Preparation and Characterization of Cattail-Derived Biochar and Its Application for Cadmium Removal

Abstract

Biochars produced from aquatic plants have attracted increasing attention for the removal of heavy metals from the environment. Therefore, biochars derived from the roots (CBR), stems (CBS) and leaves (CBL) of cattail were investigated in this paper for their higher adsorption capacity, particularly for Cd(II). The adsorption characteristics and the leaching of alkali (soil) metals within biochars obtained from the different tissues of cattail were also discussed. The results showed that the specific surface area of cattail root biochar reached 15.758 m² g⁻¹. Langmuir, Freundlich and D-R isotherm equations were used to fit the experimental data, and the last equation revealed the best fitting result. The adsorption kinetics for Cd(II) removal were determined by using two different models. The experimental data for CBR and CBS were in good agreement with the pseudo second-order model, whereas the pseudo first-order model provided a better fit for CBL. The amount of leached K reached 73.214 mg g⁻¹ in CBS (55.087 mg g⁻¹ in CBL), which was almost an order of magnitude higher than those of Mg and Ca. The experimental data showed that the leached Mg and Ca metals in CBL had maximum levels of 6.543 and 10.339 mg g⁻¹, respectively. The mechanism of Cd(II) sorption by the biochar is complex and probably involves a combination of mass transfer, ion exchange, and mineral precipitation through the macropores and micropores of the biochar in the sorption process.

1. Introduction

With the rapid development of industry and social progress, a large number of heavy metals from various anthropogenic sources are discharged into the natural water [1]. Cadmium (Cd) is a toxic element that aroused concerns worldwide due to food safety issues and potential health risks [2,3]. According to the report on the national general survey of soil contamination in China, the average content of Cd in local samples was in the range of 1.09–27.9 mg kg⁻¹, which is above the maximum limit (1.0 mg kg⁻¹) of the standard [4]. The area of farmland with Cd pollution exceeded 20 × 10⁴ hm², and the product quantity with excessive Cd content reached 14.6 × 10⁸ kg annually. In the natural ecosystem, the average Cd concentration was reported as 0.2 mg kg⁻¹, and the data has multiplied as a result of increased industrial activities [5,6]. Industrial interventions such as lead–zinc mining, non-ferrous metal smelting, electroplating and the use of cadmium compounds as raw materials or catalysts discharge high amount of Cd(II)-containing wastewater. Excessive cadmium absorption causes poisonous effects in biology through the bones, kidneys, liver, immune system and reproductive system, and can cause diseases such as bone pain, diabetes, emphysema and hypertension, and even cancer [7].

Cadmium can only undergo morphological changes and migration, and does not to become harmless by self-decomposition. Therefore, various conventional technologies revolve around the removal of Cd(II) or changes designed to reduce its activity, such as chemical precipitation [8], ion exchange [9] and chelation [10]; furthermore, Cd(II) is sometimes gathered and removed from the medium by using methods such as phytoremediation [11] or microbial remediation [12,13]. However, some of these methods are expensive or have a risk of secondary contaminations. In terms of cost and environmental friendliness, adsorption technology has gradually become a more common choice because it is rapid and the adsorbents exhibit strong affinities for pollutants [14]. Therefore, the selection of adsorbents is the key to the removal of heavy metals from contaminated wastewater [15]. Biochar is a low-cost adsorbent, and it has recently received increased attention due to its many potential environmental applications and benefits [16].

Biochar is produced from pyrolysis (300 °C–700 °C) under anaerobic or oxygen-limited conditions [17,18]; it is a carbonaceous solid with high porosity, stability and aromaticity [19,20]. Natural materials such as plant waste are widely available in large quantities and can be used as raw materials for biochar preparation, resulting in lowered production costs and energy demands. Currently, substantial attention is being paid to the application of biochar in limiting resource consumption, supplying crop nutrition and remediating environmental pollution [21]. Numerous studies have revealed that some of the properties of biomass-derived adsorbents are very similar to those of activated carbon, such as high porosity, a large specific surface area, and structures rich in carbon [22,23]. This similarity provides a basis for the effective substitution of biochar in practical applications. At present, many studies concentrate on the removal of heavy metal ions by biochar [24,25], but few concentrate on the adsorption of Cd(II) ions by biochar made from aquatic plants, especially with respect to different organs from biomass. The technical superiority of aquatic plants over terrestrial species is well established with regard to rapid growth (biomass quantified) and high photosynthetic efficiency [26]. At the same time, the former does not need to occupy cultivated land and has a certain regulating effect on the surrounding air environment. The mechanisms responsible for the adsorption of heavy metals on the biochar include functional group complexation, ionic exchange, electrostatic interactions, and precipitation with minerals [27]. The dominance of different mechanisms is probably attributable to the intrinsic properties of the biochar, which mainly hinge on the feedstock type and pyrolytic strategy.

