdsamrgovcn/hb/search/stdHBDetailed?id=EC4CA0031C60D5C5E05397BE0A0A70BA
Yaashikaa P, Kumar P, Varjani S, Saravanan A (2020) A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy. Biotechnology Reports 28,e00570. https://doiorg/101016/jbtre2020e00570
Yang X, Wan Y, Zheng Y, He F, Yu Z, Huang J, Wang H, Ok YS, Jiang Y, Gao B (2019) Surface functional groups of carbon-based adsorbents and their roles in the removal of heavy metals from aqueous solutions: a critical review. Chem Eng J 366:608–621. https://doiorg/https://doiorg/101016/jcej201902119
Chen Q, Zhang TC, Ouyang L, Yuan S (2022) Single-Step Hydrothermal Synthesis of Biochar from H₃PO₄-Activated Lettuce Waste for Efficient Adsorption of Cd(II) in Aqueous Solution. Molecules 27(1), 269. https://doiorg/103390/molecules27010269
Deng J, Liu Y, Liu S, Zeng G, Tan X, Huang B, Tang X, Wang S, Hua Q, Yan Z (2017) Competitive adsorption of Pb(II), Cd(II) and Cu(II) onto chitosan-pyromellitic dianhydride modified biocha. J Colloid Interface Sci 506, 355–364. https://doiorg/101016/jjcis2017070690
Hart J, May P, Allan N, Hallam K, Claeyssens F, Fuge G, Ruda M, Heard P (2013) Towards new binary compounds: Synthesis of amorphous phosphorus carbide by pulsed laser deposition. J Solid State Chem 198,466–474. https://doiorg/101016/jjssc201211008
Taşar S, Kaya F, Özer A (2014) Biosorption of lead(II) ions from aqueous solution by peanut shells: Equilibrium, thermodynamic and kinetic studies. J Environ Chem Eng 2(2), 1018–1026. https://doiorg/101016/jjece201403015
Mo G, Xiao J, Gao X (2022) To enhance the Cd²⁺ adsorption capacity on coconut shell-derived biochar by chitosan modifying: performance and mechanism. Biomass Convers Biorefinery 13:16737–16752
Xiao J, Hu R, Chen G (2020) Micro-nano-engineered nitrogenous bone biochar developed with a ball-milling technique for high-efficiency removal of aquatic Cd(II), Cu(II) and Pb(II). J Hazard Mater 387,121980. https://doiorg/101016/jjhazmat2019121980
Xiao J, Hu R, Chen G, Xing B (2020) Facile synthesis of multifunctional bone biochar composites decorated with Fe/Mn oxide micro-nanoparticles: Physicochemical properties, heavy metals sorption behavior and mechanism. J Hazard Mater 399 123067. https://doiorg/101016/jjhazmat2020123067
Xiao J, Jing Y, Wang X, Yao Y, Jia Y (2018) Preconcentration of Uranium(VI) from Aqueous Solution by Amidoxime-Functionalized Microspheres Silica Material: Kinetics, Isotherm and Mechanism Study. ChemistrySelect 3, 12346–12356,. https://doiorg/101002/slct201802472
Koodyńska D, Krukowska J, Thomas P (2016) Comparison of Sorption and Desorption Studies of Heavy Metal Ions From Biochar and Commercial Active Carbon. Chem Eng J 307353-363. https://doiorg/101016/jcej201608088
Lucie C, Fouad L, Mario A, Aurore R, Sylvain B, Christian D, Philippe D (2016) Phosphorus and nitrogen derivatization as efficient route for improvement of lignin flame retardant action in PLA. Eur Polym J 84,652–667. https://doiorg/101016/jeurpolymj201610003
Wang L, Zheng X, Yan L, Song W, Li Y, Wu B, Li X (2023) Multiphasic MoS₂ activates peroxymonosulfate for efficient removal of oxytetracycline: The dominant role of surface reactive species. Sep Purif Technol 317,123907. https://doiorg/101016/jseppur2023123907
Hakanson L (1980) An ecological risk index for aquatic pollution controla sedimentological approach. Water Res 14(8),975–1001. https://doiorg/101016/0043-1354(80)90143-8

No competing interests reported.

Download PDF

Reviews received at journal
02 Dec, 2025
Reviewers agreed at journal
29 Sep, 2025
Reviewers agreed at journal
29 Sep, 2025
Reviewers agreed at journal
29 Sep, 2025
Reviewers agreed at journal
29 Sep, 2025
Reviewers agreed at journal
29 Sep, 2025
Reviewers invited by journal
29 Sep, 2025
Editor assigned by journal
25 Sep, 2025
Submission checks completed at journal
03 Sep, 2025
First submitted to journal
02 Sep, 2025

You are reading this latest preprint version

Phytic Acid-Mediated Hydrothermal Valorization of Woody Biomass Containing Heavy Metals into Functional Hydrochar: Mechanistic Insights and Sustainable Resource Pathways

Status:

Version 1

Abstract

Figures

Highlights

1. Introduction

2. Experimental section

2.1 Experimental Materials

2.2 HTC Procedure

2.3 Hydrochar Characterization

2.4 Batch adsorption experiments

2.5 Soil experiments and safety risk assessment

2.6 Statistical analysis

3 Results and Discussion

3.1 Hydrochar production and distribution of HMs during HTC process

3.1.1 Hydrochar yield

3.1.2 Distribution of HMs during the HTC process

3.2 Hydrochars Structure and surface properties

3.2.1 Hydrochars composition

3.2.2 Surface properties

3.2.3 Structure properties

3.2.4 Thermal stability and fuel traits

3.3 Adsorption capacity of hydrochars to solution HMs

3.3.1 Batch adsorption experiments

3.3.2 The effect of solution pH on adsorption

3.3.3 Adsorption isotherm and kinetics

3.3.4 Adsorption mechanisms

3.4. Hydrochars capacity in HMs immobilization in soil

3.4.1 The physio-chemical properties of soil

3.4.2 The chemical speciation of HMs in soil

3.4.3 Bioavailability of HMs

3.5 Risk assessment of hydrochars in soil remediation

4 Conclusion

Abbreviations

Declarations

CRediT authorship contribution statement

Declaration of competing interest

Funding

Author Contribution

Acknowledgements

Data availability

References

Additional Declarations

Supplementary Files

Status:

Version 1