Trace metal adsorption on inorganic solid phases under estuarine conditions

Marine Chemistry, 32 ( 1991 ) 225-233

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Elsevier Science Publishers B.V., Amsterdam

Trace metal adsorption on inorganic solid phases under estuarine conditions H. Bilinski ~, S. Kozar ~, M. Plav~i6 2, 7,. Kwokal 2 and M. Branica 2'3 tDepartment of Physical Chemistry, 2Centre for Marine Research Zagreb, "Rudjer Bogkovi6" Institute, P. O. Box 1016 41 O01 Zagreb (Yugoslavia) 3Institute of Applied Physical Chemistry, Nuclear Research Center (KFA), Julich, P.O. Box 1913, tV5170 Julich (F.R.G.) (Received December 7, 1989; revision accepted May 10, 1990)

ABSTRACT Bilinski, H., Kozar, S., Plavgid, M., Kwokal, ~. and Branica, M., 1991. Trace metal adsorption on inorganic solid phases under estuarine conditions. Mar. Chem., 32: 225-233. The adsorption of trace metals (Pb, Zn, Cu, Cd and Hg) on calcite, kaolinite and bentonite has been studied in Krka river water of various salinities ( S = 3, 20 and 38%0). Model experiments were performed at pH 8 in natural water and in UV-irradiated water samples to determine the importance of natural soluble organic matter. Cold vapour atomic absorption spectrometry was used for the Hg determination, and a differential pulse anodic stripping voltammetry (DPASV) method for determination of other trace metals, using either a hanging mercury drop electrode or a mercury-coated glassy carbon working electrode. The following results have been obtained on calcite: ( 1 ) a powerful absorption of Pb and Zn was observed for both natural and UV-irradiated waters at S = 20%0; (2) a strong adsorption of Hg was observed in natural water, and significantly lower adsorption in UV-irradiated water at all three salinities, particularly at low salinity; (3) Cd adsorption was of medium intensity in both waters at S = 20%0; (4) Cu was practically not adsorbable in both waters at S = 20%0 if the total Cu ion concentration was lower than the copper complexing capacity value; ( 5 ) Cu adsorption was of a medium intensity at S = 38%0. It has not influenced by the EDTA model ligand. Trace metals Pb, Zn and Cu are adsorbable on kaolinite and bentonite at S=38%0, whereas Cd is practically not adsorbed. The Krka River is a calcareous river, which supplies the sea with calcite and aluminosilicates. In the light of our model experiments, we suggest that the self-purification of the Krka River is remarkable in respect to Pb, Zn and Hg; this occurs to a lesser extent for Cu, and is negligible in the case of Cd.

INTRODUCTION

The Krka River, situated on the eastern Adriatic coast of the Mediterranean Sea, is a typical calcareous river of medium alkalinity. The content of calcium and magnesium ions determined at the mouth of 0304-4203/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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the river is 0.0024 and 0.0005 mol dm-3, respectively. Both ions showed clearly conservative dilution with respect to salinity (Bilinski et al., unpublished data, 1991 ). An important water parameter, such as pH, changes only within a pH unit, with the highest variations observed at the freshwater/seawater interface (FSI). Concentrations of trace metals in the Krka River and its sediments have been studied for a number of years and a few papers have been published on the subject (Branica et al., 1985; Kniewald et al., 1987; Prohi6 and Jura~ir, 1989; Martinri6 et al., 1989; Mikac et al., 1990). The fate of trace metals in the estuary is governed by adsorption/desorption processes on both organic and inorganic particles. An important part of the particulate matter in the Krka River consists of living and dead organisms, with increased concentrations at the FSI (Vili~i6 and 7.utir, 1986; Zuti6 and Legovir, 1987 ). There is also evidence of the existence of inorganic particles. Prohi6 and Jura~i6 (1989) determined the prevalence of carbonate minerals in suspended matter and in sediments. Quartz and aluminosilicates were found in samples progressing towards the sea. Aluminosilicates, such as montmorillonite, illite and kaolinite, contribute to the non-carbonate fraction of sediments. In spite of the varied forms of particulate matter, dramatic changes in the adsorption of trace metals with changes in salinity are not expected, owing to the composition of the river. According to the results obtained by Hunter and Liss ( 1979 ), an almost linear slight decrease in a particle's negative mobility occurs with an increase in salinity when river water with a high calcium ion content is mixed with seawater. Thus, dramatic changes in the Krka River are not to be expected with respect to adsorption/desorption. This paper is based on a study of the adsorption of metal ions such as Cu e +, Pb 2+, Zn 2+, Cd 2+ and Hg 2÷ on selected model adsorbents (calcite, bentonite and kaolinite). EXPERIMENTAL

