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Site Energy Distribution and X‑ray Analyses of Nickel Loaded on Heterogeneous Adsorbents Bei Yan,† Catherine Hui Niu,*,†,‡ and Renfei Feng§ †

School of Environment and Sustainability, University of Saskatchewan, 117 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C8 ‡ Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A9 § Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 2V3 S Supporting Information *

ABSTRACT: The approximate site energy distributions were estimated to describe the adsorption of nickel ions on the heterogeneous surfaces of barley straw adsorbents based on the Langmuir−Freundlich model. The average site energy and standard deviation of the site energy distributions of the adsorbents were determined to analyze the interaction between the adsorbents and adsorbate, and the adsorption site heterogeneity. With higher site energy, the pretreated barley straw (PBS) has higher nickel adsorption capacity (59.5 mg/g) than most of the reported adsorbents. The bonding of nickel on PBS was investigated by X-ray absorption spectroscopies. X-ray absorption near edge structure results indicated that the nickel adsorbed on PBS remained as Ni(II). Extended X-ray absorption fine structure data indicated that the adsorbed Ni was bonded to 6 oxygen atoms of carboxyl groups and/or water molecules. The results and methodology in this work are transferable to investigate other adsorption systems for separation applications.

1. INTRODUCTION Nickel has been employed in many industries including mineral processing, electroplating, production of paints and batteries, manufacturing of sulfate, and porcelain enameling. However, large quantities of nickel wastewater, which are associated with diseases (e.g., dermatitis, nausea, chronic bronchitis, gastrointestinal distress, and lung cancer) and threaten human health, were generated.1,2 There is a need to effectively remove nickel ions from wastewater to ensure adequately treated effluent quality for various uses. Adsorption partitioning of a compound between the fluid phase and the solid phase, has been considered as one of the most effective and efficient technologies for removal of contaminants from aqueous systems. In previous research, barley straw, an abundantly generated agricultural byproduct mainly composed of cellulose, hemicellulose, and lignin, was used in adsorption of nickel from aqueous solution, showing its potential for removing contaminants.3,4 The electrostatic attraction between the negatively charged barley straw surface and positively charged nickel ions was suggested as the dominant interaction. However, the removal capacity needs to be improved and the secondary pollution caused by organic compounds release from adsorbents into aqueous solution needs to be addressed. It has been reported that acidic functional groups such as carboxyl (−COOH) were capable of adsorbing metal ions.5 Thus, increase of acidic functional groups on adsorbents may facilitate the nickel adsorption. © 2017 American Chemical Society

H3PO4 with heating was able to increase the percentage of acidic groups on wood surface and enhance the porous structures favorable for adsorption.6 Using a microwave radiation method is a potential way to solve the problems of thermal gradient and high cost of heating preparation.7 It was documented that adsorption properties associated with site energy distribution. Carter et al. proposed that adsorption affinity between adsorbate and adsorbent was associated with average site energy, and energetical heterogeneity of adsorbent surface could be evaluated by the width of site energy distribution.8 The analysis of site energy including its distribution is important for explaining adsorption data and mechanisms.9 Such work has not been done for nickel adsorption on heterogeneous adsorbents. In the present work, barley straw, representative as cellulosic byproducts from agriculture and related industries, was used as a model material to develop a methodology of making costeffective adsorbents with high adsorption capacity and stability for adsorption of nickel, a model heavy metal from aqueous solution. The nickel adsorption and desorption equilibrium were investigated. The adsorption mechanisms were investigated via analyses of site energy distribution, and bonding of Received: Revised: Accepted: Published: 8283

April 7, 2017 June 23, 2017 July 7, 2017 July 7, 2017 DOI: 10.1021/acs.iecr.7b01445 Ind. Eng. Chem. Res. 2017, 56, 8283−8291

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Industrial & Engineering Chemistry Research

into eq 1, the isotherm qe(Ce) can be written as a function of E*, expressed as qe(E*),

adsorbed Ni by X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS).

qe =

2. THEORY 2.1. Adsorption Isotherm. The Langmuir−Freundlich model10 has been widely and successfully used to describe adsorption phenomena. This equation was also selected for fitting the experimental data in this work and presented below: qe =

