Behavior of kaolinite and illite-based clays as landfill barriers

Applied Clay Science 42 (2009) 497–509

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y

Behavior of kaolinite and illite-based clays as landfill barriers Jaime Cuevas a,⁎, Santiago Leguey a, Antonio Garralón b, Manuel Rodríguez Rastrero a, Jesús R. Procopio a, M. Teresa Sevilla a, Nicanor Sánchez Jiménez a, Rafaél Rodríguez Abad c, Adrián Garrido d a

Universidad Autónoma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain Centro de Investigaciones Medioambientales y Tecnológicas (CIEMAT), Spain Centro de estudios y experimentación de obras públicas, Laboratorio de Geotecnia, Alfonso XI, 3-5, 28014 Madrid, Spain d Geotecnia y Cimientos, S.A. Los Llanos de Jeréz 10-12, Coslada, Madrid, 28820 Madrid, Spain b c

A R T I C L E

I N F O

Article history: Received 21 January 2008 Received in revised form 13 June 2008 Accepted 22 June 2008 Available online 8 July 2008 Keywords: Ceramic clays Illite Kaolinite Carbonates Anion diffusion Heavy metal ions

A B S T R A C T The technical requirements for the landfill of municipal wastes in the European Union (EU) are given in the Council Directive 1999/31/EC. A geological barrier of at least 1 m thickness with a hydraulic conductivity (HC) of 1 · 10− 9 m/s is required. Where the geological barrier does not naturally meet the above conditions, a geological barrier of at least 0.5 m thick must be artificially established. We studied at controlled conditions, the ability of some clays (kaolinite or illite based) to act as landfill barriers. Several Spanish ceramic clays were compacted in columns (0.5 m length) and characterized for mineralogical, physical–chemical and heavy metal ions adsorption properties after 10 months leaching experiments. Zn, Cd, Pb and Cr salts were dosed in the synthetic leachates in order to test their in-depth retention. The specific surface area decreased in the material located near the clay–leachate interface region (b 6 cm) due to biofilm formation around clay particles, but bulk mineralogical properties were not affected. Although all the clays fulfilled the HC requirements, the diffusion of anions (chloride) reached more than 30 cm in kaolinite–illite or pure illite clays. The presence of significant traces of smectite (b 5%) was critical in anion transport retardation. Heavy metal ions were generally retained in the first 10 cm but in high surface-area illitic clays, the anions and heavy metal ions transport was stopped at b 6 cm. The clay mineralogy and the presence of carbonates and soluble salts greatly influenced the behavior of the barrier materials. Carbonate dissolution and precipitation affected b 6 cm depth. Divalent cations of carbonates selectively occupied the exchangeable positions and inhibited the retention of Na+, NH+4 and K+, in the leachates. Sulphate was reduced at a depth of 20–30 cm. This biogeochemical process contributed to cadmium retention, presumably precipitated as sulphide. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Municipal solid waste (MSW) disposal may cause both mechanical and geochemical perturbation in the underlying geological substrata. The technical requirements for the landfill of municipal wastes in the European Union (EU) are given in the Council Directive 1999/31/EC and in council decision (2003/33/CE). In this legislation it is established that a geological barrier of at least 1 m thick with a hydraulic conductivity (HC) of 1 · 10− 9 m/s is required. Where the geological barrier does not naturally meet the above conditions, a geological barrier of at least 0.5 m thick must be artificially established. Then, a geological barrier with the ability to confine waste and buffer the hazardous leachates is a key factor in the multibarrier criteria, considered to fulfill protection and security

⁎ Corresponding author. Dpto. Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. Tel.: +34 1 497 3047; fax: +34 1 497 4900. E-mail address: [email protected] (J. Cuevas). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.06.017

objectives in final waste disposal (Savage, 1995; Bilitewski et al., 1997; Astudillo, 2001). Clay minerals, due to their small particle size and complex porous structure, constitute the natural materials with lowest permeability. Their high specific surface area allows strong physical and chemical interactions with fluids and dissolved species which are subjected to electrostatic repulsion, sorption or specific cation exchange reactions. These interactions are responsible for the retention of leachate components (Davis and Kent, 1990; Sposito, 1990 Stumm, 1992, Rowe et al., 1995, Sawney, 1996; Michael et al., 2002). In fact, clays are sealing fluid transport and retarding the migration of solutes. These hydro-geochemical functions may change due to rock–leachate interactions, because different clay minerals have different geochemical behavior and may undergo different physical and chemical responses to factors as pH, salinity, blocking cations (ammonium or potassium), or organic phases and components. Thus, it is necessary to study the behavior of clays in contact with MSW leachates, for relevant time periods, in order to establish the basis for assessing the suitability of materials and the predictability of their longevity in their geological barrier function.

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Table 1 Averaged chemical composition of landfill leachates collected in MawGreen (England, UK; Owen and Manning, 1997); El Garráf (Barcelona, Spain; GEOCISA, 2005); and Tremonti (Ímola, Italy; Frascari et al., 2004) Leachate

pH

NH+4

Na+

7125 10,500 5050

1127 4575 1330

1406 4500 nd

581 2300 nd

374 111 nd

420 5 nd

Cl−

SO2− 4

Acetatea

Propionate

Butirate

Isobutirate

190 76 225

754 nd nd

390 nd nd

322 nd nd

39 nd nd

COD

K+

Mg2+

Ca2+

7.0 8.6 8.4

t (Y) MawGreen Garráf Tremonti

1.5 N 10 N 10

4001 8966 3130

Mineralogical composition Clay minerals F

K

I

S

72 69 74

11 b1 62

60 69 12

1 – b1

Quartz

f-K

f-Na

Dolomite

Calcite

Others

14 17 20

4 4 1

10 b1 1

b1 7 2

b1 b1 1

Y (b1) H (2) Sd (1)

b 1: b 1 wt.%,; F: total amount of clay minerals; K: kaolinite; I: Illite; S: Smectite; f-K: Kfeldspar (microcline); f-Na: albite; Y: gypsum; H: hematite (Fe2O3); Sd (siderite).

