Removal of ammonium from water by a bentonite biochar composite

doi:10.21203/rs.3.rs-4723030/v1

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Removal of ammonium from water by a bentonite biochar composite

https://doi.org/10.21203/rs.3.rs-4723030/v1

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A new adsorbent of bentonite biochar composite (BRK) from natural bentonite and rice husk was synthesized for removal of ammonium (NH₄⁺) from water. The preparation of the adsorbent consisted of a pyrolysis process at 400 °C and activation of biochar with KOH to obtain BRK. Various advanced techniques were applied to characterize the investigated adsorbent, including Fourier-transform infrared spectroscopy (FTIR), N₂ adsorption analysis, scanning electron microscopy (SEM) integrated with Energy-Dispersive X-ray (EDX) Spectroscopy. The point of zero charge of BRK was 9.1. The pH solution strongly affected BRK’s adsorption capacity to NH₄⁺ ions in the solution. The removal efficiencies of NH₄⁺ were considerably diminished in the presence of coexisting cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺). The Langmuir adsorption capacity of BRK for NH₄⁺ was in the following order: 22.51 mg/g (10 ^oC) > 20.57 mg/g (30^oC) > 16.22 mg/g (50 ^oC). The kinetic experiments demonstrated that the adsorption equilibrium was achieved after 30 mins of contact. The ion-exchange was found to be the main adsorption mechanism for removing NH₄⁺ by BRK. This study proved that BRK is a low-cost and sustainable adsorbent derived from natural bentonite and rice husk and it is advantageous for successfully removing NH₄⁺ from water.

adsorption

ammonium

agricultural waste

bentonite

composite

water treatment

Ammonium (NH₄⁺) is an inorganic ion of nitrogen that is formed when organic molecules containing nitrogen are broken down and disposed of in urban wastewater and domestic wastewater. Ammonium contaminations are serious environmental issues in surface and groundwater which cause significant risks to ecosystems and human health [1]. This is due to the fact that NH₄⁺ has the potential to produce eutrophication and cause dissolved oxygen depletion. Thus, high NH₄⁺ concentrations may result in biodiversity loss, degradation of ecosystem functions, and toxicity to natural habitats and aquaculture [1, 2]. According to the World Health Organization [3] in humans, NH₄⁺ concentrations above 200 mg/kg of body weight have the potential to be toxic or harmful to health.

Many treatment approaches, such as biological treatment, adsorption, photoelectronic, chemical precipitation, air aeration, and ion exchange, were investigated to reduce the negative consequences of NH₄⁺ [1, 4]. Among them, the biological process is the most cost-effective and successful approach for treating NH₄⁺ in a domestic wastewater treatment plant. The procedure and development rate of the microorganisms used, however, are impacted by variations in environmental and water variables, such as temperature, thus critically affecting the operation of the water treatment system. These above factors are known as disadvantages of this technology. While considering the removal efficiency, cost-effectiveness, and simple operation, adsorption has emerged as a promising method to tackle NH₄⁺ problems [1]. Still, there is always a challenge in developing suitable adsorbent materials that have high efficiency, low cost, and abundance [1].

Unlike metal-organic framework (MOF) materials, which are generally only used in laboratory settings due to their high cost, the adsorbents originating from agricultural waste and natural minerals are gaining much attention and are widely applied in both laboratory and field scales. This is because of their abundance of practicability, cost-effectiveness, stability, and renewability. Among these natural adsorbents such as bentonite, zeolite, and other natural clays (e.g. kaolinite, montmorillonite, vermiculite, and sepiolite), bentonite is widely used as clay adsorbent, owing to its cost-benefit, high content of montmorillonite (well-known clay mineral with high adsorption capacity), and worldwide distribution [1, 5]. Bentonite possesses a 1:2 type layered structure, which consists of one silicon-oxygen tetrahedron sheet and two aluminum oxygen octahedron sheets. The layer might have a net negative charge, which could be readily balanced by several cations in aqueous solution (i.e., Na⁺, K⁺, Ca²⁺, and Mg²⁺) and contribute to the ion exchange capability of bentonite [6]. Furthermore, bentonite has the ability to bind to strengthen composites with controlled particle size due to the high content of montmorillonite mineral in its structure [7]. Eventually, bentonite is eco-friendly and non-toxic to the subsequent treatment, thus also confirming its applicability and sustainability for water treatment purposes.

