Preparation of high adsorption performance and stable biochar granules by FeCl₃-catalyzed fast pyrolysis

Abstract

As a low-cost and readily available adsorbent for removal of pollutants, biochar has been intensively studied for adsorption performance and its mechanism. However, it is still far from its practical applications due to difficulties of separation after adsorbing pollutants. In this study, we significantly improved mechanical stability and adsorption performance of biochar by fast co-pyrolysis of a mixture of sawdust, FeCl₃, and kaolin. Results indicated that dosing FeCl₃ during biomass pyrolysis can drastically increase the yield of biochar, and also increase the strength of granular biochar (GBC). The GBC prepared at 650 °C with 5 mmol g⁻¹ of FeCl₃ dosage (Fe₅-GBC₆₅₀) exhibited a 4-chlorophenol adsorption capacity of 35.71 mg g⁻¹, which was twice the adsorption capacity without FeCl₃ (Fe₀-GBC₆₅₀), although pure biochars in both Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ have a maximum 4-CP adsorption capacity of 250 mg g⁻¹. The compressive strength of Fe₅-GBC₆₅₀ was 4.86 MPa, which was four times higher than that of Fe₀-GBC₆₅₀ (1.12 MPa). Moreover, the scatter ratio of Fe₅-GBC₆₅₀ was only 2.58%, which was significantly lower than that of Fe₀-GBC₆₅₀ (45.61%). Multiple characterization techniques including SEM, FTIR, XPS, and scanning acoustic microscope imaging were conducted to explain the underlying mechanism. The preferable adsorption capacity of Fe₅-GBC₆₅₀ may be attributed to catalytic decomposition of the biomass and the reductive deposition of carbon (e.g., CH₄, C₂H₄, and C₂H₂) by FeCl₃ that increases the yield of biochar and the specific surface area of GBC. The high stability may have resulted from the binding interaction of FeCl₃ products. This work may facilitate the replacement of granular activated carbon by low cost biochar.

---

Introduction

4-Chlorphenol (4-CP) is widely used as an intermediate or raw material in the synthesis of pesticides, herbicides, higher chlorinated congeners, and certain dyes. 4-CP is also a typical persistent organic pollutant once discharged in water and posing serious risks to the environment.¹ It is toxic to humans and animals even at a low concentration, and thus removing it from contaminated water is essential.² A series of methods have been studied and used for 4-CP removal, such as photocatalytic degradation,^3,4 electrochemical degradation,^3,5 degradation by different Fenton systems,^6,7 de-chlorination with reductive zero-valent metals,^8,9 and biodegradation.¹⁰ Despite the availability of all these methods, adsorption is still very widely used because it produces few secondary pollutants, exhibits high efficiency, and generally removes different types of organic and inorganic pollutants simultaneously.¹¹ Therefore, many attempts have been made to develop highly efficient adsorbents, such as activated carbon, industrial waste, resin, and clay.^12–17 Although most of these adsorbents are environmentally friendly, some disadvantages such as complex preparation and high cost still need to addressed.

Biochar, a by-product of biomass pyrolysis for generating renewable bio-oil, is widely used to remediate a contaminated environment.^18–20 Because it is inexpensive and exhibits high adsorption capacity, studies on the removal of pollutants using biochar have been intensively documented. For example, Zhu et al.²¹ studied the adsorption of single-ring organic compounds using wood char prepared under different thermochemical conditions and found that steric exclusion and π–π electron donor–acceptor interactions affect the adsorption process. Harvey et al.²² analyzed metal interactions at the biochar–water interface and found that sorption occurs predominantly via two distinct cation–π bonding mechanisms, i.e., cation–π bonding to soft ligands such as C=O and bonding with electron-rich domains on aromatic structures. Oh and Seo²³ and Shih et al.²⁴ attempted to remove 4-CP with biochar and explained that electrostatic interactions and hydrophobic sorption are the main mechanisms of sorption. On the basis of previous reports, we developed a series of modified biochar to efficiently remove pollutants such as phenol,²⁵ tetracycline,²⁶ and copper ions.²⁷ However, similar to other common powder adsorbents, fine biochar particles are difficult to recycle from a suspension. Therefore, its practical applications are significantly limited.

Besides magnetizing, adsorbent granulation is also an effective method to improve its separation properties. Granular adsorbents have been widely studied and industrially applied, particularly granular activated carbon.^28–32 Ren et al.²⁸ developed a granular biochar derived from cotton stalks and loaded with ferric oxides to remove phosphate from water, and found that both granulation and loading with ferric oxides could increase the surface areas and phosphate adsorption capacity. Song et al.²⁹ compared the efficacies of powder activated carbon (PAC) and granular activated carbon (GAC) as amendments for the immobilization of volatile compounds in soil and concluded that applying GAC was the more promising approach. Wu et al.³¹ reported a fabrication strategy of hollow spherical sludge carbon to remove organic pollutants in aqueous solution, and the adsorption performance could be controlled by shell thickness.

