Takumi
Saito
*ab,
Motoki
Terashima
c,
Noboru
Aoyagi
d,
Seiya
Nagao
e,
Nobuhide
Fujitake
f and
Toshihiko
Ohnuki
b
aNuclear Professional School, School of Engineering, The University of Tokyo, 2-22 Shirakata Shirane, Tokai-mura, Ibaraki, 319-1188, Japan. E-mail: saito.takumi@jaea.go.jp; takumi.saito@nuclear.jp; Fax: +81-29-282-5927; Tel: +81-29-284-3518
bAdvanced Science Research Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai-mura, Ibaraki, 319-1195, Japan
cRadioactive Waste Processing and Disposal Research Department, JAEA, 4-33 Muramatsu, Tokai-mura, Ibaraki, 319-1194, Japan
dNuclear Science and Engineering Center, JAEA, 2-4 Shirakata, Tokai-mura, Ibaraki, 319-1195, Japan
eInstitute of Natural and Environmental Technology, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan
fGraduate School of Agricultural Science, Kobe University, Rokkodai1, Kobe 657-8501, Japan
First published on 2nd July 2015
Humic substances (HSs) are ubiquitous in various aquatic systems and play important roles in many geochemical processes. There is increasing evidence of the presence of HSs in deep groundwater; nevertheless, their ion binding properties are largely unknown. In this study we investigated the physicochemical and ion-binding properties of humic and fulvic acids extracted from deep sedimentary groundwater. The binding isotherms of protons (H+) and copper (Cu2+) were measured by potentiometry and fitted to the NICA-Donnan model, and the obtained parameters were compared with the generic parameters of the model, which are the average parameters for HSs from surface environments. The deep groundwater HSs were different from surface HSs, having high aliphaticities, high sulfur contents, and small molecular sizes. Their amounts of acidic functional groups were comparable to or slightly larger than those of surface HSs; however, the magnitude of Cu2+ binding to the deep groundwater HSs was smaller. The NICA-Donnan model attributed this to the binding of Cu2+ to chemically homogeneous low affinity sites, which presumably consist of carboxylic groups, via mono-dentate coordination at relatively low pH. The binding mode tended to shift to multi-dentate coordination with carboxylic groups and more heterogeneous alcoholic/phenolic groups at higher pH. X-ray absorption spectroscopy also revealed that Cu2+ binds to O/N containing functional groups and to a lesser extent S containing functional groups as its divalent from. This study shows the particularity of the deep groundwater HSs in terms of their physicochemical and ion-binding properties, compared with surface HSs.
Environmental impactFor future use of deep underground space it is necessary to monitor and protect the quality of deep groundwater. Development of mechanistic models that can describe reactions of pollutants with components in groundwater is mandatory, as is the case for surface water systems. Humic substances (HSs) play important roles in the speciation of metal ions; nevertheless, the details of ion binding to deep groundwater HSs are largely unknown. This study reveals the particularity of the physicochemical and ion-binding properties of the HSs extracted from sedimentary groundwater by comparing them to those of surface HSs. |
Ion binding to HSs has been an active topic of research for decades.11–13 The particularity of HSs as ligands lies in their chemical heterogeneity and polyelectrolyte nature.9,13 The former is manifested in the distribution of the affinity constant of a HS for a given metal ion due to the diversity of the environments surrounding its functional groups. The latter originates from negative charges located on its carbon backbone, which creates a negative electrostatic potential that attracts cations and excludes anions.14,15 Recent mechanistic models for ion binding to HSs such as the NICA-Donnan model9,11 and the Model VI and its successor12,16 take these aspects into account and can successfully describe the binding of various cations over a wide range of conditions. These models have been well tested for HSs extracted from different surface environments.12,17,18 There are certain similarities in the obtained model parameters, once HAs and FAs are separately discussed.17 Thus, so-called “generic” parameters have been proposed for these two groups of HSs12,17,18 and are widely used to estimate the level of metal binding to HSs for which specific model parameters are unavailable.19,20
There is increasing evidence showing the presence of HSs in deep underground environments either as dissolved forms in pore water or bound to host rocks.21–29 Deep underground environments are rather different from surface aquatic systems, as manifested by slow groundwater flow, which leads to prolonged interaction between rocks and dissolved/suspended components, low oxygen concentration, and no direct energy input from the sun. It is likely that HSs in deep underground environments are different from their counterparts in surface environments with respect to their structures and ion binding properties.21,23 Underground HSs may originate from surface waters transported by downward recharge, dissolution of sedimentary organic materials, or in situ production from the remains of microorganisms or algae in connate water, and have experienced long-term diagenesis. Ratios of dissolved and bound HSs in deep underground environments are different from site to site, and they may have rather different properties even in a given geological setting.30
