Environmental performances of hydrochar-derived magnetic carbon composite affected by its carbonaceous precursor

Abstract

The effects of hydrochar properties on the environmental performances of its derived magnetic carbon composites have been overlooked. In the present work, various hydrochars (produced at different hydrothermal carbonization temperatures, 160–300 °C) were selected as the carbonaceous precursors. Then, magnetic carbon composites were fabricated by simultaneously carbonizing hydrochar, ZnCl₂ and FeCl₃ (namely simultaneous activation and magnetization). It was observed that a magnetic carbon composite with high porosity, acid resistance and adsorption capacity for roxarsone, and low graphitization degree was prepared from a hydrochar with low hydrothermal carbonization temperature. More importantly, strong linear correlations were obtained between hydrochar properties (recalcitrance index, H/C and O/C atomic ratios) and the environmental performances of its derived magnetic carbon composites (porosity, acid resistance, degree of graphitization, and adsorption capacity for roxarsone). The adsorption of ROX molecules onto the as-prepared magnetic carbon composites were mainly regulated by the pores of materials under certain pH of the solution. This work provides novel insights into the role of hydrochar properties in determining the environmental performances of its derived magnetic carbon composites.

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1. Introduction

In the past few years, magnetic carbon composites have been received much attention due to their facile separation and high adsorption capacity for pollutants removal.^1,2 Biomass and its derived char, an abundant and sustainable carbonaceous precursor, have been considered as a renewable material for the preparation of advanced magnetic carbon composites.^3–5

Recently, hydrochar (a solid product from the hydrothermal carbonization (HTC) of biomass) proved to be an excellent carbonaceous precursor for the fabrication of advanced carbon composite, due to its higher carbon content and lower ash content than raw biomass.^6–9 Given the low porosity of hydrochar (<10 m² g⁻¹), considerable progress has been made in the optimization of the external conditions for hydrochar activation, such as activation temperature and activator loading etc., in order to obtain a magnetic carbon composite with high porosity.^7,10–13

In the current literature, the properties of hydrochar have been reported to be greatly affected by its production conditions. For example, the carbonization degree of hydrochar increases with increasing HTC temperature.^14–16 Very recently, hydrochar was activated to prepare hydrochar-based porous carbon (namely activated carbon) by a ZnCl₂ activator, and the correlations between the hydrochar properties and the porosity of the hydrochar-based porous carbon (namely activated carbon) were also observed by our group.¹⁷

Unlike porous carbon, the magnetic carbon composite should have improved acid resistance and degree of graphitization, due to addition of iron salt (precursor of magnetic medium).¹⁸ The degree of graphitization and acid resistance of magnetic carbon composite were closely related to its adsorption capacity and practical reuse. In addition, due to the catalytic effect of iron salt, the porosity of magnetic carbon composite would be changed. Therefore, it is of great importance to explore the roles of hydrochar properties (namely internal activation conditions) in the environmental performances (porosity, acid resistance, graphitization degree and adsorption capacity) of magnetic carbon composite.

Roxarsone (4-hydroxy-3-nitrophenylarsonic acid, ROX) was selected as a model adsorbate in the examination of the adsorption characteristics of as-prepared magnetic carbon composites. ROX is an organoarsenic additive and has been widely used as herbicide, pesticide and antimicrobial growth promoter.^19,20 Most of the ROX molecules are excreted via manure with unchanged structures. It has been reported that arsenic was found within manure in China with concentrations of 89.3 mg kg⁻¹.²¹ The ROX molecule possesses low toxicity, but inorganic arsenic species, such as arsenite and arsenate, may be produced from the biogeochemical degradation of ROX.^14,15 This may result in potential risks to both the environment and human health.

Although carbonaceous materials and iron oxides exhibited great performance for organoarsenic adsorption,^22–25 the disadvantage of hard collection and high iron leaching hindered their practical application. However, magnetic carbon composites were versatile adsorbents and could provide both adsorption sites and a facile collection characteristic. Therefore, there is a need to investigate ROX adsorption onto such adsorbents from water bodies in the exploration of a facile and efficient adsorption process.

