Magnetic Nanoscale Zerovalent Iron Assisted ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9673-9682

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Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance Shishu Zhu,† Shih-Hsin Ho,*,† Xiaochen Huang,† Dawei Wang,∥ Fan Yang,‡ Li Wang,*,† Chengyu Wang,§ Xinde Cao,‡ and Fang Ma† †

State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin 150090, People’s Republic of China ‡ School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China § College of Materials Science and Engineering, Northeast Forestry University, 26 Hexing Road, Harbin 150040, People’s Republic of China ∥ Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Floyd Ave, Richmond, Virginia 23219, United States S Supporting Information *

ABSTRACT: It has been reported that zerovalent iron can help biochar improve efficiency in heavy metal (HM) absorption, but the surface chemical behaviors and HM removal mechanisms remain unclear. We successfully synthesized the magnetic nanoscale zerovalent iron assisted biochar (nZVI-BC). The porosity, crystal structure, surface carbon/iron atom state, and element distribution were comprehensively investigated to understand nZVI-BC’s interfacial chemical behaviors and HM removal mechanisms. We clearly revealed the formation of a nanoscale Fe0 core− Fe3O4 shell on the surface/pores/channels of biochar. With the combination of iron nanoparticles and biochar, C−O/COOH groups were cracked with the formation of CO/CC, indicating the C−O− Fe acted as an electron acceptor during the reduction reaction. We also demonstrated that the stabilization was dramatically improved in the nZVI-BC, while more reduced iron and better homogeneity were observed. These results, showing the surface chemical behaviors of nZVI-BC, would help increase our understanding of the HM removal mechanisms. Moreover, our demonstration of the superior removal ability of multiple HM (Pb2+, Cd2+, Cr6+, Cu2+, Ni2+, Zn2+) from a solution can provide a breakthrough in making a feasible material for removing HM from polluted water resources. KEYWORDS: Biochar, Nanoscale, Zerovalent iron, Heavy metal removal, Carbon sequestration

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lead to the formation of larger particles (more than 3 μm) with a significant drop in reactivity and mobility.11,12 Surface modification (e.g., CMC13), a viscosity modifier (e.g., guar gum14), supporter addition (e.g., carbon porous material15), and protective coatings (e.g., clay16) have been employed to improve nZVI’s reactivity to HMs. Recently, porous carbon materials (e.g., graphene nanosheets/nanotubes, activated carbon) have attracted much attention as supporters, owing to their high surface area and specific pore/channel structure that could prevent oxidation and aggregation. Biochar (BC), an organic porous material derived from biological waste via thermal decomposition, is a potential candidate for use as nZVI supporter. Compared with active carbon, biochar is more suitable as a platform or matrix for synthetic composites

INTRODUCTION Due to being highly toxic and easily accumulated in the human body,1,2 heavy metal (HM) pollution raises significant health concerns. Because HM ions are stable, persistent, and usually coexist in various chemical formats, the related remediation process is always challenging. The nanoscale zerovalent iron (nZVI), a well-controllable nanosized particle with environmentally friendly characteristics,3 has shown high potential to remediate various HMs due to its high reductive reactivity, large specific surface area, and massive surface absorption sites,4,5 along with its specific structure of a metallic iron core and iron oxide shell.6,7 Notably, the specific material characteristics of nZVI can be applied to capture HMs by means of reduction or coprecipitation.8−10 The highly reactive reducing agent, low dosage, low toxicity, and nontoxic intermediate/end product lead to lower environmental risk of nZVI.3 However, the intrinsic chemical and physical properties of nZVI, such as large surface area, high surface energy, and reactivity with the surrounding media, would cause severe agglomeration and then © 2017 American Chemical Society

Received: February 21, 2017 Revised: August 4, 2017 Published: September 14, 2017 9673

DOI: 10.1021/acssuschemeng.7b00542 ACS Sustainable Chem. Eng. 2017, 5, 9673−9682

Research Article

ACS Sustainable Chemistry & Engineering

Moreover, to investigate the capacity and mechanisms of the HM removal, different pH’s (4, 6, 8, and 10) of the multiple HMs were then adjusted. Adsorption isotherm models were studied using Langmuir and Freundlich models, and regeneration ability was also studied using DI water (pH = 7.0 ± 0.5) as the desorption agent. The details were shown in sections 3 and 4 in the Supporting Information.

