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Frontiers in Environmental Chemistry ·

Volume 7 - 2026 | https://doi.org/10.3389/fenvc.2026.1912197

Vetiver biochar application for heavy metal sequestration in water treatment: bridging perfumery waste and environmental sustainability

  • Sameer Neve1
  • Viravid Na Nagara1
  • Zhiming Zhang2
  • Rupali Datta3
  • Dibyendu Sarkar1*corresponding author
  1. 1Department of Civil, Environmental and Ocean Engineering, Stevens Institute of Technology, Hoboken, NJ, United States
  2. 2Department of Civil and Environmental Engineering, Rowan University, Glassboro, NJ, United States
  3. 3Department of Biological Sciences, Michigan Technological University, Houghton, MI, United States
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Abstract

Converting industrial byproducts into functional adsorbents supports circular and sustainable approaches to water treatment. This study evaluated biochar derived from spent vetiver (Chrysopogon zizanioides) roots, a byproduct of the perfumery essential-oil industry, for the competitive adsorption of copper (Cu), cadmium (Cd), zinc (Zn), and lead (Pb). Vetiver root biochar produced at 500 °C for 60 min exhibited a high surface area of 308.2 m 2 /g and a yield of 53.8%. Batch adsorption experiments were performed in triplicate at a pH of 5.5 ± 0.2, in 0.1 M NaNO 3, at 24 °C ± 2 °C, using 1% (w/v) biochar (10 g/L), with initial metal concentrations of 100 mg/L for each metal, along with a control. After 24 h, removal efficiencies reached 100% for Pb, 99.9% for Cu, 92.6% for Zn, and 77.0% for Cd. Adsorption was rapid, with nearly complete removal of Pb and Cu and approximately 60% removal of Zn and Cd within the first 30 min. High maximum adsorption capacities were achieved particularly for Pb, along with other tested metals, although competition in multi-metal systems influenced adsorption performance. Adsorption edge analysis and surface complexation modeling indicated that chemisorption was the dominant removal mechanism, supporting strong metal retention under the tested extraction conditions. Mild extraction released <3.5% of adsorbed metal, and TCLP/SPLP leachates were below U.S. EPA thresholds. Overall, these findings demonstrate that spent vetiver roots can be converted into high-performance biochar adsorbents, offering a sustainable, value-added strategy for heavy-metal remediation in water treatment systems.

Keywords

1 Introduction

Heavy metals enter surface waters from various sources, such as industrial effluents, stormwater runoff, and daily human activities, posing severe threats to aquatic ecosystems and human health if discharged without sufficient treatment (Aguado et al., 2009). Commonly reported heavy metals include lead (Pb) and cadmium (Cd) from car batteries, zinc (Zn) from tires, and copper (Cu) from the excessive wearing of brake pads. Per the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), Pb and Cd are among the top 10, and Zn and Cu are ranked 75th and 125th, respectively, according to their toxicities (Srivastava and Majumder, 2008). These metals disrupt the essential metabolic processes and antioxidant defenses in organisms (Rehman et al., 2018). Lead affects multiple organs, notably impairs neurological function, and with long-term accumulation, can contribute to anemia and renal damage. Cadmium, a probable human carcinogen (Group 2B), accumulates in kidneys and causes oxidative stress, reproductive toxicity, and bone fragility (Valko et al., 2006; Wallin et al., 2016; Prozialeck and Edwards, 2012). Though Zn and Cu are essential micronutrients, elevated exposures can cause anemia, hepatic and renal dysfunction, gastrointestinal distress, and can adversely affect aquatic life.

Among various conventional water treatment technologies, such as ion exchange, membrane separation, and electrochemical treatment, adsorption has been proven to be an efficient removal process (Bhatnagar and Sillanpää, 2010; Gupta et al., 2009). Various organic and inorganic adsorbent media have been proven to eliminate various heavy metals from aqueous media (Gupta et al., 2009). Biochar, a solid product of biomass thermochemical conversion under oxygen-limited conditions, is emerging as a cost-effective and sustainable adsorbent for heavy metals (Biochar. biochar-international, 2022). Derived from diverse biomass sources (e.g., lignocellulosic residues, manure, and agricultural by-products), biochar exhibits high surface area, porous structure, mineral content (Si, Ca, K, Mg), and functional groups (e.g., –OH, C–O, C=O, –COOH) that promote metal binding (Inyang et al., 2016; Jha et al., 2010; Lu et al., 2013; Lehmann and Joseph, 2015; Ali, 2010). These physical and chemical characteristics enable biochar to be used directly as an adsorbent for the removal of multiple heavy metals, such as Cu, Pb, Zn, and Cd (Srivastava and Majumder, 2008; Bhatnagar and Sillanpää, 2010; Ali, 2010). Recent research has shown the removal of heavy metals (e.g., Cu and Zn) by biochar generated from wood or maize straw (Chen et al., 2011a). Also, Mohan et al. (Mohan et al., 2007) reported that the removal efficiency of oak bark char for Pb and Cd was equivalent to that of Calgon F-400, a commercially available activated carbon. Kong et al. (Kong et al., 2011) found that soybean stalk biochar successfully removed 75%–87% of mercury (Hg) from an aqueous solution. Additionally, the use of various engineering techniques in the production of biochar, such as the pretreatment of feedstock or modification of char surfaces, has led to the creation of various high-efficiency and economically engineered biochars with adsorption capacities similar to or even higher than some commercial activated carbons. For example, the sorption capacity of biochar for Cu could be significantly improved by physically treating the feedstock by pulverizing prior to pyrolysis (Tong et al., 2011). Physical and chemical activation of biochar has also been known to enhance its physicochemical characteristics, leading to improved adsorption performance (Sajjadi et al., 2019a; Sajjadi et al., 2019b).

