Structure–Activity Mechanism of Sodium Ion Adsorption and Release Behaviors in Biochar

Abstract

Biochar is a soil amendment that has the potential to effectively improve soil salinization. However, there is a paucity of studies on sodium adsorption using biochar, and the adsorption mechanism remains unclear. To better understand the adsorption mechanism of Na⁺ on the surface of biochar, both pyrochar and hydrochar were produced at different temperatures. The capacity and influencing factors of Na⁺ adsorption in biochar were analyzed via batch adsorption experiments. Pore filling dominated the Na⁺ adsorption in the concentration of the NaCl solution when it was ≤100 mg/L, where wheat straw pyrochar (WB, 3.95–4.94 mg/g Na) and poplar wood chip pyrochar (PB, 0.62–0.70 mg/g Na) presented the release and adsorption of Na⁺, respectively. When the concentration of the NaCl solution was >100 mg/L, the adsorption capacity of WB (25.44–36.45 mg/g) was significantly higher than PB (4.46–6.23 mg/g). Both the adsorption and release of Na⁺ in hydrochar was insufficient. In a high concentration of NaCl solution, ion exchange became the key mechanism determining the adsorption of Na⁺ in pyrochar, in which K⁺ contributed to more than 94% of the Na⁺ adsorption. The findings proposed strategies for the structural design of biochar used for Na adsorption. These will promote the utilization of solid biowaste for sodium adsorption and the potential of soil salinization amendment for agriculture in arid lands.

1. Introduction

Soil salinization occurs on more than 833 million hectares of global land worldwide [1], severely impacting crop yields and posing a threat to global food security. Sodium ion is the critical factor causing soil salinization. Currently, numerous techniques have been developed for the elimination of excessive sodium ions in saline soils, including chemical, biological, agricultural and irrigation engineering methods [2,3,4,5,6].

Biochar is a carbonaceous porous material obtained using biomass pyrolysis under oxygen-limit conditions [7]. Biochar is prepared from a variety of raw materials, for instance, agricultural waste [8], sludge [9] and manure [10], which leads to different physical and chemical properties [11]. To date, it has been shown that biochar is able to improve soil nutrient utilization [12], enhance carbon sequestration [13] and reduce pollutant bioavailability [14]. Adsorption ability is an essential function of biochar for environmental remediation. The adsorption effect of biochar is related to its well-developed pore structure, large specific surface area, abundant functional groups, etc. [15,16,17]. The strong adsorption capacity of biochar for organic pollutants and heavy metals has been substantially reported [18,19,20,21]; however, few studies have discussed the adsorption of sodium ions using biochar and its application in arid regions.

Biochar significantly reduces soil salinity and alleviates salt stress in plants. It has been reported that pyrolysis biochar (pyrochar) and hydrothermal biochar (hydrochar) can effectively amend saline soil and promote plant growth [22,23,24,25,26]. In soda saline alkali land, where the soil pH was 8.48 and its salt content was 0.82%, the application of 20 ton/ha corn straw biochar resulted in a reduction in Na⁺ by approximately 33.70% and 47.47% in 0–20 cm soil at the seed and harvest stages, respectively, and this ultimately promoted maize growth [22]. In the saline alkali soil of the western Songnen Plain, China, the application of 15 ton/ha acidic corn stalk biochar resulted in a reduction in Na⁺ in the soil by 18.23% and 41.42% at the seed and harvest stages, respectively, while simultaneously increasing sorghum yield [23]. In saline soil with a pH of 8.8 and a total salt content of 8.1 mg/kg, the application of biochar resulted in a significant reduction in Na⁺ levels in the soil, with a 2.8–5.5 fold decrease observed [25]. This led to a notable alleviation of the adverse effects of salt on the growth of Miscanthus. The application of rice straw biochar resulted in a significant reduction in Na⁺ uptake by cotton, with a range of 8.21~39.47% observed in saline soil from Xinjiang with a pH of 8.54 and conductivity of 6061 μS/cm [24]. The application of biochar derived from different pyrolysis temperatures resulted in a reduction of 5.26–9.47% of exchangeable Na⁺ in saline soil with a pH of 9.06 and an exchangeable Na⁺ content of 1.10 cmol/kg [26]. This approach effectively mitigated yield reduction caused by salinity. In short, the abovementioned studies using biochar to diminish Na⁺ in saline soil expanded biochar application in saline soil remediation, improving saline soil and agricultural development. Nevertheless, the precise mechanism through which biochar improves saline soils and alleviates salt stress remains unclear. Consequently, further research is required to elucidate the adsorption mechanism of biochar in relation to Na⁺, with the aim of enhancing the ameliorative effect of biochar and mitigating the adverse effects of salinity on crops.