F(E*) =

1 + bCen

(1)

where Ce is the concentration of the adsorbate in the liquid phase at equilibrium (mg/L), qm is the maximum adsorption capacity of the adsorbent (mg/g), and b is the adsorption equilibrium constant related to the binding energy of the adsorption system (L/mg)n; n is indicative of the surface site heterogeneity of the adsorbent. 2.2. Approximate Site Energy Distribution Function. Equilibrium adsorption capacity associates with site energy distribution of the adsorbent. The total adsorption (qe) of an adsorbate by a heterogeneous surface could be obtained by the integration below:

∫0

1 + bCsne−nE * /RT

F(E*) =

∫0

where qh(E,Ce) is the homogeneous isotherm over local adsorption sites with adsorption energy E and F(E) is the site energy frequency distribution over a range of sites with homogeneous energies. Adsorption energy E refers to the difference of adsorption energies between the solute (adsorbate, nickel) and solvent (water) for a given adsorption site.8,11 The limits on the integration are most appropriately based on the minimum and maximum adsorption energies,12 while they are typically not known. Thus, it is generally assumed that the limits of E on the integral range from zero to infinity.12,13 In order to determine the site energy distribution, the Cerofolini approximation was applied,14,15 in which the equilibrium liquid phase concentration (Ce) of the adsorbate is related to the energy of adsorption (E) and given by

(3)

E* = E − Es

(4)

⎛ E* ⎞ ⎟ Ce = Cs exp⎜ − ⎝ RT ⎠

(5)

(7)

dE *

qmnbCsne−nE * /RT RT (1 + bCsne−nE * /RT )2

(8)

+∞

F(E*) dE* = qm

(9)

3. MATERIALS AND METHODS 3.1. Materials. The raw barley straw (RBS) was provided by the Poultry Centre of the University of Saskatchewan in Saskatoon of Canada. It was sun dried, crushed, and sieved to the sizes of 0.425−1.18 mm, and then the straw was further dried in an oven at 378.15 K and kept in a desiccator. Nickel solutions were prepared by dissolving hydrated nickel sulfate (NiSO4·6H2O, >98.9 wt %, Fisher Scientific) in deionized water. 0.1 M sulfuric acid (H2SO4, >98.0%, E.M Science) and 0.1 M sodium hydroxide (NaOH, >98.3 wt %, Fisher Scientific) were prepared for pH adjustment. Ni(CH3COO)2 (>99 wt %) as reference of nickel crystal structure, and 85% (w/v) H3PO4 were purchased from Fisher Scientific. 3.2. Pretreatment Methods. PBS was prepared according to the previous work.17 A total of 20 g of dried barley straws were mixed with 400 mL of a 5% (w/v) H3PO4 solution, stirred by magnetic force at 100 rpm for 24 h, and then filtered. The wet samples were transferred into a microwave furnace (Rival, 700 W) for 9 min. During the heating process, the temperature increased from room temperature to the maximum value of 844.15 K. After radiation, the samples were mixed with deionized water and heated to 353.15−363.15 K for 30 min to remove residual H3PO4 and other salts until the filtrate pH became constant at about 4. The wet samples were dried at 378.15 K. The characterizations of PBS, such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) surface area, were done in the author’s previous works.17,18 3.3. Batch Adsorption−Desorption Experiments. Adsorption experiments were performed in a titrator (SCHOTT Instruments TITRONIC universal) in which the solution pH was autoadjusted to the desired values and kept constant at 298.15 ± 0.50 K. A total of 600 mg of adsorbent (RBS or PBS) were added into 300 mL of a nickel sulfate solution and magnetically stirred at a speed of 200 rpm. The initial concentrations of nickel (Ni2+) ranged from 25 to 1000 mg/L. Adsorption experiments were run for 5 h to ensure that the adsorption equilibrium was reached. Nickel concentration

(2)

⎡ ⎛ E − Es ⎞⎤ ⎟⎥ Ce = Cs exp⎢ −⎜ ⎣ ⎝ RT ⎠⎦

−dqe(E*)

Equation 8 was used to evaluate F(E*) of the adsorbents based on barley straw. Because the resulting site energy distributions are not normalized, the area under the distribution is equal to the maximum adsorption capacity qm:

+∞

qh(E , Ce)F(E) dE

(6)

