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Article

Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves

by
Kyung Bin Oh
1,†,
Saerom Park
2,†,
Ye Jin Kim
1,
Gyu Won Lee
1,
Jeong Wook Jo
1,
Jae Hun Kim
1,
Ji Eun Kim
3,
Gwangnam Kang
3,
Sang Hyun Lee
1,
Hyung Joo Kim
1,* and
Yong-Keun Choi
1,2,*
1
Department of Biological Engineering, Konkuk University, Seoul 05029, Republic of Korea
2
R&D Center, Choilab Inc., Seoul 01811, Republic of Korea
3
R&D Center, ATE Corporation, Seoul 07532, Republic of Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2025, 17(9), 4233; https://doi.org/10.3390/su17094233
Submission received: 28 March 2025 / Revised: 26 April 2025 / Accepted: 6 May 2025 / Published: 7 May 2025
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Abstract

:
The ability of biochars derived from fallen (F-BC) and non-fallen (NF-BC) maple leaves to adsorb Cu2+ ions from aqueous solutions was examined. Biochars were produced at pyrolysis temperatures of 350, 550, and 750 °C. Higher pyrolysis temperatures resulted in enhanced specific surface areas and promoted CaCO3 crystallization while limiting MgCO3 formation. The Cu2+ adsorption capacity depended on the biochar type and pyrolysis conditions. Although the Cu2+ adsorption efficiency of NF-BCs decreased with increasing pyrolysis temperature, F-BC550 exhibited a higher Cu2+ adsorption capacity than F-BC750. Additionally, the Cu2+ adsorption performance of both NF-BC350 and F-BC550 improved with increasing solution pH. Cu2+ adsorption onto NF-BC350 and F-BC550 followed the two-compartment first-order (involving fast and slow adsorption compartments) and Langmuir (meaning homogeneous monolayer adsorption) models, respectively. The maximum Cu2+ adsorption capacity of F-BC550 (147.3 mg Cu/g BC) was 7.8-fold higher than that of NF-BC350 (18.8 mg Cu/g BC), as determined by isotherm studies. The enhanced adsorption performance of F-BC550 may be attributable to physical adsorption facilitated by increased surface area and multiple mechanisms, including cationic attraction via functional groups, ion exchange (Ca and Mg), and van der Waals interaction facilitated by increased surface area. These findings suggest that F-BC550, derived from waste biomass, exhibits superior Cu2+ adsorption performance compared to NF-BCs, making it a promising adsorbent for wastewater treatment applications.

