INFLUENCE OF METAL SPECIATION IN LANDFILL LEACHATES ON KAOLINITE SORPTION

PII: S0043-1354(97)00288-1

Wat. Res. Vol. 32, No. 3, pp. 882±890, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00

INFLUENCE OF METAL SPECIATION IN LANDFILL LEACHATES ON KAOLINITE SORPTION M M MAURO MAJONE1** , MARCO PETRANGELI PAPINI1 and ENRICO ROLLE2*

Department of Chemistry, University of Rome ``La Sapienza'', P.le Aldo Moro, 5-00185, Rome, Italy and 2Department of Environmental Engineering, University of Rome ``La Sapienza'', Via Eudossiana, 18-00184, Rome, Italy

1

(First received December 1995; accepted in revised form June 1997) AbstractÐThe sorption onto kaolinite of Pb, Cd, Ni and Cu from a land®ll leachate was studied in relation to the metal speciation in the liquid phase. Metal speciation was determined by two di€erent experimental procedures based on the exchangeability on a cation chelating resin (Chelex100) and on the separation by dialysis with membranes at di€erent molecular weight (MW) cut o€. The speciation procedures were applied on the leachate before and after equilibration with clay, in order to determine the contribution of the di€erent fractions to the total sorption. As determined by the MW-based procedure, large fractions of dissolved metals were associated to substances with high MW (>1000 and >12000 daltons), even if these substances represent only 18% of the total organic content (as determined by chemical oxygen demand, COD). These high-MW fractions contribute to metal sorption onto kaolinite, as also con®rmed by the concurrent removal of COD and phenolic substances. As determined by the exchange-based procedure, the main contribution to metal sorption derived from free/labile (rapidly exchangeable) or slowly exchangeable fractions. However, Pb and Cd were also removed from the stable/inert (not exchangeable) fraction. Because metal sorption is partially due to species that are not exchangeable on Chelex100 and partially to species that have MW more than 1000 daltons, these experimental evidences strongly support that free metals are not the only species participating in the sorption onto kaolinite and that some metal species are adsorbed without previous dissociation. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐland®ll leachates, heavy metals, speciation, soil sorption

INTRODUCTION

Land®lling is the most common method for disposing of solid wastes. One of the major problems associated with land®lling is the generation of large quantities of heavily polluted leachate. Land®ll leachate is identi®ed as a potential source of ground and surface water contamination, as it may percolate through soils and subsoils causing extensive pollution of streams, creeks and water wells. Moreover, migration of leachates is often slow and the environmental impact of an improper management of the land®ll plant might become evident only after a long time. Among the di€erent pollutants occurring in leachates, heavy metals can be particularly dangerous. Their movement through soils is mainly related to the ¯uid dynamics and to the sorption on the solid phase, the latter being in turn controlled by the composition of both solid and liquid phases. With regard to this, metal distribution among the individual physico-chemical forms in the liquid phase, i.e. *Author to whom all correspondence should be addressed. 882

speciation, has to be considered more than their total concentration. As a matter of fact, the complexing ability of land®ll leachates has been demonstrated to be crucial in determining metal mobility (GarciaMiragaya and Page, 1976; Elliott and Denneny, 1982; Christensen and Kjeldsen, 1989). Because of the high variability in the leachate composition (Johansen and Carlson, 1976; Fuller et al., 1979; Ghassemi et al., 1984; Lema et al., 1988), the use of measurable broad parameters such as total organic carbon (TOC), pH or concentration of soluble common salts could be misleading in the assessment of the transport of metals through soils. In particular, the organic fraction of land®ll leachates is highly variable in composition, ranging from simple ligands, like acetate or aminoacids, to high molecular weight compounds with many phenolic, carboxylic and amminic chelating groups, like fulvic and humic substances (Chian, 1977; Chian and De Walle, 1977; Sawhney and Kozlosky, 1984; Millot et al., 1987; Gourdon et al., 1989; Oman and Hynning, 1993). Metal speciation in leachates has been studied by both experimental and theoretical methods, either

