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This work reports the use of Mangifera kemanga leaves (MKL) treated with ethanol and nitric acid as an adsorbent to remove Cr(VI) and Pb(II) ions at different pHs, adsorbent dosages, times, initial metal ion concentrations, and temperatures. The MKL's maximum adsorption capacity was 213 mg/g for Cr(VI) at pH 2 and 89 mg/g for Pb(II) at pH 6. Batch experiments showed that MKL follows the Langmuir isotherm model in absorbing Cr(VI) and Pb(II). The Cr(VI) and Pb(II) adsorption kinetics are best suited by the pseudo-second-order kinetics model. Thermodynamic data studies indicated that surface complexation coexisted with ion exchange as the preliminary removal means. The results implied that MKL could potentially remove Cr(VI) and Pb(II).

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  • Kemang leaves were used as biosorbent for Cr(VI) and Pb(II) removal from aqueous solution.

  • The biosorption capacity of the biosorbent for Cr(VI) was 212.76 mg/g.

  • The biosorption capacity of the biosorbent for Pb(II) was 89.29 mg/g.

  • Both Cr(VI) and Pb(II) adsorption were well suited by a pseudo-second-order model.

  • Biosorbent may be regenerated and reused, indicating materials’ economic benefits.

adsorption, biosorbent, heavy metals, Kemang, leaves, removal
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Surface water pollution is important globally. Unfortunately, industrialization and irresponsible industrial practices have resulted in severe environmental damage and contamination of ecosystems (Nkutha et al. 2021). Toxic pollutants from industrial operations have contaminated surface waters, damaged many ecosystems, and caused severe health issues worldwide (Azizi et al. 2023).

Cr(VI) and Pb(II) harm living organisms and do not biodegrade (Shooto 2020). Unfortunately, these pollutants are present in freshwater resources due to careless discharge by mines, metal plating, metallurgy and chemical industries, and factories that produce plastics, paper, fertilizers, pesticides, batteries, etc. (Ali et al. 2019; Emamy et al. 2021; Nkutha et al. 2021).

When heavy metals are discharged without proper treatment, they can harm ecosystems due to their high reactivity with plants and animals. High concentrations of Cr(VI) can be harmful to human health since it is both carcinogenic and teratogenic (Lala et al. 2023) because it can penetrate cells, resulting in health problems like skin itch and gastric damage, and harm to essential organs such as the liver, kidneys, and lungs (Shooto 2020). Exposure to lead has also been linked to interference in the developing nervous systems of young juveniles, which can cause dyslexia, mental impairment, attention deficit disorder, and antisocial behavior (Lim et al. 2019).

Recently, different methods have been studied for treating water contaminated by heavy metals. Conventional technologies include ion exchange, chemical precipitation, biological treatment, electrochemistry, membrane filtration, and adsorption to remove Cr(VI) and Pb(II) from the solution. Adsorption involves using an adsorbent to remove these ions through ion exchange, complexation, and adsorption (Azizi et al. 2023), which offers low cost, energy consumption, and environmental impact, with high efficiency and ability to regenerate briefly (Arrisujaya et al. 2019).

Various agriculture-based materials have been used as adsorbents for the removal of Cr(VI) and Pb(II) ions recently, including Sweet flag (Shooto 2020), Juniperus procera leaves (Ali et al. 2019), green tea leaves (Jeyaseelan & Gupta 2016), Diospyros discolor seed (Arrisujaya et al. 2019), areca nut leaf sheath (Pant et al. 2022), sugar palm fruit shells (Nazaruddin et al. 2014), Terminalia catappa shell (Hevira et al. 2020), rice husk (Lala et al. 2023), Hura crepitans (Abadi et al. 2023), Pomelo leaves (Lim et al. 2019), Lawsonia inermis (Mehrmand et al. 2022), and pineapple fiber (Tangtubtim & Saikrasun 2019) to remove metal ions. These materials are plentiful, cheap, reasonable, reusable, and safe in the environment for living beings and the ecosystem (Shooto 2020). The biosorbents were created using plant components and natural resources with functional groups like hydroxyl, aldehyde, amine, amide, and aliphatic acid (Arrisujaya et al. 2020; Hevira et al. 2020).

Kemang (Mangifera kemanga Blume) is a member of the Anacardiaceae family and grows naturally along rivers in Java, especially West Java, Indonesia. Different parts of the plant exhibit antioxidant, anti-degenerative, and anticancer properties due to active chemical components such as flavonoids, tannins, triterpenoids, alkaloids, and phenylpropanoids (Darsono et al. 2022). The use of M. kemanga Blume for wastewater treatment has yet to be widely studied. Previous research indicated that kemang seeds were reliable for removing phenol (Mardiah et al. 2022), but further investigation is needed to determine their effectiveness in removing other pollutants, including heavy metals. This study examines the adsorption capacity of M. kemanga leaves (MKLs) in removing Cr(VI) and Pb(II) ions from water solutions.