because of its abundant surface oxygen-containing functional groups (e.g.,−OH, −COOH, C−O/CO).17 nZVI-BC composites have been widely applied in environmental remediation projects focused on organic contaminant (e.g., 2,4-dinitrotoluene) removal.18,19 However, to date there have been few investigations of HM removal by nZVI-BC. Although the superior characteristics of nZVI-BCs (e.g., magnetization, high specific surface area, and high reactivity) are known, there has been no report about systematic studies of the chemical reactions between iron atoms and the organic carbon matrix of biochar’s surface, with regard to the embedded iron core−shells and individual-textured surface structure. In addition, although huge amounts of BC absorption sites (e.g., micropores and mesopores) appear to which Fe0 nanoparticles (NPs) can attach, excess concentrations of Fe may cause the Fe NPs aggregate on the surface of BC, resulting in lower reactivity and on occasion interacting with HMs.20 Therefore, optimizing the Fe/BC impregnation mass ratio to enhance the performance of HMs removal is required. In this study, the nZVI-BC composites with different Fe/BC impregnation mass ratios were synthesized. The interfacial chemical behaviors during preparation of nZVI-BCs were then investigated via Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), BET specific surface area, X-ray diffraction meter (XRD), scanning electron microscope (SEM), and element mapping. The removal efficiencies of different HMs (e.g., Pb2+, Cd2+, Cr6+, Cu2+, Ni2+, Zn2+) by nZVI-BCs were demonstrated, along with the analysis of multiple HM adsorption isotherms. Moreover, the stability of nZVI-BC was identified, including the effect of pH and regeneration. The aim of this work was to determine the interfacial chemical behaviors occurring on the surface of biochar and the HMs removal capacity of the composites. The knowledge obtained was useful in assessing the applicability of nZVI-BC as HMs absorbents and enhancing understanding of HMs absorption mechanisms.

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RESULTS AND DISCUSSION Elemental Composition, Porosity, and Crystal Structure of nZVI-BCs. The content of C, H, and N in nZVI-BCs decreased significantly as compared with BC (Table S1, Supporting Information). The Fe content (59.12−79.67 wt %) in the nZVI-BCs approximately conformed to the corresponding Fe/BC mass ratios, while the contents of Ca, K, Na, and Mg were significantly decreased after the addition of Fe (Table S1, Supporting Information). The specific surface area of nZVI-BCs decreased from 181.2 to 53.3−105.3m2/g due to the embedding of Fe NPs.24 Figure 1 shows the XRD

MATERIALS AND METHODS

Figure 1. XRD patterns of BC and nZVI-BC with different Fe/BC impregnation mass ratios (1:1, 2:1, and 4:1, rectangle-Fe3O4, cycleFe0).

Preparation of Nanoscale Zerovalent Iron Assisted Biochar (nZVI-BC). A wetland plant reed was chosen as the biochar precursor, which owned a low ratio of ash.21 Subsequently, according to the previous study,22 the reed biochar was used to synthesize nZVI-BC with a different Fe/BC impregnation mass ratio, which synthesized with NaBH4 under anaerobic conditions. Detailed information on the synthesis process is shown in section 1 in the Supporting Information. Characterization of the Chemical Properties. The X-ray diffraction meter (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), index R50,nZVI‑BC,23 scanning electron microscope (SEM), and surface area pore analyzer were employed to identify the characteristics of the pristine and nZVI-BC. Detailed information on the synthesis process is shown in section 2 in the Supporting Information. Determination of Heavy Metal (HM) Removal. Mono HM and coexisting HMs (Pb2+, Cd2+, Cr6+, Cu2+, Ni2+, Zn2+) with a concentration of 50 mg/L dissolved in a 0.01 M NaNO3 solution were used to investigate the removal capacities of composites with different Fe/BC impregnation mass ratios. BC or nZVI-BC (0.05g) was added into a 30 mL plastic reactor containing 25 mL of 50 mg/L HM solution. To meet the practical situation, the pH of the HM solution was adjusted to 6.0 ± 0.2. All experiments were performed in a shaker for 24 h at 30 °C. After the removal reaction, solid slurry was removed with a magnet, and 10 mL of the aqueous sample was filtered through a 0.22 μm filter. The filtered sample was then analyzed by ICP-AES to determine the residual HM concentration. The HM solution without the addition of BC/nZVI-BC was used as a control.