Then differentiating the isotherm, qe(E*) with respect to E*, an approximate site energy distribution F(E*) is obtained as follows:

qmbCen

qe(Ce) =

qmbCsne−nE * /RT

where Cs is the maximum solubility of NiSO4 in water (6.25 × 10 5 mg/L, 293.15 K), E s is the adsorption energy corresponding to Ce = Cs,16 R is the universal gas constant, and T is the absolute temperature (K). The E* refers to the difference of adsorption energies between the adsorbate and solvent to the adsorbent surfaces based on the reference point Es, and it can be calculated by incorporation of the known values of Cs and Ce into eq 5. In addition, assuming that the Langmuir−Freundlich isotherm model eq 1 is applicable to the adsorption system in this work (it was confirmed valid by the modeling work in the section of results in this paper), by incorporation of eq 5 8284

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In the adsorption process, nickel ions exchanged with protons of the acidic functional groups on PBS and were adsorbed via electrostatic attraction, evidenced by the fact that protons were released into the solution; therefore, the solution pH decreased during the process of nickel adsorption. As a result, to maintain constant solution pH, it had to be instantly automatically adjusted by the SCHOTT titrator via adding 0.1 M sodium hydroxide. Thus, electrostatic attraction between the negatively charged surface of PBS and positively charged nickel ions was suggested to dominate the adsorption process. The solution pH 7.0 resulted in a more negatively charged surface of PBS and therefore stronger adsorption affinity (electrostatic interaction) which enhanced the nickel adsorption capacity of PBS. As such, the nickel adsorption isotherm of PBS was determined at pH 7.0 ± 0.1 with equilibrium nickel concentrations from 1.7 to 932 mg/L. Each of the experiments was performed in duplicate. The equilibrium nickel adsorption data of PBS and RBS are presented in average value with standard deviation and illustrated in Figure 1. The model of

was measured by atomic absorption spectroscopy (Aurora Instruments Ltd., AI 1200). Nickel adsorption capacity per unit dry mass of the fresh adsorbent was calculated by nickel mass balance in the adsorption samples. Each of the experiments was performed in duplicate, and the results were presented in average value with standard deviation in the figures. To perform the desorption experiments, PBS was first loaded with nickel, separated using a filter, dried and kept in oven 378.15 K for 24 h. Then 600 mg of nickel-loaded PBS was added into 300 mL of deionized water, and the pH was adjusted to 2 by 2 M H2SO4. The mixture was shaken at a speed of 200 rpm for 24 h. The ratio of eluted nickel from PBS loaded with nickel to the initially loaded nickel, called as elution or desorption efficiency, was determined. This method has been effectively used in the previous research.3 3.4. X-ray Spectroscopic Study. X-ray fluorescence (XRF) and X-ray absorption spectroscopy (XAS) in this work were carried on the Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron (VESPERS) beamline at the Canadian Light Source in Saskatoon, Saskatchewan, Canada.19,20 For XRF measurements, the X-ray beam with an incident energy of 10 keV was employed to excite the sample. The emitted XRF spectrum was recorded by a 4-element Vortex silicon drift detector, which mainly contains Ni Kα and Kβ characteristic emission lines. Ni K-edge X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) were measured using fluorescence mode with a double crystal Si (111) monochromator. Energy calibration was performed with a nickel metal foil (K-edge at 8333 eV). All experiments were carried out under ambient conditions. The obtained XANES and EXAFS data was analyzed by the free software IFEFFIT, which is a suite of interactive programs for XAS analysis. IFEFFIT combines highquality and well-tested XAS analysis algorithms, tools for general data manipulation, and graphical display of data.21 The collected data were Fourier transformed over the k-region of 3−12 Å−1 with k2 weighting. The fitting was performed in kspace yielding optimal values for the coordination numbers (CN), atomic distance (r), Debye−Waller factors (σ2), and Rfactor which represents the relative error of the crystallographic model fitting results to the experimental data.

Figure 1. Nickel adsorption isotherms of PBS and RBS. 300 ± 2 mL of nickel solution, 600 ± 2 mg of PBS or RBS, 298.15 ± 0.50 K, and pH 7.0 ± 0.1. Error bars represent the standard deviation.