Motivated by the advantages of granular activated carbon, we propose the use of granular biochar (GBC) for organic pollutant removal. However, shaped absorbents do not usually exhibit good adsorption and mechanical performance because high pressure or binders, often adopted to enhance the mechanical strength of granular material, reduce porosity and adsorbate transmission. The main objective of this study was to verify the feasibility of obtaining GBC with both high mechanical stability and adsorption capacity for 4-CP. To this end, we (1) prepared GBC by dosing kaolin powder and tested its adsorption performance for 4-CP, (2) improved both the mechanical stability and the adsorption performance of GBC by adding FeCl₃, and (3) explained the interaction of the components and adsorption capacity or stability using scanning acoustic microscope (SAM) imaging and other characterization methods.

Experimental methods

Materials

FeCl₃, NaCl, Fe(NO₃)₃ (AR) and kaolinite (Al₂O₃·2SiO₂·2H₂O, CP) were purchased from Sinopharm Chemical Reagent Co., Shanghai, China, and used without further purification. Sawdust, which was from a kind of existing pine, was obtained from a wood-working factory in Hefei, China, and shattered into particles with a high-speed rotary grinder. Particles with sizes between 100 and 200 mesh were collected and dried at 80 °C overnight. Kaolin powder with particle sizes smaller than 1500 mesh was purchased from Xinyang, China. The components of the kaolin are shown in Table S1.† Deionized water was used in all the experiments.

Preparation of FeCl₃ loaded sawdust. Sawdust loaded with FeCl₃ was prepared with a rotary evaporator. Briefly, a certain amount of FeCl₃ and 10.0 g sawdust were mixed in a flask with the assistance of ultrasonic dispersion, and shaken in a constant temperature oscillator at 180 rpm for 12 h. Then, the mixture was dehydrated with a rotary evaporator at 50 °C, and dried at 80 °C overnight in drying oven. After being ground to a uniform powder, the sawdust loaded with FeCl₃ at certain loading ratios (1, 3, and 5 mmol g⁻¹) was obtained for further use.

Preparation of GBC. 3 : 1 (mass ratio) of kaolin and sawdust was uniformly mixed by adequately grinding with a glass mortar. Deionized water was then added to make the mixture doughy. The mixture was pressed into a plastic template with cylindrical holes under certain pressure. These cylindrical precursors of GBC, together with template, were dried at 80 °C for 4 h, then the granules were easily separated from the pore plate due to dehydration and shrinking. After drying at 105 °C for 12 h, the obtained granules were fast-pyrolyzed in a pyrolyzer under nitrogen flow (400 mL min⁻¹) at a certain temperature for an hour. The obtained products were denoted as Fe_x-GBC_y, where x stands for the loading ratio of FeCl₃ to sawdust (x = 0, 1, 3, or 5 mmol g⁻¹), and y stands for the pyrolysis temperature (y = 500, 650, or 800 °C).

Adsorption experiments

Batch sorption experiments were performed in 250 mL glass conical flasks with stoppers. A stock aqueous solution with a 4-CP concentration of 1.0 g L⁻¹ was prepared in advance, stored at 4 °C, and diluted to a required concentration when needed. After mixing the adsorbent and 4-CP solution, the reactor was transferred to a reciprocating shaking water bath oscillator at 25 °C and 150 rpm with a light shelter. A 3 mL aliquot of the sample was filtered through a 0.22 μm membrane at a given time interval from the flask to quantify adsorption. All the sorption experiments were performed in duplicate for each data point. The 4-CP concentrations were determined using an ultraviolet-visible spectrophotometer (UV/V-1800, Mapada Instrument Co., China) at 280 nm.

For kinetic characterization, the initial pH of a 4-CP solution was approximately 6.0 without any adjustment. In a typical run, the GBC was introduced to 150 mL solution with a 4-CP concentration of 100 mg L⁻¹. Samples were taken after 2, 6, 12, 24, 36, 48, 72, and 96 h to measure 4-CP concentrations. In some cases, the initial pH of the 4-CP solution was adjusted to 5.0 to 9.0 with 0.1 mol L⁻¹ of HCl and 0.1 mol L⁻¹ of NaOH to investigate the effect of pH. Adsorption isotherms were obtained using a batch equilibrium method. A certain amount of GBC was transferred to 150 mL of the 4-CP solution with a concentration of 30, 60, 100, 150, 200, and 300 mg L⁻¹. Samples were taken after 144 h to determine the 4-CP concentration.