1. Introduction

The natural shedding of leaves by trees allows them to adapt to seasonal variations, enabling them to endure the cold autumn and winter months [1,2]. Consequently, large quantities of fallen leaves continuously accumulate as waste. Although region-specific, the annual generation of leaf waste has been reported to range from 3.4 to 5.5 t/ha/year in China and from 3.3 to 4.5 t/ha/year in Brazil [3]. While these leaf wastes can be naturally decomposed by bacteria and fungi, they often persist and accumulate [2]. Moreover, while leaf wastes can be used as fertilizers, their decomposition releases greenhouse gasses such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) [2]. In addition, the accumulation of leaf waste negatively affects the condition and appearance of roads, degrades water quality, and obstructs waterways [1]. These challenges contribute to increasing socioeconomic costs. To mitigate these issues, traditional disposal techniques, such as incineration, composting, and landfilling, have been employed. However, these methods exhibit several limitations, such as the emission of noxious particulates and greenhouse gasses, extensive processing time, and production of leachate and CH4 [2]. Most notably, particulate matter (PM) containing toxic air pollution (e.g., polycyclic aromatic hydrocarbons (PAHs)) is emitted during combustion. Noblet et al. reported 34.7 g/kg and 45.0 mg/kg of PM and PAH emission factors, respectively, when burning leaf waste [4].
Researchers have explored the use of leaf waste (i.e., fallen leaves) as an adsorbent for removing pollutants from water, such as dyes (e.g., methylene blue, crystal violet, Acid Blue 113, and Acid Violet 17) [5,6,7,8,9,10,11] and heavy metals (e.g., Pb, Cu, and Co) [11,12,13,14,15,16]. However, natural adsorbents exhibit various limitations, such as low surface area, underdeveloped pore structure, and a limited number of binding sites. Despite these drawbacks, fallen leaves possess unique properties, including mineral composition, color, and moisture content, depending on environmental factors such as temperature and daylight duration [2]. Thus, to improve the adsorption ability of fallen leaves, various activation techniques have been examined. For example, NaOH- and H2SO4-treated fallen leaves of Ficus racemosa displayed a 2.6-fold increase in adsorption capacity for Acid Violet 17 compared with untreated fallen leaves [9]. Similarly, fallen leaves of Prunus dulcis activated with cetyl trimethyl ammonium bromide, a cationic surfactant, exhibited significantly improved adsorption capacity for Acid Blue 113 compared with NaOH-functionalized fallen leaves [10]. Moreover, NaOH treatment enhanced the Pb adsorption capacity of fallen leaves of Diospyros kaki [16].
Pyrolysis is another widely used technique for enhancing adsorbent performance. Biochar, a carbonaceous material produced through pyrolysis under O2-limited conditions, is a promising adsorbent because of its low production cost and abundance of adsorptive sites, such as porous structures, functional groups, and minerals [17,18]. Thus, the use of biochar for environmental remediation has been extensively explored [19,20]. The adsorption capacity of biochar depends on various factors, such as feedstock type, pyrolysis temperature, and surface characteristics [21,22]. Among these, feedstock selection is particularly important for biochar production. Numerous studies have investigated biochar derived from different feedstocks, such as lignocellulosic wastes, microalgae, animal manure, and industrial wastes [23,24,25,26]. Fallen leaves have also been considered a promising feedstock for producing biochar with the ability to remove hazardous substances (e.g., heavy metals, medicines, dyes, and herbicides) [13,24,27,28,29,30,31,32,33,34]. For example, Tan et al. evaluated the adsorption efficiency of biochars produced at low temperatures (300 °C) from various fallen leaves (Ginkgo biloba, Platanus sp., Prunus cerasifera, Magnolia denudata, Cinnamomum camphora, and Magnolia grandiflora) for Pb removal and found that adsorption mechanisms varied depending on the feedstock type [30]. In addition, biochars derived from Magnolia grandiflora fallen leaves have been studied for methylene blue adsorption [32,33], and biochars produced from Cinnamomum camphora fallen leaves have demonstrated potential for atrazine adsorption, attributable to high aromaticity and π–π interactions [34]. Furthermore, biochars produced from maple (Acer palmatum and A. saccharum) fallen leaves (M-BC) have been examined for the remediation of metal ions, dyes and pharmaceuticals, and other contaminants in water [24,31] and soil [35,36,37]. Kim et al. reported that M-BC pyrolyzed at 750 °C exhibited a maximum adsorption capacity of 361 mg/g [24]. However, for simultaneous dye removal, M-BC prepared at 550 °C showed superior removal efficiency compared to that pyrolyzed at 750 °C [31]. Derakhshan-Nejad and co-workers employed M-BC pyrolyzed at 550 °C to remediate heavy metals (e.g., Cd, Cu, Pb, Zn, As, etc.) on soil from a mining area [35,36,37].
However, these studies have demonstrated the potential of biochar derived from fallen leaves as an effective adsorbent. Researchers have focused on studying adsorbents derived fallen leaves from the perspective of leaf waste recycling. The potential differences in physicochemical properties and adsorption behaviors between fallen and non-fallen leaves remain to be extensively investigated. As these two types of leaves may exhibit distinct physicochemical characteristics, a systematic comparison is necessary to understand biomass derived-biochars as adsorbents.
Therefore, this study aimed at examining the physicochemical properties of fallen and non-fallen leaf-derived biochars and their role in Cu2+ adsorption. Fallen and non-fallen leaves collected from deciduous maple trees were used to produce biochar at various pyrolysis temperatures (i.e., 350, 550, and 750 °C). The derived biochars were characterized using scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), elemental analysis (EA), and X-ray photoelectron spectroscopy (XPS). Additionally, the Cu2+ adsorptive behaviors of both types of biochars were evaluated through kinetic (pseudo-first-order, pseudo-second-order, Elovich, two-compartment first-order, intraparticle diffusion, and liquid film diffusion models) and isotherm modeling (Langmuir, Freundlich, and Temkin models) to clarify the underlying adsorption mechanisms.

