Improved adsorption capacity of ammonium from aqueous solution by modified biogas residue biochar

Abstract

Residue biochar can be utilized as an adsorbent for ammonium nitrogen (NH₄⁺-N) to prevent non-point source pollution. However, the limited adsorption capacity has restricted its extensive application. In this study, biochar was modified with hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), and sodium hydroxide (NaOH) to enhance its adsorption performance. A comparative analysis of the biochar surface characteristics was used to investigate the adsorption systems. The results indicated that the adsorption capacities of the modified biochar (MB) were significantly enhanced compared with the raw biochar (RB). At the highest NH₄⁺-N concentration of 150 mg L⁻¹, the adsorption capacities of RB-H₂O₂, RB-NaOH, and RB-KMnO₄ increased to 3.0, 3.2, and 4.0 times that of RB, respectively. As predicted by the Langmuir isotherm model, the maximum adsorption capacities of these three MB were 13.93, 41.00, and 68.15 mg g⁻¹, respectively. Ammonium adsorption on the MB surfaces was affected by surface adsorption, liquid membrane diffusion, and intra-particle diffusion. The specific surface area and pore volume of RB-KMnO₄ were significantly enhanced, with an increase in active sites on the pore surfaces, thereby strengthening its adsorption capacity for NH₄⁺-N. In contrast, the adsorption of NH₄⁺-N by RB-H₂O₂ and RB-NaOH primarily relied on the substantial increase in –C–O functional groups, with additional contributions from other oxygen-containing functional (e.g. –OH, –COOH, and Fe–O). In conclusion, RB-KMnO₄ exhibited the highest adsorption efficiency, with pore-based adsorption playing a dominant role over functional group-based adsorption. These findings highlight the critical role of pore structure optimization in enhancing the biochar adsorption capacity for NH₄⁺-N.

Highlights

The biogas residue biochar was modified using H₂O₂, KMnO₄ and NaOH.

RB-KMnO₄ created a superior ammonium adsorption capacity because of its exceptional pore structure.

RB-H₂O₂ and RB-NaOH showed an enhanced adsorption performance attributed to its increased surface –C–O functional groups.

Enhancing the pore structure of biochar more effectively boosted ammonium ion adsorption capacity.

Introduction

Biochar, a carbon-rich, porous material, is derived from organic feedstock through a thermal combustion process involving limited oxygen at temperatures ranging from 200 to 700 ℃ (Gsior and Tic 2016). Various organic wastes such as agricultural and biogas residues can serve as feedstocks for biochar production. Biochar not only has exceptional porosity, large specific surface area (SSA), high cation-exchange capacity, and low electrical conductivity (EC), but also has robust adsorption capability, which can adsorb different forms of nitrogen and reduce nitrogen loss in farmland. This provides feasibility for solving the problem of non-point source pollution caused by excessive nitrogen in farmland (Li et al. 2018; Kaur et al. 2021).

Ammonia nitrogen (NH₄⁺-N) is one of the main indicators of agricultural non-point source pollution, which mainly comes from fertilizer application, livestock and poultry breeding emissions, and farmland drainage in agricultural activities (Feng et al. 2023a, b; Li et al. 2023). It is necessary to take measures to adsorb NH₄⁺-N, which can maintain soil acid–base balance, protect farmland water environment, ensure crop safety and human health (Raza et al. 2020; Zhu et al. 2024). Therefore, developing a novel efficient biochar material for adsorbing NH₄⁺-N has become the focus of our work. However, previous studies showed that the adsorption capacity of raw biochar (RB) for NH₄⁺-N was only 3–7 mg g⁻¹ (Zheng et al. 2020; Hailegnaw et al. 2019). The adsorption limit of the 0–10 cm soil layer is 1.11 mg g⁻¹, which is much lower than our fertilization amount (Wang et al. 2022). Even with the application of biochar, only 9–20% of the NH₄⁺-N can be adsorbed, and there is still a risk of NH₄⁺-N migrating to groundwater (Liu et al. 2019; Wang et al. 2022). Therefore, it is urgent to take effective measures to increase the load amount of NH₄⁺-N on biochar.

The reason why biochar, a carbon-based material, has good adsorption capacity is due to its inherent characteristics such as high porosity, large specific surface regions, and rich surface functional groups (Duygan et al. 2021; Rashid et al. 2021). The types of raw materials used in the preparation of biochar, the conditions for biochar preparation, and the methods of biochar modification are all important factors that affect the expression of these characteristics (Tan et al. 2023; Wang et al. 2023). It was found that biochar prepared at 60 min and 5.0 °C min⁻¹ under 700 °C presented the best development of smaller microporous and highest Brunauer–Emmett–Teller surface area of biochar in our previous studies (Cong et al. 2022; Zheng et al. 2020). However, the effectiveness of improving biochar characteristics by controlling preparation conditions is limited. Therefore, the focus has been shifted to the exploration of improving the loading capacity of NH₄⁺-N through biochar modification. Previous studies have found that modified biochar (MB) can greatly improve its adsorption capacity, among which acid, alkali, and metal ion treatments are modification methods that can increase adsorption capacity (Shi et al. 2013; Qin 2017; Shang 2019; Sun 2020). Normally, a higher pH value is beneficial for the adsorption of NH₄⁺-N, as NH₄⁺ is more easily dissociated into ammonia (NH₃), which is more easily adsorbed (Jiang et al. 2024). In contrast, strong acid treatment may lead to the destruction of some active sites, while metal ion amendments, although able to introduce specific adsorption sites, may affect the porous structure of biochar itself, thereby reducing its overall adsorption capacity (Liu et al. 2024a). Therefore, in this case, more consideration is given to using strong bases as modifiers. The advantages of strong oxidants as modifiers are more reflected in their ability to significantly alter the surface chemical properties of biochar. Some strong oxidants can introduce more oxygen-containing functional groups on the surface of biochar, such as carboxyl (–COOH), hydroxyl (–OH), etc. These functional groups can increase the negative charge density of the biochar surface, thereby enhancing the electrostatic attraction to cations such as NH₄⁺-N (Liu et al. 2024b). Numerous acidic functional groups (e.g., carboxyl groups, phenolic hydroxyl groups, hydrocarbon bonds, and hydroxyl and ethylenic bonds) have been reported to enhance the adsorption capacity of biochar derived from crop straw and oak sawdust (Wang et al. 2015; Liang et al. 2016). The improvement in acidic oxygen-containing functional groups and the formation of novel substituents, cationic exchange, and surface complexation of oxygen-containing functional groups, including carbohydrate C–OH and carboxylic C = O (C–O), significantly contributed to the adsorption of NH₄⁺-N in the modified biochar (Cao and Harris 2010; Hou et al. 2016; Yu et al. 2016; Zhang et al. 2020; Chen et al. 2021). Through oxidation, more micropores and mesopores can be formed on the surface of biochar, increasing the specific surface area and providing more adsorption sites. In addition, strong alkali treatment can further optimize pore structure and improve adsorption performance by removing some ash and minerals (Chen et al. 2022).

