Probing Surface Functionality on Amorphous ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Probing Surface Functionality on Amorphous Carbons Using X‑ray Photoelectron Spectroscopy of Bound Metal Ions Andrew J. Carrier,† Inusa Abdullahi,† Kelly A. Hawboldt,‡ Barrie Fiolek,§ and Stephanie L. MacQuarrie*,† †

Department of Chemistry, Cape Breton University, 1250 Grand Lake Road, B1P 6L2 Sydney, Nova Scotia, Canada Department of Process Engineering, Memorial University of Newfoundland, Prince Philip Drive, A1B 3X5 Saint John’s, Newfoundland and Labrador, Canada § B. W. BioEnergy Inc., 1615 Grand Lake Road, B1M 1A3 Sydney, Nova Scotia, Canada ‡

S Supporting Information *

ABSTRACT: The surface functionality of amorphous carbons is difficult to directly measure because of a lack of crystallinity and overwhelming signals derived from the bulk material. Biochar, a form of amorphous carbon containing considerable oxygen surface functionality, was probed using metal ions and X-ray photoelectron spectroscopy to simultaneously measure the presence and proximity of functional groups and determine the preferred binding modes of a variety of metal ions. These binding motifs were correlated to the efficiency of metal adsorption as determined using the Langmuir isotherm and stability with respect to leaching. Three binding motifs were apparent: physisorption (Cd2+, Mn2+, and Zn2+), chelation (Cu2+, Ni2+, and Zn2+), and hydrolysis/precipitation (Cu2+, Ni2+, and Pb2+).

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INTRODUCTION The surface chemistry of heterogeneous amorphous materials, such as activated charcoals, is difficult to examine because of limited direct detection techniques.1 The lack of crystallinity precludes X-ray diffraction and using solid-state nuclear magnetic resonance and infrared spectroscopy typically results in overwhelming signals from the bulk material obscuring those of the surface. Limited information regarding surface functionality can be assessed using the Boehm titration method,2,3 where the number of functional groups deprotonated by successively stronger bases can elucidate the quantities of some functional groups, e.g., phenols and carboxylic acids, but this yields an approximation of these groups and does not indicate their relative proximityinformation useful to those looking for specific surface interactions. Thus, developing chemical probes that report specific surface interactions that identify surface motifs would be useful for examining surface functionality and could be used to develop specific applications for amorphous adsorbents. (Post)transition metal ions are of concern to human and environmental health and cannot be degraded.4−8 As such, they © XXXX American Chemical Society

must be removed from potable water supplies, wastewater discharge, and pharmaceutical processes where they may be used as catalysts. This can be accomplished by adsorption,9−12 precipitation,13 and reverse osmosis,14−16 each of which is widely used in specific situations on the basis of efficiency and cost. Where precipitation is not applicable, adsorption, which generally requires costly ion exchange membranes9 or zeolites,10,11 is used because inexpensive unfunctionalized activated charcoals are inefficient scavengers of most metal ions.17 Biochar, a less expensive form of amorphous carbon, generated at relatively low temperature (ca. 400 °C), has considerable oxygen-containing functionality derived, in the case of woody biomass feedstocks, from cellulose, hemicellulose, and lignin.18,19 This surface functionality results in sometimes dramatic increases in metal ion scavenging compared with the case of activated charcoals having much larger surface areas.20−23 Adsorbed metal ions can also act as Received: June 27, 2017 Revised: October 30, 2017 Published: October 31, 2017 A

DOI: 10.1021/acs.jpcc.7b06311 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

concentration at 30 °C for 5 days to equilibrate. The doped biochars were filtered and dried overnight in a 115 °C laboratory oven while the filtrates were analyzed to determine the metal ion loadings based on residual metal ion concentrations using the following equation:

chemical probes of surface functionality when subjected to Xray photoelectron spectroscopy (XPS), which can reveal the oxidation state of the metals and their bound ligands. This allows for an indirect probe of many functional groups and simultaneously identifies preferred adsorption sites for a variety of metal ions. Preferred adsorption sites also correlate with the efficiency of metal binding and resistance to leaching. Biochar has a much greater affinity for most metal ions than commercially available activated carbon; our objective was to elucidate the accessible functional groups responsible for this enhanced binding by examining bound metal ions (and their surrounding environments) rather than directly examining the amorphous carbon, as the majority of the material is not directly interacting with adsorbates, even near surface sites. Our interest is also in establishing a correlation between the binding affinity measured through adsorption isotherms and binding modes determined through XPS.

