The mercury adsorption properties of carbon-based materials prepared from jacaranda (Jacaranda mimosifolia) and guava (Psidium guajava) seed wastes are reported and compared in this paper. These adsorbent samples were obtained via pyrolysis and hydrothermal carbonization. Mercury adsorption equilibrium was studied at pH 4 and 20–40 °C, and the adsorption enthalpy changes were calculated for all adsorbent samples. The results showed that jacaranda-based materials contained a higher amount of acidic functional groups than guava seed-based adsorbents, and consequently, their mercury adsorption properties were better. The surface area of these adsorbents was <10 m2/g thus being classified as low-porosity materials. Elemental analysis indicated that all adsorbents were mainly composed of oxygen (4–25%) and carbon (75–95%). The calculated adsorption capacities at saturation of the best adsorbent were 18.05–30.09 mg/g under the tested experimental conditions. Statistical physics calculations also indicated that the adsorption mechanism of HgCl2 species was multi-molecular and endothermic. Ligand exchange and van der Waals forces were involved in generating the mercury–adsorbent interface. These results highlight the importance of comparing and optimizing biomass thermochemical conversion routes to tailor the surface properties of adsorbents used for water purification.

  • Assessment and modeling of mercury adsorption.

  • Mercury adsorption properties of carbon-based materials prepared from jacaranda (Jacaranda mimosifolia) and guava (Psidium guajava) seed wastes.

  • Importance of comparing and optimizing biomass thermochemical conversion routes.

The existence of dissolved mercury in water is caused by both natural and anthropogenic pollution sources where the residual discharge from various industries (e.g., mining, energy, battery manufacturing, aerospace, and textiles) plays a significant role in its environmental release (Velempini & Pillay 2019; Prasetya et al. 2020). This heavy metal is toxic and non-biodegradable, and its bioaccumulation in the environment can affect living organisms and pose a risk to public health (Prasetya et al. 2020; Li et al. 2021; He et al. 2023; Shukla et al. 2023; Zhang et al. 2023a). A chronic exposure to mercury has been associated to heart diseases, neurological damage, and cancer in humans (Velempini & Pillay 2019; Prasetya et al. 2020). Toxicological results using laboratory animals have also indicated that the mercury exposure can generate kidney damage, neuropathological effects, decrease in fertility, besides showing carcinogenic activity; while growth retardation has been observed for plants exposed to mercury-polluted water (Syversen & Kaur 2012).

Effluents and wastewater containing mercury can be treated using various physicochemical methods including traditional coagulation and precipitation operations and advanced purification technologies such as membrane-based separation and electrochemical processes (Al-Jaaf et al. 2022; Ali et al. 2022a; Yoo et al. 2023). Particularly, the adsorption-based processes have proved to be effective for the removal of inorganic and organic pollutants from water. For example, Al-Jaaf et al. (2022) showed the benefits of using an eggplant peel-based adsorbent to purify a domestic wastewater. Therefore, the depollution of water containing mercury via adsorption is characterized by its lower cost and higher removal efficiency than those of other treatment technologies (Luo et al. 2020; Duman 2021; Jung et al. 2021). Zeolites, clay, polymers, industrial residues, activated carbons, chars, metal oxides, and their composites have been used as adsorbents to reduce the mercury concentrations in polluted streams (Zhu et al. 2012; Raji & Pakizeh 2014; Ganzagh et al. 2016; Saman et al. 2016; Shen et al. 2018; Prasetya et al. 2020; Jumah et al. 2021; Li et al. 2021; Liu et al. 2022; He et al. 2023). The performance of some of these materials is promising for large-scale water treatment due to their high adsorption capacities, but the main limitation is usually their depollution cost. The operational cost of adsorption systems is mainly related to the production of the solid phase used as the separation medium. Consequently, it is important to identify new feedstocks and low-cost and sustainable preparation routes to improve the economy of water treatment.

Adsorbents prepared by the thermochemical conversion of biomass wastes and subproducts are an interesting alternative for reducing the water purification cost associated with the mercury removal (Jung et al. 2021; Tran et al. 2021; Qin et al. 2023; Zhang et al. 2023a). Carbon-based adsorbents are typically obtained via the thermochemical conversion of biomass, in addition to surface functionalization, to tailor their surface properties. The adsorbent removal performance depends on the precursor feedstock, the synthesis route (e.g., temperature and dwell time), and the chemicals utilized in their functionalization. The quantity and type of adsorbent functional groups (i.e., active sites to bind the water pollutant) are determined by manipulating the synthesis parameters. Therefore, the analysis of the preparation conditions is crucial for establishing the final surface chemistry of the adsorbent.