Obviously, the adsorption of ammonium onto biochar is mainly influenced by its surface characteristics and pore structure. From the perspective of adsorbing NH₄⁺-N, strong bases and strong oxidants have better applicability. However, it is not clear whether these modifiers can significantly increase the adsorption capacity of biochar for NH₄⁺-N, and the adsorption mechanisms of different modifiers are also unclear. In this study, biogas residue (BR) was still used as the raw material for preparing biochar based on previous experimental results. The carbonization utilization of biogas residue waste is of great significance for the development of biogas engineering, ecological environment construction, and the development of green and low-carbon agriculture (Chen et al. 2021; Ragauskas et al. 2006; Stefaniuk et al. 2016). Hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), and sodium hydroxide (NaOH) were selected as modifiers to modify biochar from BR. We assume that these modifiers alter the pore structure and surface functional groups of the original biochar, resulting in a significant increase in the adsorption performance of biochar, manifested in a sharp improvement in the adsorption quantity of NH₄⁺-N. Thus, this study aims to (i) prepare and characterize modified biochar for NH₄⁺-N adsorption, (ii) investigate the kinetics and isotherms of NH₄⁺-N adsorption by RB-H₂O₂, RB-KMnO₄, and RB-NaOH, and (iii) elucidate the mechanisms underlying NH₄⁺-N adsorption by MB.

Materials and methods

Materials

The precursor substance employed in the biochar synthesis was obtained from an Anaerobic Digestion Residue (ADR), a byproduct of the aerobic digestion process of distillery grains (DG). This process took place in a 500 m³ biogas plant situated in Pingdu town, Qingdao, China (36° 28′ N, 119° 35′ E). Comprehensive information concerning the production and characteristics of the ADR-DG has been thoroughly discussed in previous studies (Zheng et al. 2018a, b). The processing method and characteristics of BR before preparing it into biochar are detailed in the supplementary materials.

The residue biochar was prepared based on parameters such as the maximum adsorption capacity for NH₄⁺, the optimal residence time (RT) during pyrolysis, and the heating rate (HR), as reported in our previous study (Zheng et al. 2021). The detailed preparation procedure is provided in the Supplementary Materials. The prepared RB was used for subsequent modification experiments, and its fundamental physicochemical properties are presented in Table 1.

Table 1. The basic characteristic of unmodified and modified biochars

	RB	RB-H2O2	RB-KMnO4	RB-NaOH
C%	59.61 ± 1.24c	60.78 ± 0.45b	79.51 ± 0.62a	58.93 ± 0.61c
H%	2.54 ± 0.39a	1.65 ± 0.02b	2.65 ± 0.29a	2.8 ± 0.08a
N%	1.24 ± 0.03c	1.42 ± 0.03b	1.89 ± 0.03a	1.25 ± 0.02c
S%	1.83 ± 0.15a	0.55 ± 0.02d	0.92 ± 0.14c	1.15 ± 0.06b
C/N	50.28	43.98	43.21	49.29
C/H	27.38	32.87	29.85	22.12
pH	10.95 ± 0.01a	9.79 ± 0.02c	10.60 ± 0.01b	8.46 ± 0.03d
ash%	0.48 ± 0.02a	0.47 ± 0.05a	0.53 ± 0.02a	0.15 ± 0.06b
pHpzc	6.97	6.98	8.26	6.46

Biochar modification

Hydrogen peroxide modification

The 2-h dispersion of 3 g of biochar in 20 mL of 15% H₂O₂ solution was conducted at room temperature, followed by three rinses with deionized water. After each rinse, the biochar was filtered using a 0.45 μm membrane, and was dried at 80 ℃.

NaOH modification

A 10 g sample of biochar was accurately weighed and placed in a Teflon beaker. Subsequently, 20 mL of 5 mol L⁻¹ NaOH solution was added (Teli et al. 2024). The mixture was heated at 70 ℃ for a duration of 4 h. The first drying of material was made in a Muffle furnace at 110 ℃, followed by 2-h heating at 600 °C under N₂ gas (10 psi = 68.95 kPa). The biochar was subjected to multiple rounds of rinsing with distilled water, resulting in the production of modified biochars. The modification process was considered complete when the pH of the rinse solution was approximately 7. After each rinse, the biochar was filtered with a 0.45 nm filter membrane, and dried at 70 ℃.