Metal speciation in land®ll leachates

separately or in combination with each other. Knox and Jones (1979) combining the use of ion exchange and speci®c ion electrode methods investigated the complexation of cadmium by the organic fraction of sanitary land®ll leachates at di€erent fermentation stages. The complexing ability varied considerably in the di€erent leachates and was attributed in one case to carboxylic acids and in another to high molecular weight compounds (MW>10,000 daltons). By using gel-permeation on two types of land®ll leachates, Harmsen (1983) showed that Fe and Pb were mainly complexed by organic substances with high MW, whereas Zn was associated to compounds with MW < 1000 daltons. Christensen and Lun (1989) developed a speciation scheme based on the use of a cation chelating resin (Chelex100) and involving a batch±column±batch sequence for determination of Cd species in solid waste leachates. The procedure was then applied to ten leachates (Lun and Christensen, 1989) and the di€erent Cd speciations were associated to the leachate characteristics. By combining the exchangebased experimental procedure of speciation with soil sorption tests, Christensen (1989) showed that Cd sorption was mainly due to the free/labile fraction while stable complexes did not contribute. The use of micro and ultra®ltration techniques allowed Gounaris et al. (1993) to reveal the presence of stable colloids in several leachate samples. The experimental procedure and equilibrium calculations made it possible to evaluate the association of Zn, Pb and Cr to the di€erent colloidal fractions and to assess the metal mobility depending on the size and nature of the colloids. Holm et al. (1995) applied dialysis and ion exchange methods, as well as computer calculation (GEOCHEM code), for the speciation of Cd in leachates from soil, compost, land®ll and industrial wastes. By combining the results from the di€erent approaches, the authors estimated the fractions of free, organic and inorganic complexed cadmium. Majone et al. (1993) used theoretical speciation (MINTEQA2/ PRODEFA2 code) for the interpretation of the pH e€ect on lead adsorption onto kaolinite. The state of art shows that few data are available about the in¯uence of metal speciation in the liquid phase on metal sorption onto solid phases. The aim of this paper was to study the sorption of several heavy metals (Pb, Ni, Cu and Cd) from a land®ll leachate onto kaolinite, in relation to metal speciation in the leachate. The metal speciation in the leachate was experimentally determined by two di€erent operative procedures: an exchange-based procedure carried out by using a cation chelating resin (Chelex100) and a molecular weight (MW)based procedure carried out by using dialysis with membranes at di€erent molecular weight cut o€ (MWCO). To establish the contribution of the di€erent classes to the sorption onto the kaolinite, both speciation procedures were applied on the lea-

883

chate before and after the equilibration with the kaolinite. EXPERIMENTAL

Leachate characterisation The leachate used for this research was collected from the leachate collection system of a municipal sanitary land®ll in operation and stored in polyethylene bottles at 48C. The analytical characterisation was performed on the leachate after separation of the solid phase by centrifugation at 4000 rpm (3.3 g) for 15 min and subsequent ®ltration with 0.45 mm acetate cellulose ®lters (Millipore). Kjeldahl nitrogen (inorganic and organic) and chemical oxygen demand (COD) were measured by using standard procedures (APHA, 1995). Dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were determined on a Strholein total organic carbon analyser (model CMAT 550 PC). Alkaline, alkaline-earth and heavy metals were determined by inductively coupled plasma atomic emission spectrometry (Jobin Yvon JY38 Plus) according to a previously optimised procedure (Petrangeli Papini et al., 1994). Inorganic anions were determined by ion chromatography with chemical suppression and conductimetric detection (Dionex 2000i, AG4A guard column and AS4A analytical column, eluent NaHCO3 1.7 mmol lÿ1±Na2CO3 1.8 mmol lÿ1, regenerant H2SO4 50 mmol lÿ1, eluent ¯ow rate 1.5 ml minÿ1). Volatile fatty acids were determined by gas chromatography (Ottenstein and Bartley, 1971). Phenolic substances were measured by the spectrophotometric method of Folin-Ciocalteau (Box, 1983). The leachate composition is shown in Table 1. The humic substances were determined according to the procedure of Artiola-Fortuny and Fuller, (1982): a known volume of the leachate, separated from the solid phase by centrifugation and subsequent ®ltration with 0.45 mm acetate cellulose ®lters (Millipore), was acidi®ed to pH 1.5 by concentrated HCl. After equilibration for 24 h at room temperature, the precipitate (humic acids) was separated by centrifugation. The precipitate was washed several times with aliquots of an acid solution (pH 1.50, and then Table 1. Composition of the leachate (metal concentrations after spike of the leachate are reported in parentheses) pH COD Ninorganic Norganic Corganic (DOC) Cinorganic (DIC) Phenolic substances Pb Cd Cu Ni Al Fe Zn Cr Mn Ca Mg Na K Sulphate Chloride Nitrate Acetate Propionate i-butyrate Butyrate Valerate a

(mg lÿ1) (mg lÿ1) (mg lÿ1) (mg lÿ1) (mg lÿ1) (mg lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1) (mol lÿ1)

8.29 6378 1626 218 2284 1971 711 <9.6 10ÿ7a <1.8 10ÿ7a 6.3 10ÿ7 1.6 10ÿ5 2.5 10ÿ4 5.5 10ÿ4 5.2 10ÿ6 8.6 10ÿ6 2.4 10ÿ5 4.7 10ÿ3 1.2 10ÿ2 8.0 10ÿ2 4.2 10ÿ2 7.5 10ÿ4 8.6 10ÿ2 1.8 10ÿ5 2.4 10ÿ1 8.6 10ÿ3 8.0 10ÿ4 9.0 10ÿ3 3.0 10ÿ3

Detection limit of the analytical technique.