Pyrolysis and hydrothermal carbonization are reliable biomass conversion routes to obtain materials to reduce the concentration of water pollutants, such as mercury (Correa et al. 2019; Zhang et al. 2023a). They differ in terms of energy consumption, process operating conditions, adsorbent yields, and final product properties (Rattanachueskul et al. 2017; Nguyen et al. 2022). The degradation of the natural polymers contained in biomass determines the development of the porous adsorbent structure and occurs at different temperatures for each preparation route. Therefore, adsorbents prepared from the same feedstock using hydrothermal carbonization and pyrolysis can differ substantially in their surface functionalities and textural properties (Zhang et al. 2023a). The tailoring of adsorbent properties implies the identification of the best biomass conversion conditions to improve the removal of the target adsorbate. Adsorbents for liquid-phase separation can be prepared via the pyrolysis and hydrothermal carbonization of biomass residues such as coconut shells (Jain et al. 2015), sugarcane bagasse (Rattanachueskul et al. 2017), tree leaves (Yang et al. 2019), palm kernel shells (Beri et al. 2021), rice husk (Jung et al. 2021), and corn straw (Zhang et al. 2023a). The porous solids synthesized from the conversion of lignocellulosic biomass residues typically contain oxygenated functional groups. Carboxylic, phenolic, lactonic, ketone, and hydroxyl groups are important for the mercury adsorption on carbon-based surfaces (Jain et al. 2015; Duman 2021; Nguyen et al. 2022). Consequently, it is important to compare the removal performance and properties of the adsorbents obtained using both hydrothermal carbonization and pyrolysis for the available variety of feedstock to identify the best alternative. However, this comparative approach has not been implemented in the preparation of carbon-based adsorbents in previous studies, which usually implies the definition of arbitrary adsorbent preparation conditions that are expected to be far from the optimum separation efficacy–cost balance.

A comparative analysis of the mercury removal properties of adsorbents obtained from pyrolysis and hydrothermal carbonization of Jacaranda mimosifolia fruit and guava (Psidium guajava; Psidium guajava, Linn) seeds is reported in this manuscript. Note that other studies have introduced these biomass wastes as feedstock to obtain activated carbons for the adsorption of pharmaceuticals, heavy metals, phenolic compounds, and dyes (Abdelwahab et al. 2007; Elizalde-González & Hernández-Montoya 2009; Treviño-Cordero et al. 2013; Anisuzzaman et al. 2016; Pezoti et al. 2016; Aly et al. 2019; Ortíz-Gutiérrez et al. 2020; Georgin et al. 2021; Ali et al. 2022b; Pindiga et al. 2022). For example, Elizalde-González & Hernández-Montoya (2009) used the guava seeds as precursor of carbon-based adsorbents for the removal of acid dyes. Anisuzzaman et al. (2016) obtained activated carbon from guava seeds for chlorinated phenol adsorption. This biomass was also employed by Pezoti et al. (2016) as feedstock to prepare activated carbon for amoxicillin uptake. The guava biomass (including leaves, tree bark, and seeds) has also been applied as an adsorbent for the removal of arsenic, fluoride, heavy metals, phenol, and dyes (Domínguez & Serrano 2004; Abdelwahab et al. 2007; Valencia-Leal et al. 2012; Sánchez-Sánchez et al. 2013; Aly et al. 2019; Mohan et al. 2019; Mandal et al. 2020; Ortíz-Gutiérrez et al. 2020; Georgin et al. 2021; Ali et al. 2022b; Behera et al. 2022; Pindiga et al. 2022). Abdelwahab et al. (2007) and Ortíz-Gutiérrez et al. (2020) evaluated the performance of guava seeds for the adsorption of hexavalent chromium. Elizalde-González & Hernández-Montoya (2009) conducted experiments to assess the effectiveness of this residue as an adsorbent of different acid dyes. The guava leaves were employed for the removal of cadmium (Ali et al. 2022b; Pindiga et al. 2022), lead (Pindiga et al. 2022), and dyes (Aly et al. 2019). The adsorption of geogenic pollutants (e.g., arsenic and fluoride) using guava waste was also studied by Mohan et al. (2019), Behera et al. (2022), Sánchez-Sánchez et al. (2013) and Valencia-Leal et al. (2012). On the other hand, Mandal et al. (2020) reported the adsorption of phenol on guava tree bark. Regarding the J. mimosifolia, Treviño-Cordero et al. (2013), Georgin et al. (2021), and Domínguez & Serrano (2004) reported the synthesis of activated carbons using this biomass as precursor for the adsorption of heavy metals, dyes, and ketoprofen.

In this study, the application of the carbon-based materials prepared from these biomass wastes was extended to analyze the mercury removal at pH 4 and 25–40 °C. The main surface chemistry and textural parameters of these adsorbents were also studied and compared. Adsorption equilibrium was investigated using a statistical physics model to characterize the interactions involved in generating the interface responsible for mercury removal. Therefore, the aim of the present study was to assess the surface properties of these alternatives adsorbents to remove mercury from aqueous solutions. The novelty of this study lies on a reliable comparison and analysis of the application of pyrolysis and hydrothermal carbonization of residual biomass precursors to obtain new adsorbents for the removal of mercury as a priority water pollutant.

Chemicals and analytical equipment

Deionized water (Mapla), analytical-grade HgCl2 (Sigma-Aldrich), standard NaOH and HCl solutions (Fisher), and KBr (Sigma-Aldrich) were used in this study. The analytical equipment included: Empyrean (Malvern PANalytical) X-ray diffractometer, Nicolet iS10 (Thermo Scientific) FTIR spectrometer, TM3000 (Hitachi) scanning electron microscope with a coupled energy dispersive system (Nano XFlash, Bruker), ICE 3000 (Thermo Scientific) atomic absorption spectrophotometer and ASAP 2020 (Micromeritics) porosimeter.

