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Humic acids coagulation: influence of divalent cations
Applied Geochemistry 18 (2003) 1573–1582 www.elsevier.com/locate/apgeochem
Humic acids coagulation: influence of divalent cations Nathalie A. Wall1, Gregory R. Choppin* Department of Chemistry, Florida State University, Tallahassee, FL 32306, USA Received 6 June 2002; accepted 4 February 2003 Editorial handling by W.M. Edmunds
Abstract The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca2+ and Mg2+ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca2+ and Mg2+ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg2+ and Ca2+. In the absence of the divalent cations, the coagulation has a constant value for pcH> 8, but, in the presence of Mg2+ and Ca2+, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction The Waste Isolation Pilot Plant (WIPP), a repository for transuranic wastes from US Department of Energy (DOE) defense programs, has been created about 50 km east of Carlsbad, NM, in the bedded salt of the Salado Formation of Permian age, approximately 650 m below the surface. In the 1996 WIPP Compliance Certification Application (CCA) (DOE, 1996), DOE asserted that humic substances, which are found in all natural waters as the result of biodegradation of animal and plant matter, would be present in the WIPP disposal room brines, or might form in situ, due to the presence of soils, nutrients, and cellulosic substrates for microbial action in the waste. Humic substances are important to consider in the performance assessment (PA) calculations of WIPP,
* Corresponding author. Fax: +1-850-644-8277. E-mail address: [email protected] (G.R. Choppin). 1 Present address: Sandia National Laboratories, 4100 National Parks Highway, Carlsbad, NM 88220, USA.
because they are known to have strong complexing ability with actinides (e.g. Bertha and Choppin, 1978; Moulin et al., 1992a; Kim and Czerwinski, 1996; Labonne-Wall et al., 1999) and can play a significant role in the environmental behavior of actinides (LabonneWall et al., 1997; Czerwinski et al., 1996; Kim et al., 1994; Moulin et al., 1992b; Buffle, 1988). The literature reports humic concentrations in natural waters from a few mg/l of dissolved organic C (DOC) (Marley et al., 1992; Minai et al., 1992) to more than a few hundreds mg/l DOC (Kim et al., 1997; Tiller and O’Melia, 1993). But no strong experimental or literature basis could be developed to predict humic concentration in WIPP. In the 1996 CCA, the project has bounded possible humic effects by assuming that soluble humic materials are present during the 10 ka WIPP performance period. However, the concentration of humics might not be appreciable during the 10 ka regulatory period, as humics are limited in their solubilities due to their rapid coagulation by brine constituents. Most of the literature results regarding humic coagulation are based on techniques employed for drinking
0883-2927/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00046-5
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water treatment (i.e. for low ionic strength systems) (Vik and Eikebrokk, 1989); at such low ionic strength the presence of divalent cations such as Ca2+ and Mg2+ (significant components of WIPP brines) were found to increase drastically the coagulation of humic acids in solution (Pefferkorn, 1997; Ong and Bisque, 1968; Hong and Elimelech, 1997; Ro¨mkens and Dolfing, 1998; Tipping and Ohnstad, 1984). Also, due to their near-colloidal size, humic acid concentrations are decreased by high ionic strengths. To evaluate whether humic acids (HA) would be stable in solution or would coagulate in conditions relevant to WIPP, the authors determined the HA coagulation over a range of pcH (log H+ concentration) for solutions of NaCl, LiCl and KCl over a range of ionic strengths and at concentrations of Ca2+ and Mg2+ comparable to those of WIPP [0.03 and 1.4 M (Choppin, 1999)]. The pcH formalism was used because the organic ligands/actinide binding constants experimentally determined for the WIPP project were established at fixed pcH values (e.g. Wall et al., 2002)
2. Experimental 2.1. Humic acid purification Humic acid (Aldrich Inc.) was purified by methods described earlier (Bertha and Choppin, 1978; Nash and Choppin, 1980; Kim et al., 1989). The HA was dissolved in a 102 M NaOH solution to obtain about 1 g/l HA and the solution was stirred for 5 h, filtered through glass wool and concentrated HCl was added to precipitate the HA. The humic slurry was centrifuged for 30 min and the isolated precipitate freeze-dried. After heating a sample at 500 C for 5 h; the ash content was found to be 50–60% for the original Aldrich humic acids, and less than 5% for the purified HA. The purified sample contained about 50% total organic C (TOC) (measurements performed with TOC 5050, Shimadzu). The solid state 13C-NMR of the purified compound is presented in Fig. 1 and the relative intensities of the different types of C are listed in Table 1. Other published solid state 13C NMR spectra of purified
Fig. 1. Solid state 13C NMR of purified Aldrich HA and integration curve.
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Aliphatic
Ether, alcohol, carbohydrate, amine
Aromatic, olefin
Carboxylate, carbonyl
Labonne-Wall et al. (1999) Labonne-Wall et al. (1997) This work
(0–50 ppm) 41% (0–90 ppm) 57% (0–50 ppm) 41%
(50–96 ppm) 5% (90–110 ppm) 0% (50–96 ppm) 10%
(108–165 ppm) 40% (110–160 ppm) 34% (108–165 ppm) 33%
(165–190 ppm) 14% (160–190 ppm) 9% (165–190 ppm) 11%
Aldrich HA (Kim et al., 1990; Shin et al., 1999) are quite similar to that in Fig. 1. 2.2. pcH/pHr determination A pH-electrode (Corning semi-micro combination electrode, connected to an Accumet Fisher 950 pHmeter) was calibrated daily using buffer solutions at pH 4, 7 and 10 (Fisher Chemical). The electrode was calibrated to obtain the relationship between pcH and pHr (pH value read on the pH-meter) as a function of the salt and ionic strength by titration at fixed ionic strength (NaCl, KCl or LiCl), using 0.1 M NaOH and 0.1 M HCl solutions. Such a technique has been widely used in the authors’ laboratory (e.g. Wall et al., 2002). For the 3 salts, the equations relating pcH, pHr and the ionic strength, based on the salt concentrations in molality, were determined to be: LiCl medium: pcHpHr ¼ 0:2347 ILiCl þ 0:3331 ð1aÞ NaCl medium: pcHpHr ¼ 0:2212 INaCl þ 0:0881 ð1bÞ KCl medium: pcHpHr ¼ 0:1338 IKCl -0:1117
ð1cÞ
2.3. Solution preparation Solutions of CaCl2 and MgCl2 (Fisher Chemical) were prepared in solutions of various ionic strengths of LiCl, NaCl and KCl. All the solutions were prepared with E-Pure water of 18 M/cm (Barnstead). The solutions were titrated with standard EDTA (Aldrich) to determine the divalent cation concentration, according to the method of Flaschka (1959). Humic acid solutions were prepared with the purified Aldrich HA at known ionic strength (LiCl, NaCl or KCl). All the experiments, except the ones performed to study the effect of pcH, were buffered (pcH 8–9) with 102 M CAPSO buffer (3-cyclohexylamino-2hydroxy-1-propanesulfonic acid, Aldrich). Humic solutions were fractionated with 100 kD filters (Amicon, Inc.). For the study of the effect of the filter size, HA solutions were fractionated by filtration through 10, 50, 100 and 300 kD filters (Amicon, Inc.). All the fractionations were performed using an ultrafiltration cell pressurized with N2, in which the solution was continuously stirred.