Vetiver grass, known for its deep (up to 3–4 m) robust root system, drought tolerance, and high biomass yield, has been valued for its scent, therapeutic uses, and ability to prevent soil erosion (Neve et al., 2022; Datta et al., 2013; Gnansounou et al., 2017; Gupta and Singh, 2021; Adams et al., 2004). Owing to the vast application in the perfumery industry, the roots are primarily used for extracting essential oils, while the shoots are proven to generate high-quality bioethanol (Neve et al., 2022). Once the essential oil is extracted, the roots are generally disposed of as waste. However, previous studies of biochar generation with lignocellulosic biomass have demonstrated unparalleled potential for their use as a feedstock (Neve et al., 2024).

We hypothesized that biochar produced from spent vetiver roots, owing to its high surface area and oxygen-rich functional groups, would chemically bind divalent heavy metals to achieve irreversible removal from water, including competitive multi-metal conditions. This study aims to explore the removal of heavy metals by biochar generated from spent vetiver roots. The biochar generation conditions were optimized to achieve high removal efficiencies. In addition, the adsorption performance for single metal and multiple metal systems varies because various metals may compete for the same binding sites on an adsorbent, necessitating a study to simulate natural and industrial waters that frequently have multiple metal ions present at once (Alam et al., 2018a). Vetiver root biochar was evaluated for the first time in this study for the competitive sorption of multiple metals. The specific objectives are to (1) analyze the physicochemical characteristics of vetiver root biochar, (2) investigate the impact of biochar dosage on metal(s) adsorption, (3) compare single- and multi-metal adsorption characteristics, (4) develop surface complexation models to predict metal removal performance and understand the adsorption mechanism, and (5) evaluate extraction and leaching potential of the spent vetiver root biochar.

2 Materials and methods

2.1 Batch adsorption experiments

2.1.1 Adsorption kinetics

Three initial test conditions were evaluated to determine the optimal biochar dosage for divalent metals removal: (i) 100 mg/L of metal ion at 0.1% (w/v) of biochar, (ii) 100 mg/L of metal ion at 1% (w/v) of biochar, and (iii) 1,000 mg/L of metal ion at 1% (w/v) of biochar. These concentrations span the range reported across diverse metal-bearing waters, from lower-strength sources such as urban stormwater runoff (Smith et al., 2025), landfill leachate (Sarkar et al., 2025), and municipal (Chames and Nyutu, 2025) and agricultural wastewater (Topare and Wadgaonkar, 2023) to high-strength industrial effluents from electroplating, battery manufacturing, mining, and metal-finishing operations, and provide sufficient loading to evaluate adsorption capacity under saturating conditions. Biochar was generated via pyrolysis at 500 °C for 60 min from air-dried vetiver roots (post–essential oil extraction), under continuous N2 purge with a 10 °C min-1 ramp following the protocol described in the Supplementary Material (SI, S2.1). These pyrolysis conditions were selected based on our prior optimization study, which identified 500 °C and 60 min as the conditions yielding the highest surface area and char yield for spent vetiver roots (Neve et al., 2024). Metal stock solutions were prepared from analytical-grade salts as detailed in the SI (S2.2). Since preliminary results indicated that 1% (w/v) biochar dosage at an initial metal concentration of 100 mg/L yielded the highest removal efficiency, this condition was used for all subsequent adsorption experiments.

Batch adsorption kinetic tests of Cu2+, Cd2+, Pb2+, and Zn2+ on single- and multi-metal systems were conducted to evaluate their competitive adsorptive behavior on biochar. 400 mL of each metal solution was mixed with 4 g of biochar. Sodium nitrate (NaNO3, 0.1 M) was used as the background electrolyte. The solution pH was adjusted to 5.5 ± 0.2 with 0.1 M HNO3 or NaOH to minimize metal precipitation while maintaining realistic environmental conditions. As it would not be realistic and would prevent the metal surfaces from becoming complex, pH buffer was not used to avoid interference with metal-surface complexation (Smičiklas et al., 2009). At 24 °C ± 2 °C, all tests were agitated for 24 h, before representative samples were obtained at various time intervals (0, 0.1, 0.25, 0.5, 1, 2, 6, 12, and 24 h) in triplicate. Then, the samples were filtered through a 0.45-µm nylon syringe filter and subjected to analysis by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES, 5100 Agilent Technologies, CA, United States). Multi-metal experiments followed the same protocol but contained all four metals in the initial solution.