Characterization and analytic methods

Structural features of the GBC samples were measured by nitrogen adsorption–desorption isotherm experiments at 77 K in a Micromeritics Gemini apparatus (Tristar II 3020M, Micromeritics Instrument Co., USA). The Brunauer–Emmett–Teller (BET) method was used to calculate specific surface areas of the samples, and their pore volumes were determined utilizing Barrett–Joyner–Halenda (BJH) method. Scanning electron microscopic (SEM) images were taken using a field-emission scanning electron microscope (Sirion200, FEI) to analyze surface morphology. Compressive strengths were determined using a Instron electronic static and dynamic fatigue testing apparatus (E3000K8953, Instron Co., USA) with a crushing speed of 0.003 mm s⁻¹. The sizes of the GBC samples were measured with a Vernier calliper. Impulse SAM equipped with a micro acoustic focal system (Laboratory of Acoustic Microscopy, IBCP, RAS) was utilized to investigate the interior microstructure of GBC. A high-frequency acoustic lens with low aperture (operation frequency of 50 MHz, 11° aperture, and focal distance of 27 mm in water) was used to generate ultra-short probe pulses. The spatial resolution was approximately 50 μm. X-ray diffraction (XRD) patterns of the prepared samples were acquired with an 18 kW rotating anode X-ray diffractometer (MXPAHF, Rigaku, Japan) using a nickel-filtered Cu Kα radiation source (30 kV/160 mA, λ = 0.154 nm). Samples were scanned from 10° to 70° with a scan rate (2θ) of 0.02° s⁻¹, and the diffraction peaks were assigned to corresponding crystalline phases using the XRD data analysis software (MDI JADE 5.0) and its corresponding powder diffraction file (PDF) database. The results of FTIR were recorded by using a VERTEX 70 FTIR (Bruker Co., Germany) spectrometer equipped with a ZnSe ATR crystal window. XPS analysis was studied with an ESCALAB 250 xi system.

Scatter ratio (SR) determination. To determine stability of the GBC in a flowing water system, SRs were obtained by conducting a simulation experiment. In a typical run, the weight of 10 sample granules was measured and recorded as M₀ after drying at 105 °C for 2 h. The granules were added to 30 mL water in a 100 mL glass conical flask. The flask was shaken in a reciprocating oscillator with a shaking speed of 200 rpm at 25 °C for 12 h. Granular remainders larger than 1.5 mm were collected using a Buchner filter, and the scraps were cleaned out with pure water. The granular remainders were dried at 105 °C for 12 h, and their weight, M, was measured. SR was calculated using the following equation:

Three parallel experiments were conducted for each kind of GBC, and the average was calculated as its SR result.

Biochar content in GBC. To determine the mass ratio of biochar, the pyrolyzed product of FeCl₃-preloaded sawdust was ultrasonically washed in 6 M hydrochloric acid (HCl) in a 60 °C water bath for 12 h to remove the iron species. After separating the solid sample using high-speed centrifugation, it was washed to neutral pH with pure water several times and dried at 105 °C overnight. The sample mass before and after treatment are represented by M_b1 and M_b2, respectively. To ensure complete removal of the iron species, the washed sample was digested for analysis of residual iron content with an atomic absorption spectrometer (AAS-4530F, Shanghai Precision and Scientific Instrument Co., Ltd., China). Biochar obtained from single sawdust pyrolysis was processed in the same manner to eliminate error caused by dissolved soluble substances. Mass ratio of the remainder was represented by R_i.

Thus, the production mass and rate of biochar in Fe₀-GBC₆₅₀ (M_a, r_a) or Fe₅-GBC₆₅₀ (M_b, r_b) can be calculated with eqn (2)–(5):

M_a = m_a − m_a0 × 3R_a × R_c (2)

where m_a0, m_a stand for the mass of Fe₀-GBC₆₅₀ before and after heat treatment at 650 °C, respectively; m_b0, m_b denote the mass of Fe₅-GBC₆₅₀ before and after heat treatment at 650 °C, respectively; R_a, R_b represent the mass ratio of sawdust in the raw material for Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ preparation, which is 25.0% and 20.8%, respectively; R_c is the production rate of granular kaolin (GK₆₅₀) after heat treatment at 650 °C.

Results and discussion

Characterization of GBC

Nitrogen adsorption–desorption analysis. Using a nitrogen adsorption–desorption method, the microstructure feature (e.g., surface area and pore structure) of the GBC was analyzed. According to IUPAC classification, the nitrogen sorption isotherms of both Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ exhibit typical type IV patterns accompanied by adsorption–desorption hysteresis (Fig. 1a). This behavior implies that the GBC is mesoporous, and thus adsorption occurs via multilayer adsorption followed by capillary condensation.³³ The hysteresis loops at high relative pressures are type H3, which suggests the presence of slit-like pores with various sizes and indicates that the shape is formed by aggregates (loose assemblages) of plate-like particles. Based on the quantity of adsorbed nitrogen at different relative pressures, the surface areas and pore volumes of the GBC samples were calculated, as shown in Table S2.† Fe₅-GBC₆₅₀ has a larger surface area (78.62 m² g⁻¹) and pore volume (0.054 cm³ g⁻¹) than Fe₀-GBC₆₅₀ (38.46 m² g⁻¹ and 0.030 cm³ g⁻¹, respectively). However, pore size decreases after FeCl₃ doping, as shown in Fig. 1b.

Fig. 1 (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀.

SEM imaging. Details on morphology and structure of the obtained samples were examined by SEM. As shown in the SEM images (Fig. 2), GBC is piled up with many plate-like, micron-sized particles. As shown in the low-magnification image, the particles in Fe₅-GBC₆₅₀ assemble much closer than those in Fe₀-GBC₆₅₀, and they appear to be bonded to each other by some adhesive. This observation corresponds to the results of the SR in the succeeding section. FeCl₃ may act as a binder that effectively enhances the strength of GBC. Higher magnification images show that Fe₅-GBC₆₅₀ is more compact than Fe₀-GBC₆₅₀.

Fig. 2 SEM photographs of GBC surface at different magnifications ((a and b): Fe₀-GBC₆₅₀; (c and d): Fe₅-GBC₆₅₀).