patterns of BC and nZVI-BCs. In the BC, the carbon matrix mainly existed in amorphous forms. In the nZVI-BCs, the characteristic peaks at 44.76° were detected as α-Fe0 planes (110) (PDF 01-087-0722) obviously, indicating that nZVI existed in the metallic form with a face-centered cubic crystal (cell parameters of a = 2.8608 Å, b = 2.8608 Å).25 According to the Scherrer equation, the Fe0 particles obtained were nanosized (10.77−13.34 nm),26 which could prevent nZVI aggregation significantly. Moreover, the peak intensity of α-Fe0 planes increased with an increase in the Fe/BC impregnation mass ratio. The sharp peaks at 30.29°, 35.69°, 43.37°, 53.81°, 57.37°, and 63.01° were signals of iron oxide (Fe3O4), corresponding to the following planes: (220), (311), (400), (422), (511), and (440) (PDF 01-071-6338). Similar to the findings of a related report,27 an amount of oxides (e.g., Fe3O4) was produced, for the following reasons: (1) the thin surface of the Fe0 particles may be oxidized under the drying and fabricating process; (2) the functional oxygen-containing groups on the surface of BCs (e.g., carboxyl, hydroxyl, and aliphatic ethers) might form surface oxygen-containing complexes with Fe ions through stabilization absorption 9674


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Figure 2. XPS results of BC and nZVI-BC with different Fe/BC impregnation mass ratios (1:1, 2:1, and 4:1): (a) C 1s binding state levels, (b) Fe 2p binding state levels.

configurations,28−30 as described in reactions 1−8 (section 5 in the Supporting Information). Surface Chemical Behaviors of nZVI-BCs. FTIR spectra were measured to investigate the functional groups on the surface of BC and nZVI-BCs, as depicted in Figure S1 (Supporting Information). The bands at 3430 and 1427 cm−1 were assigned to the O−H stretching vibration mode of hydroxyl and carboxyl groups, respectively. In addition, the bands at 950 to 1710 cm−1 were associated with the presence of oxygen-associated functional groups.31 Compared with BC, many bands in nZVI-BCs disappeared or shifted. For example, the bands of the C−O (1010 cm−1) and O−H (1427 cm−1) vibration modes of the hydroxyl groups disappeared and the bands of the aromatic CC (1600 cm−1), CO (1695 cm−1), and O−H (1427 cm−1) vibration modes shifted, indicating that Fe3+/Fe0 interacted with the surface’s functional groups. This was in a good agreement with previous findings, which showed that Fe ions interacted with oxygen containing groups in the organic backbone and then converted to inorganic oxides.32 The bands at 460 and 578 cm−1 were assigned to the Fe−O31 and C−Cl vibration modes,33 respectively, indicating that Fe− O complexes or iron oxides existed on the surface of BCs. The Fe−O might come from iron-surface functional groups’ complexation,34 which led to the formation of chelate bonds or bidentate bridge coordination bonds.25 The XPS full survey (Figure S2, Supporting Information) showed that BC was comprised of C 1s and O 1s, while nZVIBCs incorporated Fe 2p, C 1s, and O 1s. Furthermore, after the calibration of binding energy was done with the C 1s peak at 284.8 eV, the various states of Fe 2p, C 1s, and O 1s were determined via deconvolution using the software XPS Peak 4.1, and the relative atomic percentages were calculated quantitatively according to the deconvolution peak area. The XPS relative atomic percentages and various core levels are shown in Table S2 (Supporting Information) and Figure 2. Four peaks of C 1s were ascribed to carbon atoms in the form of C−C/CC (284.8 eV), C−O (286.3 eV), CO (287.9 eV), and O−C O (288.8 eV).35 In the BC, the C 1s spectrum was comprised of three peaks, which were assigned to C−C/CC (284.8 eV), C−O (286.1 eV), and O−CO (288.3 eV). After adding the same proportion of iron to BC, extra CO (287.5 eV) emerged in nZVI1-BC with a slight increase in C−C/CC (284.9 eV) content from 70.3% to 71.8%; meanwhile, the content of C−O (286.2 eV) and O−CO (289.5 eV) decreased from 22.3% and 7.3% to 18.3% and 3.3%,