2. Materials and Methods

2.1. Chemicals

Maple (Acer palmatum Thunb.) leaves were collected as feedstock for biochar production from Konkuk University (Gwangjin-gu, Seoul, Republic of Korea). Copper (II) sulfate pentahydrate (CuSO4∙5H2O) for Cu2+ adsorption studies was purchased from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Preparation of Biochar

Biochar was produced using two types of biomasses (i.e., fallen and non-fallen maple leaves). Before pyrolysis, collected fallen and non-fallen maple leaves were dried at 60 °C and pounded to a powder size of 500–2360 μm. The pulverized leaves were subjected to pyrolysis for 2 h after attaining temperatures of 350, 550, and 750 °C in an MTI furnace (OTF-1200X-S, MTI Co., Ltd., Richmond, CA, USA) using a quartz boat crucible under a N2 atmosphere. The produced biochar was immediately immersed in deionized water (DIW) for cooling, then washed three times with DIW to remove impurities, and dried at 105 °C overnight. The dried biochar was ground and sieved to obtain particles smaller than 140 μm. The biochar samples were labeled as F-BC350, F-BC550, F-BC750, NF-BC350, NF-BC550, and NF-BC750, based on the biomass type (F-BC: fallen leaf-derived; NF-BC: non-fallen leaf-derived) and pyrolysis temperature. The yields of NF-BC350 and F-BC550 were 28% and 21%, respectively.

2.3. Biochar Characterization

The biochar structures and morphologies were examined using SEM (TM4000 Plus, Hitachi Co., Tokyo, Japan) after platinum sputter-coating. To investigate the mineral contents (N, P, K, Ca, Fe, and Mg), biochars were digested via a microwave digestion method with 6 mL of HNO3 and 2 mL of H2O2 and then measured using inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP7400 DUO, Thermo Fisher Scientific, Waltham, MA, USA). To assess surface crystallization, XRD patterns were obtained using a Rigaku SmartLab device (Rigaku Co., Tokyo, Japan) operating at 40 kV with a scanning range of 5–80°. The pH at the point of zero charge (pHpzc) was determined using the method described by Choi and Kan [38]. A total of 0.01 g biochar was added to 50 mL of 0.01 M NaCl solution, which was adjusted to a pH of 3–10 using 0.1 M HCl and NaOH. To prevent pH changes due to CO2, the headspace was purged with N2 gas. Subsequently, the mixture was incubated for 48 h at 25 °C. The pHpzc value was calculated at DpH (pHfinal − pHinitial) = 0. The specific surface area was determined using N2 adsorption isotherms based on the BET method (3Flex, Micromeritics, GA, USA). The elemental composition (C, N, and H) was analyzed using an elemental analyzer (EA, FLASH EA 1112, Thermo Scientific, Waltham, MA, USA). The oxygen content was estimated by difference. The characteristics of functional groups were determined using ATR-FTIR spectroscopy (Jasco FR/IR-4600, Tokyo, Japan). The chemical compositions and binding energies of biochar surfaces before and after Cu2+ adsorption were explored using an XPS analyzer (Thermo Scientific, MA, USA).

2.4. Batch Adsorption Studies

Batch adsorption experiments were conducted to evaluate Cu2+ removal using biochar derived from different feedstocks (F-BCs and NF-BCs) and pyrolysis temperatures (350, 550, and 750 °C). In each study, 0.005 g of biochar was added to 40 mL of 5 mg/L Cu2+ solution (pH 6) in a 50 mL conical tube. The Cu2+ solution containing biochar was continuously agitated in a shaking incubator at 200 rpm and 20 °C. To clarify the influence of the initial solution pH, adsorption experiments were performed using F-BC550 and NF-BC350. Specifically, the pH of the 5 mg/L Cu2+ solution (40 mL) was adjusted to 2, 3, 4, 5, and 6 using 0.1 M NaOH and 0.1 M HCl before adding 0.005 g of biochar.

2.5. Adsorption Kinetic and Isotherm Studies

Adsorption kinetics and isotherm studies for Cu2+ were evaluated using F-BC550 and NF-BC350. For the kinetic study, 0.005 g of biochar was added to 40 mL of 5 mg/L Cu2+ solution (pH 6), and the mixture was stirred continuously for 720 min (NF-BC350) and 1440 min (F-BC550) at 200 rpm and 20 °C. The collected samples were filtered using a 0.2 μm syringe filter (PVDF, Whatman, Maidstone, UK), and Cu2+ adsorption capacities were measured. The experimental data were assessed using different kinetic models, including the pseudo-first order, pseudo-second order, Elovich, intraparticle diffusion, liquid film diffusion, and two-compartment first-order equations (Table S1). Cu2+ adsorption isotherm studies were performed using initial Cu2+ concentrations ranging from 5 to 100 mg/L. Specifically, 0.005 g of biochar was added to 40 mL Cu2+ solution (pH 6), and the mixture was continuously shaken for 48 h at 200 rpm and 20 °C. The experimental data were analyzed using Freundlich, Langmuir, and Temkin isotherm models (Table S1).