The reduction in Na⁺ might be as a result of the adsorption of Na⁺ by biochar. Nguyen et al. [27] found that the Na⁺ adsorption capacity of the biochars derived from rice husk, corn stalks, longan branch and coconut coir are 33.9, 30.9, 17.9 and 15.1 mg/g, respectively. Yu et al. [28] demonstrated that the Na⁺ adsorption capacity of pristine and sulfuric acid-modified corn cob biochar exhibits a Na⁺ adsorption capacity of 20.6 and 67.7 mg/g, respectively. The nature of biochar, e.g., functional groups, is an important factor affecting the adsorption capacity [28]. Pore filling and ion exchange may be the main mechanism of Na⁺ adsorption by biochar [27,28,29]. However, the quantitative research for the adsorption mechanism is still lacking in arid land.

To deeply understand the adsorption mechanism of Na⁺ in biochar, our study aimed (1) to investigate biochar’s adsorption capacity of Na⁺ with different physical structures and chemical properties; (2) to seek the key factors influencing adsorption behaviors and quantitatively understand the adsorption mechanism of Na⁺ in biochar. Wheat straw and poplar wood chips were used as raw materials for the preparation of pyrochar at 350, 550 and 750 °C and hydrochar at 180 and 220 °C. The rate and capacity of Na⁺ adsorption in biochar were observed by adsorption kinetics and isotherms experiments. The pH effect and ion-exchange experiments were applied to find out the influencing factors of Na⁺ adsorption. The structure–activity of Na⁺ adsorption in biochar was discussed to fill in the knowledge gap of the quantitative mechanism of Na⁺ adsorption. The mechanism of Na⁺ adsorption proposed in our study sheds new light on biochar application in saline soil.

2. Materials and Methods

2.1. Experimental Materials and Reagents

To establish the structure–effect of Na⁺ adsorption in biochar, the biochar with a significant difference in both structure and composition should be used. Since it is known that the feedstock influences the physiochemical properties, wheat straw (Figure S1c) and poplar wood chips (Figure S1d) were selected as the raw materials used for the preparation of biochar. The wheat straws and poplar wood chips were collected from Fukang, Xinjiang and Kaifeng, Henan, China, respectively. The raw materials were dried in an oven at 60 °C for 12 h, then crushed to pass through a 100-mesh sieve. The sieved biomasses were sealed in bags for further usage.

The main chemical reagents used in this study were sodium chloride (99.5%, Solarbio, Beijing, China), hydrochloric acid (36.0–38.0%, Kelong, Jiande, China), ammonia solution (30.0%, CNW, Frankfurt, Germany), and potassium hydroxide (99.9%, Macklin, Shanghai, China). The ultra-pure water that was used was from a water purifier (SO-K1-20TY, Saiouhuachuang, Beijing, China).

2.2. Biochar Preparation

In this study, two distinct types of biochar were prepared using disparate preparation conditions: pyrochar (pyrolysis biochar) and hydrochar (hydrothermal biochar). The pyrochar was prepared based on the pyrolysis method [30]: each sieved biomass was pyrolyzed at the target temperature (i.e., 350, 550 and 750 °C) for 6 h under oxygen-limited conditions in a muffle furnace (KSL-1200X-M, Kejing, Shenyang, China) (Figure S1a). The rate of temperature increase in the muffle furnace was 5 °C/min. At different temperatures, wheat straw pyrochar (WB) was labeled as WB350, WB550 and WB750, and poplar wood chips pyrochar (PB) was labeled as PB350, PB550 and PB750.