The main objective of this paper was the determination of correlations between the characteristics of hydrochar precursors and the environmental performances of their derived magnetic carbon composites. To achieve this purpose, hydrochar was produced at different HTC temperatures due to the fact that the properties (such as thermal stability) of hydrochar were more sensitive to HTC temperature than HTC retention time and feedstock type (evaluated by a heterogeneity index). Then, the resulting hydrochar materials were further activated to prepare magnetic carbon composites through a simultaneous activation and magnetization method. The hydrochar materials were quantitatively characterized with thermal recalcitrance and atomic ratios, and the resultant magnetic carbon composites were quantitatively characterized with porosity, acid resistance, degree of graphitization, and ROX adsorption capacity. The mechanism for ROX adsorption onto an as-prepared magnetic carbon composite was also examined.

2. Methods

Hydrochar preparation

Sawdust, obtained from a furniture factory, was hydrothermally transformed for the preparation of hydrochar. The cellulose, hemicellulose and lignin contents (analytical method: GB/T2677.10-1995 NREL and GBT10337-2008 NREL) of the raw material were 39.9, 19.4 and 17.7%, respectively. For each hydrothermal reaction, 15 g of sawdust and 150 g of deionized water were placed in an autoclave (250 mL) and sealed. The reactor was heated up at different temperatures (160 to 300 °C) with a constant retention time of 60 min and at an autogenic pressure of 0–8 MPa. The variations of the HTC temperatures and pressures as functions of reaction time are presented in Fig. S1.†

The collected solid material was thoroughly washed three times with 1 M hydrochloric acid (HCl) to reduce the inorganic salts and successively washed three times with deionized water. The resulting hydrochar was denoted as H–T, where H refers to hydrochar and T is HTC temperature in °C (i.e., 160, 200, 240, 270 or 300).

Simultaneous activation and magnetization process of hydrochar

The hydrochar-derived magnetic carbon composites were prepared by using the simultaneous activation and magnetization method, as shown in following suggestion. Briefly, 2 g of FeCl₃·6H₂O, 8 g of ZnCl₂ and 8 g of hydrochar were mixed in 50 mL deionized water. The FeCl₃ and ZnCl₂ were of analytical reagent (Sinopharm, China). This mixture was stirred for 24 h and then dried at 80 °C for 4 h in air. Subsequently, these dried mixtures were heated at 600 °C for 90 min under a nitrogen gas (N₂) flow of 1 L min⁻¹. The concentration of the activator (ZnCl₂) and the activation temperature were used according with previous work.¹³ The crude magnetic carbon composite was successively washed with 0.1 M HCl, ethanol and deionized water. The resulted magnetic carbon composite was denoted as MC–T, where MC refers to the magnetic carbon composite, T is the production temperature of the hydrochar (i.e., the precursor of magnetic carbon composite) in °C (i.e., 160, 200, 240, 270 or 300).

Characterizations of hydrochar and hydrochar-derived magnetic carbon composite

The hydrochar samples were characterized by thermogravimetry (TG) and derivative thermogravimetry (DTG), with heating conditions from room temperature to 1000 °C under an air atmosphere at a rate of 20 °C min⁻¹. The novel recalcitrance index for the hydrochar samples, R₅₀, was also quantitatively calculated as:
where T_50,hydrochar and T_50,graphite were the temperature values corresponding to 50% weight loss by oxidation and volatilization of the hydrochar sample and graphite, respectively. Values for T_50,hydrochar and T_50,graphite were obtained directly from TG curves that had been corrected for water and ash content. Graphite was a reference substance and purchased from Alfa Aesar (purity 99.9995%, 100 mesh). More details on the calculation of the R₅₀ index were provided in Harvey et al.²⁶

The elemental compositions of samples were analyzed with an elemental analyzer (Vario EL III). The combustion temperature was 950 °C. The functional groups of the samples were examined using Fourier transform infrared spectroscopy (FTIR, Nexus 470). FTIR spectra of samples were collected at a resolution of 2 cm⁻¹, and the wavenumber ranged from 4000 to 400 cm⁻¹. The phase structure was characterized with powder X-ray diffraction (XRD). The scan rate and step size for the XRD analysis was 4° min⁻¹ and 0.02° in 2θ. Raman spectra were obtained from the LabRam-1B spectrometer with He–Ne laser operating at a wavelength of 514 nm, and the curve fitting were performed with the combination of Gaussian line shapes that gave the minimum fitting error.

Transmission electron microscopy (TEM) images were obtained on a TECNAI-G2 (FEI) transmission electron microscope at an accelerating voltage of 200 kV. Samples dispersed at an appropriate concentration in ethanol were cast onto a carbon-coated Cu double-grid. Energy-Dispersive X-ray (EDX) spectra were measured on the TEM instrument with an EDX spectrometer.