2.6. Analytical Determination of Cu2+ Concentration

The concentration of Cu2+ in the solution was determined using Humas test kits (HS-Cu, Humas Co., Ltd., Daejeon, Republic of Korea) and a water analyzer (HS-3700, Humas Co., Ltd., Daejeon, Republic of Korea). A 5 mL of aliquot was transferred into a HS-Cu kit and gently inverted 10 times for mixing. Subsequently, 0.5 mL of HS-Cu solution was added and incubated for 10 min at room temperature after inverting 10 times. The reaction solution was measured at 484 nm.

3. Results and Discussion

3.1. Physicochemical Properties of NF-BCs and F-BCs

The physicochemical properties of NF-BCs and F-BCs were evaluated using SEM, ICP-OES, BET, XRD, EA, FTIR, and XPS. As shown in Figure S1, both NF-BCs and F-BCs possessed porous structures, with varying characteristics depending on the pyrolysis temperature. The specific surface area decreased in the following order: F-BC750 > F-BC550 > NF-BC550 > NF-BC750 > NF-BC350 ≈ F-BC350 (Figure 1). The specific surface area of F-BCs (2.1–191.1 m2/g) increased with pyrolytic temperature, whereas the corresponding values for NF-BCs (2.3–6.0 m2/g) were lower. These results may be due to differences in the properties of non-fallen and fallen maple leaves such as moisture content, element composition, volatile compound content, and the cellulose/hemicellulose/lignin ratio depending on season [22,39].
F-BCs (6.3–9.3) exhibited higher pHpzc values than those of NF-BCs (6.2–7.7), and the pHpzc of F-BCs pyrolyzed at temperatures above 550 °C exceeded 9. Both NF-BCs and F-BCs contained minerals such as N, Ca, and Mg (Table 1). In particular, the Ca and Mg contents increased with pyrolysis temperature. These findings suggest that higher pyrolysis temperatures promoted the condensation of alkaline compounds in biochar, thereby increasing its pHpzc [27]. Qian et al. reported that the removal of minerals from biochar decreases its pHpzc, indicating a correlation between pHpzc and the alkali mineral content [40]. Furthermore, as shown in Figure S2, biochars prepared at 550 °C and 750 °C exhibited peaks corresponding to calcite (CaCO3), whereas those pyrolyzed at 350 °C presented peaks for nesquehonite (MgCO3), attributable to the different thermal stabilities of carbonates. These results suggest that increasing the pyrolysis temperature promoted the formation of carbonate compounds, which are alkaline, contributing to the higher pHpzc of biochars [22,41,42].
Additionally, the H/C and O/C ratios were analyzed to examine the aromaticity and hydrophobicity of NF-BCs and F-BCs. As shown in Figure 2, the H/C and O/C values of biochars produced at high temperatures (750 °C) were lower than those of biochars produced at low temperatures (350 °C). These results demonstrate that high pyrolysis temperatures enhance the aromaticity and reduce the polarity of biochars through demethylation and decarboxylation during the pyrolysis process [41,43,44].

3.2. Screening of NF-BCs and F-BCs for Cu(II) Adsorption

To determine the most effective adsorbent for Cu2+ removal, the adsorption capacities of NF-BCs and F-BCs were estimated and compared. The Cu2+ adsorption ability of NF-BCs (2.6–14.1 mg Cu/g BC) decreased with increasing pyrolysis temperature (Figure 3) owing to the loss of functional groups. In contrast, among the F-BCs, F-BC550 and F-BC750 exhibited significantly higher Cu2+ adsorption capacities (24.2 mg Cu/g BC and 21.64 mg Cu/g BC, respectively) than F-BC350 (4.5 mg Cu/g BC). Although the specific surface area (SSA) of F-BC750 (191.1 m2/g) was higher than that of F-BC550 (112.3 m2/g), its Cu2+ adsorption capacity was lower. These results suggest that Cu2+ adsorption is influenced not only by the SSA but also the physicochemical properties of the biochars. Based on these findings, NF-BC350 and F-BC550 were identified as the best adsorbents for Cu2+ removal, and their adsorption behavior was further investigated.