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Materials

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MKLs were obtained from local communities' plantations in Cijayanti-Bogor, Indonesia. A stock solution of Cr(VI) and Pb(II) was made by dissolving K2Cr2O7 and Pb(NO3)2 in double-distilled water. All solutions were made with double-distilled water and other analytical-grade chemicals such as HNO3, HCl, and NaOH. An atomic absorption spectrophotometer (AA-7000 Shimadzu) was used to measure the concentration of Cr(VI) and Pb(II) before and after contact with the adsorbent.

Enhanced MKLs

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The leaves were rinsed in distilled water repeatedly to eliminate dissolved impurities and dirt particles and dried for 24 h in an air oven at 333 K. They were then dried, ground, and sieved using a 100 mesh sieve to obtain a homogeneous particle size. The uniform fine powder was steeped and agitated in ethanol (96%) for 2 h (Arrisujaya et al. 2019; Mardiah et al. 2022), then activated in 0.01 mol L nitric acid for 2 h at room temperature (Hevira et al. 2020).

Batch adsorption studies

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The MKL biosorbent (1 g) was added to the adsorbate solution (100 mL Cr(VI) or Pb(II) ions) in 100 mL Erlenmeyer flasks. They were shaken at 125 rpm for 30 min to establish equilibrium. The adsorption controlling parameter was determined by altering the initial pH (2, 3, 4, 5, and 6), using an Ohaus Starter 3100 pH meter; the adsorbent dosage (1, 2, 3, and 4 g/L); contact time (30, 60, 90, and 120 min); and adsorbate concentration (50, 100, 150, and 200 mg/L). Finally, the adsorbate-mixed solution was filtered using paper.

The adsorption capacity (qe (mg/g)) of Cr(VI) and Pb(II) onto the MKL biosorbent at equilibrium is determined using Equation (1):
(1)
where C0 and Ce are the initial and equilibrium concentrations of the adsorbate (mg/L), respectively, V represents the adsorbate solution volume added (L), and m is the mass of the MKL biosorbent (g).
To conduct the kinetic studies, a sequence of varying initial concentrations (50, 100, and 150 mg/L) of the adsorbate solution was used. The adsorption capacity onto the biosorbent (qt (mg/g)) is determined using Equation (2):
(2)
where Ct is the adsorbate solution concentration at any time t (mg/L).

The equilibrium isotherm studies used the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. The kinetics studies were based on three models: pseudo-first-order, pseudo-second-order, and intra-particle diffusion. The Van't Hoff Equation was used for thermodynamic analysis.

Desorption studies

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The MKL biosorbent used previously for the Cr(VI) and Pb(II) adsorption of an initial metal concentration of 100 mg/L was separated from the adsorbate solution by filter paper. The adsorbate-loaded MKL was agitated for 2 h with 100 mL of 0.01 mol L HNO3 solution. Desorption efficiency Ed is defined in Equation (3).
(3)
where Qd and Qe are the amount of metal desorbed and loaded (mg), respectively.
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Effect of pH

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The level of acidity or alkalinity in an aqueous solution plays a crucial role in metal ion adsorption. The effect of pH on the Cr(VI) and Pb(II) adsorption onto MKL is shown in Figure 1(a), with Cr(VI) adsorption capacity falling as the pH rises from 2 to 6. Similar decreasing trends have previously been observed in research on heavy metal adsorption onto several biosorbents (Emamy et al. 2021; Nkutha et al. 2021; Azizi et al. 2023). In acidic pH conditions, Cr(VI) ions behave as anions (⁠⁠, ⁠, or ⁠), and protonation occurs on the binding sites on the MKL biosorbent surface (Arrisujaya et al. 2019). The surface of the MKL biosorbent carries a considerable positive charge, strengthening the electrostatic interactions between binding sites on the MKL biosorbent and Cr(VI) ions (Pant et al. 2022). The MKL's protonated surface diminishes steadily with increasing pH, suggesting that the anions (OH-, ⁠, ⁠, and ⁠) compete for adsorption, resulting in a decrease in Cr(VI) adsorption capacity (Arrisujaya et al. 2019; Lala et al. 2023).
Figure 1
Cr(VI) and Pb(II) adsorption controls onto MKL. (a): Adsorbate solution pH; (b) biosorbent dose; (c) contact time; and (d) initial adsorbate concentration.
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Cr(VI) and Pb(II) adsorption controls onto MKL. (a): Adsorbate solution pH; (b) biosorbent dose; (c) contact time; and (d) initial adsorbate concentration.