The removal percentage and adsorption capacity of heavy metals were calculated using the following Equation (Feng et al., 2013): where C 0 and C *e* represent the initial and equilibrium concentrations of metal ions (mg/L), V is the solution volume (L), q *e* represents the adsorption capacity (mg/g), and W denotes the dosed biochar mass (g).

R e m o v a l p e r c e n t a g e % = C 0 − C e C 0 × 100 % ,

q e = C 0 − C e V W ,

The kinetic data were fitted to pseudo-first order (PFO), pseudo-second order (PSO), and intraparticle diffusion models using the following Equations (Hammo et al., 2021; Kołodyńska et al., 2017):

PFO : q t = q e 1 − e − k 1 t ,

PSO : q t = k 2 q e 2 t 1 + k 2 q e t ,

Intraparticle Diffusion Model: q t = k i t 1 / 2 + C.

Where q t and q e (mg g-1) are the numbers of contaminants adsorbed at time t (h) and equilibrium, respectively, k 1 (h-1) and k 2 (g mg-1 h-1) are the PFO and PSO model constants, respectively, k *i* (mg g-1 hr-1/2) denotes the intraparticle diffusion rate constant, and C represents the intercept.

2.1.2 Adsorption isotherms

Adsorption isotherm experiments were conducted to determine the efficacy of biochar and identify equilibrium sorption parameters. Biochar (0.4 g; 1% w/v) was added to 40 mL of single-metal solutions with initial concentrations of 1, 2, 5, 10, 20, 30, 50, 75, and 100 mg/L in 50 mL centrifuge tubes with three replicates each. The solution pH was adjusted to 5.5 ± 0.2 as described above. Samples were shaken at 24 °C ± 2 °C for 24 h at 150 rpm, filtered (0.45 µm nylon), and analyzed via ICP-OES. The batch adsorption data were fitted to both nonlinear Langmuir and Freundlich adsorption isotherm models using the following Equation 34:

Langmuir Model : q e = q m K L C e 1 + K L C e ,

Freundlich Model: q e = K F C e 1 / n,

where q m (mg/g) denotes the maximum adsorption capacity, q e (mg/g) represents the adsorption capacity of the biochar at equilibrium, C e (mg/L) indicates the metal concentration at equilibrium, and K L (L/mg) denotes a Langmuir constant, K F ((mg/g) (L/mg)1/n) is a Freundlich isotherm constant, and 1/n (heterogeneity factor) is the adsorption intensity. Each C0 was run in independent triplicate with a paired biochar-free, pH-matched control. Final pH was recorded for every tube. All models were fitted to untransformed data by nonlinear least squares in OriginPro 2023. Model performance was assessed using R2, the root mean square error (RMSE), and the residual sum of squares (SSE). Equilibrium concentrations below the detection limit were excluded from the fitting.

2.1.3 Blanks and controls

Reagent blanks consisting of 0.1 M NaNO3 background electrolyte without metals and without biochar were run in triplicate alongside the batch experiments to confirm the absence of background metal contamination from glassware, electrolyte, or acid/base additions. All target metals were below the ICP-OES method detection limit in these blanks.

2.2 Adsorption edge study and surface complexation modeling

One of the critical elements that affects sorption properties the most is pH. The adsorption edge study was conducted similarly to the adsorption isotherm study, with the exception that the target concentrations of the heavy metals were kept constant at 100 mg/L, and the solution pH varied from 3 to 11 (i.e., 3, 5, 7, 9, and 11) using HNO3 and NaOH. This pH range was selected based on the assumption that the background concentration of the solute may vary based on various contaminating sources. After 24 h, the final pH and concentration of the metals in the solution were measured.

Based on the adsorption edge study data, surface complexation models were performed using MINEQL+ (v.5.0, Environmental Research Software, ME, United States) and MINFIT (Xie et al., 2016). This study employed the diffuse layer model, which was successfully used to model the adsorption of Cu on biochar in Burch (Burch, 2022). A single generic surface site (≡XOH) and monodentate complexation stoichiometry (≡XOH + M2+ = ≡XOM+ + H+) were assumed. Protonation/deprotonation reactions with equilibrium constants were used in the model of metal sorption by biochar (Zhang et al., 2014). The protonation/deprotonation reactions with equilibrium constants are listed in Table 2. Monodentate complexation of Cd, Cu, Pb, and Zn, was considered for this study based on the successful model sorption of Cu, Mn, and Zn on biochar demonstrated by Zhang et al. (Zhang et al., 2014). The metal log K values were the only adjustable parameters and were optimized by nonlinear least squares in MINFIT against the measured removal at each pH, with the biochar and solution parameters given in Table 3. Total surface site density was adopted from Zhang et al. (Zhang et al., 2014), which was determined for a municipal-organic-waste biochar, rather than measured for the present material. Independent titration data for vetiver root biochar were not available, and the fitted log K values should therefore be regarded as conditional on this assumption. Independent potentiometric titration of vetiver root biochar, which was not performed here, would be required to constrain the log K values and the model is therefore presented as exploratory.