The hydrochar was obtained using the hydrothermal method [31]: each sieved biomass was added to a 100 mL stainless-steel autoclave (Figure S1b) with deionized water in the ratio of 1:10 (w:w) and hydrothermally charred at the aimed temperatures (i.e., 180 and 220 °C) for 6 h. The rate of temperature rise in the muffle furnace was 5 °C/min. Vacuum filtration was used to separate the hydrochar from the mixture in the hydrothermal reaction. The hydrochar was oven-dried after several rinses and again passed through a 100-mesh sieve. Wheat straw hydrochar (HWB) at different temperatures was denoted as HWB180 and HWB220, respectively; the poplar wood chips hydrochar (HPB) was denoted as HPB180 and HPB220, respectively.

2.3. Biochar Characterization

The Brunauer–Emmett–Teller (BET) surface area and pore volume of biochar were measured using a fully automated physical adsorption instrument (NOVA2000e, Quantachrome, Drive Boynton Beach, FL, USA). The pH of biochar was measured using a pH meter (SevenExcellence, Mettler Toledo, Greifensee, Switzerland) with a ratio of biochar to water of 1:20 (w:w) [32,33]. The ash content was measured using mass balance after calcination at 800 °C for 4 h [34]. The elemental contents of C, H, N and S were measured using an elemental analyzer (Vario EL cube, Elementar, Langenselbold, Germany). The O content was calculated using mass balance on an ash-free condition [35,36]. The elemental contents of Na, K, Ca and Mg of the biochar were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, ICAP RQ, ThermoFisher Scientific, Waltham, MA, USA) after microwave digestion (Mars6, CEM, Charlotte, NC, USA). In addition, the elemental composition of the biochar surface was measured using Energy Dispersive Spectroscopy (EDS, X-Flash SDD 5010, Bruker, Ettlingen, Germany). The crystal structure of biochar was measured using X-ray Diffraction Spectroscopy (XRD, D8-ADVANCE, Bruker, Germany).

2.4. Batch Adsorption Experiments

2.4.1. Adsorption Kinetics

The adsorption kinetics of WB and PB were carried out by adding 20 mg pyrochar and 8 mL 5 mg/L Na⁺ solution in glass vials. The mixture was shaken for 20 min, 1, 3, 6, 12, 24, 48, 72, 96 and 120 h at 25 ± 2 °C. For HWB and HPB, 20 mg hydrochar and 8 mL 1 mg/L Na⁺ solution was added to glass vials. The same approach of shaking was the same as for pyrochar, while the setup time was 24, 48, 72, 120 and 168 h. In addition, owing to the Na in pristine biochar, to distinguish the dynamic of the release of internal Na and the adsorption of external Na in biochar, the release kinetics of internal Na from pristine biochar to water were set as the control. In the release kinetics, the ratio of solid to water and the shaking duration of each biochar were the same as the adsorption kinetics. All the treatments were conducted in duplicate. The solution was separated from the mixture using a 0.45 μm filter. The Na⁺ concentration in the filtrate was measured using ion meter (PXSJ-227L, REX, Shanghai, China) with a sodium ion selective electrode (972207, REX, Shanghai, China). The standard curves of the ion meter at different Na⁺ concentration intervals are shown in Figure S2. To maintain the sensitivity of the electrode, a 0.1 mL ammonia solution was added to the filtrate to keep the pH higher than 10.0. The amount of Na⁺ adsorbed or released per unit mass of biochar was calculated using Equations (1) and (2).

$$q_{\mathrm{e}}={\displaystyle \frac{\left(C_0-C_{\mathrm{e}}\right)V}{m}}$$

(1)

$$q_{\mathrm{r}}={\displaystyle \frac{\left(C_{\mathrm{e}}-C_0\right)V}{m}}$$

(2)

where q_e (mg/g) and q_r (mg/g) are the equilibrium amounts of adsorption and release, respectively; C₀ (mg/L) and C_e (mg/L) are the initial and equilibrium concentrations of Na⁺ in solution; V (L) is the volume of the solution; m (g) is the mass of biochar. The change in adsorption capacity of Na⁺ by biochar over time was analyzed using the pseudo-first-order (Equation (3)) and pseudo-second-order (Equation (4)) kinetic models [37].