q=

(C0 − Ce)V m

where q is the metal ion loading, V is the solution volume, m is the biochar mass, and C0 and Ce are the initial and equilibrium metal ion concentrations, respectively. Several Cu2+-doped samples were prepared with varying Cu2+ loadings to determine the effect of metal loading on pore structure using nitrogen physisorption analysis. The doped biochar samples were submitted to XPS analysis to examine the metal ion binding environments. Adsorption Equilibria and Kinetics. A virgin biochar sample was leached using 0.1 M HNO3 for 3 days and the extract and blank nitric acid were analyzed to determine the extractable endogenous (post)transition metal content of the starting material. The nitric acid blank contained no quantifiable metal ions; however, the biochar leached 1200 ± 10 and 2710 ± 50 mg kg−1 of extractable Fe2+/3+ and Zn2+, respectively. Unbuffered metal ion stock solutions were prepared at concentrations of ca. 50, 100, 150, and 200 mg L−1. Aliquots (5 mL) of each concentration were dispensed in test tubes containing ca. 100 mg of either biochar or activated charcoal, or as a blank. The mixtures were incubated at 30 °C for 7 days before being centrifuged and diluted 10-fold (100-fold for Zn2+) before FAAS analysis. The Pb2+ samples required additional experiments using 15−20 mg of biochar to compensate for the complete adsorption of Pb2+at 100 mg biochar loading. To assess the adsorption kinetics, biochar samples (ca. 1 g) were incubated with 50 mL of ca. 100 mg L−1 metal ions solutions with 1 mL samples being withdrawn on days 0, 1, 2, 3, 6, and 7 for analysis. Leaching and Recycling. Metal-doped biochars (ca. 100 mg) were leached using either 5 mL of deionized water or 0.1 M HNO3 for 24 h at 30 °C three times. The collected supernatants were analyzed and cumulative metal leaching was determined on the basis of the initial metal loading of the doped-biochars. Pb2+-doped biochars were tested for recyclability by desorption using 0.1 M HNO3 followed by attempted readsorption of Pb2+.