Langmuir−Freundlich10 was used to fit the experimental data, and the regressed parameters of the model are listed in Table 1.

4. RESULTS AND DISCUSSION 4.1. Adsorption−Desorption Experiments. 4.1.1. Adsorption Equilibrium. It is well-known that solution pH affects the charge state of adsorbent and speciation of adsorbate and therefore the adsorption process. The value of point of zero net charge (PZNC) of PBS was determined to be 4.1.17 As the pH is lower than PZNC, the surface of PBS was positively charged, otherwise negatively charged at pH higher than that. To ensure that the surface of PBS becomes negatively charged, the solution pH has to be higher than 4.1. In addition, the fact that nickel precipitation occurs at solution pH 8 and above should be considered.22 Thus, effect of solution pH on adsorption capacity of nickel (Ni2+) was first examined at pH 5.0 ± 0.1 and 7.0 ± 0.1 with initial nickel concentration of 1000 mg/L, respectively. Adsorption experiments were run for 5 h to ensure that the adsorption equilibrium was reached. Each of the experiments was performed in duplicate. The achieved equilibrium nickel adsorption capacity 55.8 mg/g at pH 7.0 was higher than 29.3 mg/g at pH 5.0.

Table 1. Langmuir−Freundlich Modeling Results of Nickel Adsorption on PBS equations

qe =

a

qmbCen 1 + bCen

parameters

PBS

RBS

qm (mg/g) b (L/mg) n n R2 RSS (mg/g)2

59.5 ± 3.0 0.196 ± 0.019 0.619 ± 0.085 0.998 46.29

10.2 ± 2.0 0.070 ± 0.006 0.460 ± 0.074 0.999 0.01

RSS residual sum of squares (mg/g)2.

The experimental data of PBS and RBS were well fitted by the Langmuir−Freundlich model with the high values of coefficients of determination R2 (≥0.998). Compared with RBS, the b value of PBS was much higher than that of RBS indicating the higher binding energy of PBS with nickel ions than that of RBS. In addition, the qm value of PBS obtained from the Langmuir−Freundlich modeling (59.5 ± 3.0 mg/g) 8285

DOI: 10.1021/acs.iecr.7b01445 Ind. Eng. Chem. Res. 2017, 56, 8283−8291

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Industrial & Engineering Chemistry Research was much higher than that of RBS (10.2 ± 2.0 mg/g). It demonstrated that, after H3PO4 modification, more binding sites on RBS are available for nickel adsorption. Previous FTIR analyses of PBS and RBS17 indicated that carboxyl groups were created on PBS. The proton dissociation constants (pKa) of the carboxyl group are 3.5−4.5.23,24 This may lead to the fact that the PZNC of PBS (4.1) was lower than that of RBS (5.9), which demonstrated that PBS had more acidic groups to bind nickel ions than RBS. In addition, it was determined that the specific surface area of PBS was 1314 ± 10 m2/g, while that of RBS was just 1.8 m2/g.18 The surface morphology of PBS and RBS characterized by scanning electron microscopy17 also demonstrated that PBS has an enhanced porous structure compared with RBS. The welldeveloped pores of PBS facilitated nickel ions effectively accessing the functional groups on PBS. As a result, the significantly enhanced nickel adsorption capacity of PBS could be mainly contributed by the aforementioned acidic functional groups and porous structure PBS. Furthermore, the total organic carbon (TOC) released into nickel solution from the adsorbents significantly reduced from 34.4 ± 0.9 mg/g RBS to 0.9 ± 0.2 mg/g PBS, which indicated the enhanced stability of PBS. The pretreatment method was successful in reducing the secondary pollution, i.e., TOC release from the adsorbent into the solution. For further comparison, the nickel adsorption capacity of PBS (qm, obtained from Langmuir−Freundlich fitting) and other adsorbents reported in the literature are summarized in Table 2. Nickel adsorption capacity of PBS determined in this