$$q_{\mathrm{t}}=q_{\mathrm{e}}\left(1-e^{-k_{1}t}\right)$$

(3)

$$q_{\mathrm{t}}=q_{\mathrm{e}}\left(1-{\displaystyle \frac{1}{1+q_{\mathrm{e}}{k}_{2}t}}\right)$$

(4)

where q_t (mg/g) is the adsorption capacity at time t; t (h) is the duration of the adsorption reaction; k₁ (1/h) and k₂ (g/mg/h) are the rate constants.

2.4.2. Adsorption Isotherm

The isothermal adsorption experiments were carried out by weighing 10 mg pyrochar and 8 mL Na⁺ solution of concentrations ranging from 10 to 2500 mg/L in glass vials. The isothermal adsorption equilibrium time was 96 h based on the adsorption kinetics. The Langmuir (Equation (5)) [27], Freundlich (Equation (6)) [38] and Dubinin–Ashtakhov (D–A) (Equation (7)) [39,40] model were utilized to fit the curve of adsorption isotherms.

$$q_{\mathrm{e}}={\displaystyle \frac{q_{\max }{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}}$$

(5)

$$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{{\displaystyle \frac{1}{\mathrm{n}}}}$$

(6)

$$\log q_{\mathrm{e}}=\log Q^0-{\left(\epsilon /\mathrm{E}\right)}^{\mathrm{b}}$$

(7)

where q_max (mg/g) is the maximum adsorption capacity; K_L (L/mg) represents the affinity coefficient; K_F (mg⁽¹⁻ⁿ⁾∙Lⁿ/g) represents the Freundlich affinity coefficient; n is the Freundlich linearity constant; ε (kJ/mol) = RTln(S_w/C_e) is the effective adsorption potential; R [8.314 × 10⁻³ kJ/(mol∙K)] is the universal gas constant; T (K) is the absolute temperature; S_w (mg/L) is the water solubility of adsorbents; Q⁰ (mg/g) is the adsorption capacity; E (kJ/mol) is the characteristic adsorption energy; and b is a fitting factor.

2.4.3. Effect of Solution Equilibrium pH on Na⁺ Adsorption

The effect of pH on Na⁺ adsorption in pyrochar was investigated by adding 50 mg WB and PB with 8 mL 250 and 100 mg/L Na⁺ solutions, respectively, to glass vials, and shaking for 96 h. The equilibrium pH of the Na⁺ solutions was maintained at approximately 2.0, 4.0, 6.0, 8.0, 10.0 and 12.0. In order to achieve the desired equilibrium pH, the volumes of HCl and KOH solution were calculated based on the results of a preliminary experiment. This ensured that the equilibrium pH remained within the specified range throughout the course of the experiment. The Na⁺ concentration in the filtrate was measured using a sodium ion composite electrode as described above.

2.4.4. Ion Exchange of Na⁺ on the Surface of Pyrochar

To clarify the contribution of different ions to Na⁺ exchange adsorption, ion release experiments with pyrochar in sodium solution and deionized water were setup. The mixture of 50 mg pyrochar and 8 mL 250 mg/L Na⁺ solution was shaken for 96 h. The mixture of pyrochar and deionized water was set as the control. The concentrations of metal ions in the filtrate were measured using inductively coupled plasma mass spectrometry (ICP-MS, ICAP RQ, ThermoFisher Scientific, USA), and the concentration of H⁺ was calculated from the filtrate pH. In the Na⁺ adsorption process, the relative contribution of ions in the pyrochar was calculated using Equation (8).

$$\mathrm{Relative}\mathrm{contribution}\left(\mathrm{i},\%\right)={\displaystyle \frac{q_{\mathrm{i}}}{\left(q_{\mathrm{K}}+q_{\mathrm{Ca}}+q_{\mathrm{Mg}}+q_{\mathrm{H}}\right)}}\times 100\%$$

(8)

where i presents an individual element of K⁺, Ca²⁺, Mg²⁺ and H⁺; q_i (mg/g) is the concentration difference in an individual element from NaCl solution to deionized water; q_K, q_Ca, q_Mg and q_H are the concentration differences in K⁺, Ca²⁺, Mg²⁺ and H⁺ from the NaCl solution to deionized water, respectively.