respectively. As iron proportion increased, the C−O content fell to zero, while the content of C−C/CC and CO increased from 71.8% and 6.6% (nZVI1-BC) to 88.5% and 8.4% (nZVI2-BC) and 88% and 9.4% (nZVI4-BC), respectively. Meanwhile, the O−CO content slightly decreased with an increase in the proportion of Fe. Yang et al.36 suggested that the C−O−Fe organometallic complex occurred intensively at the interfaces of BC. The other studies suggested the binding happened as a result of ligand exchange, acid−base reactions, coordination, multidentate ligand or C−π-cation interaction,37 and a cross-linking structure converted to a condensed structure or aromatic species.38 The BC derived from reed in this study had a higher surface negative potential (pHz = 1.56, Figure S3, Supporting Information) than activated carbon or BC derived from other materials39 (Table S3, Supporting Information), and this may result in the strong C−O−Fe electrostatic binding. In addition, Wang et al.40 pointed out that the functional groups of CO and O−CO would disappear after reacting with sodium borohydride in the presence of iron. Therefore, C−O−Fe would form as Fe impregnated with BC by ligand exchange, chelating or bridging, then the C−O bond was broken by electron transferring during the reduction reaction, forming CO/CC bonds or iron oxides. However, further increasing the Fe/BC impregnation mass ratio to 4:1 resulted in unstable relative contents of C−C/CC/CO, indicating that the C−O bonds were exhausted when the addition of Fe ions reached a certain level. Taken together, the presence of iron could significantly contribute to the evolution of BC’s chemical structure.41 The interfacial chemical behavior between BC and iron was described in Scheme S1 (Supporting Information). The Fe 2p spectra in BC and nZVI-BCs with different Fe/ BC impregnation mass ratios are shown in Figure 2b, and the relative content of various surface Fe atoms is summarized in Table 1b. The peaks at the binding energies of 709 and 711 eV were assigned to Fe2+ and Fe3+ (Fe 2p3/2) in octahedral coordination, while 717 eV was assigned to Fe3+ of Fe3O4 (Fe 2p3/2) in tetrahedral coordination.30,35 Moreover, the small peak at the binding energy of 706 eV was assigned to zerovalent iron (Fe 2p3/2).30,35,42 Interestingly, the highest contents of reducing Fe0 and Fe2+ of 9.14% and 35.91% were observed for nZVI2-BC, with the lowest content of Fe3O4 (Fe 2p3/2, 2.90%). This suggested that fewer iron atoms were oxidized and more Fe0 existed in nZVI2-BC. The reduction reaction occurred at the C−O bond in C−O−Fe, and the 9675


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NPs would evenly distribute in both macropores and on the surface, with an average particle size of 20−100 nm (Figure S4, Supporting Information). However, obvious NP aggregation was observed in nZVI4-BC, which was consistent with a previous report which stated that iron NPs tended to aggregate at an impregnation ratio of 4:1.30 Similar to Figure 2, the iron and oxygen elements were observed simultaneously on the surface of nZVI1-BC, resulting in the formation of iron oxides on the “porches” of the channels/pores (Figure 4a). In contrast, in nZVI2-BC, the iron elements without oxygen elements were homogeneously distributed on the channels/pores, while the oxygen elements mainly appeared on the border of the carbon skeletons (Figure 4b). However, in nZVI4-BC, more occasional contact with oxygen was observed, with the enhanced formation of iron oxides on the top layer of the channels/ pores (Figure 4c). Therefore, nZVI-BCs would trigger significant Fe0’s diffusion into the channels/pores of BC to inhibit the aggregation, as seen in the appearance of bulkier dendritic flocs,44,45 but inappropriate Fe/BC impregnation ratios (e.g., 1:1 or 4:1) were unfavorable to the homogeneity of Fe0 NPs. Thermal Stability and Oxidation Resistance of nZVIBCs. TG and DTG curves were obtained to evaluate the mass loss and thermal stabilization (Figure 5). The peak shown at 70−100 °C was ascribed to the loss of water molecules, while the peak at 330−350 °C was due to the decomposition of hydroxyl and carboxyl groups.46 The main peak at 410−550 °C indicated the further cracking and combustion of the chemical linkages of carbon. As shown in Figure 5, compared with BC, the peak at 330−350 °C in the nZVI-BCs disappeared, indicating that the hydroxyl and carboxyl groups had been cracked or C−O−Fe/CC formed in nZVI-BCs to improve the thermal stability, consistent with the results obtained in Figures S1 (Supporting Information) and 2. In addition, the peak of nZVI-BCs at 410−550 °C clearly shifted to a higher temperature as the Fe ratio increased, along with a lower mass loss of 55−65 wt % than seen with BC. Li et al.47 reported that minerals could improve the oxidation resistance of BC during the pyrolysis process, because of the enhanced formation of aromatic C. Rosas et al.48 also demonstrated that PO43− could form C−O−P with the carbon matrix acting as a physical barrier, enhancing their thermal stability. Therefore, C−O−Fe and more aromatic C might form during the nZVI-BC synthesis reactions, along with the production of Fe0/Fe3O4 on the surface of BC, leading to an improvement in their thermal stability. The recalcitrance index (R50) is an important indicator for evaluating oxidation resistance, which influences the BC’s stability and degree of carbon sequestration.47 As shown in Table S4 (Supporting Information), the R50 increased from 55.2% to 58.9−60.3% when the Fe0 were combined with BC, indicating that the iron crystals were tightly associated with the carbon matrix to enhance the oxidation resistance, in agreement with a previous study.36 Thus, the stability and oxidation resistance of the composite should be improved to enhance its sustainability of HM removal from wastewater. Magnetic Properties of nZVI-BCs. Figure S5 (Supporting Information) presented the magnetic hysteresis curves of nZVIBCs with different Fe/BC impregnation ratios. The superparamagnetic characteristic and negligible coercivity shown indicated that this composite can be easily separated from the liquid phase by an external magnetic field, which was favorable for HMs removal from polluted water resources. The saturation magnetization (M) of nZVI1-BC, nZVI2-BC, and nZVI4-BC