work (9 min in microwave) is energy- and time-saving and relatively low-cost. 4.1.2. Desorption of Nickel Adsorbed on PBS. Desorption experiments were conducted at pH 2.0 ± 0.1. Desorption efficiency of the nickel loaded on PBS, the ratio of nickel eluted from PBS loaded with nickel to the initially loaded nickel, was determined to be 91.1%. The results indicated that H+ ions with a higher value of electronegativity (EN. H+ is 2.20) was able to replace Ni2+ ions (EN. Ni2+ is 1.91),5 and that ion exchange played an important role in the nickel adsorption on PBS. Niu et al. achieved 100% elution of Au(CN)2− from the acid washed crab shells and proved that the adsorption mechanism mainly by electrostatic attraction.34 The slightly lower desorption efficiency (91.1%) in this work implied that other mechanisms might also be involved in the nickel adsorption on PBS. It has been reported that metal ions can be bonded to carboxylic groups through chelation.35 The carboxylic groups on PBS may play an important multiroles that significantly enhanced the nickel adsorption capacity of PBS compared to that of RBS. The nickel adsorption mechanism was further investigated by site energy distribution and X-ray analyses in the later part of this paper. 4.2. Site Energy and Its Distribution. The site energy E* as a function of the equilibrium nickel adsorption capacity qe is displayed in Figure 2a. E* of PBS and RBS decreased

Table 2. Comparison of Nickel Adsorption Capacity of PBS with Adsorbents Reported in the Literature adsorbents

pH

nickel adsorption capacity, mg/g

pretreated barley straw raw barley straw activated carbon from lignin gamma irradiation activated carbon Bofe clay zeolite activated carbon from sugar cane bagasse pith aerobic activated sludge lignocellulosic bagasse Irish peat moss Graphene oxide membranes magnetic porous Fe3O4−MnO2

7.0 7.0 6.4

59.5 ± 3.0 10.2 ± 2.0 14.0

this work this work Gao et al., 201325

3.9

55.7

Ewecharoen et al., 20095

5.3 6.0 6.5

1.9 26.8 140.8

Vieiraet al., 201026 Quintelas et al., 201327 Krishnan et al., 201133

7.0

24.3

Liu et al., 201228

source

5.8 5.7 5.7

2.8 21.1 62.3

Krishnani et al., 2009 Sen et al., 200830 Ping et al., 201532

7.7

55.6

Zhao et al., 201631

Figure 2. Site energy (a) and its distributions (b) of PBS and RBS for nickel adsorption.

dramatically when nickel loading increase. Such a result revealed that nickel ions first occupied the high-energy adsorption sites on PBS and RBS and then spread to lowenergy adsorption sites. The site energy distributions of nickel adsorption on PBS and RBS determined by eq 8 are plotted respectively in Figure 2b. As given in eq 9, the area under the site energy distribution curve can be interpreted as the maximum nickel adsorption capacity qm. Thus, the much bigger area under the distribution of PBS indicated that PBS had a higher maximum adsorption capacity than RBS, which was also confirmed by the nickel adsorption experimental data. The average site energy and surface energy heterogeneity of PBS and RBS were further determined. It has been documented that the average site energy can be used to depict the interaction strength between adsorbent and adsorbate.8 To determine the average site energy of PBS and RBS, the mathematical expectation of E* based on the site energy

29

work was higher than that of RBS, activated carbon from lignin,25 Bofe clay,26 zeolite,27 aerobic activated sludge,28 lignocellulosic bagasse,29 Irish peat moss,30 magnetic porous Fe3O4−MnO2,31 and close to the graphene oxide membranes.32 Although the activated carbon from sugar cane bagasse pith had a higher nickel adsorption capacity (140.8 mg/g),33 the preparation was placed in a muffle furnace maintained at 673.15 K for 1 h and then kept at 873.15 K for 2 h which cost much more energy and time. The treatment of RBS in this 8286