(2.4 (8.9 (7.9 (8.5

10ÿ5) 10ÿ6) 10ÿ5) 10ÿ5)

884

Mauro Majone et al.

recovered by centrifugation. The precipitate was then dissolved with NaOH and analysed for COD and phenolic substances. For all the following tests, the heavy metals were spiked to the ®ltered leachate to obtain ®nal concentrations of about 2.4 10ÿ5 mol lÿ1, 7.9 10ÿ5 mol lÿ1, 8.5 10ÿ5 mol lÿ1 and 8.9 10ÿ6 mol lÿ1 for Pb, Cu, Ni, and Cd, respectively. Before the tests, the leachate was maintained for a week in the dark at room temperature under slightly positive pressure of nitrogen gas, in order to allow equilibration of spiked metals (Majone et al., 1996). Speciation by exchange on Chelex100 resin Batch tests were performed by using analytical grade Chelex100 chelating resin (Biorad Laboratory, 100±200 mesh, sodium form, exchange capacity 0.61 meq gÿ1), following the procedure of Majone et al., (1996). The resin (stored under vacuum in a desiccator at room temperature) was added to the leachate at a ratio of 0.5 g/25 ml, corresponding to a high excess of exchange capacity with respect to the total concentration of heavy metals (about 30:1 on a equivalent basis). Tests were performed in closed Erlenmeyer ¯asks at 258C in a water bath under continuous stirring. Di€erent contact times from 5 min to 24 h were used. At the chosen contact time, the tests were stopped by transferring the suspension into centrifuge tubes and by separating the supernatant after centrifugation for 3 min at 4000 rpm (3.3 g). The resulting solutions were analysed for heavy metals. Due to the high bu€ering capacity of the leachate, the pH of the suspension remained constant throughout the test (8.32 0.1 units). The exchange-based procedure made it possible to divide metals into three operational classes, namely free/ labile, slowly exchangeable and stable/inert, according to their exchangeability on the resin at di€erent contact times. The free/labile fraction is de®ned by the metal removed after 1 h contact time (rapidly exchangeable), the slowly exchangeable fraction by the metal removed in the interval between 1 h and 24 h contact times, and the stable/inert fraction by the metal not removed after 24 h contact time (not exchangeable). Speciation by dialysis The dialysis experiments were performed by using dialysis tubes (cellulose esters membranes, Spectrapor) with MWCO at 1000 and 12,000 daltons. The membranes were stored at 48C in distilled water and rinsed several times before each test. The experiments were carried out by ®lling the chosen membrane with a known volume of the ®ltered leachate. The membrane was then placed in the dialysing solution maintained under stirring at 258C for 4 days. All the experiments were performed in a bu€ered ionic strength solution (0.25 mol lÿ1 as NaNO3, according to the calculated ionic strength of the leachate), maintaining the pH at 8.320.1 by means of a pH-stat apparatus (AMEL model 234, Titrator AMEL model 233, Digital Burette). The ratio between the sample and the dialysing solution volume was always 1:100. At the end of the experiment, the sample was quantitatively transferred in a volumetric ¯ask and diluted to a known volume with distilled water. The resulting solution was analysed for heavy metals, COD, and phenolic compounds. Dialysis experiments performed on synthetic solutions with heavy metals (same concentrations of the spiked leachate, NaClO4 0.25 mol/l, pH 4) showed that in the chosen experimental conditions the kinetics of the dialysis proved to be quite slow (probably because di€usion is slowed down at the high ionic strength). Indeed, after 4 days the dialysis process was not yet completed even if most of the free metal had passed through the membrane. Therefore, the experiments with synthetic solutions were