Synthesis of carbon-based adsorbents using pyrolysis and hydrothermal carbonization and their physicochemical characterization

Carbon-based adsorbents were obtained from the biomass wastes of J. mimosifolia and P. guajava using pyrolysis and hydrothermal carbonization. These biomass wastes were obtained from local agri-food industries, parks, and gardens. They were then dried, shredded, and screened for the adsorbent synthesis. The pyrolysis of biomass waste was performed in a tubular furnace at 600 °C for 2 h with 10 °C/min heating rate under N2 atmosphere. Hydrothermal carbonization was conducted in a 100-mL Teflon-lined autoclave reactor using a mass/volume ratio of 1:4 (i.e., 15 g of biomass and 60 mL of deionized water), at 180 °C for 12 h. The results reported in literature and preliminary trials were employed to define these preparation conditions. All adsorbents were washed with deionized water (until a constant pH was obtained from the residual effluent), dried in an oven for 24 h, and sieved to obtain a homogeneous particle size (∼0.67 mm) for the adsorption studies. The samples were labelled as J-PYR, G-PYR, J-HTC, and G-HTC for the solid products obtained from the pyrolysis (PYR) and hydrothermal carbonization (HTC) of jacaranda (J) and guava seed (G) wastes, respectively. A schematic representation illustrating these experimental steps is reported in Supplementary material, Figure S1.

All the adsorbents were characterized to determine their main physicochemical properties. Specifically, the X-ray diffraction (XRD) patterns of these samples were obtained to analyze the crystalline structure and presence of inorganic elements. The adsorbents were analyzed at room temperature with Cu radiation (λ = 1.5406 Å) at 45 kV/40 mA in the angular range of 10°–60° 2θ. The functional groups present on the surfaces of these adsorbents were identified using Fourier transform infrared (FTIR) analysis. FTIR spectra of KBr-adsorbent pellets were recorded (32 scans per sample) in the 4,000–400 cm−1 range with a resolution of 4 cm−1. The elemental composition and morphology of all the adsorbents were determined by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray (EDX) analysis. The samples were submitted to high vacuum with an accelerating voltage of 20 kV prior their analysis. Textural properties of adsorbent samples were estimated from N2 physisorption, which were quantified at 77 K. Finally, the total basic and acidic groups were quantified according to the method described by Boehm (1994) and following the methodology reported by Pawlicka & Doczekalska (2013). The titrations were performed with 0.1 M NaOH and HCl solutions where the corresponding experimental conditions were 1 g/L of adsorbent/solution ratio, 48 h of contact time at 25 °C. Mass balance analysis was carried out with the results from solution titrations to calculate the concentrations of the adsorbent basic and acidic sites. The estimation of pH value of the point of zero charge (pHpzc) for all the adsorbents was conducted following the methodology described by Milonjić et al. (1975).

Analysis of mercury adsorption equilibrium and isotherm modeling

The equilibrium of mercury adsorption on the four adsorbents (J-PYR, G-PYR, J-HTC and G-HTC) was studied with batch adsorbers. Adsorption studies were performed with initial mercury concentrations (Cini,Hg) from 20 to 300 mg/L. Deionized water and HgCl2 were utilized for the solution preparation. The experimental adsorption isotherms were obtained at a solution temperature of 20, 30, and 40 °C and pH 4 under constant stirring at 120 rpm for 24 h, with an adsorbent dose (w/VHg) of 5 g/L. The concentration of mercury dissolved in all the solutions was quantified via atomic absorption spectroscopy with a linear calibration curve. The adsorption capacities (qHg, mmol/g) of the carbon-based adsorbents were calculated with the mass balance
formula
(1)
where Ceq,Hg is the equilibrium mercury concentration (mg/L), VHg is the mercury solution volume (L) and w is the adsorbent mass (g).
The experimental isotherms were utilized to estimate the mercury adsorption enthalpy change (ΔH°, kJ/mol) using the van't Hoff approach and the procedure reported by Tran et al. (2017) and Lima et al. (2020) applying the next equations (Tran et al. 2017; Lima et al. 2020).
formula
(2)
formula
(3)
where R = 8.314E-03 kJ/K·mol is the universal gas constant, ΔSo (kJ/K·mol) is the adsorption entropy change, KHg is the dimensionless thermodynamic equilibrium constant of the mercury adsorption on tested adsorbent at adsorption temperature T (K), CAds,Hg is the concentration of mercury adsorbed on the adsorbent surface at the equilibrium (mg/L), γAds and γe are the activity coefficients of mercury adsorbed on carbon-based surface and dissolved in the aqueous solution, respectively. Diluted solutions were assumed to obtain the KHg values for the ΔH° calculation following the approach suggested by Tran et al. (2017).
A statistical physics-based monolayer model (Amrhar et al. 2021) was applied to determine the steric parameters associated with mercury removal by the tested adsorbents. This statistical physics model was applied to interpret and understand the mercury adsorption mechanism. Several studies have proved that the adsorption models based on statistical physics fundaments are reliable to analyze the equilibrium adsorption data of water pollutant, thus overcoming the limitations of traditional adsorption models such as Freundlich and Langmuir equations (Sellaoui et al. 2019; Amrhar et al. 2021; Valdés-Rodríguez et al. 2022). This model was defined as (Amrhar et al. 2021)
formula
(4)
where nHg is the number of adsorbed mercury species per adsorption site on the tested adsorbent, NAds is the concentration of adsorption sites on the adsorbent surface that participate in mercury removal (mg/g), and Csm,Hg is the half-saturation mercury concentration (mg/L).
The model parameters were obtained from the nonlinear regression of experimental isotherms using the Excel® solver and the next objective function
formula
(5)
where ndat is the number of experimental data points from each isotherm, cal and exp are the calculated (by the monolayer adsorption model) and experimental adsorption capacities (qHg, mg/g), and qHg,sat is the mercury adsorption capacity at the adsorbent saturation condition (mg/g). This saturation adsorption capacity was defined as
formula
(6)
Finally, the interaction energies (ΔEHg-Ads, kJ/mol) for the mercury – adsorbent interface were calculated as follows (Amrhar et al. 2021)
formula
(7)
where SHg (mg/L) is the HgCl2 solubility in the aqueous solution.