Besides the above advantages, using adsorbent-based agricultural wastes for water treatment is also of great interest as a sustainable approach for added-values, zero-waste, and circular economy goals [8]. For carbonization of biomass and increase of adsorption capacity by improving cation exchange efficiency, specific surface area, and stable structure, various methods for preparation (e.g., pyrolysis, gasification, and hydrothermal carbonization) and modification (acids, alkali, oxidizing agents, and metal ions) of biochar were adopted [9, 10]. Different oxidative reagents (e.g., HNO₃, H₂O₂, and NaOH) were widely used to boost the quantity of oxygen-containing functional groups for NH₄⁺ adsorption [1, 11]. Besides, to enhance contaminant removal capacity, the combination of biochar and additives (e.g., clay minerals, metal oxides, single metals or couple metals, and carbonaceous materials), known as biochar-based composites, were also studied [12, 13, 14]. In this concept, biochar provides a porous structure to support the distribution of additives within its lattice, and this incorporation enhances the overall sorption capacity of the composite material [12].

The composites between biochar and clay materials have been widely investigated and developed for removing both organic and inorganic pollutants such as Pb [15], Cd [16], antibiotic ciprofloxacin [17], methylene blue (MB) [14], phosphate and NH₄⁺ [18, 19]. Biochar bentonite composite has recently been used for the simultaneous removal of phosphate and NH₄⁺ from water with maximum adsorption capacities of 132.2 and 39.5 mg/g [18]. This composite was prepared by introducing peanut shell powder and bentonite and modified by the pyrolysis process and reagents (MgCl₂ and NaOH) [18]. However, little attention was given to incorporate natural bentonite and rice husk–one of the most abundant agricultural wastes in Vietnam, for NH₄⁺ removal from aqueous solution.

The National Environmental Assessment Report in the period 2016–2020 of Vietnam showed high concentrations of NH₄⁺ in the surface and groundwater which were as high as approximately 50 and 130 times higher than the allowable limits for surface (0.3 mg/L) and groundwater (1 mg/L) [20]. High NH₄⁺ concentrations (50–99 mg/L) were recorded in groundwater in the Red River Delta [21]. Of particular concern, local residents in some areas have been using groundwater contaminated with NH₄⁺ for drinking and domestic purposes [22]. Being the third largest rice exporter in the world in 2023 and with the availability of natural bentonite, a combination of bentonite and biochar derived from rice husk would contribute to not only tackling the NH₄⁺ pollution issue but also implementing waste-to-resource practice as a circular economy approach.

In this study, a composite of bentonite and rice husk waste was prepared through a pyrolysis process and treated with KOH to enhance the cation exchange capacity (BRK). The successful fabrication of BRK adsorbent was confirmed throughout several characterization steps, including Fourier-transform infrared spectroscopy (FTIR), N₂ sorption analysis, and Scanning electron microscopy (SEM). In addition, the effects of several major factors for the removal of NH₄⁺ by BRK (i.e., pH solution, contact time, initial concentrations of NH₄⁺, temperature, coexisting ions, and desorption) were evaluated by a series of batch experiments.

2.1. Materials, chemicals, and reagents

All analytical chemicals and reagents, including NH₄Cl, NaOH, HCl, Ca(NO₃)₂, Mg(NO₃)₂, NaNO₃, and KNO₃ in this study were purchased from Sigma-Aldrich and were used directly without any extra purification. A 1000 mg NH₄⁺/L stock solution was prepared by diluting 4.394 g of salt NH₄Cl into 1000 mL deionized water. The feed NH₄⁺solutions used in the batch experiments were diluted from the stock solution using distilled water.

In this study, natural bentonite was obtained from a bentonite mine in Di Linh district, Lam Dong province, Vietnam. Coarse fractions of non-clay size were removed by sieving and sedimentation and dried at 80 ^oC. The dried bentonite sample was then gently manual-milled in an agate mortar to obtain the material with a particle size of less than 40 µm. Chemical and mineral compositions of bentonite were prior characterized by Hoang-Minh et al. [23]. Raw rice husks (RH) were collected in Hanoi, Vietnam and then washed with deionized water. After drying at 80 ^oC in an oven, the rice husks were then grinned and sieved to obtain a particle size of 0.5 mm. These prepared bentonite and RH samples were stored in plastic bags for further synthesis steps.

2.2. Synthesis of bentonite-biochar composite materials

The preparation of composite materials using bentonite and rice husk biochar, as well as their modification, via one and two stages, which is depicted in Fig. 1. In particular, 50 g of bentonite was thoroughly mixed with 50 mL of deionized water in a beaker, and the mixture was agitated utilizing a magnetic stirrer for an hour. An amount of 25 g RH was then added to the bentonite mixture, and the beaker was continuously stirred for 24h. After that, the solid part was separated from the liquid phase using 0.45 µm filters and dried at 80 ^oC for the subsequent 48 h to obtain a raw mixture of RH and bentonite.