considered as blanks for experiments with leachate: in order to calculate the fraction retained at each MWCO, the residual concentration in the dialysed leachate was corrected by subtracting the residual concentration of the corresponding blank experiment. Due to an experimental accident, Pb data on speciation by dialysis are not available. Batch sorption test The clay used for the sorption tests was commercially available kaolinite (Merck, catalogue number 1906). The purity of the kaolinite was checked by XRD analysis and resulted greater than 97% (impurities due to trace amount of quartz and illite). The cation exchange capacity (C.E.C.) was 11.2 meq/100 g, as measured by the BaCl2 exchange method. The speci®c surface area was 8.8 m2 gÿ1 as measured by the BET method. The kaolinite was stored under vacuum in a desiccator at room temperature and used without any puri®cation or selection treatment. The sorption tests were carried out according to the following procedure: 25 ml aliquots of the leachate were transferred into 50 ml Erlenmeyer ¯asks. Then, di€erent amounts of kaolinite, in the range 0.4±6.4 g, were added to the samples. The equilibration of the suspensions was performed under continuous stirring at 258C. After 24 h the suspensions were centrifuged at 4000 rpm (3.3 g) for 3 min and supernatants were measured for metals, residual COD and phenolic compounds. As also reported for Chelex100 tests, pH variation of the suspension during the sorption tests was always less than 0.1 unit with respect to the initial leachate value (8.3). Blank experiments were carried out to verify the absence of heavy metals precipitation. The above reported batch procedure was applied to aliquots of leachate in the absence of kaolinite. At the end of the tests the samples were centrifuged as reported above and ®ltered with 0.45 mm acetate cellulose ®lters (Millipore). The ®ltrate was then analysed for all the metals and the residual concentrations were compared with those measured in the initial sample. The recovery of the spiked heavy metals was always greater than 95% (relative standard deviations for three replicates always less than 5%), thus indicating the absence of precipitate. Sorption tests were also performed with synthetic solutions containing heavy metals (same concentration of the leachate) in the presence of di€erent complexing agents. Glycinate and salicylate were chosen as representative of the nitrogenated and phenolic organic compounds of the leachate, respectively (Stanforth et al., 1979). EDTA was used as a reference chelating agent. All the ligands were added as sodium salt (composition reported in Table 4). The procedure was the same as for leachates sorption tests and also in this case the pH did not vary more than 0.1 units during the tests. Two tests were performed at di€erent leachate/kaolinite ratios (25 ml of leachate, 0.4 and 4.8 grams of kaolinite, respectively).

RESULTS

Metal speciation in the leachate Table 2 shows the speciation of the various metals in the leachate, as determined by the exchange-based procedure. The free/labile fraction is the main fraction for all the metals, increasing from 50 to 80% in the order Cd < Pb < Ni < Cu. The slowly exchangeable fraction is higher for Cd (47%) and Pb (28%) and considerably less for Cu (14%) and Ni (6%). As a sum, the fractions that are exchangeable on the resin are at least 80% of

Metal speciation in land®ll leachates

885

Table 2. Metal speciation in the leachate by the exchange-based procedure

Fraction

Criterion

Pb

Ni

Cu

ÿ1

ÿ1

ÿ1

mmol l

(%)

ÿ2

mmol l

(%)

ÿ2

59.3 (0.80)*

6.30 10

77.0 (0.4)*

mmol l

ÿ2

6.04 10

Cd (%) 80.2 (0.2)*

mmol lÿ1 ÿ3

5.34 10

(%) 50.0 (2.2)*

Free or labile complexed, rapidly exchangeable

removed after 1 h

1.37 10

Complexed, slowly exchangeable

removed after 24 h, excluding the previous fraction

6.52 10ÿ3

28.2

4.77 10ÿ3

5.8

1.04 10ÿ2

13.8

4.98 10ÿ3

46.7

Inert or stable complexed, not exchangeable

not removed after 24 h

2.90 10ÿ3

12.5 (3.4)*

1.41 10ÿ2

17.2 (3.6)*

4.56 10ÿ3

6.0 (4.8)*

3.56 10ÿ4

3.3 (4.5)*

2.31 10ÿ2

100

8.19 10ÿ2

100

7.54 10ÿ2

100

1.07 10ÿ2

100

Total

*Relative standard deviation of three replicate experiments.