To date, biochar has always been used as a monolithic adsorbent [28], but few studies have been performed to gather effective information on the distribution of the components in different parts of biochar that are active for Cd(II) sorption. Plant-based biomass has its own tissue structures based on the growth characteristics of different organs, resulting in significant differences in the distribution of active components [1,29]. Various aspects of biochar, such as the elemental composition, morphological features, surface structure and micropore distribution, are heavily influenced by the pyrolysis conditions [30]. These characteristics play a key role in the adsorption treatment of Cd(II) ions(article). For example, with the increase of the pyrolysis temperature, the loss of volatile substances is related to the formation of pore-channel structures, which also modifies the surface functional groups. However, knowledge regarding the impacts of alkali and alkaline earth metals on the physicochemical properties of the resulting biochar organs, as well as the associated impacts on sorption efficiency, are still limited.

A series of laboratory experiments was performed in order to evaluate the feasibility of heavy metal removal by biochars derived from the roots, stems and leaves of aquatic plant cattail. The objectives of this work were as follows: (1) prepare and characterize biochars derived from the different organs of cattail; (2) evaluate the adsorption performance of cattail biochar (CB) with Cd(II); and (3) determine the effects of the organ, component, and ionic strength on the adsorption of Cd(II) onto the biochar.

2. Materials and Methods

2.1. Reagents

The reagents were procured from Shanghai Sinopharm Chemical Reagent Co., Ltd. A stock solution (Cd(NO₃)₂·4H₂O, 1000 mg/L) was prepared by dissolving 2.754 g Cd(NO₃)₂·4H₂O in 1000 mL deionized water. The original solution was diluted to obtain the series of subsequent required concentrations.

2.2. Preparation and Characterization of the Biochar

Cattail obtained from a nursery garden (Beijing, China) was selected as the raw biomass to make the biochar from its roots (CBR), stems (CBS) and leaves (CBL). The samples were rinsed with deionized water many times to eliminate the remaining sediment, until the different organ parts were separated, natural-air dried and grinded.

Each of the powdered materials was pyrolyzed in a fixed-bed pyrolysis reactor system that ran under nitrogen gas (N₂) flowing at 100 mL min⁻¹ to maintain anaerobic conditions. A quartz reactor was charged with 15 g of the raw stock to prepare the biochar at 873.15 K under the constant flow of 30 mL N₂ min⁻¹ [21]. The desirable temperature was increased steadily for 50 min at a rate of 10 °C min⁻¹, and was set to cool down within 150–180 min. The biochar produced from each biomass (CBR, CBS and CBR) was triturated and passed through a 2 mm diameter sieve. Based on the screening, the biochar samples were packaged into a sample sack and used for the subsequent adsorption tests.

The particle morphologies were observed through emission scanning electron microscopy (JEOL JAX-840, JEOL, Tokyo, Japan) using a working voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area of the biochar was determined through nitrogen adsorption–desorption measurements (TriStar 3000, Micromeritics, Georgia, USA). An X-ray diffractometer (D/MAX 2500 V/PC, Rigaku, Tokyo, Japan) was used to determine the crystalline substances contained in the biochar, and a Fourier transform infrared spectrophotometer (Nicolet 6700, Thermo, Massachusetts, USA) was used to determine the surface functional groups of the biochar samples.