In the first step, the pristine combination of bentonite and rice husk biochar underwent a pyrolysis process at 400 and 500 ^oC for 3 hours. The most typical pyrolysis temperatures (400 ℃ and 500 ℃) were selected for the modification of adsorbents [9]. The composite was then chemically modified via the second stage using KOH 2M. It should be noted that after each stage, the adsorbent was taken out of the mixture, bathed with distilled water until the solution was pH = 7, and then dried at 80 ℃ for the next stage and used for the preliminary experiment.

Preliminary experiments for the determination of adsorbents were performed at identical conditions (i.e., pH = 7, material dosage = 2.0 g/L, initial NH₄⁺ concentration of 20 mg/L, and shaking speed = 160 rpm). The NH₄⁺ removal rate of natural bentonite, raw rice husk, biochars after pyrolysis, and biochars after activation with KOH is shown in Fig. 1. Pyrolysis process (400–500°C) helped to improve removal efficiencies of bentonite (2.67%) and rice husk (1.13%) to 21.12–25.95% (biochar) (Fig. 1). KOH treatment increased 117–119% the removal efficiencies of NH₄⁺ from water. According to the preliminary results, bentonite biochar composite (BRK) (sample 5, pyrolysis temperature = 400 ^oC, KOH activated) was highlighted as the most effective material and was selected for further experiments.

2.3. Adsorption experiment

The NH₄⁺ removal performance of BRK material was examined by batch-scale studies. The influence of the initial pH solution, contact time, and initial NH₄⁺ concentration at various temperatures on the NH₄⁺ removal was then investigated. All experiments were conducted at a constant material dosage of 2.0 g/L by adding 0.1 g of adsorbent into 50 mL of NH₄⁺ solution with predetermined concentrations in an Erlenmeyer flask. The mixture was shaken at 160 rpm in a DAIHAN Labtech (Model LSI-2) machine. The contact time was adjusted following each category of experiments. The mixtures were filtered through 0.45 µm filters to separate the liquid and solid phases after the interval period of time. The NH₄⁺ concentration in the liquid phase was then analyzed.

To examine the effect of pH solution on NH₄⁺ removal using BRK adsorbent, the study was conducted under a wide pH range from 2 to 12. The initial NH₄⁺ concentration of 20 mg/L and contact time of 24 hours was set in this experiment. The pH solution was adjusted to the appropriate level using 0.1 M HNO₃ and NaOH. The kinetic adsorption was carried out in solution with pH = 7 and an initial NH₄⁺ concentration of 20 mg/L. The samples were taken at intervals of 1 min, 5 mins, 10 mins, 30 mins, 1 h, 2 h, 4 h, 8 h, 12 h, and 24 h. The isotherm adsorption was carried out at initial NH₄⁺ concentrations ranging from 5, 10, 20, 30, 50, 70, 100, 150, 200, and 300 mg/L. The experiments were performed at three levels of temperatures (10, 30, and 50°C). To investigate the effect of coexisting cation ions (Na⁺, K⁺, Ca²⁺, and Mg²⁺) at three different initial concentrations (10, 50, and 100 mg/L) on BRK adsorbent, the test was conducted at an initial NH₄⁺ concentration of 20 mg/L and pH = 7.

2.4. Adsorbent properties and sample analysis

The characterizations of adsorbents were determined using the Scanning Electron Microscopy (SEM), the Energy-Dispersive X-ray (EDX) Spectroscopy, the Fourier-Transform Infrared Spectroscopy (FTIR), N₂ adsorption analysis, and point of zero charge. In particular, Scanning Electron Microscopy (SEM- Nano SEM FEI-450) integrated with Energy-Dispersive X-ray (EDX) Spectroscopy (TEAM Apollo XL-EDAX) was used to determine the elemental compositions of adsorbents and to visualize in detail the topography and morphology of the sample. An EDX detector connected to a scanning electron microscope (SEM) produces an elemental spectrum. This spectrum displays the presence and strength of distinctive X-ray peaks for various elements, as well as the precise weight and atomic occupancy ratio of the constituent elements that make up the adsorbents. N₂ adsorption analysis was performed by Nova touch 4LX/Anton Paar, USA. The Jasco FT/IR-6300 spectrometer, which operated within a wavenumber ranging from 4000–500 cm^− 1, was used to detect the active functional groups available on the surface of the adsorbent. OriginPro peak-fitting method (OriginLab Corporation) was used to deconvolute the FTIR spectra. The drift method was used to calculate the BRK’s point of zero charge.