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EXPERIMENTAL SECTION Materials and Apparatus. Cd2+, Cu2+, Mn2+, Ni2+, Pb2+, and Zn2+ were selected as adsorbates and surface probes. Cu(NO3)2·3H2O, MnSO4·H2O, and Zn(NO3)2·6H2O were obtained from Acros Organics (Geel, Belgium); Ni(NO3)2· 6H2O was obtained from Ward’s Scientific Plus (Rochester, NY, USA); Cd(NO3)2·4H2O was obtained from Mallinckrodt (Paris, KY, USA); and PbNO3 was obtained from Fisher Scientific (Hampton, NH, USA). All were used without further purification. Activated charcoal Norit CA1 from wood was obtained from Sigma-Aldrich (St. Louis, MO, USA) and birch biochar was provided by B. W. Bioenergy Inc. (Sydney, NS, Canada). The birch biochar was prepared by the anaerobic pyrolysis of debarked birch wood at ca. 400 °C for 30 min followed by rapid quenching in cold water. Before use, the asreceived biochar was washed, dried, ground, and sieved (see the Supporting Information). Solutions were prepared in deionized water (15 MΩ cm) obtained from an Elix Essential system (EMD Millipore, Billerica, MA, USA). Surface area and textural properties were measured using an ASAP 2020 analyzer (Micromeritics, Norcross, GA, USA). pH measurements were performed using a model 310 ORION perpHecT LogR meter (Thermo Scientific, Waltham, MA, USA) combined with an accumet epoxy body, single junction, Ag/AgCl reference electrode. Infrared spectrometry (FT-IR) was performed using KBr disks on a Thermo-Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Flame atomic adsorption spectroscopy (FAAS) was performed using a Thermo SOLAAR M6 spectrometer (Thermo Electron, Waltham, MA, USA). X-ray photoelectron spectrometry (XPS) was performed at Queen’s University, Kingston, ON, Canada, using a Microlab 310-F spectrometer (Thermo Instruments, Santa Fe, NM, USA) with a monochromatic Mg Kα (hν = 1253.6 eV) X-ray source (P = 10−9−10−10 Torr, power = 14.5 kV × 15 mA) with Thermo Scientific Avantage XPS software, with high resolution spectra for the metal ions (M = Cd, Cu, Fe, Mn, Ni, Pb, Zn). C 1s, N 1s, O 1s, S 2p were recorded at a pass energy of 20 using an energy step size of 0.05 eV, with the spectra charge corrected using the C 1s peak at 285 eV, peak fitting using optimized Gaussian−Lorentzian curves, and a Smart Shirley-type background. XPS samples were pressed onto vacuum compatible double-sided adhesive Cu tape mounted onto stainless steel stub-type holders. Metal Ion Doping. Biochar samples (ca. 500 mg) were doped with metal ions by incubating with solutions of known

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RESULTS AND DISCUSSION Adsorbent Characterization. The textural properties of the biochar and activated charcoal indicate a much greater surface area, micropore area, and micropore volume in the activated charcoal, which is not unexpected considering the additional activation this material has received (Table 1). Elemental analysis of the biochar indicated similar elemental Table 1. Textural Properties of Birch Biochars and Activated Charcoala adsorbent

SBET (m2 g−1)

Sm (m2 g−1)

Vm (cm3 g−1)

PW (Å)

Biochar Activated charcoal

259 1323

198 430

0.105 0.234

23.0 34.9

a SBET, Brunauer−Emmett−Teller surface area; Sm, micropore surface area; Vm, micropore volume; Pw, adsorption average pore width.

B


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errors for weak N 1s and Mn 2p3/2 spectra, ∼0.3 and ∼0.14 wt %, respectively), as such the degree to which each binding assignment can accurately reflect the proportion of bound ions is limited. Additionally, the relatively low resolution and signalto-noise ratio, resulting from the low doping levels achieved, prohibit the observation of satellite peak structures that would facilitate identification of the adsorbed species, e.g., for nickel oxides shake up transitions resulting from energy losses in ionized species can be observed.26 Peak assignments are therefore only based on the prominent main peaks associated with the adsorbed species. There appear to be three principle adsorption mechanisms, some metals exhibiting more than one. Binding energies corresponding to the unchanged starting materials (or a similar salt, i.e., Zn halides) were observed for Cd2+,27 Mn2+,28 and Zn2+ (Figure 2b at 1023.10 eV),29 and are indicative of physisorption of the dissolved metal ions. Binding energies corresponding to carboxylate chelates (Chart 1) were observed for Cu2+,30 Ni2+,31 and Zn2+ (Figure 2b at 1026.22 eV),32 indicating the proximity of neighboring carboxylate groups on the biochar surface, and these metal’s affinity for this binding mechanism. Hydrolysis to form oxides (and hydroxides) was observed for Cu2+,33 Mn2+,34 Ni2+,35−37 and Pb2+ (Figure 2a at 144.23 eV, the Pb XPS signals are also consistent with Pb2+ bound through physisoprtion, but the highly symmetric peaks are indicative of only one species and adsorption and desorption data below are highly suggestive of precipitation of the oxide),38 with Mn2+ apparently oxidizing to form MnO4−.34 Additionally, Auger electrons were detected in the Cu-39 and Ni-doped36 samples. Of the metals tested, Mn2+ was the least efficiently adsorbed (on both biochar and activated charcoal) because of the inability of Mn2+ or MnO4− to form carboxylate chelates or precipitates, whereas Pb2+, the most strongly bound ion, forms a highly insoluble oxide within the pores (Figure 2a). (Pb2+ is also significantly adsorbed by activated charcoal, though not as well as on biochar. Pb2+ is a very soft metal capable of forming strong physisorption interactions with carbon surfaces.) Given the amorphous nature of the biochar it is remarkable that only one type of chemisorption site is involved in binding, with only physisorption and precipitation competing. There are likely many phenolic and ketone containing sites that could potentially participate in binding, but there is no evidence of the involvement of these groups. The XPS signals associated with the biochar are common to virgin and each doped biochar. Although the penetration depth associated with XPS is generally limited to ca. 10 nm, it is unclear if any functionality detected is accessible to adsorbates, and thus the technique is similarly limited to that of FT-IR and solid-state NMR spectroscopy through obfuscation by the bulk material; i.e., the O binding energies do not change upon doping with metal ions, so examining the biochar surface directly with XPS does not eliminate the overwhelming signal from the bulk. All materials display binding energies for bulk carbon and carbon oxides,40 O binding energies corresponding to CO and CO bonds,41 and N binding energies corresponding to generic organic nitrogen.42 The formation of carboxylate chelates indicates that there are sufficient surface carboxylate groups to facilitate chelation of metal ions; however, there are likely also surface alcohol, ketone, ester, and ether groups, though these may not be accessible. Equilibrium Adsorption. Metal ion adsorption is summarized in Tables S2−4 and despite its lower surface area the biochar had superior removal capacity compared to