2.3. Cd Adsorption Isotherm Experiment

Batch adsorption experiments were carried out using aqueous solutions of Cd(II). Solutions with Cd²⁺ concentrations of 5 mg L⁻¹, 10 mg L⁻¹, 15 mg L⁻¹, 20 mg L⁻¹, 25 mg L⁻¹, and 30 mg L⁻¹ were prepared from the Cd(NO₃)₂·4H₂O stock solutions (Section 2.1). All of the solutions were treated with 0.01 mol L⁻¹ NaNO₃ as the supporting electrolyte, and the initial solution pH was adjusted to 5.5. The biochar (0.02 ± 0.006 g) was weighed in a 50 mL polyethylene centrifuge tube, and its accurate mass was recorded. Cd²⁺ solutions (20 mL) at different concentrations (5 mg L⁻¹–30 mg L⁻¹) were added and vibrated for 8 h at 298.15 K in a constant-temperature oscillator box. The supernatant was centrifuged at a high speed of 4000 r min⁻¹ for 5 min. Then, the Cd²⁺ concentration was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, ICAP6000, Thermo, USA). The quantity adsorbed by a unit of mass of an adsorbent at equilibrium or the adsorption capacity (Q_e, mg g⁻¹) and Cd(II) removal rate (RE, %) was calculated according to the following Equations (1) and (2), respectively.

$$Q_e=\frac{\left(C_0-C_e\right)V}{m}$$

(1)

$$RE=\frac{C_0-C_e}{C_0}\times 100\%$$

(2)

where C₀ (mg L⁻¹) and C_e (mg L⁻¹) represent the concentrations at the initial and equilibrium adsorption of Cd²⁺, V (L) is the solution volume, and m (g) is the mass of the biochar. All of the detected data were repeated 3 times, and the average value of the data was used as the measurement result.

2.4. Cd Adsorption Kinetics Experiment

A Cd²⁺ solution (500 mL) with a concentration of 20 mg L⁻¹ was added to a 1000 mL beaker and stirred on a magnetic stirrer for the contact time studies (0–1440 min). The biochar particles were added into the reaction device after ensuring that the pH of the initial solution was adjusted to 5.5. The temperature was maintained at 298.15 K, and the samples were taken at fixed intervals.

2.5. Effect of Ionic Strength on the Adsorption Experiments

Biochar (0.02 ± 0.006 g) was placed into a 50 mL polyethylene centrifuge tube, and 10 mL NaNO₃ solution with a selected concentration was added as the supporting electrolyte. Then, 10 mL Cd²⁺ solution was added into the centrifuge tube, and the solution was balanced for 1 day at room temperature, during which time it was shaken at a slow speed 3–4 times for 30 min. After that, the concentration of Cd²⁺ in the supernatant was determined by ICP-OES. The adsorption capacity of the biochar for Cd²⁺ was calculated from the change of the solution’s Cd²⁺ concentration before and after equilibration.

2.6. Leaching Experiments of the Inorganic Components

The effect of coexisting ions, especially alkali and alkaline earth metal ions in high proportions, on the adsorption of Cd²⁺ ions was explored. The ionic strengths supported by the background electrolyte (NaNO₃) increased from 0 to 1.0 mol L⁻¹ for the initial adsorption system, while other parameters such as the Cd(II) concentration (100 mg L⁻¹), pH value (5.5), mass of the adsorbent (0.05 ± 0.006 g), and temperature (298.15 K) were kept constant. An equilibration time of 24 h was selected to ensure sufficient contact and a complete reaction between the adsorbent and adsorbate. The concentrations of related metal elements in the solution were measured by ICP-OES. The percentage results were calculated by the comparison of the adsorption of each ion obtained for the solutions with the adjusted ionic strength.

3. Results and Discussions

3.1. Physical and Chemical Properties of the Biochar

3.1.1. Basic Properties of Cattail Biochar

The physical properties of the biochars, along with the yield data, are presented in

Table 1. Basic properties of biochar.

Materials	Surface Area (m2 g−1)	Pore Volumes (cm3 g−1)	Carbon Yield (%)
CBR	15.758	0.0399	36.323
CBS	7.631	0.0331	30.622
CBL	7.243	0.0269	29.469

