Transport of Kaolinite Colloids through Quartz Sand: Influence of Humic Acid, Ca2+, and Trace Metals

Journal of Colloid and Interface Science 253, 1–8 (2002) doi:10.1006/jcis.2002.8523

Transport of Kaolinite Colloids through Quartz Sand: Influence of Humic Acid, Ca2+ , and Trace Metals Rachid A¨ıt Akbour,∗ Jamaˆa Douch,∗ Mohamed Hamdani,∗ and Philippe Schmitz†,1 ∗ Universit´e Ibn Zohr, Facult´e des Sciences, D´epartement de Chimie B.P. 28/S 80000 Agadir, Marocco; and †Institut de M´ecanique des Fluides de Toulouse, UMR CNRS-INP-UPS 5502, All´ee du Pr Camille soula, 31400 Toulouse, France Received November 29, 2001; accepted June 6, 2002; published online August 9, 2002

groups in humic substance (6). These reactive groups are responsible for the formation of complexes with metallic cations and charged surfaces of mineral particles. In natural environments, most inorganic colloid surfaces are coated with adsorbed natural organic matter such as humic substances (7, 8). Such organic coatings are known to alter the surface charge and aggregation behavior of mineral particles. Kretzschmar and Sticher (9) have shown that the adsorption of humic acid to hematite, positively charged, caused reversal of the surface charge from positive to negative. Several other studies have found that the addition of humic substances to a suspension of clay or Fe oxide colloids can greatly increase colloidal stability (10, 11). The stabilizing effect of humic substances may be due to a combination of electrostatic and steric stabilization (10). The effect of trace metals on the colloidal stability of humiccoated mineral colloids has not been well studied; however, it is known that the adsorbed trace metals or divalent cations may further reduce the surface charge of humic-coated mineral particles and then induce their aggregation and deposition in porous media. Kretzschmar and Sticher (9) have observed that the substitution of Ca2+ by Cu2+ or Pb2+ decreased electrophoretic mobility and colloid mobility of humic-coated hematite colloids. The colloidal particles used in transport and deposition studies are usually metal oxides which are positively charged under careful controlled conditions. Whereas, in this work, the colloids used were kaolinite clay particles with a predominance of negative surface charge. Indeed, pure kaolinite particles are globally negatively charged because of the existence of permanent negative charges (pH-independent charges) which are generally located on the basal plane of kaolinite (12). According to pH value, the positive charges, promoted by the dissociation of amphotere groups (pH-dependent charges), are located on the edge surfaces of kaolinite. In this study, we investigate the influence of humic acid concentration on the transport and deposition kinetics of colloidal kaolinite particles in a saturated porous medium of pure quartz sand. The effects of several parameters such as pore water velocity, Ca2+ , and trace metal (Cu2+ , Pb2+ ) concentrations on the deposition kinetics of humic-coated kaolinite colloids were also studied in the same porous medium.

To evaluate the risk of contaminant transport by mobile colloids, it seems essential to understand how colloids and associated pollutants behave during their migration through uncontaminated soil or groundwater. In this study, we investigated at pH 4 the influence of flow velocity, humic acid, solution Ca2+ concentrations, and trace metals (Pb2+ , Cu2+ ) on the transport and deposition of kaolinite particles through a pure crystalline quartz sand as porous medium. A short-pulse chromatographic technique was used to measure colloid deposition. Adsorption of humic acid to the kaolinite increase its negative surface charge and then decrease colloid deposition. Experiments with different flow rates showed that humic-coated kaolinite colloid deposition followed a first-order kinetic rate law. The deposition rate coefficients of humic-coated kaolinite colloids increase with increasing Ca2+ concentration in the suspension. The effect of trace metals on the mobility is studied by injecting two suspensions with different concentrations of Pb2+ and Cu2+ . At very low cation concentration, the fraction of colloids retained is low and roughly independent of the nature of divalent cations. At high concentration, the deposition is higher and depends on the affinity of divalent cations toward humic-coated kaolinite colloids. C 2002 Elsevier Science (USA) Key Words: kaolinite; humic acid; colloids; deposition; divalent cations; porous media; sand; affinity.

1. INTRODUCTION

Mobile colloids in soils and groundwater aquifers can facilitate the transport of strongly sorbing contaminants (1, 2). Colloids and associated pollutants can be mobilized in situ after a change in soil or groundwater chemical composition (2–4). Field investigations have shown that the mobile subsurface colloids are mostly composed of clay minerals, (hydr)oxides of Al, Fe, silica, carbonates, and/or natural organic matter (3). The natural organic matter consists of about 60% humic substance (5). Humic substances are considered hydrophilic amorphous polyelectrolytes; their chemical structures are very complex. Carboxylic and phenolic are the most dominant functional

1 To whom correspondence should be addressed. Fax: 05-61-28-58-99. E-mail: [email protected].

1

0021-9797/02 $35.00

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AKBOUR ET AL.

