Influence of Synthesis Methods on the High-Efficiency Removal of Cr(VI) from Aqueous Solution by Fe-Modified Magnetic Biochars.

Fe-modified biochars have been widely used in removal of Cr(VI) from water due to the resulting modified surface functional groups and magnetization property. However, few studies have synthetically investigated modification methods and synthesis parameters on the improvement of the removal efficiency of Cr(VI) by Fe-modified biochars. Herein, 10 types of corn straw-based magnetic biochars were produced using pre-modification and post-modification methods with various modifier ratios, and the highest heating temperature (HHT). Cr(VI) removal results suggest that the removal efficiency of pre-modified biochars ranged from 50.7 to 98.6%, which was much higher than that of post-modified (6.6-21.6%) and unmodified biochars (0.4-7.6%). The effect of synthesis methods on Cr(VI) adsorption was in the following order: Fe-modification method > modifier ratio > HHT. The adsorption kinetics and isotherm results of three types of pre-modified biochars were well fitted with the pseudo-second-order model (R 2 > 0.99) and the Langmuir adsorption model (R 2 > 0.99), respectively, indicating the surface homogeneity of the pre-modified biochars and unilayer chemisorptions of Cr(VI). Characterization results show that iron oxides or zerovalent iron particles were successfully deposited onto the surface of biochars and magnetism was introduced. A good Pearson correlation (r = -0.9694) between the removal efficiency and pH value in modified biochar suggests that the lower pH value may offer more positive charges and promote electrostatic attraction. Therefore, the dominant mechanism for enhanced Cr(VI) adsorption on pre-modified biochar was electrostatic attraction, resulting from its distinguished acidity nature. Our findings provide new insights into the high-efficiency removal of Cr(VI) onto Fe-modified magnetic biochars and will benefit future design of more efficient magnetic biochars.

Introduction

The forms of element chromium (Cr) in the environment are hexavalent (Cr VI) and trivalent (Cr III). Cr(VI) is 500 times larger toxic to living organisms than Cr(III).[1] Cr(VI)-containing wastewater is discharged from general industrial processes, such as metal processing, pigments, manufactures of chromium salt, electroplating, catalysis, magnetic tapes, and leather tanning, etc.[2] Therefore, it is important to realize the removal and control of Cr(VI) from wastewater.[3] Various methods applied to remove Cr(VI) from wastewater include chemical precipitation, ion exchange resins, ultrafiltration, and adsorption among others.[3] Of these methods, the adsorption method is easy, economic, and efficient.[1,4] However, there is a lack of multifunctional adsorbents with sufficient adsorption capacity and easy separation for further Cr(VI) adsorption.[5]

Biochar is a low-density charred carbon material produced from the thermochemical conversion of biomass,[6,7] which has been applied to wastewater metal adsorption due to its considerable surface area, high porosity, surface functional groups, cation exchange capacity, and pH buffering ability.[8] Although, most of biochar has a net negative surface charge[9] and provides only a limited capacity to adsorb dichromate anions (HCrO4–, CrO42–, and Cr2O7[2]).[10] Additionally, the powdered biochars are not easy to separate from wastewater, which limits the large-scale application of biochars in wastewater treatment.[10] Hence, in order to change the negatively charged surface of biochar and improve the adsorption capacity for Cr(VI), magnetic particles (i.e., Fe0, γ-Fe2O3, Fe3O4, etc.) have recently been introduced into biochar matrices to form magnetic biochars.[11,12] Due to the modifier of iron and iron oxides that can react with oxyanions and superb magnetization in modified biochars, magnetic biochars play an important role in the removal of anions. Several Fe modification methods have thus been applied to develop magnetic biochars to enhance their adsorption of Cr(VI). For example, iron oxide–biochar composites produced by either pyrolyzing iron chloride-modified biomass or precipitating Fe3+/Fe2+ on biochar surfaces significantly enhanced the Cr(VI) adsorption capacity of the pristine biochars.[1,2,13] Thus, Fe-modified biochar could be used as a better alternative adsorbent to adsorb and reduce negatively charged dichromate anions as compared with existing works of valuable biochars derived from biomass feedstock. The reason for the improved Cr(VI) adsorption capacity may be not only the conventional physical and chemical adsorption such as electrostatic interactions and metal complexation but also the special redox reaction developing between Cr(VI) and the Fe-modified biochars. This redox reaction may result from the formation of a zero-valence iron(ZVI)/Fe3O4 core through pyrolysis acting as an electron donor, which could cause the reduction of Cr(VI) to Cr(III).[14]