. The specific surface area is one of the important factors determining the adsorption capacity. The specific surface area and pore volume of the cattail root biochar were the largest among the three kinds of biochar, reaching 15.758 m² g⁻¹ and 0.0399 cm³ g⁻¹, respectively, and the specific surface area was twice that of cattail root and cattail leaf biochar, which may be caused by an increase in macropores after the removal of the volatile matter [31]. The larger specific surface area improved the adsorption capacity of the biochar and provided favorable conditions for the removal of heavy metal ions. The hollow and slender skeleton facilitated the absorption of electrolytes and provided a good conductive channel for electron transfer [32]. As an aromatic carbon matrix, cattail biochar has relatively porous structures, functional groups (carbonyl, hydroxyl, etc.) and inorganic forms (calcium cations, magnesium oxide, etc.) on the surface that provide active sites capable of interacting with metallic cations. All of the biochars used in this study were porotic, with average pore volumes of 0.0399, 0.0331 and 0.0269 cm³ g⁻¹ for CBR, CBS and CBL, respectively. The higher porosity of the adsorbent material offers more opportunities for intraparticle adsorbate diffusion and adsorption onto the surface [33]. The biochar yields decreased in the order CBR > CBS > CBL. Some studies suggest that the total yield is related to the high contribution of minerals or inorganics [20]; thus, CBR provided the highest ash content during the pyrolysis. Further comparative analysis showed that the surface area of CBR was significantly higher than those of CBS and CBL, but the other performance parameters differed little. The softening, melting and fusion of ash may lead to the coalescence of adjacent pores and structural deformation caused by pore blockage, which may be the reason for the lower specific surface area and pore volumes.

3.1.2. Scanning Electron Microscopy of Cattail Biochar (SEM)

SEM analyses were conducted in order to observe the changes in the surface morphologies of the biochar (Figure 1). As shown in Figure 1a,b, disordered, rough, tightly bound surfaces with very few meso- and macropores were reflected in the CBR. The layered-stack morphology observed in Figure 1a may explain the main reason why its specific surface area was much higher than the other materials. The planer arrangement of the multilayered carbon indicated the dominance of aromatic moieties in the biochar. Figure 1c reveals that the CBS surface was characterized by visible heterogeneous, porous and honeycombed structures. The progressive volatilization of organic matter during the thermal treatment resulted in deep channels and clear pores, which can interpret the formation of various morphologies [17,34]. The SEM image of CBL (Figure 1d) shows that pores of different sizes with open cell structures had developed on the surface of the carbon material. The analysis of the image revealed that the pore sizes of CBL varied from approximately 2 to 10 μm. A deformed and irregular pore wall with a flake structure was observed, indicating that the demolition of the porous structure was possibly due to the volatilization and substitution of organic matter [35].

3.1.3. XRD Patterns of Cattail Biochar

Figure 2 shows the XRD patterns for CBR, CBS and CBL. As displayed in Figure 2, the XRD patterns of all of the samples exhibit similar diffraction peaks located at approximately 28.4°, 40.5°, and 31.6°, 42.8°, corresponding to KCl and NaCl, respectively. The XRD results suggested that the basic structure and crystal phases between CBS and CBL showed no apparent differences. However, compared to CBS and CBL, the peaks for CBR became broader and weaker, which was ascribed to the porous and ultrathin structure. Notably, new characteristic peaks at 20.8°, 26.7° and 50.1° appeared for the CBR, and these peaks arose from SiO₂. This is probably because the hydroponic system is rich in silicon, which is concentrated in roots and remains in the biochar during pyrolysis in the form of oxides.

3.1.4. FTIR Spectra of Cattail Biochar

Figure 3 is the infrared spectra of the three biochar materials, with spectral ranges of 4000 cm⁻¹ to 400 cm⁻¹. The absorption peaks in the vicinity of 2115 cm⁻¹, 1880 cm⁻¹ and 673 cm⁻¹ were assigned to unsaturated C-C stretching vibrations, C=O stretching vibrations and O-H bending vibrations, respectively, for CBR, CBS and CBL [36]. The vibration intensities were basically consistent, showing that all of the materials may contain carboxyl groups, carbonyl groups and ester groups. The previous studies showed that these oxygen-containing functional groups provided adsorption sites for the coordination of heavy metals to form complexes, thereby impeding the migration of the heavy metals in the environment [17,27]. The absorption peak for cattail biochar near 977 cm⁻¹ arose from the in-plane bending vibrations of C-H bonds. The absorption for biochar from CBS and CBL occurring between 3600 cm⁻¹ and 3800 cm⁻¹ and near 1033 cm⁻¹ can be attributed to O-H and C-O stretching vibrations [37]. The intensity of the bands at 2925 cm⁻¹ and 2854 cm⁻¹, to which the –CH stretching vibration in the alkane corresponds [38], indicates that the alkanes were present on the biochar’s surface. In addition, the absorption peaks for cattail leaves appeared at approximately 1581 cm⁻¹, which was considered to be from vibrations of C=C bonds in aromatic rings [39]. Abundant functional groups increase the adsorption capacity of biochar and provide a guarantee for the better adsorption of heavy metal ions.