2. THEORY

The transport of colloids through porous media at constant flow rate is governed by the dispersive-convective transport equation which takes into account the first-order term due to colloid deposition. It can be expressed in one dimension as ∂C ∂ 2C ∂C = D p 2 − vp − kd C, ∂t ∂x ∂x

[1]

where C is the colloid concentration in suspension, x the travel distance, Dp the dispersion coefficient of colloidal particles, vp the average travel velocity of colloidal particles (i.e., interstitial velocity of colloidal particles), and kd the colloid deposition rate coefficient. In this equation, colloid release is neglected, which is justified if the kinetics of colloid release are very slow compared to the time scale of colloid breakthrough experiments (9, 13). This condition is generally admitted in many colloid transport experiments carried out at constant flow and solution chemistry (14). The dispersion term in Eq. [1] can be neglected for column Peclet numbers, Pe, >50 (15). Thus the colloid deposition rate coefficient can be calculated by numerical integration of the breakthrough pulse as (9)   tf 1 q kd = − Ln C(t) dt , tp No

[2]

0

where q is the volumetric flow rate, No is the total amount of colloids injected in the column, tf is the time after which the colloid pulse has completely moved through the column, tp = L/vp is the average travel time of colloidal particles through the column,  q  tf and L is the column lengh. In this equation, the term C(t)dt) represents the fraction of colloids recovered in No 0 the effluent following a pulse input of colloids. It is determined by numerical integration of breakthrough peak expressed in the following reduced coordinates C/Co = f (V /V p). 3. MATERIALS AND METHODS

3.1. Kaolinite The colloidal kaolinite used in this investigation was from Charentes (GZA IV type). Its main characteristics were previously identified by Nabzar (12). The elementary analysis shows that it is made of 36% aluminium and of 46% silicium (12). The point of zero charge (PZC) determined by potentiometric titration is about 7.2 (16). The specific surface area measured by BET (N2 ) is equal to 27 ± 1 m2 /g (17). The kaolinite is used without any preliminary chemical treatment. 3.2. Humic Acid (HA) The humic acid used in the experiments is a commercial product obtained from Aldrich Chimie in the form of sodium hu-

mate. A few characteristics of this humic acid were determined (18). Its precipitation begins at pH values lower than 2. The existence of fulvic acid at a concentration always lower than 8% was clearly shown. The potentiometric titration of humic acid shows that the mean mass of one acid equivalent is about 434 ±10 g. 3.3. Porous Medium The porous medium adopted to study the transport of kaolinite in the presence of humic acid is a pure quartz sand (Nemours sand) made of 99.999% SiO2 . Its granulometry varies between 100 and 620 µm (data from the manufacturer) and its specific surface area (BET, Kr, 77 K) is about 0.03 m2 /g (19). The point of zero charge of the pure quartz sand is found to be lower than 3.5 (20). 3.4. Reagents All chemical reagents employed in this work were p.a. quality. The solutions were prepared using bidistilled water previously boiled and kept in nitrogen atmosphere. The ionic strength was imposed using calcium chloride (CaCl2 ). Solutions of HNO3 and CO2 -free NaOH were used to fix a pH of 4. Solutions of copper nitrates (Cu (NO3 )2 ) and lead nitrates (Pb (NO3 )2 ) were employed to study the effects of metallic cations (Cu2+ and Pb2+ ) on transport and deposition of kaolinite in the presence of humic acid. 3.5. Experimental Device The column setup used for short-pulse breakthrough experiments consisted of the following components: (1) a column made of Altuglas (length, 9.8 cm; diameter, 2.4 cm), which contained the porous medium; (2) two syringe pumps (Perfusor secura) to impose the flow rate, (3) a three-way valve; (4) a pH meter (Consort) allowing for the continuous measurement of pH at the column outlet; (5) an on-line UV/visible detector (UV 975, Jasco) to measure the colloid concentration in the effluent; and (6) a recorder x, t (DLH Laumann). 3.6. Column Experiments After filling the column with the pure quartz sand, the subsequent porous medium was put in vacuum and saturated by bidistilled water previously degassed. The washing of the porous medium was performed by continuous injection of water until a stable outlet pH value was obtained. The tracer breakthrough experiments were conducted to determine the pore volume,Vp , and the column Peclet number, Pe. Short pulses using 2 ml of conservative tracer (KI, 20 mg/L) were injected and the KI concentration in the effluent was monitored on-line by a flow-through UV/visible detector at 234 nm. For all the experiments, the pore volume was between 20 and 22 ml. The porosity varied from 0.44 to 0.49 and the Peclet number remained always higher than 140. The entire experiment was performed at pH 4 and under ambient temperature. The injection flow rate equaled 1 ml/min