Characteristics of adsorbents The following adsorbents were used. (1) Analytical-grade chemical, CaCO3 (Merck, Darmstadt, F,R,G. ), with a surface area of 0.55 m 2 g-t according to Franklin and Morse ( 1983 ). Calcite was identified as the only component (Ref. No. 5-058) from an X-ray diffraction pattern using the Powder Diffraction File ( 1979 ). (2) Natural iron bentonite (The Vitoligte Mine, SR Macedonia, Yugoslavia ) with a surface area of 86 m 2 g-1 (BET, Brunauer, Emmet, Teller) and cation exchange capacity of 80 mequiv ( 100 g) - ~, in which montmorillonite (Ref. No. 7-330) and quartz (Ref. No. 11-252 ) are major, and feldspar (Ref. No. 9-478 ), mica (Ref. No, 9-334 ) and chlorite (Ref. No. 12-231 ) are minor components. Analytical composition (in per cent ): K20 1.57; CaO 0.52; TiO2

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0.9; FezO3 8.35; MgO 4.69; Na20 0.88; SiO2 47.1; A1203 18.7; and loss on ignition (L.O.I.) 18.14. Trace metals determined by X-ray fluorescence spectrometry (in ppm): Cr 81.9; Cu 23.6; Zn 103.5; Pb 54.7; Br 6.7; Rb 155; Sr 157.2; Y 43.9. (3) Natural kaolinite (the Sedlec Mine, Czechoslovakia), with a surface area of 14.5 m 2 g-l (BET) and cation exchange capacity of 11 mequiv ( 100 g)-1, in which kaolinite (Ref. No. 12-447) is a major component and mica ( Ref. No. 9- 334 ) is a minor component. Analytical composition (in per cent ): Na20 0.32; K20 1.84; CaO 0.01; MgO 0.03; A1203 27.25; SiO2 47.09. Trace metals by X-ray spectrometry (in ppm): Sn 163; B 115; Zr 116; Nb 55; Mo 28; Ti 814; Mn 32; Fe 5463; Cu 14; Zn 45; Ga 35; Pb 172; Rb 148; Sr 65; Y 39. The adsorbents used were not 'cleaned' to eliminate any possible organic coatings, although we were aware that such methods are available (Bi~ran et al., 1991 ). The reason for this decision was that calcite is the most important adsorbent in the Krka River. The solid used in this work is free of organic impurity, as it is an analytical-grade chemical.

Solutions of adsorbates Reagent-grade quality nitrates of Zn, Cu, Pb and Cd and chloride of Hg (Merck, Darmstadt, F.R.G.) were dissolved in tetradistilled water, to prepare 0.1 mol d m - 3 stock solutions. EDTA (Merck, Darmstadt, F.R.G. ) was used as a model organic ligand in the case of copper ion. Stock solutions were freshly diluted and aliquots were added by Eppendorf pipette to an electrolyte solution which was equilibrated with an adsorbent for 24 h. The equilibration part of the experiments with Pb, Zn, Cd and Cu was performed in quartz 100-cm 3 beakers. Polyethylene bottles were used for the Hg systems. The systems were filtered through a Millipore filter (0.45/tm) after 24 h of further equilibration.