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RBS (previously reported in the publication)17 demonstrated that O-containing groups such as −COOH and −OH were introduced to PBS after pretreatment by H3PO4 impregnation and microwave heating. However, Figure 2b shows that the heterogeneity of PBS represented by the σ*e value (7.2 kJ/mol) slightly decreased in comparison with that of RBS (9.4 kJ/mol). Similar results have been observed in the adsorption of naphthalene, lindane, and atrazine by high-oxygen-containing carbonaceous adsorbents, such as graphene oxide, COOHfunctionalized graphene, and graphite oxide.11 The reasons leading to the results were explained as that the introduced Ocontaining moieties led to more hydrophilic particles surfaces, which facilitated the formation of water cluster.40 The increasing water cluster coverage made the surface of the carbonaceous materials more homogeneous. Moreover, the surface of PBS became more negatively charged at the solution pH of 7.0, which was above PZNC (4.1). Thus, the negatively charged adsorption sites (−COO−) may repel each other via electrostatic interaction. This may enhance the dispersibility of adsorption sites on PBS, which may reduce the heterogeneity of the adsorption sites.11 The collective effects of multiple mechanisms might eventually lead to the slightly lower heterogeneity of adsorption sites on PBS than RBS. However, the actual mechanisms need to be further investigated. 4.3. XRF and XAS Analysis. The X-ray fluorescence (XRF) spectra (Figure 3a) of PBS after exposure to nickel solution shows that the intensity of the peaks for nickel (Kα and Kβ) increased as the amounts of nickel adsorbed on PBS was increased. The XRF results confirmed that nickel ions were adsorbed on PBS. Figure 3b displays XRF spectra of PBS loaded with nickel and PBS after desorption. The results clearly demonstrated that most of the adsorbed nickel (∼90%) was eluted from PBS at solution pH 2, which verified that the majority of nickel adsorption was reversible by lowering the solution pH to 2. X-ray absorption spectroscopy (XAS) was used to characterize the chemical state and the environment of Ni atom. X-ray absorption near edge structure spectroscopy (XANES) could provide information about the coordination geometry and oxidation state of the metal, whereas extended X-ray absorption fine structure spectroscopy (EXAFS) contains information about the backscattering atoms, coordination numbers (CN), and atomic distance (r).5 Figure 4 displays the results of normalized Ni K-edge XANES spectra of PBS loaded with Ni (curve a) in comparison with that of NiSO4 (curve b, the valence of nickel is +2) and metallic Ni foil (curve c, the valence of nickel is 0) which were used as references in order to verify whether the valence of adsorbed nickel is the same as that of Ni(II) of nickel sulfate (nickel solution was prepared by dissolving NiSO4·6H2O into water in this work). The close absorption edge position of Ni adsorbed by PBS and NiSO4 clearly indicated that the oxidation state of Ni adsorbed by PBS was Ni(II), which suggested that the oxidation state of Ni unchanged during the adsorption process of nickel on PBS. Such a result was different from the adsorption of Cr(VI) from aqueous solution on polypyrrole wrapped oxidized multiwalled carbon nanotubes, in which Cr(VI) was partially reduced to Cr(III).41 The k2-weighted EXAFS spectra and the corresponding Fourier transforms into R-space for Ni adsorbed on PBS, NiSO4 (nickel solution was prepared by dissolving NiSO4· 6H2O into water in this work), and Ni(CH3COO)2 (as reference of Ni−C crystal structure) samples are illustrated in

distribution in the range from zero to infinity was calculated below: μ(E*) =

∫0

+∞

∫0

E*F(E*) dE*

+∞

F(E*) dE*

(10)

By incorporating eqs 8 and 9 into eq 10, and integrating, the average site energy can be determined μ(E*) =

RT ln(1 + bCsn) n

(11)

As illustrated in Figure 2b, there was an overall right shift of site energy of PBS compared with that of RBS, reflecting the overall increase in site energy of PBS. The average site energy μ(E*) calculated by eq 11 showed that PBS had a higher average energy (26.6 kJ/mol) for nickel adsorption than RBS (18.9 kJ/ mol). According to Carter et al.,8 the higher the value of the average energy, the stronger the adsorption affinity. The higher average site energy of PBS indicated the stronger adsorption affinity of PBS with nickel. Therefore, PBS was more favorable for nickel adsorption than RBS. This could be attributed to the increased number of oxygen-containing functional groups of PBS, especially carboxylic groups having higher affinity for nickel ions.11 The result again demonstrated the effectiveness of pretreatment methodology used in this work. Both PBS and RBS revealed site energy heterogeneity for adsorption of nickel ions, evidenced by the site energy distribution (Figure 2b). It has been reported that the width of site energy distribution relates to the diversity of energy sites, i.e., the heterogeneity of surface site energies.8 Thus, the standard deviation σe* of the distribution was applied to characterize the site energy heterogeneity of adsorbents in this work. It was quantified by the following equations, based on the relationship between variance and standard deviation and mathematical expectation theory11 *2