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TRANSPORT OF KAOLINITE COLLOIDS THROUGH QUARTZ SAND

In these two series of experiments, the porous column was preliminarily saturated by injection of 5 × 10−5 mol/L CaCl2 solution at a constant flow rate for more than 6 h. —The third series of experiments focused on the effect of Ca2+ concentration on colloid deposition. The whole injected suspensions were made of 30 mg/L kaolinite, 4 mg/L humic acid, and CaCl2 concentration that varied from 10−6 to 10−2 mol/L. Each suspension was injected in the porous column previously preconditioned with the same CaCl2 concentration as the one of the suspension. —The fourth series of experiments evaluated the concentration and the nature effects of the divalent cations Me2+ (Ca2+ , Cu2+ , and Pb2+ ) on colloid transport and deposition. In these experiments, suspensions of 30 mg/L kaolinite, 4 mg/L humic acid in the presence of 10−6 or 5 × 10−5 mol/L of Me2+ were injected in the porous column preliminarily saturated by a solution of 10−6 or 5 × 10−5 mol/L CaCl2 respectively. The experiments were repeated in their entirety at least twice for reproducibility. For clarity, the plots of breakthrough curves depict only results concerning one experiment. Of course we have verified that all the experimental points (symbols in figures) follow the Gaussian distribution (straight line) with a corresponding regression coefficient R 2 > 0.99 and a standard deviation of about 0.006. On the other figures, the error bars correspond to the standard deviations of the two experiments.

4. RESULTS AND DISCUSSION

4.1. Stability and Surface Properties of Kaolinite in Presence of Humic Acid The effects of humic acid concentration on the stability (evaluated by optical density measurements) and surface properties (estimated by potential zeta measurements) of a suspension of 30 mg/L kaolinite in 5 × 10−5 mol/L CaCl2 at pH 4, maintained at rest for 20 h, are depicted in Fig. 1. For concentrations greater than 0.2 mg/L, the presence of humic acid leads to the increase of the stability of the suspension which is related to an increase of the absolute value of zeta potential. The increase of stability of the suspension is promoted by the adsorption of humic acid, negatively charged, on the kaolinite particle surfaces. The adsorption of organic matter on the solid surfaces (oxides or clay surfaces) is due to electrostatic interactions and specific adsorption via ligand exchange (23, 24). Several authors have already observed this particular effect of humic or fulvic acid on the charge and the stability of mineral colloids (25–27). For concentration values of humic acid lower than 0.2 mg/L, the suspension is slightly destabilized and the corresponding absolute value of zeta potential slightly diminishes. This weak destabilization can be due to either a neutralization of positive charges of kaolinite and negative charges of humic acid by electrostatic interactions or a bridging mechanism where

-13

0,12

Optical density Zeta potential

-14

0,11

0,10

-15

0,09 -16 0,08

Optical density

—The first study was devoted to the effect of humic acid concentration on the deposition kinetics of kaolinite. Five different suspensions of 30 mg/L kaolinite associated with 0, 2, 4, 6, and 8 mg/L humic acid were prepared in 5 × 10−5 mol/L CaCl2 solution. Before injection, the suspensions were stirred for 30 min. —The second study was related to the effect of pore water velocity on the deposition kinetics of kaolinite in the presence of humic acid. To this end, a suspension of 30 mg/L kaolinite, 4 mg/L humic acid, and 5 × 10−5 mol/L CaCl2 is injected at various volumetric flow rates (0.25, 0.5, 0.75, 1, and 1.5 ml/min).

MALVERN Instruments under the same conditions of pH and resting time as the ones adopted for the study of colloidal stability. Zeta potentials were calculated from the measurement of the electrophorotic mobilities using the Smoluchwski relations taking into account the function of correction of Henry. The colloidal stability of different suspensions at pH 4 was estimated by optical density measurements, using a spectrophotometer UV/visible (HP 8453 type) at 280 nm, after a resting time of 20 h.

Zeta potential (mV)

(except if indicated). We used pH 4 in order to avoid the hydrolysis phenomena of trace metals, especially for Pb2+ (21, 22). The experiments on transport and deposition of kaolinite were performed by injection of 2 ml (about 0.14 Vp ) of a suspension prepared at pH 4. The effluent is controlled with UV/visible at 280 nm. The total amount of injected colloids (No ) is determined using the same experimental device but without the column. Four different series of experiments were carried out during the present study:

-17 0,07

3.7. Stability and Electrokinetic Potential of the Colloids To better understand the effect of humic acid and Me2+ concentrations, respectively, on kaolinite and humic-coated kaolinite surface charge, electrophoretic measurements were done to determine the zeta potentials of the suspensions. These measurements were performed with a Zˆetasizer 4000 apparatus of

0

1

2

3

4

[HA] (mg/l) FIG. 1. Influence of humic acid on the stability, evaluated by optical density measurements after 20 h, and on the zeta potentials of suspension of 30 mg/L kaolinite and 5 × 10−5 mol/L CaCl2 at pH 4.