3.2. Study on the Adsorption Behavior of Metal Ions by Biochar

3.2.1. Effects of the Temperature and Initial Concentration on Cd Sorption by Biochar

The adsorption performance of the biochar was easily affected by the ambient temperature and initial concentration. Therefore, these studies were conducted with fixed adsorbent dosages of CBR, CBS and CBL, and by changing the temperature and initial Cd concentration. The results are shown in Figure 4. For CBR, the rate for adsorption of Cd(II) showed a rising trend as the temperature was increased from 288.15 K to 318.15 K. The final adsorption efficiency was greater than 90%. However, under the experimental conditions, the adsorption efficiency of CBS was not far behind that of CBL, implying that temperature and concentration hardly influenced their sorption capacities, which were related to the similar functional groups and crystalline forms of the biochar.

3.2.2. Isothermal Adsorption Characteristics

The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) equation models were used for the isotherm adsorption fitting. Based on ideal single-layer positioning adsorption theory, the Langmuir isotherm equation is a theoretical derivation formula [40]. The Langmuir isothermal equation is as follows:

$$Q_e=\frac{K_L{Q}_m{C}_e}{1+K_L{C}_e}$$

(3)

where Q_m (mg g⁻¹) was the maximum adsorption capacity, Q_e (mg g⁻¹) was the equilibrium adsorption capacity, C_e (mg L⁻¹) was the equilibrium adsorption concentration, and K_L (L g⁻¹) was the Langmuir isothermal equation parameter.

The Freundlich isotherm equation belongs to the empirical formula, which postulates that the process is obtained by the multi-layer adsorption of a heterogeneous surface [41]. The Freundlich isotherm equation is given as follows:

$$Q_e=K_F{C}_e{}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}$$

(4)

where K_F represents the adsorption capacity and n is the adsorption strength. Generally, K_F and n contain all of the parameters affecting the adsorption process. The larger K_F is, the greater the adsorption capacity is; the smaller n is, the greater the adsorption strength is [14].

Based on the microporous adsorption volume filling theory, the Dubinin–Radushkevich (D–R) model can predict the adsorption free energy (E_s) [42]. The non-linear form of the D–R equation is given as follows:

$$Q_e=Q_m\exp \left(-K{\epsilon }^2\right)$$

(5)

$$\epsilon =RT\ln \left(1+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$C_e$}\right.\right)$$

(6)

where ε is the Polanyi adsorption potential. K (mol² kJ⁻²) is a constant which relates to the adsorption energy, which is related to the mean free energy per mole of adsorbed mass. Thus, the adsorption free energy (E_s, kJ mol⁻¹) can be calculated from the following formula:

$$E_s={\left(2K\right)}^{-0.5}$$

(7)

The Langmuir, Freundlich and D-R adsorption isotherms of Cd(II) onto different biochars (CBR, CBS and CBL) are shown in Figure 5. The parameters towards the Langmuir (Q_m), Freundlich parameters (K_F and n) and D-R (Q_m2 and E_s) for the adsorption isotherms are given in

Table 2. Parameters of the isotherms for Cd²⁺ adsorption by biochar.

	Langmiur		Freundlich			Dubinin–Radushkevich
	R2	Qm (mg g−1)	R2	KF	n	R2	Qm2 (mg g−1)	Es (KJ mol−1)
CBR	0.911	18.808	0.824	3.719	0.475	0.987	13.599	55.133
CBS	0.939	66.756	0.908	118.710	0.706	0.979	56.634	51.205
CBL	0.954	44.215	0.915	77.024	0.545	0.976	44.594	44.992

. Due to the highest R² values listed in Table 2, the D-R model yielded the best fit due to its high correlation coefficients (R² > 0.97). All of the calculated E_s data were greater than 16 kJ mol⁻¹, indicating that the adsorption of Cd(II) by cattail biochar involves a chemisorption process [43]. The values of n resulted in an interval of 0~1, meaning the favorable adsorption of Cd(II) onto CBR, CBS and CBL. It’s worth noting that CBS and CBL have the competitive advantage in dislodging Cd(II) due to the values of 0.706 and 0.545 (n > 0.5). The high value of the coefficient (R² > 0.91) supports the deduction that cattail biochar acted on Cd(II) belonging to the monolayer adsorption from the perspective of the current state [44].

