Isotherm and kinetic adsorption of rice husk particles as a model adsorbent for solving issues in the sustainable gold mining environment from mercury leaching

Introduction

Community mining has been conducted throughout Indonesia for hundreds of years and is currently experiencing a surge in activity. Indonesia is one of the world's top ten gold producers (www.ceicdata.com; www.gold.org/goldhub). It is home to many artisanal and small-scale gold mining (ASGM) firms and operational facilities. ASGM is defined by the Ministry of Environment and Forestry (KLHK) as gold mining undertaken by individual miners or small firms with little capital and production. In Indonesia, there are two sorts of ASGM: those that are licensed and those that are not. The majority of ASGM activities in Indonesia continue to operate illegally since they lack government authorization. Illegal ASGM is detrimental to the state since they are unregistered, do not pay royalties, cannot be regulated, and contribute to environmental damage and adverse health effects caused by mercury usage.

The majority of ASGMs in Indonesia extract gold using a traditional method involving amalgamation with mercury [1]. The mercury and gold precipitate form a mercury-gold amalgam, which is then heated at high temperatures. Finally, the gold is extracted via evaporation by only distilling 10-40 % of the contained gold [2]. Miners are often only able to extract a limited amount of gold with high amounts of mercury (more than 1000 ton of mercury are released into the environment every year) using conventional processing techniques [1], causing environmental and health issues not just for the active miners involved in the processing, but also for the non-mining neighboring communities. Hg emissions from the ASGM industry account for 37 % of all anthropogenic Hg released into the environment, making ASGM the second most prominent source of Hg pollution in Indonesia after coal burning [3, 4]. The pollution of groundwater with mercury poses a severe hazard to humans, animals, and the ecosystem on a global scale. According to the Environmental Protection Agency and the World Health Organization, the maximum permitted limits for total mercury and CH₃Hg are 5.0 and 1.6 g/kg per week, respectively. Inaddition, Government Regulation of Indonesia N82 of 2001 states that the amount of mercury found in water is 0.001 ppm. This is backed by the Indonesian Ministry of Health no 907/Menkes/SK/VII/2002, specifying that the highest concentration of permissible mercury is 0.001mg/l. Therefore, removing mercury from water that has been contaminated is of utmost significance, and this issue requires further focus and consideration. The United Nations for Minamata Convention on Mercury, signed by 128 nations in 2013, was one of the most significant acts against Hg use. Its primary mission is to restrict and reduce the use of mercury, and finally eliminate it by 2030.

Several chemical and physical processes have been applied for Hg removals from wastewater, such as ion exchange [5], coagulation/coprecipitation [6], adsorption, reverse osmosis, membrane separation, liquid-liquid extraction, precipitation, electrodeposition [7, 8]. However, from an engineering standpoint, the technique's cost and efficiency should be compromised. Nonetheless, adsorption-based technologies can remove heavy metal ions due to their simplicity, reusability of the adsorbent, environmental friendliness, ease of operation, and cost-effectiveness [9, 10].

Rice husk is the protective outer covering of rice grains. It is an abundant agricultural waste of rice milling and has various uses and applications. A summary regarding the recent potential application of rice husk and its modification is summarized in Table 1.

Some researchers have already investigated the potential of rice husk sorbents to remove heavy metals from water [11-14]. However, the studies developed using a complex and expensive method due to modifying the rice husk. Hence, this study used raw rice husk as a sorbent material to determine rice husks' adsorption properties so that ASGM small-scale miners can use this method directly in processing mercury waste from gold mining.

This work studied the ability of raw rice husk to remove Hg²⁺ under various competing situations such as initial concentration, contact time, and dosage. In addition, to comprehend the interactions between multiple sorbates and the effect of complexing ligands on the dynamics of Hg, the efficacy of raw rice husk in removing Hg²⁺ from wastewater was investigated using adsorption kinetics and isotherm models. Then, experimental and theoretical research was conducted to elucidate the mercury adsorption mechanism on rice husk and the removal capability of this adsorbent.

Table 1

Recent potential applications and modifications of rice husk

Methods

The materials in this study consisted of raw rice husk (obtained from Farm Shop, Bandung, Indonesia), standard stock solution of mercury to prepare the mercury solution (1000±2 mg/l, Merck (Germany)), nitric acid (EMSURE), and ultrapure water (18.2 MОm/cm, Milli-Q system). In this study, all chemicals used were analytical reagent class.