KMnO₄ modification

20 g of biochar was dispersed into 100 mL of 0.1 mol L⁻¹ KMnO₄ solution over 2 h. The solution was sonicated, and then dried for 24 h at 80 ℃. The material was placed in a Muffle furnace continuously filled with nitrogen to ensure an anaerobic environment. The temperature of the muffle furnace was adjusted to 600 ℃ for pyrolysis for 1 h. The resulting modified material was thoroughly rinsed three times with deionized water. After each rinse, the material was filtered through a 0.45 nm filter membrane and then dried at 80 ℃.

Characterization of biochar

A Hitachi (SU-70, Japan) microscope was used to capture scanning electron microscope (SEM) images (5000 × magnification). The specific surface areas (SSA) and pore structures of the samples were evaluated using the Brunauer–Emmett–Teller (BET) method. Nitrogen (N₂) adsorption measurements were performed at 77 K using an ASAP 2020 M + C instrument (Micromeritics, USA). Each sample, weighing between 0.07 and 0.08 g, was initially subjected to vacuum degassing with a heating rate of 10 ℃ min⁻¹ up to 200 ℃, for a duration of 4 h prior to the measurements. The Barrett-Joyner-Halenda (BJH) approach was then used for the adsorption data to determine the pore diameter, overall pore volume (Vt), ore volume (Vmic), and mesopore volume (Vmes).

Additionally, the surface functional groups were characterized using Fourier-transform infrared spectroscopy (FTIR). The FTIR analysis was performed on a Nicolet 6700 (Thermo Scientific, USA) within the wavenumber range of 400–4000 cm⁻¹ using the KBr pellet technique. About 1.0 wt% of each sample was mixed with KBr to obtain discernible spectra. The omnic spectral analysis with OriginPro2021 (OriginLab, USA) was used to describe the significant spectral features.

The thermogravimetric (TG) and derivative thermogravimetric (DTG) experiments were performed to assess the thermal characteristics of the BR sample. These explorations were conducted non-isothermally using a TAG analyzer (Rubotherm Dyntherm HP, Netherlands) within temperature range of 25–800 ℃. Approximately 50 mg of the BR sample was utilized for the analysis under a nitrogen flow of 50 cm min⁻¹ and a heating rate of 5 ℃ min⁻¹. The TG and DTG curves were then plotted as functions of temperature. Furthermore, quantitative analysis of organic elements including carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) was performed with an element analyzer (EA, Elementar Vario EL III, Germany).

Ammonium nitrogen adsorption

The batch adsorption tests were performed following a specific procedure. Specifically, 3.819 g of ammonium chloride (NH₄Cl, certified A.C. S, Fisher Scientific) was dissolved in 1000 mL of deionized water to prepare a NH₄⁺ stock solution of 1000 mg N L⁻¹. The NH₄⁺ stock solution was diluted into 5, 10, 20, 50, 100 and 150 mg L⁻¹ respectively with deionized water. Then, 40 mL of the above solution was transferred into a 50 mL centrifuge tube, and its pH was adjusted to 7.0. Then, 0.4 g of RB, RB-H₂O₂, RB-KMnO₄, RB-NaOH had been added into the centrifuge tube to initiate the adsorption process. The mixed solution was shaken at 25 ℃ (140 rpm) for 120 h, centrifuged at 4000 rpm for 10 min, and passed through a 0.45 μm filter membrane.

The concentration of NH₄⁺ in the filtrate was determined by Nessler's reagent spectrophotometry using an UV–VIS spectrophotometer (Evolution 220, Thermo Scientific, US) with an incident wavelength of 420 nm. The adsorptive capacity for NH₄⁺-N was computed utilizing Eq. (1):

1

$$Q_e=\left(C_1{C}_2\right)\times V/m$$where Q_e is the adsorption capacity (mg g⁻¹), C₁ is the initial concentration of NH₄⁺-N (mg L⁻¹), C₂ is the consistency of NH₄⁺-N after adsorption (mg L⁻¹), V is the volume of the solution (L), and m is the mass of the adsorbent material (g).

Adsorption kinetic research

Adsorption samples were collected at 3, 30, 60, 90, 120, 240, 480, 960, and 1440 min. The adsorption kinetics were evaluated using the pseudo-second- and first-orders, and intra-particle diffusion models, as shown in Eqs. (2), (3), (4), and (5).

Pseudo-first-order, nonlinear equation:

2

$$Q_t=Q_e\times \left(1-e^{-k_{1}t}\right)$$where Q_t is the adsorption quantity at a certain time (mg g⁻¹), Q_e is the equilibrium adsorption capacity (mg g⁻¹), t is the adsorption time (min), and k₁ is the pseudo-first-order adsorption rate constant (min⁻¹).

Pseudo-second-order, nonlinear equation:

3

$$Q_t=\frac{Q_e^2{k}_{2}t}{1+Q_e{k}_{2}t}$$where k₂ (g m g⁻¹ min⁻¹) represents the pseudo-second-order rate constant.

Elovich:

4

$$Q_t=\frac{1}{\beta }ln\left(1+\alpha \beta t\right)$$where α is the primary rate constant (mg g⁻¹ min⁻¹) and β is the desorption constant (mg g⁻¹).

Intra-particle diffusion:

5

$$Q_t=k_p{t}^{1/2}+C$$where k_p is the rate constant relevant to the intra-particle diffusion model (mg g⁻¹ min^−1/2). The constant C is associated with the thickness of the boundary layer (mg g⁻¹); a higher value of C implies a more pronounced influence on the limiting boundary layer.