The NH₄⁺ concentrations in water samples were measured by continuous flow analysis which named Skalar method at wavelength 630 nm (SKALAR model 21050900, Netherlands). For chlorine donation, sodium hypochlorite and nitroprusside catalysts were employed. Ammonium standard solution of 1000 mg/L and a blank solution were used to ensure the accuracy of the analysis. The analytical detection limit for NH₄⁺ is 5 µg/L.

2.5. Data analysis

All tests were duplicated, and experiment results were expressed using the mean and standard deviation data. Isotherm and kinetic adsorption parameters were calculated using the Origin program. The calculation of NH₄⁺ adsorption capacity of adsorbents at the equilibrium time (q_e, mg/g) and at time interval t (qt, mg/g) was performed using equations given in our previous publication [24]. The best-fit model was determined by calculating the coefficient of determination (R²) and the chi-squared (χ²) value. Chi-squared (χ²) values and the coefficient of determination (R²) values were calculated to identify the best-fit model. A χ² value near 0 indicates that the data acquired by model application and the experimental data are similar, while a high χ² indicates a significant bias between the model and the experiment.

3.1. Effects of pH solution

The effect of pH solution on NH₄⁺ adsorption is presented in Fig. 2. Generally, the NH₄⁺ adsorption capacity of BRK was strongly affected by pH solution (2.0–12.0). As a result, the adsorption process of NH₄⁺ on adsorbents was not favorably affected by acidic or alkaline conditions, with the pH range of 6 to 9 typically yielding the greatest equilibrium adsorption quantity.

In this study, the ability of BRK to adsorb NH₄⁺ increases following the increase of initial pH and decreases sharply when pH > 9 (Fig. 2). Essentially, pH solution is also a significant factor as it determines NH₄⁺ speciation in water [25]. According to the NH₄⁺ species stability and a function of pH diagram, NH₄⁺ and its pKa value were determined to be approximately 9.3 (pKa) [25]. In other words, NH₄⁺ often exists as ionized NH₄⁺ when the pH solution is lower than 9.3. When the pH solution is greater than 9.3, the ammonia is found to be abundant and exists with its form of no charge (NH₃). Raising the pH of the solution will hasten the NH₃’s release from the solution since NH₃ in solution can readily cause volatility. As a result, solutions with pH values greater than 9.3 are anticipated to have a limited NH₄⁺ adsorption capability.

At low pH values (2–4), proton H⁺ from the acid environment had strong competition with ion NH₄⁺ in the active side of the material, as the repulsion between positive ions and NH₄⁺ in solution; therefore, BRK adsorbed a negligible amount of NH₄⁺ Fig. 2. The removal efficiency of NH₄⁺ was remarkably reduced in an acid environment (pH < 3) [26]. This is because extremely acidic pH will enrich the solution with protons and limit the ionization process of the acidic functional groups on adsorbents, which will reduce the ion-exchange sites available for the adsorption of NH₄⁺.

Under weak acid and weak alkaline conditions (pH = 5–9), the ability to remove NH₄⁺ from the water of BRK was higher than in strong acid conditions and reached the highest value at pH = 6 (Fig. 2). As reported by Eturki et al. [27], their investigation also found that an ideal pH from 6 to 8 produced the greatest results. This change could be explained by: (1) the decrease of ion H⁺ in solution led to the competition between H⁺ and NH₄⁺ also reduced, (2) when the pH value reached 6–8, the deprotonation of some groups might occur on the surface of the material, so NH₄⁺ can be easily adsorbed onto BRK. Particularly, the prepared BRK adsorbent in this study had a point of zero charge (pH_PZC) at 9.1 (Fig. 2). These results demonstrate that the surface of BRK has a positive charge when pH solution < pH_PZC, and vice versa, the surface of BRK has a negative charge when pH solution > pH_PZC. At this time, positively charged NH₄⁺ ions were adsorbed on BRK via an ion exchange mechanism because the surface charge of BRK was positive when pH solution < pH_PZC. When pH solution > pH_PZC, the negatively charged BRK can adsorb positively charged NH₄⁺ through an electrostatic attraction mechanism.

On the other hand, with a pH value > 9, the solution was strongly alkaline and more negative due to the existence of the –OH group. Therefore, ion NH₄⁺ preferentially reacts with –OH to produce gas (NH₃). This result is in agreement with prior studies [11, 28].