The pore structure characteristics of samples were determined on a Tristar 3000 by nitrogen adsorption at −196 °C. The surface area was calculated from the isotherm using the Brunauer–Emmett–Teller (BET) equation. The volume and surface area of the micropores were obtained with the t-plot method. Magnetic measurement was carried out at room temperature by a vibrating sample magnetometer (VSM, MPMS, SQUID) with a maximum magnetic field of ±20 000 Oe.

The acid resistance (Fe leaching) was performed at 2000 mg L⁻¹ magnetic carbon composite with 24 h contact time at room temperature under pH 2.0 and 3.0. Additionally, in order to calculate the content of iron oxide (Fe₂O₃) particles within the as-prepared magnetic carbon composites, the extracted Fe element was determined by inductively coupled plasma (ICP, P-4010), following the acid digestion (HCl–HNO₃) procedure. The experiments of Fe leaching and Fe₂O₃ content were performed in duplicate.

Adsorption of roxarsone

Adsorption kinetics and isotherms were performed to examine the adsorption reactions of ROX. Analytical grade ROX was purchased from J&K Chemical Ltd. To examine the ROX adsorption kinetics, the initial concentration of ROX was set at 400 mg L⁻¹, and samples were taken from 0 to 24 h. Adsorption isotherms were run with six points (50 to 500 mg L⁻¹) for all magnetic carbon composites, the adsorption equilibrium time was 24 h. To initiate the experiments of adsorption kinetics and equilibrium, 0.02 g of adsorbent were added to a 60 mL glass vial, followed by a stock solution of ROX (500 mg L⁻¹, prepared in water) and a desired volume of deionized water. These vials contained 50 mL of solution and were stirred at 120 rpm at 25 °C. All adsorption experiments were performed in triplicate.

After adsorption equilibrium, the supernatant was separated from the magnetic carbon composite by filtration using a 0.22 μm polytetrafluoroethylene (PTFE) membrane filter. The concentrations of ROX were determined using an ultraviolet-visible (UV-vis) spectrophotometer (HACH, DR6000) at a wavelength of 268 nm.

3. Results and discussion

Chemical characteristics of hydrochar

The main physicochemical properties for the hydrochar samples are shown in Table 1. As expected, the yields and the H/C, O/C atomic ratios of the hydrochar samples (except for the atomic ratios of HC-300 sample) decreased gradually with increasing HTC temperature, due to the increasing carbonization extent of raw sawdust. It has been well documented that the H/C and O/C atomic ratios can be used to estimate the aromaticity and polarity of carbonaceous material, respectively.^14,15 Lower H/C and O/C atomic ratios indicated that the hydrochar sample contained more aromatic carbon and became less hydrophilic. Thus, hydrochar samples with high aromaticity and low polarity were observed at higher HTC temperatures. The variation of aromaticity of the hydrochar sample was also confirmed by the FTIR spectra, as indicated by an increased intensity in the aromatic C=C (1616 cm⁻¹) character with HTC temperature (see Fig. S2†). A more detailed interpretation for the evolution of FTIR spectra is provided in Fig. S2.†

Table 1 Yields, ash, pH, atomic ratios and recalcitrance index for hydrochar samples produced under different HTC temperatures

Sample	Yield^a (%)	Ash (%)	pH	H/C^b	O/C^c	R50 index^d
a Yields are on a water-free basis.b H/C: atomic ratio of hydrogen to carbon.c O/C: atomic ratio of oxygen to carbon.d R50 index: a new thermal recalcitrance index for carbonized materials.
H-160	71.2	0.05	4.10	1.53	0.74	0.36
H-200	56.3	0.12	4.10	1.48	0.58	0.37
H-240	39.5	0.16	3.63	1.25	0.45	0.44
H-270	29.9	0.42	3.99	1.01	0.21	0.50
H-300	24.9	0.52	4.12	1.02	0.28	0.54

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The TG thermograms in Fig. 1a indicate that there was a continuous increase in the T₅₀ value when the HTC temperature was increased from 160 to 300 °C, due to the gradual loss of labile organic matter and the production of more stable matter.²⁷ Accordingly, the R₅₀ value of the hydrochar samples increased with increasing HTC temperatures. Hence, the hydrochar materials became more thermally recalcitrant (Table 1), which was confirmed by the DTG thermograms (Fig. 1b). More details on the interpretation of the DTG curves for hydrochar is provided in Text S1.† Harvey reported that the thermal recalcitrance of carbonaceous material was mainly driven by its degree of aromatization.²⁶ This was also supported by a strong negative correlation between the H/C atomic ratio and the R₅₀ value of hydrochar, as indicated by Fig. 1c. Overall, the characteristics related to the elemental composition of hydrochar samples were in good agreement with data obtained through the TG characterization techniques.