Mercury adsorption and its modeling

The mercury adsorption isotherms obtained with J-HTC, G-HTC, J-PYR, and G-PYR are shown in Figure 1. The adsorbent yields obtained from the tested preparation routes were 26% (J-PYR), 24% (G-PYR), 73% (J-HTC), and 75% (G-HTC). These results confirmed the findings reported in other studies, where the yields of hydrothermal carbonization were higher than those of pyrolysis (Jian et al. 2018). The adsorption capacities of the adsorbents were 2.00 (± 0.01) – 17.05 (± 0.12), 0.60 (± 0.03) – 12.84 (± 0.57), 0.60 (± 0.02) – 30.69 (± 0.77) and 0.60 (± 0.03) – 10.23 (± 0.14) mg/g, respectively. These adsorption capacities corresponded to mercury removal percentages of 16–63, 15–72, 9–44 and 8–35% for J-HTC, J-PYR, G-HTC and G-PYR, respectively. J-PYR adsorbent exhibited the highest adsorption capacities, while G-PYR was outperformed by all the adsorbents. In fact, the adsorbent performance followed the trend: J-PYR >> J-HTC > G-HTC > G-PYR. These adsorption capacities increased by 84, 146, 77 and 127%, respectively, when the solution temperature changed from 20 to 40 °C. Mercury adsorption on these carbon-based materials was endothermic under tested experimental conditions. The calculated mercury adsorption enthalpies for J-HTC, G-HTC, J-PYR, and G-PYR were 25, 19, 40, and 39 kJ/mol, respectively. These ΔH° values indicated that the removal of this toxic heavy metal was governed mainly by physical intermolecular forces using these carbon-based adsorbents (Tran et al. 2020). The endothermic mercury adsorption on these materials was consistent with the findings reported by Yoo et al. (2023), Zabihi et al. (2010), Ismaiel et al. (2013), Park et al. (2019), Zúñiga-Muro et al. (2020), Valdés-Rodríguez et al. (2022), Gheitasi et al. (2022) and Kaveh & Bagherzadeh 2022. Hadi et al. (2015) reviewed the mercury adsorption on activated carbons and concluded that several studies have reported an endothermic process. Mercury adsorption enthalpy changes ranging from 9 to 39 kJ/mol have been reported for other activated carbons and adsorbents prepared from lignocellulosic biomass (Ismaiel et al. 2013; Gheitasi et al. 2022; Kaveh & Bagherzadeh 2022; Yoo et al. 2023). Therefore, these values agreed with the results of this study.
Figure 1

Mercury adsorption isotherms at pH 4 using adsorbents obtained from the pyrolysis and hydrothermal carbonization of guava seed and jacaranda biomass.

Figure 1

Mercury adsorption isotherms at pH 4 using adsorbents obtained from the pyrolysis and hydrothermal carbonization of guava seed and jacaranda biomass.