XRD analysis. The crystalline phases of the GBC samples were investigated with an X-ray diffractometer (XRD), and the diffraction peaks shown in Fig. 3a suggest that the original kaolin mainly consisted of kaolinite, muscovite, montmorillonite, and SiO₂ (Joint Committee on Power Diffraction Standards (JCPDS), 14-0164, 86-1384, 29-1499, and 46-1045, respectively). The peaks of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ show that high temperature treatment made the peaks of kaolinite disappear, turning into KAl₃Si₃O₁₁ (JCPDS, 46-0741), but the peaks of muscovite and SiO₂ still existed. The XRD results of GBCs with different mass ratios between kaolin and sawdust (Fig. S1†) show that peaks of magnetic species (Fe₂O₃, Fe₃C, and Fe) can be observed without kaolin addition (Fe₅-biochar), and they get weaker with an increase of kaolin content.

Fig. 3 (a) XRD and (b) FTIR spectra of GBCs; XPS C 1s spectra of Fe₀-GBC₆₅₀ (c) and Fe₅-GBC₆₅₀ (d).

FTIR analysis. FTIR spectra of GBC are shown in Fig. 3b. The strong peaks at 1041 cm⁻¹ and 1070 cm⁻¹ are ascribed to Si–O vibrations of kaolin and C–O stretching vibrations, respectively. The peaks at 3439 cm⁻¹ and 3444 cm⁻¹ in spectra of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ are assigned to O–H stretching_ENREF_1. The weak peak at 2923 cm⁻¹ corresponds to aliphatic C–H stretching vibration and the peak that appeared at 2855 cm⁻¹ can be assigned to –CH₂ stretching. The peak at 1630 cm⁻¹ can be ascribed to C=C stretching vibration in biochar.³⁴ The band at 1450 cm⁻¹ results from the carbonate, and disappears owing to carbonate dissolution during 4-CP adsorption.³⁵ The peak at 800 cm⁻¹, which only appears in spectra of Fe₀-GBC₆₅₀, can be identified as Si–O–Si bending vibration.³⁶ No strong chemical bonding was detected after 4-CP adsorption, suggesting the 4-CP adsorption onto GBC primarily relies on van der Waals' force.

XPS analysis. X-ray photoelectron spectra surveys of both Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ (Fig. S2 in ESI†) show similar peaks and contents of C 1s, O 1s, Al 2s, Al 2p, Si 2s, and Si 2p. _ENREF_5XPS spectra of C 1s shown in Fig. 3c and d indicate that both Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ comprised four peaks with differentiated binding energy values, which can be assigned to C=C (284.3 eV), graphite C (284.8 eV), C–C (285.2 eV), and C–O (286.7 eV).^37,38 As for Fe₀-GBC₆₅₀, C–C (32.3%) and graphite C (31.1%) are the main C functional groups, while they are C–C (32.8%) and C–O (30.5%) for Fe₅-GBC₆₅₀. The main functional groups and relative peak area percentages of C and O are listed in Table S3 in SI.†

Adsorption of 4-CP

Adsorption capacity and isotherms. The 4-CP kinetic adsorption performance of the obtained materials is shown in Fig. 4a. The control test shows that the volatilization loss of the adsorbate is negligible, and granular kaolin (GK₆₅₀) or FeCl₃-doped granular kaolin (Fe-GK₆₅₀) that was prepared using the same conditions and dosages as GBC exhibits low 4-CP adsorptive capacity. Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ can adsorb 4-CP in aqueous solution, but Fe₅-GBC₆₅₀ exhibits a higher significant adsorptive capacity (87%) than Fe₅-GBC₆₅₀ (55%) after 4 d under the same experiment conditions. These observations imply that the biochar moiety of GBC, not kaolin or FeCl₃, plays a major part in the sorption of 4-CP. Adding FeCl₃ during the preparation process of GBC can increase the 4-CP adsorption capacity of GBC, which can be attributed to indirect interactions. To further confirm non-restriction of the kaolin, kaolinite (the main component of kaolin) was used to prepare kaolinite-based Fe₅-GBC₆₅₀ and repeat the 4-CP adsorption experiment. As shown in Fig. S3,† the kaolinite-based Fe₅-GBC₆₅₀ presents a similar 4-CP adsorption performance to that of kaolin-based material, indicating that this work will not be restricted to the kaolin we used.

Fig. 4 (a) Adsorption kinetics of 4-CP, (b) adsorption isotherms of 4-CP by GBCs, and (c) adsorption isotherms of 4-CP by pure biochar in GBCs.

