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Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal
Accepted Manuscript Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal Fan Yang, Shuaishuai Zhang, Yuqing Sun, Kui Cheng, Jiangshan Li, Daniel C.W. Tsang PII: DOI: Reference:
S0960-8524(18)30808-3 https://doi.org/10.1016/j.biortech.2018.06.029 BITE 20046
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
3 May 2018 9 June 2018 11 June 2018
Please cite this article as: Yang, F., Zhang, S., Sun, Y., Cheng, K., Li, J., Tsang, D.C.W., Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.06.029
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Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal Fan Yanga,b, Shuaishuai Zhanga, Yuqing Sunb, Kui Chengc, Jiangshan Lib, Daniel C.W. Tsangb* a
School of Water Conservancy and Civil Engineering, Northeast Agricultural University,
Harbin 150030, China b
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong, China c
Key Laboratory of Superlight Material and Surface Technology of Ministry of Education,
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China *Corresponding Author: [email protected]
Abstract Pyrolyzing low-cost agro-waste into biochar is a promising means for waste biomass utilization. This study engineers corn stalk-derived biochar with abundant hydrophilic functional groups as a support material for iron nanoparticles impregnation (nZVI-HCS). Surface chemistry and morphology of nZVI-HCS composites is characterized by SEM, TEM, TG, XRD, FTIR, XPS, and BET techniques, which helps to elucidate the mechanisms of Pb2+, Cu2+ and Zn2+ removal from single and mixed-metal solutions in batch experiments. Equilibrium adsorption capacities can reach 195.1, 161.9 and 109.7 mg·g-1 for Pb 2+, Cu2+ and Zn2+ at neutral medium after 6-h process, respectively. The engineered biochar with 1
hierarchical pores can impregnate iron nanoparticles, serve as an adsorbent, and enhance metal reduction/precipitation. Rapid removal and high performance can be maintained after five regeneration/reuse cycles. Multiple interaction mechanisms including adsorption, precipitation, reduction and complexation are responsible for metal removal by nZVI-HCS composites, which can be a novel biowaste-derived material for wastewater treatment. Keywords: Engineered biochar; Potentially toxic elements; Waste biomass valorization; nZVI-carbon composites; Iron-based nanomaterials; Wastewater treatment.
1. Introduction Rapid urbanization and industrialization have brought about excessive release of inorganic and organic contaminants into the environment and present health threat to humans and animals (Gong et al. 2016, Zou et al. 2016). Contamination of groundwater by heavy metals such as Cu2+, Pd2+ and Zn2+ resulting from industries (metal plating, battery manufacturing, soldering, oil refining, etc.) can lead to various adverse health effects. For example, international organizations suggest the maximum limit of Pb2+ in drinking water to be 10 µg L-1 (Muthu et al. 2018), while ecotoxicity and bioaccumulation of Cu2+ and Zn2+ also lead to several adverse effects on target organs including kidney, liver, lung, skin, and nervous system (Lv et al. 2018, Ma et al. 2018, Ramezanzadeh et al. 2018). Iron-based materials with low expense and high treatment efficiency have attracted increasing interests in wastewater treatment (Shih et al. 2012, Zhang et al. 2017). Nanoscale zero valent iron (nZVI) exhibits a promising potential because that it is an environmentally-friendly reducing agents with the characteristics of small particle size, 2
abundant active sites and fast reaction (Lu et al. 2016, Xiu et al. 2010). Remediation by nZVI-based materials is commonly employed for the removal of metals/metalloids and organic pollutants from water/wastewater. For instance, Krzisnik et al. (2014) reported that nZVI could be effective for the removal of Zn2+, Zn(II)-EDTA and Zn(II)-citrate in aqueous solutions. Huang et al. (2013) compared the removal performance of Cd(II), Cr(IV), and Pb(II) in aqueous solution with nZVI particles electrosprayed or non-electrosprayed using rapid magnetic separation means. However, challenges to environmental applications of nZVI include its thermodynamic instability, easy oxidation, short duration of reactivity, poor air stability, and rapid agglomeration (Mu et al. 2017, Ren et al. 2018, Wu et al. 2018). Assembling suitable porous carriers as support of nZVI is considered as an effective strategy to enhance its stability, for which various carbon-based materials such as activated carbon (Ren et al. 2013), graphene and its derivatives (Ren et al. 2018), carbon nanotubes (Hu et al. 2012, Shao et al. 2012), biochar (Yan et al. 2015, Zhu et al. 2017a, Zhu et al. 2018), and mesoporous carbon (Teng et al. 2017) are reported as potential candidates. The utilization of agricultural product residues for environmental remediation has received tremendous attention as they are inexpensive, abundant in source and renewable (Chen et al. 2018, Peng et al. 2017, Wang et al. 2017b). Statistical results from Chinese Academy of Agricultural Sciences indicate that there are 7.18 hundred million tons of crop straws in 2015 and corn stalks account for 34.2% (Song et al. 2018), which represent huge amounts of by-product from corn production. Biochar produced by combusting corn stalks (composed of lignin, cellulose, and hemicelluloses) under limited oxygen conditions contains high specific surface area, stable structures, and a large amount of surface sites, which can be employed to remove 3
environmental pollutants (Yang et al. 2017a, Yang et al. 2017b, Yang et al. 2017c). Utilization of biochar as porous carriers for supporting nZVI can also be efficient and economically beneficial. Qian et al. (2017a) evaluated the effectiveness of nZVI-biochar composites towards the removal of hexavalent chromium (Cr(VI)) from contaminated groundwater. Chen et al. (2018) investigated the preparation of a magnetic biochar derived from persulfate-ZVI treated sludge as an environmentally friendly biosorbent for lead removal. Nevertheless, to the best of our knowledge, few studies have explored the impregnation of nZVI into waste biomass-derived carbon foam with hierarchical pores (nZVI-HCS), and scrutinized its treatment effectiveness in single-/mixed-metal solutions. In this study, waste corn stalks serve as the raw materials for the fabrication of carbon foam with hierarchical porosity for supporting nZVI via chemical reduction method using borohydride. The removal capacities for Pb 2+, Cu2+ and Zn2+ in individual and co-existing scenarios are examined. This paper aims to develop a novel nZVI-HCS composite for metal removal and realize the following specific objectives: (1) elucidate the interactions between nZVI-HCS composites with various iron contents and heavy metals; (2) determine the effects of iron mass ratio, sample dosage, pH, contact time, and initial metal concentrations on the treatment effectiveness of nZVI-HCS sample; and (3) acquire insights into the underlying mechanisms on nZVI-HCS composites in single- and mixed-metal solutions, respectively. 2. Experimental Methods 2.1 Materials and Reagents Corn stalks (CS) were washed, dried, crushed, and sieved through a 100-mesh (pore size <165µm), collected from the campus of Northeast Agricultural University, Harbin, China. 4
Ferrous sulfate heptahydrate (FeSO4·7H2O), lead(II) nitrate (Pb(NO3)2), zinc(II) nitrate hexahydrate (Zn(NO3)2·6H2O), cupric nitrate hydrate (Cu(NO3)2·3H2O), ammonium persulfate ((NH4)2S2O8), sodium borohydride (NaBH4) , sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitric acid (HNO3) were purchased from Tianjin Chemical Reagent Co., Ltd, China. All solutions were prepared with ultrapure water (Millipore, 18 MΩcm). 2.2. Preparation of adsorbent The biochar preparation process is similar to that of Garfield et al. (2014). In a typical procedure, 2 g of corn stalks was mixed with 2 g of KHCO3 via grinding in a mortar, which is referred to optimum mass ratio reported in our previous paper (Yang et al. 2017a). Then, the mixtures were put in an aluminum porcelain boat and heated in a Tube furnace under an N2 atmosphere (200 mL·min-1) condition at 800 °C for 2 h, and heating rate were kept at 10 °C·min-1. In addition, before the heating treatment, it was performed at 25 °C for 30 min to remove residual oxygen and/or moisture in the tube. The pyrolysis product was washed with HCl (0.1 mol·L-1) for removal ash and activating agent, and washed with ultrapure water to neutral pH, and then dried at 60 °C for 12 h in the drying oven. Hydrophilic treatment of biochar was carried out as described in a previous study (Qian et al. 2017b), which serves to endow biochar with hydrophilic nature and abundant amount of oxygen-containing functional groups for subsequent impregnation of iron ions and formation of iron nanoparticles. The resulting biochars and hydrophilic biochars were referred as CS and HCS, respectively. The preparation of nZVI-HCS composite was performed as follows (Su et al. 2016): 0.42 g of hydrophilic biochars was dissolved in FeSO4·7H2O (100 mL 0.075 mol·L-1) solution with stirring for 2 h in a 250 mL three-neck flasks, and then an additional 4 h of 5
continuous stirring was implemented after introduction of NaBH4 (50 mL 0.3 mol·L-1). The reduction process of the whole aqueous solution was purged with nitrogen gas (N2) and the reaction proceeded according to the following equation (Eq. 1). 2Fe2+ + BH4- + 3H2O + HCS → 2Fe0/HCS +H2BO3- +4H+ +2H2
(1)
The final products were settled and separated from the liquid phase. Afterwards, the products were washed with ethanol and deoxygenated water, respectively, for several times and vacuum dried overnight at 60 ºC. The resulting products with the content of Fe:HCS mass ratio is 1:1 using the above procedures. For comparison, nZVI-HCS with iron nanoparticles to hydrophilic CS in mass ratio of 2:1 and 1:2 were also synthesized by varying the dose of HCS and FeSO4·7H2O. The resulting nZVI-HCS composites were referred to as nZVI-HCS (1:1), nZVI-HCS (2:1) and nZVI-HCS (1:2), respectively. The fabrication process is shown in Fig. 1. The prepared products were collected for subsequent characterization and batch sorption experiments.