3.3. Influence of Solution pH on Cu(II) Adsorption onto NF-BC350 and F-BC550

Cu2+ adsorption experiments on NF-BC350 and F-BC550 were conducted in the pH range of 2–6, where Cu precipitation does not occur. In general, the optimal pH for adsorption is influenced by the pKa of the adsorbate and pHpzc of the adsorbent. The pHpzc values of NF-BC350 and F-BC550 were 6.2 and 9.2, respectively, indicating that the biochar surfaces became positively charged within the tested pH range. Additionally, Cu existed predominantly as Cu2+ owing to its pKa value of 8.0 (Table S2). As shown in Figure 4, the adsorption capacity of biochars improved with a rising solution pH. This trend may be attributable to changes in the proton concentration and electrostatic interactions [45,46]. At a low pH, excess H+ ions competed with Cu2+ for adsorption sites, causing the biochar surface to become highly protonated, with electrostatic repulsion further reducing Cu2+ adsorption. However, as the pH increased, H+ concentration decreased, reducing competition at adsorption sites and weakening the electrostatic repulsion between the positively charged biochar and Cu2+, thereby facilitating Cu2+ adsorption.

3.4. Adsorption Kinetics of NF-BCs and F-BCs

As shown in Figure 5 and Table S3, the two-compartment first-order model was the best-fitted model for NF-BC350, with the highest determination coefficient (R2 = 0.945), indicating the occurrence of complexed physical and/or chemical adsorption in both fast and slow adsorption compartments [47]. Up to 60% of Cu2+ was rapidly adsorbed within the first 30 min, and the remaining 40% was slowly removed over 370 min. These results indicate that initial Cu2+ removal in NF-BC350 occurred through rapid surface adsorption, followed by slower intraparticle diffusion, which decreased the adsorption rate over time. In contrast, Cu2+ adsorption onto F-BC550 exhibited a R2 value (>0.99) with multiple kinetic models, including the pseudo-first-order (R2 = 0.996), pseudo-second-order (R2 = 0.993), and two-compartment first-order (R2 = 0.997) models, as shown in Figure 6. These results indicate that Cu2+ adsorption onto F-BC550 was governed either by physical adsorption or by a combination of chemical and physical adsorption (Figure 6 and Table S3). However, prior studies [46,48,49,50,51,52,53] have shown that the pseudo-second-order and Elovich models, which suggest that chemisorption predominantly controls the adsorption process, are the best-fitted kinetic models for Cu2+ adsorption onto biochars derived from Miscanthus × giganteus, hardwood, corn straw, pinecone, cork, date seed, rice straw, and hickory wood (Table S4). As shown in Figure 5C,D and Figure 6C,D, the deviation of the plots from the origin indicates that both intraparticle and liquid film diffusion simultaneously governed the adsorption of Cu2+ [54,55].

3.5. Adsorption Isotherms of NF-BCs and F-BCs

Figure 7 and Table S5 present the evaluation of isotherm models for Cu2+ adsorption onto NF-BC350 and F-BC550. The results indicated that the Langmuir model, which describes monolayer adsorption, provided the best fit for Cu2+ adsorption onto the homogeneous surfaces of NF-BC350 (R2 = 0.999) and F-BC550 (R2 = 0.951). These findings are consistent with prior studies on Cu2+ adsorption onto biochars, which also identified the Langmuir model as the most suitable for describing the adsorption process (Table S4). Additionally, the Temkin constant (bT) of NF-BC350 and F-BC550 was <40 kJ/mol (Table S5). These results mean that Cu2+ adsorption onto NF-BC350 and F-BC550 was conducted via physical adsorption [56]. The maximum Cu2+ adsorption capacities of NF-BC350 and F-BC550, calculated using the Langmuir constant, were 18.8 mg Cu/g BC and 147.3 mg Cu/g BC, respectively. These results indicate that F-BC550 is a more effective adsorbent for Cu2+ than NF-BC350. Moreover, as summarized in Table S4, the maximum Cu2+ adsorption capacity of F-BC550 was considerably higher than that of biochars produced from other biomasses, including hickory wood, Miscanthus × giganteus, hardwood, sewage sludge, pinecone, dairy manure, cork, date seed, ginkgo leaf, fruitwood, and crop residues like corn, peanut, soybean, canola, and rice straw [28,46,48,49,50,51,52,53,57,58,59,60,61,62]. These findings suggest that F-BC550 exhibits superior Cu2+ adsorption capacity compared to most previously studied biochars. However, Katiyar et al. reported that biochar derived from Ascophyllum nodosum seaweed possessed an even higher Cu2+ adsorption capacity (223 mg Cu/g BC), despite having a lower surface area [63]. According to their work, Cu2+ adsorption onto Ascophyllum nodosum biochar was strongly associated with pore deposition, electrostatic attractions, and surface precipitation [63]. Hence, it is implied that multiple physical adsorption mechanisms, including van der Waals interactions, ion exchange, electrostatic attraction, and π–π interaction, contribute to the high Cu2+ adsorption performance of F-BC550. The detailed Cu2+ adsorption mechanisms onto NF-BC350 and F-BC550 are interpreted in Section 3.6.

