Influence of humic substances on the aquatic adsorption of heavy metals on defined mineral phases

EnvironmentI&mational, Vol. 22, No. 5, pp. 507-517,1996 Copytight 01996 Elsevis Science Ltd Printedin the USA. All rights reserved 0160-4120/96 S15.00+.00

Pergamon

PII S160-4120(96)00040-2

INFLUENCE OF HUMIC SUBSTANCES ON THE AQUATIC ADSORPTION OF HEAVY METALS ON DEFINED MlNElRAL PHASES F.H. Frimmel and L. Huber Engler-Bunk-lnstitut,

University of Karlsruhe, 76128 Karlsruhe, FRG

EI 9507-375 M (Received 26 April 1996; accepted 28 April 1996)

The distribution of heavy metals in the aqueous phase of aquifers is strongly dependent on the kind of solid phase and on the presence of dissolved organic matter. Batch experiments were performed to investigate the phase distribution of Cd, Pb, and Cu using Quartz, Potassium Feldspar, Sodium Feldspar, Kaolinite, Calcite, and Fe&m-coated Quartz sand as mineral phases. Natural organic matter (NOM) from a brown water lake was used to study its influence on the phase distribution of the metals. From the experimental results, Freundlich isotherms were calculated. They turned out to be suited for a description of the heterogeneous systems. Despite the obvious limitations of mechanistic interpretation, NOM increased the dissolved fraction of Cu and Pb, and decreased that of Cd.

INTRODUCTION

The protection of groundwater is important for safeguarding the drinking water supply. Besides organic micropollutants, heavy metals are a risk for water quality. Therefore, it is essential to know about the distribution and transport of metals in aquifers. Only from a detailed understanding of these processes can measures to protect aquatic systems be derived. There have been numerous attempts to determine and calculate the interactions of heavy metals with sediments and rock formations. Adsorption, ion exchange, and precipitation or surface complexation have been identified as major mechanisms (Farley et al. 1985; KorJ 1992). Natural soil and sediment samples, clay minerals and well defined solid phases like SiO,, Ah%, or goethite have been used to investigate the distribution of heavy metals and their dissolved species(Gerthand Brtinner 1983; Zasoski and Burau 1988; StSver and Roennefahrt 1992).

Model calculations have shown that it is still difficult to apply the data obtained for well defined systems to the complex mixtures in nature. This is most obvious for cases where the ubiquitous natural organic matter (NOM) is taken into account (Buffle 1989). The role of protons in complexation reactions of metals was studied thoroughly by Perdue (1990). Tipping (1986) focused on the phase distribution of humic substances caused by iron oxides and hydroxides, and Marinski and Ephraim (1986) applied the polyelectrolyte model to explain the interactions of NOM and metals. Recently, Benedetti et al. (1995) showed that copper competes much more effectively with protons bound to the phenolic type groups of NOM than calcium and cadmium. An attractive approach to improve the predictive power of models uses phenomenological and

F.H. Frimmel and L. Huber

508

mechanistic parts. The application of adsorption isotherms seems to be a relatively uncomplicated way which leads to useful information on the phase distribution of water constituents. This has been demonstrated mainly for the elimination of organic micropollutants and NOM with activated carbon (Sontheimer et al. 1988; Johannsen and Worth 1994). It is promising to gain data sets also on the behaviour of metals in systems with NOM and defined mineral components of aquifer material, and by this to set a basis for the description and simulation of real aquifer systems. The aim of this work was to: 1) Investigate the phases quartz (Qz), sodium feldspar (NaFs), potassium feldspar (Kfs), kaolinite (Kaol), calcite (Cal), and iron/manganese hydroxides (MnFe) as sorbents for dissolved cadmium, copper, and lead; 2) Determine the phase distributions in the presence and absence of NOM; and, 3) Describe the equilibria by means of Freundlich isotherms. THEORETICAL