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AKBOUR ET AL. 0,4

a - - - tracer 0 mg/l HA 2 mg/l HA 4 mg/l HA 6 mg/l HA

0,8

0,6

0,3

0,2

0,4 0,1

C/Co (suspension)

C/Co (tracer)

1,0

0,2

0,0

0,0

b

1

14

V/ Vp

2

3

0,5

Recovered fraction K d (h-1 )

12

0,4

K d (h-1)

10 8

0,3

6

0,2

4

Recovered fraction

0

0,1 2 0

0,0 0

2

4

6

8

[HA] (mg/l) FIG. 2. Influence of humic acid on the transport and deposition kinetics of kaolinite in pure quartz sand. (a) Breakthrough curves for a conservative solute tracer (KI) and kaolinite as a function of humic acid (HA) concentrations. (b) Evolution of both kaolinite deposition rate coefficients and fraction of colloids recovered in the presence of different HA concentrations. The sand column is preconditioned with 5 × 10−5 mol/L CaCl2 electrolyte solution at pH 4. The suspensions containing 30 mg/L kaolinite and 5 × 10−5 mol/L Ca2+ at pH 4 were injected at a constant flow velocity of 1 ml/min.

anionic polyelectrolyte is adsorbed on the surface of several particles (28). 4.2. Effect of Humic Acid Concentration on Kaolinite Mobility The effect of humic acid on the transport of kaolinite through a column of pure quartz sand is depicted in Fig. 2a. In the absence of humic acid, low quantities of kaolinite are detected in the effluent. In the presence of humic acid, the mobility of colloids increases considerably. The breakthrough curves obtained as a function of humic acid concentration show that the colloids traveled considerably faster through the porous column than the conservative tracer. This can be attributed to chromatographic exclusion phenomenon (or volumetric exclusion), in which colloidal particles are excluded from small pores and hence from part of the total pore space. This phenomenon was also observed in several other studies devoted to colloidal particle transport and macromolecule transport in porous media (9, 29). In the present

work, the exclusion volume is found to be about 10% of the total pore volume. The values of the deposition rate coefficient, K d , determined from the integration of peak areas using Eq. [2] are presented in Fig. 2b. It can be seen that the deposition rate coefficients decrease and progressively stabilize as the amount of humic acid increases. The corresponding stable value is about 2.76 h−1 for a concentration of humic acid of 6 mg/L. Parallelly, the fraction of colloid recovered in the column effluent increases as the concentration of humic acid increases as can be seen in Fig. 2b. It is finally stabilized when the polyelectrolyte concentration is higher than 6 mg/L. It can be assumed that the decrease in the deposition rate of colloids in porous media is related to repulsive interactions between kaolinite surface covered by humic acid and negatively charged surfaces of sand with respect to its PZC lower than 3.5 despite the presence of CaCl2 at 5 × 10−5 mol/L. Indeed the adsorption of humic acid, both on the edge of kaolinite (positively charged at pH 4) and on the basal face (by means of divalent cations Ca2+ ) (30), increases the global negative charge of clay, as confirmed by zeta potential measurements depicted in Fig. 1. The fraction of colloids deposited in the column, which can be considered relatively high (70% for 4 mg/L of humic acid), is probably promoted by specific interactions between Ca2+ and the matrix surface (9), which induce patches of positive charges. To illustrate the effect of humic acid concentration on colloid transport, we calculated the maximum travel distance, L T , after which 99.9% of the colloids are immobilized assuming that there is no colloid release. This distance can be expressed as   −vp C LT = ln , kd Co

[3]

with C/Co = 0.001. The values of L T as a function of concentration in humic acid are reported in Table 1. It can be seen that the presence of humic acid drastically increases the distance L T which is found to be four times higher in the presence of 4 mg/L humic acid in the suspension than without humic acid.

TABLE 1 Maximum Travel Distance (LT ) of Kaolinite Particles Versus Humic Acid in 5 × 10−5 mol/L CaCl2 Solutions and Humic-Coated Kaolinite Colloids at Different CaCl2 Concentrations [HA] (mg/L)

L T (cm)

0

1

2

18

23

31

3

4

6

8

38

55

75

75

[Ca2+ ] (mol/L)

L T (cm)

10−6

10−5

5 × 10−5

10−4

10−3

2 × 10−3

5 × 10−3

147

79

55

37

22

16

14

Note. Experimental details are given in Figs. 2 and 5.