The uniqueness of deep underground HSs has been pointed out by several researchers.21,23–26,29,31,32 Schäfer et al.23 studied possible sources of FAs in the Gorleben aquifer, based on isotopic data and C and S X-ray absorption near-edge spectroscopy (XANES). They reported that FAs derived from the deep brine groundwater at around −216 m below ground level (bgl) had similar carbon backbone structures to those of FA in the corresponding shallow recharge groundwater and that HAs and FAs originated from lignite in Miocene sediments were highly aromatic. Alberts et al.32 reported that the properties of HA and FA extracted from groundwater at −30 to −70 m bgl were different from those of their counterparts from surface water, but copper binding to them was similar. Courdouan et al.30 studied the binding of trivalent metal ions to extracts of organic matter from pore water and rocks in the Opalinus clay (OPA) and the Callovo-Oxfordian formations and found stronger binding of curium to pore-water organic matter from OPA. Although some properties of deep underground HSs such as elemental composition, 13C NMR carbon distribution, optical properties, and molecular-size distribution have been reported,21,24–26 their ion binding properties over a wide range of conditions remain largely unknown.31–33 This is particularly the case for deep groundwater HSs, as large-scale extraction of HSs from deep groundwater is limited.
Deep groundwater HSs can be extracted from pumped groundwater, using boreholes from the surface. Some countries are operating underground research laboratories (URLs) for feasibility tests of geological disposal of nuclear wastes, where large amounts of groundwater samples are available with less contamination or alteration.34–37 This makes URLs appropriate places for extraction of deep groundwater HSs.
Considering the future uses of deep underground space by mankind such as geological disposal of nuclear wastes and potential deterioration of groundwater quality, the ion binding properties of deep groundwater HSs are to be studied and the applicability of the aforementioned mechanistic models is to be tested, as is the case for surface HSs. Thus, the objective of this study is to reveal the physicochemical and ion-binding properties of HA and FA isolated from sedimentary groundwater at the Horonobe URL of the Japan Atomic Energy Agency (JAEA).35 The physicochemical properties of the Horonobe HSs, which are denoted as HHSs hereafter, were compared with those of surface HAs and FAs to discuss their structural differences. Binding isotherms of protons (H+) and copper (Cu2+) were measured over a wide range of conditions by potentiometric titration and fitted to the NICA-Donnan model.9,11 The results were compared to the model calculations with the generic parameters proposed by Milne et al.,18 which capture average trends of ion binding to HSs from surface environments. The oxidation state and local coordination environment of Cu2+ bound to the HA fraction of the HHSs were also assessed by X-ray absorption spectroscopy (XAS). Copper was chosen as a representative divalent metal ion in this study to examine the general metal binding properties of the HHSs, as it can be easily quantified by an ion selective electrode (ISE) and its binding to surface HSs has been well studied.18,38,39 Copper is also an essential trace element for organisms at low concentration and becomes toxic at elevated concentrations.40,41 It could be introduced to deep groundwater systems by exploitation of underground space, as it is an important constituent of various materials used in modern industries. Thus, studies on the speciation Cu2+ in the presence of groundwater HSs are relevant for its fate in deep groundwater environments. Although the HA and FA from a single groundwater source are examined in this study, the outcomes can be applied or be a good starting point to estimate the degree of metal binding to HSs in sedimentary groundwater similar to this study.
HA and FA were extracted from groundwater collected at the −250 m gallery of the Horonobe URL located in the northern part of Hokkaido Prefecture, Japan. The geology and geochemistry of the site are described elsewhere.35,42 The −250 m gallery is located at the boundary of the Pliocene Koetoi and the Miocene Wakkanai formations, which are composed of diatomaceous and siliceous mudstones, respectively. Groundwater at the sampling location is a weakly alkaline Na+/HCO3− type with relatively high total organic carbon (TOC) and Cl− levels. Groundwater after filtration and acidification was passed through a column packed with DAX-8 resins (Supelite DAX-8, Sigma-Aldrich). Separation and extraction of HA and FA fractions from the loaded resins and subsequent purification was performed in a laboratory on the surface, according to the protocol recommended by the International Humic Substances Society (IHSS).43 In total 6.6 g of HA and 3.5 g of FA were obtained by treating approximately 6000 L of the groundwater. These values correspond to approximately 8.5 and 4.5% of the TOC in the groundwater, respectively. Hereafter, the HA and FA fractions are denoted as HHA and HFA, respectively.