Close modal

As indicated, the mercury adsorption properties of carbon-based adsorbents synthesized from jacaranda biomass were better than those of guava waste-based adsorbents, independent of the chosen preparation route (i.e., hydrothermal carbonization or pyrolysis). This performance may be related to the polymeric composition (i.e., lignin, hemicellulose, and cellulose) of these residues, which affects the adsorbent surface chemistry. Other studies have indicated that jacaranda biomass contains a polymeric fraction of 98% (i.e., cellulose 50%, hemicellulose 21%, and lignin 27%), whereas the polymeric content of guava seed biomass is 77% (i.e., cellulose 61%, hemicellulose 9%, and lignin 7%) (Elizalde-González & Hernández-Montoya 2009; Treviño-Cordero et al. 2013). The type of thermal treatment applied to prepare each adsorbent also affects the presence of surface functionalities, because temperature promotes the degradation of natural polymers contained in the biomass precursor. Hemicellulose starts its thermal decomposition at ∼160 °C, while lignin and cellulose begin to degrade at ∼ 220 and 180 °C, respectively (Heidari et al. 2019). The degradation degree of these polymers depends on the adsorbent preparation process and its conditions (Heidari et al. 2019). Therefore, the type of biomass feedstock is a key parameter affecting the adsorption performance and physicochemical properties of carbon-based materials (Han et al. 2017). Note that the lignin content of the biomass precursor has been associated with a significant impact on the adsorbent surface chemistry, mainly on oxygenated functional groups (Arias-Arias et al. 2017). For instance, previous studies have shown that this polymer undergoes condensation reactions and polymerizes to form the adsorbent structure during hydrothermal carbonization of biomass (Plavniece et al. 2022). The jacaranda waste has a higher lignin content than guava seed residues, thus causing wetting and water diffusion inside the particles to become more constrained (Rodríguez-Correa et al. 2019). This may cause insoluble lignin to generate a highly condensed adsorbent structure (Plavniece et al. 2022), which could limit the mass transfer of dissolved mercury in aqueous solutions, thus affecting the adsorption performance. Consequently, the J-PYR adsorbent sample obtained by pyrolysis exhibited a higher mercury adsorption capacity than J-HTC prepared via hydrothermal carbonization. Wang et al. (2022) also found similar results for the removal of cadmium ions with adsorbents prepared from Napier grass. Table 1 reports the concentrations of the acidic groups found in the tested adsorbent samples. The concentration of acidic sites followed the order: J-HTC > J-PYR ≅G-HTC > G-PYR. The jacaranda-based adsorbents had a higher concentration of oxygenated functionalities (e.g., hydroxyl and carboxyl groups) for mercury binding than other adsorbents (G-HTC and G-PYR). It is also clear that G-HTC outperformed G-PYR because of its higher acidic functional group content. These findings agreed with those reported in the literature regarding the importance of oxygenated functional groups for heavy metal adsorption from water (Arias-Arias et al. 2017; Othmani et al. 2021). On the other hand, it was found that that the leaching of trace elements (e.g., Mg, Fe, Ca, K, Na) from adsorbent surface did not occur in the aqueous solution after mercury removal.

Table 1

Chemical and textural properties of carbon-based adsorbents prepared from pyrolysis and hydrothermal carbonization of jacaranda fruit and guava seeds

AdsorbentTotal groups, mmol/g
pHpzcTextural properties
BasicAcidicBET surface area, m2/gPore size, Å
G-PYR 6.70 ± 0.14 9.41 ± 0.10 7.13 ± 0.13 3.4 15.84 
G-HTC 6.49 ± 0.11 9.52 ± 0.09 6.30 ± 0.12 <1 8.75 
J-PYR 6.43 ± 0.08 9.53 ± 0.07 5.08 ± 0.23 2.4 57.48 
J-HTC 6.32 ± 0.13 10.16 ± 0.08 6.28 ± 0.08 <1 9.66 
AdsorbentTotal groups, mmol/g
pHpzcTextural properties
BasicAcidicBET surface area, m2/gPore size, Å
G-PYR 6.70 ± 0.14 9.41 ± 0.10 7.13 ± 0.13 3.4 15.84 
G-HTC 6.49 ± 0.11 9.52 ± 0.09 6.30 ± 0.12 <1 8.75 
J-PYR 6.43 ± 0.08 9.53 ± 0.07 5.08 ± 0.23 2.4 57.48 
J-HTC 6.32 ± 0.13 10.16 ± 0.08 6.28 ± 0.08 <1 9.66 

The four adsorbents exhibited different pHpzc values ranging from 5.08 (J-PYR) to 7.13 (G-PYR), see Table 1. At tested solution pH, all the adsorbents were positively charged, and the mercury adsorption capacity of the tested adsorbents decreased as pHpzc increased. This trend indicated that electrostatic forces were a minor contributor to the interaction forces during the removal of this metal cation. The corresponding speciation diagram shows that HgCl2 was mainly present in the aqueous solution at pH 4 (see Supplementary material, Figure S2), thus explaining the negligible effect of the electrostatic forces on the adsorption mechanism. HgCl2 can interact with the acidic functional groups (mainly hydroxyl and carboxyl groups) of these adsorbents via ligand exchange, as described below (Arias-Arias et al. 2017; Kaveh & Bagherzadeh 2022). All adsorbents showed specific surface areas < 10 m2/g and could be classified as low-porosity materials, see Table 1. These results indicated that the surface area of these adsorbents did not play a relevant role for mercury adsorption, which was confirmed with a statistical analysis. Consequently, it could be expected that the interfacial phenomenon for the separation of mercury from the aqueous solution occurred mainly via the interaction with the functional groups located on the external surface of these adsorbents, with a very limited contribution from intraparticle diffusion.