BASIS

For the description of phase distribution equilibria in solid/liquid systems, isotherms according to Freundlich and Langmuir, or distribution coefficients are suited (Stumm 1992). For aqueous systems, the Freundlich approach is commonly used (Sontheimer et al. 1988). The adsorption equilibrium between the adsorbed and dissolved part of the sorbate is described by Eq. 1: q=K&’

(1)

where, q is the amount of sorbate adsorbed on solid phase (e.g., umol/g); and, K, is the Freundlich constant; the better the adsorption the higher the number. In the double logarithmic plot of q versus c, the isotherm is linear for n = 1; for n > 1 adsorption isotherms are called ‘unfavourable’ according to the low load of the solid phase at low equilibrium concentrations in solution. The experimental determination of the isotherms can be done by using the initial concentration (c,,)and the concentration remaining in the solution after equilibration (c). According to Eq. 2, the amount of adsorbed matter can be calculated:

q = 2 (co-c) mS

(2)

Fig. 1. Competing reactions between metals and ligands (L) in a solid/liquid system (X, Y, Z different sites on the solid phase surface).

where m, is the mass of sorbent (e.g., in g); and, V, is the volume of the solution (e.g., in L). The complex formation in solution is described as a 1: 1 metal/ligand interaction according to Eq. 3 :

iv+

L &ML

(3)

where, L is an independent functional group of NOM. The different reactions in the heterogeneous system of an aquifer can be expressed as shown in Fig. 1. The complexity of the interactions of metals, ligands, and sites in a heterogeneous system is obvious. The kinetics of the adsorption of NOM on the mineral phases can be calculated (Stumm 1992; Cosovic 1990). For the calculation of the adsorption isotherms, the analysis developed for activated carbon adsorption was used (Frick and Sontheimer 1983; Johannsen and Worth 1994). The method is applied to undefined mixtures (e.g., NOM) and uses fictitious components with different adsorption properties. It is based on the ideal adsorption solution theory (IAS-theory) (Radke and Prausnitz 1972). The relative amount (initial concentration) of the fictitious components and their two Freundlich parameters (Kr and n) are varied until a good fit with the experimental data is obtained. Normally only a few (e.g., 3) components are necessary and a constant number for the Freundlich value n is selected. EXPERIMENTAL

DETAILS

Mineralphases

Quartz (SiOd was obtained from Quarzwerke Freenen as type F 32. According to the classification system of DIN 18123 (1983) it is a middle sand which has practically no basic or other impurities. Sodium feldspar (albite, NaAlSisO,) and potassium feldspar (orthoclase,

Influence of humic substances on aquatic adsorption of heavy metals on defined mineral phases

509

Table 1. Composition of mineral phases (main components), numbers as weight %, average values from x = 5 determinations. L.O.I. = loss of ignition; n.m. = not measured.

SO,

Qz

KFs

NaFS

Kaol

98.3

66.0

68.8

57.7

1.00

4.73

18.4

18.8

28.6

0.32

7.01

Calc

MnFe

-4’203

0.13

Fe*03

0.06

0.30

0.13

0.18

0.71

TiO,

0.02

0.02

0.01

0.58

0.01

n.m.

K,O

0.01

2.45

3.76

0.38

1.52

Na&

co.01

2.82

7.11

0.19

0.06

1.02

CaO

0.05

0.46

1.75

0.01

MgO

0.06

0.02

0.02

0.02

0.35

MnO,

co.01

co.01

co.01

co.01

0.02

0.01

0.01

0.02

0.05

0.01

PA

11.6

BaO L.O.I.

53.9

5.22 1.25 53.4 n.m.