The UV/Vis absorption spectra of the HHS as well as those of the standard or reference HSs from the international humic substances society (IHSS) and the Japanese humic substance society (JHSS) and purified Aldrich HA (PAHA)46 were measured in this study by a UV/Vis spectrometer (UV-3100, Shimadzu). The samples were prepared as 50 mg L−1 HS solutions in 0.01 M NaHCO3 buffer.47 The size distributions of HHA and HFA were determined by flow-field flow fractionation (Fl-FFF) with 1 kDa polyethersulfone membrane (AF2000, Postnova), according to Lukman et al.48 The electron accepting capacities (EAC) of the HHS and the standard HSs from the IHSS and JHSS were determined by the mediator electrochemical reduction (MER) in a similar way to Aeschbacher et al.,49 using diquat dibromide monohydrate (99.5%, Supelco) as a mediator. The details of the MER measurement are given in the ESI.†
Proton binding isotherms of the HHSs were obtained by acid–base titration, as described elsewhere.46 30 mL of a 1 g L−1 HHA or HFA solution was first titrated to pH 4 and stirred for 1 hour; then three-sets of forward and backward titrations were performed. The salt concentration of the solution was increased by adding a 1 M NaClO4 solution (Merck) between the different sets of titrations. At every point of the titration the reading of the glass electrode was recorded when the drift became less than 0.1 mV min−1 or after 30 min. The relative positions of the charge (−q)–pH curves of the HHSs at the different salt levels were determined from the amounts of base and acid titrants necessary to back-titrate H+ released in the pH-stat salt titration. The absolute position of the curves was then determined by optimizing the initial negative charge, q0, at the beginning of the titration in the NICA-Donnan fitting,11 as described in the ESI.† The results of the forward base titration were used in the subsequent fitting, as the hysteresis between the forward and backward titrations at a given salt level was small. The uncertainty in the determination of the HHS charge, q, was estimated to be less than 0.1 meq g−1, using a typical standard deviation of the glass electrode calibration (0.05 pH unit).
Copper binding isotherms to the HHSs were measured by pH-stat titration at three pH levels (pH 4, 6, and 8) and 0.1 M NaClO4.38 At pH 4 the additional titration of 0.01 M NaClO4 was performed. The Cu2+ titration to HFA at pH 8 failed most likely because of poor pH buffering (see the discussion below). 30 mL of a 1 g L−1 HHA or HFA solution was first titrated to pH 4 and stirred for 1 hour, and then to a desired pH and equilibrated within 0.2 mV (0.003 pH unit) for 30 min. After equilibration a 0.1 M Cu(ClO4)2 solution or 10−3 M Cu(ClO4)2 solution in 0.1 M NaClO4 was added. The pH of the sample solution was back-titrated to the original value and kept within 0.2 mV for 20 min by addition of the acid and base titrants. Readings of the electrodes were recorded after their drifts became less than 0.1 mV min−1 or after 20 min. At each titration point the solution was checked for the formation of Cu(OH)2(s) (logKsp = −19.32 (ref. 52)) and the Cu2+ binding amount ([Cu2+]bound) was calculated by subtracting the sum of the concentrations of free Cu2+ and its hydrolysis products from the total concentration, using the hydrolysis constants of Cu2+.52 The magnitude of the uncertainty in log[Cu2+]bound was estimated to be no more than 0.2, using a typical error of the Cu ISE calibration (0.06 as the logarithm of Cu2+ activity, log
aCu).
The obtained H+ and Cu2+ binding isotherms to HHA and HFA were fit to the NICA-Donnan model,9,11 using an in-house MATLAB® program. The details of the model as well as the fitting procedure are given in the ESI.† First, the maximum density of H+ binding sites, Qmaxj,H of the site j (j = 1 and 2 for the low-affinity and high-affinity sites, respectively), the median values of the affinity constants of the site j for H+, j,H, the apparent heterogeneity parameters of the site j, mj, the Donnan parameter, b, and q0 were optimized, using the charge/pH curves. Then, the median values of the affinity constants of the site j for Cu2+,
j,Cu, the ion-specific non-ideality parameters of the site j for H+ and Cu2+, nj,H, nj,Cu, and the heterogeneity parameters of the site j, pj, which correspond to the reciprocal of the width of the affinity distribution, were optimized by fitting to the Cu2+ binding isotherms, while nj,H × pj. was kept equal to mj.11 The lower and upper boundaries were set to 0 and 1 for the mj, nj,i and pj parameters.