The steric parameters calculated for mercury removal using the tested adsorbents are listed in Table 2. Statistical physics calculations indicated that the adsorption of this toxic metal was multi-molecular, and up to two HgCl2 molecules could interact with each oxygenated functional group on the adsorbent surface. Specifically, nHg values were 1.9–2.0, 1.6–2.3, 2.0–2.5 and 1.3–2.4 for G-PYR, G-HTC, J-PYR and J-HTC, respectively. The calculated concentrations of adsorption sites involved in the binding of mercury species were 2.41–5.22, 2.41–8.22, 8.02–15.04 and 7.22–14.64 mg/g for G-PYR, G-HTC, J-PYR and J-HTC, respectively. These results indicated that less than 1% of the acidic functional groups available on the adsorbent surface interacted with and adsorbed this heavy metal. The modeling results and textural parameters listed in Table 1 are the basis for inferring that the mercury adsorption occurred mainly on the external surfaces of these adsorbents. The calculated interaction energies ranged from 16 to 20 kJ/mol, confirming that physical interaction forces were responsible of the mechanism of mercury adsorption for these adsorbents. Finally, the calculated saturation adsorption capacities followed the trend: J-PYR (18.05–30.09 mg/g) > J-HTC (10.03–18.05 mg/g) > G-HTC (6.02–14.04 mg/g) > G-PYR (4.01–10.03 mg/g), see Figure 2. J-PYR adsorbent can outperform mercury removal properties reported for other activated carbons and chars obtained via pyrolysis of different lignocellulosic biomass (Hadi et al. 2015; Zúñiga-Muro et al. 2020; Valdés-Rodríguez et al. 2022). In fact, the adsorption properties of J-PYR were better than those of carbon-based adsorbents prepared by activation with H2O, H2SO4, HCl, H2O2, and air, with surface areas of 280–870 m2/g (Hadi et al. 2015). For illustration, Table 3 reports a comparison of the mercury adsorption capacities reported for different materials. The adsorbents obtained from jacaranda fruit and guava seed biomass showed reasonable removal performance in comparison with other biomass-based adsorbents, although they were outperformed by silicon-based materials such as SBA-15 and MCM-41. However, it is important to highlight that the carbon-based materials prepared from residual biomass and its subproducts usually offer more benefits in terms of environmental protection, low production cost, biomass waste minimization and final disposal.
Table 2

Results of monolayer statistical physics model for mercury adsorption using carbon-based adsorbents

AdsorbentT, °CnHgNads, mg/gR2
G-PYR 20 2.0 2.41 0.995 
 30 1.9 3.21 0.995 
 40 2.0 5.22 0.996 
G-HTC 20 2.3 2.41 0.998 
 30 1.8 4.01 0.996 
 40 1.6 8.22 0.988 
J-PYR 20 2.3 8.02 0.993 
 30 2.5 9.63 0.997 
 40 2.0 15.04 0.999 
J-HTC 20 1.3 7.22 0.986 
 30 1.3 8.42 0.993 
 40 1.3 14.64 0.977 
AdsorbentT, °CnHgNads, mg/gR2
G-PYR 20 2.0 2.41 0.995 
 30 1.9 3.21 0.995 
 40 2.0 5.22 0.996 
G-HTC 20 2.3 2.41 0.998 
 30 1.8 4.01 0.996 
 40 1.6 8.22 0.988 
J-PYR 20 2.3 8.02 0.993 
 30 2.5 9.63 0.997 
 40 2.0 15.04 0.999 
J-HTC 20 1.3 7.22 0.986 
 30 1.3 8.42 0.993 
 40 1.3 14.64 0.977 
Table 3

Mercury adsorption capacities reported for different adsorbents

AdsorbentpHTemperature, °CAdsorption capacity, mg/gReference
Activated carbon from beer barley husk 25 109.37 Gheitasi et al. (2022)  
Activated carbon from grape pomace 25 4.21 Valdés-Rodríguez et al. (2022)  
Activated carbon from guava seeds   61.18  
Activated carbon from J. mimosifolia seeds   130.58  
Activated carbon from sugarcane bagasse   104.71  
Biochar from rice straw biogas residue 25 209.65 Liu et al. (2022)  
Grape bagasse-based char 30 32.29 Zúñiga-Muro et al. (2020)  
Corn husk biomass 25 18.86 Núñez-Zarur et al. (2018)  
Spanish broom plant 19.86 Arias-Arias et al. (2017)  
Coconut pit char 4.5 30 46.14 Saman et al. (2016)  
Activated carbon from coconut pit 142.42 
Candlenut shell charcoal – 25 76.35 Mariana et al. (2022)  
Candlenut shell particles 37.18 
Magnetic rice straw-derived biochar 25 ∼73.00 Lim et al. (2023)  
Magnetic activated carbon – 166.60 Kaveh & Bagherzadeh (2022)  
Thiol functionalized SBA-15 25 ∼225.00 Shen et al. (2018)  
SBA-15/Ag nanocomposite 27 42.26 Ganzagh et al. (2016)  
MCM-41 modified by ZnCl2 20 87.00 Raji & Pakizeh (2014)  
Amine-grafted MCM-41 25 118.35 Zhu et al. (2012)  
β-cyclodextrin/MCM-48 composite 20 173.40 Jumah et al. (2021)  
Guava seed hydrochar (G-HTC) 30 6.88 This study 
Jacaranda hydrochar (J-HTC) 10.38 
Guava seed char (G-PYR) 5.82 
Jacaranda char (J-PYR) 23.47 
AdsorbentpHTemperature, °CAdsorption capacity, mg/gReference
Activated carbon from beer barley husk 25 109.37 Gheitasi et al. (2022)  
Activated carbon from grape pomace 25 4.21 Valdés-Rodríguez et al. (2022)  
Activated carbon from guava seeds   61.18  
Activated carbon from J. mimosifolia seeds   130.58  
Activated carbon from sugarcane bagasse   104.71  
Biochar from rice straw biogas residue 25 209.65 Liu et al. (2022)  
Grape bagasse-based char 30 32.29 Zúñiga-Muro et al. (2020)  
Corn husk biomass 25 18.86 Núñez-Zarur et al. (2018)  
Spanish broom plant 19.86 Arias-Arias et al. (2017)  
Coconut pit char 4.5 30 46.14 Saman et al. (2016)  
Activated carbon from coconut pit 142.42 
Candlenut shell charcoal – 25 76.35 Mariana et al. (2022)  
Candlenut shell particles 37.18 
Magnetic rice straw-derived biochar 25 ∼73.00 Lim et al. (2023)  
Magnetic activated carbon – 166.60 Kaveh & Bagherzadeh (2022)  
Thiol functionalized SBA-15 25 ∼225.00 Shen et al. (2018)  
SBA-15/Ag nanocomposite 27 42.26 Ganzagh et al. (2016)  
MCM-41 modified by ZnCl2 20 87.00 Raji & Pakizeh (2014)  
Amine-grafted MCM-41 25 118.35 Zhu et al. (2012)  
β-cyclodextrin/MCM-48 composite 20 173.40 Jumah et al. (2021)  
Guava seed hydrochar (G-HTC) 30 6.88 This study 
Jacaranda hydrochar (J-HTC) 10.38 
Guava seed char (G-PYR) 5.82 
Jacaranda char (J-PYR) 23.47 
Figure 2