Fig.1 Scheme of the preparation procedures of nZVI-HCS composites. 2.3 Surface Characterization The microstructures of samples were imaged on scanning electron microscopy (SEM, ZEISS SUPRA40) and transmission electron microscopy (TEM, Tecnai-G20). Thermo gravimetric (TG) analysis was performed with a synchronous thermal analyzer (SDT-Q600, TA Instruments, United States) to test thermal property of samples. X-ray diffraction (XRD) 6
patterns of the samples were analyzed on Rigaku TTR III with Cu Kα radiation in the range of 5∼90° (2θ). Fourier transform infrared spectrometry (FTIR) data was measured on a Nicolet Avatar 370DTGS spectrophotometer (Thermo Fisher Scientific, USA) with the scan region ranging from 400 to 4000 cm-1. X-ray photoelectron spectroscopy was made by using Al Kα radiation (XPS, Thermo ESCALAB 250). N2 adsorption/desorption measurements were measured at 77 K, using Micromeritics ASAP 2020. The specific surface area and pore volume
were
calculated
using
Brunauer-Emmett-Teller
(BET)
method
and
Barret-Joyner-Halenda (BJH) method, respectively. The metal concentrations in the aqueous solutions were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Varian Liberty 200. 2.4 Batch Sorption Experiments The stock solutions were prepared by dissolving appropriate amounts of Pb(NO3)2, Cu(NO3)2·3H2O, and Zn(NO3)2·6H2O in ultrapure water, respectively. Bath adsorption experiments were carried out by adding 10 mg of nZVI-HCS composites (or CS) to a 40 mL ultrapure water and 50 mg·L-1 Pb2+, Cu 2+, and/or Zn2+ solution at the different time intervals (0.16, 0.5, 1, 2, 4 and 6 h) in a 150-rpm thermostatic reciprocating shaker at room temperature (25±1 ºC). The solutions were immediately filtered through 0.45 µm pore size membranes filters with an initial pH of 7 adjusted by using either 0.1 mol·L-1 HCl or 0.1 mol·L-1 NaOH. The equilibrium adsorption capacity (qe) and removal efficiency (R%) of Pb2+, Cu2+ and Zn2+ were calculated using the following equations: q = R=
×100 %
(2) (3) 7
Where C0 and Ce are the initial and equilibrium metal concentrations in solution (mg·L-1), respectively; V is the volume of adsorbate solution (mL), and m is the mass of the adsorbent (mg). 2.4.1 Kinetics The pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data as shown below: q = q 1 − e ) q =
(4)
5
Where qe (mg·g-1) is the adsorption capacity, qt (mg·g-1) is the adsorption capacity at time (h); k1 (h-1) and k2 (g·(mg·h)-1) are the rate constants for the pseudo-first-order and pseudo-second-order adsorption kinetic models, respectively. 2.4.2 Isotherms The isotherms experiments were performed in flasks containing 10 mg nZVI-HCS (2:1) and initial concentrations of Pb 2+ (5, 10, 30, 50, 70, and 100 mg·L-1). Langmuir and Freundlich isotherm models were applied to fit the experimental data of nZVI-HCS. The equilibrium models were shown below: q =
#
q = K ! C
(Langmuir model)
(6)
(Freundlich model)
(7)
Where q m is the maximum amount of coverage in the monolayer (mg·g-1 ); KL is the Langmuir constant (L·mg-1 ); Ce is the metal concentration at equilibrium (mg·L-1 ); and qe is
the amount of metals adsorbed at equilibrium (mg·g-1); KF (( mg·g-1 )( L·mg-1 )1/n ) and are $
the Freundlich constants. 8
2.4.3 Competitive studies The competitive experiments for Pb 2+ and Cu2+ (or Zn2+) in solution to 10 mg of nZVI-HCS sample were performed using initial concentrations of 50 mg·L-1 for each metal added to 40 mL of ultrapure water. Subsequently, the adsorbents were separated as previously described in the adsorption experiments, then the analysis of residual Pb2+ concentrations in the supernatant. 2.4.4 Recyclability of nZVI-HCS composites The reusability of nZVI-HCS sample was tested by repeated Pb2+adsorption and desorption cycles for five consecutive cycles. The regeneration of the saturated adsorbent was conducted under using filtration and desorbed using 30 mL ultrapure water for 10 h. In each cycle, 10 mg of fresh or regenerated nZVI-HCS sample was suspended in 40 mL of 50 mg·L-1 Pb2+ solution at 25 ºC under shaking for 6 h. The removal efficiency was measured and compared after each cycle. 3. Results and Discussion 3.1 Characterization of nZVI-HCS composites The surface morphology of nZVI-HCS sample was observed by SEM and TEM analysis (Supporting Information) and the mass ratio of nZVI-HCS at 2:1 is chosen as an example. It is evident that as-fabricated nZVI-HCS sample had abundant porous structure with relatively rough and irregular surfaces. It can be observed that iron nanoparticles synthesized from FeSO4·7H2O reduction by borohydride were uniformly dispersed on the surface without obvious accumulation and the size of iron particles was small in nanoscale. Low- and highmagnification TEM images can provide more detailed structural information, that is, iron 9
nanoparticles have shown in the shape of round with size of down to 100 nm, which are well dispersed within the biochar matrix, in agreement with the observation from SEM analysis. In general, the as-prepared iron particles was often found to agglomerate together, hence, the application of HCS as a porous support effectively prevents the as-formed nanoparticles from irreversible aggregation, which can provide large surface area and maintain high reactivity for subsequent metal removal. Thermal stabilities of the nZVI-HCS composite and porous carbon carrier were estimated using TG analysis (Supporting Information). Both nZVI-HCS composite and carbon carrier displayed obvious difference in their thermal stability. The weight loss for nZVI-HCS composite is about 10.3 % over the temperature range from 30 °C to 300 °C, mainly due to the removal of pre-adsorptive water molecules on samples. The weight of nZVI-HCS composite kept almost unchanged with the increasing of the temperature from 600 °C to 800 °C, indicating its high stability in the tested temperature range. Ash contents after combustion in the air atmosphere was only 2.9 % for porous carbon carrier, much lower than that for nZVI-HCS composite, suggesting existence of Fe0 or iron oxides in the as-prepared composite (Baikousi et al. 2015). The crystalline structure of nZVI-HCS samples was analyzed by XRD spectra (Supporting Information). The as-prepared three samples exhibited a broad and weak diffraction peak centered at 2θ=21.6°, attributed to the sp3 hybridized carbon in amorphous form. As shown, three strong diffraction peaks of 44.7°, 65.0° and 82.3° were present on nZVI-HCS composite, corresponding to plane (100), (200) and (211) of iron element (JCPDS, NO.87-0721), respectively. These results revealed the formation of well-crystallized 10
zero-valent iron on the hydrophilic biochars. Some minor peaks of Fe-O compounds, for example, peak at 35.7° for Fe2O3, were also observed, which might result from oxidation and corrosion during synthesis and drying process. For comparison, XRD planes of other nZVI-HCS samples are also shown, which were consistent with the report from Ahmed et al. (2017), indicating the formation of zero-valent iron on the surface of HCS. The surface functional groups are recognized as a critical factor that determines the adsorption behavior of metal removal. FTIR spectra for different samples were carried out in the range of 400-4000 cm-1 (Supporting Information). The broad absorption band centered at 3431 cm-1 can be attributed to O-H stretching vibration of alcohol, phenol or carboxylic groups due to the presence of hemicelluloses, lignin or cellulose. The absorbance peak at 1628 cm-1 is associated with C=O stretching vibration of carbonyl and carboxyl groups. The band at 1378 cm-1 can be attributed to the COO- groups of nZVI-HCS (Su et al. 2016), and the band (1126 cm-1) is assigned to the C-C bond vibrations. The band at 1126 cm-1 corresponds to the C-O or C-O-C stretching vibration modes. Compared with spectrum of CS and HCS exhibited in our previous report (Yang et al. 2018), the weak peaks of Fe-O fall in the ranges of 730-470 cm-1, confirming that Fe3+ was successfully attached to carbon surface by chelating with oxygen-containing groups, which confirmed the formation of metal oxides in HCS surface (Prabu et al. 2017). The FTIR spectrum indicated that the as-prepared nZVI-HCS contained abundant oxygen functional groups, which could act as available adsorption sites. The oxygen atoms in C=O and O-H groups could provide free pairs of electrons to interact with the empty orbital of metal ions (Pb 2+,Cu2+ and Zn2+), which could form coordinately banded complexes. Thus, the abundance of oxygen functional groups 11
could play a vital role in the metal removal process, and further discussion will be provided in later section. The chemical composition of nZVI-HCS samples were further studied via XPS spectra (Supporting Information). The peaks at around 284.85 eV, 531.66 eV, and 712.22 eV are shown in the XPS survey spectrum of nZVI-HCS samples, which corresponds to C, O and Fe elements, respectively. Fe 2p spectra of nZVI-HCS sample can be deconvoluted into two main bands at 724.7 eV and 711.2 eV assigned to oxidized iron binding energies of Fe 2p 1/2 and Fe 2p 3/2, respectively (Xiao et al. 2016). Both the O 1s and C 1s regions could be deconvoluted into three peaks, suggesting the presence of oxygen-containing functional groups (such as C-O, O-H and C=O) (Yang et al. 2017a). There can be also observed three different peaks assigned to Fe-O-H, Fe-O-C, and Fe-O-Fe, demonstrating the interaction between iron nanoparticles and porous carbon support. In addition, Fe 2p spectra of nZVI-HCS samples with various mass ratios were also displayed. The BET specific surface area, average pore diameter and total pore volume for nZVI-HCS sample were calculated as 603.4 m2·g-1, 3.14 nm and 0.474 cm3·g-1, respectively (Supporting Information). The specific surface areas of CS and HCS are 1236 and 1216 m2·g-1, respectively, in our previous studies (Yang et al. 2018). However, iron particles loaded on the surface of biochar blocked the micropores of biochar, resulting in the significant decrease in specific surface area. The pore-size distribution is similar to type-II model with H4 type hysteresis loop at medium relative pressure (P/P0=0.4-0.95) resulting from capillary condensation, demonstrating the existence of micropores and mesopores (Asmel et al. 2017, Qiu et al. 2014). The pore size distributions of the resulting samples are obtained using the 12
Barret-Joyner-Halenda (BJH) analysis. The curve presents the continuous distribution in the range from 0-100 nm, with the major distributions in the size range of < 2 nm, which would endow large specific surface area of nZVI-HCS composites for adsorption capability. 3.2 Single-/mixed-metal removal by nZVI-HCS composites 3.2.1 Kinetics
Fig. 2 Kinetic studies for Pb2+ (a), Cu2+ (b) and Zn2+ (c) adsorption over the nZVI-HCS composites. Removal efficiency of Pb2+, Cu2+ and Zn2+ in single-metal solutions (d) (metal concentration: 50 mg· L-1, T = 25 °C, adsorbent dose =10 mg).
The kinetic studies were performed to compare the removal efficiencies of heavy metals (Pb 2+, Cu 2+ and Zn2+) by the synthesized nZVI-HCS composites and CS in Fig. 2a-c. For CS sample, the removal rate of Pb 2+, Cu 2+ and Zn2+ increased rapidly within contact time from 0 13
to 2 h and reached equilibrium with increasing contact time. For nZVI-HCS composites obtained from different iron content, they have shown high adsorption capacity for Pb2+ removal, following the order of nZVI-HCS (2:1) > nZVI-HCS (1:1) > nZVI-HCS (1:2) > CS. The removal progress of heavy metals may be involved into multistep reactions (as discussed in Section 3.3) and it gradually increased with increasing contact time before slowly attaining apparent equilibrium. After 6-h reaction (Fig. 2d), the Pb2+, Cu2+ and Zn2+ removal efficiency of nZVI-HCS (2:1) sample was 97.5%, 81.0%, and 54.8%, respectively, which was therefore chosen as a model material for further investigation. Moreover, there were consistent results for the improved metal removal efficiency with higher Fe content on biochar surface, which indicates that iron nanoparticles play a key role in the removal process. The removal mechanisms of Pb2+, Cu2+ and Zn2+ may also depend on the loading content of iron in the nZVI-HCS composites, which were discussed later.
14
Fig. 3 Adsorption kinetic plots of Pb2+ on nZVI-HCS composite fitted by pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) (heavy metals concentration: 50 mg· L-1 , T = 25 °C, adsorbent dose =10 mg); Adsorption isotherms of Pb2+ by nZVI-HCS sample fitted with Langmuir (c) and Freundlich model (d).