3.2.3. Dynamic Adsorption Model

In order to further clarify the adsorption characteristics of heavy metals, the static adsorption kinetic curves were fitted by pseudo first-order and pseudo second-order kinetic models, which are shown in Equations (8) and (9). The relevant parameters are shown in

Table 3. The pseudo first-order and pseudo second-order model parameters.

	Experiment (mg g−1)	Pseudo First-Order			Pseudo Second-Order
		Qe (mg g−1)	k1 × 10−3 (min−1)	R2	Qe (mg g−1)	k2 × 10−3 (g mg−1 min−1)	R2
CBR	12.640	11.623	0.635	0.706	12.616	12.378	0.985
CBS	19.905	7.430	7.160	0.870	18.650	6.120	0.999
CBL	19.955	18.984	11.020	0.987	22.957	0.660	0.937

, and the fitting results are shown in Figure 6.

$$\ln \left(Q_e-Q_t\right)=\ln Q_e-k_{1}t$$

(8)

where Q_t and Q_e are the adsorption capacity at time t and equilibrium, respectively, mg g⁻¹. k₁ is the first-order kinetic equation rate constant, min⁻¹.

$$\frac{t}{Q_t}=\frac{1}{k_2{Q}_e^2}+\frac{t}{Q_e}$$

(9)

where k₂ is the rate constant of the pseudo second-order kinetic equation, g mg⁻¹ min⁻¹.

Table 3 shows that the whole adsorption process for CBR and CBScan be described well by a pseudo second-order kinetic equation (R² > 0.98), which gave a higher fit than the first-order model. The linear fitting parameters directly explained the relationship between the fitted and experimental data. For instance, the calculated equilibrium adsorption quantities of the three biochars fitted from the pseudo second-order were displayed as 12.378, 18.650 and 22.957 mg g⁻¹, which were relatively close to the experimental values. However, for CBS, the result predicted by the pseudo first-order kinetics was actually far from the actual situation; therefore, it was difficult to evaluate the real process. According to the above analysis, a pseudo second order kinetic equation can be used to describe the whole adsorption process of Cd²⁺ by CBR, CBS and CBL, and the root part showed the best adsorption efficiency. Based on above results, CBL had a higher adsorption capacity compared to the other sorbents (CBR and CBS). It can be concluded that CBL shows strong potential as an inexpensive and easily available alternative adsorbent for the removal of heavy metals in wastewater treatment systems. Furthermore, the adsorption characteristics of CBR, CBS and JFC were compared with various adsorbents reported in the literature (

Table 4. Adsorption characteristics of Cd(II) compared with various plant-based biochars.

Feedstock	Pyrolytic Conditions	Qm (mg g−1)	Reference
S. hermaphrodita	973.15 K (240 min)	35.71	[45]
Pine bark waste	1223.15 K (120 min)	17.793	[46]
Hickory wood	873.15 K (60 min)	28.100	[47]
Buffalo weed	973.15 K (240 min)	13.369	[48]
E. crassipes	773.15 K (120 min)	36.899	[49]
Cattail-root	873.15 K (50 min)	18.808	This paper
Cattail-stem	873.15 K (50 min)	66.756	This paper
Cattail-leaf	873.15 K (50 min)	44.215	This paper

). The results show that cattail-derived biochar has higher adsorption potential than that of many corresponding materials, especially for CBS and CBL.

3.2.4. Effects of the Ion Intensity on the Adsorption of Heavy Metal Ions by Three Kinds of Cattail Biochar

In practical applications, the adsorption process is affected by many factors, and the change of the ion strength, as one of the factors, will also have different effects on the adsorption, with a complex trend [50]. The change of the ion strength may lead to the increase or decrease of electrostatic interaction between the adsorbents and adsorbents, or the formation of ion pairs with adsorbents, thereby affecting the adsorption process [31,51]. Figure 7 shows the effect of the ion intensity on the Cd²⁺ adsorption by CBR, CBS and CBL. It can be seen that when using different biochars to adsorb Cd²⁺, the ionic strength has different influences. The adsorption of Cd²⁺ by CBS and CBL was affected by the ion intensity in a basically consistent manner. When the ion intensity gradually increased from 0, the adsorption capacity of the two biochars slightly decreased, which showed that the adsorption action was not affected by the variation of the ion strength. But for CBR, the adsorption capacity changed irregularly with the increasing ion strength. This may be due to the enrichment of various ions in the root, which is related to the interaction of heavy metals.