Calculated saturation adsorption capacities of the carbon-based adsorbents for mercury removal at pH 4.

Figure 2

Calculated saturation adsorption capacities of the carbon-based adsorbents for mercury removal at pH 4.

Close modal

Physicochemical characterization of carbon-based adsorbents used in mercury removal

XRD diffractograms of the four adsorbents with and without loaded mercury are shown in Figure 3. All samples showed the characteristic patterns of carbon-based materials derived from lignocellulosic biomass. Specifically, the XRD patterns of J-HTC and G-HTC contained wide diffraction peaks at ∼ 15°, 22°, and 34° 2θ, attributed to cellulose crystallization (Wang et al. 2022), as shown in Figure 3(a). This result indicated that only a fraction of biomass cellulose was converted during the hydrothermal reaction (Han et al. 2017; Li et al. 2021). However, the diffraction peaks of the cellulose crystals disappeared in the diffraction pattern of the pyrolysis-based adsorbents (Figure 3(b)), leading to the formation of carbon planes characteristic of graphitic structures, which corresponded to the two diffraction peaks at ∼22° and 43° 2θ (Nanda et al. 2012; Li et al. 2021; Pan et al. 2021; He et al. 2023). The amorphous behavior of these samples suggested that the microcrystalline structure of cellulose was destroyed (Han et al. 2017). This result was associated with the thermal treatment conditions and degradation temperatures of the structural biopolymers (i.e., cellulose, hemicellulose, and lignin) contained in both precursors (Heidari et al. 2019; Li et al. 2020; Pan et al. 2021). Other diffraction peaks at ∼29.3°, 35.9°, 39.4°, 43.2°, 47.4°, and 48.4° 2θ corresponding to the crystalline structure of calcium carbonate (ICDD: 00-005-0586) were identified in the G-PYR sample. Calcium is a component of the mineral composition of guava seeds, including iron, copper, zinc, magnesium, and other trace elements (Pezoti et al. 2016; Silveira-Junior et al. 2020). After the loading of mercury on the adsorbent surfaces, a decrease in the crystallinity of all samples was observed. This change in the diffraction patterns was caused by the incorporation of this heavy metal on the adsorbent surface (Marciniak et al. 2019; Li et al. 2020). New diffraction peaks (at ∼21.4°, 28.1°, 31.6°, 32.8°, 40.2°, 43.7°, 46.1°, and 52.8° 2θ) associated with mercury chloride (ICDD: 01-073-1247) were identified in the XRD results of the adsorbent samples that were synthesized via hydrothermal carbonization. Note that analogous results were reported by Liu et al. (2022). The interaction between mercury and the main oxygenated functional groups of carbon-based adsorbents can imply a ligand exchange, which is represented as follows (Chaudhuri et al. 2022)
formula
(8)
formula
(9)
Figure 3

X-ray diffraction patterns of carbon-based adsorbents synthesized from jacaranda fruits and guava seeds biomass using (a) hydrothermal carbonization and (b) pyrolysis. Label ‘ + Hg’ indicates the adsorbent samples loaded with mercury

Figure 3

X-ray diffraction patterns of carbon-based adsorbents synthesized from jacaranda fruits and guava seeds biomass using (a) hydrothermal carbonization and (b) pyrolysis. Label ‘ + Hg’ indicates the adsorbent samples loaded with mercury

Close modal

In addition, van der Waals forces may have been present during the mercury adsorption (Lim et al. 2023).