8.63 0.2

0.30

0.2

0.09

42.3 5.2

W sum

18.4

99.2

99.8

99.4

KAlSi,O,) were from SCR Sibelco (Antwerpen, Netherlands) and had the size of middle and fine sand. Kaolinite (Al,(OI-I)@i,O,) was from Gebriider Dorfher GmbH & Co of the type Dorfner-Kaolin ZY. The particle size equals that of a clay-fraction. Calcite (CaCO,) was from Mathis GmbH & Co, and its origin was a quarry of the Tuniberg near Freiburg. Manganese and iron oxihydrate was supplied by Stadtwerke Karlsruhe (water works Rheinwald) and originated from a quartz sand filter which functioned for elimination of iron and manganese from raw water. The chemial composition of kaolinite and calcite are given in Tables 1 and 2. Numbers are averaged from x = 5 determinations. Some characteristic properties of the solid phases are given in Table 3 (Hodel et al. 1995). Particle size distribution was obtained by dry sieving according to DIN 18 123. Most of the particles had a

99.8

99.1

97.8

fairly homogeneous chemical composition. The exception was Mn/Fe-oxihydrate. A core of quartz-particles (d ca. 0.5 mm) was coated with Mn(IV)- and Fe(III)hydroxide (ca. 120 pm). To increase the yield in the isotherm experiments, mineral phases (except MnFe) were ground which led to specific surfaces of 670 (Qz), 940 (KFs), 750 (NaFs), 860 (Kaol), and 1250 (Calc) m2/kg. Solutions and chemicals

Experiments were performed with deionized water (dw) and synthetic water. The synthetic water had an ion balance similar to groundwater from calcite dominated aquifers, with electric conductivity K (20°C) = 375 @/cm and ionic strength u = 5.18 mmol/L. Concentration of Ca” was 0.76 mmol/L and of Mg2+

F.H. Frimmel and L. Huber

510

Table 2. Composition of mineral phases (trace elements), numbers in mgkg, average values from x = 5 determinations. n.d.: < limit of detection.

Qz

KFs

NaFs

Ni

n.d.

n.d.

n.d.

CU

9.30

9.31

9.68

Zn

5.78

7.14

7.98

Pb

4.91

55.7

33.3

As

n.d.

n.d.

n.d.

Sr

8.25

52.7

52.5

2.98

7.98

n.d.

n.d.

n.d.

Th

n.d.

n.d.

n.d.

Rb

n.d.

Cd

0.02

n.d.

0.02

Tl

n.d.

0.71

Bi

0.02

co

n.d.

Zr

22.8

Nb

Kaol

MnPe

Calc

8.26

9.97

26.0

11.6

73.3 9.68

3.84 52.4

15.9 8.87

192

2.05

9.66

n.d.

301

335

115

6.71

n.d.

16.8

n.d.

n.d.

16.7

n.d.

0.2

2.91

n.d.

n.d.

0.04

0.53

0.25

n.d.

0.02

0.22

0.15

1.27

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

26.0

392

161

165

111

121

The analytically determined concentrations of the trace metals show standard deviations of up to 10%. Table 3. Characteristic properties of the mineral phases.

Properties

Qz

density, g/cm3

2.65

Co,, weight %

0.07

cation exchange capacity, mmol(eq)/kg

6.0

11

19

specific surface, m2/kg

9.2

16.5

12.6

average particle size (based on 50 % sample ma=), Pm

300

NaFs

Kaol

talc

2.57

2.605

2.6

2.72

2.68

0.17

0.22

0.15

0.29

KFS

220

300

20

860 14

101

6.5 550

MnEe

493

63 000 1300

511

Influence of humic substances on aquatic adsorption of heavy metals on defined mineral phases

i

cKaoIinite

‘i

0.4-

1 0.3

0

!

I 100

I 200

I 300

I 400

s60

I 600

I 700

time [h] Fig. 2 Kinetics of the adsorption of brown water NOM on the mineral phases calcite, kaolinite, sod iron/manganese

was 0.31 mmol/L; pH (20°C) was 8.4. The water was in thermodynamic equilibrium with respect to CaCO,. In addition, water from a brown water lake (Hohlohsee, Black Forest) was used as source for NOM in its original composition or diluted with dw. The original brown water had an electric conductivity K (20°C) = 48 @/cm. Concentrations of Ca*’ and M$+ were 0.05 mmol/L; concentration of DQC was 20 mg/L; A (254 nm) / DOC was 4 L/(mg - m); pH (20°C) was 4. The water was filtered through a 0.45 urn membrane filter (cellulose-nitrate, Sartorius) after sampling. All chemicals used for pH adjustment and analytical purposes were of analytical grade (puriss. p. a.). Analytical methods

Metal concentrations were determined with atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) according to standardized methods (Fachgruppe Wasserchemie 1995). The detection limits were 0.1 ug/L (Cd), 0.6 ug/L (Cu), and 0.6 ug/L (Pb). The concentrations of dissolved organic carbon (DOC) were determined with a Carbon-analyzer (Dohrmann). The detection limit was 0.2 mg/L. Standard deviations for the methods were three times the detection limits. The pH-values were determined with a glass electrode.

oxihydrate.