Adsorption isotherm research

The adsorption isotherms were subsequently analyzed using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models, as shown in Eqs. (6), (7), (8), (9), (10), and (11).

Langmuir:

6

$$Q_e=\frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}$$where K_L is the Langmuir adsorption isotherm constant (L mg⁻¹), Q_m is the maximum monolayer adsorption capacity (mg g⁻¹), Q_e (mg g⁻¹) is the equilibrium adsorption capacity, and C_e is the equilibrium ammonium concentration (mg L⁻¹).

Freundlichadsoption isotherm:

7

$$Q_e=K_F{C}_e^{1/n}$$where K_F is the Freundlich adsorption capacity (mg^(1−1/n) L^−1/n g⁻¹). The Freundlich equation constant, 1/n (dimensionless), also referred to as the Freundlich adsorption intensity, signifies the reaction intensity between the ammonium molecules and adsorbent, which varies with the nonuniformity of the adsorbent. The value of 1/n is within the range of 0–1. A value within the range of 0.1 to 0.5 implies a higher adsorption intensity, suggesting favorable adsorption conditions.

In the context of the Temkin model, indirect mutual effects among adsorbates lead to a linear decline in the enthalpy of adsorption as a function of surface coverage. Consequently, the enthalpy of adsorption decreased linearly with the quantity of adsorbed species.

8

$$Q_e=\frac{\mathit{RT}}{b}ln\left(K_T{C}_e\right)$$

In this equation, b means a constant associated with the enthalpy of adsorption (J mol⁻¹), b < 4.2 kJ mol⁻¹. The equilibrium constant, K_T (L mg⁻¹), signifies the maximum binding energy in the context of material analysis and detection.

Dubinin-Radushkevich isotherm, adsorption by uniformed pores:

9

$$Q_e=Q_m{e}^{-K_{\mathit{DR}}{\epsilon }^2}$$

10

$$\epsilon =RTln\left(1+\frac{1}{C_e}\right)$$

11

$$E=\frac{1}{\sqrt{2K_{\mathit{DR}}}}$$where Q_m is the maximum adsorption capacity of ions (mg g⁻¹); K_DR is the Dubinin-Radushkevich (D-R) constant (mol² kJ⁻²) related to the energy of adsorption. The Polanyi potential is symbolized by ε. The average free energy of adsorption is estimated by E (kJ mol⁻¹). It is noteworthy that when 1 < E < 16 kJ mol⁻¹, physical adsorption is the leading function, whereas when E > 16 kJ mol⁻¹, chemisorption predominates. The universal gas constant is represented by R, with a value of 8.314 kJ mol⁻¹. T refers to the standard temperature, set at 298.15 K.

Furthermore, the factors influencing ammonium-nitrogen adsorption, including solution pH, competitive ions, and initial NH₄⁺-N concentration, were also characterized and are detailed in the Supplementary Materials.

Statistical analysis

The collected data were subjected to one-way analysis of variance (ANOVA), followed by a post-hoc Duncan’s multiple range test using SPSS Statistics software, version 25. The threshold for statistical significance was P < 0.05. All graphical representations were created using OriginPro2021 (OriginLab, USA).

Results and discussion

Characterization of biochar

SEM

Figure 1 shows the SEM images of the morphological texture on the surface of RB and modified biochar (RB-H₂O₂, RB-NaOH, and RB-KMnO₄). Modifications with H₂O₂, NaOH, and KMnO₄ significantly altered the surface characteristics of the RB. The RB exhibited a honeycomb-like porous structure, with most of the pores being regularly shaped (Fig. 1a). After modification, the biochar morphology became notably rougher with porous structures displaying diverse shapes and sizes. In the RB-H₂O₂ sample, the rectangular pores were interconnected by adjacent pores (Fig. 1b), which could be attributed to the volatilization and release of organic compounds (Pariyar et al. 2020). Successful biochar loading was observed in the form of attachments on the surfaces of RB-NaOH and RB-KMnO₄ (Zhang et al. 2020). In particular, the pores of RB-KMnO₄ became more abundant and deeper, and the surface exhibited a significant collapse, thereby enhancing the roughness of the biochar surface (Fig. 1d).

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Fig. 1

The SEM images of biochars: RB (a), RB-H₂O₂ (b), RB-NaOH (c), RB-KMnO₄ (d). RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Physicochemical properties of biochar

The pH in the MB treatment decreased significantly (Table 1). This implied an increase in the electronegative surface charge of the biochar (Qiu et al. 2008; Tripathy et al. 2021). The enhancement in electronegativity was demonstrated to be beneficial for the adsorption of NH₄⁺-N. Upon modification, the sulfur content of the biochar notably diminished, whereas the carbon and nitrogen contents of the RB-KMnO₄ increased. The C/H ratio and ash content of RB-NaOH decreased, indicating weakening of the aromatic construction of the biochar and an improvement in its hydrophilicity, which facilitated the fixation of NH₄⁺-N (Chen et al. 2021). The observed pH values of the biochar at the zero-charge point (pH_pzc) exceeded the theoretical values, as shown in Table 1. This suggests that the biochar surface possessed a negative charge, which is conducive to the NH₄⁺ ions electrostatic adsorption.