3.6. Mechanisms for Cu(II) Adsorption onto NF-BC350 and F-BC550

As indicated in Table 2, NF-BC350 and F-BC550 exhibit notable differences in physicochemical properties, including SSA, functional groups, and mineral content (e.g., Ca, Mg, N, and Fe). These variations may be attributable to seasonal changes in the composition of maple leaves. The high SSA of F-BC550 provided a favorable environment for Cu2+ adsorption, resulting in 7.8-fold improvement in adsorption capacity compared to NF-BC350. Moreover, the mineral composition caused by seasonal variation can affect the formation of crystalline structures and functional groups on the biochar surface, further influencing the adsorption behavior of the derived biochars. The FTIR and XPS analyses conducted before and after Cu2+ adsorption (Figure 8 and Figure 9) provided evidence for the role of the chemical forms and functional groups of biochars in Cu2+ adsorption. Specifically, functional groups such as CO32−, C–O, C–H, and C=O were more pronounced on the surface of F-BC550, whereas NF-BC350 exhibited fewer and weaker functional groups (Figure 8). After Cu2+ adsorption, the peak intensities decreased, suggesting an interaction between these functional groups and Cu2+ ions. In particular, the decline in the CO32− peak in F-BC550 indicated cationic metal attraction with Cu2+ via electrostatic interactions. Furthermore, XRD analysis suggested that Ca2+ from CaCO3 likely underwent ion exchange with Cu2+. This result was further supported by changes in binding sites observed in the XPS spectra (Figure 9). The XPS result showed a decline in C1s and Ca2p (CaCO3) peak intensities and an increase in O1s peak intensity after Cu2+ adsorption, implying that Cu2+ was primarily adsorbed through physical interactions. In the case of NF-BC350, the changes in XPS peak intensities after Cu2+ adsorption were relatively lower, which is consistent with the FTIR results. Additionally, as shown in Table 1, NF-BC350 exhibited approximately 1.3-fold higher Mg content compared to F-BC550, but its Ca content was about 2.2-fold lower. The lower ion exchange capacity of Mg2+ observed in the XRD analysis of NF-BC350 led to a lower adsorption capacity for Cu2+ compared to F-BC550.
As shown in Figure 10, multiple mechanisms, including cationic metal attraction, van der Waals interactions, and ion exchange, were involved in Cu2+ adsorption onto these biochars. Therefore, although both F-BC550 and NF-BC350 followed the same best-fit kinetic model (i.e., the two-compartment first-order model) and isotherm model (i.e., the Langmuir model), the difference in adsorption capacity could be attributed to variations in their physicochemical properties.

4. Conclusions

Biochars derived from fallen and non-fallen maple leaves were produced at different pyrolysis temperatures, and their physicochemical properties and Cu2+ adsorption performances were investigated. The feedstock type and pyrolysis temperature significantly influenced the physicochemical characteristics of the biochars, including the SSA, mineral composition, and functional groups. Among the tested biochars, F-BC550 and NF-BC350 exhibited the highest Cu2+ adsorption capacity within their corresponding groups. Kinetic and isotherm studies showed that adsorption onto both biochars could be explained by the two-compartment first-order and Langmuir models, indicating monolayer adsorption and complex physical and chemical adsorption mechanisms. However, the maximum adsorption capacity of F-BC550 was significantly higher than that of NF-BC350. The superior adsorption performance of F-BC550 compared with NF-BCs suggests that fallen leaves, when appropriately pyrolyzed, can serve as an efficient and sustainable material for heavy metal remediation. These findings provide valuable insights into biochar optimization for environmental applications, particularly in wastewater treatment. Therefore, future studies will evaluate Cu2+ adsorption efficiency under various environmental conditions (e.g., anions and cations), the reusability of biochars, and column tests for practical applications. Additionally, comparative studies on the adsorption behaviors of various heavy metal ions (e.g., Pb2+, Cd2+, Zn2+, Ni2+, etc.) will be conducted to assess the selectivity and competitive adsorption capacity of biochars, providing further insights into their performance under multi-contaminant conditions relevant to real wastewater systems.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17094233/s1, Figure S1: SEM images of NF-BCs ((A) NF-BC350, (C) NF-BC550, (E) NF-BC750) and F-BCs ((B) F-BC350, (D) F-BC550, (F) F-BC750); Figure S2: XRD patterns of NF-BCs (A) and F-BCs (B) (●: nesquehonite, ■: calcite); Table S1: Adsorption isotherm and kinetic models used in this study; Table S2: Surface charge of NF-BC350, F-BC550, and Cu2+ at various pH values; Table S3: Kinetic parameters of Cu2+ adsorption onto NF-BC350 and F-BC550; Table S4: Comparison of Cu2+ adsorption performance of biochars; Table S5: Isotherm parameters of Cu2+ adsorption onto NF-BC350 and F-BC550.