Fig. 1 Water and ash content-corrected TG (a) and DTG thermograms (b) of hydrochar samples produced under different HTC temperatures, and correlation between H/C atomic ratio and R₅₀ index for hydrochar.

Porous textural characteristics of hydrochar-derived magnetic carbon composite

As shown in Table 2, increased production temperatures of hydrochar had a positive effect on the yield of the magnetic carbon composite, suggesting that the environmental performance of the resultant magnetic carbon composite would be affected by the type of hydrochar. Interestingly, strong correlations were obtained between the yields of the magnetic carbon composite and the TG and elemental characteristics of the hydrochar precursor, including the R₅₀ index and the H/C and O/C atomic ratios (Fig. S3†). Thus, under given activation conditions, the yield of hydrochar-derived magnetic carbon composites could be forecast accurately by the properties of their precursors.

Table 2 Yield, surface area and pore volume for different hydrochar-derived magnetic carbon composites

Sample	Yield^a (%)	SBET^b (m^2 g^−1)	Smic^c (m^2 g^−1)	Smic/SBET (%)	Vt^d (cm^3 g^−1)	Vmic^e (cm^3 g^−1)	Vmic/Vt (%)
a Yields calculated from the magnetic carbon composite’s weight to its initial hydrochar precursor’s weight.b Brunauer–Emmett–Teller (BET) surface area calculated on the basis of N2 adsorption data in the P/P0 range of 0.04 to 0.3.c Micropore surface area calculated using the t-plot method.d Total pore volume determined at P/P0 = 0.99.e Micropore volume calculated using the t-plot method.
MC-160	44.7	1470	1392	94.7	0.749	0.679	90.7
MC-200	47.7	1239	1180	95.2	0.642	0.586	91.2
MC-240	62.8	1142	1083	94.8	0.587	0.531	90.5
MC-270	74.5	953	828.3	86.9	0.528	0.409	77.6
MC-300	80.2	879	822	93.6	0.469	0.406	86.6

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The N₂ adsorption isotherms and pore size distribution of different hydrochar-derived magnetic carbon composites are shown in Fig. S4.† Obviously, the first three magnetic carbon composites (MC-160, MC-200 and MC-240 samples) exhibited a type I isotherm, which is typical for microporous materials. And the MC-270 and MC-300 samples exhibited a type IV isotherm, indicating the development of mesoporous character. The majority of pores in the as-prepared magnetic carbon composites possessed a diameter of less than 2 nm, which further confirmed the microporous structure of the resultant materials. However, there was a continuous downward shift in the N₂ adsorption of the magnetic carbon composites with increasing production temperature of the hydrochar precursor. The BET surface area (S_BET), micropore surface area (S_mic), total pore volume (V_t) and micropore volume (V_mic) of the magnetic carbon composites decreased with increasing production temperature of the hydrochar (see Table 2), suggesting that the production temperature of the hydrochar strongly affected the porosity of its derived magnetic carbon composites. This trend can be attributed to the higher carbonization extent that was developed to produce micoropores when a low-temperature hydrochar served as precursor. This explanation was consistent with the increase in the yield of magnetic carbon composite as the production temperature of the hydrochar increased (Table 2). In addition, the proportion of microporosity (indicated by S_mic/S_BET and V_mic/V_t ratios) remained unchanged with the production temperature of the hydrochar (except for the MC-270 sample).

The correlations between the porosity of magnetic carbon composites and the properties of their hydrochar precursors, including the R₅₀ index, the H/C and O/C atomic ratios, were also investigated. As shown in Table S1,† the porosity (S_BET, S_mic, V_t and V_mic) of the magnetic carbon composites was negatively correlated with the R₅₀ index of the hydrochar precursor and positively correlated with the H/C and O/C atomic ratios of the hydrochar. This indicated that the chemical activation reaction of hydrochar material by ZnCl₂ and FeCl₃ was resisted by the aromatic carbon of thermal recalcitrance. Hence, the characteristics of hydrochar were good indicators of porosity for its derived magnetic carbon composite.