Copper K-edge X-ray absorption spectra (XAS) were measured in fluorescence mode at 148 K using a liquid N2 cryostat equipped with Kapton® windows (CoolSpek, UNISOKU) for the HA samples and in transmission mode at room temperature for the reference materials. A Si(111) double crystal monochromator was detuned by about 50% to reject higher harmonic intensity. Reduction and theoretical fitting of the obtained XAS data was performed by the Athena and Artemis software packages53 and FEFF 6.54 The details of the data reduction and fitting are given in the ESI.†
Elemental compositiona (%) | 13C NMRb (%) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
C | H | N | O | S | Ash | CI | CII | CIII | CIV | CV | |
a Ash free basis. b CI: carbonyl C (190–220 ppm), CII: carboxyl C (165–190 ppm), CIII: aromatic C (110–165 ppm), CIV: methoxyl and carbohydrate C (48–110 ppm), CV: aliphatic C (5–48 ppm). c Not detected. | |||||||||||
HHA | 62.29 | 6.40 | 3.36 | 25.44 | 2.51 | N.D.c | 3.0 | 12.7 | 26.4 | 17.9 | 40.6 |
HFA | 60.23 | 6.84 | 2.06 | 29.00 | 1.87 | N.D.c | 4.2 | 13.9 | 21.4 | 15.5 | 45.6 |
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Fig. 1 van Krevelen plot of the HHSs, the IHSS and JHSS reference and standard HSs, and PAHA. The types of HS and origins are designated by different symbols and colors. |
The densities of oxygen-containing carboxylic and phenolic functional groups of the HHSs determined by conventional end-point acid–base titration in Table S1† are comparable to those of the surface HSs. This means that the oxygen depletion indicated by the small O/C ratios of the HHSs occurs in functional groups other than carboxylic and phenolic-type groups, as described by Thurman.56 The UV/Vis optical properties of the HHSs are characterized by relatively small A250/210 and A350/280 ratios (Table S1†). This points to the presence of small conjugated systems in the HHSs48 and again in accordance with their low aromaticities.
The size distributions of the HHSs measured by Fl-FFF are compared to those of the IHSS and JHSS standard HSs and PAHA in Fig. 2. For the surface HSs the sizes of the HAs are larger than those of the FAs. The size distributions of the HAs are largely overlapping, although their shapes are somewhat different from each other; EHA, IHA, and LHA possess multiple peaks. The size distributions of the FAs are all mono-modal, and the peak locations are different, depending on their sources. It seems that the JHSS FAs (BFA, DFA, and IFA) are somewhat smaller than the IHSS FAs (SRFA and NFA). The size distributions of HHA and HFA are mono-modal with the modal sizes of 0.6 and 0.3 nm, respectively, which are appreciably smaller than those of the surface HSs. Relatively small sizes of deep underground organic matter have been reported in the literature.57,58 Bouby et al.58 reported that organic matter in the Gorleben groundwater had a modal size of 1 nm. Saito et al.57 compared the size distribution of organic matter in granitic and sedimentary groundwater by Fl-FFF. The sedimentary groundwater was taken from a borehole at the same depth in the Horonobe URL as in this study and exhibited a mono-modal size distribution with a peak around 2 nm. This may indicate that HA and FA fractions may account for only a part of the dissolved organic matter in this groundwater.
The EAC of a HS corresponds to the number of electrons transferred to the HS from the mediator, normalized by the mass of the HS, and represents its redox capacity. For the HHSs, the EAC values are relatively small compared with those of the IHSS and JHSS HSs (Fig. S2 in the ESI†). As in Aeschbacher et al.,49 we found a linear relationship between the EAC and the aromaticities of the HSs investigated (Fig. S2†). This is because the concentration of quinone moieties, that are predominantly responsible for redox reactions in HSs, tends to be proportional to the amount of aromatic carbon.42 Thus, the HHSs with low aromaticities have small redox capacities compared with the surface HSs.