Figure 1
Cr(VI) and Pb(II) adsorption controls onto MKL. (a): Adsorbate solution pH; (b) biosorbent dose; (c) contact time; and (d) initial adsorbate concentration.
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Cr(VI) and Pb(II) adsorption controls onto MKL. (a): Adsorbate solution pH; (b) biosorbent dose; (c) contact time; and (d) initial adsorbate concentration.

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In contrast, Pb(II) adsorption capacity increased to about 29 mg/g from about 7 as the initial solution pH rose from 2 to 6 – i.e., MKL's Pb(II) adsorption capacity is more effective in an average acidic solution than in a strong one (Emamy et al. 2021; Nkutha et al. 2021). The effect of solution pH on Pb(II) adsorption might arise from competition on the protonated MKL surface between protons (H+) and Pb(II) cations, resulting in lower polarity (Shooto 2020). Protonation decreases as pH increases, and the OH- groups increase in conjunction with the more prominent deprotonated MKL surface, indicating a partial negative charge on the latter. Hence, the positively charged Pb(II) adsorption onto MKL increases through electrostatic interaction and surface complexation (Ali et al. 2019; Shooto 2020).

Biosorbent dosage

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Cr(VI) and Pb(II) adsorptions onto MKLs were influenced considerably by biosorbent dosage (Figure 1(b)). Cr(VI) and Pb(II) adsorption capacity appeared to decrease with the MKL dosage increasing from 1 to 4 g/L. This could arise because more binding sites are available on the MKL surface (Hevira et al. 2020). At the same time, the Cr(VI) and Pb(II) initial concentrations are restricted to the value of 100 mg/L used in this study of the effect of the MKL biosorbent dosage.

Contact time

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Contact time is an essential metric because it determines the period to achieve equilibrium and the optimum term for heavy metal ion adsorption. Figure 1(c) displays the time-dependent adsorption of Cr(VI) and Pb(II) onto MKL. The concentration of adsorbate increased and ultimately stabilized once it attained equilibrium. The physicochemical nature of the adsorption process was determined, and it was found to be unaffected by energy. It was further observed that the electrostatic forces played a crucial role in the process (Ali et al. 2019). Various steps can control adsorption, including film, external and intra-particle diffusion, and adsorption on the adsorbent surface (Lin et al. 2017). It was confirmed that equilibrium adsorption of Cr(VI) and Pb(II) arises at approximately 90 min, so, for all studies, 90-min adsorption contact time was used.

Initial adsorbate concentration

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Adsorbate displacement, such as metal ions between the liquid and solid phases, heavily relies on their initial concentration. The effect of initial Cr(VI) and Pb(II) concentration on the MKL adsorbate was considered using standard 50, 100, 150, and 200 mg/L working solutions at 298 K and a 1 g/L adsorbent dose (Figure 1(d)). As the initial concentration was increased, MKL adsorbed more Cr(VI) and Pb(II), indicating a direct relationship between them. As the initial adsorbate solution concentration increased, MKL's adsorption ability increased – i.e., mass transfer was increased in solutions with elevated initial concentrations because there are more possibilities for metal ions to collide with binding sites (Lim et al. 2019) on the MKL surface. Cr(VI) and Pb(II) adsorption on MKL followed similar patterns (Figure 1(d)). The maximum Cr(VI) adsorption capacity on MKL from the 200 mg/L working solution was about 105 mg/g, while, for Pb(II), it was 71 mg/g – i.e., MKL removed Cr(VI) better than Pb(II).