The practise of sanitary landfilling of MSW was commonly adopted from the 1970's at the same time as the first national waste management laws came out (Bilitewski et al., 1997). However, even in the 1990's between 20 and 30% of MSW landfilled in Spain were uncontrolled. Now, less than 4% is estimated to be out of the EU multibarrier concept standards, (MMA, 2005). Today, waste disposal facilities are expensive and have complex installations that need a careful planning for the extraction of the mobile phases (fluid and gas) produced by the fermentation of biowastes (Smith et al., 2001). This facilitates the long-term stability of waste and the future possibility for land reclamation (Rowe et al., 1995). Even though landfills are safe emplacements, the scenario of leachate infiltration has to be evaluated. In doing this we should consider the main stages in degradation of domestic waste (Williams, 1998):

Table 3 Physical–chemical properties: surface area, exchangeable cations and hydraulic conductivity SBET

Mg+

Ca+

Cl+

SO2− 4

mmol/L

O.C.a

CE

C%

µS/cm

mg/L

Table 2 Semi-quantitative mineralogical composition of clay raw materials by means of X-ray diffraction analysis

Pantoja Carboneros Ariño

K+

CE: electrical conductivity. a O.C.: Organic carbon was determined in a TOC 5000.

COD: Chemical oxygen demand. nd: not determined. t (Y): time of disposal of waste cells. a There were small quantities of other organic acids.

Mass %

Na+

Pantoja 16.1 ± 1.1 0.3 ± 0.0 5.8 ± 0.7 1.3 ± 0.1 1.8 ± 0.1 10.5 ± 1.3 0.06 2550 ± 61 Carboneros 1.1 ± 0.1 0.5 ± 0.1 0.5 ± 0.0 0.3 ± 0.0 0.2 ± 0.0 b0.01 280 ± 3 Ariño 0.7 ± 0.1 0.4 ± 0.1 2.4 ± 0.2 6.1 ± 0.4 0.2 ± 0.1 10.1 ± 0.6 1.18 1381 ± 71

mg/L MawGreen Garráf Tremonti

Table 4 Ions in aqueous extract (1:5 clay/distilled water); organic carbon content of the clay

CEC

Na+

K+

Mg2+

Ca2+

m2/g cmol(+)/kg

Pantoja 42 ± 1 15.7 ± 0.0 Carboneros 39 ± 1 12.7 ± 2.1 Ariño 20 ± 1 12.3 ± 0.9

k (m/s) 2 g/cm3 dry density

1.91 ± 0.05 0.70 ± 0.03 15.4 ± 0.3 0.18 ± 0.01 0.97 ± 0.07 9.3 ± 0.3 0.01 ± 0.00 0.33 ± 0.02 2.5 ± 0.1

8.8 ± 0.0 16.3 ± 1.1 8.8 ± 0.7

1.7 · 10− 10 8.2 · 10− 10 4.0 · 10− 10

SBET: external specific surface area by N2 (g) adsorption. (CEC): cation exchange capacity by sodium saturation at pH = 8, ethanol washing and Mg2+ displacing at pH 5. Exchangeable cations distribution by extraction with NH4–OAc at pH 7. k: hydraulic conductivity measured in a triaxial cell.

Aerobic stage: The available O2 within the waste is consumed during the first month of disposal. This is an exothermic stage in which the temperature may rise to 70 °C. Anaerobic acetogenic stage (1–2 years): This stage is characterized by carbohydrates hydrolysis aided by bacteria. The by-products are mainly simple sugars, from which fermentation produces light organic acids, CO2 and H2. Anaerobic methanogenic stage: Methanogenic bacteria predominate and the main by-products are CH4 and CO2. The last stages can be active for several years and are responsible in the generation of leachates. The organic acids are oxidized by sulphate-reducing bacteria until the sulphate available is consumed. The degradation process is continued by methanogenic bacteria, (Lovely and Klug, 1986). Degradation reactions of organic material and redox reactions(in order of increasing redox potential: sulphate, Fe3+, nitrate and O2) regulate the spatial distribution of the contamination plume by establishing a redox zoning in non-controlled landfills (Brun and Engeesgaard, 2002). Due to these processes, a leachate composition changes with time. Robinson (1990) demonstrated that the behavior of dissolved Na+ and Cl− is conservative with time (1–3 years) and that the mature leachates produced during the methanogenic stage are composed mainly of inorganic salts. As a general rule, acetogenic leachates are rich in both inorganic salts and acetic or propionic acid, at slightly acidic to near neutral pH (Owen and Manning, 1997). Methanogenic leachates have less dissolved species and reach higher pHs. In spite of this, two significant examples of long-lived (N10 years) landfills showed mixed compositions with high pHs and high contents of dissolved organic species (Table 1). According to Rowe et al. (1995) and Pivato and Raga (2006), general characteristics of landfill leachates are pHs from 7 (young cells) to 9 (long-lived cells); high concentrations of inorganic solutes (ammonium and sodium chlorides) and variable concentrations of organic solutes (mainly acetate).