content (87% C, 2.6% H, 0.3% N, 0.1% S, and >1% ash) comparable to the activated charcoal (88% C, 0.5% H, 0.5% N and 3−4% ash), but its hydrogen content was slightly higher and its ash content was very low. However, where these atoms exist in the material cannot be assessed using this technique. Surface functionality was assessed using the Boehm titration method, with the number of acidic and basic functional groups on the biochar surface (nCSBF) being 3.84 and 0.90 mmol g−1, respectively.2,3 Its pH was 6.7 following a standard method.24 Although the Boehm titration method can assess the number of accessible functional groups of sufficient acidity or basicity to react, it reveals no detailed information as to the functional group identity or their relative proximity. An intense IR band at 3430 cm−1 was attributed to water and diminished upon drying, but a broad peak at 1635 cm−1 is indicative of carbonyl functionality, though this evidence of oxygen functionality is limited because the relative concentrations of surface sites to bulk carbon are small (Figure S1). Scanning electron microscopy (Figure 1) shows the biochar particles retain the

Figure 1. Scanning electron microscopy (SEM) images of the birch biochar after pyrolysis and quenching.

bulk wood morphology with visible capillaries. These macropores facilitate diffusion of adsorbates throughout the particles where mass transit becomes limited into the micropores contained in the capillary walls. The particles are of an irregular nonuniform size distribution. Titration of the biochar pores with Cu2+ ions did not appear to alter the surface and micropore area and volume, or the average pore width (Figures S16−S19). Binding Mechanisms. The (post)transition metal doped biochars were submitted to XPS analysis to determine the nature of metal ion binding on the biochar surface, particularly to identify the reason biochar exhibits enhanced binding of metal ions compared to activated charcoal on both mass and surface area bases. The XPS spectra for the Pb- and Zn-doped biochar are given in Figure 2, with the remainder in Figures S2−S7. Electron binding energies (Table 2, and in detail in Table S1) were compared to known compounds in the NIST XPS database.25 The experimental binding energies were recorded as the local energy maxima of optimized Gaussian− Lorentzian curves, and their relative areas were determined after subtraction of the Smart Shirley-type background. Errors in the relative areas were inversely proportional to relative peak intensity (and thus the elemental composition of the samples, with low errors for intense C 1s spectra, ∼90 wt %, and high C