the total for all metals. The highest concentrations in the stable/inert fraction were measured for Ni (17%) and Pb (12%). The di€erent speciation of the various metals qualitatively agrees with that previously reported on a leachate of similar composition (Majone et al., 1996), even though in the present case a general shift towards more exchangeable species can be noticed. The metal speciation based on the fractionation by dialysis is reported in Table 3, together with the fractionation of the COD and the phenolic compounds. All the metals are mainly present in the fraction with MW below 1000 daltons. Considering the considerable duration of the test (four days), this fraction could be overestimated because of the possible dissociation of labile complexes with higher MW, if originally present. However, the remaining fractions at higher MW are not negligible. In particular, about 45% of the total Cu content is retained at 12,000 daltons as also about 10% of other metals, whereas the intermediate fraction accounts for 8±25% of the total metal contents. The main part of the COD is present in the fraction at the lowest MW. By converting data in Table 1 in terms of COD, it results that volatile fatty acids account for about 88% of the fraction and 72% of the total COD content. Fractions at high MW (>1000 and >12,000 daltons), contribute for about 10% each to the total COD content. Phenolic substances are present in all the fractions. However, the phenolic content of the fraction decreases as MW increases.

The high MW compounds (>12,000 daltons) are probably humic compounds. Independent determinations of humic compounds obtained by separation after acid precipitation, showed that the ratio between phenolic substances and COD associated to humic compounds were almost correspondent to that obtained in the fraction retained at the highest MW. This ratio is quite a lot higher in the fractions at lower MW, where fulvic substances are probably prevailing. The MW of fulvic compounds in several matrixes is usually suggested to be about 1000 daltons (De Mora and Harrison, 1983) and the occurrence of fulvic substances with this MW has been detected by gel-permeation in land®ll leachates (Harmsen, 1983). However, the retention of fulvic acids from di€erent matrices at higher MW is also reported (Alberts et al., 1976; Truitt and Weber, 1981). The comparison of Tables 2 and 3 shows that exchangeability and MW are not directly related. Because most metal species are in general low MW and exchangeable, some overlapping of these two classes occurs. It can be calculated that at least 76%, 87% and 95% of low-MW forms of Ni, Cu and Cd are also exchangeable. For Cu, however, at least 58% and 87% of the high-MW species (>12,000 daltons) are labile or slowly exchangeable, respectively. Metal sorption on clay Figure 1 shows the experimental sorption isotherms in terms of metal concentration in the solid phase as a function of equilibrium metal concen-

Table 3. Speciation of heavy metals, phenolic substances and COD by the MW-based procedure Ni Fraction

mmol lÿ1 ÿ2

Cu %

mmol lÿ1 ÿ2

Cd %

mmol lÿ1 ÿ3

phenolic substances

COD

%

mg lÿ1

%

mg lÿ1

%

66.7 (9.0)*

405

57.0 (14.4)*

5249

82.3 (2.6)*

MWCO < 1000

5.98 10

1000 < MWCO < 12,000

1.31 10ÿ2

16.0

6.29 10ÿ3

8.3

2.67 10ÿ3

25.0

232

32.6

536

8.4

MWCO>12,000

9.03 10ÿ3

11.0

3.40 10ÿ2

45.0

8.90 10ÿ4

8.3

74

10.4

593

9.3

Total

8.19 10ÿ2

100

7.55 10ÿ2

100

1.07 10ÿ2

100

711

100

6378

100

73.0 (5.9)*

3.53 10

*Relative standard deviation of three replicate experiments.

46.7 (10.6)*

7.12 10

886

Mauro Majone et al.

Fig. 1. Adsorption isotherms of the metals on kaolinite from leachate (pH 8.32 0.1, 24 h contact time, 258C, kaolinite/leachate ratio 0.4±6.4 g/25 mL).

tration in the leachate. All isotherms show upward concavity (which is unlikely to be due to the presence of precipitation, as reported in the experimental section). This behaviour was already observed in a previous research on lead adsorption onto kaolinite from a land®ll leachate of similar composition (Majone et al., 1993), were an S-shaped isotherm was found operating in a larger concentration range. The occurrence of an S-shaped isotherm has been interpreted as being due to the presence of cooperative adsorption or to the presence of competitive e€ects of other solutes (Giles et al., 1974a and b). Sorption tests were also performed with synthetic solutions containing di€erent organic ligands (Table 4). For Pb, Ni, and Cd, metal sorption from the leachate was less than that observed from glycinate and salicylate solutions and higher than that from the EDTA one. This behaviour with respect

to sorption onto kaolinite corresponds to that already observed for the exchangeability on Chelex100 (Majone et al., 1996). On the contrary, Cu is removed from the leachate more than from the synthetic solutions. To evaluate the contribution of the di€erent forms of the metals to the total sorption, both speciation procedures were applied on the leachate after equilibration with the clay (kaolinite/leachate ratio 4.8 g/25 ml, contact time 24 h, pH = 8.320.1). Table 5 shows the metal speciation obtained by the exchange-based procedure. For all the metals, the residual free/labile fraction is decreased with respect to the initial one (Table 2). Thus, sorption mainly involves metal species from this fraction. Other fractions are in general increased, the only exception being that Cd is not present in the stable/ inert fraction. The contribution of the di€erent