Morphology, magnetic properties and acid resistance of magnetic carbon composite

The TEM image of a typical hydrochar-derived magnetic carbon composite is shown in Fig. 2, the sample was mainly composed of a multi-layer carbon sheet. The distributions of C, Fe, and O elements have been investigated by the corresponding elemental mapping images (EDX analysis). The central region of the particle appeared bright due to the presence of higher atomic weight Fe atoms.²⁸ It can be seen that Fe and O elements were uniformly distributed in the carbon surfaces. Therefore, it can be assumed that some Fe–O moieties were grown on the carbon surface.

Fig. 2 The TEM image and elemental mapping spectra of C, Fe and O elements for the hydrochar-derived magnetic carbon composite (MC-160).

As shown in Fig. S5,† the crystalline phases within the as-prepared magnetic carbon composites were iron oxides (γ-Fe₂O₃ and α-Fe₂O₃) via the thermal decomposition of FeCl₃,^13,29,30 partly confirmed the results of Fe and O elemental mapping. The peaks at 2θ of 30.0°, 35.3°, 42.9°, 53.5°, 56.9° and 62.4° were assigned to γ-Fe₂O₃. Moreover, the weak peak at 2θ of 36.5° was related to the presence of α-Fe₂O₃. The characteristics of the XRD peaks showed no apparent change on the basis of their type and intensity.

Obviously, the γ-Fe₂O₃ particles were the main magnetic composition. As shown in Fig. S6a,† weak magnetic hysteresis loops were also observed. The saturation magnetization of the as-prepared magnetic carbon composites ranged from 13.6 to 16.3 emu g⁻¹, due to their different contents of Fe₂O₃ particles (Table S2†). This magnetic characteristic ensured that the as-prepared magnetic carbon composites were suitable for the separation of particles from the aqueous solution, as indicated by the Fig. S6.†³¹

Table S2† shows the effect of the hydrochar type on the Fe leaching concentrations under acidic conditions (pH values of 2.0 and 3.0). Obviously, low-temperature hydrochar-derived magnetic carbon composites exhibited strong acid resistance, due to the lower dissolution capability of Fe₂O₃ particles under acidic conditions. As shown in Fig. S7,† the Fe leaching concentrations under acidic conditions were well controlled by the porosity of the magnetic carbon composites, especially under pH 2.0 conditions, indicating that high porosity magnetic carbon composites possessed strong acid resistance. This can be attributed to the Fe₂O₃ particles derived from a high-porosity magnetic carbon composite being well wrapped by its carbon matrix, due to the strong interaction between the Fe₂O₃ particles and the carbon matrix.

The above porosity analysis showed the porosity of magnetic carbon composites was well correlated with the properties of hydrochar. Hence, it was expected that Fe leaching concentrations of magnetic carbon composites also could be further linked with the properties of hydrochar. As anticipated, strong correlations were observed between the Fe leaching concentrations of the magnetic carbon composites and the properties (the R₅₀ index, the H/C and O/C atomic ratios) of the hydrochar precursors (Fig. 3). These correlations suggested that, acid resistance of the as-prepared magnetic carbon composites were also regulated by the properties of their precursors. It should be noted that a low-temperature hydrochar-derived magnetic carbon composite not only had high porosity, but also exhibited high acid resistance, indicating preferable practical application for pollutant removal.

Fig. 3 Correlations between Fe leaching concentration of magnetic carbon composites and the hydrochar properties, including R₅₀ index (a), H/C atomic ratio (b) and O/C atomic ratio of hydrochar.

FTIR and Raman analysis of magnetic carbon composite

As shown in Fig. 4a, the FTIR spectra of the as-prepared magnetic carbon composites showed similar characteristics and weak differences in the adsorption intensities and shifts due to different precursors. The adsorption of the C=C vibration in the aromatics group at 1596 cm⁻¹ for the MC-160, 200 and 240 samples was shifted to lower wave numbers of 1537 and 1561 cm⁻¹ for MC-270 and MC-300 products, respectively, due to the formation of weaker C–O–Fe bonds. The band assigned to the C–O vibration in ester or ether also exhibited a similar observation, also confirming the formation of C–O–Fe bonds.³² The bands at 658 and 565 cm⁻¹ (Fe–O stretching) confirmed the formation of γ-Fe₂O₃ on the composites.³³ It should be noted that the high-temperature hydrochar-derived magnetic carbon composites had higher FTIR adsorption intensities for the above two bands, possibly due to a reduced coating impact of the carbon matrix. These results further confirmed that a high-temperature hydrochar-derived magnetic carbon composite possessed weak acid resistance.