Equilibrium isotherm modeling

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MKL adsorption of Cr(VI) and Pb(II) was examined using four adsorption isotherm models such as Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich. Adsorption isotherm models revealed the equilibrium distinction between adsorbate concentration in the solution and the concentration of adsorbate enclosed in the MKL biosorbent at specific temperatures (Azizi et al. 2023). They could be plotted based on information on MKL biosorbent adsorption capacity versus the initial adsorbate concentration (Arrisujaya et al. 2019; Shooto 2020). The Langmuir isotherm model can be simplified to a linear form – Equation (4) – that applies to adsorption onto a surface with limited uniform sites:
(4)
where qe is the adsorption capacity (mg/g) at equilibrium; qm is the maximum adsorption capacity (mg/g); Ce is the adsorbate equilibrium concentration (mg/L); and KL is the Langmuir adsorption equilibrium constant (L/mg). Figure 2(a) is a plot of 1/qe versus 1/Ce, illustrating Cr(VI) and Pb(II) adsorption onto the MKL surface. Table 1 shows the values of KL and qm values, which were calculated by the intercept and slope of the 1/qe versus 1/Ce. Additionally, the dimensionless constant separation factor (RL) values, a fundamental attribute of the Langmuir isotherm, provide considerable evidence of sorption nature (Ali et al. 2019). This tool effectively clarifies the attractions and interactions between the sorbent and sorbate. The value of RL is determined using Equation (5):
(5)
where C0 denotes the highest initial adsorbate concentration (mg/L). If RL = 1, the process is assumed to be linear and if RL = 0, it is expected to be irreversible (Emamy et al. 2021). The process is favorable if 0 < RL < 1, but unfavorable if RL > 1 (Heidari et al. 2021; Lala et al. 2023). In this analysis, RL revealed that Cr(VI) and Pb(II) adsorption onto MKL were favorable, with values of 0.58 and 0.15, respectively. Based on the high correlation coefficients (0.9822 for Cr(VI) and 0.957 for Pb(II)), the Langmuir isotherm model seems a suitable fit for the experimental data. The maximum monolayer Cr(VI) and Pb(II) adsorption capacities for MKL are 213 and 89 mg/g, respectively.
Table 1

Isotherm and desorption parameters for Cr(VI) and Pb(II) adsorption onto the MKL biosorbent

Isotherm parametersCr(VI)Pb(II)
Langmuir isotherm 
 qmax (mg/g) 212.76 89.29 
 KL (L/mg) 0.0036 0.027 
 R2 0.9822 0.957 
Freundlich isotherm 
 Kf 1.7737 7,2193 
 n 0.8761 2.0644 
 R2 0.9506 0.9423 
Temkin isotherm 
 KT (L/mg) 1.0029 1.0016 
 B1 60.649 22.682 
 R2 0.9477 0.9085 
 bt (J/mol) 40.851 109.23 
Dubinin–Radushkevich isotherm 
 qD (mg/g) 173.0 78.48 
 BD (mol2/kJ2) 0.0113 0.0037 
 R2 0.9648 0.9017 
 E (kJ/mol) 6.6533 11.6279 
Desorption parameter by 0.01 mol/L HNO3 
 Qe (mg/g) 45.22 44.68 
 Qd (mg/g) <0.02 21.28 
 % desorption – 47.68 
Isotherm parametersCr(VI)Pb(II)
Langmuir isotherm 
 qmax (mg/g) 212.76 89.29 
 KL (L/mg) 0.0036 0.027 
 R2 0.9822 0.957 
Freundlich isotherm 
 Kf 1.7737 7,2193 
 n 0.8761 2.0644 
 R2 0.9506 0.9423 
Temkin isotherm 
 KT (L/mg) 1.0029 1.0016 
 B1 60.649 22.682 
 R2 0.9477 0.9085 
 bt (J/mol) 40.851 109.23 
Dubinin–Radushkevich isotherm 
 qD (mg/g) 173.0 78.48 
 BD (mol2/kJ2) 0.0113 0.0037 
 R2 0.9648 0.9017 
 E (kJ/mol) 6.6533 11.6279 
Desorption parameter by 0.01 mol/L HNO3 
 Qe (mg/g) 45.22 44.68 
 Qd (mg/g) <0.02 21.28 
 % desorption – 47.68 
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Figure 2
Adsorption isotherm models for Cr(VI) and Pb(II) using MKL. (a) Langmuir; (b) Freundlich; (c) Temkin; and (d) Dubinin–Radushkevich.
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Adsorption isotherm models for Cr(VI) and Pb(II) using MKL. (a) Langmuir; (b) Freundlich; (c) Temkin; and (d) Dubinin–Radushkevich.

Figure 2
Adsorption isotherm models for Cr(VI) and Pb(II) using MKL. (a) Langmuir; (b) Freundlich; (c) Temkin; and (d) Dubinin–Radushkevich.
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Adsorption isotherm models for Cr(VI) and Pb(II) using MKL. (a) Langmuir; (b) Freundlich; (c) Temkin; and (d) Dubinin–Radushkevich.