3. Results and Discussion

3.1. Characterization of Biochar

3.1.1. Surface Area and Pore Volume of Pyrochar and Hydrochar

The surface area and pore volume of pyrochar gradually increased with the increasing temperature (

Table 1. Physicochemical properties of pyrochar and hydrochar.

Biochar	SBET (m2/g)	Vmeso (cm3/g)	Vmicro (cm3/g)	Ash (%)	pH	Elemental Composition (%)					Atomic Ratio
						C	H	O	N	S	H/C	O/C
WB350	4.63	0.008	0.072	19.71 ± 0.03	8.6	56.37 ± 0.04	3.64 ± 0.02	19.47 ± 0.03	0.73 ± 0.01	0.09 ± 0.00	0.06	0.35
WB550	194.66	0.012	0.108	25.67 ± 0.34	11.0	60.54 ± 0.02	2.14 ± 0.03	10.94 ± 0.01	0.63 ± 0.04	0.08 ± 0.00	0.04	0.18
WB750	417.34	0.021	0.141	26.91 ± 0.10	10.6	59.96 ± 0.04	1.62 ± 0.00	10.81 ± 0.00	0.66 ± 0.04	0.05 ± 0.00	0.03	0.18
PB350	109.05	0.006	0.114	0.45 ± 0.07	6.0	70.45 ± 0.01	3.76 ± 0.02	25.03 ± 0.00	0.26 ± 0.02	0.07 ± 0.00	0.05	0.36
PB550	484.27	0.025	0.164	2.06 ± 0.80	9.0	82.23 ± 0.04	2.88 ± 0.02	12.41 ± 0.05	0.37 ± 0.01	0.06 ± 0.01	0.04	0.15
PB750	601.90	0.125	0.207	1.95 ± 0.06	10.0	85.76 ± 0.04	2.26 ± 0.02	9.51 ± 0.07	0.44 ± 0.05	0.09 ± 0.00	0.03	0.11
HWB180	8.46	0.033	0.019	9.81 ± 0.33	4.6	45.53 ± 0.02	4.98 ± 0.09	39.17 ± 0.15	0.48 ± 0.04	0.04 ± 0.00	0.11	0.86
HWB220	12.54	0.055	0.033	9.96 ± 0.61	4.0	52.34 ± 0.02	5.85 ± 0.04	31.30 ± 0.00	0.52 ± 0.02	0.03 ± 0.00	0.11	0.60
HPB180	2.15	0.005	0.011	1.19 ± 0.35	4.5	49.33 ± 0.04	6.36 ± 0.05	42.91 ± 0.11	0.16 ± 0.01	0.05 ± 0.01	0.13	0.87
HPB220	3.47	0.011	0.030	0.15 ± 0.07	4.2	55.36 ± 0.04	6.64 ± 0.02	37.66 ± 0.08	0.14 ± 0.05	0.06 ± 0.01	0.12	0.68

, Figures S3 and S4), as the higher temperature can promote the development of the pore structure of pyrochar [41,42,43]. The surface areas of WB750 (417.34 m²/g) and PB750 (601.90 m²/g) were 90.14 and 5.52 times higher than those of WB350 (4.63 m²/g) and PB350 (109.05 m²/g), respectively. At the same temperature, the surface area of PB was significantly higher than that of WB. Consequently, both temperature and feedstock are critical factors affecting the pore structure of biochar [44]. In addition, the surface area and pore volume of the hydrochar were smaller than those of the pyrochar, which may be attributed to the lower temperatures causing in a lower degree of pyrolysis [43].