As shown in Fig. 3a, the adsorption of 50 mg·L-1 Pb2+ by nZVI-HCS sample reached equilibrium within 30 min, whereas the removal of Zn2+ and Cu2+ sharply increased in 10 min and gradually increased till reaching equilibrium in 4 h. Compared to pseudo-first-order kinetic model, the higher correlation coefficient obtained from pseudo-second-order kinetic model (Fig. 3b, R2=0.999) demonstrated that the chemical adsorption process on nZVI-HCS sample may be the rate limiting step, of which all the kinetic parameters are shown in Table 1. Ma et al. (2017) prepared graphene oxide/Fe3O4 composite and reported that the adsorption 15
capacity of graphene oxide/Fe3O4-g-G3.0 approached equilibrium at 100 min and the adsorption capacity for Pb2+ was 181.4 mg·g-1 at 298K. Ngah et al. (2010) have synthesized chitosan-tripolyphosphate bead and its equilibrium time and adsorption capacity for Pb2+ were 100 min and 57.3 mg·g-1, respectively. Hence, nZVI-HCS composites introduced in this study displayed highly competitive performance and removal rate. Table 1. Kinetic model parameters for the adsorption of Pb2+, Cu2+ and Zn2+ onto nZVI-HCS sample. Pseudo-first order Qe.(mg· g-1)
k1
Pseudo-second order R2
Qe. (mg· g-1)
k2
R2
Pb2+
194.7
23.5
0.999
195.3
1.22
0.999
Cu2+
146.1
2.41
0.819
158.3
0.0248
0.911
Zn2+
98.5
7.25
0.920
105.5
0.0968
0.963
3.2.2 Isotherms The equilibrium adsorption isotherms of nZVI-HCS sample after 6-h reaction at different temperatures (25 °C, 35 °C and 55 °C) are shown in Fig. 3c-d. The parameters of Langmuir and Freundlich isotherm models are summarized in Table 2. The data were better fitted with Langmuir model (R2=0.94-0.95) than Freundlich model (R2=0.82-0.89). The nZVI-HCS composite showed significant adsorption affinity towards Pb2+ with maximum Pb2+ adsorption of 291.3, 288.2 and 323.7 mg g-1 at three different temperatures (25 °C, 35 °C and 55 °C), respectively. It is important to note that the adsorption capacity of nZVI-HCS composites in this study was obviously higher than other reported materials (Supporting Information). For example, Tran et al. (2010) reported that the maximum adsorption capacities for Pb2+ and Ni2+ were 63.3 and 52.6 mg·g-1 on chitosan/magnetite composite
16
beads under room temperature. These comparisons demonstrated the potential of nZVI-HCS as an effective adsorbent. Table 2. Isotherm model parameters for the adsorption of Pb2+ on the nZVI-HCS sample at different temperatures. Freundlich T(°C)
KF -1
-1 1/n
(mg· kg )/(mg· L ) Pb2+
Langmuir
1 n
R2
Qmax (mg· g-1)
KL -1
R2
(L· mg )
25
132.7
0.263
0.888
291.3
1.19
0.950
35
137.7
0.260
0.822
288.2
2.02
0.939
55
165.7
0.275
0.826
323.7
2.58
0.945
3.2.3 Kinetics in mixed-metal solutions
Fig. 4 Comparative mixed removal performance of Pb 2+, Cu 2+ and Zn2+ solution using 17
nZVI-HCS sample. The removal kinetics of bi-solute system (Pb2+ and Cu2+ or Zn2+) by nZVI-HCS sample is shown in Fig. 4. There was a significant decrease for Pb2+ adsorption with co-existence of Cu2+ (or Zn2+) ions, suggesting competitive adsorption between Pb 2+ and Cu2+ (or Zn2+) such that the adsorption capacity was decreased from 195 mg ·g-1 to 137.6 mg·g-1 (or 154.8 mg· g-1). More detailed discussion on the removal mechanisms in bi-solute system is presented in Section 3.3. 3.2.4 Recyclability Recyclability is one of the most important indicators evaluating the applicability of nZVI-HCS. The decrease from 195.0 mg·g-1 to 98.4 mg·g-1 after five cycles probably because the iron nanoparticles were exhausted by chemical reduction and adsorbed Pb2+ ions were not totally extracted (Supporting Information). As shown, nZVI-HCS maintained ~50% of the Pb2+ removal efficiency after several adsorption-desorption cycles, suggesting that the nZVI-HCS sample has good reusability and stability. 3.3 Discussion on adsorption mechanisms by nZVI-HCS composites The redox potentials of Cu 2+/Cu0 (+0.34 V) and Pb2+/Pb0 (-0.13 V) were much higher than Fe2+/Fe0 (-0.44 V) and Zn2+/Zn0 (-0.76 V). Therefore, Cu2+ and Pb 2+ can be more easily reduced by nZVI than Zn2+, which implies that Zn2+ removal occurs via co-precipitation and complexation rather than reduction. However, the removal of Pb2+ was more significant than Cu2+ by nZVI-HCS sample (Fig. 2), which might be due to the higher electronegativity and smaller hydrated radius of Pb2+ that accounted for its higher affinity toward organic functional groups. A significant decrease was observed using nZVI-HCS sample when Pb2+ 18
and Cu2+ (or Zn2+) co-existed, suggesting the co-existence of ions exert significant influence on the equilibrium adsorption capacity and existence of competitive adsorption between Pb2+ and Cu2+ (or Zn2+). The co-adsorption of Pb 2+ and Cu2+ (or Zn2+) involved competitive active sites, leading to a decline in the adsorption of Pb2+. In addition, our previous work (Zhang et al. 2018) has indicated that higher Pb2+ removal by biochar may be owing to the formation of Pb-phosphate precipitate β-Pb9(PO4)6 and hydrocerussite Pb3(CO3)2(OH)2 with the minerals in biochar, which can be expressed as follows: + + 6OH → Pb PO + 6H O 6HPO+ 0 * 1 + * + 9Pb + 2HCO + 4OH → Pb3 CO3 + OH + + 2H+O 3 + 3Pb
(8) (9)
The possible reactions of Pb2+, Cu 2+, or Zn2+in nZVI-HCS composite materials and CS are briefly concluded as follows: M2+ + OH/ C=O/COOH-Corn stalks→M− − OH/ C=O/COOH-Corn stalks (coordination complexes)
(10)
M2+ + Fe0→M− −Fe2+ (adsorption)
(11)
M2+ + Fe0→M + Fe2+ (reduction)
(12)
+ M2+ + HCO 3 / HPO* -Corn stalks→M0 PO* 1-Corn stalks (precipitation)
(13)
(M2+= Pb2+, Cu2+, or Zn2+) To gain further insights into the removal mechanisms, XPS, FTIR and XRD analysis of nZVI-HCS sample was conducted before and after the adsorption of Pb2+. As evident from the XRD pattern of nZVI-HCS (2:1) after sorption of Pb2+ (Supporting Information), the characteristics peak of nZVI-HCS (2:1) was observed at 2θ = 36.4° in correspondence to the (200), which was ascribed to the Pb0 resulting from the reduction of Pb 2+. After sorption of 19
Pb2+, XRD characteristic peaks of Fe0 were not significant in nZVI-HCS (2:1). As the reaction proceeded, the Fe0 on the surface of biochar was gradually transformed into iron oxides. The XPS full survey (Supporting Information) clearly shows an apparent weak peak at 138.77 eV after reaction, indicating that Pb 2+ was attached onto the nZVI-HCS surface. For the Fe 2p survey before and after Pb2+ treatment, nZVI-HCS sample peaks before removal at 725.31 eV and 711.4 eV could be assigned to the binding energies of Fe 2p 1/2 and Fe 2p 3/2 (Luo et al. 2014, Zhu et al. 2017b), which can be decomposed into two or three peaks, respectively. The peaks located at approximately 711.26 eV and 725.08 eV were in correspondence with binding energies of 2p 3/2 and 2p 1/2 of Fe3+. A fitting peak at 709.18 eV could be assigned to the 2p 3/2 peak of Fe0, suggesting the presence of Fe0 species in the nZVI-HCS sample. Photoelectron peaks at 712.5 eV and 723.93 eV were observed in the nZVI-HCS sample, indicating the co-presence of Fe2+. These peaks represent the binding energies of the Fe 2p that mainly originated from the iron oxide in the nZVI-HCS sample. The species of Fe2+, Fe0 and Fe3+ account for a proportion of about 30.9%, 19.0% and 50.1% in the total iron surface atoms before Pb2+ adsorption, respectively. However, the proportion of Fe2+ and Fe3+ in the total iron atoms increases after Pb 2+ adsorption (39.6% and 60.4%, respectively), indicating Fe0 has been mostly converted to Fe2+ and Fe3+ during the Pb2+ remove process. Binding energy profiles of O (O1s) and Pb (Pb 4f) after adsorption in nZVI-HCS sample are shown in the Supporting Information. The appearance of the Pb 4f7/2 and Pb 4f5/2 peak at 139.02 eV and 143.79 eV after adsorption confirms that nZVI-HCS sample can adsorb Pb2+. 20
It is worth noting that the existence of Pb0 species in Pb 4f spectrum of the Pb2+-adsorbed sample further indicates that reduction reaction may occur during the removal progress. The FTIR peaks at 3431, 1628, and 1378 cm-1 also decreased in intensity after the loading of Pb2+ on nZVI-HCS sample, which could be attributed to complexation of the C=O and O-H with Pb2+ as reported earlier (Wang et al. 2017a). Therefore, it can be concluded that after reaction with nZVI-HCS sample, most of iron nanoparticles on the surface were consumed and transformed into iron oxide, resulting in a decrease in the removal capacity of the composites. The possible interaction mechanism for metal removal (Pb2+ as an example) includes two key steps: (i) adsorption of free Pb2+ from water solution onto active sites of nZVI-HCS sample; (ii) then the reduction of surface adsorbed Pb 2+ to Pb0 by the metallic Fe. Moreover, CS plays an important role in Pb2+ immobilization because of the aggregate-preventive function for iron nanoparticles and its intense inherent adsorption affinity for Pb2+. In addition, the removal of Pb2+ is also partly attributed to the mineral contents of biochar containing N, S, P elements. Hence, the metal removal by nZVI-HCS composite was controlled by surface adsorption via electrostatic attraction, abiotic reduction, and formation of precipitates and surface complexes as vividly illustrated in the Supporting Information. 4. Conclusions Highly efficient performance for metal removal can be realized using corn stalk-derived hydrophilic biochar supported nZVI (e.g., 291.3 mg·g-1 for Pb2+ following pseudo-second order kinetics). The synthesized nZVI-HCS sample was characterized by homogeneous impregnation of iron nanoparticles on the surface of biochar with hierarchical pores. The 21
porous biochar carrier can facilitate the metal sequestration by alleviating aggregation of iron nanoparticles, enhancing metal reduction, and acting as an adsorbent itself. The co-existence of ions exerts notable influence on the removal capacity via competitive adsorption. Regeneration/reuse tests further suggested that nZVI-HCS composite can be potentially applied for wastewater treatment.
Acknowledgment We gratefully acknowledge the financial support of University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2017018), the National Nature Science Fund for Young Scholars (31600413), “Young Talents” Project of Northeast Agricultural University (17XG03), and Hong Kong Research Grants Council (PolyU 15222115 and 15223517).