Electrolytes Adsorption measurements on calcite were performed using water from the Krka River, and seawater ( S = 38%o) from the ~ibenik area (in the vicinity ) was used for adsorption measurements on bentonite and kaolinite. A 150-W UV lamp (Hanau, F.R.G. ) was used for 16 h, for the preparation of UV-irradiated water without addition of H202 and acids. The aim was to preserve natural pH, although our previous work (Plav~i6 et al., 1987 ) shows that only ~ 60% of the organic matter could be destroyed by such a procedure. The water samples were collected on June 14, 1989 by a scuba diver, in a manner previously described by Kniewald et al. ( 1987 ). Water samples collected at depths of 0.15 m (S = 3%00), 1.30 m (S = 20%0 ) and 6.5 m ( S = 38%0) were chosen as representative electrolytes for freshwa-

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ter (FW), the freshwater/seawater interface (FSI) and seawater (SW), respectively.

Instruments and procedure All voltammetric measurements were performed using a PAR 174-A polarographic analyzer connected to a Hewlett-Packard 7045 A X - Y recorder. Measurements were performed in a differential pulse mode. The electrode system consisted of either a hanging mercury drop electrode (HMDE, Metrohm Herisau) or a mercury-coated glassy carbon working electrode (TMFE; Magjer and Branica, 1977 ), with a coiled platinum wire as a counter electrode and an Ag/AgCI reference electrode. Pre-electrolysis was performed at the potential of - 1.30 V for Zn, - 0 . 9 5 V for Cd, - 0 . 9 0 V for Cu and - 0 . 8 0 V for Pb. The voltammograms were recorded after bubbling nitrogen through the filtered system for ~ 20 min. Acidification was necessary for the successful application of cold vapour atomic absorption spectrometry (CVAAS; by a Perkin-Elmer model 410 ). The procedure initially developed by Fitzgerald et al. (1974) was described in detail by Stoeppler (1983). The pH of the solutions was measured with a Metrohm Herisau E 603 pH-meter. The bentonite and kaolinite were analyzed for trace elements by X-ray fluorescence spectroscopy (Philips Roentgen Apparatus, Model PW 1010/30 ). RESULTS AND DISCUSSION

The calcite of the Krka River water system, to which trace metals were added, represents in this work a simple surrogate of the conditions found in any calcareous estuary. Table 1 shows selected typical model adsorption data for Zn, Pb, Cd, Cu and Hg ions. The experiments were performed in natural and UV-irradiated water samples of three different salinities. A powerful adsorption of Pb and Zn was observed for both water samples; this suggests that natural organic ligands in the Krka River have no significant influence on the adsorption of Pb and Zn. According to Zachara et al. (1988), surface exchange of Pb 2÷ and Zn 2÷ with Ca 2÷ can be assumed at such a low concentration, rather than coprecipitation of the corresponding carbonates (Schindler et al., 1969; Bilinski and Schindler, 1982). A powerful adsorption of Hg, slightly decreasing with salinity, was observed in natural water, whereas it was significantly lower in a UV-irradiated water, particularly at low salinity. This suggests an important role of natural organic ligands in promoting absorption of Hg on calcite. We are not sure whether Hg entering an estuary from the atmosphere in the form of Hg ° (Wood, 1975), remains in this form, or becomes oxidized to HgOHCI °, HgC~ and HgC142- (Anfalt et al.. 1968 ), as the oxidation rate is, to our knowledge, still unknown (Bilinski and Jusufi, 1980).

Hg

8 × 10 - s 8>{ 10 - s 8 × 10 - s 8 × 10 - s

Pb Zn Cd Cu

5>( 10 -9 5 × 10 -9

5 × 10 - 9 5>( 10 - 9

5 × 10 -9 5X 10 -9

7 X 10 - 7

7 × 10 -7 7 × 10 - 7 7>( 10 -7 7X 10 -7 7 × 10 - 7

2 × 10 - 7 2>{ 10 - 7

Added total metal ion (mol dm -3 )

Trace metal

0.3 0.3 0.3 0.3 0.35 5 0.10 0.25 0.48 0.54 1.04 1.40 1 1 1 3 3 3

Added CaCO3 ( g d m -3 )

20 20 20 20 38 38 38 38 38 38 38 38 3 20 38 3 20 38

Salinity (%0)

DPASV DPASV DPASV DPASV DPASV DPASV DPASV DPASV DPASV DPASV DPASV DPASV CVAAS CVAAS CVAAS CVAAS CVAAS CVAAS

(HMDE) (HMDE) (HMDE) (HMDE) (TMFE) (TMFE) (HMDE) (HMDE) (HMDE) (HMDE) (HMDE) (HMDE)