3.3. Alkali and Alkali Soil Metals Affected the Cd²⁺ Sorption on Biochar

Aquatic plants, in the growth process, absorb some inorganic components to meet their own nutritional and life needs. When biochars are formed by the pyrolysis of the biomass and are used in adsorption process, the inorganic components released can be used as nutrient or adsorption sites, and some can even co-precipitate with heavy metals to realize the removal behavior [39]. In this paper, the influence of the ion strength on the leaching of alkaline earth metals (mainly K, Mg and Ca) from biochar was analyzed, and the change of the leaching amount with time was explored.

Figure 8 shows the effect of the ion intensity on the amount of inorganic components extracted from cattail biochar. As can be seen, the overall leaching amount of the three elements was basically consistent. The leaching amount of K was the highest, which gradually increased at the beginning and then leveled off, and finally decreased slightly. The leaching amount of Mg and Ca changed little with the ionic strength. The amounts of K and Ca were the largest for the CBS, followed by CBL and CBR. The amount of Mg increased in the following order: CBL < CBS < CBR.

Figure 9 shows the change of the leaching amount of inorganic components over time. It can be seen that in the three kinds of biochar, the leaching amount of K, Mg and Ca elements first increased and then tended to balance, among which the amount of K increased almost linearly in the first 30 min of CBR. Under the same conditions, the amount of K extracted from CBS reached 73.214 mg g⁻¹, followed by CBL with 55.087 mg g⁻¹, and then CBR as 33.254 mg g⁻¹. Compared with CBR (1.212 mg g⁻¹), the amounts of Mg extracted from the stem and leaf were up to 3.058 mg g⁻¹ and 6.537 mg g⁻¹, respectively. For Ca (1.912 mg g⁻¹), the amount of biochar extracted from the cattail root was the smallest, and the amount of biochar extracted from the cattail stem and leaf was not much different, i.e., about 8.344 mg g⁻¹ and 10.339 mg g⁻¹, respectively.

3.4. Mechanisms of Cd(II) Removal by Cattail Biochar

On account of the adsorption data fitted through thermodynamics and kinetics, we hypothesized that the mechanism of the metal ion sorption by the biochar is complex, and is probably a combination of external mass transfer and intraparticle diffusion through the pore distribution structure of the biochar in the sorption process.

First, Cd(II) species were transferred to the surface of the biochar with the help of a mass transfer driving force (Table 1). Second, with the participation of hydrogen ions and electron donors from the biochar (porous structure, protonated carboxylic and hydroxyl groups; Figure 1 and Figure 3), the cations were partly replaced by Cd(II) through ion exchange, and mineral precipitation occurred simultaneously [52]. Finally, some of the converted soluble cations were released to the solution (Figure 2 and Figure 8), with the rest being complexed with function groups in the absorbent. Whether the instantaneous reaction on the surface of the solid–liquid or the whole adsorption process is dominated by various chemical reactions, the adsorption rate is decided by the concentration of the adsorbent and the number of active sites on the surface of the adsorbent.

4. Conclusions

This paper studied the Cd(II) adsorption capacities of root, stem and leaf biochar derived from cattail. The adsorption characteristics were analyzed with isotherm and kinetic models, and the effects of the ion intensity were studied in order to explore the influence of Cd(II) adsorbed on CBR, CBS and CBL. Additionally, the different effects of the leached alkali (soil) metals from cattail biochar were also investigated. The equilibrium data were better fitted with D-R and Langmuir isotherm models, which confirms that the Cd(II) removal was due to chemisorption with monolayer adsorption. The maximum adsorption capacity was obtained with the application of the Langmuir isotherm model as 66.756 mg g⁻¹ of CBS, which is a comparatively good adsorption capacity. The kinetics results showed that the CBR, CBS and CBL represent better fitting results according to the pseudo second-order kinetic model. The mechanism of the Cd(II) sorption by the biochar is complex, and probably involves a combination of mass transfer, ion exchange and mineral precipitation through the macropores and micropores of the biochar in the sorption process. Due to the high potassium content contained in the plant, the three kinds of biochar exhibited the highest K leaching concentration. Potassium has the strongest potential for replacement by Cd(II). CBS and CBL would be more suitable for utilization as adsorbents for their more superior performances, while CBR can be applied to improve the soil condition and increase the carbon sequestration on the basis of its higher carbon yield. This also provides a direction to maximize the utilization of biochar pyrolyzed from aquatic plants for desirable reclamation.