Table 1. XPS Results for the Relative Atomic Percentages of C 1s and Fe 2p Binding State on BC and nZVI-BC with Different Fe/BC Impregnation Mass Ratios (1:1, 2:1, and 4:1): (a) C 1s, (b) Fe 2p.a (a) C 1s atomic % sample

C−C/CC

BC nZVI1-BC nZVI2-BC nZVI4-BC

70.3 71.8 88.5 88.1

C−O

CO

22.3 18.3 n.d. n.d. (b) Fe 2p3/2

n.d. 6.6 8.4 9.4

O−CO 7.4 3.3 3.1 2.6

atomic %

a

0

sample

Fe

BC nZVI1-BC nZVI2-BC nZVI4-BC

n.d. 5.3 9.2 7.1

Fe

2+

n.d. 24.1 35.9 26.6

Fe3+ n.d. 34.0 24.6 30.3

Fe3O4 n.d. 14.1 2.9 9.4

n.d.: not determined.

conjoint iron may be converted to oxides. Thus, the few iron atoms in nZVI1-BC would cause a lack of iron shell to protect the Fe0 exposed to the outside O2. Moreover, because BC was oxidized from the outside surface to the interior structure,43 more Fe0 would settle onto the surface of the carbon matrix in nZVI4-BC, resulting in the severe oxidation of Fe0 NPs. These phenomena could also be confirmed by determining the different ratios of O 1s and Fe 2p (Table S2, Supporting Information), with the results demonstrating that the lowest O/ Fe ratio of 0.9 was obtained in nZVI2-BC. To further investigate the presence and distribution of elements, SEM images together with elemental mapping were shown in Figures 3 and 4. The surface of BC was smooth, with many formatted channels and internal macropores, while large amounts of Fe0 NPs were observed on the BC’s surface in nZVI-BCs (Figure 3). In the nZVI1-BC, the NPs mainly existed in the interior macropores of the BC. But in the nZVI2-BC, the

Figure 3. SEM images of BC and nZVI-BC with different Fe/BC impregnation mass ratios (1:1, 2:1, and 4:1): (a) BC (10000×), (b) nZVI1-BC (5000×), (c) nZVI2-BC (5000×), (d) nZVI4-BC (5000×). 9676


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Figure 4. SEM elemental mapping (blue stands for Fe, red stands for O, green stands for C) and EDS spectra for nZVI-BC with different Fe/BC impregnation mass ratios (1:1, 2:1, and 4:1): (a) nZVI1-BC, (b) nZVI2-BC, (c) nZVI4-BC.