However, previous studies reported that the properties of magnetic biochars were greatly dependent on the properties of their biomass, metal nanoparticles, modification methods, and process parameters including the modifier ratio and pyrolysis temperature.[5] First, biochar modification with different nanoparticles may change the chemical nature, surface functional groups, and surface area of the biochar that may influence Cr(VI) removal.[15−17] Second, various modification methods significantly influenced the metal removal efficiency; for example, previous study showed that phenol removal efficiency was in the order: pre-treated > post-treated > untreated.[18] Also, the modification method greatly influenced the pH nature in Fe-modified biochar in aqueous solutions and then changed the adsorption capacity for metals, which was dependent on the pH in water.[11] The pH values in solutions play a key role in Cr(VI) removal by influencing the existing form of chromium.[19] The reason may be that electrostatic interactions are prominent for the removal of anionic metals at lower pH, whereas a higher pH is suitable for removal of cationic metals by electrostatic interactions.[5] However, the wide range of pH values in Fe-modified biochars can be due to (1) the different methods for synthesis of Fe-modified biochar, (2) the various raw materials used for Fe-modified biochar, and (3) the different physicochemical properties of Fe-modified biochar.[11] However, few studies systematically disclosed the connection between the Cr(VI) removal efficiency and pH property in Fe-modified biochars via a pre-modification method or post-modification method. Third, the pyrolysis temperature played a key role in the porous structures and magnetic properties of magnetic biochar.[6,9,20] On the one hand, Liu et al. found that the Cr(VI) adsorption capacities of magnetic biochar synthesized at various temperatures (650–800 °C) were increased with increasing pyrolysis temperature.[21] On the other hand, Chen et al. reported that the increased pyrolysis temperature not only decreased the adsorption capacity due to both the characteristics of pollutants and surface chemistry of biochar but also led to Fe3O4 evolving into FeO resulting in being less ferromagnetic.[22] Lastly, the literature showed that the ratio of metal to biomass/biochar should be optimized as a higher ratio may result in the metal particles covering most of the active sites of the biochar and greatly influencing adsorption capacity.[5,6] However, to the best of our knowledge, a comprehensive study toward suitable modification methods and optimum synthesis parameters, such as pre-modification, post-modification, pyrolysis temperature, and the ratio of Fe to C, on the removal of Cr(VI) from aqueous solution by Fe-modified biochars has not been achieved to date.

The objective of the present study was two-fold. First, 10 types of biochars using various modification methods, pyrolysis temperature, and the mass ratio of biochar to iron were prepared to enhance the Fe-modified biochar adsorption capacity for Cr(VI). Second, characterization and the Pearson correlations between the Cr(VI) removal efficiency and biochar properties were carried out to demonstrate the dominant mechanism for enhanced Cr(VI) adsorption on Fe-modified magnetic biochars.

Results and Discussion

Screening of Fe-Modified Biochar

The effects of the synthesis methods and additional amounts on the removal of Cr(VI) by Fe-modified biochars were determined with the results shown in Figure . Figure a presents the highest treatment temperature (HTT) effect on Cr(VI) adsorption onto the unmodified biochars and modified biochars. For unmodified biochars, BC@500 had a Cr(VI) removal efficiency of 7.61%, while negligible amounts of Cr(VI) onto BC@350 (< 5%) and BC@650 (<1%) were observed. The less removal efficiency for BC@650 may be due to the collapse of the carbon skeleton, the decrease of the specific surface area, and the degradation of surface functional groups with the increase of HTT.[10] For modified biochars, the removal efficiencies for 5BC-Fe@B350, 5BC-Fe@B500, and 5BC-Fe@B650 were 95.54%, 98.59% and 98.52%, respectively, while those for 5BC-Fe@A350, 5BC-Fe@A500, and 5BC-Fe@A650 was 16.30%, 14.80%, and 21.60%, respectively. This result suggested that both modification methods may improve the Cr(VI) removal efficiency, but the HTT had a slight effect on the Cr(VI) removal efficiency of modified biochars, although HTT was key for the production of biochar with the desired surface area, pore characteristics, surface functional groups, and crystalline structure.[23]

Figure 1

Removal efficiency of Cr(VI) from aqueous solution by (a) unmodified, pre-modified, and post-modified biochars produced at different pyrolysis temperatures, (b) pre-modified and post-modified biochars produced at different mass ratios of biochar to iron, and (c) pre-modified biochar (5BC-Fe@B500) with different dosages.