Fig. 4 FTIR (a) and Raman (b) spectra of hydrochar-derived magnetic carbon composites.

As shown in Fig. 4b, the Raman spectra of the as-prepared magnetic carbon composites consisted of two prominent characteristic peaks, namely the D band assigned to the presence of defects at ∼1332 cm⁻¹ and the G band for the graphitic sheet at ∼1590 cm⁻¹.^33,34 It was interesting to note that the G band in all the as-prepared magnetic carbon composites was located at a higher frequency than in graphite (1590 versus 1580 cm⁻¹),³⁵ indicating an interaction, such as charge transfer, between graphite and Fe₂O₃ particles.³² Because of the coating effect of the carbon matrix, the band assigned to the Fe₂O₃ (∼500 cm⁻¹) in the Raman spectra were not observed, confirming the strong acid resistance of the as-prepared magnetic carbon composites.

Typically, the relative intensity ratio of the D band to the G band (I_D/I_G) provides the degree of graphitization of the as-prepared products.³⁴ Obviously, high-temperature hydrochar-derived magnetic carbon composites possessed a lower I_D/I_G value, revealing that increasing the hydrochar production temperature substantially increased the graphitization degree of the final activation material. Hence, the graphitization degree of hydrochar-derived magnetic carbon composites was also regulated by their precursors.

To elucidate the correlations between the I_D/I_G values of magnetic carbon composites and their precursors, the I_D/I_G values were also plotted versus the R₅₀ index, H/C and O/C atomic ratios, as shown in Fig. 5. Interestingly, a linear decrease in the I_D/I_G values as a function of the R₅₀ index was observed, whereas linear increases in the I_D/I_G value as functions of the H/C and O/C atomic ratios were obtained, suggesting that a hydrochar-derived magnetic carbon composite with a low I_D/I_G value resulted from a precursor with a high degree of carbonization (indicated by high thermal recalcitrance, high aromaticity and low polarity). Hence, under given activation conditions, the graphitization degree of hydrochar-derived magnetic carbon composites could also be predicted based on the properties of the precursor material.

Fig. 5 Correlations between I_D/I_G value of magnetic carbon composites and the hydrochar properties, including R₅₀ index (a), H/C atomic ratio (b) and O/C atomic ratio of hydrochar.

Adsorption of roxarsone onto magnetic carbon composite

The pseudo-second-order kinetics and Langmuir equation were used to fit the adsorption kinetics and isotherms of ROX onto the as-prepared magnetic carbon composites, respectively, as shown in Fig. S8 and S9.† A more detailed interpretation for the ROX adsorption kinetics and isotherms is provided in Text S2.† The fitted data are shown in Table 3. The as-prepared magnetic carbon composites exhibited fast adsorption rates and large adsorption capacities for ROX removal from aqueous solutions, indicating that these materials could be excellent candidates for the design of a separable material for ROX removal.

Table 3 Regressed pseudo-second-order kinetic parameters and Langmuir equation parameters for ROX adsorption onto as-prepared magnetic carbon composites

Sample	Pseudo-second-order kinetics parameters			Langmuir equation parameters
	qe (mg g^−1)	k2 (g mg^−1 min^−1)	R^2	b (L mg^−1)	qm (mg g^−1)	R^2
MC-160	515.5	13.2 × 10^−4	0.99	0.063	588.2	0.99
MC-200	502.5	10.3 × 10^−4	0.99	0.061	555.6	0.99
MC-240	427.4	17.4 × 10^−4	0.99	0.050	500.0	0.99
MC-270	421.9	5.6 × 10^−4	0.99	0.072	454.5	0.99
MC-300	381.7	3.2 × 10^−4	0.99	0.048	416.7	0.99

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It has been well documented that the high porosity of porous carbon material can enhance the adsorption of organic pollutants, due to the pronounced pore filling effect.³⁶ As shown in Fig. S10,† strong positive correlations between the ROX adsorption uptake at equilibrium (q_e, obtained from the pseudo-second-order kinetics equation) and the porosity of as-prepared magnetic carbon composites were observed. Similar trends were observed between the ROX maximum adsorption capacity (q_m, obtained from the Langmuir equation) and the porosity of the as-prepared magnetic carbon composites (see Fig. S11†), due to the effect of pore filling. Although it has been reported that ROX adsorption onto an iron oxide composite was partly driven by surface complex interaction, including an As(V) moiety of the ROX molecule interacting with the surface of iron oxide.³⁷ However, it should be emphasized that the Fe₂O₃ particles within as-prepared magnetic carbon composites were well wrapped by the carbon matrix. Therefore, the ROX adsorption mechanism onto the as-prepared magnetic carbon composites was mainly dominated by the effect of pore filling at certain pH of the solution, as further indicated by Fig. S10 and S11.†