The existence of ordered structures and aromatic carbons in RB, RB-H₂O₂, RB-KMnO₄, and RB-NaOH was confirmed by Raman spectroscopy, as illustrated in Fig. 2. Spectroscopic analysis provided evidence of the structural integrity and chemical composition of these materials, underscoring the presence of aromatic carbon configurations. All the biochars under investigation displayed two primary non-graphitic carbon bands, the G band at 1579 cm⁻¹ and the D band at 1326 cm⁻¹, as depicted in Fig. 2a. The G-band represents the ideal graphite lattice, while the D-band represents the disordered graphite lattice. The presence of these G and D bands suggests the existence of sp2 bonded carbon within the biochar, as reported by Hossain et al. (2018). Notably, the ID/IG ratio of the modified biochar decreased significantly, implying a more structured carbon arrangement (Tripathy et al. 2021).

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Fig. 2

Raman analysis of RB, RB-H₂O₂, RB-NaOH and RB-KMnO₄. Different letters indicate a significant difference (P < 0.05). RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Thermogravimetric analysis was conducted to evaluate the thermal steadiness and weight loss action of biochar under an N₂ atmosphere from 30 to 1000 ℃, as depicted in Fig. 3. Primarily, the evaporation of water involved van der Waals forces of attraction between molecules, and adsorption on the MB surface resulted in the loss of weight up to a temperature of 110 ℃ (Tripathy et al. 2021). Figure 3 indicates that there were three stages of weight loss for the unmodified biochar, whereas only two stages of mass loss were observed in the modified biochar. Thus, the thermal stability of RB-KMnO₄ was significantly improved.

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Fig. 3

Thermogravimetric analysis of RB, RB-H₂O₂, RB-NaOH and RB-KMnO₄. RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

The BET analysis results for RB and MB are presented in Table 2. Notably, the micropore area, BET surface area, and pore size of RB-KMnO₄ were higher than those of MB. All the biochars exhibited microporosity (pores within the scope of 2–16 nm according to Fig. 4a). The mean pore diameter fluctuated within a range of 3.72–4.54 nm. The nitrogen adsorption–desorption isotherms of biochar exhibited characteristics consistent with a Type II isotherm (Fig. 4b), which indicated that a physical adsorption process occurred on nonporous or macroporous solid surfaces, a phenomenon pertinent to the field of soil biophysics. The isotherm displayed a distinct H4 hysteresis loop, implying that the modified biochar had narrow slit-like pores and an incomplete pore network (Gopinath et al. 2021).

Table 2. Pore structure of biochars

Biochar	SBETa (m2 g−1)	SMicrob (m2 g−1)	SExtc (m2 g−1)	VTotd (cm3 g−1)	VMicroe (cm3 g−1)	VMesof (cm3 g−1)	DTotg (nm)
RB	236.80	202.16	34.64	0.1164	0.0769	0.0395	3.72
RB-H2O2	256.68	238.40	18.28	0.1144	0.0897	0.0247	4.02
RB-KMnO4	301.69	266.25	35.45	0.1524	0.1012	0.0512	4.05
RB-NaOH	199.00	180.33	18.67	0.1064	0.0673	0.0391	4.54

^aspecific surface area calculated using the Brunauer-Emmet-Teller (BET) method

^bt-plot micropore area

^ct-plot external surface area

^dsingle point adsorption total pore volume of pores at P/P0 ≈ 0.994

^et-plot micropore volume

^fV_Meso = V_Tot − V_Micro, mesopore volume

^gaverage pore diameter of total pore

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Fig. 4

Pore size distribution (a) and quantity adsorbed (b) of RB, RB-H₂O₂, RB-NaOH and RB-KMnO₄. RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Furthermore, the N₂ adsorption–desorption isotherm of RB-KMnO₄ was substantially higher than those of the other treatments, implying the effectiveness of modification with KMnO₄. However, the N₂ adsorption–desorption isotherm of RB-NaOH was the lowest. One extremely possible reason is that under high concentration sodium hydroxide (> 2 mol L⁻¹) modification conditions, sodium carbonate (Na₂CO₃) crystals are formed on the surface of RB-NaOH, which can hinder the formation of pores or block the original pores (Table 2) (Hu et al. 2018). This implies that when using high concentrations of modifiers for modification, we also need to coordinate appropriate reaction conditions to efficiently exert the effect of the modifier.

FTIR analysis

Characterization of the infrared spectra and investigation of the surface functional groups of the biochars were conducted using FTIR, as depicted in Fig. 5. Upon alteration, an increase in the intensity of the O–H tensile vibration peak was observed at approximately 3400 cm⁻¹. The peak within the scope of 1612–1585 cm⁻¹ was caused by the aromatic carbon structure, specifically the C = C and C = O bonds. Additionally, the out-of-plane bending vibration and stretching of C–H were enhanced by the two distinct bands at 801 and 462 cm⁻¹. The relative content of the C = C functional groups of RB-KMnO₄ was 88.12%, which was higher than those of the other biochars, as shown in Table 3. The peak value of the asymmetric stretching vibration of aromatic C–O–C (C–O stretching vibration of easily degradable carbohydrates such as polysaccharides) at 1085 cm⁻¹ was significantly enhanced, particularly in RB-NaOH. This suggests an increase in the aromaticity of RB-NaOH, which enhances the adsorptive characteristics of the biochar. The relative content of C–O–C functional groups in RB-H₂O₂ is intermediate. Furthermore, the stretching vibration peak of Fe–O at 586 cm⁻¹ in RB-NaOH suggests an increased capacity for ammonium adsorption.