Raw rice husk was prepared by saw-milled, and sieved to get the desired particle sizes (Yayasan Bumi Publikasi Nusantara, Indonesia) with a hole size of 74, 125, 250, 500, 1000, and 2000μm, and then preserved at room temperature. The rice husk is placed into a conventional batch-type blade-milling apparatus. Typically, this milling apparatus includes a milling tube made of SAN material with a diameter of 16.6 and a length of 13.5cm, along with stainless steel blades measuring 6cm. The milling procedure was conducted at room temperature for 10 min.

Nicolet 6700 Fourier-transform infrared (FTIR) spectrometer, manufactured by Thermo Fisher Scientific Co., USA, was used to determine the functional groups on rice husk at wavelength range 400-4000cm^–1. Additionally, a field emission scanning electron microscope spectroscopy JSM-6360 was utilized to characterize the surface morphology of the adsorbent. Then, the specific surface area and average pore size of the adsorbent were measured using the Brunauer − Emmett − Teller (BET) method at a temperature of 77K using Quantachrome NOVA 2200E, China. Finally, to utilize the chemical elements in the solution used the atomic absorption Spectrophotometers Agilent 200 Series AA manufactured by Agilent Technologies.

Adsorption Procedure

To conduct the batch adsorption experiments, Hg(II) solutions were prepared by using an analytical grade reagent (Hg(NO₃)₂) and using ultrapure water as the solvent. The pH of the prepared solutions was adjusted using 0.05M solutions of HNO₃ to ensure the presence of metal ions based on the pH-induced chemical speciation.

Various experimental tests were conducted to assess the effectiveness of rice husk in removing Hg from 100 ml aqueous solutions. To determine the most favorable adsorption condition, several factors were investigated, such as contact time (ranging from 30 to 180min), adsorbent dose (0.02 to 0.2g), and initial concentration (5 to 20 mg/l). Subsequently, the optimal values, which yielded the highest percentage of removal, were obtained. Then, using the obtained optimum values, kinetics, and isothermal tests were conducted using different contact times.

The mixture solution was shaken at 200 rpm using a magnetic stirrer-SP88850105 manufactured by Thermo Fisher Scientific. After adsorption, the solution was filtered using a 0.45 µm nylon membrane filter to remove adsorbent that might interfere with mercury determination. The residual concentration of Hg(II) was determined at the end of each experiment using Agilent 200 Series AAatomic absorption spectrophotometers (AAS). Then, the adsorption capacity and Hg_removal were calculated:

where C₀ and C_e are the initial and residual concentrations of metal ions, mg/l; C_t denotes the analyte concentration at time t, mg/l; V is the volume of solution used, l; m is the adsorbent mass, g.

Kinetic Procedure

The kinetic parameters are necessary to anticipate the rate at which Hg(II) is removed from aqueous solutions, to comprehend the adsorption mechanism, and to define the step that determines the adsorption [31]. In this work, the data obtained from the effect of contact time on the quantity of Hg(II) adsorbed at the surface of rice husk were used in Lagergren pseudo-first-order [32]

and pseudo-second-order [31, 32] have been used

where Q_e is the adsorption capacity at equilibrium, mg Hg/g rice husk; Q_t is the adsorption capacity at time t; mg Hg/g rice husk; k₁ is the rate constant of the Lagergren first-order kinetic model, min⁻¹; tis the time of the experiment, min; k₂ is the rate constant of the pseudo-second-order kinetic model, min⁻¹.

Adsorption kinetics are typically thought to be heavily dependent on the physicochemical qualities of the used substance (adsorbent) [18].

Results and discussion

FTIR analysis of the raw rice husk is conducted after and before adsorption (Fig.1). The band is located at 3282 cm^–1 be attributed to (–OH) related to the stretching of cellulose and lignin of rice husk before the adsorption process [33-35]. Other peaks in the spectrum of rice husk before Hg²⁺ adsorption, which is the band at around 2918cm^–1, assigned to C–H in CH and C–H₂, the bands at 2160-1978.65cm^–1 due to C≡C group, and the band around 1650.69-1514.86cm^–1 attributed to the C=O (ester and carboxylic group from hemicellulose) and C=C aromatic ring from lignin, while 1455.11-1330.89cm^–1 contributed to –COOH [36]. The sharp vibration peak around 1073.75 and 1032.02cm^–1 can be correlated to organic silicone (Si–O–Si), while the peak of transmittance appears in the wave number area 788.75-557.36cm^–1 is a typical characteristic of amorphous silica (Si–OH) spectrum [35].