The adsorption processes of 4-CP were fitted using the Langmuir equation.³⁹ The relevant parameters are shown in Fig. 4b, where the symbols represent the experimental data and the dotted lines are the Langmuir isotherms. For both samples, the Langmuir model can model the equilibria data reasonably well, with correlation coefficients (R²) above 0.990. The maximum 4-CP adsorption capacity of Fe₅-GBC₆₅₀ (containing kaolin, FeCl₃, and biochar) is 35.71 mg g⁻¹, which is twice that of Fe₀-GBC₆₅₀ (17.54 mg g⁻¹). As shown in Fig. 4a, kaolin and FeCl₃ (Blank sample) can barely adsorb 4-CP so the sole factor in 4-CP adsorption must be pure biochar. Therefore, the adsorption capacity value of pure biochar was calculated by dividing 4-CP removal amount by biochar mass in the GBC composite. The isotherms of pure biochar in GBCs (Fig. 4c) show that the maximum 4-CP adsorption capacity of pure biochar in the Fe₅-GBC₆₅₀ composite is 250.0 mg g⁻¹, similar to that of Fe₀-GBC₆₅₀ (243.9 mg g⁻¹) and higher than that of most biochars reported (Table S4†). Combining the results of XPS and FTIR with their adsorption performances suggests that the dosage of FeCl₃ significantly increased the adsorbability of the GBC composite, but did not affect the 4-CP adsorption capacity of biochar in GBC.

Effects of FeCl₃ dosage and pyrolytic temperature. As discussed earlier, FeCl₃ significantly affected the adsorption performance of GBCs. In order to investigate the action mechanism of FeCl₃, the effects of FeCl₃ dosage were tested, and the results are shown in Fig. 5a. Increasing FeCl₃ dosages up to 3 mmol g⁻¹ improved the adsorption performance of the GBC material toward 4-CP, but adding more FeCl₃ did not enhance the adsorption further. Enlightened by our previous work, we calculated the biochar content in GBCs and found that the adsorption performance was consistent with the biochar yield level in GBC (Table S5†). When the FeCl₃ dosage increased from 0 to 3 mmol g⁻¹, the yield of biochar increased significantly from 20.6% to 48.9%, and then it slightly increased to 52.2% when the FeCl₃ dosage was increased to 5 mmol g⁻¹. Therefore, FeCl₃ overdose may restrain the adsorption performance because FeCl₃, which acts as a binder, can fill the holes in GBC and prevent 4-CP from transferring to the biochar.

Fig. 5 (a) Effect of FeCl₃ dosage on 4-CP adsorption. (b) Effect of heat treatment temperature on 4-CP adsorption. (c) Effect of pH on 4-CP adsorption.

Interestingly, not all ferric species and chlorides positively affect the adsorption performance of GBC. Control experiments were designed to investigate the contributions that catalyze biochar. The results are presented in Fig. S4.† During the process of preparing metal–salt loaded sawdust, an equal mole of chloride ions or ferric ions instead of FeCl₃ was loaded onto sawdust in the form of NaCl or Fe(NO₃)₃ or both. The follow-up preparation conditions were similar to those for Fe₅-GBC₆₅₀, and all the 4-CP adsorption experiments were conducted under the same conditions of the former batch sorption experiment. The experimental results show that the adsorptive performance of both NaCl-GBC₆₅₀ and Fe(NO₃)₃-GBC₆₅₀ are similar to that of Fe₀-GBC₆₅₀ and inferior to that of Fe₅-GBC₆₅₀. Therefore, improved biochar yield can be attributed to the synergetic effects of Fe³⁺ and Cl⁻. The performance of NaCl-(FeNO₃)₃-GBC₆₅₀, in which Fe³⁺ and Cl⁻ coexist, corroborates this assumption. The adsorption performance of NaCl-(FeNO₃)₃-GBC₆₅₀ is close to that of Fe₅-GBC₆₅₀ and is better than that of Fe₀-GBC₆₅₀. According to reports, chloride can catalyze the pyrogenic decomposition of biomass and increase the specific surface area of a product.⁴⁰ Iron has been proven to be an effective catalyst to deposit small molecular hydrocarbons, such as CH₄, C₂H₄, and C₂H₂, which are produced from biomass pyrolysis.⁴¹ In our previous work,⁴² we determined that only FeCl₃ could catalyze carbon fiber generation during sawdust pyrolysis and not CuCl₂, NiCl₂, Fe₂(SO₄)₃, or Fe(NO₃)₃. In this work, FeCl₃ exhibited a similar effect on the carbon-depositing process and increased the yield of biochar, but it did not catalytically produce nanofibers. Briefly, just as shown in Fig. S5,† when the sawdust was pyrolyzed with FeCl₃, Cl⁻ can catalyze the pyrolysis of lignin, cellulose, and hemicellulose to release more small molecular hydrocarbons. Meanwhile, the dispersive iron species can in situ catalyze their deposition on biochar, and thus increase the yield of biochar, which leads to the enhancement of adsorption capacity of Fe₅-GBC₆₅₀ compared with Fe₅-GBC₆₅₀.

Fig. 5b shows that the pyrolytic temperature slightly affects the adsorptive performance of GBCs. Reports have mentioned that increasing pyrolytic temperature decreases biochar yield, whereas the surface area of biochar generally increases until it reaches the temperature at which deformation occurs; thus, the surface area is subsequently reduced.⁴³ Generally, a higher biochar yield and larger surface area facilitate adsorption. Therefore, the sorption superiority of Fe₀-GBC₆₅₀ over Fe₀-GBC₅₀₀ or Fe₀-GBC₈₀₀ may be attributed to its high biochar yield and large surface area. Conversely, the sorption superiority of Fe₅-GBC₅₀₀ over Fe₀-GBC₆₅₀ or Fe₀-GBC₈₀₀ may result from its higher biochar yield at a relatively lower temperature.