Fig. 6 presented the TEM-EDX elemental mappings of C, As, Fe, and O elements for the hydrochar-derived magnetic carbon composite adsorbed with ROX molecule, which indicated that As elements (typical from the ROX molecule) were homogeneously adsorbed on the carbon surface. However, the Fe elements were concentrated on the central region. This confirmed that the ROX adsorption onto the as-prepared magnetic carbon composites was mainly dominated by the carbon surface.

Fig. 6 The TEM image and elemental mapping spectra of C, As, Fe and O elements for the hydrochar-derived magnetic carbon composite (MC-160) adsorbed with the ROX molecule.

As suggested in Table S1,† the porosity of hydrochar-derived magnetic carbon composites was notably correlated with the hydrochar properties. In order to determine a direct link between ROX adsorption characteristics (q_e and q_m) and hydrochar properties, the values of q_e and q_m were also plotted against various properties of hydrochar, including the R₅₀ index, H/C and O/C atomic ratios, as shown in Fig. 7 and S12.† As expected, strong correlations between ROX adsorption characteristics and hydrochar properties were observed, and magnetic carbon composite derived from low temperature hydrochar sample exhibited high adsorption capacity for ROX removal. Hence, the adsorption performance of hydrochar-derived magnetic carbon composites was also greatly dependent on the properties of the precursor hydrochar.

Fig. 7 Correlations between q_m value (obtained from Langmuir equation) for ROX adsorption onto the as-prepared magnetic carbon composites and the hydrochar properties, including R₅₀ index (a), H/C atomic ratio (b) and O/C atomic ratio (c) of hydrochar.

4. Conclusions

Overall, the environmental performances of hydrochar-derived magnetic carbon composites, including porosity, acid resistance, degree of graphitization and adsorption capacity, was well correlated with the TG and elemental characteristics of their hydrochar precursor (produced at different temperatures). A magnetic carbon composite with high porosity, high acid resistance and low graphitization degree was derived from a hydrochar with the characteristic of low aromaticity and thermal recalcitrance, and high polarity. In addition, the as-prepared magnetic carbon composite exhibited high potential for the removal of ROX and could be easily separated from the aqueous solution.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 21407027), National Key Technology Support Program (No. 2015BAD15B06), and Shanghai Talent Development Fund (No. 201414).