Filtered humic solutions were stored in a refrigerator (at 3–5 C) as, under such conditions, the humic molecular size distribution remained constant during the time of the study. 2.4. Determination of humic acid concentration UV–visible spectra of humic acid solutions were measured between 300 and 800 nm with a CARY 14 using the OLIS Spectroscopy Operating System. The absorbance measurements were performed immediately after preparing the solutions (filtration, pcH adjustment). The ratios of the absorbances of the humic solutions at 465 and 665 nm, referred to as E4/E6, were measured. Salt and halide free samples of humic acid for TOC analysis were obtained by adding a small amount of concentrated HNO3 to an aliquot of each sample, followed by centrifugation for 45 min at 12,000 g (Beckman centrifuge model J2-HC, rotor JA-20). The supernatant was discarded and the precipitated HA was dissolved in 102 M aqueous NaOH. The pHr of the solution was adjusted to 4.0 0.5, the solution was purged with N2(g) and the TOC of the solution measured. The amount of humic compound lost during the HA recovery was estimated from measurement by this procedure with standard HA solutions and yielded the following correction factor: TOC before HA extraction ¼ 1:23 ð0:03Þ TOC after HA extraction
ð2Þ
The combined error in the reproducibility of the HA recovery technique and the correction factor was 8%. UV–visible spectra and TOC content were measured immediately after 300, 100, 50 and 10 kD filtration of a HA solution (pcH 8.0 0.1, 3 M NaCl). The pcH of an unfiltered HA solution containing 60 mg/l TOC (3 M NaCl) was adjusted to various values between 7 and 12. The volumes of acid or base added to adjust the pcH were negligible. The UV–visible spectrum of each sample was measured immediately after pcH adjustment. 2.5. Coagulation experiments Buffered samples for the coagulation experiments were prepared with known volumes of filtered HA
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solution and variable amounts of Mg2+ and Ca2+ solution. For most of the work, the total volume of the samples was 25 ml; for the experiments requiring filtration to obtain different sized fractions, the total volume of the samples was 100 ml. The sample solutions were stored in glass vials at room temperature and either exposed to light (2 15 W fluorescent lamps for 16 h/day) or stored in the dark for fixed equilibration times. The samples were filtered with a 100 kD filter to obtain HA of < 100 kD size and with filter-sizes of 10, 50, 100 and 300 kD to obtain HA samples for the experiments with variable size fractions. The total organic C (TOC) of these HA stock solutions were determined. Then, for all the coagulation experiments, a variety of solutions was prepared from the HA stock solutions, as described below; after recorded times the solutions were passed through filters of sizes of 10, 50, 100 or 300 kD. Defining TOCi and TOCf as the amount of organic C initially present in solution and the amount of organic c present in solution after the filtration taking place at the end of the experiment, respectively, the amount of coagulated HA was calculated as: TOCi TOCf 100 TOCi
ð3Þ
HA solutions containing 30 mg/l TOC (NaCl 3 m, pcH 8, filter size 100 kD) were left undisturbed for periods of 2 h, 4 days and 48 days and then refiltered to evaluate the kinetics of humic acid coagulation; these samples were exposed to light. Other experiments were performed to study the influence of ionic strength and HA concentration over a 50 days equilibration period using buffered HA solutions of a 4100 kD fraction in NaCl media at pcH 8.4 0.5. The influence of pcH was studied over a 1-day equilibration period using solutions of a 4100 kD fraction in 3 M NaCl. To study the influence of the alkali cation, the coagulation was measured in NaCl (pcH 8.4 0.5), LiCl (pcH 8.5 0.5) and KCl (pcH 8.1 0.4) solutions of 4100 kD HA with 50 days equilibration. To investigate the effect of divalent cations, solutions of the 4100 kD fraction containing 40 mg/l ( 8) TOC and variable amounts of Ca2+ and Mg2+ were prepared in NaCl media at pcH 8.3 0.3. The influence of pcH in the presence of the divalent cations was studied using 4100 kD fraction solutions containing 0.05 M Ca2+ or Mg2+ prepared in 3 M NaCl. The effect of light was studied with a4100 kD fraction solution prepared in 0.1 M NaCl containing TOC 40 mg/l 8 and 0.01 M Ca2+, pcH 8.6 0.5 in the dark for 50 days and with a similar sample exposed to light for the same time period. To evaluate the filter size effect, HA solutions in 0.1 and 3.0 M ionic strengths NaCl were fractionated by filtration through 10, 50, 100 and 300 kD filters while a sample of the solution was kept unfiltered. After 50
days, each sample was fractionated again by filtration through 10, 50, 100 and 300 kD filter. All the experiments were performed exposed to the normal atmosphere.
3. Results 3.1. Measurements of humic acid absorbance The data in Table 2 show the influence of the HA filter size and the pcH on the absorbance of a HA solution: the absorbances at 465 nm of different filter sized HA solutions are not linearly dependent on TOC and increased pcH led to an increase of HA absorbance, although the concentration of HA was kept constant. The E4/E6 ratio varies with the size of the filtered HA and with pcH. 3.2. Kinetics The HA solutions containing 30 mg/l TOC (NaCl 3 M, pcH 8, filter size 100 kD), exposed to light for different periods of times, have different amounts of coagulated humic material: none after 2 h, 14% after 4 days and 71% after 48 days. 3.3. Influence of ionic strength, humic concentration and pcH value on HA coagulation In the low range of HA concentration (0–25 mg/l), the humic coagulation increases with an increasing humic concentration (Fig. 2), in agreement with literature data (Ro¨mkens and Dolfing, 1998). Also, the results in Fig. 2 show that increasing ionic strength, from 0.1 to 1.0 M,
Table 2 Influence of filter size and pcH on the absorbance of HA solution A. Filter size (pcH=8.00.1, 3.0 M NaCl) Filter size (kD)
TOC (mg/l)
Absorbance (465 nm)
E4/E6
10 50 100 300
27.4 2 34.3 3 37.1 3 50.4 4
0.90 0.51 0.47 0.40
13.5 6.0 7.0 7.6
B. pcH (unfiltered samples, TOC=60 mg/l, 3.0 M NaCl) pcH 7.79 8.71 9.86 10.60 11.76
Absorbance (465 nm) 0.59 0.60 0.64 0.66 0.69
E4/E6 4.08 4.88 4.91 5.03 5.12
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Fig. 2. HA (4100 kD fraction) coagulation in NaCl media, of different concentrations at pcH 8.4 0.5, after 50 days of equilibration.