Table 5 Synthetic leachates and collected landfill leachate (“El Garráf”, Barcelona, Spain)

pH (mg/L) TOC (CH3–COOH) NH+4 K+ Na+ Mg+2 Ca+2 Cl− SO2− 4 Cr Pb Zn Cd

Acetogenic acidic (L2)

Methanogenic basic (L3)

Real “El Garraf”(L4)

5.0

8.0

8.6

3508 8772 819 538 973 307 717 1791 209 10 10 10 10

81 204 30 225 150 125 250 1154 35 0.005 0.023 0.10 0.10

2272

TOC: total organic carbon.

4575 2300 4500 111 5 8966 76 –– –– –– ––

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Fig. 1. Sampling location and biofilm at the top of the column of Carboneros clay with the “El Garráf” leachate.

The objective of this study is to consider the direct interaction between leachates and different clay-type based barriers to identify geochemical perturbations by: (1) mineralogical alteration, (2) changing of clay surface properties, (3) transport properties of solutes and (4) retention of heavy metals. 2. Experimental 2.1. Materials Three clays were selected from significant ceramic raw materials quarried in Spain. The selection criteria was (1): to have a minimum clay content of 60% of non-expandable clays (b5% smectite), to reach mechanical stability and to assure k b1 · 10− 9 m/s; and (2): to have high potential resources and a broad distribution across the Spanish territory (Tables 2 and 3). The Ariño (Teruel) quarry (Cretacic age; González López et al., 2005) provides high grade kaolinitic clay. Miocene clays from Pantoja (Toledo) quarry (García Calleja, 1991) are illitic (di-octahedral), with significant contents of kaolinite and smectite (di- and tri-octahedral). Illite–smectite mixed-layer minerals layers are not significant in this clay, as far as either anomalous shifts in c-spacings or changing effects on reflection profiles under glycerol solvation are not observed (Fig. 6). Finally, clays from the Carboneros (Jaén) quarry (Vazquez and Jimenez-Millan, 2004) are Triasic and are only illite. These materials also contain a wide spectrum of accessory mineralogy with the presence or absence of carbonates. In the Ariño or Pantoja clays, although gypsum is not detected, a significant quantity of sulphate can be dissolved in an aqueous extract (Table 4). The organic matter is low, and is virtually absent from Pantoja or Carboneros (Table 4). All the materials were quarried in situ (500 kg) and stored. 100 kg were dried in a rotary kiln at 60 °C, then ground to b5 mm size, homogenized and packaged in 20 kg polyethylene bags, which were distributed for the different testing labs. All materials fulfilled the technical requirements of the Landfill EU directive (10− 9 m/s). 2.2. Experimental design Compacted clay columns of 0.5 m length were packed at Proctor's water content/density optimum (virtually water saturated) in 0.25 m

diameter methacrylate 1 m length cells. Four cells per clay were prepared, adding different leachates: distilled water (L1), two synthetic solutions (L2,L3) and one landfill leachate (L4), (Table 5). The composition of the synthetic solutions was formulated according to Tchobanoglous et al. (1994) young and long-term typical landfill leachates. Zn, Cd, Pb and Cr were dosed in the synthetic leachates to test their in-depth retention. The system was left to interact for ten months. The contact of the clay with the liquid displayed an altered zone of 2–5 cm that had expanded in relation to the bulk column. Samples were taken at 2, 4, and 6 cm depth measured from the liquid contact (Fig. 1). Another sample was taken from a section at 45–40 cm measured from the cell bottom with an average depth of 10 cm from the liquid contact. These samples were analyzed for mineralogy, surface properties and heavy metal retention. In addition, the entire column was tested, every 5 cm, for chlorides and sulphates to study ionic transport through the clay. The study of organics–clay interaction will be studied in future research. 2.3. Analytical methods Mineralogy was studied by X-ray diffraction (XRD) after dismantling of the columns. Bulk sample was analyzed by the random powder method and the oriented slides method was applied for the b2 μm size fraction, which was saturated in Mg2+ solution (Moore and Reynolds, 1989). To semi-quantify the minerals in the bulk sample the procedure proposed by Schultz (1964) was used, (UNE 22-161-92 standard method for mineralogical quantification of clay samples containing sepiolite). To compare and to check differences in the very similar diffraction patterns obtained (Figs. 2, 3 and 4), 10% of ZnO was mixed as an internal standard with the bulk sample (Srodon et al., 2001). The patterns were normalized with respect to the area of the ZnO reflection at 2.475 Å in the original clay powder pattern. Although a 10% relative error is assumed for this semi-quantification method, the relative differences between samples could be appreciated when pattern profiles were represented together. The XRD estimation for the content of clay minerals in the clay fraction (mainly: illite (10 Å), kaolinite or chlorite (7.1 Å) and smectite (18 Å under glycerol solvation)) was performed as proposed by Barahona (1974).

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Fig. 2. Random powder XRD patterns of the Pantoja clay after 10 months interaction. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. M0: Original Clay. 2, 4, 6, 10 cm: interface samples in Fig. 1. F: clay minerals. Q: quartz, f-Na,K: albite and microcline. ZnO: reflections of ZnO internal standard. (060) reflections of the clay minerals represented at the right hand side.