Upon analysis of the FTIR spectra after the adsorption of mercury (Fig.1), a discernible peak at 432.08cm^–1 was observed. This peak can be attributed to the vibration of oxygen that is perpendicular to the Si–O–Si planes [35]. In addition, rice husk spectra after the removal of Hg²⁺ have revealed a noticeable decrease in the wavelength peaks at 2160.82, 2034.84, 1978.65, and 1073.75cm^–1 due to the oxygenated functional groups. The outcomes obtained from the adsorbent materials produced from agroindustry waste bear resemblance to those published in the literature [31-37]. Earlier research has indicated that mercury compounds display characteristic FTIR signals at 3500, 1750, and 600cm^–1 [38]. In the case of the rice husk material, the band is noticed at 3500cm^–1, which corresponds to the –OH and Hg interaction. These results are in line with Guo et al. [36] that phenolic hydroxyl groups on the adsorbent surface undergo complexation reactions with Hg²⁺.

In addition, the SEM image of the rice husk adsorbent is shown in Fig.2. The SEM analysis of the surface of the adsorbent revealed an irregular layer of silica and natural resins in the rice husk structure. The SEM image of rice husk before adsorption (Fig.2,a,b) reveals the presence of surface pores, which serve as the ideal location for capturing Hg²⁺ molecules from aqueous solutions [39]. Following the adsorption process, the SEM image of rice husk displays the successful capture of Hg²⁺ molecules within its surface pores (Fig.2,c,d). This was accomplished through a process of diffusion and migration with Hg²⁺ molecules moving from the aqueous solution to the rice husk surface and eventually filling the available pores [40]. The physical adsorption of Hg²⁺ at the surface of rice husk was likely due to mechanical adhesion through electrostatic force, which is consistent with previous research [39-41].

Figures 2,b,d display the EDX spectra of the rice husk. Figure 2,c shows that the predominant elements in the prepared rice husk are carbon and oxygen, indicating the presence of oxygen functional groups on the carbon surface. Additionally, there are small quantities of silica, calcium, magnesium, and phosphorus from the source. Nevertheless, following the occurrence of the adsorption procedure (Fig.2,d), there was a decrease in the cations present in rice husk.

Also, the appearance of Hg peaks in the EDX was found, confirming that Hg fills the pore structure. This outcome suggests that an ion exchange mechanism might have taken place [36].

Nitrogen adsorption isotherms utilizing Quantachrome NOVA 2200E and the BET formalism were employed to determine the isotherm shape, specific surface area, and pore volume of the adsorbent. The outcome of the nitrogen adsorption isotherm test displayed a Type IV pattern as classified by IUPAC and H3 hysteresis (Fig.3,a). This suggests a mesoporous substance with a continuous facile pore connection. Additionally, it has a limited multilayer structure that matches the complete filling of the capillaries [42-45]. The BET-specific surface area of the adsorbent was 173.782m²/g, and the BJH pore distribution method showed a pore size diameter of about 1.714nm and a pore volume of about 0.307cm³/g (Fig.3,b). These results outperformed previous studies [37, 46]. Our findings demonstrate the outstanding quality of the adsorbent.

Adsorption study

Several factors were identified:

• Effect of contact time. To design effective batch adsorption experiments, it is crucial to assess the impact of contact time required to reach equilibrium. Therefore, the effect of contact time on the adsorption of Hg was studied at various adsorbent dosages within 30-180min. The results indicated that, at room temperature, the mercury concentrations in water decreased within the first 30min (Fig.4,a). The fast initial rates of adsorption are due to the presence of a significant number of binding sites for adsorption at the beginning, followed by saturation and equilibrium [47]. In addition, the presence of major functional groups such as oxygenated aromatic rings in the rice husk structure confirmed by FTIR spectra images (see Fig.1) allows for Hg-π conjugation and stacking interactions with Hg ions [48]. As shown by SEM images (see Fig.2), the presence of cellulose and lignin can also play a role in the removal of heavy metal mercury [37]. The optimal contact time for Hg removal was found to be around two hours for all rice husk dosages, as the percentage removal did not significantly increase beyond this duration. This information is depicted in Fig.4,a.
• Effect of adsorbent dosage. Typically, adsorbents have a limited number of active sites which means that their dosage must be optimized. To treat a 100 ml aqueous solution with an initial concentration of 200µg/l of mercury within 120min under ambient conditions, various rice husk dosages were evaluated. Figure4,b shows that the percentage of Hg removal increased with increasing adsorbent dosage, reaching 78.8% at a dosage of 0.2g. This is because the abundance of active sites on the surface of the adsorbent increases with dosage, allowing it to bind more mercury [32, 37, 47, 48]. However, increasing the adsorbent dosage also decreased the amount of metal ions adsorbed due to the reduced concentration of adsorbate in the solution. Thus, the driving force for the adsorption process was also reduced. As a result, not all available adsorption sites can be occupied (unsaturated), leading to a decrease in adsorption capacity [49, 50].
• Effect of Hg initial concentrations. In Fig.4,c,d, the impact of the initial concentrations of the adsorbate (Hg) on the efficiency of the adsorption process using 0.05g of rice husk is presented. The study investigated the impact of different levels of Hg concentration on adsorption properties using a mercury solution as a model for wastewater. The initial concentrations of the mercury solution were set at 5, 10, and 20mg/l. This concentration is taken based on the actual concentration of mercury in the contaminated wastewater (river) in Indonesia. Based on the data presented in Fig.4,c,d, it is evident that as the initial concentration of mercury increases, both the adsorption capacity and removal of mercury also increase. When the mercury concentration is higher (20 mg/l), the maximum removal efficiency is approximately 93.1%, with an adsorption capacity of around 37.3mg/g.

In comparison, for lower mercury concentrations of 10 and 5mg/l, the removal efficiencies are approximately 78 and 52% respectively, with adsorption capacities of approximately 28.6 and 5.2mg/g after 120min of contact time. This is because a high initial concentration provides the necessary driving force to overcome adsorbate mass transfer processes between the aqueous and solid phases [47, 51, 52].

Adsorption kinetics

To understand the rate and mechanism of Hg(II) removal from an aqueous solution, knowledge of its kinetic parameters is required, hence a kinetics study was conducted. By graphing kinetic equations (1) and (2) against experimental data, linear connections were formed. Thus, kinetic parameters were determined and provided in Table 2. The effectiveness of the kinetic models was evaluated by the coefficient of determination values R². The calculated kinetic constants demonstrated that the pseudo-second-order kinetic model had the highest correlation coefficient R² and provided the best fit. Additionally, the results showed that the Hg-uptake obtained through pseudo-first-order kinetic was significantly lower than the experimental value (Q_e(cal) ⊥ Q_e(exp)). In contrast, the calculated Hg-uptake in pseudo-second-order kinetic was much closer to the experimental value. This finding was consistent with previous research conducted [31, 32]. These results confirm that the best-fit kinetic model is the pseudo-second-order kinetic model and that Hg adsorption on rice husk is most likely through the chemisorption mechanism [8,53].

Table 2

Kinetic parameters for the mercury adsorption over rice husk

Adsorption Isotherms

To study the adsorption mechanism, a brief overview of ten adsorption isotherms and the computation for obtaining curves from data fitting results are shown in Table 3. In prior work [54], the calculation of the isotherm adsorption is given in detail.

Table 3

A summary of ten adsorption isotherm models

Note: slope – slope factor; intercept – linear part of approximation equation.

A summary of ten adsorption isotherm models

The analysis of adsorption isotherms is crucial for determining how the adsorbent surface interacts with adsorbate molecules. The interaction between the metal ion and the adsorbent material, as well as the adsorption capacity of the material, can be determined using isothermal models. The ten isotherm adsorption models were used to match experimental mercury adsorption onto rice husk. The rice husk parameters derived from the ten studied models are presented in Table 4. The experimental values are reconstructed based on the charting of several parameters (using plot equations in Table 3). Then, linear correlation coefficients R² are employed to demonstrate the compatibility of correlation curves between experimental data and linearized isotherm models.