In summary the HHSs can be viewed as relatively small organic matter with abundant aliphatic carbons and sulfurs. The densities of acidic functional groups are comparable to those of the surface HSs. The differences between HHA and HFA are small. The cluster analysis (Fig. S3 in the ESI†) performed for the physicochemical properties compiled in Table S1† clearly indicates that they are different from the IHSS and JHSS HSs. BFA is an exception, being clustered into the same group as the HHSs. This may indicate the presence of similar formation processes among them.
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Fig. 3 Charge–pH curves (symbols) of HHA (a) and HFA (b) at the three salt concentrations. The solid curves represent the results of fitting to the NICA-Donnan model for the HHSs and the dotted curves correspond to the calculation of the NICA-Donnan model with the generic parameters in Table 2.18 The negative charges (−q) are plotted in the ordinates. |
HHA | HFA | GHA | GFA | |
---|---|---|---|---|
a The values in italic are constrained in physically meaningful ranges of the corresponding parameters (see the text of the ESI for details). b The correlation coefficients of the fitting for H+ and Cu2+. | ||||
q 0 (eq kg−1) | −0.64 | −0.53 | — | — |
b | 0.81 | 0.87 | 0.49 | 0.57 |
Q max1,H (eq kg−1) | 4.38 | 5.64 | 3.15 | 5.88 |
p 1 | 1 | 1 | 0.62 | 0.59 |
Q max2,H (eq kg−1) | 4.44 | 4.09 | 2.55 | 1.86 |
p 2 | 0.36 | 0.27 | 0.41 | 0.70 |
log![]() ![]() |
3.74 | 3.63 | 2.93 | 2.34 |
n 1,H | 0.82 | 1 | 0.81 | 0.66 |
log![]() ![]() |
10.62 | 10.48 | 8.00 | 8.60 |
n 2,H | 1 | 1 | 0.63 | 0.76 |
log![]() ![]() |
1.32 | 1.16 | 2.23 | 0.26 |
n 1,Cu | 1 | 1 | 0.56 | 0.53 |
log![]() ![]() |
14.43 | 15.05 | 6.85 | 8.26 |
n 2,Cu | 0.28 | 0.29 | 0.34 | 0.36 |
R 2 (H+)b | 0.9970 | 0.9975 | — | — |
R 2 (Cu2+)b | 0.9951 | 0.9876 | — | — |
The Cu2+ binding isotherms to HHA and HFA are presented in Fig. 4. The isotherms are similar between them. The binding amounts of Cu2+ to the HHSs increases with the Cu2+ concentration and pH, as is expected for metal binding to surface HSs.18,38 The observed pH dependency of Cu2+ binding is the result of diminished H+ competition to the functional groups of the HHSs with an increase of pH. The Cu2+ binding amounts also slightly decrease with an increase of salt concentration due to screening of the negative electrostatic potential of the HHSs. In Fig. 4 the Cu2+ binding isotherms calculated by the NICA-Donnan model with the generic parameters given in Table 2 are presented for comparison. The binding amounts of Cu2+ to the HHSs are smaller than those of the model calculations regardless of pH and salt concentration. Weak binding of europium is also reported for HA and FA extracted from groundwater collected at −495 to 550 m bgl through a surface borehole in the Horonobe URL.33 At pH 4 the slopes of the isotherms to the HHSs are close to 1 in the log–log plot, which are larger than those of the model calculations with the generic parameters at the same pH. Interestingly, the differences in the slopes became smaller at higher pH, and at pH 8 for HHA it becomes similar to the slope of the calculated Cu2+ isotherms. The slope of a metal-binding isotherm of HS in a log–log plot is determined by the combination of the chemical heterogeneity of sites and the ion-specific non-ideality such as stoichiometry of the binding.9 The slope close to 1 in the isotherms of the HHSs at pH 4 together with the relatively large slopes of their charge–pH curves at acidic pH (Fig. 3) indicate Cu2+ binding to relatively homogeneous sites via mono-dentate coordination. At higher pH it seems that the binding mode tends to shift to coordination with greater denticity such as bi-dentate coordination.