provokes an increase of humic coagulation, which has been reported previously (Wen et al., 1997; Hong and Elimelech, 1997; Sholkovitz and Copland, 1981), while an increase in the ionic strength above 1.0 m does not affect the humic coagulation. Therefore, the critical coagulation concentration [i.e., the minimum concentration of electrolyte (NaCl) required to generate a rapid coagulation of the humic acids] lies between 0.1 and 1.0 M. This agrees with Tombacz and Meleg (1990) who reported this value to be 0.6 M, at pH 7.5. Coagulation decreases rapidly between pcH 4 and 7–8 (Fig. 3). A decrease in humic coagulation with pcH (or pH) was reported at low ionic strengths, (< 0.7 M) (Sholkovitz and Copland, 1981; Henning et al., 1997; Shaban and Mikulaj, 1998), but other authors found little or no influence of pH on humic coagulation in very low ionic strength media ( <0.01 M) (Tipping and Ohnstad, 1984; Gosh and Schnitzer, 1980; Yoon et al., 1998). This difference in results is due to the difference in ionic strength and pH. The critical pH value below which HA coagulates is a function of the ionic strength: pH 2.5 in 103 M NaCl, pH 4 in 0.1 M NaCl and pH 7 in 3 M NaCl (Dentel, 1988; Dempsey et al., 1984; Tombacz and Meleg, 1990). 3.4. Effect of the alkali cation The plots of the 4100 kD fraction TOC measured after 50 days equilibration (TOCf) versus the initial
4100 kD fraction TOC (TOCi) are linear, for the experiments conducted in NaCl, LiCl, and KCl media. The linear regression equations (TOCf=a TOCi+b) and their regression coefficient (r2) are presented in Table 3 for the systems LiCl, NaCl and KCl. For the media LiCl and NaCl, the regression equations have one set of a and b values for the ionic strength 0.1 M and a different set for the ionic strengths above 1.0 M; however, the humic coagulation is similar in all KCl media (0.1–3.0 M). At 0.1 M ionic strength, the best fit lines have similar slopes for the 3 salts (between 0.63 and 0.67) but the intercepts follow the order K+ < Na+ < Li+, which indicates that the alkali cations have an increasing efficiency in destabilizing the humic acids in the order Li+
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Fig. 3. HA (4100 kD fraction, initial total TOC=515 mg/l) coagulation as a function of pcH, in 3 M NaCl, after 1 day of equilibration.
Table 3 Data from linear regression analysis of 4100 kD fraction at 0 (TOCi) and 50 (TOCf) days of equilibration for 10 mg/l
LiCl NaCl KCl
pcH
8.50.5 8.40.5 8.10.5
0.1 M
51.0 M
a
b
r2
a
b
r2
0.640.04 0.670.03 0.640.02
4.692.67 0.791.86 3.101.35
0.98 0.99 0.98
0.460.04 0.370.05 0.640.02
8.422.6 3.332.0 3.101.35
0.94 0.80 0.98
previous humic acid studies (Hong and Elimelech, 1997; Yoon et al., 1998; Ro¨mkens and Dolfing, 1998; Ong and Bisque, 1968; Tipping and Ohnstad, 1984; Sholkovitz and Copland, 1981) however, Ro¨mkens and Dolfing (1998) expressed doubt about the generalization of such values for humic acids of different origin. An increase of the ionic strength (using the alkali halides) diminishes the coagulating efficiency of Ca2+ (Fig. 4). The Ca2+ critical coagulation concentration is about 0.005 M in media of ionic strengths greater than 3 M. The Mg2+ critical coagulation concentration is between 0.01 and 0.05 M at 0.1 m (Fig. 4), in agreement with previous studies (Ong and Bisque, 1968; Sholkovitz and Copland, 1981). The cation Mg2+ is a much less
efficient coagulating agent than Ca2+, as previously reported (Grace et al., 1997; Popiel, 1978). In the presence of Ca2+ or Mg2+, the humic coagulation decreases from pcH 4 to about 9 and increases for pcH values above 9 (Fig. 5). Hong and Elimelech (1997) and Yoon et al. (1998) observed the same trend. These experiments were repeated under N2(g) atmosphere to remove CO2(g) and identical results were obtained. 3.6. Effect of the light exposure The coagulation experiments performed in the dark showed that coagulation occurs in both the absence and the presence of light: in the presence of 0.01 M Ca2+, 65% ( 10) HA coagulated for the system kept in the
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Fig. 4. HA (4100 kD fraction, initial total TOC=40 8 mg/l) coagulation in NaCl media in presence of varying concentration of CaCl2 or MgCl2, at pcH 8.3 0.3, after 50 days of equilibration.
Fig. 5. HA (4100 kD fraction, initial total TOC=515 mg/l) coagulation in 3 M NaCl media, in presence of 0.05 M CaCl2 or MgCl2, after 1 day of equilibration.
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Table 4 HA (%) retained on different size filters (kD) 50 days after initial filtration Filter size (50 d): Initial filter size
<10
10–50
50–100
100–300
>300
of the aromatic C network and an increased degree of aliphatic structure (Kononova, 1966). These results also indicate the limitations of UV-visible spectrophotometry for determination of HA concentration. 4.2. DLVO theory and its application
A: 0.1 m NaCl medium, pcH 8.30.2 10 kD 475 10 6 06 50 kD 404 26 6 108 100 kD 415 28 7 38 300 kD 425 52 9 010 No filtration 303 14 4 146
105 59 48 39 37
516 209 249 239 387
B: 3.0 M NaCl medium, pcH 7.80.2 10 kD 829 4 10 50 kD 607 0 7 100 kD 415 23 6 300 kD 455 31 7 No filtration 212 10 3
127 010 08 06 04
318 357 348 486 674
09 010 48 08 84
dark and 80% ( 10) for the system kept under light. However, while the values in dark and in light are within the error limits of the measurements, the uncertainty is too large to eliminate the possibility of only a small difference in coagulation between the two systems. 3.7. Effect of the filter size The results of the filter size experiments are presented in Table 4 as the percentage of C present in the solution filtered after 50 days over a range of initial and final filter sizes. In 0.1 and 3.0 M ionic strengths, the unfractionated material (i.e. not filtered at time 0 and filtered after 50 days) has little mid-size Aldrich HA, as previously reported (Kim et al., 1989; Shin et al., 1999). This is particular to the Aldrich HA, as other humic acids have shown different size distributions (Eyrolle et al., 1996; Henning et al., 1997). After 50 days, the fractionated humic acids regain size distributions similar to the original unfractionated humic solution.
4. Discussion 4.1. Determination of humic acid concentration The influence of the HA size and of pcH on the UV– visible spectra (Table 2) can be interpreted in terms of the E4/E6 ratio results. Values of the E4/E6 ratio, commonly used for characterization of HAs, has been reported to be dependant on the source of the humic acids but is independent of the concentration of humic materials from 100 to 500 mg/l (Kononova, 1966; Schnitzer and Khan, 1978). Increases of E4/E6 ratio with decreased filter size from 50 to 10 kD and increased pcH has been attributed to a reduced degree of condensation
Humic coagulation has been discussed in terms of molecular conformation change (Ghosh and Schnitzer, 1980; Senesi et al., 1997; von Wandruska et al., 1997), of a more microscopic approach (Gregory, 1973) and of the Deryaguin, Landau, Verwey and Overbeek (DLVO) theory (Buffle et al., 1998). The DLVO theory considers the repulsive electrostatic (double layer) and the attractive (van der Waals) contributions to the potential energy of the interaction of colloidal particles and can be applied to humic colloids as a first approximation (Buffle et al., 1998; Hunter, 1993). Humic molecules can possess high negative charge due to the deprotonation of the carboxylic and phenolic groups; as a result, cations are attracted in a compact layer adjacent to the humic surface (i.e. the Stern layer), with a gradually decreasing concentration with increased distance from the humic surface, resulting in a diffuse double layer. A colloidal system is destabilized (coagulation, precipitation) when double-layered systems interact with each other, due to their Brownian motion. In the DLVO theory, the colloidal stability depends on several parameters; for example, coagulation increases with increases in z, the charge of the ion present in the double layer, and/or increases in , the Debye–Hu¨ckel parameter, which is directly proportional to I1/2 [1/ is referred to as the double layer thickness (Hunter, 1993; Hiemenz, 1977)]. The present study shows an increase of coagulation with increasing cationic charge (Fig. 4), increasing ionic strength (Fig. 2), or decreasing size of the hydrated ions present in the double layer, in the case of alkali (Table 3) or divalent cations (Figs. 4 and 5). The radii of the hydrated cations are inversely related to that of the ionic radii; i.e., for hydrates, the radii vary Li+ > Na+ > K+ and Mg2+ > Ca2+ (Ong and Bisque, 1968). Also, based on the DLVO theory, as the HA surface potential decreases, the inter-particle repulsion decreases and the system becomes less electrically stable (Tombacz and Meleg, 1990). As a result, the HA coagulation increases for a decreased pcH due to protonation of the HA, as illustrated by Fig. 3. The increase of humic coagulation above pcH 9 for the systems containing 0.05 M Ca2+ or Mg2+ (Fig. 5) indicates a change of charge at the humic surface, possibly due to the formation of a mixed HA–Ca–OH complex although the presence of such a complex has not been proven thus far. A mixed carbonate complex is eliminated as a possibility because experiments repeated under CO2(g) free conditions showed similar results as the experiments performed under normal atmosphere.