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Fig. 5

The functional groups of RB, RB-H₂O₂, RB-NaOH and RB-KMnO₄ under FTIR spectra. RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Table 3. The relative content functional groups (%)

Functional groups	eV	RB	RB-H2O2	RB-KMnO4	RB-NaOH
C = C(C–C)	284.49	86.57	75.39	88.12	66.99
C–O	285.69	12.95	24.61	11.88	33.01
C = O	286.99	0.48	0	0	0
–COOH	288.89	0	0	0	0

eV binding energy

Adsorption performance

Adsorptioon before and after modification

Figure 6 shows the adsorption capabilities of both unaltered and chemically enhanced biochars for NH₄⁺-N across a spectrum of NH₄⁺-N concentrations. The modified biochar exhibited superior adsorption performance compared to the unmodified biochar in solutions with higher NH₄⁺-N concentrations (> 20 mg L⁻¹), especially for RB-KMnO₄. In the 150 mg L⁻¹ NH₄⁺-N solution, the adsorbance of RB-H₂O₂, RB-KMnO₄, and RB-NaOH increased 2.99, 3.99, and 3.21 times compared with that of RB. This might be caused by the increase in the pore volume and surface area of RB-KMnO₄, which provides more adsorption sites (Table 1). In addition, an increase in the C = C functional group content further increased the ammonium adsorption capacity (Fig. 5).

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Fig. 6

Influence of initial concentration on RB, RB-H₂O₂, RB-NaOH and RB-KMnO₄ adsorption capacity. RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Adsorption kinetics

The kinetic behavior of NH₄⁺-N adsorption onto both the modified and unmodified biochars was investigated, as illustrated by the corresponding fitted curves in Fig. 7. A detailed summary of the fitting results is provided in Table 4. The simulation results indicated that the NH₄⁺-N adsorption process on both biochar types was characterized by an initial rapid increase within the first 1.5 h, followed by a more gradual increase in adsorption capacity until saturation was achieved at approximately 2 h. Notably, the adsorption capacity of RB-KMnO₄ surpassed that of the other chars. The pseudo-first and second-order kinetic models (Fig. 7a, b), as indicated by their lower R² values, were insufficient to accurately characterize the adsorption process of NH₄⁺-N, especially in the case of RB-KMnO₄ (R² < 0.66) and RB-H₂O₂ (R² < 0.88).

[See PDF for image]

Fig. 7

Adsorption kinetics of NH₄⁺-N on RB (a), RB-H₂O₂ (b), RB-NaOH (c) and RB-KMnO₄ (d). Q_t means the adsorption quantity at a certain time (mg g⁻¹). RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Table 4. Adsorption kinetics parameters of modified biochar

Kinetic models	Parameters	RB	RB-H2O2	RB-KMnO4	RB-NaOH
Pseudo-first-order	k1 (min−1)	0.0080	0.0215	0.0279	0.0166
	Qe (mg g−1)	0.7241	1.8440	2.5054	1.9794
	R2	0.9183	0.9010	0.4461	0.7973
Pseudo-second-order	k2 (min−1)	0.0115	0.0183	0.0346	0.0131
	Qe (mg g−1)	0.8140	1.9454	2.5053	2.0894
	R2	0.9339	0.8815	0.6689	0.8804
Elovich	α (mg g−1 min−1)	0.0167	0.5185	5.6783	0.3972
	β (mg g−1)	6.3670	3.8836	3.7587	3.5015
	R2	0.9403	0.8035	0.8629	0.9647
Intra-particle diffusion	Kp1(mg g−1 min−1/2)	0.0207	0.1648	0.1216	0.1108
	C1 (mg g−1)	0.1011	0.0320	0.9180	0.3820
	R2	0.9972	0.8710	0.8057	0.9044
	kP2 (mg g−1 min−1/2)	0.0103	0.0093	0.0113	0.0250
	C2 (mg g−1)	0.3993	1.5607	2.2390	1.2594
	R2	0.9506	0.9521	0.9802	0.9705

The NH₄⁺-N adsorption process by both modified and unmodified biochars can be delineated into two distinct phases, as per the intra-particle diffusion kinetic equation (Fig. 7d). This equation, which exhibited a superior R² value (> 0.95), offered a more accurate depiction of NH₄⁺-N adsorption on modified biochar when juxtaposed with other kinetic models. The fitting parameters R² for the intra-particle diffusion kinetics equation were 0.9972 (RB, first stage), 0.9521 (RB-H₂O₂, second stage), 0.9802 (RB-KMnO₄, second stage), and 0.9705 (RB-NaOH, second stage) (Fig. 7d). These values were significantly higher than those obtained from other equations, thereby underscoring the efficacy of the diffusion kinetics equation within the particles in describing the NH₄⁺-N adsorption process.

In Fig. 7d, the first stage of the intra-particle diffusion model is attributed to rapid attachment of ammonium ions to the outer surface and macropores of biochar after undergoing liquid film diffusion. On the one hand, the concentration of NH₄⁺-N in the solution is higher, which increases the contact probability between ammonium ion and biochar material, providing conditions for the increase of adsorption capacity. On the other hand, this process is also influenced by the adsorption sites of ammonium ions on the particle surface (Hai et al. 2025; Skic et al. 2024). As the process transitioned to the subsequent gradual adsorption phase, the solute concentration decreased, causing an increase in the diffusion resistance within the carbonaceous material. The adsorption sites on the surface of biochar tend to be saturated gradually, and the concentration difference of NH₄⁺-N in the liquid film on the surface of biochar material decreases gradually, resulting in the equilibrium state of adsorption rate and desorption rate. The following was the second stage—slow diffusion of ammonium ions within the micropores of biochar. Thus, the curve exhibits a relatively flat state. The intra-particle diffusion is influenced by the pore structure of biochar. Its pore size distribution and specific surface area both affect the migration rate (Ai et al. 2024; Kang et al. 2024). The above diffusion process well explained the reason why the intraparticle diffusion model did not pass through the origin, which further illustrated that the potential kinetic mechanism of adsorption was a multifaceted process, including liquid film diffusion, surface adsorption, and intra-particle diffusion (Jiang et al. 2019; Wu et al. 2014). This intricate mechanism integrates various aspects of material interactions, demonstrating the multifaceted nature of adsorption kinetics.