Appendix A. Supplementary data
References 1. Ahmed, M.B., Zhou, J.L., Ngo, H.H., Guo, W., Johir, M.A., Sornalingam, K., Belhaj, D. and Kallel, M. 2017. Nano-Fe0 immobilized onto functionalized biochar gaining excellent stability during sorption and reduction of chloramphenicol via transforming to reusable magnetic composite. Chem. Eng. J. . 322, 571-581. 2. Asmel, N.K., Yusoff, A.R.M., Krishna, L.S., Majid, Z.A. and Salmiati, S. 2017. High 22
concentration arsenic removal from aqueous solution using nano-iron ion enrich material (NIIEM) super adsorbent. Chem. Eng. J. . 317, 343-355. 3. Baikousi, M., Georgiou, Y., Daikopoulos, C., Bourlinos, A.B., Filip, J., Zbořil, R., Deligiannakis, Y. and Karakassides, M.A. 2015. Synthesis and characterization of robust zero valent iron/mesoporous carbon composites and their applications in arsenic removal. Carbon. 93, 636-647. 4. Chen, Y.-d., Ho, S.-H., Wang, D., Wei, Z.-s., Chang, J.-S. and Ren, N.-q. 2018. Lead removal by a magnetic biochar derived from persulfate-ZVI treated sludge together with one-pot pyrolysis. Bioresour. Technol. 247, 463-470. 5. Garfield, L.D., Dixon, D., Nowotny, P., Lotrich, F.E., Pollock, B.G., Kristjansson, S.D., Doré, P.M. and Lenze, E.J. 2014. Effect of synthesis methods on magnetic Kans grass biochar for enhanced As(III, V) adsorption from aqueous solutions. Biomass & Bioenergy. 71, 299-310. 6. Gong, Y., Tang, J. and Zhao, D. 2016. Application of iron sulfide particles for groundwater and soil remediation: A review. Water Res. 89, 309-320. 7. Hu, J., Yang, S. and Wang, X. 2012. Adsorption of Cu(II) on β‐cyclodextrin modified multiwall carbon nanotube/iron oxides in the absence/presence of fulvic acid. J. Chem. Technol. Biot. 87, 673-681. 8. Huang, P., Ye, Z., Xie, W., Chen, Q., Li, J., Xu, Z. and Yao, M. 2013. Rapid magnetic removal of aqueous heavy metals and their relevant mechanisms using nanoscale zero valent iron (nZVI) particles. Water Res. 47, 4050-4058. 9. Lu, H.J., Wang, J.K., Ferguson, S., Wang, T., Bao, Y. and Hao, H.X. 2016. Mechanism, 23
synthesis and modification of nano zerovalent iron in water treatment. Nanoscale. 8, 9962-9975. 10. Luo, S., Lu, T., Peng, L., Shao, J., Zeng, Q. and Gu, J.D. 2014. Synthesis of nanoscale zero-valent iron immobilized in alginate microcapsules for removal of Pb(II) from aqueous solution. J. Mater. Chem. A. 2, 15463-15472. 11. Lv, D., Liu, Y., Zhou, J., Yang, K., Lou, Z., Baig, S.A. and Xu, X. 2018. Application of EDTA-functionalized bamboo activated carbon (BAC) for Pb(II) and Cu(II) removal from aqueous solutions. Appl. Surf. Sci. 428, 648-658. 12. Ma, L., Wang, F., Yu, Y., Liu, J. and Wu, Y. 2018. Cu removal and response mechanisms of periphytic biofilms in a tubular bioreactor. Bioresour. Technol. 248, 61-67. 13. Ma, Y.X., Kou, Y.L., Xing, D., Jin, P.S., Shao, W.J., Li, X., Du, X.Y. and La, P.Q. 2017. Synthesis of magnetic graphene oxide grafted polymaleicamide dendrimer nanohybrids for adsorption of Pb(II) in aqueous solution. J. Hazard. Mater. 340, 407. 14. Mu, Y., Jia, F., Ai, Z. and Zhang, L. 2017. Iron oxide shell mediated environmental remediation properties of nano zero-valent iron. Environ. Sci.: Nano. 4, 27-45. 15. Muthu, M., Santhanam, M. and Kumar, M. 2018. Pb removal in pervious concrete filter: Effects of accelerated carbonation and hydraulic retention time. Construction and Building Materials. 174, 224-232. 16. N, K., A, M., AS, Š., L, Š., J, Š. and R, M. 2014. Nanoscale zero-valent iron for the removal of Zn2+, Zn(II)-EDTA and Zn(II)-citrate from aqueous solutions. Sci. Total Environ. 476-477, 20-28. 17. Peng, X., Liu, X., Zhou, Y., Peng, B., Tang, L., Luo, L., Yao, B., Deng, Y., Tang, J. and 24
Zeng, G. 2017. New insights into the activity of a biochar supported nanoscale zerovalent iron composite and nanoscale zero valent iron under anaerobic or aerobic conditions. RSC Adv. 7, 8755-8761. 18. Prabu, D., Parthiban, R., Ponnusamy, S.K., Anbalagan, S., John, R. and Titus, T. 2017. Sorption of Cu(II) ions by nano-scale zero valent iron supported on rubber seed shell. Iet Nanobiotechnology. 11, 714-724. 19. Qian, L., Zhang, W., Yan, J., Han, L., Chen, Y., Ouyang, D. and Chen, M. 2017a. Nanoscale zero-valent iron supported by biochars produced at different temperatures: Synthesis mechanism and effect on Cr(VI) removal. Environ. Pollut. 223, 153-160. 20. Qian, X., Meng, R., Yao, Z., Yue, D., Yu, H., Jia, J. and Zhao, Y. 2017b. Visible Light Assisted
Heterogeneous
Fenton-Like
Degradation
of
Organic
Pollutant
via
α-FeOOH/Mesoporous Carbon Composites. Environ. Sci. Technol. 51, 21. Qiu, B., Gu, H., Yan, X., Guo, J., Wang, Y., Sun, D., Wang, Q., Khan, M., Zhang, X. and Weeks, B.L. 2014. Cellulose derived magnetic mesoporous carbon nanocomposites with enhanced hexavalent chromium removal. J. Mater. Chem. A. 2, 17454-17462. 22. Ramezanzadeh, M., Asghari, M., Ramezanzadeh, B. and Bahlakeh, G. 2018. Fabrication of an efficient system for Zn ions removal from industrial wastewater based on graphene oxide nanosheets decorated with highly crystalline polyaniline nanofibers (GO-PANI): Experimental and ab initio quantum mechanics approaches. Chem. Eng. J. . 337, 385-397. 23. Ren, L., Dong, J., Chi, Z. and Huang, H. 2018. Reduced graphene oxide-nano zero value iron (rGO-nZVI) micro-electrolysis accelerating Cr(VI) removal in aquifer. Journal of Environmental Sciences. 25
24. Ren, X., Li, J., Tan, X. and Wang, X. 2013. Comparative study of graphene oxide, activated carbon and carbon nanotubes as adsorbents for copper decontamination. Dalton Trans. 42, 5266-5274. 25. Shao, D., Chen, C. and Wang, X. 2012. Application of polyaniline and multiwalled carbon nanotube magnetic composites for removal of Pb(II). Chem. Eng. J. . 185-186, 144-150. 26. Shih, Y.-h., Chou, H.-L., Peng, Y.-H. and Chang, C.-y. 2012. Synergistic effect of microscale zerovalent iron particles combined with anaerobic sludges on the degradation of decabromodiphenyl ether. Bioresour. Technol. 108, 14-20. 27. Song, D.-l., Hou, S.-p., Wang, X.-b., Liang, G.-q. and Zhou, W. 2018. Nutrient resource quantity of crop straw and its potential of substituting Journal of plant nutrition and fertilizer. 24, 1-21. 28. Su, H., Fang, Z., Tsang, P.E., Fang, J. and Zhao, D. 2016. Stabilisation of nanoscale zero-valent iron with biochar for enhanced transport and in-situ remediation of hexavalent chromium in soil. Environ. Pollut. 214, 94-100. 29. Teng, W., Fan, J., Wang, W., Bai, N., Liu, R., Liu, Y., Deng, Y., Kong, B., Yang, J., Zhao, D. and Zhang, W.-x. 2017. Nanoscale zero-valent iron in mesoporous carbon (nZVI@C): stable nanoparticles for metal extraction and catalysis. J. Mater. Chem. A. 5, 4478-4485. 30. Tran, H.V., Tran, L.D. and Nguyen, T.N. 2010. Preparation of chitosan/magnetite composite beads and their application for removal of Pb(II) and Ni(II) from aqueous solution. Materials Science & Engineering C. 30, 304-310. 31. Wang, N., Ouyang, X.K. and Yang, L.Y. 2017a. Fabrication of a magnetic cellulose 26
nanocrystal/metal-organic framework composite for removal of Pb(II) from water. ACS Sustainable Chem. Eng. 5, 10447–10458. 32. Wang, S., Zhou, Y., Gao, B., Wang, X., Yin, X., Feng, K. and Wang, J. 2017b. The sorptive and reductive capacities of biochar supported nanoscaled zero-valent iron (nZVI) in relation to its crystallite size. Chemosphere. 186, 495-500. 33. Wu, Y., Yue, Q., Ren, Z. and Gao, B. 2018. Immobilization of nanoscale zero-valent iron particles (nZVI) with synthesized activated carbon for the adsorption and degradation of Chloramphenicol (CAP). J. Mol. Liq. 262, 19-28. 34. Xiao, F., Li, W., Fang, L. and Wang, D. 2016. Synthesis of akageneite (beta-FeOOH)/reduced graphene oxide nanocomposites for oxidative decomposition of 2-chlorophenol by Fenton-like reaction. J. Hazard. Mater. 308, 11-20. 35. Xiu, Z.-m., Jin, Z.-h., Li, T.-l., Mahendra, S., Lowry, G.V. and Alvarez, P.J.J. 2010. Effects of nano-scale zero-valent iron particles on a mixed culture dechlorinating trichloroethylene. Bioresour. Technol. 101, 1141-1146. 36. Yan, J., Han, L., Gao, W., Xue, S. and Chen, M. 2015. Biochar supported nanoscale zerovalent iron composite used as persulfate activator for removing trichloroethylene. Bioresour. Technol. 175, 269-274. 37. Yang, F., Sun, L., Zhang, W. and Zhang, Y. 2017a. One-pot synthesis of porous carbon foam derived from corn straw: atrazine adsorption equilibrium and kinetics. Environ. Sci.: Nano. 4, 625-635. 38. Yang, F., Zhang, S., Li, H., Li, S., Cheng, K., Li, J.-S. and Tsang, D.C.W. Corn straw-derived biochar impregnated with α-FeOOH nanorods for highly effective copper 27
removal. Chem. Eng. J. . 39. Yang, F., Sun, L., Xie, W., Jiang, Q., Gao, Y., Zhang, W. and Zhang, Y. 2017b. Nitrogen-functionalization biochars derived from wheat straws via molten salt synthesis: An efficient adsorbent for atrazine removal. Sci. Total Environ. 607-608, 1391-1399. 40. Yang, F., Zhang, W., Li, J., Wang, S., Tao, Y., Wang, Y. and Zhang, Y. 2017c. The enhancement of atrazine sorption and microbial transformation in biochars amended black soils. Chemosphere. 189, 507-516. 41. Zhang, Y., Cao, B., Zhao, L., Sun, L., Gao, Y., Li, J. and Yang, F. 2018. Biochar-supported reduced graphene oxide composite for adsorption and coadsorption of atrazine and lead ions. Appl. Surf. Sci. 427, 147-155. 42. Zhang, Z.-Z., Xu, J.-J., Shi, Z.-J., Bai, Y.-H., Cheng, Y.-F., Hu, H.-Y. and Jin, R.-C. 2017. Unraveling the impact of nanoscale zero-valent iron on the nitrogen removal performance and microbial community of anammox sludge. Bioresour. Technol. 243, 883-892. 43. Zhu, S., Ho, S.-H., Huang, X., Wang, D., Yang, F., Wang, L., Wang, C., Cao, X. and Ma, F. 2017a. Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance. ACS Sustainable Chem. Eng. 5, 9673-9682. 44. Zhu, S., Ho, S.H., Huang, X., Wang, D., Yang, F., Wang, L., Wang, C., Cao, X. and Ma, F. 2017b. Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance. ACS Sustainable Chem. Eng. 5, 45. Zhu, Y., Li, H., Zhang, G., Meng, F., Li, L. and Wu, S. 2018. Removal of hexavalent chromium from aqueous solution by different surface-modified biochars: Acid washing, 28
nanoscale zero-valent iron and ferric iron loading. Bioresour. Technol. 261, 142-150. 46. Zou, Y., Wang, X., Khan, A., Wang, P., Liu, Y., Alsaedi, A., Hayat, T. and Wang, X. 2016. Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review. Environ. Sci. Technol. 50, 7290-7304.
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30
Highlights •
Corn stalk biochar is engineered with hydrophilic functionality and hierarchical pores.
•
Fe0-HCS composites combine advantages of porous biochar with iron nanoparticles.
•
Fast kinetics and high adsorption capacity of Pb 2+, Cu 2+ and Zn2+ are demonstrated.
•
Multiple mechanisms are involved in metal removal by Fe0-HCS composites.