Method of analysis

Selected model adsorption experiments of trace metals on calcite added to the water of Krka Estuary

TABLE 1

0.98 0.87 0.54 0 0.49 0.68 0.35 0.59 0.58 0.74 0.78 0.78 0.96 0.92 0.87 0.93 0.89

Natural water

0.33 0.72 0.61 0.34 0.65 0.65

0.98 0.91 0.49 0.16

UV-irradiated water

Fraction of adsorbed metal, Y

0.53 0.67

Natural water + EDTA 2XI0-TM

tJ t~

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H. B I L I N S K I E T AL.

A

1.0

;~

"

v~--~o

Pb

X= KAOLINITE

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•

0 L

-2 =0

- 3.o [ogX

,1~v

- ~.o ( k g d m -3)

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Fig. l. A d s o r b e d f r a c t i o n ( Y ) o f P b , Cu, Z n and C d on ( A ) k a o l i m t e and ( B ) bentonite, each

plotted vs. log X(kg d m - 3 ), where X is the amount of kaolinite, according to Schindler's model ( 1975 ). Total amount of added trace metals was 8 × l 0-7 tool d m - 3 and salinity S = 38°/00.

Mercury was added in the oxidized form in the model experiment. We assume that the organic coating on calcite promotes the adsorption of any zerocharged species of mercury and that inorganic mechanisms can be excluded. Experiments with artificial seawater (H, Bilinski and Z. Kwokal, unpublished results) show negligible adsorption of Hg ions on calcite, which indicates that Hg 2+ substitution for Ca 2+ in calcite does not take place. The pure mercury carbonate phase, HgCO3-2HgO, will certainly not precipitate, as the added concentration of Hg is far below its solubility product(Bilinski et al., 1980 ), A completely different behaviour was observed in the case of Cu. No

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231

adsorption was observed in natural water at S = 20%0, and only negligible absorption in an UV-irradiated water. However, at S = 38%0, adsorption was of a medium intensity. The model ligand EDTA has no influence on Cu adsorption. We found earlier (Plav~i6 et al., 1987) that CuL (where L is a natural ligand) was adsorbed on calcite at S = 38%0. The present results show that the adsorption of Cu ion is apparent only at concentrations which exceed the complexing capacity value of the water sample studied (Plav~i6 et al., 1982 ). The fact that adsorption of Cd was approximately equal in natural and UVirradiated water at FSI leads to the conclusion that natural organics have a rather weak effect on Cd adsorption on calcite at S = 20%0. A similar conclusion was recently drawn by Kozar et al. ( 1988 ), concerning cadmium adsorption on bentonite added to seawater. Although calcite is most important for the removal of trace metals in the Krka River estuary, we have performed some adsorption experiments on kaolinite (Fig. la), and on bentonite (Fig. lb), using a DPASV ( H M D E ) technique, at S = 38%0. The concentration of added Pb, Zn, Cu and Cd ions was in all cases 8 × 10-7 mol din-3, under which conditions a possible influence of natural organic ligands is of minor importance. In the case of kaolinite, Pb, Zn and Cu show very similar adsorption properties, whereas in the case of bentonite the adsorption intensity decreases in the order Zn > Cu > Pb. In both cases, cadmium shows negligible adsorbability. Further research in the Krka River concerning Hg, Cu and Cd with respect to natural ligands is being carded out. CONCLUSION

The model experiments performed in this work have clearly demonstrated that the Krka River, which has calcium carbonate precipitating in the water column and calcite in surface sediments, and brings aluminosilicates to the sea, has a remarkable self-purification ability with respect to Pb, Zn and Hg trace ions. Even Cu ions should remain partly adsorbed under seawater conditions. Cadmium is the only one of the trace metals studied which is released into the water column at higher salinities. The surface sediments containing calcite and aluminosilicates can be considered as the sink for most trace metals in the Krka River estuary. ACKNOWLEDGEMENTS

This work was supported by the Self-Management Community of Interest for Scientific Research of SR Croatia. This work is part of the joint project 'Environmental Research in Aquatic Systems' of the Institute of Applied Physical Chemistry, Nuclear Research Center (KFA) Julich, and the Center for Marine Research Zagreb, 'Rudjer Bo~koviC Institute, Zagreb. Financial