References

1. B. Qiu, H. Gu, X. Yan, J. Guo, Y. Wang, D. Sun, Q. Wang, M. Khan, X. Zhang and B. L. Weeks, J. Mater. Chem. A, 2014, 2, 17454–17462.
2. S. Zhang, M. Zeng, J. Li, J. Li, J. Xu and X. Wang, J. Mater. Chem. A, 2014, 2, 4391–4397.
3. Z. H. Ruan, J. H. Wu, J. F. Huang, Z. T. Lin, Y. F. Li, Y. L. Liu, P. Y. Cao, Y. Fang and J. Xie, J. Mater. Chem. A, 2015, 3, 4595–4603.
4. A. Jain, S. Jayaraman, R. Balasubramanian and M. Srinivasan, J. Mater. Chem. A, 2014, 2, 520–528.
5. Y. L. Chen, S. Q. Chen, K. Tian, J. J. Chen and H. Jiang, J. Mater. Chem. A, 2015, 3, 9843–9850.
6. L. Dai, B. Wu, F. Tan, M. He, W. Wang, H. Qin, X. Tang, Q. Zhu, K. Pan and Q. Hu, Bioresour. Technol., 2014, 161, 327–332.
7. W. Hao, E. Bjorkman, M. Lilliestråle and N. Hedin, Ind. Eng. Chem. Res., 2014, 53, 15389–15397.
8. W. Hao, E. Björkman, Y. Yun, M. Lilliestråle and N. Hedin, ChemSusChem, 2014, 7, 875–882.
9. M. Zhang, B. Gao, J. Fang, A. E. Creamer and J. L. Ullman, RSC Adv., 2014, 4, 28171–28175.
10. M. Sevilla, A. Fuertes and R. Mokaya, Energy Environ. Sci., 2011, 4, 1400–1410.
11. A. Romero-Anaya, M. Ouzzine, M. Lillo-Ródenas and A. Linares-Solano, Carbon, 2014, 68, 296–307.
12. X. Zhu, Y. Liu, C. Zhou, S. Zhang and J. Chen, ACS Sustainable Chem. Eng., 2014, 2, 969–977.
13. X. Zhu, Y. Liu, G. Luo, F. Qian, S. Zhang and J. Chen, Environ. Sci. Technol., 2014, 58, 5840–5848.
14. A. Budai, L. Wang, M. G. Gronli, L. T. Strand, M. J. Antal, S. Abiven, A. Dieguez-Alonso, A. Anca-Couce and D. P. Rasse, J. Agric. Food Chem., 2014, 62, 3791–3799.
15. X. Cao, K. S. Ro, J. A. Libra, C. I. Kammann, I. Lima, N. Berge, L. Li, Y. Li, N. Chen and J. Yang, J. Agric. Food Chem., 2013, 61, 9401–9411.
16. G. K. Parshetti, S. Chowdhury and R. Balasubramanian, Bioresour. Technol., 2014, 161, 310–319.
17. X. Zhu, Y. Liu, F. Qian, C. Zhou, S. Zhang and J. Chen, ACS Sustainable Chem. Eng., 2015, 3, 833–840.
18. X. Zhang, Y. Li, G. Li and C. Hu, RSC Adv., 2015, 5, 4984–4992.
19. X. D. Zhu, Y. J. Wang, C. Liu, W. X. Qin and D. M. Zhou, Chemosphere, 2014, 107, 274–281.
20. M. Yoshinaga and B. P. Rosen, Proc. Natl. Acad. Sci., 2014, 111, 7701–7706.
21. L. Shi, W. Wang, S. Yuan and Z. Hu, Environ. Sci. Technol., 2014, 48, 7951–7958.
22. D. Arts, M. Abdus Sabur and H. A. Al-Abadleh, J. Phys. Chem. A, 2013, 117, 2195–2204.
23. W. R. Chen and C. H. Huang, J. Hazard. Mater., 2012, 227, 378–385.
24. J. Hu, Z. Tong, Z. Hu, G. Chen and T. Chen, J. Colloid Interface Sci., 2012, 377, 355–361.
25. S. Depalma, S. Cowen, T. Hoang and H. A. Al-Abadleh, Environ. Sci. Technol., 2008, 42, 1922–1927.
26. O. R. Harvey, L.-J. Kuo, A. R. Zimmerman, P. Louchouarn, J. E. Amonette and B. E. Herbert, Environ. Sci. Technol., 2012, 46, 1415–1421.
27. S. Kang, X. Li, J. Fan and J. Chang, Ind. Eng. Chem. Res., 2012, 51, 9023–9031.
28. L. Ling and W. X. Zhang, J. Am. Chem. Soc., 2015, 137, 2788–2791.
29. Y. Li, C. Zhu, T. Lu, Z. Guo, D. Zhang, J. Ma and S. Zhu, Carbon, 2013, 52, 565–573.
30. Y. Zhai, Y. Dou, X. Liu, B. Tu and D. Zhao, J. Mater. Chem., 2009, 19, 3292–3300.
31. J. D. Xiao, L. G. Qiu, X. Jiang, Y. J. Zhu, S. Ye and X. Jiang, Carbon, 2013, 59, 372–382.
32. J. Zhou, H. Song, L. Ma and X. Chen, RSC Adv., 2011, 1, 782–791.
33. A. P. Singh, M. Mishra, P. Sambyal, B. K. Gupta, B. P. Singh, A. Chandra and S. K. Dhawan, J. Mater. Chem. A, 2014, 2, 3581–3593.
34. H. J. Lee, W. Cho, E. Lim and M. Oh, Chem. Commun., 2014, 50, 5476–5479.
35. M. Baikousi, A. B. Bourlinos, A. Douvalis, T. Bakas, D. F. Anagnostopoulos, J. i. Tuček, K. r. Šafářová, R. Zboril and M. A. Karakassides, Langmuir, 2012, 28, 3918–3930.
36. M. Xie, W. Chen, Z. Xu, S. Zheng and D. Zhu, Environ. Pollut., 2014, 186, 187–194.
37. J. H. Kwon, L. D. Wilson and R. Sammynaiken, Materials, 2014, 7, 1880–1898.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07339a

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