Reactions

Batch experiments were performed in PTFE bottles (volume V = 250 mL). First, 2 g (Qz, NaFs, KFs, Kaol), 1.5 g (Calc), or 1 g (MnFe) of the mineral phases were defined metal concentrations (about 1 umol/L each) and NOM (mostly 4.7 mg/L) for 2 d at 100 rpm. After this, samples were taken, filtered (0.45 pm membranes) and analysed. Most experiments were run five times. All experiments were performed at ambient temperature. RESULTS Selection of time scale

In multicomponent systems, equilibration time has to be watched carefully. Metal adsorption is a relatively fast process. Adsorption of NOM generally requires more time. Figure 2 shows the kinetics of DOC adsorption on selected mineral phases. In the case of calcite and kaolinite, equilibrium is nearly reached after a few days. Equilibration in the MnFe system takes longer. From a pragmatic view, 2 d was chosen as the reaction time for equilibration. Stability of pH-values

Adsorption processes, especially those with metals, are strongly influenced by the proton concentration (pHvalues). Figure 3 shows the change of pH-value in the

512

F.H. Frimmel and L. Huber

8

7-

[Kaolinitef -

[lb

initial pH

1.

3-

2 0

-

3.6

*

4.8

+

6.5

*

7.3

1

I

I

I

I

I

I

I

10

20

30

40

SO

60

70

80

m (solid) I V (solution)

[g/L]

Fig. 3. Plot of pH-values of aqueous systems containing solutions with different initial pH-values and different kaolinite/solution

ratios.

Table 4. Initial concentrations of NOM and Freundlich parameters for the fictitious components adsorbed on kaolinite. Fraction

n

KF (mg/g)-(wCY

m@-

%

non adsorbable

0

3.3

17.5

1

0.5

0.16

2.0

10.5

2

1.0

0.16

5.6

30.2

3

2.0

0.16

7.8

41.8

sum:

18.7

averaged

error (Oh):

solution as a function of the relative amount of kaolinite as the solid phase. Kaolinite acts like a solid pH-buffer with a pK value around 4.6. As a consequence, the pHvalue of heterogeneous phases will be significantly dependant on the component with the highest buffer capacity. The other mineral phases showed equilibrated pHvalues of 5.8 f 0.2 for Qz, KFs, and NaFs; 8.6 f 0.1 for calcite, and 7.2 f 0.2 for MnFe. Adsorption

D°CO,i

isotherms

for NOM

The adsorption of NOM onto mineral phases can be described according to the model of Johannsen and

100

1.15

Worth (1994). It was originally developed for the description of NOM adsorption on activated carbon and is based on the assumption of fictitious components with different adsorbability given as Freundlich parameters (I& n). Table 4 shows a set of data for the fictitious components of brown water NOM which describes the experimental results of its adsorption on kaolinite. The four components include a non-adsorbable fraction (17.5% of DOC) and fractions with increasing afIinity to kaolinite. It turned out that a single value for the Freundlich exponent n was well suited to match the experimental data points with the model calculation (Fig. 4).

513

Influence of humic substances on aquatic adsorption of heavy metals on defined mineral phases

pH = 3.6 Dilution

1:l

Isotherm for original brownwater

0.0 0

4I

2

6I

Remaining concentration

I

I

8 in solution

I

10 (DOC mg/L)

1

12

Fig. 4. Adsorption isotherms (experimental data points and model calculation) for aquatic NOM on kaolinite.