31
S0960-8524(18)30808-3 https://doi.org/10.1016/j.biortech.2018.06.029 BITE 20046
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
3 May 2018 9 June 2018 11 June 2018
Please cite this article as: Yang, F., Zhang, S., Sun, Y., Cheng, K., Li, J., Tsang, D.C.W., Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.06.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and characterization of hydrophilic corn stalk biochar-supported nanoscale zero-valent iron composites for efficient metal removal Fan Yanga,b, Shuaishuai Zhanga, Yuqing Sunb, Kui Chengc, Jiangshan Lib, Daniel C.W. Tsangb* a
School of Water Conservancy and Civil Engineering, Northeast Agricultural University,
Harbin 150030, China b
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong, China c
Key Laboratory of Superlight Material and Surface Technology of Ministry of Education,
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China *Corresponding Author: [email protected]
Abstract Pyrolyzing low-cost agro-waste into biochar is a promising means for waste biomass utilization. This study engineers corn stalk-derived biochar with abundant hydrophilic functional groups as a support material for iron nanoparticles impregnation (nZVI-HCS). Surface chemistry and morphology of nZVI-HCS composites is characterized by SEM, TEM, TG, XRD, FTIR, XPS, and BET techniques, which helps to elucidate the mechanisms of Pb2+, Cu2+ and Zn2+ removal from single and mixed-metal solutions in batch experiments. Equilibrium adsorption capacities can reach 195.1, 161.9 and 109.7 mg·g-1 for Pb 2+, Cu2+ and Zn2+ at neutral medium after 6-h process, respectively. The engineered biochar with 1
hierarchical pores can impregnate iron nanoparticles, serve as an adsorbent, and enhance metal reduction/precipitation. Rapid removal and high performance can be maintained after five regeneration/reuse cycles. Multiple interaction mechanisms including adsorption, precipitation, reduction and complexation are responsible for metal removal by nZVI-HCS composites, which can be a novel biowaste-derived material for wastewater treatment. Keywords: Engineered biochar; Potentially toxic elements; Waste biomass valorization; nZVI-carbon composites; Iron-based nanomaterials; Wastewater treatment.
1. Introduction Rapid urbanization and industrialization have brought about excessive release of inorganic and organic contaminants into the environment and present health threat to humans and animals (Gong et al. 2016, Zou et al. 2016). Contamination of groundwater by heavy metals such as Cu2+, Pd2+ and Zn2+ resulting from industries (metal plating, battery manufacturing, soldering, oil refining, etc.) can lead to various adverse health effects. For example, international organizations suggest the maximum limit of Pb2+ in drinking water to be 10 µg L-1 (Muthu et al. 2018), while ecotoxicity and bioaccumulation of Cu2+ and Zn2+ also lead to several adverse effects on target organs including kidney, liver, lung, skin, and nervous system (Lv et al. 2018, Ma et al. 2018, Ramezanzadeh et al. 2018). Iron-based materials with low expense and high treatment efficiency have attracted increasing interests in wastewater treatment (Shih et al. 2012, Zhang et al. 2017). Nanoscale zero valent iron (nZVI) exhibits a promising potential because that it is an environmentally-friendly reducing agents with the characteristics of small particle size, 2
abundant active sites and fast reaction (Lu et al. 2016, Xiu et al. 2010). Remediation by nZVI-based materials is commonly employed for the removal of metals/metalloids and organic pollutants from water/wastewater. For instance, Krzisnik et al. (2014) reported that nZVI could be effective for the removal of Zn2+, Zn(II)-EDTA and Zn(II)-citrate in aqueous solutions. Huang et al. (2013) compared the removal performance of Cd(II), Cr(IV), and Pb(II) in aqueous solution with nZVI particles electrosprayed or non-electrosprayed using rapid magnetic separation means. However, challenges to environmental applications of nZVI include its thermodynamic instability, easy oxidation, short duration of reactivity, poor air stability, and rapid agglomeration (Mu et al. 2017, Ren et al. 2018, Wu et al. 2018). Assembling suitable porous carriers as support of nZVI is considered as an effective strategy to enhance its stability, for which various carbon-based materials such as activated carbon (Ren et al. 2013), graphene and its derivatives (Ren et al. 2018), carbon nanotubes (Hu et al. 2012, Shao et al. 2012), biochar (Yan et al. 2015, Zhu et al. 2017a, Zhu et al. 2018), and mesoporous carbon (Teng et al. 2017) are reported as potential candidates. The utilization of agricultural product residues for environmental remediation has received tremendous attention as they are inexpensive, abundant in source and renewable (Chen et al. 2018, Peng et al. 2017, Wang et al. 2017b). Statistical results from Chinese Academy of Agricultural Sciences indicate that there are 7.18 hundred million tons of crop straws in 2015 and corn stalks account for 34.2% (Song et al. 2018), which represent huge amounts of by-product from corn production. Biochar produced by combusting corn stalks (composed of lignin, cellulose, and hemicelluloses) under limited oxygen conditions contains high specific surface area, stable structures, and a large amount of surface sites, which can be employed to remove 3
environmental pollutants (Yang et al. 2017a, Yang et al. 2017b, Yang et al. 2017c). Utilization of biochar as porous carriers for supporting nZVI can also be efficient and economically beneficial. Qian et al. (2017a) evaluated the effectiveness of nZVI-biochar composites towards the removal of hexavalent chromium (Cr(VI)) from contaminated groundwater. Chen et al. (2018) investigated the preparation of a magnetic biochar derived from persulfate-ZVI treated sludge as an environmentally friendly biosorbent for lead removal. Nevertheless, to the best of our knowledge, few studies have explored the impregnation of nZVI into waste biomass-derived carbon foam with hierarchical pores (nZVI-HCS), and scrutinized its treatment effectiveness in single-/mixed-metal solutions. In this study, waste corn stalks serve as the raw materials for the fabrication of carbon foam with hierarchical porosity for supporting nZVI via chemical reduction method using borohydride. The removal capacities for Pb 2+, Cu2+ and Zn2+ in individual and co-existing scenarios are examined. This paper aims to develop a novel nZVI-HCS composite for metal removal and realize the following specific objectives: (1) elucidate the interactions between nZVI-HCS composites with various iron contents and heavy metals; (2) determine the effects of iron mass ratio, sample dosage, pH, contact time, and initial metal concentrations on the treatment effectiveness of nZVI-HCS sample; and (3) acquire insights into the underlying mechanisms on nZVI-HCS composites in single- and mixed-metal solutions, respectively. 2. Experimental Methods 2.1 Materials and Reagents Corn stalks (CS) were washed, dried, crushed, and sieved through a 100-mesh (pore size <165µm), collected from the campus of Northeast Agricultural University, Harbin, China. 4
Ferrous sulfate heptahydrate (FeSO4·7H2O), lead(II) nitrate (Pb(NO3)2), zinc(II) nitrate hexahydrate (Zn(NO3)2·6H2O), cupric nitrate hydrate (Cu(NO3)2·3H2O), ammonium persulfate ((NH4)2S2O8), sodium borohydride (NaBH4) , sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitric acid (HNO3) were purchased from Tianjin Chemical Reagent Co., Ltd, China. All solutions were prepared with ultrapure water (Millipore, 18 MΩcm). 2.2. Preparation of adsorbent The biochar preparation process is similar to that of Garfield et al. (2014). In a typical procedure, 2 g of corn stalks was mixed with 2 g of KHCO3 via grinding in a mortar, which is referred to optimum mass ratio reported in our previous paper (Yang et al. 2017a). Then, the mixtures were put in an aluminum porcelain boat and heated in a Tube furnace under an N2 atmosphere (200 mL·min-1) condition at 800 °C for 2 h, and heating rate were kept at 10 °C·min-1. In addition, before the heating treatment, it was performed at 25 °C for 30 min to remove residual oxygen and/or moisture in the tube. The pyrolysis product was washed with HCl (0.1 mol·L-1) for removal ash and activating agent, and washed with ultrapure water to neutral pH, and then dried at 60 °C for 12 h in the drying oven. Hydrophilic treatment of biochar was carried out as described in a previous study (Qian et al. 2017b), which serves to endow biochar with hydrophilic nature and abundant amount of oxygen-containing functional groups for subsequent impregnation of iron ions and formation of iron nanoparticles. The resulting biochars and hydrophilic biochars were referred as CS and HCS, respectively. The preparation of nZVI-HCS composite was performed as follows (Su et al. 2016): 0.42 g of hydrophilic biochars was dissolved in FeSO4·7H2O (100 mL 0.075 mol·L-1) solution with stirring for 2 h in a 250 mL three-neck flasks, and then an additional 4 h of 5
continuous stirring was implemented after introduction of NaBH4 (50 mL 0.3 mol·L-1). The reduction process of the whole aqueous solution was purged with nitrogen gas (N2) and the reaction proceeded according to the following equation (Eq. 1). 2Fe2+ + BH4- + 3H2O + HCS → 2Fe0/HCS +H2BO3- +4H+ +2H2
(1)
The final products were settled and separated from the liquid phase. Afterwards, the products were washed with ethanol and deoxygenated water, respectively, for several times and vacuum dried overnight at 60 ºC. The resulting products with the content of Fe:HCS mass ratio is 1:1 using the above procedures. For comparison, nZVI-HCS with iron nanoparticles to hydrophilic CS in mass ratio of 2:1 and 1:2 were also synthesized by varying the dose of HCS and FeSO4·7H2O. The resulting nZVI-HCS composites were referred to as nZVI-HCS (1:1), nZVI-HCS (2:1) and nZVI-HCS (1:2), respectively. The fabrication process is shown in Fig. 1. The prepared products were collected for subsequent characterization and batch sorption experiments.