3.1.2. Chemical Properties of Pyrochar and Hydrochar

With higher temperatures leading to higher ash content (Table 1), WB750 (26.91%) and PB750 (1.95%) had a higher ash content than WB350 (19.71%) and PB350 (0.45%), respectively. Moreover, at the same temperature, straw-derived biochar (WB) had a higher ash content than the wood-derived biochar (PB). The ash content of the biochar would affect the pH due to the presence of alkaline mineral oxides [45], which explains the reason that the pH of WB is much lower than PB (Table 1), although the lowest pH among pyrochar was found in PB350 with a pH of 6.0 which is still higher than all of the hydrochars. Compared to pyrochar, the lower pH (≤4.6) of hydrochar may be due to the dissolution of alkaline functional groups during the preparation.

The contents of Na, K, Ca and Mg elements in pyrochar had a tendency to rise with increasing temperature (

Table 2. The elemental content of Na, K, Ca and Mg in pyrolyzed biochar.

Biochar	Metal Element Content (mg/g)
	Na	K	Ca	Mg
WB350	3.95	49.80	4.49	3.24
WB550	4.44	58.50	6.00	4.29
WB750	4.94	60.60	5.61	4.33
PB350	0.66	2.97	3.42	0.47
PB550	0.70	3.95	4.94	0.93
PB750	0.62	5.40	6.79	1.42

). Moreover, the Na and K contents were significantly higher in WB compared to PB. The Na and K contents of WB750 were 7.97 and 11.22 times higher than those of PB750 (Table 2), respectively, demonstrating that the feedstock determines the amounts of Na and K in biochar [45]. The above findings were similar to the relative elemental content of biochar measured using EDS (Table S2). Additionally, the crystal phase of KCl in WB was observed in the XRD analysis (Figure S5). It was inferred that abundant K was mainly distributed on the WB surface in the form of KCl. Compared to the pyrochar, the hydrochar contained a relatively lower amount of Na (i.e., <0.80 mg/g), which could be caused by using pyrolysis in an aqueous environment (Table S1).

3.2. Adsorption Kinetics of Na⁺ in Biochar

For WB, the majority of Na⁺ was released rather than adsorbed (Figure S6a), while the adsorption capacity of PB increased rapidly in the first 10 h and stabilized after 96 h (Figure S6c). It is clear to see that PB released a much lower amount of Na⁺ than WB (Figure S6b,d), which was consistent with the less internal Na that was observed in PB (Table 2). The abundant amount of internal Na in WB may lead to the substantial release of Na⁺. The adsorption and release of Na⁺ in HWB and HPB were insufficient (Table 2, Figure S6e,f); thus, hydrochar was not involved in any further adsorption analyses in our study. From the fitting parameters (

Table 3. Kinetics parameters of Na⁺ adsorption on PB fitted to the pseudo-first-order model and pseudo-second-order model.

Biochar	Pseudo-First-Order Model			Pseudo-Second-Order Model
	qe (mg/g)	k1 (/h)	R2	qe (mg/g)	k2 (g/mg/h)	R2
PB350	0.32 ± 0.02	1.36 ± 0.63	0.61	0.32 ± 0.03	7.79 ± 5.52	0.58
PB550	0.38 ± 0.01	3.78 ± 0.85	0.88	0.39 ± 0.01	20.98 ± 8.33	0.88
PB750	0.23 ± 0.01	5.02 ± 2.84	0.58	0.23 ± 0.02	63.74 ± 80.61	0.57

), both the pseudo-first-order model and the pseudo-second-order model could not explain the adsorption behaviors well, revealing that the internal Na release could impact the adsorption of Na⁺ in biochar. In this case, the exact influence of internal Na on the adsorption of Na⁺ needs further exploration in our study.

3.3. Adsorption Isotherms of Na⁺ in Pyrochar

The fitting curves of Na⁺ adsorption in pyrochar using the Langmuir, Freundlich and D–A models are depicted in Figure 1. When the Na⁺ solution concentration was low (≤100 mg/L), the WB showed obvious Na⁺ release, which was consistent with the kinetic results (Figure S6). It confirmed that when the external Na is low, the internal Na of biochar would largely affect the adsorption and release behaviors of Na⁺. The fitting parameters of Na⁺ adsorption on the surfaces of WB and PB based on the three models are listed in

Table 4. Isothermal parameters of Na⁺ adsorption on WB and PB fitted to the Langmuir, Freundlich and D–A models.