L.”

1.8I

1.6-

8

.

1.2-

e 1.0$.g 1.4,o d ‘;; 0.8-S 2

/

DH-values: initial1

0.6-

hY

=

I 2

I 4

I 6

I

8

I

10

I

12

Remainingconcentrationin solution Fig.5. Adsorption

final

1

-3.7

7.6 1 93~0.07

14

I

16

I

18

I

20

(DOC mg/L)

of aquatic NOM on calcite starting from different initial pH-values.

Good agreement is obtained for the adsorption of NOM in the original brown water and in its dilutions (1: 1, 1:2, 1:3). Isotherms for the adsorption of aquatic NOM on calcite are given in Fig. 5. Even though there was quite a difference in the initial pH-value of the solutions, there is only a narrow range in the final suspensions. Obviously there is an increase in the adsorbed

NOM as the pH-value decreases. This is in good agreement with observations from the adsorption of humic substances on XAD-resins (Abbt-Braun and Frimmel 199 1). The comparison of the isotherms for the NOM adsorption on kaolinite, on calcite, and on the iron/ manganese oxihydrate is given in Fig. 6.

F.H. Frimmel and L. Huber

514

Kaolinite (2 days) DO&= 18.7 mg/L.

a, = 1250 m*/kg

s, = 63000 m*ikg 0.001

!

0

I 2

I 4

I 8

I 6

‘

I

10

Remaining concentration

I

12

in solution

I

14

16

I

18

1

(DOC mg/L)

Fig. 6. Adsorption isotherms(experimentaldata points and model calculations)for aquaticNOM on kaoliite, calcite, and iron/manganeseoxihydrate. Table 5. Dissolved and adsorbed amounts of NOM in aqueous suspensions of selected mineral phases.

c,(DOC) = 20 mgIL; solid/solution-ratio m/v, = 20 a. DOCIDOC,

DOC

%

mg/L

100

19.4

0.00

0.00

NaFs

93

17.3

0.06

0.08

MnFe

41

8.5

0.61

0.01

Calc

20

3.7

0.72

0.57

Kaol

21

3.8

0.74

0.86

Solid phase

Qz

Note that theNOM load on mineral phases is based on their surface area. It has to be kept in mind that the pHvalues in the experiments were different according to the buffer capacities of the mineral phases. For the Fe/Mn oxihydrate containing system, the reaction time was 40 d. This was according to the aim to reach thermodynamic equilibrium (Fig. 2). Despite the long reaction time, there was relatively poor coverage of the surface area determined via a nitrogen-adsorption-isotherm according to the BET-method of Brunauer et al. (193 8). One might assume that most of the determined surface is due to a porous structure and cannot easily be reached by the

Load DOC

mg/n

DOC

me/m2

NOM. A comparison of kaolinite and calcite shows that the lower pH-value ofthe kaolinite containing system is stimulating the adsorption ofNOM and obviously rules out the effect of the larger surface area of the calcite. The degree of adsorption of NOM on the different mineral phases under comparable reaction conditions is given in Table 5. There is a clear tendency for better adsorption from quartz to kaolin&e+if the load is given as mass ratio. The sequence is somewhat different on the basis of surface area. Again the special role of the Fe&In-oxihydrate is obvious.

Influence of humic substances on aquatic adsorption of heavy metals on defined mineral phases

515

I

Pb/Calci

Dissolved concentration

[pmol/L]

b

Dissolved concentration [pmol/L] Fig. 7. Adsorption isotherms for the lead/calcite system in absence a) and presence b) of NOM (c,,(DOC) = 4.7 mg/L).