2.3 Extraction tests

The spent biochar from the adsorption kinetics experiments with a single-metal system was recovered and oven-dried overnight at 80 °C. The biochar was first shaken with deionized water (DI water) for 24 h to assess the extraction associated with water-soluble fractions. Then, it was subjected to 0.1 mM NaNO3 solution for another 24 h. A solid-to-liquid ratio of 1:100 was maintained for both experiments. Samples were filtered using a 0.45-µm nylon syringe filter and analyzed using ICP-OES.

2.4 Leaching tests

The short-term leachability of heavy metals from spent biochar used in the adsorption experiments was assessed using the U.S. Environmental Protection Agency (EPA) Synthetic Precipitation Leaching Procedure (SPLP) and Toxicity Characteristic Leaching Procedure (TCLP). Experimental details and statistical analysis were described in SI (S2.3 and S2.4).

2.5 Statistical analysis

All experiments were conducted in triplicate (n = 3), and results are reported as mean ± standard deviation. Differences among biochar dosages, among metals, and between single- and multi-metal systems were evaluated by one-way analysis of variance followed by the Tukey–Kramer honestly significant difference test, which adjusts for multiple pairwise comparisons. The significance level was set at α = 0.05. Analyses were performed in JMP 15.2 (SAS Institute Inc., Cary, NC, United State), and figures were prepared in OriginPro 2023 (OriginLab Corporation, Northampton, MA, United States). Formal tests of normality and homogeneity of variance were not applied, as with three replicates per group such tests have very low power; we therefore relied on ANOVA with the conservative Tukey–Kramer procedure and report exact p-values so that readers can judge the strength of each comparison.

3 Results and discussion

3.1 Biochar characterization

The primary physicochemical properties of vetiver root biochar are summarized in Supplementary Table S1. A biochar yield of 53.76% was obtained from the pyrolysis of vetiver root, with a surface area of 308.15 m2/g and a basic pH of 10.98. As previously reported in most other types of biochar, a substantially high carbon content (68.59%) was shown in the produced biochar. The H/C (0.04) and O/C (0.25) molar ratios indicated that the biochar is more aromatic than polar. The physicochemical characteristics of vetiver root biochar (e.g., high pH) are consistent with prior reports that used biochar as a remediation agent, e.g., for soils affected by acid mine drainage and mine tailings (Neve et al., 2025).

The scanning electron microscopy (SEM) images showed a nonuniform fibrous structure of the biochar, with some honeycomb-like tube-shaped pores (Supplementary Figure S1). This porosity can provide structural conditioning and retain water and soil nutrients to promote greater plant development (Alkharabsheh et al., 2021; Li et al., 2022; Wong and Ogbonnaya, 2021). The structure of biochar can also provide high surface accessibility for sorption of contaminants, including heavy metals (Li et al., 2017; Chen et al.), making it a promising material for environmental remediation. Fourier transform infrared (FTIR) spectrum of the vetiver root biochar displayed bands corresponding to various functional groups (Supplementary Figure S1), such as O-H (∼3350 cm−1), C-H (∼2980 cm−1), C=O, and C-N (∼1650 cm−1), phenols (∼1375 cm−1), aromatic rings (∼1000 cm−1), and aromatic groups (∼800 cm−1) (Ni et al., 2019; Park et al., 2016). The presence of multiple functional groups provides a high potential for the adsorption of heavy metals at multiple sites (Yang et al., 2019; Liu and Zhang, 2022).

3.2 Impact of biochar dosage for metals removal

Single-metal removal was conducted with two levels of biochar dosages (1% and 0.1% w/v) at two concentrations (100 mg/L and 1000 mg/L) to determine the optimal biochar dosage, as compared in Figure 1. Pb and Cu achieved near-complete removal at all tested conditions whereas Zn and Cd showed better removal at 100 mg/L with the 1% w/v biochar dosage. At lower dosage of biochar (0.1%), the removal of Zn and Cd was less effective than that of Pb and Cu. This suggests that the removal of Zn and Cd was highly dependent on available sorption sites. In comparison, at low biochar dosage or high metal concentrations, the removal efficiencies for Pb and Cu were not impacted.

Effect of biochar dosage and initial concentration on metal removal efficiency. Time-dependent removal percentages for Pb, Zn, Cu, and Cd under varying initial metal concentrations (100 mg/L vs. 1000 mg/L) and solid-to-liquid ratios (0.1% vs. 1%).
Figure 1. Effect of biochar dosage and initial concentration on metal removal efficiency. Time-dependent removal percentages for Pb, Zn, Cu, and Cd under varying initial metal concentrations (100 mg/L vs. 1000 mg/L) and solid-to-liquid ratios (0.1% vs. 1%).