3.2. Adsorption kinetics

Figure 3 shows the effects of the contact time on the removal capacity of BRK toward NH₄⁺. Adsorption kinetic data showed NH₄⁺ adsorption rate by BRK was rapid within the first 5 mins, then decreased with time, and reached the equilibrium state after 30 mins. Approximately 35, 43, and 50% NH₄⁺ from the solution was absorbed onto BRK at the early stage of 1, 5, and 10 mins, which indicated that BRK had a strong affinity toward NH₄⁺ ion. Obviously, within the first contact time of 15 mins, the adsorption performance of BRK increased significantly. This can be explained by the fact that at the initial stage, a large amount of vacant sites on the adsorbent’s surface that are available and not yet occupied readily interact with NH₄⁺. Additionally, the active functional groups on BRK’s surface were available and readily for bonding with NH₄⁺ ions.

After 30 mins, the adsorption capacity reached equilibrium, with their removal efficiency reaching 53%. In this period, the BRK’s active site had already started to connect to and bond with NH₄⁺. As a result, there was a reduced space for NH₄⁺ to be adsorbed, which led to the amount of NH₄⁺ adsorbed by BRK reducing and BRK’s adsorption capacity decreasing. Therefore, the adsorption process approached saturation gradually. The adsorption capacity remained almost constant after 60 mins of adsorption when compared to the first 30 mins. The adsorption capacity and removal efficiency of BRK at 24 h was 5.34 mg/g and 54%, respectively. The adsorption capacity did not fluctuate significantly or was virtually unchanged. This is due to the fact that almost all BRK’s active site was occupied during this time.

The experimental data were fitted using the Pseudo-First Order (PFO), Pseudo-Second Order (PSO), and Elovich models (Fig. 3). Kinetic adsorption data calculated from three models for NH₄⁺ is given in Table 1. The value of Chi-squared (χ²) and the coefficient of determination (R²) for PFO, PSO, and Elovich model of BRK material were 0.13–0.95, 0.038–0.99, and 0.12–0.95, respectively. Adsorption by BRK was better described by the PSO model (with higher R² and lower χ²) than that for the two other models. While the adsorption rate of the PFO model normally depends on the diffusion of adsorbates on the adsorbent’s surface, the adsorption rate by PSO is controlled by the interaction of adsorption sites with the adsorbate. On the other hand, in the PFO adsorption kinetic model, the number of uncopied adsorption active sites on the adsorbent’s surface does not control the adsorption rate. In contrast, the PSO model was related to the uncopied active site in the adsorbent. It is considered a chemical interaction between the adsorbent and the adsorbate. This result suggests that NH₄⁺ adsorption may be controlled by the chemisorption mechanism (ion exchange, electrostatic attraction, and complexation), which is primarily responsible for managing the contaminant's adsorption [29]. Alshameri et al. [30] investigated the adsorption test of natural clay materials toward NH₄⁺; they stated that the adsorption process of NH₄⁺ onto clay materials was presented by PSO kinetics. The NH₄⁺ adsorption process can be explained by the following steps: (1) moving NH₄⁺ from the liquid phase to the liquid-solid interface; (2) transferring the solid phase on the BRK’s surface; and (3) then diffusing into the BRK’s pores.

Moreover, the experimentally derived adsorption capacities (q_{t,exp =} 5.336 mg/g ) were nearly close to the computed data (q_t,cal) from the PSO model (Table 1) (q_e = 5.31 mg/g). The adsorption capacity obtained from the PFO kinetic model (q_e = 5.17 mg/g) was lower than that of experiment adsorption capacity.

Table 1

Kinetic parameters for NH₄⁺ adsorption by BRK
Models	Unit	BRK
q_exp	mg/g	5.34
Pseudo-first order
q_e	mg/g	5.17
k₁	1/min	1.06
R²		0.95
ꭓ²		0.13
Pseudo-second order
qe	mg/g	5.31
K₂	g/ mg × min	0.29
R²		0.99
ꭓ²		0.038
Elovich model
α	mg/(g×min)	4.97 x 10^6
β	g/mg	4.21
R²		0.95
ꭓ²		0.12

3.3. Adsorption isotherms

The effect of initial NH₄⁺ concentrations (C_o = 5–300 mg/L) on the adsorption capacity by BRK were evaluated at various temperatures through adsorption isotherms (Fig. 4). The results showed that temperature was a significant factor in the NH₄⁺ adsorption process. The NH₄⁺ absorption capability of BRK reduced as the temperature rose from 10°C to 50°C. In other words, the NH₄⁺ removal by BRK reached its highest at 10 ℃, and lowest at 50 ℃. These results also suggested that the NH₄⁺ adsorption process on BRK adsorbent was exothermic and the low-temperature environment is favorable. The observed trend of temperature influence was in agreement with previous findings on activated carbon made from coconut shells [31], corncobs [11], and modified bentonite [28]. The NH₄⁺ adsorption process onto modified bentonite and corncob waste-derived activated carbon constituted an exothermic reaction [11, 28]. In addition, many others have documented comparable isotherm shape observations [28], when studying NH₄⁺ adsorption by materials derived from agriculture wastes or bentonite materials and their modified forms.