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Figure 2. X-ray photoelectron spectra (XPS) of (a) Pb-doped and (b) Zn-doped biochars. From top to bottom Pb 4f5/2,7/2 or Zn 2p3/2, C 1s, O 1s, and N 1s spectra. “Envelope” refers to the optimal fit of the experimental data and has been omitted when it obscures the identified peaks.

activated charcoal for every tested metal ion. Even metal ions with only modest adsorption onto the biochar surface displayed negligible adsorption onto activated charcoal such that insufficient metals became doped onto the activated charcoal

surface for any further experiments. The adsorption data were well modeled using the Langmuir isotherm, with the exception of Mn 2+ (Figure 3 and Table 3), because discrete chemisorption sites and monolayer formation were expected. D


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The Journal of Physical Chemistry C Table 2. Selected X-ray Photoelectron Peaks of Metal Ion Doped Biochar

Table 3. Langmuir Isotherm Model Constants for Metal Ion Adsorption onto Birch Biochara Mn+

signal

binding energy (eV)

relative area

Mn 2p3/2 Mn 2p3/2 Cu 2p3/2 Cu 2p3/2 Cu 2p3/2 Ni 2p3/2

642.7 647.1 935.0 939.2 944.1 856.5

1 0.52 1 0.39 0.48 1

Ni 2p3/2

860.7

0.44

MnSO4 MnO4− Cu(OAc)2, CuO Cu(OAc)2 CuO Ni(OAc)2, Ni(OH)2, NiO Ni(OAc)2

Ni 2p3/2 Zn 2p3/2 Zn 2p3/2 Cd 3d5/2 Cd 3d3/2 Pb 4f7/2 Pb 4f5/2

863.8 1023.1 1026.2 406.2 412.9 139.5 144.2

0.34 1 0.34 1 0.58 1 0.68

Ni(OH)2, NiO ZnCl2 Zn(OAc)2 Cd(NO3)2 Cd(NO3)2 PbO PbO

binding motif

ref

2+

Mn Cu2+ Ni2+ Zn2+ Cd2+ Pb2+

28 34 30, 33 30 33 31

qm (mg g−1)

KL (mg L−1)

RMSE

1.4 ± 0.48 5.64 ± 0.22 2.37 ± 0.20 6.64 ± 0.43 7.95 ± 2.01 17.7 ± 1.90

0.0136 ± 0.05 0.057 ± 0.07 0.019 ± 0.001 0.2269 ± 0.00065 0.0187 ± 0.0026 0.142 ± 0.01

0.108 0.17 0.06 0.38 0.57 0.87

a

qm, monolayer adsorption capacity; KL, Langmuir constant related to energy of adsorption (affinity of binding sites); and RMSE, root-meansquare error. Reported errors correspond to 1σ for n = 3 measurements.

31, 35−37 35−37 29 32 27 27 38 38

adsorbed amount corresponding to a complete monolayer, and KL is the Langmuir constant, an equilibrium constant correlated to the enthalpy of adsorption. Mn2+ is an outlying case in that, although it should also adsorb forming a monolayer, it does so slowly, and its solutions did not reach equilibrium within the weeklong time scale of the experiment, resulting in poor correlation with the Langmuir model. The constant qm represents the monolayer adsorption capacity in the Langmuir model and is useful when adsorbent capacities of different materials are compared. Nonlinear adsorption modeling for the Langmuir and Freundlich (an entirely empirical model) isotherms for the biochar is given and discussed in Figures S8−13 and Table S5 in the Supporting Information. The scattered plots observed for the activated charcoal (Figures S14 and S15, Tables S6 and S7) indicate that the data do not fit the isotherm models, likely because of very poor adsorption. The amount adsorbed corresponding to monolayer coverage, qm, determined using Langmuir plots indicated that the highest maximum monolayer capacities were obtained for Pb2+, Cd2+, and Zn2+, with the remaining ions displaying significant capacity. The order of monolayer capacity of the ions correlates to their observed binding mechanism. Mn2+, which is only bound by physisorption of the Mn2+ or MnO4− ions, had the lowest monolayer capacity. Pb2+, which precipitated as the stable oxide, PbO, had the highest monolayer adsorption capacity, whereas the ions bound as carboxylate chelates form those with intermediate adsorption capacities. An estimate of the Gibbs free energy of adsorption was calculated using the Langmuir constant and a correction suggested by Zhou et al.43 and Zhou et al.44 to correct for the dimensionless nature of ΔG°:

Chart 1. Cations Bound as Carboxylate Chelates Include Cu2+, Ni2+, and Zn2+

ΔG° = −RT ln(55.5 × 1000 × MW × KL)

Figure 3. Linearized Langmuir plots for biochar. Error bars correspond to 1σ for n = 3 measurements.

where R is the universal gas constant, T is the absolute temperature in K, the constant “55.5” represents the molar density of water (mol L−1), and MW is the molecular weight of the adsorbate (g mol−1). On the basis of on this approximation, all the metals showed a ΔG° < 0, indicating adsorption is favored (−15 to −30 kJ mol−1). Metal Leaching and Recycling. The doped biochars were leached using either deionized water or dilute nitric acid (Figure 4), which was selected because it was thought that protonation of carboxylate chelates would desorb the metals and each metal nitrate is water-soluble. Many environmental waters are acidic, e.g., acid mine tailings, so leaching under these conditions is significant.

The Langmuir isotherm model assumes monolayer coverage where adsorption sites bind independently of each other; i.e., occupied sites do not affect the enthalpy of binding of neighboring sites. It can be modeled in using the linearized equation: ⎛1 ⎞ Ce 1 = ⎜⎜ ⎟⎟Ce + qe qmKL ⎝ qm ⎠

where Ce and qe are the residual concentration in solution and adsorbed amount at equilibrium, respectively, qm is the E


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Figure 4. Leaching of metal-doped biochar in deionized water and (inset) 0.1 M HNO3.

Mn2+, Cu2+, and Pb2+ ions did not leach into deionized water (the small amount of Cu2+ ions detected was below the limit of quantification and may be insignificant), as can be seen in Figure 4b; however, Ni2+, Zn2+, and Cd2+ ions displayed significant leaching, prolonged across all three extractions. This indicated that desorption into solution is disfavored and may be kinetically limited as the number of retained ions, q, does not correspond to the equilibrium concentration; i.e., these systems have not reached equilibrium after 24 h of leaching. Similar amounts eluted during the second and third extractions correspond to a constant leaching rate over 24 h, whereas the higher concentrations observed during the first wash could also include metal ions that were only poorly adsorbed to the surface. Zn 2+ and Cd 2+ are both partially bound by physisorption and Ni(OH)2 has some solubility in water, so these may be contributing factors; however, Mn2+ is only physisorbed and does not leach; thus arguments correlating leaching to binding affinity are probably more complex and the leached ions may also be only loosely bound near external pores. All metal ions desorbed rapidly when leached using 0.1 M HNO3 solution (Figure 4a). This was anticipated, as the anionic sites on the biochar are protonated under those conditions, thus reducing the cation exchange capacity. Additionally, any basic sites on the biochar surface would become protonated, resulting in an overall positive surface charge. Electrostatic repulsion is likely a significant factor in the desorption process. Nitrogen physisorption analysis of acid washed biochar only indicated a modest decrease in BET surface area to 246 m2 g−1, micropore area to 186 m2 g−1, and micropore volume to 0.099 cm3 g−1, with a concomitant increase in average pore width to 23.2 Å. These are not significant changes indicative of pore collapse and cannot explain the loss of adsorptive capacity. All metal ions desorbed within two washes. The rapid desorption of Pb2+ under acidic conditions, in contrast to its rapid and thermodynamically driven adsorption under neutral conditions, highlights the importance of surface charge in its adsorption. A biochar sample was then repeatedly loaded with Pb2+ and eluted using 0.1 M HNO3 to determine its recyclability (Figure 5). Although Pb2+ was readily desorbed, subsequent reloadings