The specific surface area was determined by BET analysis (multipoint N2 adsorption, micromeritcs® GEMINI V after degassing under N2 flow during 18 h at 90 °C (UNE 22-164/94). For surface chemistry analysis, the exchangeable cations were extracted from the clay at room temperature as described by Thomas (1982). Na+ and K+ were analyzed using a Buck Scientific™ PFP-7 flame photometer; Ca2+ and Mg2+, by ICP-MS (ELAN 6000, PERKIN ELMER™); and NH+4 by ion selective potentiometry (ORION™ 9512 Ammonia Gas Sensing Electrode). The cation exchange capacity (CEC) of the original clays was determined at room temperature by Na+ homoionization (Na–

COO–CH3 1 M, pH = 8) and Mg2+ displacement (MgNO3·5H2O 0.5 M at pH = 5), Rhoades (1982). Chlorides and sulphates were measured by ion chromatography (Metrohm™ 761 Compact IC) in aqueous extracts (1:10 solid:liquid ratio). The suspension was soaked for 2 h, and then was left to settle for 24 h. The supernatant was extracted and filtered. To determine the heavy metal content in the uppermost zone of the clay column, the clays once dried, were digested in aqua-regia inside a microwave oven device (12 min at 1000 W in PFA vials). The original clay was treated in the same way. Suspensions were filtered and analyzed by Atomic Absorption

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Fig. 3. Random powder XRD patterns of the Carboneros clay after 10 months interaction. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. M0: Original Clay 2, 4, 6, 10 cm: interface samples in Fig. 1. F: clay minerals. Q: Quartz, f-Na,K: albite and microcline. ZnO: reflections of ZnO internal standard. (060) reflections of the clay minerals represented at the right hand side.

Spectrometry AAS (Unicam™ Solaar M). The same analysis was also performed in the leachate collected before sampling the clay column. 3. Results 3.1. Characterization of the leachates after ten months The most obvious change in leachates is the pH (Table 6). The Ariño kaolinitic clay lowered solution pH to around 5 in the case of both basic (L2) and acidic (L3) synthetic leachates. Using distilled water, the

value dropped below 4. Thus, the kaolinite surface may be protonated (Huertas et al., 1998; Cama et al., 2002), or a different mineral may be formed (Linares and Huertas, 1971, Drever, 1988). In the Pantoja illitic clay, in contrast, pH became basic, even the pH = 5 solution (L3) rose to 8.0. This effect was also expected for Carboneros clay in view of the presence of dolomite, but this clay dispersion remained acidic. In the case of the landfill leachate (L4), all the clay columns reached pH 9, close to the initial value 8.6. The electrical conductivity is virtually constant at each leachate test. Only in the case of L3 solution in contact with Pantoja illitic clay it

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Fig. 4. Random powder XRD patterns of the Ariño clay after 10 months interaction. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. M0: Original Clay. 2, 4, 6, 10 cm: interface samples in Fig. 1. F: clay minerals. Q: Quartz, f-Na,K: albite and microcline. ZnO: peaks of ZnO internal standard. (060) reflections of the clay minerals represented at the right hand side.

decreased by 22% accompanied by distinct physical–chemical changes. The degree of oxygen saturation allows us to determine the existence of biological activity as far as high nutrient concentration is available in L4 leachates. In the presence of distilled water or synthetic solutions, oxygen was maintained at least at 50% in relation to the initial saturation stage, but the oxygen becomes virtually depleted in the real leachate. Again, Pantoja clay, in the case of L3 acidic solution presented a specific behavior and was de-oxygenated like L4.

In relation to the metal ions, the basic pH of solutions led to precipitation of all heavy metal ions. In L3 remaining acidic also in the case of Carboneros and Ariño clay, metal ions remained in solution. At lower pH the Ariño clay showed a lower retention capacity. 3.2. Mineralogy XRD analysis (Table 7) showed insignificant changes in clay content and detrital quartz compared to the original materials (Fig. 2, 3 and 4). The differences in albite (f-Na) and microcline (f-K) contents are not

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Table 6 Physical–chemical, “in situ” parameters of leachates Clay

Sample

pHa

E.C.a

O2 dis.a

mS/cm Pantoja

L1 L2 L3 L4

7.8 7.6 8.0 9.0

1.40 5.21 11.96 37.65

82.5 69.3 5.6 6.7

Carboneros

L1 L2 L3 L4

8.1 7.4 5.9 8.9

0.21 4.30 15.65 38.16

81.0 51.6 46.9 5.0

Ariño

L1 L2 L3 L4

3.7 5.2 4.9 8.9

1.08 5.05 14.43 39.27

81.3 64.3 73.7 4.9

[Zn]

[Pb]

[Cd]

%

mg/L

mg/L

mg/L

[Cr] mg/L

L3(0)

9.13 n.d. n.d. n.d. 0.07

9.39 n.d. n.d. 0.14 n.d.

9.16 n.d. n.d. n.d. n.d.

10.77 n.d. n.d. n.d. 0.50

L2(0)

0.10 n.d. n.d. 5.58 0.08

0.023 n.d. n.d. 3.28 n.d.

0.005 n.d. n.d. 8.80 n.d.

0.10 n.d. n.d. 11.71 0.52

L4(0)

0.32 4.3 2.3 10.86 0.10

0.21 n.d. n.d. 3.25 n.d.

n.d. n.d. n.d. 9.75 n.d.