The intra particle diffusion model effectively explained the high adsorption capacity of biochar modified with KMnO₄. The higher C₂ (2.2390 mg g⁻¹) of potassium permanganate meant that more NH₄⁺ reached the surface of the particles. Then, the richer, deeper, and rougher pores of RB-KMnO₄ created conditions for its higher intraparticle diffusion rate and adsorption capacity (Fig. 1d and Table 2). Moreover, under the condition of consistent solution flow rate and temperature, the higher pH of RB-KMnO₄ solution also provided advantages for the liquid film diffusion of ammonium ions (Table 1). Because at high pH values, ammonium ions mainly exist in a free state and are more easily adsorbed (Lu et al. 2024). This further illustrates that modified biochar materials exhibit differences in adsorption kinetics during the adsorption process by comprehensively affecting the liquid film diffusion process, surface adsorption process, and intra particle diffusion process.

Adsorption isotherm

The fitting curve of the adsorption isotherm of NH₄⁺-N on the modified biochar is shown in Fig. 8, indicating an increasing trend in the adsorption capacity of NH₄⁺-N. Table 5 further delineates the association parameters (R²) for the Langmuir model, with values of 0.9920 (RB-H₂O₂), 0.9863 (RB-KMnO₄), and 0.9690 (RB-NaOH), all of which surpassed those of the Freundlich model. This suggests that isothermal NH₄⁺-N adsorption onto biochar predominantly adheres to the Langmuir isotherm model (Lingamdinne et al. 2020). Consequently, the primary mechanism underlying ammonium adsorption by MB was inferred to be monolayer adsorption on a highly homogeneous surface.

[See PDF for image]

Fig. 8

Adsorption isotherms of NH₄⁺-N on RB (a), RB-H₂O₂ (b), RB-NaOH (c) and RB-KMnO₄ (d). Q_e means the equilibrium adsorption capacity (mg g⁻¹). RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Table 5. Adsorption isotherm parameters for the 3 types of modified biochar

Isotherm models	Parameters	RB	RB-H2O2	RB-KMnO4	RB-NaOH
Langmuir	Qm (mg g−1)	9.0781	13.9269	68.1534	41.0008
	KL (L mg−1)	0.0018	0.0072	0.0016	0.0017
	R2	0.9391	0.9920	0.9863	0.9690
Freundlich	KF (mg(1−1/1n) L−1/n g−1)	0.0388	0.1585	0.1067	0.1093
	1/n	0.7788	0.7872	0.9780	0.8681
	R2	0.9514	0.9777	0.9824	0.9678
Temkin	b (J mol−1)	6600.7589	1437.1208	1101.0007	1666.4864
	KT (L mg−1)	0.3526	0.2061	0.2140	0.2701
	R2	0.7438	0.9397	0.8585	0.8531
Dubinin-Radushkevich	Qm (mg g−1)	0.9672	4.4540	4.6338	3.4812
	KDR (mol2 kJ−2)	0.0021	0.0058	0.0048	0.0036
	E (kJ mol−1)	15.4672	9.2529	10.1850	11.7688
	R2	0.5799	0.9104	0.8039	0.7997

The adsorption process is predominantly monolayer in nature and is characterized by chemisorption (Li et al. 2017). For RB, the fitting coefficients R² were greater for the Freundlich model than for the Langmuir model. Additionally, the results indicate that the Langmuir isotherm model more accurately describes the adsorption characteristics of MB for NH₄⁺-N. The equilibrium constant (K_L) of Langmuir equation is in the range of 0 < K_L < 1, indicating that adsorption is not a spontaneous process. In addition, K_L represents the adsorption capacity of biochar for NH₄⁺-N. In this study, the K_L of RB-H₂O₂ was higher than that of other biochar materials, indicating that RB-H₂O₂ had the highest affinity for NH₄⁺- N in solution.

Compared with previous studies, the fitting results of the Langmuir model demonstrated the significant adsorption advantage of modified biochar from biogas residue, with the adsorption capacity increasing from 9.08 mg g⁻¹ to 13.93–68.15 mg g⁻¹. Wang et al. (2023) modified chicken manure biochar with FeCl₃, and the maximum adsorption capacity of NH₄⁺ reached 55.29 mg g⁻¹. Song et al. (2024) modified pig manure biochar with different acid–base modification methods, and the adsorption capacity of ammonium nitrogen in neutral environment was 2.53 mg g⁻¹. It may be inferred that the adsorption performance of different biochar materials modified with different modifiers varies greatly. The types of raw materials used in the preparation of biochar, the conditions for biochar preparation, and the methods of biochar modification are all important factors that affect the porosity, specific surface area, and functional group expression of modified biochar. (Chen et al. 2021; Halder et al. 2023). The influences of solution pH, competitive ions, and initial NH₄⁺-N concentration on the NH₄⁺-N adsorption capacity of modified biochar are further analyzed and discussed in the Supplementary Materials. Therefore, in the modification process, it is also necessary to choose suitable modifiers to achieve better adsorption performance.