Figure a also shows the effects of the pre-modification or post-modification method on the Cr(VI) adsorption. Interestingly, it was evident that, under the Fe/C ratio at 5:1 and HTT at 500 °C, the pre-modified biochar contained several orders of magnitude higher Cr(VI) removal efficiency (95.54–98.59%) than the post-modified biochars (14.80–21.60%). This result may be due to the different FeO crystal structures and the amount of loading supported on the biochar surface by two modification methods.[24] This finding indicates that the Cr(VI) adsorption mechanism onto the modified biochar may include not only physical adsorption but also redox reactions, ion exchange, electrostatic attraction, precipitation, and complexation.[6]

The effect of the modifier ratio on Cr(VI) removal is shown in Figure b. The modifier ratio had a significant effect on the Cr(VI) removal efficiency. With an increase of the mass ratio of C to Fe from 1:1 to 10:1, the Cr(VI) removal efficiency of pre-modified and post-modified biochars first increased from 91.40 to 98.59%, and from 7.19 to 16.30%, respectively, then dropped greatly to 50.70% and 6.57%, respectively. The reason may be that the excessive proportion of C will entrap the iron and prevent the direct contact between the iron and the Cr(VI) and thus reduce the Cr(VI) removal.[6] This result was consistent with the previous literature.[16,24,25] Hence, the suitable modifier ratio will stimulate the Cr(VI) removal, and excessive Fe addition or biochar addition may cause a disadvantage over the Cr(VI) removal.[10]

Figure c shows the effect of the dosage of 5BC-Fe@B500 on Cr(VI) removal (reaction condition: pH = 4.5, Cr(VI) concentration = 20 mg/L, contact time = 12 h). The removal efficiency of the biochars under the lowest dosage of 1 g/1000 mL reached 99.1% in the first hour. Surprisingly, with increasing the dosage from 1 g/1000 mL to 5 g/1000 mL, the removal efficiency remained constant at approximately 99.1%, indicating that 1 g/1000 mL or less was the optimal dosage. As compared to a previous study (reaction condition: pH = 3, Cr(VI) concentration = 10 mg/L, dosage = 5 g/1000 mL),[2] the dosage of 1 g/1000 mL in this study presented better removal efficiency, owing to abundant surface active sites on 5BC-Fe@B500.

Adsorption Kinetics and Isotherms

Based on the above removal efficiency of various modified biochars, the pre-modified biochars pyrolyzed at 500 °C presenting higher removal efficiency were selected to study the kinetic and isotherm adsorption experiments.

As shown in Figure a, adsorption kinetics of Cr(VI) onto 1BC-Fe@B500, 5BC-Fe@B500, and 10BC-Fe@B500 presented two distinct phases: a rapid adsorption phase over the first hour and another gradually slow adsorption phase until equilibrium. The initial rapid adsorption was most likely the result of electrostatic attraction. The later slow adsorption suggested an involvement of intraparticle diffusion.[26] Pseudo-first-order, pseudo-second-order, and intraparticle models were used to fit the adsorption kinetic data, and the fitting parameters are presented in Table . As shown in Figure b, based on the correlation coefficients (R2), the pseudo-second-order model fitted BC-Fe@B500 (R2 = 0.9921), 5BC-Fe@B500 (R2 = 1.0000), and 10BC-Fe@B500 (R2 = 0.9956) best among them. This pseudo-second-order model suggests that the Cr(VI) adsorption onto Fe-modified biochar was mainly controlled by chemical interactions involving electron transfer between modified biochar and Cr(VI).[1,3] The k value of the Cr(VI) system with 5BC-Fe@B500 was higher than that of systems with 1BC-Fe@B500 and 10BC-Fe@B500. This suggests that the 5BC-Fe@B500 with a 5:1 modifier mass ratio achieved a larger Cr(VI) removal rate, which was consistent with the results in Figure b.[27]

Figure 2

Adsorption kinetic data (a), linear plots of the pseudo-second-order model (b), adsorption isotherm data (c), and linear plots of the Langmuir adsorption model (d) of Cr(VI) onto pre-modified biochars with different modifier ratios.