0.48 n.d. n.d. 14.28 0.50

Dissolved heavy metal ions in contact with clay. E.C.: electrical conductivity, O2: % saturation level. L1: distilled water, L2: synthetic basic (pH = 8), L3: synthetic acid (pH = 5), L4: El Garráf Leachate. (0): initial content of metal. a Determined in situ with a multi-parametric sensor at 20 ± 2 °C.

significant due to their low content and the difficulties in grinding these hard materials mixed in a soft clay matrix. The compacted clay materials showed high chemical stability. The (060) XRD reflections (Figs. 2, 3 and 4), are sensitive to subtle chemical changes in the octahedral sheet. Minor changes are detected. The

Table 7 Mineralogical semi-quantification of clay materials (mass %)

Pantoja clay L1 — 2 cm L2 — 2 cm L3 — 2 cm L4 — 2 cm L3 — 4 cm L3 — 6 cm L3 — 10 cm L4 — 4 cm L4 — 6 cm L4 — 10 cm Carboneros clay L1 — 2 cm L2 — 2 cm L3 — 2 cm L4 — 2 cm L3 — 4 cm L3 — 6 cm L3 — 10 cm L4 — 4 cm L4 — 6 cm L4 — 10 cm Ariño clay L1 — 2 cm L2 — 2 cm L3 — 2 cm L4 — 2 cm L3 — 4 cm L3 — 6 cm L3 — 10 cm L4 — 4 cm L4 — 6 cm L4 — 10 cm

F

Q

f-Na

f-K

Ca

Do

Sd

H

Y

75 79 81 78 81 80 79 75 77 75 77 71 73 70 75 73 73 69 71 75 73 71 75 79 81 77 79 77 79 78 80 80 80

17 17 13 15 15 16 16 18 17 19 18 18 19 23 20 19 19 22 22 18 19 20 22 18 17 20 18 20 18 19 18 18 17

5 2 4 5 2 3 2 5 3 4 2 b1 b1 b1 b1 b1 1 1 b1 b1 b1 1 1 1 1 1 1 1 1 1 1 1 1

2 2 2 2 2 1 2 1 2 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 b1 1 1 b1 1

b1 nd nd nd b1 b1 b1 b1 b1 b1 b1 nd b1 1 b1 b1 1 2 b1 b1 1 1 b1 b1 b1 nd b1 b1 nd b1 b1 b1 b1

1 nd nd nd b1 b1 b1 b1 1 b1 1 8 4 4 2 4 5 5 4 4 5 5 b1 nd nd nd nd nd nd nd nd nd nd

nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 0 0 0 1 b1 b1 b1 b1 b1 b1 b1 b1 1 b1

nd nd nd nd nd nd nd nd nd nd nd 2 1 1 1 2 1 2 2 1 2 2 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1

nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd b1 1 nd b1 b1

F: total amount of sheet-silicates, Q: quartz, f-Na: albite, f-K: microcline, Ca: calcite, Do: dolomite, Sd: siderite, H: hematite (Fe2O3); Y: Gypsum; 2, 4, 6, 10 cm: interface samples in Fig. 1. L1: distilled water, L2: synthetic basic (pH = 8), L3: synthetic acid (pH = 5), L4: El Garráf leachate.

Fig. 5. Details of mineralogical changes determined by XRD random powder patterns. a) Carbonates dynamics in Carboneros clay. b) Gypsum traces in Ariño clay. 2, 4, 6, 10 cm: see Fig. 1

intensity of the 1.53 A reflection of tri-octahedral sheets of Pantoja clay (chlorite, smectite) was enhanced for the interface samples with acidic or real leachate interaction. Carboneros clay is virtually unaffected and its two di-octahedral reflections (1.50, 1.49 Å) indicate the presence of two illite type phases (muscovitic and paragonitic). Finally, in the Ariño clay, the di-octahedral 1.50 Å illite reflection is enhanced in relation to the 1.49 Å kaolinite reflection, so some alteration in the kaolinite component must be considered. Two significant features were observed: (1) The dissolution of dolomite at the liquid interface (0–2 cm) in Carboneros clay was detected under L3 fluid interaction accompanied with the precipitation of calcite at 4–6 cm (Fig. 5a). (2) Gypsum was present at 6–10 cm in Ariño clay treated with L3 (Fig. 5b). This mineral was absent in the original clay but high amounts of both sulphate and calcium were dissolved in the water extract analysis (Table 4). These changes implied the existence of mass transfer processes that produced specific dissolution and precipitation reactions. Regarding the study of the b2 μm size fraction (Figs. 6, 7 and 8), the observed changes were also of minor importance. Both, Pantoja and Ariño clays contain highly disordered (very broad peaks) expandable minerals (smectites) that change their basal spacing reflection from 14 to 16–18 Å upon glycerol solvation. Pantoja and Carboneros also had a small amount of non-expandable chlorite. Smectite maintained a regular expansion during the treatments, from 14 to 18 Å, in Pantoja clay and the smectite component was stable. However, in Ariño clay, the reflection shift from 14 to 15.9 Å was lost in the clays interacting with several leachates, so the small amount of smectite was altered. 3.3. Physical–chemical properties 3.3.1. Specific surface area In contrast to the subtle mineralogical changes, the specific surface area decreased in the interface zone (near 50 cm from the cell base), mainly for the landfill leachate experiments (Fig. 9). As far as no mineralogical changes were detected, the effect has to be attributed to the organic coatings that glued clay particles in this biofilm affected zone. The acidic leachate produced a minimum of surface area in the Carboneros clay, near the calcite precipitation zone, and a maximum