Author Contributions

Conceptualization, K.B.O., Y.-K.C. and H.J.K.; methodology, K.B.O. and S.P.; software, S.P.; validation, Y.-K.C. and H.J.K.; formal analysis, K.B.O., Y.J.K. and G.W.L.; investigation, K.B.O., J.W.J. and J.H.K.; resources, J.E.K. and G.K.; data curation, K.B.O. and Y.-K.C.; writing—original draft preparation, S.P. and Y.-K.C.; writing—review and editing, Y.-K.C. and S.H.L.; visualization, K.B.O. and S.P.; supervision, Y.-K.C. and H.J.K.; project administration, Y.-K.C. and H.J.K.; funding acquisition, Y.-K.C. and H.J.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Korea Water Cluster “2023 Demand-based carbon neutral water technology demonstration supporting project” (Project No. B00806120001292-202303-0401) and by the Rural Development Administration as a Cooperative Research Program for Agriculture Science and Technology Development (Project No. RS-2021-RD009728). This research was supported by a grant from the Development of Technology for Biomaterialization of Marine Fishery Byproducts of the Korea Institute of Marine Science and Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (KIMST-20220128) and by the Korea Forest Service as an R&D Program for Forest Science Technology (RS-2022-KF002028).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article and Supplementary Materials. Further inquiries can be directed at the corresponding author.