Figure 5. Recycling of biochar for Pb2+ adsorption after 0.1 M HNO3 desorption.

after the first were limited. This indicated the biochar surface was modified during the desorption. Rinsing the biochar with deionized water until the washes became neutral was insufficient to deprotonate the basic sites on the biochar surface, and the residual positive charge resulted in electrostatic repulsion. Dilute nitric acid also likely oxidized the surface. This is interesting because Pb2+ is adsorbed as PbO, which should not have specific interactions with the surface, but rather precipitate within the pores. The functional groups responsible for this conversion must have been altered or the electrostatic repulsion completely restricted the Pb2+ ions from reaching sites that promote hydrolysis. Adsorption Kinetics. Adsorption of (post)transition metals was assessed as a function of time (Figure 6). Pb2+ was most rapidly removed from solution, whereas smaller metal ions were absorbed more slowly and less strongly. The trends appear to follow those of equilibrium adsorption and are likely dictated by the same processes. The thermodynamic sink of precipitation or chelation drives the adsorption to completion. Adsorption should approach an equilibrium qe; however, there appeared to be a slowing of the adsorption for most metal ions visible at 2−3 days before continuing to an equilibrium value. There appears to be two separate adsorption processes occurring, an initial rapid adsorption into shallow pore spaces with limited capacity followed by a slower mass transfer into deeper, larger pores with some delay between the processes. This is unclear for Pb2+ because its overall adsorption is much faster than the others. However, detailed kinetic analysis of this slow process is hindered by large sample sizes required for FAAS analysis. Gathering sufficient data points would generate F



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AUTHOR INFORMATION

Corresponding Author

*S. L. MacQuarrie. Tel: 1 (902) 563-1302. Fax: 1 (902) 5631880. E-mail: [email protected]. ORCID

Stephanie L. MacQuarrie: 0000-0002-0183-6622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of NSERC Engage and Mitacs, the expertise in X-ray photoelectron spectroscopy provided by Dr. Gabriele Schatte and Professor Hugh Horton at Queen’s University, Kingston, ON, Canada, and B. W. BioEnergy Inc. for providing samples of birch biochar.

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Figure 6. Adsorption of metal ions onto birch biochar at 30 °C as a function of time. Error bars correspond to 1σ for n = 3 measurements.

a large volume of contaminated aqueous waste. However, the general time scale of adsorption of these ions has been demonstrated.

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CONCLUSIONS The birch biochar demonstrated modest adsorption of Mn2+, Cu2+, Ni2+, Zn2+, and Cd2+ ions, and excellent adsorption of Pb2+, and was superior to the commercial wood-based activated charcoal despite its significantly lower surface area. This correlated well with the binding modes observed via XPS, which revealed a propensity of accessible carboxylate functional groups on the biochar surfaces, which would otherwise be masked by the bulk material in most characterization analyses; i.e., the C 1s, N 1s, and O 1s XPS spectra do not change upon metal binding because the majority of their signals originate from the near-surface bulk, but the bound metal ions, Cu2+, Ni2+, and Zn2+, indirectly reveal the presence of neighboring carboxylate groups capable of forming metal chelates. The XPS data supported the formation of carboxylate chelates for modestly adsorbed metal ions, whereas strongly adsorbed species formed insoluble oxides within the biochar pores. Mn2+ was poorly adsorbed on both biochar and activated charcoal because it does not form a carboxylate chelate or insoluble precipitate within the pores. Additionally, biochar is a readily available adsorbent for heavy metal contaminants where activated charcoal fails. Adsorbed metal ions can be selectively leached from biochar, and although it cannot be reused, it can be disposed of as innocuous waste.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06311. Carbon washing and preparation procedures, infrared spectroscopy, detailed XPS data and graphs, detailed adsorption data for activated and acid washed carbons, linearized Langmuir and Freundlich plots and nonlinear fitting, Cu2+ titrated textural properties of biochar, and acid washed biochar adsorption kinetics (PDF) G


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