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Fig. 6. Oriented slides XRD patterns of b2 μm fraction (air dried and glycerol solvated) of Pantoja clay. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. At 2 cm see Fig. 1. Peak numbers in angstroms (Å). S: smectite, K: kaolinite, I: illite, Cl: chlorite.

near the surface, where dolomite is dissolved. All the cases showed a maximum of specific surface area with distilled water at the interface due to the higher degree of dispersion of individual clay particles. This effect is also observed, but with lower intensity, in the basic leachate. Finally, the Pantoja clay achieved a high specific surface area with the acidic leachate, but no mineral reaction was observed. This property was demonstrated to be sensitive to precipitation or dissolution of cementing materials. Hence, some surface modification must be taking place in the Pantoja clay. 3.3.2. Cation exchange Table 8 shows cation exchange properties measured after the experiments in comparison with the initial determinations. The sum of exchangeable cations (ΣE.C.) exceeded the cation exchange capacity (CEC) in Pantoja and Carboneros clays. This is caused by the dissolution of dolomite (Carboneros). In the case of Pantoja clay extraction of exchangeable cations was performed at pH 7 (whereas the CEC determination was performed in the alkaline dispersion. The

Ariño kaolinitic clay showed ΣE.C. lower than CEC of the initial clay. This was caused by the positive variable charges in the acidic leachates which eliminated cationic exchange sites. The behavior of the three clays becomes similar in L4, as far as a basic pH condition was produced. The distribution of cation species changed mostly for L3 and L4. Ammonium and alkaline cations displaced divalent Ca2+ and Mg2+. The adsorption of monovalent cations is higher in Pantoja clay with higher specific surface and CEC than in Ariño clay (Fig. 10). In the case of Carboneros, the dissolution of carbonates produced a strong competition between divalent cations and monovalent ones and showed a minimum of monovalent cation adsorption. 3.4. Anion transport The distribution patterns of Cl− determined in 1:10 (solid:liquid) aqueous extracts are plotted in Fig. 11. A first insight shows two

Fig. 7. Oriented slides XRD patterns of b2 μm fraction (air dried and glycerol solvated) of Carboneros clay. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. At 2 cm see Fig. 1. Peak numbers in angstroms (Å). S: smectite, K: kaolinite, I: illite, Cl: chlorite.

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Fig. 8. Oriented XRD patterns of b2 μm fraction (air dried and glycerol solvated) of Ariño clay. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate. At 2 cm see Fig. 1. Peak numbers in angstroms (Å). S: smectite, K: kaolinite, I: illite, Cl: chlorite.

alternative behaviors. The Pantoja clay acts as an effective barrier for chlorides, but this is not the case for Carboneros or Ariño in which this anion was transported to 40–50 cm below the leachate interface zone. SO2− 4 is depleted in the first 30–40 cm in Pantoja clay with both acid and landfill leachate, which must be related to sulphate reduction phenomena and not to anion transport towards the leachate interface. Dark patches had developed at the same depth in this clay that rapidly turned yellow under air exposure in a few days, which means that the dark colours were sulphides and not organic materials. Ariño clay showed sulphate depletion, but just at the interface expanded zone (45–55 cm) and not towards the bottom of the cell column, which is consistent with the gypsum traces found in the 10 cm (40 cm measured from the bottom) sample. This depletion can also be explained by sulphate diffusion towards the leachate. In fact, Ariño clay was the only material in which sulphates were detected in 300– 1500 mg/L levels in L4 and L3, respectively (results not shown). The thickness in which chlorides had penetrated in the clay columns is consistent with diffusion transport rather than an advective transport process. Hydraulic conductivities of 1–10 · 10− 10 m/s at hydraulic gradient equal to 1 should produce a solute front penetration of less than 3 cm in 10 months. However, when estimating the penetration distance in 10 months for molecular diffusion as 2√Dat (Bose–Einstein equation, Richardson and McSween, 1989), with = De 1 · 10− 10 m2/s (Rowe et al., 1995) a 10–30 cm distance was calculated, in agreement with the experimental results. We have calculated apparent diffusion coefficients (Da) for the case of the landfill leachate, in which the difference in chloride concentration between the clay and the liquid contact was higher. We have assumed that Da is independent of the position. Then, the concentration can be solved as a function of distance to a constant high concentration source as (Crank, 1975), C¼

M 1=2

ðπDa t Þ

from the liquid/column interface. The value of Da is calculated when representing LnC in front of x2. A straight line is obtained with a slope of (4Dat)− 1 (Idemitsu et al., 1998). Fig. 12 shows linear regressions, LnC in front of x2, for the three clay columns data. Da in Pantoja clay was calculated from three data points

  1 2 x exp − 4Da t

for initial boundary conditions of (t = 0, x = 0, C(0) = ∞ and t = 0, x N 0, C(x) = 0). In our experiments non-null concentrations exist along the clay column, but are well below the concentration at the liquid/ column interface for contact for L4. We have normalized concentrations to that of the original clay values in order to obtain Da: CCl = (Cx − Ca)/(Cx = 0), Ca = background chloride concentration in unaltered clay; Cx: chloride concentration at a distance x measured

Fig. 9. Specific surface area (m2/g) of clays in the column experiments. L1: distilled water, L2: basic leachate, L3: acid leachate and L4: real “El Garráf” leachate.