The isothermal model fitting for NH₄⁺ adsorption onto BRK is provided in Fig. 4. In this study, the adsorption process and adsorption behavior of NH₄⁺ on BRK were adequately described by the fit models, which comprised the Langmuir and Freundlich. The isotherm model fit data was calculated and presented in Table 2. The results show that the Langmuir isotherm model was the best match for the experimental data of BRK at all three investigated temperatures (10, 30, and 50 ℃). This is due to R² and χ2 values at 10, 30, and 50 ℃ of the Langmuir model, which were larger and smaller, respectively, when compared to those of the Freundlich model. The NH₄⁺ maximum adsorption of BRK obtained from the Langmuir model were 22.51, 20.57, and 16.22 mg/g at 10, 30, and 50 ℃, respectively. Therefore, when the temperature increases to 50 ℃, the capability to absorb NH₄⁺ decreases approximately 28%, indicating that the interaction between BRK and NH₄⁺ becomes weaker following the increase in temperature. In addition, the results of the experiments demonstrated a good agreement with the Langmuir model, suggesting that the adsorption behavior of NH₄⁺ on the BRK was mainly determined by the formation of the adsorption monolayer. The adsorption process of NH₄⁺ on six natural clay materials (NCM) are also monolayer due to the isotherm adsorption data obtained which were coincident with the Langmuir model [30].

Table 2

Parameters of the adsorption isotherm onto BRK material
Model	Unit	Temperatures
Model	Unit	10 ℃	30 ℃	50 ^℃
Langmuir model
Q_max	mg/g	22.51	20.57	16.22
K_L	L/mg	0.037	0.04	0.054
R²		0.99	0.98	0.99
X²		0.47	0.83	0.32
Freundlich model
K_F	(mg/g)/(mg/L)ⁿ	3.15	3.07	2.94
n_F		0.36	0.35	0.32
R²		0.93	0.91	0.90
X²		3.11	3.63	2.63

Table 3 presents a comparison between the Langmuir maximum adsorption capacity of BRK and some other materials studied in the literature on removal of NH₄⁺. As, expected, BRK adsorbents (Q_max = 20.57) demonstrated a higher adsorption capacity of NH₄⁺ than other materials such as the pristine biochar derived from maple wood (5.44 mg/g) [32], pine sawdust (5.38 mg/g) [33], the natural bentonite from Algeria (19.01 mg/g) [5], bentonite from Indonesia (12.37 mg/g) [34], the modified biochar of waste spruce sawdust (17.96 mg/g) [35], of corncob (17.03 mg/g) [11], the modified bentonite (5.66 mg/g) [28], and the composite of bentonite and hydrochar (23.67 mg/g) [34].

Table 3

The Langmuir maximum adsorption capacity of some investigated adsorbents toward NH₄⁺
Adsorbent	Initial concentrations	pH	m/V	Q_max	References
BRK	5-300	7	2	20.57	This study
Maple wood biochar	0–100	-	10	5.44	[32]
Pine sawdust biochar	0–100	6	3	5.38	[33]
Bentonite from Algeria	10–10000	7	5	19.	[5]
Bentonite from Indonesia	200	6	0.1–10	12.37	[34]
Modified Biochar	10–110	7	2	17.96	[35]
Corncob activated carbon	10–105	7	2	17.03	[11]
Chemically Activated Biochar	50–600	8.4	4	14.34	[36]
Modified bentonite	0–200	5–9	–	5.66	[28]
Hydrochar from Koi fish	200	6	0.1–10	12.37	[34]
Bentonite hydrochar composite	200	6	0.1–10	23.67	[34]

3.4. Effects of existing ions

In this study, four commonly encountered cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) have been selected to examine the (inhibitory) influence of these ions on the NH₄⁺ removal process by BRK (Fig. 5). These cations were tested at three initial concentrations of 10, 50, and 100 mg/L. The results showed that the removal efficiencies of NH₄⁺ had significantly decreased due to the strong competition between cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) and NH₄⁺ ions in aqueous solution. Generally, when the concentration of co-existence cation was at 100 mg/L, the removal capacity of BRK toward NH₄⁺ decreased in the order of no competitor > K⁺ > Na⁺ > Mg²⁺ > Ca²⁺. Similar trends were found in other studies [11, 35, 37].