μ(E ) =

∫0

+∞

∫0

E*2F(E*) dE*

+∞

F(E*) dE*

(12)

again incorporating eqs 8 and 9 and integrating the above equation, the following equation was obtained: 2(RT )2 n2

∫0

bCsn

1 ln(1 + bCen)d(bCen) bCen

(13)

then the standard deviation σ*e of the site energy distribution can be calculated σe* =

μ(E*2) − μ(E*)2

(14)

Through eq 14, the values of σe* of PBS and RBS for nickel adsorption were determined to be 7.2 and 9.4 kJ/mol, respectively, which were also given in Figure 2b. The results demonstrated the heterogeneity of site energy of both adsorbents. Generally, the heterogeneity of adsorption sites of carbonaceous adsorbents originated from the defect structures, as well as the cross-linking and disordered arrangement of various carbon structure. The heterogeneous adsorption sites of graphitized carbons for organic pollutants have been attributed to these.36−38 In addition, the heterogeneity of adsorption sites could also be derived from the grafted functional groups (chemical composition heterogeneity), especially oxygencontaining functional groups.39 The FTIR spectra of PBS and 8287

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Figure 5. k2-weighted EXAFS spectra (a) and their Fourier transforms (b) of Ni adsorbed on PBS (pH 7.0 and qe = 35.8 mg/g), NiSO4, and Ni(CH3COO)2.

Table 3. Structure Parameters Obtained from the EXAFS Analysis of Nickel Adsorbed on PBS Figure 3. XRF spectra analysis of nickel adsorbed on PBS (a) and after desorption (b). 300 ± 2 mL of nickel solution, 600 ± 2 mg of PBS or RBS, 298.15 ± 0.50 K, and pH 7.0 ± 0.1.

nickel samples NiSO4·6H2O NiCO343 Ni-adsorbed on PBS

shell

CN

r (Å)

Ni−O Ni−O Ni−C Ni−O

6 6 6 6

2.036 ± 0.012

2.043 ± 0.013

r (Å) XRD 2.01642 2.076 2.932

σ2 (Å2) 0.0040

0.0065

This indicated that Ni was surrounded by six oxygen atoms after adsorbed on PBS. The peaks 3 and 4 that were fitted by those of Ni(CH3COO)2 implied that the adsorbed Ni(II) might be bonded to the carboxylic groups on PBS. The result was in accordance with adsorption of copper ion on soil in which copper was suggested to be bonded to six oxygen atoms of the organic matter of soil.44 Thus, the interactions between the adsorbent PBS and adsorbate nickel could be proposed: pH-dependent electrostatic attraction and/or cation exchange dominated the adsorption, which was supported by release of proton and higher desorption efficiency (91.1%) at pH 2.0, and chelation between nickel and carboxyl groups of PBS might also exist though being insignificant, which could explain the small amount of undesorbed nickel on PBS and the higher average site energy of the distribution of PBS. For aqueous environmental systems, multiple bonding mechanisms are expected to operate simultaneously. The actual mechanisms of nickel adsorption by PBS need further investigation. It was pointed out that this work focused on the adsorption equilibrium, site energy, and major mechanisms using nickel as a model ion. More work on kinetics and additional characteristics of nickel and metal adsorption could be done in the future by reference to the work in the literature.45−50

Figure 4. Ni K-edge XANES spectra: (a) Ni adsorbed on PBS (blue, pH 7.0 and qe = 35.8 mg/g); (b) NiSO4 (red); (c) nickel foil (green).