The preferential removal of Pb and Cu over Zn and Cd is consistent with the established order of affinity for biochar, which prioritizes Pb2+ and Cu2+ due to their smaller hydrated radii and greater tendency for specific surface complexation and precipitation (Park et al., 2016; Trakal et al., 2011). The high dependency of Zn and Cd removal on dosage is expected for lower-affinity metals, as an increased biochar application rate provides more surface area and functional groups necessary to achieve high removal efficiencies, particularly when the initial metal concentration is high (Trakal et al., 2011; Hefny et al., 2020).

3.3 Adsorption kinetics

Single- and multi-metal adsorption kinetic data are shown in Figure 2. In both systems, Pb and Cu reached complete removal within 6 min, followed by the gradual adsorption of Zn and Cd on biochar. Over 60% of Zn and Cd were removed from the solution in the first 30 min in the adsorption test for both single- and multi-metal systems, indicating the biochar’s potential for quick water purification in environmental applications. Translating these rapid kinetics to packed-bed operation requires column breakthrough experiments, which were not in scope of this study. Compared to the single metal adsorption test, the presence of other metals in the multi-metal system did not significantly (p > 0.05) affect the removal of Pb, Cu, and Zn. However, Cd exhibited faster and more complete removal in the multi-metal system. This likely reflects synergistic co-precipitation, in which the strongly bound Pb and Cu raise the local surface pH and form metal-hydroxide nuclei on the biochar that provide additional binding sites for the weaker-affinity Cd, rather than the simple site competition expected in mixed-metal environments. Previous research demonstrated that the surface characteristics of biochar, including functional groups and pore structure, are essential factors in the process for metal adsorption. The presence of various metals may modify these characteristics, hence amplifying the adsorption of certain metals. For instance, Park et al. (2016) discovered that the porous nature of biochar and the presence of oxygen-containing functional groups are responsible for its significant ability to adsorb heavy metals (Park et al., 2016). In comparison to Cd (4.26 Å), Pb has a smaller hydrated radius (4.01 Å) and a lower pKH (negative log of hydrolysis constant) (7.71) than that of Cd (10.1). Due to the inner surface complexation or adsorption processes, Pb is more preferentially removed by biochar than Cd due to these metal properties (Ni et al., 2019). The sharp initial adsorption phase may be caused by sorption activities at the easily accessible solid-solution interface of the biochar’s outer surface (Na et al., 2022).

Comparative adsorption kinetics in single-versus multi-metal systems. Normalized concentration decay (C/C<sub>0</sub>) over time for Zn, Cu, Cd, and Pb in **(a)** non-competitive single-metal solutions and **(b)** a competitive quaternary multi-metal mixture.
Figure 2. Comparative adsorption kinetics in single-versus multi-metal systems. Normalized concentration decay (C/C0) over time for Zn, Cu, Cd, and Pb in (a) non-competitive single-metal solutions and (b) a competitive quaternary multi-metal mixture.

Adsorption kinetic modeling showed that the PSO model best described the data for all metals (R 2 > 0.942) in either the single- or the multi-metal systems (Supplementary Table S2). Besides the indication by higher R 2 values, further evidence that PSO model fitted best for the experimental kinetic data was that the q *e* values estimated from the PSO model are more comparable to the q *e* values directly obtained from the experiment. In comparison, with much lower fit (R 2 < 0.616), the intraparticle diffusion model did not fully govern the adsorption processes of these metals onto the biochar, indicating the adsorption followed a two-step mechanism wherein the first phase for the diffusional mass transport of metals in the biochar pores and the second phase for metal adsorption on the biochar surface sites.

3.4 Adsorption isotherm

The parameters for Langmuir and Freundlich adsorption isotherm models are summarized in Supplementary Table S3. In single-metal systems, the Freundlich model provided the best fit (R 2 > 0.965) for most heavy metals, except Cu, which fit the Langmuir model better (R 2 = 0.982). This suggests heterogeneous adsorption sites for most metals and a more monolayer-dominated mechanism for Cu. In contrast, in the multi-metal system, the Langmuir adsorption isotherm model fitted best for all heavy metals (R 2 > 0.937) except Pb, which still aligned better with the Freundlich model (R 2 = 0.960). The model-derived maximum adsorption capacity in the single-metal system was in the order of Pb (189.58 mg/g) > Cu (8.486 mg/g) > Cd (7.844 mg/g) > Zn (5.569 mg/g). The competitive adsorption of these metals affected the maximum adsorption capacity in the multi-metal system, where Pb (8.29 mg/g) > Cd (8.289 mg/g) > Cu (3.977 mg/g) > Zn (3.752 mg/g). The significant change in the adsorption capacities of Pb and Cu in the multi-metal system can be attributed to multiple metals attaching to various sorption sites available on the biochar. The competitive order in the multi-metal system, Pb > Cd > Cu > Zn, could be explained by differences in the crystal ionic radii and hydrolysis behavior. Pb2+ has the largest crystal ionic radius (0.119 nm) and lowest first hydrolysis constant (pKH 7.7), giving it the strongest affinity and least competition for surface sites, followed by Cd2+ (0.095 nm), whereas Cu2+ and Zn2+ bind more weakly under competition. This sequence is consistent with reported affinity series for biochar surfaces.