Fig.1 Scheme of the preparation procedures of nZVI-HCS composites. 2.3 Surface Characterization The microstructures of samples were imaged on scanning electron microscopy (SEM, ZEISS SUPRA40) and transmission electron microscopy (TEM, Tecnai-G20). Thermo gravimetric (TG) analysis was performed with a synchronous thermal analyzer (SDT-Q600, TA Instruments, United States) to test thermal property of samples. X-ray diffraction (XRD) 6
patterns of the samples were analyzed on Rigaku TTR III with Cu Kα radiation in the range of 5∼90° (2θ). Fourier transform infrared spectrometry (FTIR) data was measured on a Nicolet Avatar 370DTGS spectrophotometer (Thermo Fisher Scientific, USA) with the scan region ranging from 400 to 4000 cm-1. X-ray photoelectron spectroscopy was made by using Al Kα radiation (XPS, Thermo ESCALAB 250). N2 adsorption/desorption measurements were measured at 77 K, using Micromeritics ASAP 2020. The specific surface area and pore volume
were
calculated
using
Brunauer-Emmett-Teller
(BET)
method
and
Barret-Joyner-Halenda (BJH) method, respectively. The metal concentrations in the aqueous solutions were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Varian Liberty 200. 2.4 Batch Sorption Experiments The stock solutions were prepared by dissolving appropriate amounts of Pb(NO3)2, Cu(NO3)2·3H2O, and Zn(NO3)2·6H2O in ultrapure water, respectively. Bath adsorption experiments were carried out by adding 10 mg of nZVI-HCS composites (or CS) to a 40 mL ultrapure water and 50 mg·L-1 Pb2+, Cu 2+, and/or Zn2+ solution at the different time intervals (0.16, 0.5, 1, 2, 4 and 6 h) in a 150-rpm thermostatic reciprocating shaker at room temperature (25±1 ºC). The solutions were immediately filtered through 0.45 µm pore size membranes filters with an initial pH of 7 adjusted by using either 0.1 mol·L-1 HCl or 0.1 mol·L-1 NaOH. The equilibrium adsorption capacity (qe) and removal efficiency (R%) of Pb2+, Cu2+ and Zn2+ were calculated using the following equations: q = R=
×100 %
(2) (3) 7
Where C0 and Ce are the initial and equilibrium metal concentrations in solution (mg·L-1), respectively; V is the volume of adsorbate solution (mL), and m is the mass of the adsorbent (mg). 2.4.1 Kinetics The pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data as shown below: q = q 1 − e ) q =
(4)
5
Where qe (mg·g-1) is the adsorption capacity, qt (mg·g-1) is the adsorption capacity at time (h); k1 (h-1) and k2 (g·(mg·h)-1) are the rate constants for the pseudo-first-order and pseudo-second-order adsorption kinetic models, respectively. 2.4.2 Isotherms The isotherms experiments were performed in flasks containing 10 mg nZVI-HCS (2:1) and initial concentrations of Pb 2+ (5, 10, 30, 50, 70, and 100 mg·L-1). Langmuir and Freundlich isotherm models were applied to fit the experimental data of nZVI-HCS. The equilibrium models were shown below: q =
#
q = K ! C
(Langmuir model)
(6)
(Freundlich model)
(7)
Where q m is the maximum amount of coverage in the monolayer (mg·g-1 ); KL is the Langmuir constant (L·mg-1 ); Ce is the metal concentration at equilibrium (mg·L-1 ); and qe is
the amount of metals adsorbed at equilibrium (mg·g-1); KF (( mg·g-1 )( L·mg-1 )1/n ) and are $
the Freundlich constants. 8
2.4.3 Competitive studies The competitive experiments for Pb 2+ and Cu2+ (or Zn2+) in solution to 10 mg of nZVI-HCS sample were performed using initial concentrations of 50 mg·L-1 for each metal added to 40 mL of ultrapure water. Subsequently, the adsorbents were separated as previously described in the adsorption experiments, then the analysis of residual Pb2+ concentrations in the supernatant. 2.4.4 Recyclability of nZVI-HCS composites The reusability of nZVI-HCS sample was tested by repeated Pb2+adsorption and desorption cycles for five consecutive cycles. The regeneration of the saturated adsorbent was conducted under using filtration and desorbed using 30 mL ultrapure water for 10 h. In each cycle, 10 mg of fresh or regenerated nZVI-HCS sample was suspended in 40 mL of 50 mg·L-1 Pb2+ solution at 25 ºC under shaking for 6 h. The removal efficiency was measured and compared after each cycle. 3. Results and Discussion 3.1 Characterization of nZVI-HCS composites The surface morphology of nZVI-HCS sample was observed by SEM and TEM analysis (Supporting Information) and the mass ratio of nZVI-HCS at 2:1 is chosen as an example. It is evident that as-fabricated nZVI-HCS sample had abundant porous structure with relatively rough and irregular surfaces. It can be observed that iron nanoparticles synthesized from FeSO4·7H2O reduction by borohydride were uniformly dispersed on the surface without obvious accumulation and the size of iron particles was small in nanoscale. Low- and highmagnification TEM images can provide more detailed structural information, that is, iron 9
nanoparticles have shown in the shape of round with size of down to 100 nm, which are well dispersed within the biochar matrix, in agreement with the observation from SEM analysis. In general, the as-prepared iron particles was often found to agglomerate together, hence, the application of HCS as a porous support effectively prevents the as-formed nanoparticles from irreversible aggregation, which can provide large surface area and maintain high reactivity for subsequent metal removal. Thermal stabilities of the nZVI-HCS composite and porous carbon carrier were estimated using TG analysis (Supporting Information). Both nZVI-HCS composite and carbon carrier displayed obvious difference in their thermal stability. The weight loss for nZVI-HCS composite is about 10.3 % over the temperature range from 30 °C to 300 °C, mainly due to the removal of pre-adsorptive water molecules on samples. The weight of nZVI-HCS composite kept almost unchanged with the increasing of the temperature from 600 °C to 800 °C, indicating its high stability in the tested temperature range. Ash contents after combustion in the air atmosphere was only 2.9 % for porous carbon carrier, much lower than that for nZVI-HCS composite, suggesting existence of Fe0 or iron oxides in the as-prepared composite (Baikousi et al. 2015). The crystalline structure of nZVI-HCS samples was analyzed by XRD spectra (Supporting Information). The as-prepared three samples exhibited a broad and weak diffraction peak centered at 2θ=21.6°, attributed to the sp3 hybridized carbon in amorphous form. As shown, three strong diffraction peaks of 44.7°, 65.0° and 82.3° were present on nZVI-HCS composite, corresponding to plane (100), (200) and (211) of iron element (JCPDS, NO.87-0721), respectively. These results revealed the formation of well-crystallized 10
zero-valent iron on the hydrophilic biochars. Some minor peaks of Fe-O compounds, for example, peak at 35.7° for Fe2O3, were also observed, which might result from oxidation and corrosion during synthesis and drying process. For comparison, XRD planes of other nZVI-HCS samples are also shown, which were consistent with the report from Ahmed et al. (2017), indicating the formation of zero-valent iron on the surface of HCS. The surface functional groups are recognized as a critical factor that determines the adsorption behavior of metal removal. FTIR spectra for different samples were carried out in the range of 400-4000 cm-1 (Supporting Information). The broad absorption band centered at 3431 cm-1 can be attributed to O-H stretching vibration of alcohol, phenol or carboxylic groups due to the presence of hemicelluloses, lignin or cellulose. The absorbance peak at 1628 cm-1 is associated with C=O stretching vibration of carbonyl and carboxyl groups. The band at 1378 cm-1 can be attributed to the COO- groups of nZVI-HCS (Su et al. 2016), and the band (1126 cm-1) is assigned to the C-C bond vibrations. The band at 1126 cm-1 corresponds to the C-O or C-O-C stretching vibration modes. Compared with spectrum of CS and HCS exhibited in our previous report (Yang et al. 2018), the weak peaks of Fe-O fall in the ranges of 730-470 cm-1, confirming that Fe3+ was successfully attached to carbon surface by chelating with oxygen-containing groups, which confirmed the formation of metal oxides in HCS surface (Prabu et al. 2017). The FTIR spectrum indicated that the as-prepared nZVI-HCS contained abundant oxygen functional groups, which could act as available adsorption sites. The oxygen atoms in C=O and O-H groups could provide free pairs of electrons to interact with the empty orbital of metal ions (Pb 2+,Cu2+ and Zn2+), which could form coordinately banded complexes. Thus, the abundance of oxygen functional groups 11
could play a vital role in the metal removal process, and further discussion will be provided in later section. The chemical composition of nZVI-HCS samples were further studied via XPS spectra (Supporting Information). The peaks at around 284.85 eV, 531.66 eV, and 712.22 eV are shown in the XPS survey spectrum of nZVI-HCS samples, which corresponds to C, O and Fe elements, respectively. Fe 2p spectra of nZVI-HCS sample can be deconvoluted into two main bands at 724.7 eV and 711.2 eV assigned to oxidized iron binding energies of Fe 2p 1/2 and Fe 2p 3/2, respectively (Xiao et al. 2016). Both the O 1s and C 1s regions could be deconvoluted into three peaks, suggesting the presence of oxygen-containing functional groups (such as C-O, O-H and C=O) (Yang et al. 2017a). There can be also observed three different peaks assigned to Fe-O-H, Fe-O-C, and Fe-O-Fe, demonstrating the interaction between iron nanoparticles and porous carbon support. In addition, Fe 2p spectra of nZVI-HCS samples with various mass ratios were also displayed. The BET specific surface area, average pore diameter and total pore volume for nZVI-HCS sample were calculated as 603.4 m2·g-1, 3.14 nm and 0.474 cm3·g-1, respectively (Supporting Information). The specific surface areas of CS and HCS are 1236 and 1216 m2·g-1, respectively, in our previous studies (Yang et al. 2018). However, iron particles loaded on the surface of biochar blocked the micropores of biochar, resulting in the significant decrease in specific surface area. The pore-size distribution is similar to type-II model with H4 type hysteresis loop at medium relative pressure (P/P0=0.4-0.95) resulting from capillary condensation, demonstrating the existence of micropores and mesopores (Asmel et al. 2017, Qiu et al. 2014). The pore size distributions of the resulting samples are obtained using the 12
Barret-Joyner-Halenda (BJH) analysis. The curve presents the continuous distribution in the range from 0-100 nm, with the major distributions in the size range of < 2 nm, which would endow large specific surface area of nZVI-HCS composites for adsorption capability. 3.2 Single-/mixed-metal removal by nZVI-HCS composites 3.2.1 Kinetics
Fig. 2 Kinetic studies for Pb2+ (a), Cu2+ (b) and Zn2+ (c) adsorption over the nZVI-HCS composites. Removal efficiency of Pb2+, Cu2+ and Zn2+ in single-metal solutions (d) (metal concentration: 50 mg· L-1, T = 25 °C, adsorbent dose =10 mg).
The kinetic studies were performed to compare the removal efficiencies of heavy metals (Pb 2+, Cu 2+ and Zn2+) by the synthesized nZVI-HCS composites and CS in Fig. 2a-c. For CS sample, the removal rate of Pb 2+, Cu 2+ and Zn2+ increased rapidly within contact time from 0 13
to 2 h and reached equilibrium with increasing contact time. For nZVI-HCS composites obtained from different iron content, they have shown high adsorption capacity for Pb2+ removal, following the order of nZVI-HCS (2:1) > nZVI-HCS (1:1) > nZVI-HCS (1:2) > CS. The removal progress of heavy metals may be involved into multistep reactions (as discussed in Section 3.3) and it gradually increased with increasing contact time before slowly attaining apparent equilibrium. After 6-h reaction (Fig. 2d), the Pb2+, Cu2+ and Zn2+ removal efficiency of nZVI-HCS (2:1) sample was 97.5%, 81.0%, and 54.8%, respectively, which was therefore chosen as a model material for further investigation. Moreover, there were consistent results for the improved metal removal efficiency with higher Fe content on biochar surface, which indicates that iron nanoparticles play a key role in the removal process. The removal mechanisms of Pb2+, Cu2+ and Zn2+ may also depend on the loading content of iron in the nZVI-HCS composites, which were discussed later.
14
Fig. 3 Adsorption kinetic plots of Pb2+ on nZVI-HCS composite fitted by pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) (heavy metals concentration: 50 mg· L-1 , T = 25 °C, adsorbent dose =10 mg); Adsorption isotherms of Pb2+ by nZVI-HCS sample fitted with Langmuir (c) and Freundlich model (d).