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Table 8 Cation exchange properties ΣE.C.

Na+

Pantoja CEC

15.7 ± 0.0

cmol(+)/kg

L1-2 L2-2 L3-2 L3-4 L3-6 L3-10 L4-2 L4-4 L4-6 L4-10 M0

17.9 20.9 22.2 26.3 17.9 23.7 20.1 24.4 25.4 24.5 26.8 ± 0.3

1.0 1.2 1.3 1.3 1.5 1.4 3 4.1 4.0 4.4 1.9 ± 0.1

Carboneros CEC 12.7 ± 2.1

cmol(+)/kg

L1-2 L2-2 L3-2 L3-4 L3-6 L3-10 L4-2 L4-4 L4-6 L4-10 M0

12.0 12.9 10.6 24.7 27.9 19.7 16.5 22.5 21.9 17.8 26.8 ± 1.2

0.07 0.11 0.27 0.25 0.29 0.36 2.2 2.4 1.8 1.6 0.2 ± 0.0

Ariño CEC

12.3 ± 0.9

cmol(+)/kg

L1-2 L2-2 L3-2 L3-4 L3-6 L3-10 L4-2 L4-4 L4-6 L4-10 M0

6.49 10.0 11.9 13.2 14.1 11.3 16.1 22.3 19.3 18.6 11.6 ± 1.0

0.03 0.08 0.56 0.57 0.41 0.36 2.5 3.9 3.1 1.6 0.01 ± 0.00

K+

Mg+2

Ca+2

NH+4

0.90 1.5 5.5 3.6 1.8 1.5 4.7 6.3 5.9 5.3 0.70 ± 0.0

9.9 7.6 5.0 3.7 3.2 4.1 4.5 6.8 7.0 5.0 15.4 ± 0.3

4.8 9.3 5.8 11.8 6.6 13.6 5.0 4.4 5.2 6.1 8.8 ± 0.0

1.3 1.3 4.6 5.9 4.8 3.1 2.9 2.8 3.3 3.7 ––

1.4 1.5 1.4 1.4 1.5 1.4 3.8 4.6 3.6 2.8 1.0 ± 0.1

3.5 3.2 2.0 2.9 3.3 2.5 2.9 3.0 3.1 3.2 9.3 ± 0.3

5.9 7.2 4.7 20.1 22.0 14.6 6.3 10.9 10.8 10.2 16.3 ± 1.1

1.1 0.92 2.2 0.6 0.8 0.8 1.3 1.6 2.6 2.1 –

0.28 1.7 1.9 1.1 0.97 0.36 3.3 3.3 2.8 1.9 0.33 ± 0.02

1.9 1.7 2.2 1.1 1.5 1.5 2.0 1.3 1.6 1.2 2.54 ± 0.08

3.6 5.6 4.1 6.9 7.8 8.1 5.4 9.1 11.8 7.9 8.76 ± 0.71

0.68 0.91 3.1 3.5 3.7 1.0 2.9 4.7 3.9 6 –

CEC: cation exchange capacity; ΣE.C.: sum of exchangeable cations.

because the difference in chlorides between measured and original data was zero after 5 cm. Carboneros or Ariño clays showed two data regions. In the uppermost altered zone (2–5 cm expanded bed), slopes are more negative (higher Da), and non-linear. The remaining points (N5 cm from the liquid/column interface), showed a linear trend caused by a uniform

density (constant Da) in the non-expanded zones. Calculated Da are in the order of magnitude of low-medium density compacted clay materials (50–60% total porosity), (Rowe et al., 1995), with the exception of Pantoja clay values, which lay in the field of highly compacted clays (b10% porosity). As the used points were located in an expanded altered zone, we can expect lower values in the deeper zones. 3.5 Heavy metals retention The Zn, Pb and Cr metal ions in the acidic leachates were mostly retained to a depth of 10 cm (Fig. 13). However, Cd ions were not initially present. Carboneros or Ariño clays showed measurable levels of Cd ions at 10 cm. This difference can be explained by the fact that pH was maintained at 5–6 in these clays but in the Pantoja clay the pH rose to 8. At this pH Cd is precipitated as carbonates. Taking into account that sulphate was reduced, cadmium sulphide (greenockite) could also be formed (Brookins, 1988). 4. Discussion This work indicated the significant anion (chloride) penetration in relatively thick clay barriers. Comparing Pantoja clay (illite + traces of stable smectite phases) with Ariño clay, both with high clay mineral (N70%) content in the clay size, without carbonates, we can consider the role of illite and kaolinite. Acidic medium recharges the kaolinite surface to positive, thus favouring anion diffusion. However, this cannot explain the chloride diffusion under basic pH. The lower specific surface area of this clay, and the higher particle size and higher pore size should be taken into account. Pantoja clay conditioned a basic pH (8–9) for the whole set of leachates. The permanent negative charged surfaces at basic pH could favour anion repulsion and hence, the observed anion barrier properties. The high specific surface of Pantoja clay could be related both to the presence of lower size pores and also to the presence of low crystalline size smectites, in which solute–surface interactions are more significant. This is in agreement with the differences shown by Shackleford, (1988) for effective diffusion coefficients of chloride ions, higher for kaolinite than for smectitic clays. The role played by anion exclusion from permanent charged negative clay surfaces has been considered to explain this anion behavior (Smith et al., 2004). The reduction of sulphates developed more on illitic than in kaolinite clays, in the absence of carbonates. Sulphate-reducing bacteria preceded the metanogenic stage of landfill organic matter degradation (Williams, 1998), which establish conditions of basic pH

Fig. 10. Exchangeable cations distribution as function of depth in the clay columns. Open circles are: Ca2+ + Mg2+ in charge (cmol(+)/kg. Solid circles: Na+ + K+. Squares are exchangeable NH+4. Arrow line indicates the decreasing trend of adsorption of monovalent cations.