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Fig. 4 Copper binding isotherms to HHA (a) at pH 4, 6, and 8 and to HFA (b) at pH 4 and 6, measured in the presence of 0.1 M NaClO4. For pH 4, the results with 0.01 M NaClO4 are also presented. The solid and dashed curves correspond to the results of the NICA-Donnan fitting at 0.1 and 0.01 M NaClO4, respectively. The dotted and chained curves correspond to the calculation of the NICA-Donnan model with the generic parameters in Table 2 at 0.1 and 0.01 M salt concentrations.18 |
For H+ binding the model successfully reproduces the charge–pH curves, especially at pH < 6. At neutral to alkaline pH the model somewhat underestimated the magnitude of the salt effect. The electrostatic part of the NICA-Donnan model (eqn (S4) and (S5) in the ESI†) assumes a relatively simple functional form for the so-called Donnan volume, which depends only on the salt concentration.14 Although the Donnan model is relatively simple with only one adjustable parameter and advantageous over other more sophisticated but complex models, its potential flaw for small FAs has been recognized.59 Considering the sizes of HHA and HFA, which are smaller than the Debye length of the solutions (1 and 3 nm for 0.1 and 0.01 M NaClO4), electrostatic potential calculation by the rigid-sphere or ion-permeable sphere model would be more realistic.14 The discrepancy observed at pH > 6 may also indicate the presence of a pH-dependent conformational change in the HHSs. Such conformational changes are common for linear aliphatic polyelectrolytes such as polymethacrylic acid.60 For HHA, Cu2+ binding is well reproduced by the model over a wide range of conditions, using the single set of the parameters. For HFA the model overestimated the salt effect and failed to describe the Cu2+ binding at 0.01 M NaClO4 and pH 4. It is likely that the model mishandles the electrostatic potential of HFA.
The optimized NICA-Donnan parameters for HHA and HFA are more or less similar to each other except for Qmax1,H, which is larger for HFA. The maximum densities of H+ binding sites are similar between the high (j = 1) and low (j = 2) affinity sites in HHA. For HFA the density of the latter group was smaller by 1.5 meq g−1. The obtained parameters can be compared to those of surface HSs with various origins and the generic parameters in Table 2.18 The values of Qmax1,H of the HHSs are in the ranges reported for the surface HSs, while those of Qmax2,H are larger.17 The median affinity constants of H+ of the HHS are larger than those of most of the surface HSs and Milne's generic parameters. The heterogeneity parameter, pj, and ion-specific non-ideality parameter, nj,H are also relatively large for the HHSs. This is especially the case for the low affinity carboxylic-type sites, reflecting the large slopes of their charge/pH curves (Fig. 3) at pH < 6. The Donnan parameters, b, are also larger for the HHSs. Considering the large aliphaticity of the HHSs as discussed in the physicochemical characterization, the log2,H values of HHA (10.62) and HFA (10.48), which are larger than the corresponding values of the generic HA (8.60) and FA (8.00), may indicate a larger contribution of alcoholic hydroxyl groups to the sites of the HHSs than those of surface HSs, although the presence of phenolic groups with large pKa cannot be entirely neglected as the HHSs still contain a certain amount of aromatic carbons (Table 1). This can also explain the weak H+ buffering by the HHSs at neutral to alkaline pH (Fig. 3).
The NICA-Donnan parameters of Cu2+ binding to HHSs are rather different from those of the generic parameters derived by Milne et al. (Table 1).18 For the low-affinity sites log1,Cu is larger for HHA and smaller for HFA than the corresponding generic parameters; whereas n1,H for both HHA and HFA are 1 and larger than the corresponding generic parameters (0.56 for GHA and 0.53 for GFA). For the high-affinity sites log
2,Cu of HHA and HFA are larger than those of the generic parameters, and n2,H are smaller. Thus, the Cu2+ binding to the HHSs are characterized by n1,Cu = 1 for the low-affinity sites and large log
2,Cu and small n2,Cu values for the high-affinity sites. A ratio of the parameter nj,i of a metal ion and proton is a good indicator of the underlying denticity of the complexation reaction (ni,j/nj,H close to 1 for mono dentate binding and 0.5 for bi-dentate binding). Thus, the optimized NICA-Donnan parameters for the HHSs suggest the mono-dentate nature of Cu2+ binding to the chemically homogeneous low-affinity sites at acidic pH. Considering the comparable or larger densities of the sites, Qmax1,H, of the HHS to/than those of surface HSs, this is most likely caused by sparsely distributed carboxylic groups on the aliphatic backbones of the HHSs, with which it is hard to form bi-dentate coordination with Cu2+. This can also explain the weak Cu2+ binding to the HHSs (Fig. 4). The relatively small n2,Cu/n2,H values for the HHSs together with the relatively small p2 values indicate that the more heterogeneous phenolic/alcoholic-type groups are involved in the binding of Cu2+ at neutral to alkaline pH via multi-dentate coordination. It is of interest to compare the NICA-Donnan parameters for the HHSs to those optimized for other groundwater HSs. Marang et al.61 reported the NICA-Donnan parameters for the binding of divalent and trivalent metal ions including Cu2+ to HA extracted from deep groundwater (−139 m bgl) of the Gorleben aquifer in Germany. The NICA-Donnan parameters of Cu2+ for this HA are more like those of GHA than those of HHA, suggesting a diversity of the ion binding properties of groundwater HSs. Further research is needed to relate it to the origin and genesis of groundwater HSs.