surface complexes with −OH/−COOH groups or delocalized π electrons.50−52 Moreover, the surface of nZVI-BCs was covered with Fe0/Fe3O4 crystals, which could improve the adsorption capacities through transferring into H−Fe−O−Mn+ species in the liquid phase53,54 and providing more surface adsorption sites.55 Wang et al.24 suggested the Fe−O−Mn+ compexation governed the mechanisms due to oxide crystals. In this study, FTIR was employed to verify the formation of complexation. The results and detailed illustration are shown in Figure S6 (Supporting Information), indicating that the interactions between surface functional groups and HM ions happened during the removal process. Moreover, Liu et al.10 discovered that about 47% of Pb2+ could be removed by the reduction reaction in novel nZVI-Mg(OH)2 composites. Diao et al.56 also investigated the intermediate products and suggested that nZVI could reduce Cr6+ into Cr3+ followed by the formation of insoluble Cr2O3/Cr(OH)3. Therefore, the abundant surface functional groups (−OH/−COOH/Fe−O; Figures S1, Supporting Information) would react with Pb2+ and Cu2+ to form complexes, while the reduction reaction occurred toward Pb2+, Cu2+, and Cr6+ and the reduced Cr3+ was introduced to the oxygen containing groups in succession. As such, both the reduction and complexation might play vital roles during the removal of Pb2+, Cu2+, and Cr6+ from the liquid phase.57 Notably, the removal efficiencies of Pb2+ between BC and nZVI2-BC were similar (95.0% and 98.9%), indicating that due to the better affinity of Pb2+ against organic functional groups, the main mechanism of Pb2+ removal was perhaps mainly attributed to complexation. But, the variance of Cu2+ and Cr6+ between BC and nZVI2-BC was significant; therefore, reduction was the main mechanism of Cu2+ and Cr6+ removal. In addition, the highest removal efficiencies for Pb2+, Cu2+, and Cr6+ of nZVI2-BC were obtained from higher reduction reactivity and complexation adsorption sites, which resulted from its high Fe0/ Fe2+ ratio (Figures 2 and 4) and preferable

Figure 5. TG and DTG curves for BC and nZVI-BC with different Fe/ BC impregnation mass ratios (1:1, 2:1, and 4:1).

reached 26.2, 48.7, and 38.2 emu g−1 at 15 000 Oe, respectively, better than those reported in many of the related studies.32,35,49 In addition, nZVI2-BC had the highest relative content of Fe0 and best NPs homogeneity, leading to the highest saturation magnetization (M), while the relatively higher content of Fe3O4 in nZVI4-BC resulted in better magnetization than seen in nZVI1-BC. Heavy Metals Removal Performance of nZVI-BCs. The removal efficiencies for Pb2+, Cd2+, Cr6+, Cu2+, Ni2+, and Zn2+ using nZVI-BCs with different Fe/BC impregnation ratios were individually investigated (Figure 6). The removal efficiencies of Pb2+, Cu2+, and Cr6+ were enhanced for nZVI-BCs compared with BC. Moreover, the removal efficiencies reached the highest removal efficiencies of 99.4%, 99.1%, and 66.7% for Pb2+, Cu2+, and Cr6+, as the Fe/BC impregnation ratio rose from 1.0 to 2.0 g/g. However, the ratio further increasing to 4:1 resulted in a marked drop of removal efficiencies. It has been reported that the sorption of Pb2+, Cu2+, and Cr6+ could be attributed to the 9677


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Figure 6. Heavy metal removal performance of BC and nZVI-BC with different Fe/BC impregnation mass ratios (1:1, 2:1, and 4:1): (a) monometal removal rates, (b) multimetal removal rates.

homogeneity. As for Cd2+, Ni2+, and Zn2+ removal, clearly different phenomena were seen compared with Pb2+, Cu2+, and Cr6+, in which the removal efficiencies of nZVI1-BC (26.9%, 25.9%, and 51.0%) were lower than those of BC (43.1%, 50.9%, 61.3%; Figure 6a), because the redox potentials of Pb2+/Pb0 (−0.13 V), Cu2+/Cu0 (+0.34 V), and Cr6+/Cr3+ (+1.36 V) were much higher than those of Cd2+/Cd0 (−0.40 V), Ni2+/Ni0 (−0.24 V), and Zn2+/Zn0 (−0.76 V). Thus, Pb2+, Cu2+, and Cr6+ could be more easily reduced by nZVI, as the reduction reactions were for them. In contrast, coprecipitation and complexation were the major removal channels for Cd2+, Ni2+, and Zn2+. The removal efficiencies of Cd2+, Ni2+, and Zn2+ increased to 98.8%, 99.9%, and 98.8% in nZVI2-BC, resulting from the more iron content and surface functional groups in nZVI2-BC. Sharp drops in the removal efficiencies of Cd2+ (13.8%), Ni2+ (13.6%), and Zn2+ (17.8%) were found (Figure 6a), due to the severe aggregation on the nZVI4-BC surface. Furthermore, despite the absorption process being related to the high surface area and porosity of BC, the contribution from the surface area or pores in the study was weak due to the decrease in BET surface area in nZVI-BC. The main removal mechanisms of HMs by nZVI-BC are shown in Scheme 1. To meet practical demand, an investigation of the removal ability for multiple HMs from the liquid phase was shown in Figure 6b. In general, due to the superior status of the