As shown in Fig. 3a, the adsorption of 50 mg·L-1 Pb2+ by nZVI-HCS sample reached equilibrium within 30 min, whereas the removal of Zn2+ and Cu2+ sharply increased in 10 min and gradually increased till reaching equilibrium in 4 h. Compared to pseudo-first-order kinetic model, the higher correlation coefficient obtained from pseudo-second-order kinetic model (Fig. 3b, R2=0.999) demonstrated that the chemical adsorption process on nZVI-HCS sample may be the rate limiting step, of which all the kinetic parameters are shown in Table 1. Ma et al. (2017) prepared graphene oxide/Fe3O4 composite and reported that the adsorption 15
capacity of graphene oxide/Fe3O4-g-G3.0 approached equilibrium at 100 min and the adsorption capacity for Pb2+ was 181.4 mg·g-1 at 298K. Ngah et al. (2010) have synthesized chitosan-tripolyphosphate bead and its equilibrium time and adsorption capacity for Pb2+ were 100 min and 57.3 mg·g-1, respectively. Hence, nZVI-HCS composites introduced in this study displayed highly competitive performance and removal rate. Table 1. Kinetic model parameters for the adsorption of Pb2+, Cu2+ and Zn2+ onto nZVI-HCS sample. Pseudo-first order Qe.(mg· g-1)
k1
Pseudo-second order R2
Qe. (mg· g-1)
k2
R2
Pb2+
194.7
23.5
0.999
195.3
1.22
0.999
Cu2+
146.1
2.41
0.819
158.3
0.0248
0.911
Zn2+
98.5
7.25
0.920
105.5
0.0968
0.963
3.2.2 Isotherms The equilibrium adsorption isotherms of nZVI-HCS sample after 6-h reaction at different temperatures (25 °C, 35 °C and 55 °C) are shown in Fig. 3c-d. The parameters of Langmuir and Freundlich isotherm models are summarized in Table 2. The data were better fitted with Langmuir model (R2=0.94-0.95) than Freundlich model (R2=0.82-0.89). The nZVI-HCS composite showed significant adsorption affinity towards Pb2+ with maximum Pb2+ adsorption of 291.3, 288.2 and 323.7 mg g-1 at three different temperatures (25 °C, 35 °C and 55 °C), respectively. It is important to note that the adsorption capacity of nZVI-HCS composites in this study was obviously higher than other reported materials (Supporting Information). For example, Tran et al. (2010) reported that the maximum adsorption capacities for Pb2+ and Ni2+ were 63.3 and 52.6 mg·g-1 on chitosan/magnetite composite
16
beads under room temperature. These comparisons demonstrated the potential of nZVI-HCS as an effective adsorbent. Table 2. Isotherm model parameters for the adsorption of Pb2+ on the nZVI-HCS sample at different temperatures. Freundlich T(°C)
KF -1
-1 1/n
(mg· kg )/(mg· L ) Pb2+
Langmuir
1 n
R2
Qmax (mg· g-1)
KL -1
R2
(L· mg )
25
132.7
0.263
0.888
291.3
1.19
0.950
35
137.7
0.260
0.822
288.2
2.02
0.939
55
165.7
0.275
0.826
323.7
2.58
0.945
3.2.3 Kinetics in mixed-metal solutions
Fig. 4 Comparative mixed removal performance of Pb 2+, Cu 2+ and Zn2+ solution using 17
nZVI-HCS sample. The removal kinetics of bi-solute system (Pb2+ and Cu2+ or Zn2+) by nZVI-HCS sample is shown in Fig. 4. There was a significant decrease for Pb2+ adsorption with co-existence of Cu2+ (or Zn2+) ions, suggesting competitive adsorption between Pb 2+ and Cu2+ (or Zn2+) such that the adsorption capacity was decreased from 195 mg ·g-1 to 137.6 mg·g-1 (or 154.8 mg· g-1). More detailed discussion on the removal mechanisms in bi-solute system is presented in Section 3.3. 3.2.4 Recyclability Recyclability is one of the most important indicators evaluating the applicability of nZVI-HCS. The decrease from 195.0 mg·g-1 to 98.4 mg·g-1 after five cycles probably because the iron nanoparticles were exhausted by chemical reduction and adsorbed Pb2+ ions were not totally extracted (Supporting Information). As shown, nZVI-HCS maintained ~50% of the Pb2+ removal efficiency after several adsorption-desorption cycles, suggesting that the nZVI-HCS sample has good reusability and stability. 3.3 Discussion on adsorption mechanisms by nZVI-HCS composites The redox potentials of Cu 2+/Cu0 (+0.34 V) and Pb2+/Pb0 (-0.13 V) were much higher than Fe2+/Fe0 (-0.44 V) and Zn2+/Zn0 (-0.76 V). Therefore, Cu2+ and Pb 2+ can be more easily reduced by nZVI than Zn2+, which implies that Zn2+ removal occurs via co-precipitation and complexation rather than reduction. However, the removal of Pb2+ was more significant than Cu2+ by nZVI-HCS sample (Fig. 2), which might be due to the higher electronegativity and smaller hydrated radius of Pb2+ that accounted for its higher affinity toward organic functional groups. A significant decrease was observed using nZVI-HCS sample when Pb2+ 18
and Cu2+ (or Zn2+) co-existed, suggesting the co-existence of ions exert significant influence on the equilibrium adsorption capacity and existence of competitive adsorption between Pb2+ and Cu2+ (or Zn2+). The co-adsorption of Pb 2+ and Cu2+ (or Zn2+) involved competitive active sites, leading to a decline in the adsorption of Pb2+. In addition, our previous work (Zhang et al. 2018) has indicated that higher Pb2+ removal by biochar may be owing to the formation of Pb-phosphate precipitate β-Pb9(PO4)6 and hydrocerussite Pb3(CO3)2(OH)2 with the minerals in biochar, which can be expressed as follows: + + 6OH → Pb PO + 6H O 6HPO+ 0 * 1 + * + 9Pb + 2HCO + 4OH → Pb3 CO3 + OH + + 2H+O 3 + 3Pb
(8) (9)
The possible reactions of Pb2+, Cu 2+, or Zn2+in nZVI-HCS composite materials and CS are briefly concluded as follows: M2+ + OH/ C=O/COOH-Corn stalks→M− − OH/ C=O/COOH-Corn stalks (coordination complexes)
(10)
M2+ + Fe0→M− −Fe2+ (adsorption)
(11)
M2+ + Fe0→M + Fe2+ (reduction)
(12)
+ M2+ + HCO 3 / HPO* -Corn stalks→M0 PO* 1-Corn stalks (precipitation)
(13)
(M2+= Pb2+, Cu2+, or Zn2+) To gain further insights into the removal mechanisms, XPS, FTIR and XRD analysis of nZVI-HCS sample was conducted before and after the adsorption of Pb2+. As evident from the XRD pattern of nZVI-HCS (2:1) after sorption of Pb2+ (Supporting Information), the characteristics peak of nZVI-HCS (2:1) was observed at 2θ = 36.4° in correspondence to the (200), which was ascribed to the Pb0 resulting from the reduction of Pb 2+. After sorption of 19
Pb2+, XRD characteristic peaks of Fe0 were not significant in nZVI-HCS (2:1). As the reaction proceeded, the Fe0 on the surface of biochar was gradually transformed into iron oxides. The XPS full survey (Supporting Information) clearly shows an apparent weak peak at 138.77 eV after reaction, indicating that Pb 2+ was attached onto the nZVI-HCS surface. For the Fe 2p survey before and after Pb2+ treatment, nZVI-HCS sample peaks before removal at 725.31 eV and 711.4 eV could be assigned to the binding energies of Fe 2p 1/2 and Fe 2p 3/2 (Luo et al. 2014, Zhu et al. 2017b), which can be decomposed into two or three peaks, respectively. The peaks located at approximately 711.26 eV and 725.08 eV were in correspondence with binding energies of 2p 3/2 and 2p 1/2 of Fe3+. A fitting peak at 709.18 eV could be assigned to the 2p 3/2 peak of Fe0, suggesting the presence of Fe0 species in the nZVI-HCS sample. Photoelectron peaks at 712.5 eV and 723.93 eV were observed in the nZVI-HCS sample, indicating the co-presence of Fe2+. These peaks represent the binding energies of the Fe 2p that mainly originated from the iron oxide in the nZVI-HCS sample. The species of Fe2+, Fe0 and Fe3+ account for a proportion of about 30.9%, 19.0% and 50.1% in the total iron surface atoms before Pb2+ adsorption, respectively. However, the proportion of Fe2+ and Fe3+ in the total iron atoms increases after Pb 2+ adsorption (39.6% and 60.4%, respectively), indicating Fe0 has been mostly converted to Fe2+ and Fe3+ during the Pb2+ remove process. Binding energy profiles of O (O1s) and Pb (Pb 4f) after adsorption in nZVI-HCS sample are shown in the Supporting Information. The appearance of the Pb 4f7/2 and Pb 4f5/2 peak at 139.02 eV and 143.79 eV after adsorption confirms that nZVI-HCS sample can adsorb Pb2+. 20
It is worth noting that the existence of Pb0 species in Pb 4f spectrum of the Pb2+-adsorbed sample further indicates that reduction reaction may occur during the removal progress. The FTIR peaks at 3431, 1628, and 1378 cm-1 also decreased in intensity after the loading of Pb2+ on nZVI-HCS sample, which could be attributed to complexation of the C=O and O-H with Pb2+ as reported earlier (Wang et al. 2017a). Therefore, it can be concluded that after reaction with nZVI-HCS sample, most of iron nanoparticles on the surface were consumed and transformed into iron oxide, resulting in a decrease in the removal capacity of the composites. The possible interaction mechanism for metal removal (Pb2+ as an example) includes two key steps: (i) adsorption of free Pb2+ from water solution onto active sites of nZVI-HCS sample; (ii) then the reduction of surface adsorbed Pb 2+ to Pb0 by the metallic Fe. Moreover, CS plays an important role in Pb2+ immobilization because of the aggregate-preventive function for iron nanoparticles and its intense inherent adsorption affinity for Pb2+. In addition, the removal of Pb2+ is also partly attributed to the mineral contents of biochar containing N, S, P elements. Hence, the metal removal by nZVI-HCS composite was controlled by surface adsorption via electrostatic attraction, abiotic reduction, and formation of precipitates and surface complexes as vividly illustrated in the Supporting Information. 4. Conclusions Highly efficient performance for metal removal can be realized using corn stalk-derived hydrophilic biochar supported nZVI (e.g., 291.3 mg·g-1 for Pb2+ following pseudo-second order kinetics). The synthesized nZVI-HCS sample was characterized by homogeneous impregnation of iron nanoparticles on the surface of biochar with hierarchical pores. The 21
porous biochar carrier can facilitate the metal sequestration by alleviating aggregation of iron nanoparticles, enhancing metal reduction, and acting as an adsorbent itself. The co-existence of ions exerts notable influence on the removal capacity via competitive adsorption. Regeneration/reuse tests further suggested that nZVI-HCS composite can be potentially applied for wastewater treatment.
Acknowledgment We gratefully acknowledge the financial support of University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2017018), the National Nature Science Fund for Young Scholars (31600413), “Young Talents” Project of Northeast Agricultural University (17XG03), and Hong Kong Research Grants Council (PolyU 15222115 and 15223517).