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The Freundlich isotherm – Equation (6) – is used to analyze adsorption data for multiple layers with different adsorption energies:
(6)
where KF is the Freundlich constant correlated to the biosorbent adsorption capacity and 1/n is the Freundlich constant ascribed to the adsorption intensity. Validation of the Freundlich isotherm required the use of a graph depicting ln qe and ln Ce (Nkutha et al. 2021; Mardiah et al. 2022). The KF and n values ascertained from the intercept and slope of Figure 2(b) are presented in Table 1. Based on the correlation coefficients (0.9506 for Cr(VI) and 0.9423 for Pb(II)), the experimental data appear to align well with the Freundlich isotherm model. The model's n values determine the nonlinearity between concentration and adsorption, offering insight into sorption favorability (Lim et al. 2019). Generally, the sorption characteristics are good when n is between 2 and 10. If it is between 1 and 2, it is moderately strict, and if it is below 1, sorption is deficient. The value of n for Cr(VI) adsorption was 0.87, indicating poor characteristics. It exceeded 2 for Pb(II), indicating that the ions are adsorbed favorably by MKL.
The Temkin isotherm model helps to understand material behavior. The model assumes that the adsorption heat of all molecules decreases linearly with the increase in the coverage of the adsorbent surface, and that adsorption is characterized by a uniform distribution of binding energies, up to a maximum binding energy (Sahoo & Prelot 2020). The linear form of the Temkin isotherm model is shown in Equation (7):
(7)
where B1 = RT/bt, R is the universal gas constant, T is the absolute temperature at 298 K, the variable bt is connected to the heat of adsorption (J/mol), and the Temkin adsorption potential (KT) is measured in L/mg. Figure 2(c) shows the plots of qe versus ln Ce, which depict Cr(VI) and Pb(II) adsorption onto the MKL biosorbent. Table 1 shows the values of KT and B1, which were obtained by calculating the slope and intercept of the qe versus ln Ce plots. Based on the high correlation coefficients of 0.9477 for Cr(VI) and 0.9085 for Pb(II), it can be concluded that the Temkin model provides a suitable fit for the experimental data. The values of bt of Cr(VI) and Pb(II) adsorption were determined as 40.85 and 109.23 J/mol, respectively. Studies indicate that heats of sorption below 20 kJ/mol are characteristics of physisorption (Lim et al. 2019; Emamy et al. 2021). The low bt value, in this case, suggests a weak link to the MKL, indicating physical adsorption (Ali et al. 2019; Azizi et al. 2023).
The Dubinin–Radushkevich isotherm model can distinguish between chemical and physical adsorption, describing how adsorbates are adsorbed, and is used to calculate the energy required for the absorption process (Mehrmand et al. 2022; Abadi et al. 2023). The linearized model is given by Equation (8):
(8)
and
(9)
where qD is the adsorbent's maximum adsorption capacity (mg/g); BD (mol2/kJ2) is the adsorption energy constant; and is the Polanyi potential (Ali et al. 2019; Abadi et al. 2023). The values of BD and qD (Table 1) were defined from the intercept and slope of ln qe versus RT ln (1 + 1/Ce) (Figure 2(d)). The correlation coefficients for Cr(VI) and Pb(II) adsorption are 0.9648 and 0.9017, indicating that the experimental data align well with the model.
The average adsorption energy required can be estimated using Equation (10):
(10)

The value of E can give information about how adsorption occurs. The energy required when transferring one mole of ions can determine the adsorption process (Mehrmand et al. 2022). Between 1 and 8 kJ/mol, it is physisorption; between 8 and 16 kJ/mol, it is adsorption followed by ion exchange, while 20–40 kJ/mol indicates chemisorption. The 6.65 kJ/mol for Cr(VI) suggests physisorption (Ali et al. 2019; Mehrmand et al. 2022). The value of E for Pb(II) adsorption is 11.62 kJ/mol, indicating ion exchange.

Adsorption kinetics

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To gain a better consideration of the adsorption rate of Cr(VI) and Pb(II) by MKL within a specific time, several kinetic models were examined: pseudo-first-order, pseudo-second-order, and intra-particle diffusion (Arrisujaya et al. 2019; Lala et al. 2023). The results for constant rate studies for various initial concentrations of adsorbate solution (Cr(VI) or Pb(II)) by all kinetic models are listed in Table 2. The pseudo-first-order kinetic model is founded on the idea that the rate of solute absorption increases with time in proportion to the variation between the level of saturation concentration and the quantity of solid uptake during a specific duration remains an essential factor (Sahoo & Prelot 2020; Shooto 2020). The kinetic model commonly represents the initial stage of adsorption. The pseudo-first order kinetic model is represented by Equation (11):
(11)
Table 2