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TRANSPORT OF KAOLINITE COLLOIDS THROUGH QUARTZ SAND

b

0,5

a

101

1,0

0,4

0,3

0,2

0,1

0,2

0,0

0,0 0

1

2

Kd (h-1)

C/Co (tracer)

0,6

C/Co (suspension)

0,4

- - - tracer 0.25 ml/min 0.75 ml/min 1 ml/min 1.5 ml/min

0,8

100

3

V/ Vp

101

102

pore water velocity (cm / h)

Recovered fraction

c 0,5 0,4 0,3 0,2 0,1 0,0

101

102

pore water velocity (cm / h) FIG. 3. Influence of flow rate on the mobility and deposition kinetics of humic-coated kaolinite colloids in columns packed with sandy soil material. (a) Typical breakthrough curves for KI tracer and colloids resulting from short-pulse inputs. (b) Deposition rate coefficients as a function of pore water velocity. (c) Fraction of colloids recovered in the column effluents. The sand column is preconditioned with 5 × 10−5 mol/L CaCl2 electrolyte solution at pH 4. The suspensions contained 30 mg/L kaolinite, 4 mg/L HA, and 5 × 10−5 mol/L Ca2+ at pH 4.

4.3. Effect of the Pore Water Velocity

4.4. Effect of Divalent Cations

In this series of experiments we have studied the effect of volumetric flow rate on the mobility of humic-coated kaolinite colloids (30 mg/L kaolinite and 4 mg/L humic acid) in the presence of CaCl2 (5 × 10−5 mol/L) through a column of pure quartz sand. The breakthrough curves resulting from short pulses of colloids for different flow rates are reported in Fig. 3a. The deposition rate coefficients and the fraction recovered as a function of pore water velocity, v (i.e., interstitial velocity of fluid), are depicted in Figs. 3b and 3c, respectively. As expected, colloid breakthrough occurred considerably earlier than breakthrough of the tracer. Colloid fraction detected in porous column effluents drastically decreases when the pore water velocity decreases (Fig. 3c). In log–log scale, the deposition rate coefficient increases linearly as a function of pore water velocity, the slope of the corresponding straight line is 0.20. This result means that the deposition rate coefficient depends on the pore water velocity to the power of 0.2 (i.e. K d ∝ v 0.2 ). This is consistent with the recent theoretical analysis of colloid deposition rates that predict slopes in the range of 0 to 1/3 (31).

4.4.1. Stability and properties of kaolinite surface in presence of humic acid and divalent cations. The effect of concentration of divalent metallic cations Me2+ (Ca2+ , Cu2+ , and Pb2+ ) on the stability of suspensions (30 mg/L kaolinite and 4 mg/L humic acid) after a resting time of 20 hours, can be seen in Fig. 4. The values of zeta potential of the suspension are reported in Table 2. We can see that the increase in divalent cation TABLE 2 Influence of Me2+ (Ca2+ , Cu2+ , and Pb2+ ) Concentrations on Zeta Potential (ζ) of Humic-Coated Kaolinite Colloids (30 mg/L Kaolinite and 4 mg/L HA at pH 4) Me2+ (mol/L)

Ca2+ Cu2+ Pb2+

0

10−6

5 × 10−5

−27.1 −26.9 −27.7

−26.7 −26.4 −25.9

−16.7 −16.4 −16.1

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AKBOUR ET AL.

0,08

0,04

0,00 -6

-5

-4

-3

Log[Me2 + ] FIG. 4. Influence of Ca2+ , Cu2+ , and Pb2+ concentrations on the stability of suspension evaluated by optical density measurements after 20 h. The suspensions containied 30 mg/L kaolinite and 4 mg/L humic acid at pH 4.

concentration induces the decrease of the absolute value of zeta potential. It can be explained by the combination of two phenomena: (i) The increase in ionic strength due to the addition of cation reduces the diffuse double layer thickness resulting in the decrease of the electric potential at the shear plane (29). (ii) the increase in divalent cation concentration increases the number of sites complexed due to the complexation with the functional groups of humic acid, therefore reducing the global negative charge of polyelectrolyte (32). For a given concentration, the three divalent cations have almost the same effect on the charge of colloid surface. The variations in the stability of colloidal suspensions, prepared under the same conditions as before (see previous sections), have been studied as a function of metallic cations Me2+ by measuring the optical density of the solution (Fig. 4). In this study, results show that the stability depends on both the concentration and the nature of the cation. For concentrations of Me2+ higher than 2 × 10−5 mol/L, the suspension aggregates and subsequently sedimentation occurs more or less rapidly depending on the nature of cations. The sedimentation is more rapid in the presence of Pb2+ than in the presence of Cu2+ or Ca2+ . Thus we assume that Pb2+ favors the formation of relatively dense and big aggregates that sediment easier than the ones formed in the presence of other metallic cations. 4.4.2. Effect of concentration of CaCl2 . We have chosen Ca2+ as the metallic cation because this element is present in the chemical composition of the main soils or groundwaters. Moreover Ca2+ cation can affect colloid transport through a natural porous media. The effect of Ca2+ concentration on breakthrough peaks of humic-coated kaolinite is presented in Fig. 5a. It can be seen that the mobility of colloids decreases considerably as the salt concentration increases. Furthermore it can be noticed that the injected colloidal suspension is completely deposited inside the porous column for a concentration of 10−2 mol/L of Ca2+ (results not presented here). The results obtained are qualitatively in good agreement with Derjaguin, Landau,