Appendix A. Supplementary data
References 1. Ahmed, M.B., Zhou, J.L., Ngo, H.H., Guo, W., Johir, M.A., Sornalingam, K., Belhaj, D. and Kallel, M. 2017. Nano-Fe0 immobilized onto functionalized biochar gaining excellent stability during sorption and reduction of chloramphenicol via transforming to reusable magnetic composite. Chem. Eng. J. . 322, 571-581. 2. Asmel, N.K., Yusoff, A.R.M., Krishna, L.S., Majid, Z.A. and Salmiati, S. 2017. High 22
concentration arsenic removal from aqueous solution using nano-iron ion enrich material (NIIEM) super adsorbent. Chem. Eng. J. . 317, 343-355. 3. Baikousi, M., Georgiou, Y., Daikopoulos, C., Bourlinos, A.B., Filip, J., Zbořil, R., Deligiannakis, Y. and Karakassides, M.A. 2015. Synthesis and characterization of robust zero valent iron/mesoporous carbon composites and their applications in arsenic removal. Carbon. 93, 636-647. 4. Chen, Y.-d., Ho, S.-H., Wang, D., Wei, Z.-s., Chang, J.-S. and Ren, N.-q. 2018. Lead removal by a magnetic biochar derived from persulfate-ZVI treated sludge together with one-pot pyrolysis. Bioresour. Technol. 247, 463-470. 5. Garfield, L.D., Dixon, D., Nowotny, P., Lotrich, F.E., Pollock, B.G., Kristjansson, S.D., Doré, P.M. and Lenze, E.J. 2014. Effect of synthesis methods on magnetic Kans grass biochar for enhanced As(III, V) adsorption from aqueous solutions. Biomass & Bioenergy. 71, 299-310. 6. Gong, Y., Tang, J. and Zhao, D. 2016. Application of iron sulfide particles for groundwater and soil remediation: A review. Water Res. 89, 309-320. 7. Hu, J., Yang, S. and Wang, X. 2012. Adsorption of Cu(II) on β‐cyclodextrin modified multiwall carbon nanotube/iron oxides in the absence/presence of fulvic acid. J. Chem. Technol. Biot. 87, 673-681. 8. Huang, P., Ye, Z., Xie, W., Chen, Q., Li, J., Xu, Z. and Yao, M. 2013. Rapid magnetic removal of aqueous heavy metals and their relevant mechanisms using nanoscale zero valent iron (nZVI) particles. Water Res. 47, 4050-4058. 9. Lu, H.J., Wang, J.K., Ferguson, S., Wang, T., Bao, Y. and Hao, H.X. 2016. Mechanism, 23
synthesis and modification of nano zerovalent iron in water treatment. Nanoscale. 8, 9962-9975. 10. Luo, S., Lu, T., Peng, L., Shao, J., Zeng, Q. and Gu, J.D. 2014. Synthesis of nanoscale zero-valent iron immobilized in alginate microcapsules for removal of Pb(II) from aqueous solution. J. Mater. Chem. A. 2, 15463-15472. 11. Lv, D., Liu, Y., Zhou, J., Yang, K., Lou, Z., Baig, S.A. and Xu, X. 2018. Application of EDTA-functionalized bamboo activated carbon (BAC) for Pb(II) and Cu(II) removal from aqueous solutions. Appl. Surf. Sci. 428, 648-658. 12. Ma, L., Wang, F., Yu, Y., Liu, J. and Wu, Y. 2018. Cu removal and response mechanisms of periphytic biofilms in a tubular bioreactor. Bioresour. Technol. 248, 61-67. 13. Ma, Y.X., Kou, Y.L., Xing, D., Jin, P.S., Shao, W.J., Li, X., Du, X.Y. and La, P.Q. 2017. Synthesis of magnetic graphene oxide grafted polymaleicamide dendrimer nanohybrids for adsorption of Pb(II) in aqueous solution. J. Hazard. Mater. 340, 407. 14. Mu, Y., Jia, F., Ai, Z. and Zhang, L. 2017. Iron oxide shell mediated environmental remediation properties of nano zero-valent iron. Environ. Sci.: Nano. 4, 27-45. 15. Muthu, M., Santhanam, M. and Kumar, M. 2018. Pb removal in pervious concrete filter: Effects of accelerated carbonation and hydraulic retention time. Construction and Building Materials. 174, 224-232. 16. N, K., A, M., AS, Š., L, Š., J, Š. and R, M. 2014. Nanoscale zero-valent iron for the removal of Zn2+, Zn(II)-EDTA and Zn(II)-citrate from aqueous solutions. Sci. Total Environ. 476-477, 20-28. 17. Peng, X., Liu, X., Zhou, Y., Peng, B., Tang, L., Luo, L., Yao, B., Deng, Y., Tang, J. and 24
Zeng, G. 2017. New insights into the activity of a biochar supported nanoscale zerovalent iron composite and nanoscale zero valent iron under anaerobic or aerobic conditions. RSC Adv. 7, 8755-8761. 18. Prabu, D., Parthiban, R., Ponnusamy, S.K., Anbalagan, S., John, R. and Titus, T. 2017. Sorption of Cu(II) ions by nano-scale zero valent iron supported on rubber seed shell. Iet Nanobiotechnology. 11, 714-724. 19. Qian, L., Zhang, W., Yan, J., Han, L., Chen, Y., Ouyang, D. and Chen, M. 2017a. Nanoscale zero-valent iron supported by biochars produced at different temperatures: Synthesis mechanism and effect on Cr(VI) removal. Environ. Pollut. 223, 153-160. 20. Qian, X., Meng, R., Yao, Z., Yue, D., Yu, H., Jia, J. and Zhao, Y. 2017b. Visible Light Assisted
Heterogeneous
Fenton-Like
Degradation
of
Organic
Pollutant
via
α-FeOOH/Mesoporous Carbon Composites. Environ. Sci. Technol. 51, 21. Qiu, B., Gu, H., Yan, X., Guo, J., Wang, Y., Sun, D., Wang, Q., Khan, M., Zhang, X. and Weeks, B.L. 2014. Cellulose derived magnetic mesoporous carbon nanocomposites with enhanced hexavalent chromium removal. J. Mater. Chem. A. 2, 17454-17462. 22. Ramezanzadeh, M., Asghari, M., Ramezanzadeh, B. and Bahlakeh, G. 2018. Fabrication of an efficient system for Zn ions removal from industrial wastewater based on graphene oxide nanosheets decorated with highly crystalline polyaniline nanofibers (GO-PANI): Experimental and ab initio quantum mechanics approaches. Chem. Eng. J. . 337, 385-397. 23. Ren, L., Dong, J., Chi, Z. and Huang, H. 2018. Reduced graphene oxide-nano zero value iron (rGO-nZVI) micro-electrolysis accelerating Cr(VI) removal in aquifer. Journal of Environmental Sciences. 25
24. Ren, X., Li, J., Tan, X. and Wang, X. 2013. Comparative study of graphene oxide, activated carbon and carbon nanotubes as adsorbents for copper decontamination. Dalton Trans. 42, 5266-5274. 25. Shao, D., Chen, C. and Wang, X. 2012. Application of polyaniline and multiwalled carbon nanotube magnetic composites for removal of Pb(II). Chem. Eng. J. . 185-186, 144-150. 26. Shih, Y.-h., Chou, H.-L., Peng, Y.-H. and Chang, C.-y. 2012. Synergistic effect of microscale zerovalent iron particles combined with anaerobic sludges on the degradation of decabromodiphenyl ether. Bioresour. Technol. 108, 14-20. 27. Song, D.-l., Hou, S.-p., Wang, X.-b., Liang, G.-q. and Zhou, W. 2018. Nutrient resource quantity of crop straw and its potential of substituting Journal of plant nutrition and fertilizer. 24, 1-21. 28. Su, H., Fang, Z., Tsang, P.E., Fang, J. and Zhao, D. 2016. Stabilisation of nanoscale zero-valent iron with biochar for enhanced transport and in-situ remediation of hexavalent chromium in soil. Environ. Pollut. 214, 94-100. 29. Teng, W., Fan, J., Wang, W., Bai, N., Liu, R., Liu, Y., Deng, Y., Kong, B., Yang, J., Zhao, D. and Zhang, W.-x. 2017. Nanoscale zero-valent iron in mesoporous carbon (nZVI@C): stable nanoparticles for metal extraction and catalysis. J. Mater. Chem. A. 5, 4478-4485. 30. Tran, H.V., Tran, L.D. and Nguyen, T.N. 2010. Preparation of chitosan/magnetite composite beads and their application for removal of Pb(II) and Ni(II) from aqueous solution. Materials Science & Engineering C. 30, 304-310. 31. Wang, N., Ouyang, X.K. and Yang, L.Y. 2017a. Fabrication of a magnetic cellulose 26
nanocrystal/metal-organic framework composite for removal of Pb(II) from water. ACS Sustainable Chem. Eng. 5, 10447–10458. 32. Wang, S., Zhou, Y., Gao, B., Wang, X., Yin, X., Feng, K. and Wang, J. 2017b. The sorptive and reductive capacities of biochar supported nanoscaled zero-valent iron (nZVI) in relation to its crystallite size. Chemosphere. 186, 495-500. 33. Wu, Y., Yue, Q., Ren, Z. and Gao, B. 2018. Immobilization of nanoscale zero-valent iron particles (nZVI) with synthesized activated carbon for the adsorption and degradation of Chloramphenicol (CAP). J. Mol. Liq. 262, 19-28. 34. Xiao, F., Li, W., Fang, L. and Wang, D. 2016. Synthesis of akageneite (beta-FeOOH)/reduced graphene oxide nanocomposites for oxidative decomposition of 2-chlorophenol by Fenton-like reaction. J. Hazard. Mater. 308, 11-20. 35. Xiu, Z.-m., Jin, Z.-h., Li, T.-l., Mahendra, S., Lowry, G.V. and Alvarez, P.J.J. 2010. Effects of nano-scale zero-valent iron particles on a mixed culture dechlorinating trichloroethylene. Bioresour. Technol. 101, 1141-1146. 36. Yan, J., Han, L., Gao, W., Xue, S. and Chen, M. 2015. Biochar supported nanoscale zerovalent iron composite used as persulfate activator for removing trichloroethylene. Bioresour. Technol. 175, 269-274. 37. Yang, F., Sun, L., Zhang, W. and Zhang, Y. 2017a. One-pot synthesis of porous carbon foam derived from corn straw: atrazine adsorption equilibrium and kinetics. Environ. Sci.: Nano. 4, 625-635. 38. Yang, F., Zhang, S., Li, H., Li, S., Cheng, K., Li, J.-S. and Tsang, D.C.W. Corn straw-derived biochar impregnated with α-FeOOH nanorods for highly effective copper 27
removal. Chem. Eng. J. . 39. Yang, F., Sun, L., Xie, W., Jiang, Q., Gao, Y., Zhang, W. and Zhang, Y. 2017b. Nitrogen-functionalization biochars derived from wheat straws via molten salt synthesis: An efficient adsorbent for atrazine removal. Sci. Total Environ. 607-608, 1391-1399. 40. Yang, F., Zhang, W., Li, J., Wang, S., Tao, Y., Wang, Y. and Zhang, Y. 2017c. The enhancement of atrazine sorption and microbial transformation in biochars amended black soils. Chemosphere. 189, 507-516. 41. Zhang, Y., Cao, B., Zhao, L., Sun, L., Gao, Y., Li, J. and Yang, F. 2018. Biochar-supported reduced graphene oxide composite for adsorption and coadsorption of atrazine and lead ions. Appl. Surf. Sci. 427, 147-155. 42. Zhang, Z.-Z., Xu, J.-J., Shi, Z.-J., Bai, Y.-H., Cheng, Y.-F., Hu, H.-Y. and Jin, R.-C. 2017. Unraveling the impact of nanoscale zero-valent iron on the nitrogen removal performance and microbial community of anammox sludge. Bioresour. Technol. 243, 883-892. 43. Zhu, S., Ho, S.-H., Huang, X., Wang, D., Yang, F., Wang, L., Wang, C., Cao, X. and Ma, F. 2017a. Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance. ACS Sustainable Chem. Eng. 5, 9673-9682. 44. Zhu, S., Ho, S.H., Huang, X., Wang, D., Yang, F., Wang, L., Wang, C., Cao, X. and Ma, F. 2017b. Magnetic Nanoscale Zerovalent Iron Assisted Biochar: Interfacial Chemical Behaviors and Heavy Metals Remediation Performance. ACS Sustainable Chem. Eng. 5, 45. Zhu, Y., Li, H., Zhang, G., Meng, F., Li, L. and Wu, S. 2018. Removal of hexavalent chromium from aqueous solution by different surface-modified biochars: Acid washing, 28
nanoscale zero-valent iron and ferric iron loading. Bioresour. Technol. 261, 142-150. 46. Zou, Y., Wang, X., Khan, A., Wang, P., Liu, Y., Alsaedi, A., Hayat, T. and Wang, X. 2016. Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review. Environ. Sci. Technol. 50, 7290-7304.
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Highlights •
Corn stalk biochar is engineered with hydrophilic functionality and hierarchical pores.
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Fe0-HCS composites combine advantages of porous biochar with iron nanoparticles.
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Fast kinetics and high adsorption capacity of Pb 2+, Cu 2+ and Zn2+ are demonstrated.
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Multiple mechanisms are involved in metal removal by Fe0-HCS composites.
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