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support by the International Bureau of Standards KFA Julich, within the framework of a bilateral agreement between the F.R.G.and Yugoslavia, is gratefully acknowledged. We thank Moria ~panovi6 for correcting the English and for typing the manuscript. REFERENCES Anfalt, T., Dyrssen, D., lvanova, E. and Jagner, D., 1968. Kemiska tillstandet hos tvavart kvicksilver i naturliga vatten, Svensk Kem. Tidskr., 80: 340-342. Bilinski, H. and Jusufi, S., 1980. Model studies of the solubility of inorganic mercury in the polluted coastal marine environment; Croat. Chem. Acta, 53: 93-99. Bilinski, H. and Schindler, P., 1982. Solubility and equilibrium constants of lead in carbonate solutions (25°C, •=0.3 mol din-3). Geochim. Cosmochim. Acta, 46:921-928. Bilinski, H., Markovi6, M. and Gessner, M , 1980. Solubility and equilibrium of mercury (I1) in carbonate solutions (25 ° C, I = 0.5 mol d m - 3). Inorganic Chem., 19: 3440-3443. Big6an, J., Rhebergen, J., Jura~i6, M., Martin, J:M. and Mouchel, J.M., 1991. Surface properties of suspended solids in stratified estuaries (Krka River estuary and Rh6ne River delta ). Mar. Chem., 32: 235-252. Branica, M., Peharec, Z., Kwokal, Z. and Kozar, S., 1985. Trace metals in the Sibenik aquatorium. P-1. Concentrations ofZn, Cd, Pb and Cu analyzed in 1983/84 period. Rapp. Comm. Int. Mer Medit., 28:111-113. Fitzgerald, W.F., Lyons, W.B. and Hunt, C.D., 1974. Gold-trap preconcentration method for the determination of mercury in seawater and other natural materials. Anal. Chem, 46:18821885 Franklin, M.L. and Morse, J.W., 1983. The interaction of manganese(II ) with the surface of calcite in dilute solutions and in seawater. Mar. Chem., 12: 241-245. Hunter. K.A. and Liss, P.S., 1979. The surface charge of suspended particles in estuarine and coastal waters. Nature, 282: 23-25. Kniewald, G., Kwokal, Z. and Branica, M., 1987. Marine sampling by scuba diving. 3. Sampling procedures for measurement of mercury concentrations on estuarine waters and seawater. Mar. Chem., 22: 343-352. Kozar, S., Bilinski, H. and Branica, M., 1988. Adsorption of cadmium and lead on bentonite in seawater and under estuarine conditions. Xth Int. Syrup. on Chemistry Of the Mediterranean, Primogten, Yugoslavia, May 1988. Book of Abstracts, pp. 106-107. Magjer, T. and Branica, M., 1977. A new electrode system with efficient mixing of electrolyte. Croat. Chem. Acta, 49: 1-5. Martin~i6, D., Kwokal, ~., Stoeppler, M. and Branica, M., 1989. Trace metals in sediments from the Adriatic Sea. Sci. Total Environ., 43: 27-39. Mikac, N., Kwokal, Z., May, K. and Branica, M., 1989. Mercury distribution in the Krka River Estuary (Eastern Adriatic coast). Mar. Chem., 28: 109-I 26. Plavgi6. M., Krznari6, D. and Branica, M., 1982. Determination of the apparent complexing capacity of seawater by anodic stripping voltammetry. Mar. Chem., l I : 17-31. PlavgiC M., Bilinski, H. and Branica, M., 1987. Voltammetric study of adsorption of C u ( l l ) species on solid particles added to seawater. Mar. Chem., 21: 151-160. Powder Diffraction File, 1979. Search Manual, Hanawalt method. Inorganic International Center for Diffraction Data, Swaltmore. Prohi~, E. and Jura~i6, M., 1989. Heavy metals in sediments. Problems concerning determination of the anthropogenic influence. Study in the Krka Estuary, eastern ,Adriatic coast. Yugoslavia. Environ. Geol., Water Sci., 12: 145-151.

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