Biochar	Freundlich Model			Langmuir Model			D–A Model
	n	KF	R2	qmax	KL	R2	Q0	E	b	R2
WB350	1.77 ± 0.19	0.21 ± 0.09	0.9564	25.44 ± 1.68	0.0008 ± 0.0001	0.9883	21.54 ± 1.42	18.97 ± 0.14	5.13 ± 0.54	0.9965
WB550	1.52 ± 0.14	0.10 ± 0.04	0.9688	30.21 ± 3.15	0.0005 ± 0.0001	0.9859	24.88 ± 3.83	18.28 ± 0.22	4.47 ± 0.85	0.9926
WB750	1.44 ± 0.11	0.09 ± 0.03	0.9786	36.45 ± 3.19	0.0004 ± 0.0001	0.9927	29.47 ± 2.10	18.02 ± 0.10	4.29 ± 0.34	0.9989
PB350	2.11 ± 0.16	0.13 ± 0.03	0.9624	6.23 ± 0.53	0.0017 ± 0.0004	0.9524	13.72 ± 7.39	19.42 ± 2.43	1.90 ± 0.66	0.9674
PB550	2.67 ± 0.23	0.24 ± 0.05	0.9452	4.46 ± 0.21	0.0036 ± 0.0005	0.9718	5.67 ± 0.79	23.51 ± 0.50	3.09 ± 0.54	0.9780
PB750	2.52 ± 0.26	0.23 ± 0.07	0.9265	5.41 ± 0.19	0.0031 ± 0.0003	0.9862	5.85 ± 0.54	22.71 ± 0.39	4.09 ± 0.61	0.9814

. The Langmuir and D–A models were better fitted with adsorption data rather than the Freundlich model according to R². This demonstrated that the adsorption of Na⁺ in pyrochar is nonlinear, thus, the parameters of the Langmuir and D–A models are primarily focused on in our further discussion. The adsorption capacity represented by the q_max or Q⁰ of WB increased with the pyrolysis temperature, implying that the Na⁺ adsorption may relate to the surface area increment and metal ion enrichment. Considering the well-accepted adsorption mechanisms, this suggests that the adsorption of Na⁺ on WB is most likely based on either pore filling or ion exchange [27]. The saturated adsorption capacity of WB350 was 4.18 times higher than that of PB350, while the surface area of WB350 was 4.24% of that of PB350. Although PB has a significantly larger surface area compared to WB, its adsorption capacity for Na⁺ was notably smaller. This indicated that pore filling cannot be the dominant mechanism for Na⁺ adsorption in pyrochar.

3.4. Effect of Equilibrium pH on Na⁺ Adsorption in Pyrochar

At a pH range of 2 to 12, the adsorption and release behaviors of Na⁺ in pyrochar were significantly different. The behaviors of WB and PB in Na⁺ solution changed from releasing Na⁺ to adsorbing Na⁺ with an increasing equilibrium pH (Figure 2). At an acidic condition, Na⁺ was released from both WB and PB, suggesting electrostatic repulsion between Na⁺ and H⁺ in the solution and the competitive effect of H⁺ on the adsorption site [46,47]. In the alkaline condition, both WB and PB exhibited Na⁺ adsorption. In particular, the low-temperature pyrochar showed greater adsorption than the pyrochar from the higher temperature, although the pore volume of the low-temperature pyrochar was much lower than the higher-temperature pyrochar. The detailed causes of this phenomenon are not yet understood and need to be further researched. The specific surface area and pore volume of PB decreased after impregnation in an alkaline (pH = 12.0) solution (Table S6). This suggests that the pore volume does not determine the Na⁺ adsorption capacity of pyrochar under alkaline conditions. The strong electrostatic attraction of deprotonated functional groups may favor the adsorption of Na⁺ in biochar [48,49].