To contextualize the adsorption performance of vetiver root biochar, its maximum adsorption capacities were compared with those of other biochars reported in the literature (Table 1). For Pb, vetiver root biochar (189.6 mg/g) outperformed most pristine, unmodified biochars, including sesame straw (102 mg/g), oak bark char (13.1 mg/g), and long-root water hyacinth biochar (39.1 m/g), and was exceeded only by banana stem and leaf biochar (302.2 mg/g) and by process-optimized or chemically engineered materials. The present data with the extrapolated values support is a strong Pb affinity relative to Cu, Cd, and Zn, evidenced by complete Pb removal under all tested conditions. This strong Pb affinity is consistent with the dominant role of surface complexation and precipitation identified in the present study. For Cu, Cd, and Zn, the capacities of vetiver root biochar were more modest and fell within the range typical of unmodified lignocellulosic biochars, reflecting the lower binding affinity of these metals and the absence of any activation or surface modification step.

TABLE 1. Comparison of vetiver root biochar with other reported biochars for heavy metal adsorption from aqueous solution. Values are extrapolated single-metal maximum adsorption capacities (qmax) from the Langmuir model unless noted.

Adsorbent (feedstock)Pyrolysis T (°C)Metal (s)qmax (mg/g)Best-fit isothermDominant mechanism/noteReferences
Vetiver root biochar500Pb, Cu, Cd, ZnPb 189.61; Cu 8.5; Cd 7.8; Zn 5.6Freundlich (Cu: Langmuir)Surface complexation; chemisorption; strong short-term retentionThis study
Sesame straw biochar700Pb, Cu, Cd, ZnPb 102; Cd 86; Cu 55; Zn 34Langmuir/FreundlichCompetitive sorption; Cd readily displaced(Park et al., 2016)
Long-root water hyacinth biochar (LEC500)500Pb, Cu, Cd, ZnCu 48.2; Zn 45.4; Cd 44.0; Pb 39.1LangmuirEndothermic, spontaneous; carboxyl/phosphate binding(Li et al., 2018)
Hardwood biochar450Cu, ZnCu 6.8; Zn 4.5LangmuirCation exchange; pristine wood char(Chen et al., 2011b)
Corn straw biochar600Cu, ZnCu 12.5; Zn 11.0LangmuirCation exchange(Chen et al., 2011b)
Oak bark char400Pb, CdPb 13.1; Cd 5.4LangmuirComparable to commercial activated carbon (Calgon F-400)(Mohan et al., 2007)
Banana stem and leaf biochar600Pb, CdPb 302.2; Cd 32.0LangmuirMineral precipitation dominant (∼85–89%)(Liu et al., 2022)
Water hyacinth biochar, RSM-optimized (Cu-BC/Zn-BC)∼425Cu, ZnCu 210.6; Zn 223.3LangmuirProcess optimization; vs. unoptimized Cu 177.7/Zn 146.1(Zhou et al., 2022)

1Model-extrapolated value.

The comparison highlights two points relevant to practical application. First, the metal-specific performance of vetiver root biochar follows the affinity sequence widely reported for biochar surfaces, with Pb consistently favored over Cu, Cd, and Zn, confirming that the present material behaves in line with established adsorption mechanisms rather than as an anomaly. Second, several of the higher-performing materials in Table 1 achieve their capacities only through chemical activation, mineral doping, or response-surface optimization of pyrolysis conditions, whereas vetiver root biochar attains competitive Pb removal in its pristine form using a waste feedstock with no additional processing. Reported qmax values in the literature are obtained under widely differing adsorbent doses, initial concentration ranges, pH values, ionic strengths, and contact times, and are in many cases Langmuir extrapolations, rather than experimentally saturated capacities. This indicates clear potential for further enhancement of Cu, Cd, and Zn capacities through activation or surface functionalization in future work, while underscoring the present material’s value as a low-cost adsorbent produced within a circular resource-recovery framework, in which a perfumery-industry residue is converted into an effective water-treatment material. The metal-laden biochar from this study was diverted from the landfill and instead used as an aggregate in an ultra-high-performance concrete study (Neve et al., 2023).

3.5 Adsorption edge study and surface complexation modeling

Figure 3 shows the pH-dependent adsorption behavior for Cd, Cu, Pb, and Zn. Adding biochar to the solution increased the initial pH from 3 to ≥ 5.5 in the case of Cu and to ≥ 6 for Cd, Pb, and Zn, which was consistent with its alkaline nature. Biochar did not significantly affect the higher pH levels at 9 and 11 (p > 0.05).

Influence of solution pH on the adsorption of heavy metals onto vetiver root biochar. Comparison of experimental data (points) and model predictions (solid lines) for the removal of **(a)** Cd, **(b)** Cu, **(c)** Pb, and **(d)** Zn across a pH range of 2–12.
Figure 3. Influence of solution pH on the adsorption of heavy metals onto vetiver root biochar. Comparison of experimental data (points) and model predictions (solid lines) for the removal of (a) Cd, (b) Cu, (c) Pb, and (d) Zn across a pH range of 2–12.