Table 1

Best-Fit Parameters for Kinetics Models of Cr(VI) Adsorption onto Pre-modified Biochars

	pseudo-first order			pseudo-second order			intraparticle diffusion
adsorbent	qe	k1	R2	qe	k2	R2	C	k3	R2
1BC-Fe@B500	2.78	1.803	0.715	3.10	0.466	0.992	0.21	0.076	0.501
5BC-Fe@B500	4.08	12.044	–0.155	4.08	25.229	1.000	0.01	0.005	–0.249
10BC-Fe@B500	2.27	1.732	0.808	2.33	2.223	0.996	0.26	0.093	0.200

Adsorption isotherms of Cr(VI) onto 1BC-Fe@B500, 5BC-Fe@B500, and 10BC-Fe@B500 are shown in Figure c. The Langmuir, Freundlich, and Temkin models were applied to fit the isotherm data, and the model parameters are shown in Table . As shown in Figure d, the Langmuir model fitted the data better, and the values of R2 for 1BC-Fe@B500, 5BC-Fe@B500, and 10BC-Fe@B500 were 0.957, 0.998, and 0.974, respectively, suggesting a monolayer adsorption process.[6,19] The maximum adsorption capacities of Cr(VI) on 1BC-Fe@B500, 5BC-Fe@B500, and 10BC-Fe@B500 calculated from the Langmuir model were 11.75, 361.01, and 23.08 mg/g, respectively, which indicates potential maximum adsorption abilities. Hence, 5BC-Fe@B500 showed several orders of magnitude of Cr(VI) adsorption capacity higher than that of 1BC-Fe@B500 and 10BC-Fe@B500. This result may be due to the suitable acidity, abundant functional groups, and magnetic structures on 5BC-Fe@B500, which may cause the synergistic effects on the Cr(VI) adsorption.[2,19,28]

Table 2

Best-Fit Parameters for Isotherm Models of Cr(VI) Adsorption onto Pre-modified Biochars

	Langmuir			Freundlich			Temkin
adsorbent	kL	qm	R2	kF	n	R2	A	b	R2
1BC-Fe@B500	0.016	11.754	0.957	1.309	2.971	0.734	0.336	1286.215	0.869
5BC-Fe@B500	0.001	361.011	0.998	0.404	1.217	0.955	0.045	135.668	0.911
10BC-Fe@B500	0.005	23.084	0.974	0.522	1.863	0.849	0.097	746.344	0.989

Characteristics’ Analysis

The detailed analysis of the various synthesis methods on the physicochemical characteristics are as follows. BET, SEM, and TGA characterization results are discussed in Section S-I of the Supporting Information. In general, BET results in Figure S1 show that all modified biochars contained mesoporous structures, which were beneficial to the accessibility of Cr(VI) to the adsorption sites, and due to that, the mesoporous structure could promote the transportation and diffusion of electrolyte ions.[29] SEM images in Figures S2–S6 suggests that the inorganic salts FeCl3·6H2O or FeSO4·7H2O were successfully transformed to microparticles onto pre-modified or post-modified biochar surfaces, respectively, resulting in magnetic carbonaceous materials as expected. TGA results in Figure S7 show that there was a steep and steady weight loss in the pre-modified magnetic biochars in the range of 450–800 °C due to the degradation of Fe2O3HCl,[10,30] indicating the better association of the iron composite in the pre-modified biochars, compared to post-modified biochars.

Biochar physical and chemical properties are presented in Table S1. Pre-modified biochars are acidic in nature with a pH range of 1.85–3.01 and have a strong oxidized characteristic with ORP values at the range of 185.50–251.00 mV. Conversely, all unmodified and post-modified biochars presented alkalinity with a pH value of 10.02–10.20 and 8.47–9.58, respectively, and their ORP values were negative. The decreases in pH and alkalinity of pre-modified biochars are likely due to H+ generation from hydrolysis and precipitation of aqueous Fe(III).[27] Hence, the acidic nature of the pre-modified biochars indicates that the surface functional groups such as hydroxyl and carboxyl groups are dominated by positive groups rather than negative groups.[18] The presence of positive hydroxyl and carboxyl groups on pre-modified biochars is known to have electrostatic attraction with the negative dichromate anions (HCrO4–, CrO42–, and Cr2O72).[1,31] Meanwhile, the OH–1 ions at alkaline conditions may compete with Cr(VI) for active sites under strong alkaline conditions, resulting in the blocking of Cr(VI) adsorption on the surface of the modified biochar.[32] This result was consistent with the excellent Cr(VI) removal efficiency on pre-modified biochars.