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Fig. 5 XANES spectra of Cu2+ bound to HHA and PAHA at pH 4 and 7 together with those of the CuII and CuI reference materials. |
The k3-weighted EXAFS spectra of Cu2+ bound to HHA and PAHA at pH 4 and 7 (Fig. 6(a)) exhibit systematic differences between the two HAs at 5.7, 6.5, and 7.4 Å−1. In Fig. S4 of the ESI† the same spectra are shown as an overlapped plot. The magnitudes of the corresponding Fourier transforms (Fig. 6(b) and S5(b) in the ESI†) are also different between them. For both HAs the Fourier transformed magnitudes are dominated by the intense peaks around 1.5 Å, which correspond to the scattering from the nearest O (and, to a lesser extent, N). For HHA additional peaks are noticeable around 2 Å, although their magnitudes are small. These peaks most likely originate from the Cu–S path as suggested by others.49,64 This assignment agrees with the high S content of HHA (Table 1). Quantitative modeling of the first coordination sphere of Cu2+ (Fig. 5(b) and Table 3) shows that approximately four O exist at 1.93 Å for PAHA, which corresponds to the Jahn–Teller distorted coordination geometry around Cu2+. For HHA at pH 4 the coordination number (CN) of O in the first shell is decreased and a small but non-negligible number of S (CN = 0.4 ± 0.2) is found at 2.35 Å. At pH 7 the presence of S in the second shell is inconclusive. Note that this does not necessarily mean bi-dentate coordination of Cu2+ with O/N and S in HHA. Considering the results of the NICA-Donnan fitting (Fig. 4 and Table 2), it seems more likely that the two independent mono-dentate sites exist in HHA, as an EXAFS spectrum is a one-dimensional representation of the coexisting coordination environments of a target element. The S containing functional group could be the thioacetic group, R–COSH, which then should be a part of the low affinity sites in the NICA-Donnan modeling, as the pKa value of thioacetic acid (CH3COSH, pKa = 3.33) is close to that of acetic acid (pKa = 4.76) and smaller than that of methanethiol (CH3SH, pKa = 10.33).65 It is also noteworthy to mention that the contribution of S to Cu2+ binding to HHA might be underestimated, compared with that in the original groundwater, as the functional groups containing reduced S may be oxidized during the extraction and storage. For more detailed discussion, S speciation in the HHSs should be determined by soft X-ray absorption spectroscopy.23
First shell (Cu–O/N) | Second shell (Cu–S) | ΔE0 (eV) | |||||
---|---|---|---|---|---|---|---|
R (Å) | CNa | σ 2 (103 Å2) | R (Å) | CN | σ 2 (103 Å2) | ||
a Coordination number. b Debye–Waller factor. The Debye–Waller factor of the Cu–S shell was set to be equal to that of the Cu–O/H shell. | |||||||
HHA | |||||||
pH 4 | 1.95 ± 0.01 | 2.7 ± 0.6 | 3 ± 1 | 2.35 ± 0.04 | 0.4 ± 0.2 | 3 ± 1 | 1.53 ± 2.36 |
pH 7 | 1.95 ± 0.02 | 3.1 ± 0.9 | 4 ± 3 | 2.35 ± 0.16 | 0.1 ± 0.3 | 4 ± 3 | 1.52 ± 3.51 |
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PAHA | |||||||
pH 4 | 1.94 ± 0.01 | 3.7 ± 0.8 | 5 ± 2 | — | — | — | 0.71 ± 2.80 |
pH 7 | 1.93 ± 0.01 | 3.6 ± 0.5 | 4 ± 1 | — | — | — | 1.19 ± 2.07 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5em00176e |
This journal is © The Royal Society of Chemistry 2015 |