Among investigated co-existing cations, divalent cations (Ca²⁺ and Mg²⁺) were found to possess more sway than monovalent cations (Na⁺ and K⁺) (Fig. 5). Because divalent cations have a divalent charge as opposed to monovalent cations, as a result, they have occupied more adsorption sites. In other words, they were stronger competitors for the adsorption site leading to reducing the removal efficiency of BRK [33, 35]. Furthermore, it was shown that the two major components of water hardness, Ca and Mg, had concentration ranges between 80 and 200 mg/L. In fact, hard water inhibits the NH₄⁺ removal efficiency in wastewater treatment plants. Furthermore, the removal capacity of the adsorbent is observed to be influenced by the concentrations of cations. An increased amount of co-existing cations leads to a lower adsorption capacity (Fig. 5).

3.5. Adsorbent characteristics

Figure 6a displays the scanning electron microscopy (SEM) images of BRK before NH₄⁺ adsorption. The surface of BRK displays a multitude of holes and huge grooves of varying widths, indicating the composite of bentonite and biochar derived from rice husk materials had a porous and asymmetric surface structure. With its composition containing biochar materials, BRK had a rich porous structure. The SEM images of BRK showed that the activation mediated by KOH may promote BRK pore development, leading to the adsorption capacity of materials. This result is similar to those reported by others [38, 39]. In addition, via the SEM image, it can be concluded that the BRK not only contains biochar adsorbent but also has bentonite in its composition. The presence of bentonite clay can be confirmed through the overlaid layer or plate-like appearance of bentonite (Fig. 6a). Et-Tayea et al. [40] stated that the surface microscopic morphology of bentonite features aggregates of spherical, heterogeneous-sized bentonite grains with an impressive compact structure, as well as layered, overlapping layers. Or Ashiq et al. [17] prepared the composite of biochar based bentonite (MSW-BC) for removing antibiotic ciprofloxacin. The MSW-BC surface shares many similarities with the BRK surface. Its SEM image is characterized by a significant amount of pores, which constitute a property of biochar, and a plate-like layer, known to be a feature of bentonite. These results suggest that the BRK was successfully prepared.

Figure 6a also indicates the EDX results for composite of bentonite and biochar derived from rice husk (BRK) treated by KOH solution. Obviously, the BRK’s composition possesses C, O, Si, K, Ca, Fe. The presence of a large amount of K could be explained for the successful activation by KOH solution.

Figure 6b shows that there are no visible changes in the surface morphology of BRK after NH₄⁺ adsorption. The NH₄⁺-laden BRK still exhibited a lot of pore and had a porous structure. There is one significant change in the weight percent of metal elements (K, Ca, Fe) in the NH₄⁺-laden BRK. EDX data indicated that the weight percent of K, Ca, Fe decreased from 6,43–0.36%; 1,39% to 1,06%; 2,22 5 to 1.04%, respectively. Among them, the K element indicated a remarkably decreased rate than two other elements. This result is in line with the reports of Alshameri et al. [30] and Yu et al. [41]. This result suggests that it could be due to ion exchange of these elements with NH₄⁺ or called ion exchange mechanism.

The FTIR spectra provide valuable information on the surface chemistry of BRK both before and after adsorption (Fig. 7). It can be seen that BRK possesses a lot of common functional groups that are found in other biochar adsorbents, such as OH, C = O, C = C, and CO [39]. They are pieces of evidence for confirming the presence of the biochar adsorbent in its component of BRK. The stretching vibration of –OH is specifically responsible for a noticeable broad peak located at 3425 cm^− 1 on the surface of the BRK adsorbents before and after NH₄⁺ adsorption. The peak at about 1622 cm^− 1 was identified as the stretching vibration of C = O in carboxyl groups or aliphatic ketone, whereas the bands at around 2372 cm^− 1 were associated with the stretching vibrations of C = C and C-H [38, 42]. In addition, a small peak at about 1031 cm^− 1 observed in BRK pristine material was featured as the stretching vibration of Si-O, which is commonly found in bentonite clay materials [23, 43]. All things considered, BRK was rich in functional surface groups which provided the basis for removing NH₄⁺ ions from aqueous solution. The infrared spectrum also revealed that the distinctive peaks at –OH had clearly shifted following adsorption, which was caused by the complexation of NH₄⁺ with oxygen-containing functional groups [44].