Kinetic model rate constants for Cr(VI) and Pb(II) adsorption onto MKL

Adsorbate Cr(VI) (mg/L)Pseudo-first order
Pseudo-second order
Intra-particle diffusion
K1 (min−1)qe1 (mg/g)R2K2 (g/(mg min))qe2 (mg/g)R2kiCiR2
50 2.1991 42.37 0.629 0.0139 42.01 0.998 0.3824 37.582 0.5371 
100 5.5593 84.74 0.739 0.0011 89.28 0.994 2.1363 59.721 0.9143 
150 38.926 147.05 0.992 0.0001 142.85 0.996 8.2395 22.18 0.9473 
Pb(II) (mg/L)                   
50 2.8652 31.34 0.276 0.0037 33.67 0.991 0.5682 25.176 0.5653 
100 3.6862 39.21 0.599 0.0051 38.31 0.993 0.8079 48.795 0.6436 
150 14.277 50.5 0.953 0.0012 48.78 0.997 7.0489 129.26 0.9458 
Adsorbate Cr(VI) (mg/L)Pseudo-first order
Pseudo-second order
Intra-particle diffusion
K1 (min−1)qe1 (mg/g)R2K2 (g/(mg min))qe2 (mg/g)R2kiCiR2
50 2.1991 42.37 0.629 0.0139 42.01 0.998 0.3824 37.582 0.5371 
100 5.5593 84.74 0.739 0.0011 89.28 0.994 2.1363 59.721 0.9143 
150 38.926 147.05 0.992 0.0001 142.85 0.996 8.2395 22.18 0.9473 
Pb(II) (mg/L)                   
50 2.8652 31.34 0.276 0.0037 33.67 0.991 0.5682 25.176 0.5653 
100 3.6862 39.21 0.599 0.0051 38.31 0.993 0.8079 48.795 0.6436 
150 14.277 50.5 0.953 0.0012 48.78 0.997 7.0489 129.26 0.9458 
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Table 3

Comparing the maximum adsorption capacity of different natural adsorbent materials

AdsorbentsAdsorbatesqmax (mg/g)pHDosage (g/L)Reference
Sweet flag Cr(VI) 24.48 1.0 0.2 Shooto (2020)  
Juniperus procera leaves Cr(VI) 23 4.0 10 Ali et al. (2019)  
Green tea leaves Cr(VI) 26.39 2.0 40 Jeyaseelan & Gupta (2016)  
Areca nut leaf sheath Cr(VI) 109.89 2.0 1.25 Pant et al. (2022)  
Rice husk Cr(VI) 42.37 2.0 40 Lala et al. (2023)  
MKL Cr(VI) 213 2.0 1.0 This work 
Sweet flag Pb(II) 88.08 8.5 0.2 Shooto (2020)  
Juniperus procera leaves Pb(II) 30.3 4.6 10 Ali et al. (2019)  
Pomelo leaves Pb(II) 207.2 4.0 2.0 Lim et al. (2019)  
Lawsonia Inermis Pb(II) 15.41 5.0 5.0 Mehrmand et al. (2022)  
Pineapple fiber Pb(II) 165 5.0 0.1 Tangtubtim & Saikrasun (2019)  
MKL Pb(II) 89 6.0 1.0 This work 
AdsorbentsAdsorbatesqmax (mg/g)pHDosage (g/L)Reference
Sweet flag Cr(VI) 24.48 1.0 0.2 Shooto (2020)  
Juniperus procera leaves Cr(VI) 23 4.0 10 Ali et al. (2019)  
Green tea leaves Cr(VI) 26.39 2.0 40 Jeyaseelan & Gupta (2016)  
Areca nut leaf sheath Cr(VI) 109.89 2.0 1.25 Pant et al. (2022)  
Rice husk Cr(VI) 42.37 2.0 40 Lala et al. (2023)  
MKL Cr(VI) 213 2.0 1.0 This work 
Sweet flag Pb(II) 88.08 8.5 0.2 Shooto (2020)  
Juniperus procera leaves Pb(II) 30.3 4.6 10 Ali et al. (2019)  
Pomelo leaves Pb(II) 207.2 4.0 2.0 Lim et al. (2019)  
Lawsonia Inermis Pb(II) 15.41 5.0 5.0 Mehrmand et al. (2022)  
Pineapple fiber Pb(II) 165 5.0 0.1 Tangtubtim & Saikrasun (2019)  
MKL Pb(II) 89 6.0 1.0 This work 
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The chemisorption step limits the adsorption rate by the pseudo-second-order model (Figure 3(a) and 3(d)). The kinetic model of this form can possess adsorption behavior across the complete range. The pseudo-second-order kinetic model is represented by Equation (12):
(12)
where K2 is the equilibrium rate constant of the pseudo-second-order model (g/mg min), at all initial concentrations of Cr(VI) and Pb(II) studied, very high correlation coefficients (>0.99) were obtained working with the pseudo-second-order kinetic model (Figure 3(b) and 3(e)). K2 decreases as Cr(VI) concentration increases for the MKL biosorbent in Table 2. This behavior arises because of reduced adsorption competition on surface sites at low concentrations. The K2 value for Pb(II) adsorption is frequently lower than that of Cr(VI), indicating more rapid adsorption of Cr(VI) by MKLs.
Figure 3
Adsorption kinetic models for Cr(VI) and Pb(II) using MKL. (a) and (d) Pseudo-first order; (b) and (e) pseudo-second order; and (c) and (f) intra-particle diffusion.
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Adsorption kinetic models for Cr(VI) and Pb(II) using MKL. (a) and (d) Pseudo-first order; (b) and (e) pseudo-second order; and (c) and (f) intra-particle diffusion.