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To assess the performance of the WIPP, DOE investigated the consequences of inadvertent drillings through the disposal rooms, before the 10 ka years regulatory period. Such a situation could provoke a contaminated material release to the accessible environment, as material entrained in the circulating drilling fluid would be brought to the surface. The Castile Formation lying between the Delaware Mountain Group and the overlying Salado Formation, which host the WIPP, contains pressurized pockets of Castile brines. Therefore, one of the scenarios considered is an inadvertent penetration of a waste panel that also penetrates a pressurized Castile brine reservoir. A simulated Castile brine presents an ionic strength of ca. 5 M and contains 12 mM Ca2+ and 19 mM Mg2+ (DOE, 1996). As shown on Fig. 4, such a concentration of Ca2+ would be sufficient to coagulate the humics in a solution of low ionic strength, but the coagulating efficiency of Ca2+ decreases with increased ionic strength due to competition between Ca2+ and Na+ (from the ionic medium electrolyte NaCl) to populate the colloid double layer. At higher ionic strengths, the stabilizing effect of Na+ is greater than the coagulating properties of Ca2+ (Hunter, 1993). Fig. 4 shows also that the Mg2+ concentration in the Castile brine is not sufficient either to reach 100% HA coagulation. Of course, one cannot pretend to predict the behavior of humic acids in WIPP based on simplified experiments; nevertheless, this study confirms the DOE approach of non-negligible humic concentrations.
Acknowledgements The authors acknowledge the help of Donald E. Wall in several aspects of this study. This research was supported by Sandia National Laboratories. Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy, supported this research under contract DE-AC0494AL85000. References Bertha, E.L., Choppin, G.R., 1978. Interaction of humic and fulvic acids with Eu(III) and Am(III). J. Inorg. Chem. 40, 655–658. Buffle, J., 1988. Complexation Reactions in Aquatic Systems: An Analytical Approach. Ellis Horwood, Chichester, UK. Buffle, J., Wilkinson, K.J., Stoll, S., Filella, M., Zhang, J., 1998. A generalized description of aquatic colloidal interactions: the three-colloidal component approach. Environ. Sci. Technol. 32, 2887–2899. Choppin, G.R., 1999. Near field and far field interactions and data needs for geologic disposal of nuclear waste. In: Reed,
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D.T., Clark, S.B., Rao, L. (Eds.), Actinide Speciation in High Ionic Strength Media. Kluwer Academic/Plenum Publishers, New York, pp. 3–10. Czerwinski, K.R., Kim, J.I., Rhee, D.S., Buckau, G., 1996. Complexation of trivalent actinide ions (Am, Cm) with humic acid: the effect of ionic strength. Radiochim. Acta 72, 179–187. Dempsey, B.A., Ganho, R.M., O’Melia, C.R., 1984. The coagulation of humic substances by means of aluminum salts. J. Am. Water Works Assoc. 76, 141–150. Dentel, S.K., 1988. Application of the precipitation-charge neutralization model of coagulation. Environ. Sci. Technol. 22, 825–832. DOE (US Department of Energy), 1996. Title 40 CFR Part 191 Compliance Certification Application for the Waste Isolation Pilot Plant. DOE/CAO-1996-2184. US Department of Energy, Carlsbad Area Office, Carlsbad, NM. Eyrolle, F., Benedetti, M.F., Benaim, J.Y., Fe´vrier, D., 1996. The distributions of colloidal and dissolved organic carbon, major elements and trace elements in small tropical catchments. Geochim. Cosmochim. Acta 60, 3643–3656. Flaschka, H.A., 1959. EDTA Titrations: An Introduction to Theory and Practice. Pergamon Press, New York. Ghosh, K., Schnitzer, M., 1980. Macromolecular structures of humic substances. Soil Sci. 129, 266–276. Grace, M.R., Hislop, T.M., Hart, B.T., Beckett, R., 1997. Effect of saline groundwater on the aggregation and settling of suspended particles in a turbid Australian river. Colloids Surf. A. 120, 123–141. Gregory, J., 1973. Rates of flocculation of latex particles by cationic polymers. J. Colloid Interface Sci. 42, 448–456. Henning, K., Steffes, H.-J., Fakoussa, R.M., 1997. Effects on the molecular weight distribution of coal-derived humic acids studied by ultrafiltration. Fuel Proc. Technol. 52, 225–237. Hiemenz, P.C., 1977. Principles of Colloid and Surface Chemistry: Undergraduate Chemistry, Vol. 4. Marcel Dekker, New York. Hong, S., Elimelech, M., 1997. Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes. J. Membrane Sci. 132, 159–181. Hunter, R.J., 1993. Introduction to Modern Colloid Science. Oxford University Press, New York. Kim, J.I., Czerwinski, K.R., 1996. Complexation of metal ions with humic acid: metal ion charge neutralization model. Radiochim. Acta 73, 5–10. Kim, J.I., Buckau, G., Li, G.H., Duschner, H., Psarros, N., 1990. Characterization of humic and fulvic acids from Gorleben groundwater. Fresenius J. Anal. Chem. 338, 245–252. Kim, J.S., Chian, E.S.K., Saunders, F.M., Perdue, E.M., Giabbai, M.F., 1989. Characteristics of humic substances and their removal behavior in water treatment. In: Suffert, I.H., MacCarthy, P. (Eds.), Aquatic Humic Substances. Influence on Fate and Treatment of Polluants. American Chemical Society, Washington, DC, pp. 473–497 (Chapter 29). Kim, J.I., Delakowitz, B., Zeh, P., Klotz, D., Lazik, D., 1994. A column experiment for the study of colloidal radionuclide migration in Gorleben aquifer systems. Radiochim. Acta 66/ 67, 165–171. Kim, J.I., Rhee, D.S., Buckau, G., Morgenstern, A., 1997. Americium (III)-humate interaction in natural groundwater:
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influence of purification on complexation properties. Radiochim. Acta 79, 173–181. Kononova, M.M., 1966. Soil Organic Matter: Its Nature, Its Role in Soil Formation and in Soil Fertility. Pergamon Press, Oxford. Labonne-Wall, N., Choppin, G.R., Lopez, C., Monsallier, J.-M., 1999. Interaction of uranyl with humic and fulvic acids at high ionic strength. In: Reed, D.T., Clark, S.B., Rao, L. (Eds.), Actinide Speciation in High Ionic Strength Media. Kluwer Academic/Plenum Publishers, New York, pp. 199– 211. Labonne-Wall, N., Moulin, V., Vilarem, J.-P., 1997. Retention properties of humic substances onto amorphous silica: consequences for the sorption of cations. Radiochim. Acta 79, 37–49. Marley, N.A., Gaffney, J.S., Orlandini, K.A., Picel, K.C., Choppin, G.R., 1992. Chemical characterization of sizefractionated humic and fulvic materials in aqueous samples. Sci. Total Environ. 113, 159–177. Minai, Y., Choppin, G.R., Sisson, D.H., 1992. Humic material in well water from the Nevada Test Site. Radiochim. Acta 56, 195–199. Moulin, V., Tits, J., Moulin, C., Decambox, P., Mauchien, P., de Ruty, O., 1992a. Complexation behaviour of humic substances towards actinides and lanthanides studied by timeresolved laser-induced spectrofluorometry. Radiochim. Acta 58/59, 121–128. Moulin, V., Tits, J., Ouzounian, G., 1992b. Actinide speciation in the presence of humic substances in natural water conditions. Radiochim. Acta 58/59, 179–190. Nash, K.L., Choppin, G.R., 1980. Interaction of humic and fulvic acids with Th(IV). J. Inorg. Nucl. Chem. 42, 1045– 1050. Ong, H.L., Bisque, R.E., 1968. Coagulation of humic colloids by metal ions. Soil Sci. 106, 220–224. Pefferkorn, E., 1997. Structure and stability of natural organic matter/soil complexes and related synthetic and mixed analogues. Adv. Colloid Interface Sci. 73, 127–200. Popiel, W.J., 1978. Introduction to Colloid Science. Exposition Press Inc, Hicksville, NY. Ro¨mkens, P., Dolfing, J., 1998. Effect of Ca on the solubility and molecular size distribution of DOC and Cu binding in soil solution samples. Environ. Sci. Technol. 32, 363–369.
Schnitzer, M., Khan, S.U., 1978. Soil Organic Matter. Elsevier, New York. Senesi, N., Rizzi, F.R., Dellino, P., Acquafredda, P., 1997. Fractal humic acids in aqueous suspensions at various concentrations, ionic strengths and pH values. Colloids Surf. A. 127, 57–68. Shaban, I.S., Mikulaj, V., 1998. Impact of an anionic surfactant addition on solubility of humic acid in acid-alkaline solutions. Chem. Papers 52, 753–755. Shin, H.-S., Monsallier, J.-M., Choppin, G.R., 1999. Spectroscopic and chemical characterization of molecular size fractionated humic acid. Talanta 50, 641–647. Sholkovitz, E.R., Copland, D., 1981. The coagulation, solubility and adsorption properties of Fe, Mn, Cu, Ni, Cd, Co and humic acids in a river water. Geochim. Cosmochim. Acta 45, 181–189. Tiller, C.L., O’Melia, C.R., 1993. Natural organic matter and colloidal stability: models and measurements. Colloids Surf. A. 73, 89–102. Tipping, E., Ohnstad, M., 1984. Aggregation of aquatic humic substances. Chem. Geol. 44, 349–357. Tombacz, E., Meleg, E., 1990. A theoritical explanation of the aggregation of humic substances as a function of pH and electrolyte concentration. Org. Geochem. 15, 375–381. Vik, E.A., Eikebrokk, B., 1989. Coagulation process for removal of humic substances from drinking water. In: Suffet, I.H., MacCarthy, P. (Eds.), Aquatic Humic Substances. Influence on Fate and Treatment of Polluants. American Chemical Society, Washington, DC, pp. 385–408 (Chapter 24). von Wandruszka, R., Ragle, C., Engebretson, R., 1997. The role of selected cations in the formation of pseudomicelles in aqueous humic acids. Talanta 44, 805–809. Wall, N., Borkowski, M., Chen, J., Choppin, G., 2002. Complexation of americium with humic, fulvic, and citric acids at high ionic strength. Radiochim. Acta 90, 1–6. Wen, L.-S., Santschi, P.H., Tang, D., 1997. Interactions between radioactively labeled colloids and natural particles: evidence for colloidal pumping. Geochim. Cosmochim. Acta 61, 2867–2878. Yoon, S., Lee, C., Kim, K., Fane, A., 1998. Effect of calcium ion on the fouling of nanofilter by humic acid in drinking water production. Water Res. 32, 2180–2186.
Humic acids coagulation: influence of divalent cations Nathalie A. Wall1, Gregory R. Choppin* Department of Chemistry, Florida State University, Tallahassee, FL 32306, USA Received 6 June 2002; accepted 4 February 2003 Editorial handling by W.M. Edmunds
Abstract The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca2+ and Mg2+ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca2+ and Mg2+ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg2+ and Ca2+. In the absence of the divalent cations, the coagulation has a constant value for pcH> 8, but, in the presence of Mg2+ and Ca2+, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction The Waste Isolation Pilot Plant (WIPP), a repository for transuranic wastes from US Department of Energy (DOE) defense programs, has been created about 50 km east of Carlsbad, NM, in the bedded salt of the Salado Formation of Permian age, approximately 650 m below the surface. In the 1996 WIPP Compliance Certification Application (CCA) (DOE, 1996), DOE asserted that humic substances, which are found in all natural waters as the result of biodegradation of animal and plant matter, would be present in the WIPP disposal room brines, or might form in situ, due to the presence of soils, nutrients, and cellulosic substrates for microbial action in the waste. Humic substances are important to consider in the performance assessment (PA) calculations of WIPP,
* Corresponding author. Fax: +1-850-644-8277. E-mail address: [email protected] (G.R. Choppin). 1 Present address: Sandia National Laboratories, 4100 National Parks Highway, Carlsbad, NM 88220, USA.