To examine the surface properties of Fe-modified biochars, FTIR analysis for the samples was carried out. As shown in Figure S8, free OH of alcohols and phenols occurred at 3600–3650 cm–1, while H-bonded OH appeared as a broad peak at 3200–3500 cm–1.[33] The peaks at 2949 and 2866 cm–1 were assigned to aliphatic C–H and aliphatic CH, respectively. The peak at around 1600 cm–1 was assigned to C=C stretching vibrations,[34] and the peak at 1375 cm–1 was attributed to phenolic O-H bending, and the signal at 1100–1020 cm–1 can be ascribed to overlapping P–O and C–O peaks.[1,34] Peaks between 700 and 900 cm–1 were associated with aromatic out-of-plane C–H bending vibrations.[35] Also, the peaks at 880, 670, and 578 cm–1 were indicative of Fe-OH.[2,25,26,36] Biochar contained the characteristic band of Si–O–Si at 466 cm–1, indicating that Si–O–Si was involved in the process of Fe-modified biochar.[3] Based on FTIR analysis, there were evident peaks attributed to hydroxyl, alkenes, and alkyl groups in biochars and modified biochars, suggesting that these functional groups were engaged in the adsorption of hexavalent chromium on the surface of the prepared adsorbents.[31] This finding was also in line with the mechanism of acidity characteristics of pre-modified biochars shown in Table S1.

The crystalline structure properties of pre-modified, post-modified, and unmodified biochars are shown in Figure S9. The presence of quartz (SiO2), sylvite (KCl), and calcite (CaCO3) were confirmed in all biochars derived from corn straw pyrolyzed at 350, 500, and 650 °C.[37] Thus, solid corn straw residue is a good biochar precursor for the synthesis of magnetic biochar-based adsorbents. Furthermore, the diffraction peak at 26.2° was assigned to the (002) plane of the graphite structure due to the formation of carbon structures with some degree of graphitic order during pyrolysis under HHT. The characteristic peaks at 30.1°, 35.5°, and 57.4° were attributed to the (220), (311), and (511) reflections of magnetite (Fe3O4), respectively.[19,30] The characteristic peaks at 14.1°, 28.5°, 36.4°, and 46.9° agreed with the (020), (040), (130), and (002) reflections of iron oxide hydroxide (FeOOH), respectively. The diffraction at 44.7° was attributed to the (100) plane of cubic Fe.[19] These iron oxides may play a key role in the Cr(VI) adsorption. The observed minerals in XRD, such as quartz, calcite, and magnetite, were in agreement with the presence of Si–O–Si, C–O, and Fe–O in the corresponding FTIR spectra, respectively. Moreover, the intensity of characteristic peaks of iron oxides for pre-modified biochars was significantly stronger than unmodified and post-modified biochars. Therefore, these iron oxide-related minerals suggested that Fe3O4 particles were introduced by Fe3+ during pyrolysis or co-precipitation and ZVI particles were formed by partly Fe2+ reduction. The transformation of FeCL3·6H2O to Fe3O4 during the hydrolysis and pyrolysis process could be explained by the following reactions( R1–R6).[5,38] Meanwhile, the reduction procedure of FeSO4·7H2O to ZVI was explained by the following reaction (.[39]

As shown in Figure S10, the XRD patterns were further supported by magnetization results, which were investigated using the magnetic hysteresis loop range of 20 kOe. The saturation magnetization values for 5BC-Fe@B500, 5BC-Fe@A500, and BC@500 were found to be approximately 19.45, 6.21, and 1.00 emu/g, respectively. The results reveal that pre-modified biochars contained more ferromagnetic properties than post-modified biochars. Also, the unmodified biochars had a negligible ferromagnetic property. The magnetic substances might be Fe3O4, which were proven by the XRD results.[3,30] Furthermore, the synthesis effects on the ferromagnetic properties was in the following order: modification method ≫ mass ratio of BC to Fe > HHT. The results suggest that the pre-modification method could enhance the superparamagnetic property so as to ensure the separation characteristics of pre-modified biochars, which might be due to the production of the soft magnetic crystal structure.[3] Hence, the lower magnetic property of post-modified biochars may be due to the weak combination of ZVI and biochars, resulting in easy removal by DI water and ethanol. This finding was consistent with previous literature.[30] Interestingly, the saturation magnetization for 5BC-Fe@B500 was higher than that of pre-modified biochars with other modifier ratios. This result was consistent with the peak intensity of iron oxides in XRD patterns.