Figure 3
Adsorption kinetic models for Cr(VI) and Pb(II) using MKL. (a) and (d) Pseudo-first order; (b) and (e) pseudo-second order; and (c) and (f) intra-particle diffusion.
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Adsorption kinetic models for Cr(VI) and Pb(II) using MKL. (a) and (d) Pseudo-first order; (b) and (e) pseudo-second order; and (c) and (f) intra-particle diffusion.

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Weber and Morris developed an intra-particle mass transfer diffusion model to determine diffusion in adsorption (Sahoo & Prelot 2020). The model is expressed by Equation (13):
(13)
where ki is the rate of intra-particle diffusion (mg/g min−1/2), and the rate constants ki and Ci (mg/g) are determined from the slope and intercept of the regression line (the plots of qt as a function of t0.5). To determine the boundary layer thickness accurately, it is essential to analyze the C values thoroughly, high values indicate more significant boundary layer effects. The linear plot's intercept (Figure 3(c) and 3(f)) suggests that absorption involves more than intra-particle diffusion as a rate-controlling step (Sahoo & Prelot 2020). The process is complex and includes multiple diffusion resistances, implying that the surface biosorbent worked simultaneously with intra-particle diffusion (Shooto 2020) in the adsorbate and MKL interactions.

Temperature effect and thermodynamic studies

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The effect of temperature at 303, 308, and 313 K for the adsorption of Cr(VI) and Pb(II) ions onto MKL is shown in Figure 4. The adsorption capacity of MKL toward Cr(VI) ions increased when the temperature was raised from 303 to 313 as shown in Figure 4. This is significant because it shows that high temperatures give the metal ions in the solution the kinetic energy necessary to overcome all factors impeding the adsorption processes. This was caused by an enhanced diffusion rate. This demonstrated that the adsorption processes were endothermic. However, the adsorption of Pb(II) slightly decreased when the temperature of the system was increased. The adsorption of Pb(II) decreased with increased temperature of the system; this suggested that the reactions were exothermic in nature.
Figure 4
Temperature effect on the adsorption of Cr(VI) and Pb(II) ions onto MKL.
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Temperature effect on the adsorption of Cr(VI) and Pb(II) ions onto MKL.

Figure 4
Temperature effect on the adsorption of Cr(VI) and Pb(II) ions onto MKL.
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Temperature effect on the adsorption of Cr(VI) and Pb(II) ions onto MKL.

Close modal
To analyze adsorption thermodynamics, equilibrium constants and thermodynamic parameters that vary with temperature must be calculated, including the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) (Hevira et al. 2020). These can be determined using Equations (14)–(16) inclusive:
(14)
(15)
(16)
ΔH and ΔS were determined from the slope and intercept of the linear plot of ΔG versus T. Figure 5 shows the thermodynamic parameters obtained. The negative values of ΔG confirm that Cr(VI) adsorption by MKL is favorable and spontaneous (Tangtubtim & Saikrasun 2019). The Gibbs free energy values for Pb(II) adsorption are positive, however, and it is noted that the adsorption process is non-spontaneous (Shooto 2020). The positive value of ΔG also indicates that there is an energy barrier.
Figure 5
Thermodynamic parameters for Cr(VI) and Pb(II) adsorption by MKL (Cr(VI) ΔH: 66.028 kJ/mol, ΔS: 219.8 J/mol K and Pb(II) ΔH: −13.456 kJ/mol, ΔS: −46.6 J/mol K).
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Thermodynamic parameters for Cr(VI) and Pb(II) adsorption by MKL (Cr(VI) ΔH: 66.028 kJ/mol, ΔS: 219.8 J/mol K and Pb(II) ΔH: −13.456 kJ/mol, ΔS: −46.6 J/mol K).