a

0,8

1,0

- - - tracer 10-6 M

0,8

0,6

Ca2 +

5.10-5 M Ca2 +

0,6

10-3 M 5.10

0,4

-3

Ca2 +

0,4

M Ca2 + 0,2

0,2

C/Co (Suspession)

Ca2 +

C/Co (tracer)

Optical density

Cu2 +

Verwey, and Overbeek theory, known as DLVO theory (33). Indeed, the increase in CaCl2 concentration compresses the diffuse double layer therefore reducing the repulsive forces between kaolinite and matrix surface. As a consequence, the energy barrier is reduced, which favors an increase of particle deposition. Similar results were previously observed by Kretzschmar and Sticher (9). The corresponding deposition rate coefficients, calculated from the integrated peak areas using Eq. [2], are reported in Fig. 5b. In this range of concentrations (10−6 –5 × 10−3 mol/L), the deposition rate coefficients vary with Ca2+ concentration. In log–log scale, the deposition rate of humic-coated kaolinite colloids increases as a power law with increasing Ca2+ concentration. Approximate values of the characteristic distance L T as a function of calcium concentration are reported in Table 1. Even if it is rather difficult to apply these values to real conditions (three-dimensional space), the effect of Ca2+ on the transport of colloids in porous media is clearly shown in this work.

0,0

0,0 0

1

V/ Vp

2

3

b 101

kd (h -1)

Pb

0,12

2+

100

10-6

10-5

10-4

[Ca

2+

10-3

] (mol/l)

FIG. 5. Influence of CaCl2 on the transport and deposition kinetics of humiccoated kaolinite colloids in columns packed with pure quartz sand. (a) Breakthrough curves for a conservative solute tracer (KI) and colloids as a function of CaCl2 concentrations. (b) Variation of the deposition rate coefficients with CaCl2 concentrations. The sand column is preconditioned with the same CaCl2 electrolyte concentrations at pH 4 as in the suspensions. The suspensions contained 30 mg/L kaolinite and 4 mg/L HA at differents CaCl2 concentrations. The constant flow velocity is 1 ml/min.

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TRANSPORT OF KAOLINITE COLLOIDS THROUGH QUARTZ SAND

4.5. Effect of Trace Metals on Colloid Mobility The effect on colloid transport is examplified by performing two comparative studies with the three divalent cations (Ca2+ , Cu2+ , and Pb2+ ) at two different concentrations (10−6 and 5 × 10−5 mol/L) in presence of 30 mg/L of kaolinite and 4 mg/L of humic acid. Before the injection of the suspension, the porous column is preliminarily saturated with a solution of CaCl2 at the same concentration as the one of Me2+ in the suspension. Breakthrough peaks resulting from short pulses are depicted in Figs. 6a and 6b. The fraction of colloid retained appears to be relatively low when the cation concentration equals 10−6 mol/L since only 35, 36, and 37% of colloids are retained in the presence of Pb2+ , Cu2+ , and Ca2+ , respectively. For these low concentrations, the fraction of colloid deposited can be considered as independent of the nature of the cation. For a higher divalent cation concentration (5 × 10−5 mol/L), the fraction of colloid retained is much higher, 93, 82, and 70% of collo¨ıds de-

a

1,0

- - - tracer Pb2 +

0,8

0,8

Cu2 +

0,6

Ca2 +

0,6

0,4

0,4

C/Co (suspension)

C/Co (tracer)

1,0

0,2

0,2

0,0

0,0 0

1

2

3

V/ Vp

b 0,3

0,8

- - - tracer Ca2 +

0,6

Cu2 +

0,2

Pb2 +

0,4

0,1

C/Co (suspension)

C/Co (tracer)

1,0

0,2 0,0

0,0 0

1

V/Vp

2

3

FIG. 6. Breakthrough curves demonstrating the nature and concentration effects of divalent cations Me2+ (Ca2+ , Cu2+ , and Pb2+ ) on the deposition of humic-coated kaolinite colloids in columns packed with pure quartz sand. (a) Breakthrough curves for a conservative solute tracer (KI) and the suspensions containing 30 mg/L kaolinite, 4 mg/L HA, and 10−6 mol/L Me2+ at pH 4. (b) Same conditions as above with 5 × 10−5 mol/L as Me2+ concentration. The sand column is preconditioned with (a)10−6 and (b) 5 × 10−5 mol/L CaCl2 electrolyte concentrations at pH 4. The constant flow velocity is 1 ml/min.