To further discover the valence states of Fe in modified biochars, the X-ray photoelectron (XPS) spectra are shown in Figure . The XPS peaks of Fe were detected in both Fe-modified samples in Figure a. The Fe 2p XPS peaks of the pre-modification biochar and post-modification biochar were obtained and are shown in Figure b,c, respectively. It can be seen that the existence form of Fe on the surface of the samples was similar between the two modification samples. The peaks with a binding energy of 711 and 725 eV corresponded to Fe3+, while the peaks of 713 and 728 eV corresponded to Fe4+, Ranjusha et al. had reported that the active component of Fe3+/Fe4+ can effectively improve the reaction activity.[40] It should be noted that the interaction between carbon-based materials and Fe oxides results in the coexistence of high and low Fe on the surface of the samples, which is obviously beneficial to improve the reaction activity of the samples. However, it can be found that the activity of the two methods is still different, which may be due to the different proportions of surface Fe3+/Fe4+.[40] By comparing the XPS spectra of the two modified samples, it can be found that pyrolysis has a direct effect on the proportion of different Fe valence states. The sample 5BC-Fe@B500 that was modified before pyrolysis showed a peak area ratio of Fe3+ to Fe4+ of 0.64:0.36, while the peak area ratio of Fe3+ to Fe4+ is 0.34:0.66 in 5 BC-Fe@A500. Obviously, the proportion of Fe4+ was decreasing in 5BC-Fe@B500, which may be due to the effect of volatile matters in the pyrolysis process. Meanwhile, it not only adjusted the proportion of Fe3+/Fe4+ but also brought better reactivity to 5BC-Fe@B500.

Figure 3

XPS spectra of representative unmodified (BC@500), pre-modified (5 BC-Fe@B500) and post-modified biochar (5 BC-Fe@A500) (a), and deconvoluted XPS Fe 2p spectra of 5BC-Fe@B500 (b) and 5BC-Fe@A500 (c).

Based on the above detailed characterization analysis, comparative analysis of the characterization results of representative unmodified, pre-modified, and post-modified biochars pyrolyzed at 500 °C is summarized in Figure . To sum it up, the acidity, the thermal mass loss of the magnetic substance, ferromagnetic behaviors, Fe–O functional group, and Fe3O4 peak intensity in pre-modified biochars were stronger compared with unmodified and post-modified biochars. Furthermore, the specific surface areas of Fe-modified biochars were not greatly influenced by various synthesis methods and were kept in the narrow range between 10.59 and 28.47 m2/g, which were much lower than activated carbons,[28] but the responding Cr(VI) removal rate of these pre-modified magnetic biochars was significantly improved to approximately 99%. Thus, the Cr(VI) removal reaction process of these Fe-modified biochars was necessary to study to disclose the removal mechanism.

Figure 4

Characterization results of representative unmodified (BC@500), pre-modified (5BC-Fe@B500), and post-modified biochars (5BC-Fe@A500): (a) SEM image of BC@500, (b) SEM image of 5BC-Fe@B500, (c) SEM image of 5BC-Fe@A500, (d) surface area and pore size, (e) pH and ORP values, (f) DTG curves, (g) FTIR spectra, (h) XRD patterns, and (i) magnetization properties of three representative unmodified and modified biochars.

Mechanism Analysis

To confirm the adsorption mechanism, the Pearson correlations between the Cr(VI) adsorption efficiency and properties of biochars with or without Fe modification are shown in Figure S11. The following increasing strength of the Pearson correlation was observable with Cr(VI) adsorption efficiency: Vtotal (|r| = 0.05) ≪SBET (|r| = 0.29) ≈ (|r| = 0.31) ≈ dpore (|r| = 0.32) < Vmicro/Vtotal (|r| = 0.47) < EC (|r| = 0.69) < magnetization (|r| = 0.74) < pH (|r| = 0.97). These trends suggest that the most important key element for an enhanced Cr(VI) removal rate is to increase the acidity character in Fe-modified biochars (Figure S11a) followed by the magnetization properties (Figure S11h). The excellent Cr(VI) adsorption capacity of the Fe pre-modified biochars stood out among the rest (Table S1), achieving higher acidity and greater magnetization shown in the characterization analysis. This confirms that the Fe pre-modification method to produce modified biochars (in our case, FeCl3·6H2O as the modifier) is among the most promising for enhanced Cr(VI) adsorption properties. Additionally, the specific surface areas of these Fe-modified biochars had a slight influence on the Cr(VI) removal capacity as the Pearson correlation was only 0.29. This result may be attributed to the fact that the Cr(VI) removal process may not depend on the physical adsorption but other reactions occurring after magnetization, which stabilized Fe oxides on the biochar surface and then caused a higher Cr(VI) removal efficiency.[41] The good metal adsorption capacity onto biochars with the pre-modified method was consistent with previous reports.[18,30]