In practical applications, pH varies from 5.0 to 9.0. Fig. 5c shows that the pH of 4-CP aqueous solution has a slight influence on the adsorption process. Therefore, GBC can be used in most surface water or types of soil.

Stability of GBC

Aside from its adsorptive performance against organic pollutants, the chemical and physical stabilities of GBC are crucial factors that affect its environmental application. We used SR (eqn (1)) and compressive strength to assess the physical stability of GBC.

Scatter ratio. The SRs of different GBCs under different conditions are listed in Table 1, which correspond to the concentrations of suspended substance (R_SS) shown in Table S6.† As shown in the table, SR decreases with increasing FeCl₃, and elevating the pyrolytic temperature can also reduce the SRs of GBCs. The SR of Fe₅-GBC₆₅₀ is 2.58%, which is significantly lower than that of Fe₀-GBC₆₅₀ (45.61%). This finding indicates that Fe₅-GBC₆₅₀ is more recyclable than Fe₀-GBC₆₅₀. Ferric species and chlorides also affect SR. Both NaCl-GBC₆₅₀ and Fe(NO₃)₃-GBC₆₅₀ exhibit even worse performance than Fe₀-GBC₆₅₀, and the SR of NaCl-(FeNO₃)₃-GBC₆₅₀, where Fe³⁺ and Cl⁻ coexist, is between that of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀. These results indicate that the positive effect of Fe³⁺ and Cl⁻ on the SR of GBC only occurs when both ions are present. An aqueous solution with a pH of 5.0 to 9.0 slightly affected the SR of GBC. This result implies that GBC, particularly Fe₅-GBC₆₅₀, can stably work in practical applications (Table S7†).

Table 1 Scatter ratios of different kind of GBCs

Sample	SR (%)	Sample	SR (%)
Fe0-GBC500	47.87	Fe5-GBC500	3.02
Fe0-GBC650	45.61	Fe5-GBC650	2.58
Fe0-GBC800	44.74	Fe5-GBC800	1.79
Fe1-GBC650	11.73	NaCl-GBC650	70.91
Fe3-GBC650	6.63	Fe(NO3)3-GBC650	86.10
		NaCl-Fe(NO3)3-GBC650	29.06

---

The combination of clay and Fe₂O₃ or iron ore has been reported to enhance the mechanical strength and physical stability of materials by forming a hardened gel or crystals.^44,45 Enhancing the physical stability of GBC by dosing with FeCl₃ may result from a similar mechanism. FeCl₃ and its derivatives may form a solid bridge of hardened gel or crystals to strengthen the particle contact points during high-temperature treatment. Therefore, the mechanical stability of GBC is enhanced.

Compressive strength. Compressive strength is commonly used to determine the mechanical stability of solid materials. The compressive strength test was conducted using randomly selected 3 Fe₅-GBC₆₅₀ and Fe₀-GBC₆₅₀ particles. The results of the compressive strength tests of Fe₅-GBC₆₅₀ show that the compressive stresses of different particles slightly change (Fig. 6). This change could be caused by the incomplete distribution of the Fe/Cl components. By contrast, different particles without Fe/Cl (Fe₀-GBC₆₅₀) almost have the same compressive stresses. This finding suggests that Fe/Cl components significantly affect the mechanical stability of GBC. The average maximum compressive stress of Fe₅-GBC₆₅₀ is 4.86 MPa, which is four times higher than that of Fe₀-GBC₆₅₀ (1.12 MPa). This result demonstrates the dominant effect of Fe/Cl components with the mechanical stability of GBC. On the basis of these SR results, we can infer that Fe₅-GBC₆₅₀ has a high physical stability and can steadily work even with a turbulent water system. Similar to the SR result, NaCl-GBC₆₅₀ and Fe(NO₃)₃-GBC₆₅₀ have smaller compressive strengths than Fe₀-GBC₆₅₀, and the comprehensive strength of NaCl-(FeNO₃)₃-GBC₆₅₀ is between that of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ (Table S8†). Based on the results mentioned earlier, FeCl₃ can be concluded to be an effective binder to shape kaolin and biomass powder, and simultaneously improve the adsorptive capacity of GBC.

Fig. 6 Performances of GBCs on compressive stress. Table insert lists the average result of each kind of GBC.

Ultrasonic scanning microscopy analysis

The SAM method is a technique used for investigating the bulk microstructure of GBC materials, as it allows the probe beam to penetrate into a specimen body using a special acoustic focused system. Ultrasound waves penetrate into a solid bulk material and reflect on any internal interface of the microstructure inside the specimen. Using these reflected ultrasound waves from the subsurface of GBC samples, acoustic images can be obtained to observe the internal structure of GBCs (Fig. 7a).⁴⁶ Fig. 7b shows an A-Scan reflection signal from one point in the GBC sample. Similarly, the reflection signals can be gathered from B- and C-Scans.

Fig. 7 SAM images of Fe₀-GBC₆₅₀ (c and e) and Fe₅-GBC₆₅₀ (b, d and f). (a) is the simulated diagram of different scanning modes; (b) is A-Scan image; (c and d) are C-Scan images; (e and f) are part 3D images of GBCs.