Mechanism of NH₄⁺-N adsorption on altered biochar

Compared with unmodified biochar, the adsorption capacity of MB was significantly enhanced. However, the mechanisms underlying the improved adsorption performance varied among the three types of modified biochar (Fig. 9). In the above, we elucidated the adsorption advantages of RB-KMnO₄ from the perspective of the intra particle diffusion model. The enhanced adsorption capacity was primarily attributed to the improved pore structure of biochar, which was characterized by a significant increase in both pore volume and specific surface area (Table 2). Oxidation treatment can create additional micropores and mesopores on the biochar surface, thereby increasing the specific surface area and providing more adsorption sites (Chen et al. 2022). However, modification of biochar using another oxidizing agent, H₂O₂, only resulted in a minor improvement in its adsorption performance. Unlike KMnO₄, the oxidation by H₂O₂ did not significantly enhance the pore structure of biochar but instead led to a notable increase in the –C–O functional groups on the surface of RB-H₂O₂ (Table 2), accompanied by minor increases in –OH and –COOH functional groups (Fig. 5). These oxygen-containing functional groups can elevate the negative charge density on the biochar surface, thereby enhancing the electrostatic attraction to cations such as NH₄⁺ (Liu et al. 2024a, b). The adsorption mechanism of RB-NaOH was similar to that of RB-H₂O₂, both of which significantly increased the –C–O functional groups on the biochar surface (Table 3). However, RB-NaOH also slightly introduced Fe–O functional groups (Fig. 5). Although RB-NaOH exhibited relatively lower specific surface area and total pore volume, its adsorption capacity surpassed that of RB-H₂O₂ (Table 5 and Fig. 6). This discrepancy can be attributed to the fact that NaOH modification was more effective than H₂O₂ modification in increasing the relative content of –C–O functional groups on the biochar surface (Table 3). However, neither modification strategy surpassed the adsorption performance of RB-KMnO₄, indicating that pore adsorption is more advantageous than surface functional group adsorption for NH₄⁺-N. There are three factors attributed to this superiority. Firstly, the increased internal surface area, particularly the presence of micropores and mesopores, provides a larger adsorption surface, allowing more ammonium ions to enter and be immobilized within these pores (Cai et al. 2022). Secondly, the diffusion and accommodation mechanism play a crucial role. Once ammonium ions enter the pores of biochar, they are less likely to desorb due to spatial confinement effects. This intrinsic trapping effect significantly enhances adsorption efficiency. In contrast, although surface functional groups can capture ammonium ions through chemical bonding, their capacity is limited (Joshi et al. 2023). Thirdly, competitive adsorption sites pose a challenge for surface functional groups. When multiple pollutants or ions coexist, surface functional groups may engage in competitive adsorption, reducing the effective adsorption capacity for specific targets such as ammonium ions. In contrast, pore structures are less affected by competition because they primarily rely on physical adsorption processes rather than chemical interactions (Sumaraj et al. 2020). Besides, it is also important to consider the potential for KMnO₄ to react with organic components in biochar, forming more stable complexes that enhance biochar stability and adsorption efficiency (Shang et al. 2020).

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Fig. 9

Adsorption mechanisms of RB-H₂O₂, RB-NaOH and RB-KMnO₄. RB means residue biochar; RB-H₂O₂ means residue biochar modified by H₂O₂; RB-NaOH means residue biochar modified by NaOH; RB-KMnO₄ means residue biochar modified by KMnO₄

Conclusion

As the concentration of NH₄⁺-N solution increased, the MB exhibited a more pronounced adsorption advantage. The maximum adsorption capacities of these three MB were 13.93, 41.00, and 68.15 mg g⁻¹, respectively, from the Langmuir isotherm model. Ammonium adsorption on the MB surfaces was affected by surface adsorption, liquid membrane diffusion, and intra-particle diffusion. The adsorption mechanisms of ammonium ions by different MB were distinct. Relative to RB, the modification with KMnO₄ enhanced the specific surface area and pore volume of biochar, thereby optimizing its pore structure. This led to a significant increase in the number of active sites on RB-KMnO₄, which in turn enhanced its adsorption capacity for NH₄⁺-N. The adsorption of NH₄⁺-N by RB-NaOH primarily relied on functional group interactions, including –C–O and Fe–O, while RB-H₂O₂ adsorbed NH₄⁺-N mainly through –C–O functional groups, with weaker contributions from –OH and –COOH. The modified biochar tended to exhibit more efficient adsorption performance under neutral pH conditions, with lower concentrations of competitive ions and higher concentrations of NH₄⁺-N solution. Overall, biochars modified with KMnO₄, H₂O₂, and NaOH exhibited superior performance in adsorbing ammonium nitrogen. Among these, RB-KMnO₄ demonstrated the highest adsorption efficiency, with pore-based adsorption playing a dominant role over functional group-based adsorption. These findings highlight the critical role of pore structure optimization in enhancing the adsorption capacity of biochar for ammonium nitrogen.

Acknowledgements

Not applicable.

Author contributions

Ping Cong: investigation, data analysis, original draft preparation; Shuhui Song: conceptualization, supervision and methodology; Yanmei Zhu: supervision, data curation; Xinwei Ji and Shuai Liu: writing-review and editing; Shuai Kuang: writing-original draft preparation; Yanli Xu and Qiuqiang Hou: project administration; Xuebo Zheng: data analysis, supervision and methodology; Wenjing Song: data curation and supervision.

Funding

This study was supported by the National Natural Science Foundation of China (32301969; 31901195), Shandong Provincial Natural Science Foundation (ZR2024MC159), the Agricultural Science and Technology Innovation Program of the Chinese Academy of Agricultural Sciences (ASTIP-TRIC03), and Yunnan Branch of China National Tobacco Corporation (2024530000241031).

Data availability

The datasets generated during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.

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