Table 4. Metal removal by kaolinite from di€erent matrices (ionic strength of all the synthetic solutions 0.25 mol lÿ1 by NaClO4, pH = 8.3 20.1) Kaolinite (g/25 ml) 0.4

4.8

Matrix leachate glycinate (10ÿ2 mol/l) salicylate (10ÿ1 mol/l) EDTA (10ÿ3 mol/l) leachate glycinate (10ÿ2 mol/l) salicylate (10ÿ1 mol/l) EDTA (10ÿ3 mol/l)

Pb (%)

Ni (%)

Cu (%)

Cd (%)

57.1 100.0 92.4 15.8 82.9 100.0 93.9 74.0

7.9 10.6 24.6 5.1 53.6 49.0 82,2 19.7

20.4 3.8 1.8 8.2 37.0 ÿ1.9 29.6 7.1

20.5 81.3 61.2 6.2 36.7 100.0 99.6 51.3

Metal speciation in land®ll leachates

887

Table 5. Metal speciation in the leachate after equilibration with the kaolinite (4.8 g of kaolinite, 25 ml of spiked leachate, pH 8.3 2 0.1) by the exchange-based procedure

Fraction

Criterion

Pb

Ni

Cu

ÿ1

ÿ1

ÿ1

mmol l

ÿ3

(%)

mmol l

ÿ2

(%)

Cd (%)

mmol lÿ1

(%)

ÿ2

ÿ3

mmol l

Free or labile complexed, rapidly exchangeable

removed after 1 h

1.69 10

42.7

2.03 10

53.4

2.06 10

43.4

1.87 10

27.6

Complexed, slowly exchangeable

removed after 24 h, excluding the previous fraction

1.21 10ÿ3

30.5

3.58 10ÿ3

9.4

2.08 10ÿ2

43.7

4.89 10ÿ3

72.4

Inert or stable complexed, not exchangeable

note removed after 24 h

1.06 10ÿ3

26.8

1.41 10ÿ2

37.2

6.14 10ÿ3

12.9

0

0

3.96 10ÿ3

100

3.80 10ÿ2

100

4.75 10ÿ2

100

6.76 10ÿ3

100

Total

Table 6. Metal removal from the di€erent fractions after equilibration with kaolinite (4.8 g of kaolinite, 25 ml of spiked leachate, pH 8.3 20.1), according to the exchange-based procedure (data obtained from Tables 2 and 5) Fraction Removed with respect to the initial content of the fraction (%)

rapidly exchangeable slow exchangeable not exchangeable rapidly exchangeable slow exchangeable not exchangeable

Removed with respect to the total quantity removed (%)

forms to the total sorption can be calculated from the data presented in Tables 2 and 5: Table 6 shows data on metal removal after equilibration with kaolinite both in terms of percent removal with respect to the initial content of the fraction and in terms of percent contribution of the fraction to the total quantity removed. Pb is removed from all fractions and the removal percentage of each fraction decreases as the exchangeability on Chelex100 decreases. The same trend is observed for the contribution to total sorption which decreases from 63% to 10%. Cd removal is mainly due to the free/ labile fraction and to a lesser extent to the stable/ inert fraction (89% and 9% contribution with respect to the total quantity removed, respectively). However, the stable/inert fraction is completely removed while the free/labile only partially (65% removal with respect to the initial content of the fraction). Ni is mainly removed from the free/labile fraction (97% of total sorption which corresponds to 68% of the initial content of the fraction). Also for Cu the sorption seems to be due only to the free/labile fraction but its behaviour is quite anomalous because the concentration in the other frac-

Pb

Ni

Cu

Cd

87.7 81.4 63.4 62.7 27.7 9.6

67.8 24.9 0.0 97.3 2.7 0.0

65.9 ÿ100 ÿ34.6 143.4 ÿ35.8 ÿ5.7

65.0 1.8 100 88.8 2.3 9.1

tions signi®cantly increases (negative removals in the table). Table 7 reports the speciation of the residual metals in the leachate after the equilibration with kaolinite according to the MW-based procedure, whereas Table 8 reports the contribution of each fraction to the sorption (as calculated from Tables 3 and 7). Both tables also report data on COD and phenolic substances. It is noteworthy that both COD and phenolic substances are partially removed after the equilibration with the clay (total removal 10% and 49%, respectively). Moreover, both metals and organic substances from the fraction at the highest MW contribute to total removal. In particular, for Cu and COD, the contribution from the fraction at the highest MW represents 79% and 58% of the total removal, respectively, while for Cd, Ni, and phenolic substances the prevailing contribution to the sorption comes from the fraction at low MW. With respect to the initial content of the fraction, 30% to 65% of metals, 60% of COD, and 96% of the phenolic substances are removed from the fraction at the highest MW. For Cu, as well as for COD, the anomalous behaviour of the intermediate fraction (MW between 1000 and 12,000