Metal adsorption

An example of the influence of NOM on the adsorption of heavy metals on mineral phases is given in Fig. 7. The experiments were done in model water (groundwater type). The two lines in Fig. 7a show the isotherms for the individual application of lead as a single metal (upper line) and for th e 1ea d application simultaneous with the other metals (lower line). The isotherms for the lead adsorption on calcite simultaneously applied with the other metals and, in the presence of NOM (Fig. 7b), show the numbers obtaitied from balance calculations according to Eq. 2 (uptier line), and the data points gained from extractions of the sorbent (lower line). There was a smaller yield of the experimental extraction which led to an apparent smaller load of the calcite. The

reason for that may be seen in a partly irreversible fixation of the metal in the calcite. This has to be kept in mind in case of a quantitative use of the data. The discrepancy was also shown for the other systems but to a smaller extent. (More details are given in Hodel et. al. (1995).) Mineral phase specific effects can be deduced. Comparison of the isotherms for the lead adsorption in the presence and absence of NOM (Fig. 7: a, lower line and b, upper line) proves the vehicle function of NOM for lead which remains in solution to a higher extent if NOM is present. The simultaneous adsorption of the metals Cd, Pb, and Cu on the different mineral phases is given in Table 6. The influence of the NOM on the phase distribution of the metals is given in Table. 7. The values of the Freundlich parameters are only given for correlation coefficients ? > 0.5 1. The K, values were calculated

F.H. Frimmel and L. Huber

516

Table 6. Freundlich isotherms for the simultaneous adsorption of cadmium, lead, and copper (c,, (each) = 1 urnoIL) on different mineral phases; K, in (pmobkg) - (umo!/L)-“. Cd

Solid phase QZ

KF

6.0

n

cu

Pb 3

K,

n

rr

K,

n

?

1.8

0.95

140

1.8

0.98

150

1.4

0.99

KFs

22

1.9

0.89

170

1.2

0.98

71

1.2

0.81

NaFs

17

0.2

0.84

270

1.1

0.92

340

1.5

0.94

Kaol

42

1.8

0.96

350

0.8

0.87

1200

2.1

0.92

Calc

230

0.9

0.96

300

1.2

0.92

210

1.5

0.89

MnFe

220

1.0

0.87

0.51

400

1.6

0.83

Table 7. Freundlich isotherms for the adsorption of cadmium, lead, and copper (cO(each) = 1 pmol/L) on different mineral phases in the presence of NOM (c,(DOC) = 4.7 mg/L); KF in (umolkg) * (umol/L)“. Cd

Solid

Pb

cu

KF

n

?

K,

n

3

K,

n

rs

Qz

2100

6.5

0.98

390

8.0

0.98

730

23

0.93

KFs

680

3.7

0.98

800

5.6

0.99

NaPs

3700

7.4

0.94

220

4.3

0.91

140

17

0.86

Kaol

415

2.2

0.96

470

2.4

0.99

510

12

0.76

Calc

590

1.4

0.92

120

1.6

0.97

MnFe

1200

1.5

0.93

phase

according to Eq. 1, using six data points for q in pmol/kg and c in pmol/L. Note that the linearized isotherms are only valid for a fairly narrow concentration range which, for copper, for example, is 0.79 umol < c(Cu) < 0.87 pmol. The values of n which are extremely high in case of copper can only partly be explained by the kg-based loads. This shows clearly that an interpretation similar to the explanation of phase equilibria of purely organic compounds cannot be done. Because of the empirical character of Freundlich isotherms and the necessity to work in analytically manageable concentration ranges, no general mechanistic information can be derived. In addition, the conditions under which

0.05

0.51

0.04 200

1.9

0.94

it is reasonable to work with the different model systems can vary significantly (e.g., pH, size distribution). This implies a limitation of the direct comparability of the results. Despite those pitfalls, the results show some general trends. For copper, to a lesser extent, there is a mobilization effect caused by NOM. Cadmium is better adsorbed in the presence of NOM. This was also observed in breakthrough experiments using columns filled with the mineral phases (Huber and Frimmell994). The experimental results so far cannot be fully explained by the stability of the metal/NOM-complexes which increases from Cd to Cu.