because they are known to have strong complexing ability with actinides (e.g. Bertha and Choppin, 1978; Moulin et al., 1992a; Kim and Czerwinski, 1996; Labonne-Wall et al., 1999) and can play a significant role in the environmental behavior of actinides (LabonneWall et al., 1997; Czerwinski et al., 1996; Kim et al., 1994; Moulin et al., 1992b; Buffle, 1988). The literature reports humic concentrations in natural waters from a few mg/l of dissolved organic C (DOC) (Marley et al., 1992; Minai et al., 1992) to more than a few hundreds mg/l DOC (Kim et al., 1997; Tiller and O’Melia, 1993). But no strong experimental or literature basis could be developed to predict humic concentration in WIPP. In the 1996 CCA, the project has bounded possible humic effects by assuming that soluble humic materials are present during the 10 ka WIPP performance period. However, the concentration of humics might not be appreciable during the 10 ka regulatory period, as humics are limited in their solubilities due to their rapid coagulation by brine constituents. Most of the literature results regarding humic coagulation are based on techniques employed for drinking
0883-2927/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00046-5
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water treatment (i.e. for low ionic strength systems) (Vik and Eikebrokk, 1989); at such low ionic strength the presence of divalent cations such as Ca2+ and Mg2+ (significant components of WIPP brines) were found to increase drastically the coagulation of humic acids in solution (Pefferkorn, 1997; Ong and Bisque, 1968; Hong and Elimelech, 1997; Ro¨mkens and Dolfing, 1998; Tipping and Ohnstad, 1984). Also, due to their near-colloidal size, humic acid concentrations are decreased by high ionic strengths. To evaluate whether humic acids (HA) would be stable in solution or would coagulate in conditions relevant to WIPP, the authors determined the HA coagulation over a range of pcH (log H+ concentration) for solutions of NaCl, LiCl and KCl over a range of ionic strengths and at concentrations of Ca2+ and Mg2+ comparable to those of WIPP [0.03 and 1.4 M (Choppin, 1999)]. The pcH formalism was used because the organic ligands/actinide binding constants experimentally determined for the WIPP project were established at fixed pcH values (e.g. Wall et al., 2002)
2. Experimental 2.1. Humic acid purification Humic acid (Aldrich Inc.) was purified by methods described earlier (Bertha and Choppin, 1978; Nash and Choppin, 1980; Kim et al., 1989). The HA was dissolved in a 102 M NaOH solution to obtain about 1 g/l HA and the solution was stirred for 5 h, filtered through glass wool and concentrated HCl was added to precipitate the HA. The humic slurry was centrifuged for 30 min and the isolated precipitate freeze-dried. After heating a sample at 500 C for 5 h; the ash content was found to be 50–60% for the original Aldrich humic acids, and less than 5% for the purified HA. The purified sample contained about 50% total organic C (TOC) (measurements performed with TOC 5050, Shimadzu). The solid state 13C-NMR of the purified compound is presented in Fig. 1 and the relative intensities of the different types of C are listed in Table 1. Other published solid state 13C NMR spectra of purified
Fig. 1. Solid state 13C NMR of purified Aldrich HA and integration curve.
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Aliphatic
Ether, alcohol, carbohydrate, amine
Aromatic, olefin
Carboxylate, carbonyl
Labonne-Wall et al. (1999) Labonne-Wall et al. (1997) This work
(0–50 ppm) 41% (0–90 ppm) 57% (0–50 ppm) 41%
(50–96 ppm) 5% (90–110 ppm) 0% (50–96 ppm) 10%
(108–165 ppm) 40% (110–160 ppm) 34% (108–165 ppm) 33%
(165–190 ppm) 14% (160–190 ppm) 9% (165–190 ppm) 11%
Aldrich HA (Kim et al., 1990; Shin et al., 1999) are quite similar to that in Fig. 1. 2.2. pcH/pHr determination A pH-electrode (Corning semi-micro combination electrode, connected to an Accumet Fisher 950 pHmeter) was calibrated daily using buffer solutions at pH 4, 7 and 10 (Fisher Chemical). The electrode was calibrated to obtain the relationship between pcH and pHr (pH value read on the pH-meter) as a function of the salt and ionic strength by titration at fixed ionic strength (NaCl, KCl or LiCl), using 0.1 M NaOH and 0.1 M HCl solutions. Such a technique has been widely used in the authors’ laboratory (e.g. Wall et al., 2002). For the 3 salts, the equations relating pcH, pHr and the ionic strength, based on the salt concentrations in molality, were determined to be: LiCl medium: pcHpHr ¼ 0:2347 ILiCl þ 0:3331 ð1aÞ NaCl medium: pcHpHr ¼ 0:2212 INaCl þ 0:0881 ð1bÞ KCl medium: pcHpHr ¼ 0:1338 IKCl -0:1117
ð1cÞ
2.3. Solution preparation Solutions of CaCl2 and MgCl2 (Fisher Chemical) were prepared in solutions of various ionic strengths of LiCl, NaCl and KCl. All the solutions were prepared with E-Pure water of 18 M/cm (Barnstead). The solutions were titrated with standard EDTA (Aldrich) to determine the divalent cation concentration, according to the method of Flaschka (1959). Humic acid solutions were prepared with the purified Aldrich HA at known ionic strength (LiCl, NaCl or KCl). All the experiments, except the ones performed to study the effect of pcH, were buffered (pcH 8–9) with 102 M CAPSO buffer (3-cyclohexylamino-2hydroxy-1-propanesulfonic acid, Aldrich). Humic solutions were fractionated with 100 kD filters (Amicon, Inc.). For the study of the effect of the filter size, HA solutions were fractionated by filtration through 10, 50, 100 and 300 kD filters (Amicon, Inc.). All the fractionations were performed using an ultrafiltration cell pressurized with N2, in which the solution was continuously stirred.
Filtered humic solutions were stored in a refrigerator (at 3–5 C) as, under such conditions, the humic molecular size distribution remained constant during the time of the study. 2.4. Determination of humic acid concentration UV–visible spectra of humic acid solutions were measured between 300 and 800 nm with a CARY 14 using the OLIS Spectroscopy Operating System. The absorbance measurements were performed immediately after preparing the solutions (filtration, pcH adjustment). The ratios of the absorbances of the humic solutions at 465 and 665 nm, referred to as E4/E6, were measured. Salt and halide free samples of humic acid for TOC analysis were obtained by adding a small amount of concentrated HNO3 to an aliquot of each sample, followed by centrifugation for 45 min at 12,000 g (Beckman centrifuge model J2-HC, rotor JA-20). The supernatant was discarded and the precipitated HA was dissolved in 102 M aqueous NaOH. The pHr of the solution was adjusted to 4.0 0.5, the solution was purged with N2(g) and the TOC of the solution measured. The amount of humic compound lost during the HA recovery was estimated from measurement by this procedure with standard HA solutions and yielded the following correction factor: TOC before HA extraction ¼ 1:23 ð0:03Þ TOC after HA extraction
ð2Þ
The combined error in the reproducibility of the HA recovery technique and the correction factor was 8%. UV–visible spectra and TOC content were measured immediately after 300, 100, 50 and 10 kD filtration of a HA solution (pcH 8.0 0.1, 3 M NaCl). The pcH of an unfiltered HA solution containing 60 mg/l TOC (3 M NaCl) was adjusted to various values between 7 and 12. The volumes of acid or base added to adjust the pcH were negligible. The UV–visible spectrum of each sample was measured immediately after pcH adjustment. 2.5. Coagulation experiments Buffered samples for the coagulation experiments were prepared with known volumes of filtered HA
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solution and variable amounts of Mg2+ and Ca2+ solution. For most of the work, the total volume of the samples was 25 ml; for the experiments requiring filtration to obtain different sized fractions, the total volume of the samples was 100 ml. The sample solutions were stored in glass vials at room temperature and either exposed to light (2 15 W fluorescent lamps for 16 h/day) or stored in the dark for fixed equilibration times. The samples were filtered with a 100 kD filter to obtain HA of < 100 kD size and with filter-sizes of 10, 50, 100 and 300 kD to obtain HA samples for the experiments with variable size fractions. The total organic C (TOC) of these HA stock solutions were determined. Then, for all the coagulation experiments, a variety of solutions was prepared from the HA stock solutions, as described below; after recorded times the solutions were passed through filters of sizes of 10, 50, 100 or 300 kD. Defining TOCi and TOCf as the amount of organic C initially present in solution and the amount of organic c present in solution after the filtration taking place at the end of the experiment, respectively, the amount of coagulated HA was calculated as: TOCi TOCf 100 TOCi
ð3Þ
HA solutions containing 30 mg/l TOC (NaCl 3 m, pcH 8, filter size 100 kD) were left undisturbed for periods of 2 h, 4 days and 48 days and then refiltered to evaluate the kinetics of humic acid coagulation; these samples were exposed to light. Other experiments were performed to study the influence of ionic strength and HA concentration over a 50 days equilibration period using buffered HA solutions of a 4100 kD fraction in NaCl media at pcH 8.4 0.5. The influence of pcH was studied over a 1-day equilibration period using solutions of a 4100 kD fraction in 3 M NaCl. To study the influence of the alkali cation, the coagulation was measured in NaCl (pcH 8.4 0.5), LiCl (pcH 8.5 0.5) and KCl (pcH 8.1 0.4) solutions of 4100 kD HA with 50 days equilibration. To investigate the effect of divalent cations, solutions of the 4100 kD fraction containing 40 mg/l ( 8) TOC and variable amounts of Ca2+ and Mg2+ were prepared in NaCl media at pcH 8.3 0.3. The influence of pcH in the presence of the divalent cations was studied using 4100 kD fraction solutions containing 0.05 M Ca2+ or Mg2+ prepared in 3 M NaCl. The effect of light was studied with a4100 kD fraction solution prepared in 0.1 M NaCl containing TOC 40 mg/l 8 and 0.01 M Ca2+, pcH 8.6 0.5 in the dark for 50 days and with a similar sample exposed to light for the same time period. To evaluate the filter size effect, HA solutions in 0.1 and 3.0 M ionic strengths NaCl were fractionated by filtration through 10, 50, 100 and 300 kD filters while a sample of the solution was kept unfiltered. After 50
days, each sample was fractionated again by filtration through 10, 50, 100 and 300 kD filter. All the experiments were performed exposed to the normal atmosphere.