Table 7. Speciation of heavy metals, phenolic substances and COD by the MW-based procedure after equilibration with kaolinite (4.8 g of kaolinite, 25 ml of spiked leachate, pH 8.3 2 0.1) Ni

Cu

Cd

ÿ1

ÿ1

ÿ1

Fraction

mmol l

MWCO < 1000 1000 < MWCO < 12,000 MWCO>12,000 Total

2.06 1.31 4.26 3.80

10ÿ2 10ÿ2 10ÿ3 10ÿ2

% 54.3 34.5 11.2 100

mmol l 2.75 8.18 1.18 4.75

10ÿ2 10ÿ3 10ÿ2 10ÿ2

% 57.9 17.3 24.8 100

mmol l 4.45 1.78 6.23 6.85

10ÿ3 10ÿ3 10ÿ4 10ÿ3

Phenolic substances % 65.0 26.0 9.0 100

ÿ1

mg l 228 134 3 365

% 62.5 36.7 0.8 100

COD ÿ1

mg l 4567 956 236 5759

% 79.3 16.6 4.1 100

888

Mauro Majone et al.

Table 8. Removal of metal, phenolic substances and COD from the di€erent fractions after equilibration with kaolinite (4.8 g of kaolinite, 25 ml of spiked leachate, pH 8.3 20.1), according to the MW-based procedure (data obtained from Tables 3 and 6) Cu

Cd

Phenolic substances

Removed with respect to the initial content of the fraction (%)

MWCO < 1000 65.6 22.1 1000 < MWCO < 12,000 0.0 ÿ30.0 MWCO>12,000 53.2 65.3

37.9 33.3 30.0

43.7 42.2 5.9

13.0 ÿ78.4 60.2

Removed with respect to the total quantity removed (%)

MWCO < 1000 89.3 27.9 1000 < MWCO < 12,000 0.0 ÿ6.8 MWCO>12,000 10.9 78.6

70.1 23.1 7.0

51.2 28.3 20.5

110 ÿ67.9 57.7

Fraction

Daltons) can again be observed as in the case of the exchange-based procedure (negative removal). Cross comparison of Tables 6 and 8 makes it possible to derive some relationship between the operational classes obtained by the two procedures, with reference to their availability to clay sorption. For Cd and Ni, the removal is mostly from the free/labile fraction (exchange-based procedure) and from the low MW fraction (MW-based procedure), thus showing the two fractions are at least partially related. Under the hypothesis of no internal shifts among the fractions in the liquid phase during the equilibration with clay, 90% of the initial free/labile fraction of Ni (99% of total removal) is due to lowMW species. For Cd, the low-MW species constitute from 65 to 77% of the initial free/labile fraction (90% of total removal). However, for both metals the free/labile fraction contains also part of the high-MW fractions. For Cu, this comparison has to be made with more caution; the anomalous increase of both intermediate fractions needs further study. However, an opposite relationship might be present as the free/labile fraction seems to be mainly associated to the highest MW fraction. CONCLUSIONS

MW-based speciation showed that the organic substances in the leachate are mainly species with MW less than 1000 Daltons, even though fractions at higher MW are not negligible (18%). The substances with high MW have an important in¯uence on metal speciation: all three considered metals are bound to these substances with a rank Cu>Cd>Ni. Although data on lead were not obtained, it is also probably bound to substances with high MW (Harmsen, 1983; Gounaris et al., 1993). Exchange-based speciation showed that in the leachate most metal species are exchangeable within 24 h of contact time and, within the exchangeable fraction, free/labile forms prevail on the slowly exchangeable ones (Cu = Ni>>Pb>Cd). On the opposite side, stable/inert complexes are formed in the order Ni = Pb>Cd = Cu. Literature data referring to metal exchangeability from land®ll leachates are restricted to the Cd (Christensen and Lun, 1989; Lun and Christensen, 1989), which was found to be present mainly in the labile form.