As stated, the surface area of tested adsorbents was <10 m2/g for all samples and, consequently, they can be considered as low-porosity adsorbents. On the other hand, Figure 4 shows the surface morphologies at 100x of the four carbon-based adsorbents synthesized from jacaranda fruit and guava seed biomass. The adsorbents prepared by hydrothermal carbonization exhibited a rough, compact, and irregular morphology with low porosity. In contrast, the J-PYR and G-PYR adsorbents had irregular cavities with different shapes and sizes owing to the surface breakage and fragmentation of large to small molecules during thermal treatment (Treviño-Cordero et al. 2013; Basu 2018; Pan et al. 2021). Elemental analysis indicated that all adsorbents were mainly composed of oxygen (4–25%) and carbon (75–95%), which are the main elements in lignocellulosic-based materials (Treviño-Cordero et al. 2013; Mendoza-Castillo et al. 2014).
Figure 4

SEM micrographs of carbon-based adsorbents synthesized from jacaranda fruits and guava seeds biomass by hydrothermal carbonization and pyrolysis.

Figure 4

SEM micrographs of carbon-based adsorbents synthesized from jacaranda fruits and guava seeds biomass by hydrothermal carbonization and pyrolysis.

Close modal
FTIR spectra of all adsorbent samples are given in Figure 5. The absorption band of the stretching vibration of the hydrogen bonded to the –OH group identified at ∼3,420 cm−1 was due to alcohols linked to the polymeric content of the biomass precursor (Lima et al. 2007; Li et al. 2018, 2020; Mamaní et al. 2019; Yoo et al. 2023). The absorption bands at ∼2,930–2,850 cm−1 corresponded to the C–H stretching vibrations of aliphatic chains (Li et al. 2018, 2020; Mamaní et al. 2019). The set of absorption bands located between ∼2,000 and 1,500 cm−1 were associated with the C = C stretching vibration (∼1,600 cm−1) present in the aromatic rings and the C = O stretching vibration (∼1,700 and 1,435 cm−1) of the carboxylic groups, respectively (Lima et al. 2007; Li et al. 2018, 2020; Mamaní et al. 2019). The absorption bands at approximately 1,300–910 cm−1 were assigned to the C–O stretching vibrations of carbonyls, alcohols, carboxylic acids, phenols, ethers, and esters (Li et al. 2018, 2020; Mamaní et al. 2019), whereas the absorption bands at ∼870–700 cm−1 originated from the out-of-plane deformation produced by aromatic C-H atoms (Zhang et al. 2023b). FTIR spectra of the adsorbents synthetized via hydrothermal carbonization differed significantly than those of adsorbents prepared by pyrolysis. The absorption bands of some functional groups lose their intensity, indicating that their degradation was caused by the temperature used in the thermochemical conversion of the precursor (Sun et al. 2021). FTIR spectra of the mercury-loaded adsorbents showed a change in the intensity of absorption bands of the carboxyl and hydroxyl groups, indicating the contribution of these oxygenated functionalities during mercury adsorption (Mamaní et al. 2019; Sellaoui et al. 2019). Similar findings have been reported by Yoo et al. (2023), Goyal et al. (2009), Guo et al. (2020) and Zhao et al. (2022). The most significant changes were observed in the FTIR spectra of the pyrolysis-based adsorbents (i.e., J-PYR and G-PYR samples).
Figure 5

FTIR spectra of carbon-based adsorbents synthetized from jacaranda fruits and guava seeds biomass using (a) hydrothermal carbonization and (b) pyrolysis. Label ‘ + Hg’ indicates the adsorbent samples loaded with mercury.

Figure 5

FTIR spectra of carbon-based adsorbents synthetized from jacaranda fruits and guava seeds biomass using (a) hydrothermal carbonization and (b) pyrolysis. Label ‘ + Hg’ indicates the adsorbent samples loaded with mercury.

Close modal

The mercury adsorption properties of adsorbents prepared from hydrothermal carbonization and pyrolysis of jacaranda and guava seed wastes were analyzed and compared. Mercury adsorption on these adsorbents was endothermic and may involve a multi-molecular interaction mechanism with oxygenated functionalities. van der Waals and ligand exchange are expected to be the main interaction forces for the mercury adsorption mechanism using these adsorbents. The carbon-based materials obtained from jacaranda exhibited the highest mercury adsorption capacities where the biomass pyrolysis allowed to obtain the adsorbent with the best mercury removal. This adsorbent outperformed the adsorption capacities of other activated carbons with higher surface areas. It was expected that the mercury adsorption on these carbon-based materials occurred mainly on their external surface with a limited contribution from intraparticle diffusion. The lignin content of jacaranda waste favored the formation of oxygenated functionalities during biomass thermochemical conversion, especially pyrolysis, under the tested experimental conditions.

The support provided by the MatPore – Porous Materials National Laboratory is acknowledged.

All authors contributed to the study conception and design. V.A.M.-H., F.G.Q.-Á., D.I.M.-C., and A.B.-P. conceptualized the study; V.A.M.-H., F.G.Q.-Á., D.I.M.-C., H.E.R.-Á., I.A.A.-V., and A.B.-P. performed the methodology; F.G.Q.-Á., H.E.R.-Á., D.I.M.-C., H.E.R.-Á., I.A.A.-V., V.J.L.-S., and A.B.-P. investigated the study; F.G.Q.-Á., H.E.R.-Á., I.A.A.-V., and D.I.M.-C. wrote and prepared the original draft; D.I.M.-C. and A.B.-P. wrote, reviewed, and edited the article; D.I.M.-C. supervised the study; D.I.M.-C. did project administration . All the authors have read and agreed to the published version of the manuscript.

This research received no external funding.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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