The surface properties information of BRK obtained by the nitrogen adsorption/desorption isotherm technique is provided in Table 4. It can be seen from Table 4, that the specific surface areas of BRK pristine and NH₄⁺-laden BRK were 4.89 and 17.72 m2/g, respectively. Obviously, after NH₄⁺ adsorption, the specific surface area of BRK was gently increased. In addition, the pore volume of BRK was increased from 0.009 cm3/g to 0.024 cm3/g after the adsorption process. These particular results suggest that NH₄⁺ adsorbed on BRK via a pore-filling mechanism might be negligible.

Table 4

The surface properties information of BRK
Surface properties	BRK pristine	NH₄⁺-laden BRK
BET specific surface area (S_BET - m²/g)	4.89	17.72
Pore volume cm³/g	0.009	0.024
Pore radius (nm)	1.92	1.72

3.6. Possible adsorption mechanisms

The previous mechanism studies found that the most predominant adsorption mechanisms in removal of NH₄⁺ by the clay materials and biochar adsorbents were ion exchange [11, 30], electrostatic attraction [45], pore-filling mechanism, and surface complexation. Through a series of batch experiments conducted at a pH of 7.0 and characterization of pristine and NH₄⁺ laden BRK, the potential NH₄⁺ adsorption mechanism on BRK was identified. Firstly, the pristine BRK had a tiny specific surface area (4.89 m²/g). However, after the adsorption process, the BRK loaded with NH₄⁺ had a greater S_BET (17.72 m²/g) than the pristine BRK. The findings suggested that the pore-filling mechanism may not be crucial to the adsorption process. Moreover, according to [41], there was no relationship between the NH₄⁺ adsorption capacity and the specific surface area.

As mentioned in Section 3.1, with its pH_PZC at 9.1, BRK possessed a positive charge when pH solution of 7.0, thus positively charged NH₄⁺ ions were exchanged with positively charged BRK through an ion exchange mechanism. In other words, the ion exchange is of great significance in removing NH₄⁺ from the water environment. The ion exchange mechanism is also supported by the results of SEM/EDX that the percentage of K elements in pristine BRK decreased significantly (from 6.43–0.36%) after adsorption process, possibly due to ion exchange of K⁺ with NH₄⁺ ions in solution [30, 41]. Additionally, it was discovered by the FTIR analysis above that NH₄⁺ complexed with oxygen-containing functional groups on the surface of BRK, such as –OH, during the adsorption process.

In this study, a bentonite biochar composite (BRK) from natural bentonite and rice husk was successfully synthesized at a pyrolysis temperature of 400°C and then treated with KOH. The batch experimental data showed that the adsorption capacity of BRK for NH₄⁺ significantly depended on the pH solution (2.0–12.0) and co-existing cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺). The PSO model provided a better description of the kinetic adsorption of NH₄⁺ on BRK than the PFO and Elovich models. BRK outperformed other adsorbents in the literature with a maximum Langmuir adsorption capacity of 20.57 mg NH₄⁺/g at 30 ℃. The characterization of the adsorbent and batch investigations showed that the ion exchange was found to be the primary mechanism for the removal of NH₄⁺ by BRK. BRK is an effective and cost-effective material for the treatment of water contaminated with NH₄⁺. Column and pilot field experiments using real water should be conducted for feasible assessment for the practical application of BRK.

Acknowledgments: This study was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (grant number 105.99-2019.311).

Author contributions

Conceptualization: N.T.H., T.H.-M., N.T.H.H.; Methodology: N.T.H., T.H.-M., N.T.H.H., D.T.H., T.T.H., N.V.D., L.V.D.; Experiment: N.T.H., D.T.H., T.T.H., B.V.D.; Analysis: L.V.D., D.T.H., T.T.H.; Writing original draft: N.T.H.; Writing review and editing: T.H.-M., N.T.H.H., D.T.H., L.V.D., T.T.H., B.V.D.

Funding This study was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (grant number 105.99-2019.311).

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent to publish All the authors have agreed to publish this article.

Competing interests The authors declare no competing interests.

Availability All the data generated or analyzed during this study are available upon request.

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Removal of ammonium from water by a bentonite biochar composite

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Materials, chemicals, and reagents

2.2. Synthesis of bentonite-biochar composite materials

2.3. Adsorption experiment

2.4. Adsorbent properties and sample analysis

2.5. Data analysis

3. Results and discussion

3.1. Effects of pH solution

3.2. Adsorption kinetics

3.3. Adsorption isotherms

3.4. Effects of existing ions

3.5. Adsorbent characteristics

3.6. Possible adsorption mechanisms

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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