3. Results 3.1. Measurements of humic acid absorbance The data in Table 2 show the influence of the HA filter size and the pcH on the absorbance of a HA solution: the absorbances at 465 nm of different filter sized HA solutions are not linearly dependent on TOC and increased pcH led to an increase of HA absorbance, although the concentration of HA was kept constant. The E4/E6 ratio varies with the size of the filtered HA and with pcH. 3.2. Kinetics The HA solutions containing 30 mg/l TOC (NaCl 3 M, pcH 8, filter size 100 kD), exposed to light for different periods of times, have different amounts of coagulated humic material: none after 2 h, 14% after 4 days and 71% after 48 days. 3.3. Influence of ionic strength, humic concentration and pcH value on HA coagulation In the low range of HA concentration (0–25 mg/l), the humic coagulation increases with an increasing humic concentration (Fig. 2), in agreement with literature data (Ro¨mkens and Dolfing, 1998). Also, the results in Fig. 2 show that increasing ionic strength, from 0.1 to 1.0 M,
Table 2 Influence of filter size and pcH on the absorbance of HA solution A. Filter size (pcH=8.00.1, 3.0 M NaCl) Filter size (kD)
TOC (mg/l)
Absorbance (465 nm)
E4/E6
10 50 100 300
27.4 2 34.3 3 37.1 3 50.4 4
0.90 0.51 0.47 0.40
13.5 6.0 7.0 7.6
B. pcH (unfiltered samples, TOC=60 mg/l, 3.0 M NaCl) pcH 7.79 8.71 9.86 10.60 11.76
Absorbance (465 nm) 0.59 0.60 0.64 0.66 0.69
E4/E6 4.08 4.88 4.91 5.03 5.12
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Fig. 2. HA (4100 kD fraction) coagulation in NaCl media, of different concentrations at pcH 8.4 0.5, after 50 days of equilibration.
provokes an increase of humic coagulation, which has been reported previously (Wen et al., 1997; Hong and Elimelech, 1997; Sholkovitz and Copland, 1981), while an increase in the ionic strength above 1.0 m does not affect the humic coagulation. Therefore, the critical coagulation concentration [i.e., the minimum concentration of electrolyte (NaCl) required to generate a rapid coagulation of the humic acids] lies between 0.1 and 1.0 M. This agrees with Tombacz and Meleg (1990) who reported this value to be 0.6 M, at pH 7.5. Coagulation decreases rapidly between pcH 4 and 7–8 (Fig. 3). A decrease in humic coagulation with pcH (or pH) was reported at low ionic strengths, (< 0.7 M) (Sholkovitz and Copland, 1981; Henning et al., 1997; Shaban and Mikulaj, 1998), but other authors found little or no influence of pH on humic coagulation in very low ionic strength media ( <0.01 M) (Tipping and Ohnstad, 1984; Gosh and Schnitzer, 1980; Yoon et al., 1998). This difference in results is due to the difference in ionic strength and pH. The critical pH value below which HA coagulates is a function of the ionic strength: pH 2.5 in 103 M NaCl, pH 4 in 0.1 M NaCl and pH 7 in 3 M NaCl (Dentel, 1988; Dempsey et al., 1984; Tombacz and Meleg, 1990). 3.4. Effect of the alkali cation The plots of the 4100 kD fraction TOC measured after 50 days equilibration (TOCf) versus the initial
4100 kD fraction TOC (TOCi) are linear, for the experiments conducted in NaCl, LiCl, and KCl media. The linear regression equations (TOCf=a TOCi+b) and their regression coefficient (r2) are presented in Table 3 for the systems LiCl, NaCl and KCl. For the media LiCl and NaCl, the regression equations have one set of a and b values for the ionic strength 0.1 M and a different set for the ionic strengths above 1.0 M; however, the humic coagulation is similar in all KCl media (0.1–3.0 M). At 0.1 M ionic strength, the best fit lines have similar slopes for the 3 salts (between 0.63 and 0.67) but the intercepts follow the order K+ < Na+ < Li+, which indicates that the alkali cations have an increasing efficiency in destabilizing the humic acids in the order Li+
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Fig. 3. HA (4100 kD fraction, initial total TOC=515 mg/l) coagulation as a function of pcH, in 3 M NaCl, after 1 day of equilibration.
Table 3 Data from linear regression analysis of 4100 kD fraction at 0 (TOCi) and 50 (TOCf) days of equilibration for 10 mg/l
LiCl NaCl KCl
pcH
8.50.5 8.40.5 8.10.5
0.1 M
51.0 M
a
b
r2
a
b
r2
0.640.04 0.670.03 0.640.02
4.692.67 0.791.86 3.101.35
0.98 0.99 0.98
0.460.04 0.370.05 0.640.02
8.422.6 3.332.0 3.101.35
0.94 0.80 0.98
previous humic acid studies (Hong and Elimelech, 1997; Yoon et al., 1998; Ro¨mkens and Dolfing, 1998; Ong and Bisque, 1968; Tipping and Ohnstad, 1984; Sholkovitz and Copland, 1981) however, Ro¨mkens and Dolfing (1998) expressed doubt about the generalization of such values for humic acids of different origin. An increase of the ionic strength (using the alkali halides) diminishes the coagulating efficiency of Ca2+ (Fig. 4). The Ca2+ critical coagulation concentration is about 0.005 M in media of ionic strengths greater than 3 M. The Mg2+ critical coagulation concentration is between 0.01 and 0.05 M at 0.1 m (Fig. 4), in agreement with previous studies (Ong and Bisque, 1968; Sholkovitz and Copland, 1981). The cation Mg2+ is a much less
efficient coagulating agent than Ca2+, as previously reported (Grace et al., 1997; Popiel, 1978). In the presence of Ca2+ or Mg2+, the humic coagulation decreases from pcH 4 to about 9 and increases for pcH values above 9 (Fig. 5). Hong and Elimelech (1997) and Yoon et al. (1998) observed the same trend. These experiments were repeated under N2(g) atmosphere to remove CO2(g) and identical results were obtained. 3.6. Effect of the light exposure The coagulation experiments performed in the dark showed that coagulation occurs in both the absence and the presence of light: in the presence of 0.01 M Ca2+, 65% ( 10) HA coagulated for the system kept in the
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Fig. 4. HA (4100 kD fraction, initial total TOC=40 8 mg/l) coagulation in NaCl media in presence of varying concentration of CaCl2 or MgCl2, at pcH 8.3 0.3, after 50 days of equilibration.