Figure 5. The fitting results and structural parameters are given in Table 3. The main features of Ni adsorbed on PBS in Figure 5b (peaks 1 and 2) matched well with those from NiSO4·6H2O. However, the rest (peaks 3 and 4) were closer to Ni(CH3COO)2. Peaks 1 and 2 of Ni adsorbed on PBS could be well fitted with first oxygen shell of NiSO442 or NiCO343 at a distance of 2.043 ± 0.013 Å with a coordination number (CN) of 6, and both values of the R-factor, representing the relative error of the crystallographic model fitting results to the experimental data, were 0.008 (shown in Figures S1 and S2). 8288

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5. CONCLUSION The adsorbent (PBS) made from raw barley straw with H3PO4 impregnation and microwave heating had a significantly enhanced nickel adsorption capacity almost six times that by RBS. In addition, the TOC release of PBS decreased to 0.9 mg/ g which was over 38 times lower than that of RBS, demonstrating enhanced stability of PBS. Solution pH played an important role during the nickel adsorption process. The release of proton during adsorption and higher desorption efficiency (91.1%) at pH 2 indicated that electrostatic attraction dominated the adsorption. Furthermore, XAS studies (XANES and EXAFS) demonstrated that the adsorbed Ni remained the oxidation state (II) during the adsorption process and associated with 6 oxygen atoms from water or the functional groups on PBS such as carboxyl group, at a distance of 2.043 ± 0.013 Å. For the first time, the approximate adsorption site energy distribution based on the Langmuir−Freundlich model was determined for nickel adsorption on PBS. The results revealed that the high-energy adsorption sites on PBS were first occupied, and then nickel adsorption spread to the low-energy adsorption sites. The pretreatment of H3PO4 impregnation with microwave heating influenced the energetical heterogeneity and adsorption affinity of adsorbents for nickel. With a lower degree of heterogeneity and a higher value of weighted mean (therefore higher affinity), PBS was more favorable than RBS for nickel adsorption. The methodology of analyses of site energy distribution and XAS could also be applied to investigate adsorption of other heavy metals.

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NOMENCLATURE b adsorption equilibrium constant ((L/mg)n) Ce equilibrium nickel concentration in solution (mg/L) CN coordination numbers Cs maximum solubility of solute in water (mg/L) E* difference of adsorption energy at Ce and Cs (kJ/mol) Es value of the adsorption energy corresponding to Ce = Cs (kJ/mol) F(E) site energy frequency distribution over a range of energies F(E*) site energy distribution over a range of energies (mg· mol/(g·kJ)) k Fourier transformation weighting and it can be 1, 2, and 3 n indicator of the surface site heterogeneity of adsorbent (dimensionless) qe amount of nickel adsorption per unit mass of adsorbent at equilibrium (mg/g) qh(E,Ce) energetically homogeneous isotherm qm maximum nickel adsorption capacity of the adsorbent (mg/g) r atomic distance (Å) R gas constant (8.314 J/(mol·K)) R2 coefficient of determination R factor a relative error of the crystallographic model fitting results to the experimental data RSS residual sum of squares ((mg/g)2) T temperature (K)

Greek letter

σe* energetical heterogeneity (kJ/mol) μ(E*) average site energy (kJ/mol) σ2 Debye−Waller factors

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01445. IFEFFIT fitting results of nickel adsorbed on PBS with references of NiSO4 and NiCO3. (PDF)

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Article

Abbreviations

AUTHOR INFORMATION

Corresponding Author

*Tel: 1 306 966 2174. Fax: 1 306 966 4777. E-mail: catherine. [email protected]. ORCID

Bei Yan: 0000-0002-3541-222X Catherine Hui Niu: 0000-0002-7883-7981 Renfei Feng: 0000-0001-8566-4161

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BET Brunauer−Emmett−Teller surface area EXAFS extended X-ray absorption fine structure FTIR Fourier transform infrared spectroscopy PBS pretreated barley straw PZNC point of zero net charge RBS raw barley straw SEM scanning electron microscopy TOC total organic carbon XANES X-ray absorption near edge structure XAS X-ray absorption spectroscopy XRF X-ray fluorescence

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this project was provided by the China Scholarship Council (No. 201408530054), the Natural Science and Engineering Research Council of Canada (No. RGPIN 299061-2013), Canada Foundation for Innovation (No. 11357), and the Saskatchewan Ministry of Agriculture and the Canada-Saskatchewan Growing Forward 2-bilateral agreement (No. 20130220). The synchrotron X-ray measurements were performed at the Canadian Light Source, Saskatoon, Saskatchewan. We sincerely thank Blondin Richard of the Department of Chemical and Biological Engineering at the University of Saskatchewan for his help in analysis of flame atomic absorption spectroscopy. All of the support is highly appreciated. 8289

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