TABLE 3 Influence of Me2+ (Ca2+ , Cu2+ , and Pb2+ ) Concentrations on Deposition Rate Coefficients (kd ) and Maximum Travel Distance (LT ) of Humic-Coated Kaolinite Colloids (30 mg/L Kaolinite and 4 mg/L HA at pH 4; Flow Velocity, 1 ml/min) [Me2+ ] = 10−6 mol/L

Ca2+ Cu2+ Pb2+

[Me2+ ] = 5 × 10−5 mol/L

kd (h−1 )

L T (cm)

kd (h−1 )

L T (cm)

1.4 1.35 1.3

147 152 158

3.75 5.21 8.1

55 40 26

posited in the presence of Pb2+ , Cu2+ , and Ca2+ , respectively. Moreover, this phenomenon depends significantly on the nature of the cation in the solution. The different behaviors observed as a function of Me2+ concentration can be explained by the amount of Me2+ binding by humic-coated kaolinite and by the affinity of each cation toward this colloid. Indeed at low Me2+ concentration (10−6 mol/L), humic-coated kaolinite colloids were not very sensitive to the presence of divalent cations, the zeta potential measurements reported in Table 3 show a weak variation when 10−6 mol/L divalent cations was added to the injected suspension. On the contrary, at high Me2+ concentration (5 × 10−5 mol/L), the zeta potential variation is higher. In this latter case, a lot of ions form complexes with the functional groups of humiccoated kaolinite in the injected suspension and then induce a charge compensation. The deposition rate of humic-coated kaolinite colloids in the sand columns vary as a function of Me2+ concentration added to the suspension. However, the variation is relatively weak and practically identical when the divalent ion concentration is 10−6 mol/L. But the variation is more significant and depends on the nature of the ion when the cation concentration in the suspension is of 5 × 10−5 mol/L. In this latter case, the dependence between the deposition rate and the nature of divalent ion may be probably due to the difference of the affinity of cations for humic-coated kaolinite colloids. Indeed, a set of batch studies was conducted to evaluate the affinity of Ca2+ , Cu2+ , and Pb2+ to humic acid (34). The results of these investigations show that the cation Pb2+ is clearly more strongly bonded to humic acid than Cu2+ which is also more strongly bonded than Ca2+ . This result is qualitatively in agreement with the above results and provides an explanation for the observed behavior. Even if the cation concentration is higher than 2 × 10−5 mol/L (concentration at which the aggregation begins, see Fig. 4), the effect of the aggregation on the transport and deposition of humic-coated kaolinite in the presence of divalent cations is negligible, otherwise one will observe an inversion in the deposition rate consistent with the filtration theory (8). All the deposition rate coefficients and characteristic distances L T obtained are reported in Table 3. It can be noticed that L T varies drastically (six times longer for Pb2+ for example) as the concentration in the suspension changes from 10−6 to 5 × 10−5 mol/L.

8

AKBOUR ET AL.

5. CONCLUSION

The essentially experimental work presented in the paper allows us to show the major effect of organic matter on mineral colloid transport through porous media. In this study, we have shown that the adsorption of humic acid on the kaolinite surface leads to the increase of its mobility through the porous medium. The colloidal particles were excluded from about 10% of the total pore volume. The fraction of colloids deposited decreases with increasing flow velocity. In the presence of divalent cations, the deposition rate of humic-coated kaolinite colloids increases with increasing Me2+ concentration. In addition, the colloid deposition rate depends on the nature of the divalent ion. This dependence seems to be due to the difference in the affinity of the divalent ions toward humic acid. The problem of transport of contaminants through soils and groundwater appears to be very complex when organic matter is adsorbed on colloid particles. The results of the present paper help us to better understand these complex phenomena.

9. 10. 11. 12.

13. 14. 15. 16.

17.

18. 19.

20.

ACKNOWLEDGMENTS

21.

We gratefully acknowledge E. Pefferkorn for chemical products, P. Behra and C. Zarcone for their help, and P. Bacchin from the Laboratoire de G´enie Chimique de Toulouse for potential zeta measurements. This work is supported by the Programme Th´ematique d’Appui a` la Recherche Scientifique (PROTARS No. P2T3/04).

22. 23. 24. 25.

REFERENCES

26.