Figure 5
Thermodynamic parameters for Cr(VI) and Pb(II) adsorption by MKL (Cr(VI) ΔH: 66.028 kJ/mol, ΔS: 219.8 J/mol K and Pb(II) ΔH: −13.456 kJ/mol, ΔS: −46.6 J/mol K).
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Thermodynamic parameters for Cr(VI) and Pb(II) adsorption by MKL (Cr(VI) ΔH: 66.028 kJ/mol, ΔS: 219.8 J/mol K and Pb(II) ΔH: −13.456 kJ/mol, ΔS: −46.6 J/mol K).

Close modal

The values of ΔH for Cr(VI) are positive, indicating that adsorption is endothermic (Tangtubtim & Saikrasun 2019; Shooto 2020), while Pb(II) adsorption on MKL is exothermic (i.e., ΔH < 0) (Komárek et al. 2015). The positive value of ΔS suggests an increase in disorder during Cr(VI) adsorption onto MKL at the solid/solution interface. A negative entropy change for Pb(II) adsorption shows a decrease in entropy, and the system converts less randomly (Azizi et al. 2023). Physisorption has an energy change range of −20 to 0 kJ/mol, while that for chemisorption it is from −80 to −400 kJ/mol (Shooto 2020). This study shows that the values of ΔG do not indicate chemisorption or physisorption, but rather, different adsorption processes like ion exchange with exchangeable cations or surface complexation (Nkutha et al. 2021) with binding sites on MKLs.

Comparative studies on Cr(VI) and Pb(II) adsorption

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This study's findings suggest that MKL can potentially serve as a valuable biosorbent for Cr(VI) and Pb(II) (Table 3).

Desorption

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Desorption studies aid in explaining the process of adsorbate adsorption. In the adsorption–desorption experiments, 0.01 mol/L of HNO3 was used for a single cycle, which is considered inadequate. The desorption efficiency result is given in Table 1. The studies show that Cr(VI)-loaded MKL was not desorbed by 0.01 mol/L HNO3 (pH 2), while about 48% of Pb(II) was desorbed from loaded MKL. Cr(VI) does not affect the desorption solution at pH 2. The desorption solution works well with Pb(II) at pH 2, however. Accordingly, nitric acid selected in this study is believed to play functions in both desorption and protonation. This desorption study considered the degree of acidity or alkalinity of the desorption solution, affecting the high acid/base concentrations. The desorption study had previously considered the desorption solution's degree of acidity or alkalinity, which significantly varied the acid or base concentration. Each adsorbate may be more or less appropriate for the acidity or basicity of the desorption solution.

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The study aimed to examine the potential effectiveness of using MKLs in removing Cr(VI) and Pb(II) ions from aqueous solutions. Cr(VI) and Pb(II) adsorptions onto MKLs were based on adsorbate solution pH, biosorbent dosage, contact time, initial adsorbate concentrations, and temperature. The adsorption equilibrium data fit the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models adequately.

As per the Langmuir model, the maximum MKL adsorption capacities of Cr(VI) at pH 2 and Pb(II) at pH 6 were approximately 213 and 89 mg/g, respectively. The Langmuir isotherm RL values for MKLs for Cr(VI) and Pb(II) indicate a favorable process. The Freundlich isotherm n values showed that Cr(VI) adsorption had poor adsorption characteristics, while Pb(II) was adsorbed favorably by MKLs. The value of bt for the Temkin isotherm for Cr(VI) and Pb(II) adsorption was characteristic of physisorption. The E values for the Dubinin–Radushkevich isotherm show that the Cr(VI) adsorption by MKL was physisorption, while that of Pb(II) was chemisorption.

The pseudo-second-order kinetic model was the most suitable for Cr(VI) and Pb(II) adsorption in the kinetic study. The adsorption kinetic studies show that chemisorption may explain the process, including valency forces through sharing or exchanging electrons for Cr(VI) and Pb(II) on MKLs. The removal of Cr(VI) onto MKL was favored at high temperatures while Pb(II) favored low temperature. Thermodynamic analysis studies revealed that surface complexation coexisted with ion exchange as the primary removal mechanism.

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This work was financially supported in part by Research and Community Service Institution (LP2M), Universitas Nusa Bangsa, Indonesia, by Research Contract Number: 098/REK-UNB/SPK/VIII/2022, the fiscal year 2022.

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

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The authors declare there is no conflict.

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