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Fig. 11. Distribution of chlorides and sulphates as a function of depth measured from the bottom of the cell column.

and inorganic ammonium–sodium chloride leachates. This will aid to a local precipitation of heavy metal ions at the leachate–clay interface. In contrast, the kaolinitic clay showed resistance to sulphate reduction and a pH control at slightly acidic values as in the case of acetogenic stage of landfill (Owen and Manning, 1997). This behavior will promote eventually migration of metal ions either in the form of organic complexes (if organic acids are not degraded) or as free ions or chloride complexes in acidic medium. In addition, pH changes can modify also surface complexation of heavy metal ions. Speciation of metals and surface properties should be modelled to explain the metal transport, which is very limited in contrast to chloride migration. This task will be developed in future studies. The Carboneros clay, which contained a mixture of illite and dolomite, did not retard the diffusion of chlorides in comparison to kaolinitic clay. Although the specific surface area of this clay is very similar to Pantoja clay, we may consider the absence of smectite and the predominance of divalent cations in the exchange complex as factors influencing the pore structure of clay by forming more stable, higher size clay aggregates. The dissolution of dolomite grains produced enough divalent cations to prevent the cationic exchange retention of NH+4, Na+ or K+. Thus, dissolution–precipitation of sparingly soluble carbonates will drive to competitive adsorption phenomena between monovalent and divalent cations. On the other hand, this clay did not show reducing phenomena. The virtual absence of sulphates and the presence of significant amounts of iron oxide (hematite) will let the redox potential to be controlled by Fe3+/Fe2+ ions (Van Breukelen et al., 2004). The presence of a stable smectitic phase in small amounts in the clay barrier is critical for the barrier function itself as it stabilizes the

cation exchange retention properties as well as anion exclusion. The role of biological activity and the reduction of sulphates is not strictly a clay mineral function. It can play a significant role in pH buffering and metal ions retention by sulphide precipitation. 5. Conclusions The diffusion of anions and alkaline cations was faster than expected for advective transport (hydraulic conductivity) in clay materials. The anion front (chloride) was observed to reach more than 30 cm in some materials. Smectite traces have been demonstrated to help improve anion retention. Specific surface area decreased near the clay–leachate interface region (b5 cm). This was more pronounced kaolinite clays with smaller specific surface area. Despite the minor mineralogical changes, the specific surface area is a good indicator of clay alteration in terms of textural modifications induced by biofilm formation. Heavy metal ions were generally retained in the first 10 cm. In illitic clay with larger specific surface area, their transport was critically limited due to traces of smectite and sulphate reduction. The dissolution of carbonates also influenced the behavior of barrier materials. A competitive exchange reaction between dissolved divalent cations (Ca2+,Mg2+) and monovalent cations coming from landfill leachates was observed. Acknowledgements The authors would like to thank J.M. Rogel, P. Avellanosa, E. Muñoz and L. Martín from GEOCISA, for their valuable comments and support. Also to J. Hervás who directed the analytical determinations

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J. Cuevas et al. / Applied Clay Science 42 (2009) 497–509 Crank, J., 1975. The Mathematics of Diffusion, 2nd ed. Clarendon Press, Oxford, pp. 11–21. Davis, J.A., Kent, D.B., 1990. Surface complexation modelling in aqueous geochemistry. In: Hochella, M.F., White, A.F. (Eds.), Mineral–Water Interface Geochemistry. Reviews in Mineralogy, 23, pp. 177–248. Drever, J.I., 1988. The Geochemistry of Natural Waters. Prentice Hall, New Jersey. 435 pp. Frascari, D., Bronzini, F., Giordano, G., Tedioli, G., Nocentini, M., 2004. Long-term characterization, lagoon treatment and migration potential of landfill leachate: a case study in an active Italian landfill. Chemosphere 54, 335–343. García Calleja, M.A., 1991. Estudio petrológico y geoquímica de materias primas de la Cuenca de Madrid para su uso en la industria cementera. Tesis Doctoral de la Universidad complutense de Madrid. Centro de Estudios y Experimentación de Obras Públicas CEDEX. Madrid. 395 pp. González López, J.M., Bauluz, B., Fernández-Nieto, C., Yuste Oliete, A., 2005. Factors controlling trace-element distribution in fine-grained rocks: the Albian kaoliniterich deposits of the Oliete Basin (NE Spain). Chemical Geology 214, 1–19. Linares, J., Huertas, F., 1971. Kaolinite: synthesis at room temperature. Science 171, 896–897. Huertas, J.F., Chou, L., Wollast, R., 1998. Mechanism of kaolinite dissolution at room temperature and pressure: part 1. Surface speciation. Geochimica et Cosmochimica Acta 62, 417–431.

Fig. 12. Plot of LnC versus x2 (distance (cm) from the liquid/column interface), used in Da (apparent diffusion) coefficient calculation.

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