3.5. Ion Exchange of Na⁺ in Pyrochar

Since it was proved that pore filling does not predominantly determine the Na⁺ adsorption in pyrochar, we hypothesized that ion exchange might be the key mechanism in Na⁺ adsorption. As can be seen in Figure 3., K⁺ had a highest relative contribution (>94%) to the Na⁺ adsorption for WB, while both K⁺ and Ca²⁺ contributed to the Na⁺ adsorption for PB, indicating that K⁺ was the main metal ion undergoing ion exchange with Na⁺. This suggests that the significantly higher Na⁺ adsorption of WB compared to PB is most likely attributed to the higher K content of WB. Therefore, the metal (e.g., K) in biochar that causes ion-exchange interaction is the predominant mechanism of Na⁺ adsorption.

3.6. Mechanism of Na⁺ Adsorption in Biochar

Pore filling and ion exchange are the main mechanisms of Na⁺ adsorption in biochar [27,28,29]. Nguyen et al. [27] revealed that ion exchange occurred through Na⁺ adsorption by altering the amount of metal ions released from biochar. Yu et al. [28] improved the adsorption capacity of biochar for Na⁺ by increasing the amount of H⁺ exchanging with Na⁺. Besides ion-exchange interaction, pore filling also determined the Na⁺ adsorption in biochar. For biochar prepared from various feedstocks, the contributions of pore filling and ion exchange to Na⁺ adsorption were different. Based on our findings summarized in Figure 4, when the external Na is low (≤100 mg/L), pore filling is the dominant mechanism of Na⁺ adsorption in biochar. When the external Na was higher (>100 mg/L), ion exchange became the main mechanism of Na⁺ adsorption. The susceptibility to ion exchange between the metal ion (K⁺) and Na⁺ may be due to their binding to functional groups through electrostatic interactions [20,37,50,51].

In our study, the specific surface area and pore volumes of PB were greater than that of WB; however, WB had a higher adsorption capacity than PB, which further suggested that ion exchange may have a greater contribution to adsorption than pore filling. The adsorption capacity of Na⁺ was extracted from previous studies, in which the concentrations of external Na were all higher than 200 mg/L in these studies. The fitting (R² = 0.31, p > 0.05) between the adsorption capacity of Na⁺ and the surface area in the literature [27,28,52] was performed (Figure 5), which again demonstrated that surface area was not an important factor influencing the adsorption capacity of biochar for Na⁺. It is worth noting that this contradicts the result where there was a strong positive relationship between the surface area of the adsorbent and the adsorption capacity of the adsorbent (e.g., Cd²⁺) [53].

4. Conclusions

In this study, the Na in the pristine pyrochar and NaCl solution were defined as internal Na and external Na, respectively. (1) When the external Na is low (≤100 mg/L), the pyrochar with a higher amount of internal Na would release Na. While the concentration of internal Na was low, the pyrochar exhibited the ability of Na adsorption; pore filling is the dominant mechanism of Na⁺ adsorption in pyrochar. (2) When the external Na is higher (>100 mg/L), the adsorption capacity of Na does not significantly relate to the surface area of the pyrochar. This demonstrated that pore filling is not the dominant adsorption mechanism, where instead ion exchange becomes the main mechanism of Na⁺ adsorption. The capacity of metal ions (specifically K⁺) to engage in ion exchange with Na⁺ through electrostatic interactions determined the adsorption capacity of Na in pyrochar. Strategies were proposed for the structural design of biochar used for Na adsorption. Pore structure development should be emphasized when the external Na is ≤ 100 mg/L; in contrast, the biochar with abundant Na-exchangeable ions should be selected for Na adsorption when the external Na is >100 mg/L.

This study confirmed that the internal Na of pyrochar substantially affects the adsorption and release behavior of Na in pyrochar. Consequently, further research would be more comprehensible if the ratio of internal and external Na in biochar were considered to evaluate the adsorption behavior of biochar. Different procedures should be applied to increase the adsorption capacity of Na: (1) feedstock with high amounts of metal ions should be selected when the content of external Na is high; (2) when the content of external Na is low, it is crucial to select suitable feedstocks that exhibit well-developed pores, or alternatively, to conduct processes of pore structure enhancement during biochar preparation.