Based on the above discussion of adsorption and characterization results and the Pearson correlation coefficient, the potential reaction mechanism for higher removal efficiency of Cr(VI) by Fe pre-modified biochars than the post-modified biochars is depicted in Table and Figure .[10,14,42,43] Generally, due to the greater acidity nature in pre-modified Fe biochars and the resulting higher electrostatic attraction capacity for the alkaline dichromate, the dominant removal process for Cr(VI) was electrostatic attraction as compared with a redox reaction, physisorption, co-precipitation, surface complexation, and ion exchange. Moreover, it may be suggested that the high-efficiency removal of Cr(VI) was possibly completed in the following steps.[10,19,28] In the first step, the pre-modified biochars presented a positive nature due to the abundance of hydroxyl and carboxyl acid groups on surface; then, negatively charged Cr(VI) was adsorbed onto the Fe pre-modified surface via electrostatic attraction, especially 1BC-Fe@B500 and 5 BC-Fe@B500. Second, the Cr (VI) anions attached to the surface were transformed to Cr(III) due to abundant electron donors (Fe–O observed from FTIR analysis and iron oxides and cubic iron observed in XRD and XPS patterns) on the surface of magnetic biochars. Third, due to the basic functional groups and mesopore structure, some other mechanisms, such as physical adsorption, ion exchange, or co-complexation, might also play an important role. Lastly, Cr(III) ions adsorbed onto the surface of magnetic biochars could be separated by an external magnetic field.

Table 3

Cr(VI) Adsorption Mechanism of Fe Pre-modified Biocharsa

reaction mode	procedure
surface electrostatic attraction	protonation processes:
	Fe–O + H2O/H+ → Fe–OH++OH–
	MBC–OH + H2O/H+ → MBC–OH++OH–
	electrostatic attraction:
	Fe–OH+ + HCrO4–/CrO42–/Cr2O72– → Fe–OH+—HCrO4–/CrO42–/Cr2O72–
	MBC–OH+ + HCrO4–/CrO42–/Cr2O72– → MBC–OH+—HCrO4–/CrO42–/Cr2O72–
surface reduction	7H+ + HCrO4– + 3Fe2+ → Cr3+ + 3Fe3+ + 4H2O
	8H+ + CrO42– + 3Fe2+ → Cr3+ + 3Fe3+ + 4H2O
	14H+ + Cr2O72– + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O
physical adsorption
surface complexation	3Fe–OH+ + Cr3+ → 3Fe–O—Cr3+ + 3H+
	3MBC–R–OH + Cr3+ → 3(MBC–R–O)−—Cr3+ + 3H+
	3MBC–CHO + Cr3+ → 3(MBC–C=O)−—Cr3+ + 3H+
	3MBC-COOH+Cr3+ → 3(MBC-COO)−—Cr3++3H+
co-precipitation	3CO32– + 2Cr3+ → Cr2(CO3)3↓
	PO43– + Cr3+ → CrPO4↓
	3SiO32– + 2Cr3+ → Cr2(SiO3)3↓
ion exchange	3MBC–R–OM + Cr3+ → 3(MBC–R–O)−—Cr3+ + 3M+
	3MBC–CMO + Cr3+ → 3(MBC–C=O)−—Cr3+ + 3M+
	3MBC–COOM + Cr3+ → 3(MBC–COO)−—Cr3+ + 3M+

MBC is referred as modified biochar.

Figure 5

Schematic illustration of the Cr(VI) removal mechanism by Fe pre-modified magnetic biochar.

Conclusions

In summary, a comparative study of Cr(VI) adsorption and characterization using 10 magnetic corn straw-derived biochars prepared in different synthesis methods was conducted. Adsorption results suggested that the removal efficiency of pre-modified biochars (50.7–98.6%) was significantly higher than that of post-modified (6.6–21.6%) and unmodified biochars (0.4–7.6%). The effect of synthesis methods on the Cr(VI) adsorption was in the following order: Fe-modification method > modifier ratio > HHT. The adsorption kinetics and isotherm results of pre-modified biochars were well fitted with the pseudo-second-order model (R2 > 0.99) and the Langmuir adsorption model (R2 > 0.99), respectively.

Characterization analysis revealed that the physiochemical properties (acidity of biochars, thermal mass loss of magnetic substance, ferromagnetic behaviors, Fe–O functional group, Fe3O4 peak intensity, and valence states of Fe) of pre-modified biochars were stronger compared with unmodified and post-modified biochars. Furthermore, the high Pearson correlation coefficient (r = −0.9694) between the pH value of modified biochars and removal efficiency and the low coefficient (r = −0.2925) for the BET surface area indicated that electrostatic attraction was the dominant mechanism for enhanced Cr(VI) adsorption on pre-modification magnetic biochars, but other adsorption mechanisms including redox reactions, ion exchange, surface complexation, co-precipitation, and physical adsorption should not be ruled out.