Conflicts of Interest

Authors Saerom Park and Yong-Keun Choi were employed by the company R&D Center, Choilab Inc. Authors Ji Eun Kim and Gwangnam Kang were employed by the company R&D Center, ATE Corporation. The funders or companies employed by some of the authors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Specific surface area (SSA) and pHpzc of non-fallen and fallen leaf-derived biochars (NF-BCs and F-BCs).
Figure 1. Specific surface area (SSA) and pHpzc of non-fallen and fallen leaf-derived biochars (NF-BCs and F-BCs).
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Figure 2. Van Krevelen diagram showing the relationships between O/C and H/C ratios of NF-BCs and F-BCs.
Figure 2. Van Krevelen diagram showing the relationships between O/C and H/C ratios of NF-BCs and F-BCs.
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Figure 3. Cu2+ adsorption capacities of NF-BCs and F-BCs produced at different pyrolysis temperatures. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 48 h.
Figure 3. Cu2+ adsorption capacities of NF-BCs and F-BCs produced at different pyrolysis temperatures. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 48 h.
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Figure 4. Influence of solution pH on Cu2+ adsorption capacities of NF-BCs and F-BCs. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; 48 h.
Figure 4. Influence of solution pH on Cu2+ adsorption capacities of NF-BCs and F-BCs. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; 48 h.
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Figure 5. Adsorption kinetics of Cu2+ onto NF-BC350, fitted using the (A) pseudo-first-order, pseudo-second-order, and Elovich models, (B) two-compartment first-order model, (C) intraparticle diffusion model, and (D) liquid film diffusion model. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 12 h.
Figure 5. Adsorption kinetics of Cu2+ onto NF-BC350, fitted using the (A) pseudo-first-order, pseudo-second-order, and Elovich models, (B) two-compartment first-order model, (C) intraparticle diffusion model, and (D) liquid film diffusion model. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 12 h.
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Figure 6. Adsorption kinetics of Cu2+ onto F-BC550, fitted using the (A) pseudo-first-order, pseudo-second-order, and Elovich models, (B) two-compartment first-order model, (C) intraparticle diffusion model, and (D) liquid film diffusion model. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 24 h.
Figure 6. Adsorption kinetics of Cu2+ onto F-BC550, fitted using the (A) pseudo-first-order, pseudo-second-order, and Elovich models, (B) two-compartment first-order model, (C) intraparticle diffusion model, and (D) liquid film diffusion model. [Conditions] 40 mL of 5 mg/L; 0.005 g BC; initial pH = 6; 24 h.
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Figure 7. Adsorption isotherms of Cu2+ onto (A) NF-BC350 and (B) F-BC550, based on Langmuir, Freundlich, and Temkin isotherm models. [Conditions] 40 mL of 5–100 mg/L; 0.005 g BC; initial pH = 6; 48 h.
Figure 7. Adsorption isotherms of Cu2+ onto (A) NF-BC350 and (B) F-BC550, based on Langmuir, Freundlich, and Temkin isotherm models. [Conditions] 40 mL of 5–100 mg/L; 0.005 g BC; initial pH = 6; 48 h.
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Figure 8. FTIR spectra of NF-BC350 and F-BC550 before and after Cu2+ adsorption.
Figure 8. FTIR spectra of NF-BC350 and F-BC550 before and after Cu2+ adsorption.
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Figure 9. (A–D) XPS survey scan, (E,H) C1s, (F,I) O1s, and (G,J) Ca2p spectra for (A,B,E–G) NF-BC350 and (C,D,H–J) F-BC550 before and after Cu2+ adsorption.
Figure 9. (A–D) XPS survey scan, (E,H) C1s, (F,I) O1s, and (G,J) Ca2p spectra for (A,B,E–G) NF-BC350 and (C,D,H–J) F-BC550 before and after Cu2+ adsorption.
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Figure 10. Potential mechanisms for Cu2+ adsorption onto NF-BC350 and F-BC550.
Figure 10. Potential mechanisms for Cu2+ adsorption onto NF-BC350 and F-BC550.
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Table 1. Mineral content of non-fallen and fallen leaf-derived biochars (NF-BCs and F-BCs).
Table 1. Mineral content of non-fallen and fallen leaf-derived biochars (NF-BCs and F-BCs).
BiocharsMineral Content (%)
NPKCaMgFe
NF-BC3503.260.000.121.710.620.04
NF-BC5503.040.000.193.681.580.09
NF-BC7502.620.550.103.211.820.10
F-BC3501.150.040.103.510.250.14
F-BC5501.140.070.133.700.480.17
F-BC7500.740.070.266.300.460.35
Table 2. Comparison of physicochemical properties of NF-BC350 and F-BC550.
Table 2. Comparison of physicochemical properties of NF-BC350 and F-BC550.
PropertyNF-BC350F-BC550
Cu(II) adsorption capacity (mg Cu/g BC) *18.8147.3
Specific surface area (m2/g)2.31112.3
Main peak in XRDMgCO3CaCO3
Main functional groupsC=O, C=C, –OHCO32−, C–O, C–H, C=O, C=C
Optimal solution pHpH 6pH 6
pHpzc6.29.2
Major mineralsCa, MgCa, Mg
Best-fitted kinetic modelTwo-compartment first-orderTwo-compartment first-order
Best-fitted isotherm modelLangmuirLangmuir
* Calculated maximum Cu2+ adsorption capacity.
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Oh, K.B.; Park, S.; Kim, Y.J.; Lee, G.W.; Jo, J.W.; Kim, J.H.; Kim, J.E.; Kang, G.; Lee, S.H.; Kim, H.J.; et al. Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves. Sustainability 2025, 17, 4233. https://doi.org/10.3390/su17094233

AMA Style

Oh KB, Park S, Kim YJ, Lee GW, Jo JW, Kim JH, Kim JE, Kang G, Lee SH, Kim HJ, et al. Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves. Sustainability. 2025; 17(9):4233. https://doi.org/10.3390/su17094233

Chicago/Turabian Style

Oh, Kyung Bin, Saerom Park, Ye Jin Kim, Gyu Won Lee, Jeong Wook Jo, Jae Hun Kim, Ji Eun Kim, Gwangnam Kang, Sang Hyun Lee, Hyung Joo Kim, and et al. 2025. "Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves" Sustainability 17, no. 9: 4233. https://doi.org/10.3390/su17094233

APA Style

Oh, K. B., Park, S., Kim, Y. J., Lee, G. W., Jo, J. W., Kim, J. H., Kim, J. E., Kang, G., Lee, S. H., Kim, H. J., & Choi, Y.-K. (2025). Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves. Sustainability, 17(9), 4233. https://doi.org/10.3390/su17094233

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