Table 4

Isotherm parameters for the adsorption of Hg (II) on rice husk

The Langmuir factor R_L defines whether the adsorption is favourable. In this instance, values fall in the range R_L>1, indicating that the adsorption process is unfavourable, with maximum adsorption capacities Q_m of 68.140 mg/g. In addition, the degree of adsorbate-adsorbent interaction k_L indicates a weak contact between the adsorbate molecule and the adsorbent surface due to its small value (k_L=–0.0016) [54]. Meanwhile, the correlation coefficient value for Langmuir is R²=0.934, indicating that the adsorption process employing the present adsorbent can produce a monolayer structure. Nevertheless, the negative value of k_L shows that the data do not fit Langmuir.

The fitting performance of Freundlich is shown in Table 4. The slope derived via data fitting in the form of a gradient is n^–1, while the intercept is lnk_F. Analysis of the n value of the rice husk adsorbent revealed a negative value with correlations of n<1 and n^–1<1, indicating that adsorption happens chemically via a typical process. In addition, the high value of R²0.984 implies that the adsorption profile of rice husk in the adsorption process has a multilayer structure. The adsorption capacities denoted by k_F have a value of 0.788 mg/g.

Temkin's approach was founded on the fact that adsorption decreases the heat of adsorption of all molecules in a linear way [55]. The determined values of its parameters (B_T and A_T) are given in Table 4. The B_T and A_T have small values (2.02∙10^–56 and –1.229, respectively), which means less affinity between the adsorbent and adsorbate molecules due to their physical interactions. The correlation coefficient of Temkin (R²=0.989) suggests that there is multilayer adsorption with a homogenous distribution between the adsorbate and the surface of the adsorbent. However, because the parameter value is small, this model cannot be used.

Dubinin – Radushkevich's R²=0.892 value is the lowest in this study. This value indicates the existence of micropore size in adsorbent surfaces with multilayer structures. The examination of the E value in this model was <8 KJ/mol proving that adsorption is a physical process [55, 56].

The thermodynamic feasibility and spontaneity of an adsorption process can be predicted using the Flory – Huggins isotherm, which describes the degree of surface covering properties of the adsorbate on the adsorbent [54, 57]. The estimated n_FH in this study (Table 4) suggests that the adsorbate occupies more than one active adsorbent zone generating a multilayer structure [54], this is supported by the R² value as well R²=0.984, indicating that no molecule was bonded to any adsorption site on the rice husk adsorbent [57]. As a result, the Flory – Huggins model cannot predict the adsorption of Hg(II) onto rice husk due to small value parameters and is therefore disregarded. The negative ΔG value demonstrates that the adsorption of Hg(II) onto rice husk is thermodynamically spontaneous and validates the process's viability.

The Fowler – Guggenheim isotherm model takes into consideration the lateral interaction of adsorbate adsorption onto adsorbent [54, 58]. The R²=0.992 value of this model shows a good fit. On the contrary, the parameters (W and k_FG) show relatively small values. A negative value of W indicates that the interaction occurs repulsively between the adsorbed molecules. Meanwhile, a very small k_FG value indicates the weak interaction between the adsorbate and adsorbent due to the Van der Waals bond. Hence, this model is not suitable.

The Hill – deBoer model describes the mobile adsorption and lateral interaction of molecules that have been adsorbed [59]. Using equation(3), Hill – de Boer model adsorption parameters were determined. As seen in Table 4, the regression coefficient R² had a value of 0.969 informing of the multilayer structure in the adsorption process. Meanwhile, a negative k₂ value represents repulsion. When there is repulsion among the adsorbed species, the apparent affinity decreases with loading [59]. A modest value of k₁ indicates poor adsorbent-adsorbate interaction because the active site is ineffective in carrying out the adsorption process. Consequently, this model is inappropriate.

The adsorption parameters of the Harkin – Jura model were obtained using equation(2). The R² value of 0.970 is the result of a multilayer adsorption process on the surface of an adsorbent with heterogeneous pore distribution.

The coefficient correlation value of Halsey exhibits a good adsorption system (R²=0.984), indicating the nature of the adsorbent is heterogeneous with a multilayer structure. These results are in line with the Harkin – Jura model.

The Jovanovic isotherm model has few mechanical contacts between desorbing and adsorbing molecules [54, 58, 59]. The Jovanovic parameters exhibit the most satisfactory value (Table 4), with the correlation coefficient R² (0.999) being near unity. In addition, this model has a high maximum capacity (107.299mg/g). This indicates that the experiment consists of adsorption with a moving and localized monolayer and no lateral contact.