1. Saiers, J. E., and Hornberger, G. M., Water Resour. Res. 32, 33 (1996). 2. Grolimund, D., Borkovec, M., Barmettler, K., and Sticher, H., Environ. Sci. Technol. 30, 3118 (1996). 3. Mccarthy, J. F., and Degueldre, C., in “Environmental Particles” (J. Buffle and H. P. Van Leeuwen, Eds.), p. 247, Lewis, Boca Raton, FL, 1993. 4. Roy, S. B., and Dzomback, D. A., Colloids. Surf. A 107, 245 (1996). 5. Ochs, M., Cosovic, B., and Stumm, W., Geochim. Cosmochim. Acta 58, 639 (1994). 6. Stevenson, F. J., “Humus Chemistry Genesis. Composition, Reactions,” Wiley–Interscience, New York, 1982. 7. O’Melia, C. R., Colloids Surf. 39, 255 (1989). 8. O’Melia, C. R., in “Aquatic Chemical Kinetics: Reactions Rates of Processes in Natural Waters,” Werner Stumm, New York, 1990.

27. 28. 29.

30. 31. 32. 33. 34.

Kretzschmar, R., and Sticher, H., Environ. Sci. Technol 31, 3497 (1997). Tipping, E., and Higgins, D. C., Colloids Surf. 5, 85 (1982). Tarchitzky, J., Chen, Y., and Banin, A., Soil Sci. Am. J. 57, 367 (1993). Nabzar, L., “Stabilit´e des collo¨ıdes min´eraux en pr´esence de polym`eres hy´ drosolubles: Etude du syst`eme polyacrylamide–kaolin,” th`ese de Doctorat de l’Universit´e Louis, Pasteur, Strasbourg, 1985. Johnson, P. R., Sun, N., and Elimelech, M., Environ. Sci. Technol. 30, 3284 (1996). Mc Dowell-Boyer, L. M., Environ. Sci. Technol. 26, 586 (1992). Schweich, D., and Sardin, M., J. Hydrol. 50, 1 (1981). Ait Akbour, R., “Stabilit´e et propri´et´es acido-basiques de la kaolinite et d’une suspension d’argile extraite d’un sable naturel,” M´emoire de D.E.A de l’Universit´e Ibn Zohr, Agadir, 1997. Elfarissi F., “Adsorption d’acides humiques sur l’alumine et la kaolinite en pr´esence d’ions aluminium. Stabilit´e collo¨ıdale des syst`emes,” th`ese de Doctorat de l’Universit´e Louis Pasteur, Strasbourg, 2000. Elfarissi, F., Nabzar, L., Ringenbach, E., and Pefferkorn, E., Colloids Surf. A 131, 281 (1997). Lecarme-Th´eobald, E., “Comportement du tributyl´etain en milieu aqueux en pr´esence d’une phase solide h´et´erog`ene,” th`ese de Doctorat de l’Universit´e Henri Poincar´e de Nancy, 1998. Bueno, B., “Etude dynamique des processus desorption-d´esorption du tributy´etain sur un milieu poreux d’origine naturelle,” th`ese de doctorat de l’Universitaire de Pau et des Pays de L’Adour, 1999. Coston, J. A., Fuller, C. C., and Davis, J. A., Geochim. Cosmochim. Acta 59(17), 3535 (1995). Bargar, J. R., Brown, G. E., and Parks, G. A., Geochim. Cosmochim. Acta 62(2), 193 (1998). Parfitt, R. L, Fraser, A. R., and Farmer, V. C., J. Soil Sci. 28, 289 (1977). Vermeer, A. W. P., and Koopal, L. K., Laugmuir 14, 2810 (1998). Amirbahman, A., and Olson, T. M., Environ. Sci. Technol. 27, 2807 (1993). Kretzschmar, R., Holthoff, H., and Sticher, H., J. Colloid Interface Sci. 202, 95 (1998). Masset, S., Monteil-Rivera, F., Dupont, L., Dumonceau, J., and Aplincourt, M., Agronomie 20, 525 (2000). Wang, T. K., and Auderbert, R., J. Colloid Interface Sci. 119, 459 (1987). Grolimund, D., Elimelech, M., Borkovec, M., Barmettler, K., Kretzschmar, R., and Sticher, H., Environ. Sci. Technol. 32, 3562 (1998). Elfarissi, F., and Pefferkorn, E., J. Colloid Interface Sci. 221, 64 (2000). Song, L., and Elimelech, M., J. Chem. Soc. Faraday Trans. 89, 3443 (1993). Amirbahman, A., and Olson, T. M., Colloids Surf. A 99, 1 (1995). Israelachvili, J., “Intermolecular and Surface Forces,” 2nd ed. Academic Press, London, 1992. Ait Akbour, R., Douch, J., Hamdani, M., and Schmitz, P., submitted to Compte Rendu Acad´emie des Sciences S`erie IIc.