Fig. 5. HA (4100 kD fraction, initial total TOC=515 mg/l) coagulation in 3 M NaCl media, in presence of 0.05 M CaCl2 or MgCl2, after 1 day of equilibration.
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Table 4 HA (%) retained on different size filters (kD) 50 days after initial filtration Filter size (50 d): Initial filter size
<10
10–50
50–100
100–300
>300
of the aromatic C network and an increased degree of aliphatic structure (Kononova, 1966). These results also indicate the limitations of UV-visible spectrophotometry for determination of HA concentration. 4.2. DLVO theory and its application
A: 0.1 m NaCl medium, pcH 8.30.2 10 kD 475 10 6 06 50 kD 404 26 6 108 100 kD 415 28 7 38 300 kD 425 52 9 010 No filtration 303 14 4 146
105 59 48 39 37
516 209 249 239 387
B: 3.0 M NaCl medium, pcH 7.80.2 10 kD 829 4 10 50 kD 607 0 7 100 kD 415 23 6 300 kD 455 31 7 No filtration 212 10 3
127 010 08 06 04
318 357 348 486 674
09 010 48 08 84
dark and 80% ( 10) for the system kept under light. However, while the values in dark and in light are within the error limits of the measurements, the uncertainty is too large to eliminate the possibility of only a small difference in coagulation between the two systems. 3.7. Effect of the filter size The results of the filter size experiments are presented in Table 4 as the percentage of C present in the solution filtered after 50 days over a range of initial and final filter sizes. In 0.1 and 3.0 M ionic strengths, the unfractionated material (i.e. not filtered at time 0 and filtered after 50 days) has little mid-size Aldrich HA, as previously reported (Kim et al., 1989; Shin et al., 1999). This is particular to the Aldrich HA, as other humic acids have shown different size distributions (Eyrolle et al., 1996; Henning et al., 1997). After 50 days, the fractionated humic acids regain size distributions similar to the original unfractionated humic solution.
4. Discussion 4.1. Determination of humic acid concentration The influence of the HA size and of pcH on the UV– visible spectra (Table 2) can be interpreted in terms of the E4/E6 ratio results. Values of the E4/E6 ratio, commonly used for characterization of HAs, has been reported to be dependant on the source of the humic acids but is independent of the concentration of humic materials from 100 to 500 mg/l (Kononova, 1966; Schnitzer and Khan, 1978). Increases of E4/E6 ratio with decreased filter size from 50 to 10 kD and increased pcH has been attributed to a reduced degree of condensation
Humic coagulation has been discussed in terms of molecular conformation change (Ghosh and Schnitzer, 1980; Senesi et al., 1997; von Wandruska et al., 1997), of a more microscopic approach (Gregory, 1973) and of the Deryaguin, Landau, Verwey and Overbeek (DLVO) theory (Buffle et al., 1998). The DLVO theory considers the repulsive electrostatic (double layer) and the attractive (van der Waals) contributions to the potential energy of the interaction of colloidal particles and can be applied to humic colloids as a first approximation (Buffle et al., 1998; Hunter, 1993). Humic molecules can possess high negative charge due to the deprotonation of the carboxylic and phenolic groups; as a result, cations are attracted in a compact layer adjacent to the humic surface (i.e. the Stern layer), with a gradually decreasing concentration with increased distance from the humic surface, resulting in a diffuse double layer. A colloidal system is destabilized (coagulation, precipitation) when double-layered systems interact with each other, due to their Brownian motion. In the DLVO theory, the colloidal stability depends on several parameters; for example, coagulation increases with increases in z, the charge of the ion present in the double layer, and/or increases in , the Debye–Hu¨ckel parameter, which is directly proportional to I1/2 [1/ is referred to as the double layer thickness (Hunter, 1993; Hiemenz, 1977)]. The present study shows an increase of coagulation with increasing cationic charge (Fig. 4), increasing ionic strength (Fig. 2), or decreasing size of the hydrated ions present in the double layer, in the case of alkali (Table 3) or divalent cations (Figs. 4 and 5). The radii of the hydrated cations are inversely related to that of the ionic radii; i.e., for hydrates, the radii vary Li+ > Na+ > K+ and Mg2+ > Ca2+ (Ong and Bisque, 1968). Also, based on the DLVO theory, as the HA surface potential decreases, the inter-particle repulsion decreases and the system becomes less electrically stable (Tombacz and Meleg, 1990). As a result, the HA coagulation increases for a decreased pcH due to protonation of the HA, as illustrated by Fig. 3. The increase of humic coagulation above pcH 9 for the systems containing 0.05 M Ca2+ or Mg2+ (Fig. 5) indicates a change of charge at the humic surface, possibly due to the formation of a mixed HA–Ca–OH complex although the presence of such a complex has not been proven thus far. A mixed carbonate complex is eliminated as a possibility because experiments repeated under CO2(g) free conditions showed similar results as the experiments performed under normal atmosphere.
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To assess the performance of the WIPP, DOE investigated the consequences of inadvertent drillings through the disposal rooms, before the 10 ka years regulatory period. Such a situation could provoke a contaminated material release to the accessible environment, as material entrained in the circulating drilling fluid would be brought to the surface. The Castile Formation lying between the Delaware Mountain Group and the overlying Salado Formation, which host the WIPP, contains pressurized pockets of Castile brines. Therefore, one of the scenarios considered is an inadvertent penetration of a waste panel that also penetrates a pressurized Castile brine reservoir. A simulated Castile brine presents an ionic strength of ca. 5 M and contains 12 mM Ca2+ and 19 mM Mg2+ (DOE, 1996). As shown on Fig. 4, such a concentration of Ca2+ would be sufficient to coagulate the humics in a solution of low ionic strength, but the coagulating efficiency of Ca2+ decreases with increased ionic strength due to competition between Ca2+ and Na+ (from the ionic medium electrolyte NaCl) to populate the colloid double layer. At higher ionic strengths, the stabilizing effect of Na+ is greater than the coagulating properties of Ca2+ (Hunter, 1993). Fig. 4 shows also that the Mg2+ concentration in the Castile brine is not sufficient either to reach 100% HA coagulation. Of course, one cannot pretend to predict the behavior of humic acids in WIPP based on simplified experiments; nevertheless, this study confirms the DOE approach of non-negligible humic concentrations.
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