The R² value analysis reveals that the rice husk adsorption system is compatible with various models, excluding Dubinin – Radushkevich. The Langmuir, Freundlich, Temkin, Flory – Huggins, Hill – de Boer, Harkin –Jura, Fowler – Guggenheim, and Halsey models all showed a good fit, but the Jovanovic model provided the best results. This suggests that the adsorption process forms a monolayer structure without any lateral interactions between the adsorbate and adsorbent. Additionally, the rice husk adsorbent is believed to have a multilayer adsorption process due to its heterogeneous pore distribution, which leads to pore filling. Thus, the bond between the adsorbate and adsorbent is characterized by weak chemical and physical interaction (Fig.5). The screening and arrangement are based on an understanding of the coefficient of correlation R². In descending order: Jovanovic, Fowler – Guggenheim, Temkin, Flory – Huggins, Freundlich, Halsey, Harkin – Jura, Hill – de Boer, Langmuir, Dubinin – Radushkevich. Although the model is in good agreement with the experimental results, analyses based on Table 4, physicochemical characterization, and kinetic adsorption show that the adsorption profile follows physical and chemical adsorption with multilayer adsorption. In addition, the adsorption process occurs endothermically and spontaneously by forming a multilayer surface with a pore-filling system and mutually repulsive intermolecular interactions.

Proposed adsorption mechanism

Concerning the characterization findings and in conjunction with the reported adsorption results, we propose the following adsorption mechanism. While morphology does indeed factor into the adsorption process, the presence of functional groups on the surface of rice husk is equally significant. There exists a noticeable relationship between Hg²⁺ and the oxygenated functional groups due to their interaction with available pairs of electrons on oxygen and the presence of aromatic structures (π-type bonds) in the materials. This results in them having polar characteristics with partially negative charges and a strong tendency to attract positively charged mercury ions. Thus, the rice husk adsorbent materials predominantly undergo Hg²⁺ adsorption through three processes: electrostatic interaction between the negatively charged density at the surface of rice husk and Hg²⁺ metal cation; formation of (–COO)₂Hg and (–O)₂Hg complexes due to the interaction between Hg²⁺ and conjugate bases of carboxylic and phenolic groups (–COO– and –O–), and interactions between Hg²⁺ and π (or ion exchange), Hg–Cπ electrons from the graphene structure (C=C) and C=O present in the materials. These findings agree with previous studies reported [31, 32, 36, 60]. However, the complexation reaction between –OH groups and Hg²⁺ is the most dominant [36]. The mechanism that happens during the adsorption process is complexation.

In addition, this study is preliminary for pilot and large scale process. Some issues must be more detailed analyzed, including the analysis in the formation of Hg into the pore structure, including the final pore structure. Further information relating to surface area (using BET and XRD) and molecule interaction (using FTIR) must be also done and compared to current literature [61-63].

Conclusions

The study successfully removed Hg(II) from realistic environmental concentration levels using rice husk in batch mode. The results showed that the ability to adsorb mercury increased with longer contact time, higher adsorbent dosage, and initial concentration.

The adsorption kinetics followed a pseudo-second-order model, and the Jovanovic model accurately described the adsorption isotherm with a maximum adsorption capacity of 107.299 mg Hg/g rice husk, suggesting a combination of physical and chemical interactions. The study also found that adsorbent materials with low surface area and high oxygenated functional groups are effective in removing Hg(II) because functional groups on the material's surface attract Hg(II) through various interactions. The rice husk material was found to be highly effective in removing Hg(II) from aqueous solutions. In conclusion, this study demonstrates the effectiveness of using rice husk for removing Hg(II) from water.

This study, we summarize the additional advantages of the use of rice husk as follows:

• rice husk is largely available, especially in rice-producing countries; since it is a by-product of rice production, the cost of rice husk is free;
• rice husk is categorized as one of the agricultural wastes, making the use of rice husk as an adsorbent material to solve the current issues in disposal and waste management;
• utilization of rice husk for wastewater treatment needs a simple procedure, requiring less processing steps;
• rice husk has an excellent pore structure, making it have a good adsorption capacity; further development and treatment of this rice husk can increase its selectivity;
• this method is also possible for being applied in the adsorption of heavy metal ions;
• the utilization of rice husk for adsorbing heavy metal can achieve significant results in realizing sustainable development goals (SDGs).

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