Fig. 7c and d are the subsurface images of Fe₀-GBC₆₅₀ and Fe₅-GBC₆₅₀ particles, respectively. The bright point indicates that the signal is reflected and that a phase interface exists, whereas the dark area confirms the presence of a uniform phase that cannot reflect signals. Compared with that of Fe₀-GBC₆₅₀, the white area of Fe₅-GBC₆₅₀ is more dispersed. This phenomenon corresponds to the BET analysis that indicates that Fe₅-GBC₆₅₀ has a larger surface area than Fe₀-GBC₆₅₀. The carbon catalyzed by FeCl₃ and deposited on the surface of the GBC particles could have formed the more dispersed phase interfaces. The binder bonded some particles together to form localized and acoustically uniform media, resulting in more dispersed dark areas. The 3D images provide more monolithic and microcosmic information about the subsurface structure of GBCs. Based on the marginal area of the cube image, the distribution of the simulated particles in Fe₅-GBC₆₅₀ (Fig. 7f) appears significantly closer and larger in a 3D space than that in Fe₀-GBC₆₅₀ (Fig. 7e). These findings are consistent with the SEM result and give reasonable evidence that FeCl₃ and its resultants can bind biochar particles more effectively. The lack of smoothness of the GBC specimen causes edge diffraction on the surface⁴⁷ and can influence the detected depth. The depth of the subsurface ultrasonic signal reach is approximately 1 mm under the front surface because of the intrinsic property of GBC.

Conclusions

We demonstrated a promising method to prepare GBC with high mechanical stability and adsorption performance. The results indicate that the dominant factor influencing 4-CP adsorption of GBC is FeCl₃ and its resultants during pyrolysis. Dosing with FeCl₃ increases the adsorption capacity of 4-CP and significantly enhances the physicochemical stability of GBC as well. GBC prepared at 650 °C with 5 mmol g⁻¹ of FeCl₃ dosage (Fe₅-GBC₆₅₀) exhibited a 4-chlorophenol adsorption capacity of 35.71 mg g⁻¹, which was twice the adsorption capacity without FeCl₃ (Fe₀-GBC₆₅₀). But the maximum 4-CP adsorption capacity of pure biochar in the Fe₅-GBC₆₅₀ composite can reach 250.0 mg g⁻¹. The compressive strength of Fe₅-GBC₆₅₀ was 4.86 MPa, which was four times higher than that of Fe₀-GBC₆₅₀ (1.12 MPa). Moreover, the scatter ratio of Fe₅-GBC₆₅₀ was only 2.58%, which was significantly lower than that of Fe₀-GBC₆₅₀ (45.61%). Furthermore, both adsorption capacity and mechanical strength are barely affected by pH (5.0 to 9.0). A series of characterizations, including SEM, nitrogen adsorption–desorption, compressive stress, SR, and SAM, demonstrated that FeCl₃ dosage could significantly improve the microstructure of GBC. The excellent adsorption performance may be attributed to the catalytic deposition of carbon (e.g., CH₄, C₂H₄, and C₂H₂) by FeCl₃ that increases the yield and dispersion of biochar and expands the surface area of GBC. The high stability results from the binding interaction of the FeCl₃ products.

Acknowledgements

The authors gratefully acknowledge financial support from National 863 Program (2012AA063608-01), the Key Special Program on the S&T for the Pollution Control, and Treatment of Water Bodies (No. 2012ZX07103-001).