Under the adopted site density and protonation constants, a one-site monodentate diffuse-layer formulation provided an empirical fit to the observed pH-dependent dissolved-metal removal for Cd, Cu, Pb, and Zn, consistent with previously reported findings (Liu and Zhang, 2022; Betts et al., 2013; Qian et al., 2016). Minor deviations between the model and experimental data for Cu near pH 5.5 and Cd near pH 10.7 likely reflect the simplifying assumptions of the monodentate diffuse-layer model, which does not capture surface precipitation or multiple binding-site types; no comparable decreasing trend in Cu or Cd removal has been reported in the literature (Liu et al., 2022; Qian et al., 2016). The surface complexation reactions for Cd, Cu, Pb, and Zn with calibrated equilibrium constants are summarized in Table 2, while the parameters for surface complexation models are provided in Table 3. The two adsorption processes, physical adsorption and chemisorption, can describe the mechanisms of sorption of heavy metal ions onto biochar (Liu and Zhang, 2022; Alam et al., 2018b). In addition, the precipitation of metals onto the surface of biochar is also a possible removal mechanism owing to the pH values observed in the sorption edge study (Na et al., 2022; Betts et al., 2013). Removal by this biochar can proceed by surface complexation with oxygen-containing functional groups, cation exchange with ash-derived base cations (CEC 82 cmol kg-1), outer-sphere electrostatic attraction, and precipitation. The last cannot be discounted: the biochar is strongly alkaline (pH 10.98) and raised the solution pH from 3 to ≥5.5 for Cu and ≥6 for Cd, Pb, and Zn, conditions under which Pb and Cu hydroxide precipitation becomes increasingly favorable. We therefore do not attribute removal solely to chemisorption or surface complexation. The surface complexation model reproduces the observed pH-dependent removal but cannot by itself separate inner-sphere complexation from surface precipitation or ion exchange. We further note that the adopted site density, when normalized to the measured surface area, corresponds to an unusually high site density, reinforcing that it functions as an effective model parameter rather than a measured density of accessible surface sites.

TABLE 2. Surface and metal complexation reactions for vetiver root biochar used in the diffuse layer model, with equilibrium constants (log K) adopted from (Zhang et al., 2014).

ReactionslogKReferences
Surface reactions
≡XOH = ≡XO− + H+−12.00(Zhang et al., 2014)
≡XOH + H+ = ≡XOH2 +2.67(Zhang et al., 2014)
Surface complexation reactions
≡XOH + Cd+2 = ≡XOCd+ + H+−4.91This study
≡XOH + Cu+2 = ≡XOCu+ + H+−3.77This study
≡XOH + Pb+2 = ≡XOPb+ + H+−4.39This study
≡XOH + Zn+2 = ≡XOZn+ + H+−4.90This study

TABLE 3. Biochar and solution parameters used in the diffuse layer model. Total surface site density was adopted from (Zhang et al., 2014), determined for a municipal-organic-waste biochar; no titration data are available for vetiver root biochar.

ParametersValue
Surface area (m2/g)308.15
Solid concentrations (g/L)10
Total surface sites (mmol/g) a11
Initial concentration (mg/L)100
Ionic strength (M, NaNO3)0.1

(Zhang et al., 2014).

3.6 Extraction

The biochar used for the adsorption kinetics studies was used to assess the stability of bonding between biochar and adsorbed heavy metals. DI water and NaNO3 solution were used for the extraction of heavy metals from the heavy metal-loaded biochar, and irreversible bound was shown (Figure 4). In specific, Pb was detected in neither the water fraction nor the NaNO3 fraction. The desorption for both Cu and Zn were less than 1% for either DI water or NaNO3 extraction, while slightly higher numbers for Cd (3.10% for water and 3.35% for NaNO3 extraction), suggesting relatively weaker binding and higher exchangeability for Cd than others. These results indicated that vetiver root biochar can efficiently treat heavy metal contaminated water without releasing these metals into the water system after removal processes.

Various fractions of metal extraction in an aqueous medium. Distribution of Pb, Cu, Cd, and Zn fractions recovered by water (soluble) and NaNO<sub>3</sub> (exchangeable), compared to the unextractable (stable) fraction retained by the biochar.
Figure 4. Various fractions of metal extraction in an aqueous medium. Distribution of Pb, Cu, Cd, and Zn fractions recovered by water (soluble) and NaNO3 (exchangeable), compared to the unextractable (stable) fraction retained by the biochar.

The for Cu, Zn, and Cd (and zero for Pb) are consistent with numerous studies on pristine or unmodified biochars, which often report low desorption yields, typically less than 5% for neutral salt or water extractions, confirming that the initial strong sorption is largely irreversible (Koł et al., 2017; Trakal et al., 2011). This strong, irreversible binding is characteristic of chemical adsorption mechanisms like precipitation and surface complexation, especially for Pb2+ and Cu2+, which is why the less chemically-bound Cd2+ shows a slightly higher, yet still low, desorption rate compared to the other metals (Trakal et al., 2011).