reduction reaction, the decline of removal efficiency of Cu2+ or Cr6+ was not significant. Moreover, the higher electronegativity (2.33)58 and smaller hydrated radius (4.01 Å)50 of Pb2+ led to better affinity toward organic functional groups, resulting in high removal efficiency when competing with other coexisting HMs in the solution. However, dramatic drops of the removal efficiencies of Cd2+, Ni2+, and Zn2+ were found, indicating that the Cd2+, Ni2+, and Zn2+ removal via coprecipitation and complexation were behind the reduction, which had difficulty in competing with Pb2+. The nZVI2-BC exhibited excellent removal capacity in multiple HM solution, which could be ascribed to the following reasons: (1) There were more electron donors (Fe0/Fe2+) compared to nZVI1-BC; (2) the broken C−O/COO and lower iron content of nZVI1-BC resulted in weaker complexation and coprecipitation. The excessive iron existing in nZVI4-BC would lead to agglomeration. To further identify the possible mechanisms, the adsorption isotherms of nZVI2-BC are shown in Figure 7. On the basis of the correlation coefficient R2 values, the Pb2+, Cu2+, Cr6+, Cd2+, and Ni2+ were more fitted with the Langmuir model (0.961, 0.989, 0.994, 0.952, and 0.931), while Zn2+ was fitted as the Freundlich model (0.956). These results indicated that the adsorption of Pb2+, Cu2+, Cr6+, Cd2+, and Ni2+ was involved in a monolayer sorption on the homogeneous surface while the adsorption of Zn2+ was attributed to the heterogeneous sorption. In addition to Zn2+, the maximum adsorption capacities of Pb2+, Cu2+, Cr6+, Cd2+, and Ni2+ reached 38.31 mg/g, 30.37 mg/g, 23.09 mg/g, 39.53 mg/g, and 47.85 mg/g, respectively, suggesting that the nZVI-BC exhibited great removal capacity toward multi-HMs. Moreover, the K L (binding strength constant) determined from the Langmuir model, which indicated the affinity and binding strength of the HMs ion, was ordered as Cu2+ > Pb2+ > Cd2+ > Ni2+ > Zn2+ > Cr6+, while the slight difference of order was observed as determined by the Freundlich model (Cu2+ > Pb2+ > Ni2+ > Cd2+ > Zn2+ > Cr6+), suggesting that Cu2+ and Pb2+ showed better affinities toward the surface of nZVI-BC. However, except for surface ion affinity, the reduction reaction, hydrated radius, and electronegativity together contributed to the removal ability, which explained why the Cu2+ removal capacity was lower than that of Pb2+. Moreover, the results of the pH effect upon the multiple HMs removal and regeneration ability are shown in Figures 8 and S7 (Supporting Information). As

Scheme 1. Illustration of the Theoretical Heavy Metal (Pb2+, Cd6+, Cr2+, Cu2+, Ni2+, Zn2+) Removal Mechanisms of nZVIBC, Including Reduction, Surface Complexation, and Coprecipitation

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Figure 7. Adsorption isotherms of nZVI2-BC in the multi-HMs solution. (a) Langmuir isotherm models, (b) Freundlich isotherm models.