Notes and references

1. O. Gimeno, M. Carbajo, F. J. Beltrán and F. J. Rivas, J. Hazard. Mater., 2005, 119, 99–108.
2. C.-Y. Chen and J.-H. Lin, Chemosphere, 2006, 62, 503–509.
3. Y. Hou, X. Li, Q. Zhao, X. Quan and G. Chen, Environ. Sci. Technol., 2010, 44, 5098–5103.
4. G. Huang, S. Zhang, T. Xu and Y. Zhu, Environ. Sci. Technol., 2008, 42, 8516–8521.
5. I. F. Cheng, Q. Fernando and N. Korte, Environ. Sci. Technol., 1997, 31, 1074–1078.
6. L. Xu and J. Wang, J. Hazard. Mater., 2011, 186, 256–264.
7. L. Xu and J. Wang, Environ. Sci. Technol., 2012, 46, 10145–10153.
8. J. Su, S. Lin, Z. Chen, M. Megharaj and R. Naidu, Desalination, 2011, 280, 167–173.
9. R. Cheng, J.-l. Wang and W.-x. Zhang, J. Hazard. Mater., 2007, 144, 334–339.
10. S. A. C. Lima, M. F. J. Raposo, P. M. L. Castro and R. M. Morais, Water Res., 2004, 38, 97–102.
11. S. Andini, R. Cioffi, F. Montagnaro, F. Pisciotta and L. Santoro, Appl. Clay Sci., 2006, 31, 126–133.
12. V. M. Monsalvo, A. F. Mohedano and J. J. Rodriguez, Desalination, 2011, 277, 377–382.
13. A. K. Jain, V. K. Gupta, S. Jain and Suhas, Environ. Sci. Technol., 2004, 38, 1195–1200.
14. V. K. Gupta, I. Ali and V. K. Saini, Environ. Sci. Technol., 2004, 38, 4012–4018.
15. M. S. Bilgili, J. Hazard. Mater., 2006, 137, 157–164.
16. B. Koumanova and P. Peeva-Antova, J. Hazard. Mater., 2002, 90, 229–234.
17. J.-M. Li, X.-G. Meng, C.-W. Hu and J. Du, Bioresour. Technol., 2009, 100, 1168–1173.
18. J. J. Manyà, Environ. Sci. Technol., 2012, 46, 7939–7954.
19. L. Beesley, E. Moreno-Jiménez, J. L. Gomez-Eyles, E. Harris, B. Robinson and T. Sizmur, Environ. Pollut., 2011, 159, 3269–3282.
20. D. A. Laird, R. C. Brown, J. E. Amonette and J. Lehmann, Biofuels, Bioprod. Biorefin., 2009, 3, 547–562.
21. D. Zhu, S. Kwon and J. J. Pignatello, Environ. Sci. Technol., 2005, 39, 3990–3998.
22. O. R. Harvey, B. E. Herbert, R. D. Rhue and L.-J. Kuo, Environ. Sci. Technol., 2011, 45, 5550–5556.
23. S.-Y. Oh and Y.-D. Seo, Environ. Sci. Pollut. Res., 2015, 1–11.
24. Y.-h. Shih, Y.-f. Su, R.-y. Ho, P.-h. Su and C.-y. Yang, Sci. Total Environ., 2012, 433, 523–529.
25. W. J. Liu, F. X. Zeng, H. Jiang and X. S. Zhang, Bioresour. Technol., 2011, 102, 8247–8252.
26. P. Liu, W. J. Liu, H. Jiang, J. J. Chen, W. W. Li and H. Q. Yu, Bioresour. Technol., 2012, 121, 235–240.
27. G.-X. Yang and H. Jiang, Water Res., 2014, 48, 396–405.
28. J. Ren, N. Li, L. Li, J.-K. An, L. Zhao and N.-Q. Ren, Bioresour. Technol., 2015, 178, 119–125.
29. Y. Song, F. Wang, F. O. Kengara, Y. Bian, X. Yang, C. Gu, M. Ye and X. Jiang, Environ. Sci.: Processes Impacts, 2015, 17, 74–80.
30. J.-h. Zhou, H. Zhao, M. Hu, H.-t. Yu, X.-y. Xu, J. Vidonish, P. J. J. Alvarez and L. Zhu, Bioresour. Technol., 2015, 198, 358–363.
31. Z. Wu, L. Kong, H. Hu, S. Tian and Y. Xiong, ACS Sustainable Chem. Eng., 2015, 3, 552–558.
32. T. Karanfil and J. E. Kilduff, Environ. Sci. Technol., 1999, 33, 3217–3224.
33. M. Kruk and M. Jaroniec, Chem. Mater., 2001, 13, 3169–3183.
34. X. Zhu, Y. Liu, F. Qian, C. Zhou, S. Zhang and J. Chen, Bioresour. Technol., 2014, 154, 209–214.
35. M. Tatzber, M. Stemmer, H. Spiegel, C. Katzlberger, G. Haberhauer and M. H. Gerzabek, Environ. Chem. Lett., 2007, 5, 9–12.
36. H. Aguiar, J. Serra, P. González and B. León, J. Non-Cryst. Solids, 2009, 355, 475–480.
37. N. G. Sahoo, H. K. F. Cheng, L. Li, S. H. Chan, Z. Judeh and J. Zhao, Adv. Funct. Mater., 2009, 19, 3962–3971.
38. D. Pan, L. Guo, J. Zhang, C. Xi, Q. Xue, H. Huang, J. Li, Z. Zhang, W. Yu, Z. Chen, Z. Li and M. Wu, J. Mater. Chem., 2012, 22, 3314–3318.
39. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361–1403.
40. W. J. Liu, H. Jiang, K. Tian, Y. W. Ding and H. Q. Yu, Environ. Sci. Technol., 2013, 47, 9397–9403.
41. L. A. Edye, G. N. Richards and G. Zheng, Clean Energy from Waste and Coal, 1992, vol. 515, pp. 90–101.
42. W.-J. Liu, K. Tian, Y.-R. He, H. Jiang and H.-Q. Yu, Environ. Sci. Technol., 2014, 48, 13951–13959.
43. J. Lehmann and S. Joseph, Biochar for environmental management: science and technology, Routledge, 2012.
44. H. Li, H.-g. Xiao, J. Yuan and J. Ou, Composites, Part B, 2004, 35, 185–189.
45. Y. Huang, G. Han, T. Jiang, G. Li, Y. Zhang and D. Wang, 3rd International Symposium on High-Temperature Metallurgical Processing, John Wiley & Sons, 2012, pp. 391–399.
46. S. Liu, E. Guo, V. M. Levin, F. Liu, Y. S. Petronyuk and Q. Zhang, Ultrasonics, 2006, 44, 1037–1044.
47. J. Ding, V. Levin and Y. Petronyuk, Acoust. Imaging, 2012, 31, 299–312.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22870k

---