3.7 Leaching studies

The standard USEPA SPLP can assess the leachability of waste under simulated rainfall conditions, while the TCLP test examines whether waste may be disposed of in municipal landfills as nonhazardous waste by simulating the conditions of a typical municipal landfill. Both the SPLP and TCLP tests showed concentrations of all leachate metals from the spent biochar well below U.S. EPA thresholds (Table 4), indicating the metal-loaded biochar is nonhazardous and suitable for landfill disposal. This confirms its potential as a safe and sustainable adsorbent for heavy metal remediation.

TABLE 4. TCLP and SPLP leaching results with Resource Conservation and Recovery Act (RCRA) 8 metals plus Cu and Zn (µg L-1) from spent vetiver root biochar, with corresponding U.S. EPA regulatory limits. Values are means of independent triplicates (n = 3).

​AgAsBaCdCrHgPbSeCuZn
TCLP Limit50005000100,0001000500020050001000--
Biochar TCLPBDL21.59229.6BDL1.660BDL17.41BDLBDLBDL
Biochar SPLPBDL41.639.900BDL2.190BDL7.5201.810BDLBDL

BDL, Below the Detection Limit.

4 Limitations and future work

All experiments used synthetic solutions in 0.1 M NaNO3. In natural waters, organic matter, hardness cations, and other background constituents will compete for sites, and so, the represented values should be considered an upper bound. The tested concentrations of up to 1000 mg/L are representative of concentrated industrial effluents and should not be extrapolated to dilute matrices. Filtered biochar leachate controls and post-sorption solid-phase characterization (XPS, SEM-EDS, XRD) were not performed. These would assist to further distinguish surface adsorption from homogeneous precipitation, surface precipitation, and co-precipitation, and are thus identified as priority future work. Parameter sensitivity analysis, incorporation of precipitation reactions, and independent model validation were not performed and are required to establish the robustness of the fitted constants. TCLP and SPLP results support landfill disposal of the spent char as non-hazardous, but this valorizes the feedstock without recovering the metals. These results were obtained from batch studies, and future studies should include column breakthrough studies. Surface precipitation and co-precipitation on ash-derived phases were not excluded. Aqueous speciation and mineral saturation calculations were not performed. While the fundamental removal mechanism was elucidated using a surface complexation model, the removal mechanism could be further validated based on the surface structure and surface coordination using spectroscopic evidence in future studies, such as the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The biochar that was spent during this study was used as an aggregate in ultra-high-performance concrete. It can also be regenerated and used for metal recovery, which was not the scope of this study.

5 Conclusion

The study demonstrated that biochar produced from spent vetiver roots can serve as an effective and sustainable adsorbent for the removal of Pb, Cu, Cd, and Zn from aqueous solutions. Detailed characterization revealed that the biochar possesses a high surface area, alkaline properties, and abundant functional groups, all of which contribute to rapid and efficient heavy-metal adsorption. Batch adsorption experiments showed high maximum adsorption capacities for each metal; however, the presence of multiple metals in solution reduced the adsorption performance due to competitive interactions. Adsorption edge analysis and surface complexation modeling confirmed that chemisorption is the dominant removal mechanism and emphasized the critical role of pH in optimizing adsorption efficiency. Moreover, the strong short-term retention of metals to the biochar minimizes leaching, thereby lowering the risk of secondary environmental contamination. Thus, vetiver root–derived biochar represents a non-hazardous and environmentally friendly metal scavenger with strong potential for heavy-metal remediation and water quality improvement. This study was limited to batch experiments using synthetic single- and multi-metal solutions at a fixed temperature, and did not address continuous-flow (column) operation, regeneration and reuse over multiple cycles, the influence of natural organic matter and competing background ions such as Ca2+ and Mg2+, or long-term stability. Future research can therefore focus on validating performance in real wastewater, evaluating fixed-bed column systems and reusability, assessing the effect of coexisting water constituents, and conducting a techno-economic and life-cycle assessment to confirm the material’s viability as a scalable, circular adsorbent.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvc.2026.1912197/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Cheng-Han Lee, National Cheng Kung University, Taiwan

Reviewed by: THANGAGIRI Baskaran, Mepco Schlenk Engineering College, India

Syarifudddin Syarifudddin, Hasanuddin University, Indonesia

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

SN: Formal Analysis, Data curation, Writing – original draft, Investigation, Methodology. VN: Writing – review and editing, Methodology, Visualization. ZZ: Writing – review and editing, Methodology, Visualization, Data curation. RD: Conceptualization, Writing – review and editing, Methodology. DS: Conceptualization, Resources, Writing – review and editing, Project administration, Methodology, Supervision.

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Citation

Neve, S., et al. (2026). Vetiver biochar application for heavy metal sequestration in water treatment: bridging perfumery waste and environmental sustainability. Frontiers in Environmental Chemistry, Volume 7. https://doi.org/
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Copyright © 2026 Neve, Nagara, Zhang, Datta and Sarkar.

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*Correspondence: Dibyendu Sarkar

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