mechanisms of Pb2+ and Cu2+, the effect of electric charge might be more significant. (4) The alkaline condition would lead to metal hydroxide species,59 which contributed to the higher removal efficiencies of Pb2+ or Cu2+. Moreover, different pH values contributed to different dominant mechanisms of HM removal. For example, a slight decrease appearing at pH = 8 (compared with pH = 6) might result because the decreasing intensity of the reduction reaction was higher than the increased electrostatic force intensity from the positive charge. The regeneration ability of the nZVI2-BC was investigated toward multi-HMs removal. As shown in Figure S7 (Supporting Information), the removal efficiencies of Pb2+, Cu2+, Cr6+, Cd2+, Ni2+, and Zn2+ dropped to 60.4%, 53.5%, 24.7%, 24.6%, 20.3%, and 19% after four cycles, respectively. The higher regeneration stabilities of Pb2+ and Cu2+ were obtained, indicating that more reduction was attributed to their removal mechanisms due to the higher redox potential. In addition, the adsorption sites tended to be inhibited toward Cd2+, Ni2+, and Zn2+ after four cycles. However, because the removal efficiencies of some HMs were not very satisfactory after four-cycle repeats, a systematic study of selecting a better regeneration agent will be further considered to improve its regeneration ability/stability. Taken together, it demonstrated the nZVI-BC can be reused for multi-HMs removal, which will be beneficial for the practical applications of multi-HMs removal from wastewater. In this study, magnetic nanoscale zerovalent iron assisted biochars (nZVI-BCs) were successfully synthesized with different Fe/BC impregnation ratios. The interfacial chemical behaviors occurring on the surface of nZVI-BCs were comprehensively investigated. Through the systematic analysis of HMs removal mechanisms, we clearly demonstrated that nZVI2-BC exhibited great HM (Pb2+, Cd2+, Cr6+, Cu2+, Ni2+, Zn2+) removal performance in both mono- and multi-HMs solution. The satisfactory magnetic properties would help in the recycling of the nZVI-BC from wastewater. The sustainability of nZVI-BC was further improved as follows: (1) The thermal stability and oxidation resistance of nZVI-BC were improved,

Figure 8. Effect of pH on the multi-HMs removal efficiencies.

shown in Figure 8, the differences were observed among the HMs under different pH’s after the removal reaction. The removal efficiencies of Pb2+, Cu2+, Cd2+, Ni2+, and Zn2+ increased from 81.8%, 96.2%, 63.7%, 75.5%, and 90.4% to both >98% with increasing pH from 4 to 10, respectively. However, the removal efficiency of Cr6+ decreased from 63.7% to 48.9% when raising the pH. The phenomena might result due to the following reasons: (1) According to the zeta potential−pH curve, the pHz of nZVI2-BC was improved to around 5.5, which was significantly higher than the pristine biochar. Thus, the positive charge governed the surface of nZVI2-BC at pH = 4, which led to the electrostatic repulsion force reducing the cation adsorption sites (e.g., Pb2+, Cu2+, Cd2+, Ni2+, and Zn2+). (2) The Cr6+ existed in the form of anions (e.g., HCrO4− or CrO42−), which preferred the positive charge at a low pH value. Furthermore, the iron dissolution happened in the acid solution, which helped in removing the iron oxide shell and provided more available Fe0/Fe2+ as an electron donor to proceed to the reduction reaction. (3) Although the reduction reaction was attributed to the removal 9679



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compared with the pristine biochar; (2) Due to the protection of biochar, nZVI could avoid being further oxidated versus nZVI-alone; (3) Addition of the “green” and low cost biochar would decrease the dosage of Fe0 nanoparticles and iron leakage, leading to greater sustainability for the practical application.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00542. Detailed information on preparation of nanoscale zerovalent iron assisted biochar (nZVI-BC), characterization of the nanocomposites, adsorption isotherm models, details of the regeneration experiment, equations, FTIR curves, XPS full survey, pH-zeta potential curves, SEM image, magnetic hysteresis curves, description of FTIR before and after multi-HMs adsorption, regeneration ability, elements and specific surface areas, XPS relative atomic percentage, scheme of interfacial chemical behavior (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-451-86282069. E-mail: [email protected]; [email protected]. *Tel.: +86-451-86283008. E-mail: [email protected]. ORCID

Shih-Hsin Ho: 0000-0002-9884-1080 Dawei Wang: 0000-0002-3341-3544 Chengyu Wang: 0000-0002-6337-489X Fang Ma: 0000-0002-8849-0803 Present Address ⊥

State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin, 150090, People’s Republic of China

Funding

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2016TS07), Major Science and Technology Program for Water Pollution Control and Treatment (2012ZX07201003), National Natural Science Foundation of China (31570505), and Nanqi Ren Studio, Academy of Environment and Ecology, Harbin Institute of technology (HSCJ201708). Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Research Article

ACKNOWLEDGMENTS

The authors gratefully acknowledge the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2016TS07), the Major Science and Technology Program for Water Pollution Control and Treatment (2012ZX07201003), the National Natural Science Foundation of China (31570505), and Nanqi Ren Studio, Academy of Environment and Ecology, Harbin Institute of technology (HSCJ201708). 9680


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