Ni

COD

Working on di€erent matrixes (river waters and treated wastewaters) by a mixed procedure (Chelex100 exchange and anodic stripping voltammetry), Figura and McDue (1980) showed that Cd is mainly free and in the labile form. They also found Cu mainly in the labile and slowly exchangeable forms, and Pb mainly in the slowly exchangeable and inert forms. Our data correspond well with reference to Cu and Pb whereas Cd is exchangeable more slowly in our study than in previous ones. Reference data on Ni were not found in literature. Cross comparison of speciation by the two procedures indicates that main part of lowMW forms are also exchangeable. The e€ect of such a metal speciation on sorption onto kaolinite has been evaluated by determining the change in the speciation in the liquid phase after the equilibration onto the solid phase. Thus, it was possible to calculate the contribution of the di€erent fractions to the sorption. For Pb, the exchange-based procedure indicates that all fractions contribute to the sorption, in decreasing order from free/labile forms to stable/inert ones. For Ni and Cd, the sorption is mainly due to the free/labile fraction on the one hand and to the lowest MW fraction on the other. The material balance shows that the two fractions are mainly overlapped but also that the free/labile fraction contains some substances with high MW. For Cu, the sorption is mainly due to the free/labile fraction on one side and to the highest MW fraction on the other one. This seems to indicate the two fractions are related to each other, even though the interpretation is made somewhat uncertain due to the anomalous increase of both intermediate fractions after leachate equilibration with kaolinite. In this regard, the kaolinite was used here without any initial pretreatment and the presence of presorbed copper could explain the observed behaviour. Preliminary experiments carried out with the metals at their natural concentration in the leachate (ppb levels, pH>8.0) made it possible to observe that some Cu is extracted from the kaolinite. The distribution in the leachate of the extracted Cu among the di€erent fractions depends on the new composition of the leachate after equilibration (with particular reference to the residual organic substances). As an hypothesis, this can explain the enrichment of intermediate fractions.

Metal speciation in land®ll leachates

More generally, sorption of metals bound to high MW species is relevant for all the considered metals: 30%, 53% and 65% of the initial contents of Cd, Ni and Cu are, respectively, removed from the fraction at MW>12,000. This is in agreement with the concurrent removal of COD: in particular, phenolic substances are almost completely removed from the fraction at MW>12,000. It is also noteworthy, that for some metals like Pb and Cd, the stable/inert fraction (as de®ned by the exchangebased procedure) is also removed after contact with kaolinite. In particular, the stable/inert fraction of Cd is completely removed, unlike what was reported by Christensen (1989). These experimental evidences strongly support that free metals are not the only species participating in the sorption onto kaolinite. Even though with a di€erent percentage for the di€erent metals, some of the sorption is due to species that are not exchangeable on Chelex100 and some to species that have MW more than 1000 daltons. Thus, it is likely that metals associated to high-MW species, which are also not rapidly exchangeable, are adsorbed without previous dissociation. In this regard, studying metal adsorption on various clays from synthetic matrixes, Gagnon et al. (1992) have shown that the presence of metals, such as Cd and Ni, and phenolic ligands, such as vanillic acid and eugenol, increases the adsorption of both metals and ligands. The authors supposed that the sorption onto clays of ligands with free complexing sites increases the sorption sites available for metals and that the sorption on clays of metals facilitates the sorption of ligands by decreasing the negative surface charge of the clay. From another point of view, it can be assumed that stable complexes of metals are adsorbed on the clay as a whole, depending on the anity between the clay and the complexed species which in turn mainly depends on the features of the ligand, such as structure and MW. Following this approach, the adsorption isotherms reported here should be interpreted as the sum of di€erent adsorption isotherms for di€erent species with di€erent mechanisms. The occurrence of an Sshaped isotherm has been reported as being due to the presence of cooperative sorption or to the presence of competitive e€ects of other solutes (Giles et al., 1974a and b). Both phenomena can be present in the case of sorption from such a complex matrix as the leachate. Thus, the unusual isotherms we obtained can be considered as an indirect con®rmation of the in¯uence of the speciation on the sorption. In conclusion, for the description of the metal sorption from a land®ll leachate onto kaolinite it is necessary to consider the contribution of all the di€erent fractions. As a matter of fact, the extent of the sorption cannot be fully described in terms of the free/labile metal concentration and the role of the organic substances cannot be fully described in

889

terms of complexation equilibria. Exchange-based and MW-based speciation procedures give complementary informations about the contribution of the di€erent species to total removal and can help to describe the entire interaction of metals and organic substances in the liquid phase with the solid phase. This in turn is a key point for describing the transport of the metal due to percolation of leachate through soils.

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