The pre-modification synthesis method and suitable Fe/C ratio could promote the formation of Fe3O4 and improve the Cr(VI) removal efficiency. Moreover, the Fe pre-modification synthesis method employed in this study was simple and cost-effective and can be an alternative to produce value-added adsorbents for Cr(VI) removal in wastewater.

Materials and Methods

Preparation of Unmodified and Modified Biochars

Pre-treated feedstock: Corn straw powder was purchased from Lianyungang, Jiangsu, China, dried at 60 °C for 24 h, and sieved through 80-mesh (0.18 mm) screens for the following experiments.

Unmodified biochar (BC): Dried and sieved corn straw was pyrolyzed at 350, 500, and 650 °C for 2 h under a stream of N2 (5 mL/min flow rate) using a laboratory-scale tube furnace. Resulting biochar samples were allowed to cool to room temperature overnight under the N2 atmosphere. Biochars are hereby denoted by the biochar abbreviation and pyrolysis temperature, e.g., BC@350 represents unmodified biochar produced at 350 °C. The biochars were ground and sieved between 80-mesh (0.18 mm) and 100-mesh (0.15 mm) screens to confirm the similar particle size for the following experiments.

Modified biochar (MBC): There are two types of modification methods to prepare Fe-modified biochar, including pre-modification and post-modification.[5,17,18,27] One was to modify feedstock with FeCl3·6H2O before pyrolysis denoted as xBC-Fe@B,[44] and the other was to modify biochar with zerovalent iron (ZVI) after pyrolysis followed by a drying process, denoted as xBC-Fe@A.[43] Both modified biochars were pyrolyzed and sieved as described above. For the sample names, the letters A and B were referred as post-modification and pre-modification methods, respectively, and letter x was referred as the modifier ratio. For example, 5BC-Fe@B500 represents pre-modified biochar pyrolyzed at 500 °C with the biochar: Fe of a mass ratio of 5:1. The synthesis process is shown in Figure , and the detailed synthesis methods are shown in Section S-II of the Supporting Information.

Figure 6

Schematic illustration of the preparation of unmodified, pre-modified, and post-modified magnetic biochars.

Batch Screening Experiments

A stock solution containing 100 mg/L Cr(VI) was prepared by dissolving 0.2829 g of K2Cr2O7 in 1000 mL of deionized water. Batch screening experiments were examined by mixing 0.1 g of the adsorbent with 20 mL of 20 mg/L Cr(VI) solutions. The pH value of Cr(VI) solution was adjusted to 4.5 using a 1 M HCl or 1 M NaOH solution. The choice of an initial pH of 4.5 was because the Fe oxides in solutions could be protonated into FeOH+ under a lower initial pH, resulting in an increase of adsorption capacity of magnetic biochar due to electrostatic attraction.[3,6] Then, the mixture was shaken at 150 rpm for 12 h. After adsorption, the mixture was filtered and the concentrations of Cr(VI) in the filtrate were measured according to the PRC National Standard (GB/T 7467–1987) using an ultraviolet–visible light spectrophotometer (UV756, China) at a wavelength of 540 nm with a colorimetric method using 1,5-diphenylcarbazide.[2,19,31] The equations of adsorption capacity qe (mg/g) and the removal efficiency (%R) are shown in Section S-III of the Supporting Information.

Kinetic and Isotherm Experiments

The adsorption kinetics of selected modified biochars was determined using the above batch adsorption experiments, but the contact time was adjusted to 1, 3, 6, 12, and 24 h, respectively. In the present study, the kinetic response of Cr(VI) adsorption by modified biochar was validated with pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models.[3,10,31,45,46] The details of these kinetic models are shown in Section S-IV of the Supporting Information.

Adsorption isotherms of selected modified biochars were also determined using the batch adsorption experiments, but the initial concentration of Cr(VI) solution ranged from 0 to 600 mg/L. Equilibrium isothermal experimental results were simulated with transformed liner Langmuir, Freundlich, and Temkin isothermal models.[1−3,27] The details of these isothermal models are shown in Section S-V of the Supporting Information.

Characterization Methods

The physical–chemical characteristics of the samples, including the chemical properties, morphology, thermal degradation behavior, specific surface area (ABET), and pore characteristics, surface chemistry, crystal structure, magnetism, and the valence states of Fe have been studied by various characterization technologies, which are described in detail in Section S-VI of the Supporting Information.