Influence of humic substances on aquatic adsorption of heavy metals on defined mineral phases

CONCLUSIONS

The work shows that under some precautions Freundlich isotherms are suited to describe the phase distribution of metals also in the presence of NOM. From the isotherms of the different mineral phases, it is possible to deduce-at least for a limited concentration range-the behaviour of the metals and their species in aquifers if their mineral composition is known. The results so far are in good agreement with most data which were obtained in other investigations. Model calculations depend strongly on sound experimental data and results. Therefore, it is essential to identify the key components for the system under investigation. In addition to the influences of pH, temperature, and ionic strength, competing reactions and their degree of reversibility have to be investigated. There is no doubt about the fundamental role of NOM in the distribution, transport, and reactions of metals in aquifers. Due to the complicated structure of NOM, there is little hope for reliable mechanisms to describe the interaction with other parts of a groundwater system. Simple approaches like isotherms obtained from batch experiments in addition to column experiments will help to obtain the necessary sets of data for model calculations with greater prognostic power. hmding of the work by Stithtng Volkswagenwerk (AZ: 4710,410S) and the experimental help of Brigitte Raue and Gudrun Grabe is gratefully acknowledged.

Acknowledgment-The

REFERENCES Abbt-Braun, G.; Frimmel, F.H. Spektroskopische Strukturaufkl%rung aquatischer Huminstoffe (Spectroscopic investigations of the structure of aquatic humic substances). Vom Wasser 77: 291-302; 1991. Benedetti, M.C.; Milue, C.J.; Kimiburgh, D.G.; van Riemsdijk, W.H.; Koopal, L.K. Metal ion binding to humic substances: Application of the non-ideal competitive adsorption model. Environ. Sci. Technol. 29: 446-457; 1995. Brunauer, S.; Emmett, P.H.; Teller, E. Adsorption of gases in multimolecular layers. J. Amer. Chem. Sot. 60: 309-319; 1938. Buffle, J. Complexation reactions in aquatic systems. Chichester: John Wiley & Sons; 1989. Cosovic, B. Adsorption kinetics of the complex mixture of organic solutes at model and natural phase boundaries. In: Stumm, E., ed. Aquatic chemical kinetics. New York: John Wiley & Sons; 29 l310; 1990. Deutsches Institut fbr Normung e. V.: DIN 18123. Bestimmung der KomgroRenverteilung (Determination of grain-size distribution). 12 S. Berlin: Beuth Verlag GmbH; 1983. Fachgruppe Wasserchemie in der Gesellschatt Deutscher Chemiker (Ed.) Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung (German standard methods for the examination of water, waste water and sludge). 32. Auflage. Weinheim: VCH; 1995.

517

Farley, K.J.; Dzombak, D.A.; Morel, F.M.M. A surface precipitation model for the sorption of cations on metal oxides. J. Colloid Interface Sci. 106: 226-242; 1985. Frick, B.; Sontheimer, H. Adsorption equilibria in multisolute mixtures of known and unknown composition. In: McGuire, M.J.; Suffett, I.H., eds. Treatment of water by granular activated carbon. Advances in Chemistry Series 202. Washington, D.C.: Am. Chem. Sot.; 1983. Gerth, J.; Brtimmer, G. Adsorption und Festlegung von Nickel, Zink und Cadmium durch Goethit (a-FeOOH) (Adsorption and immobilization of nickel, zinc and cadmium by Goethite (cc-FeOOH)). Fres. Z. Anal. Chem. 316: 616-620; 1983. Hodel, M.; Huber, L.; Lehmann, M.; Lensing, H.J.; Herrling, B.; Frimmel, F.H.; Puchelt, H. Quantitative Wechselwirkungen zwischen Metallen und Mineralphasen unter Berticksichtigung des Einflusses nattirlicher organischer Wasserinhaltsstoffe und Modellienmg des StoITtransports im Grundwasser (Quantitative interactions of metals and mineral phases with respect to the influence of organic matter and transport modelling in aquifers). Veroffentlichung des Lehrstuhls fur Wasserchemie und der DVGW-Forschungsstelle am Engler-Bunte-Institut der Universitat Karlsruhe; 1995. Huber, L.; Frimmel, F.H. Zum Transportverhalten von Cadmium, Blei und Kupfer in ausgewtilten mineralischen Phasen von Grundwasserleitem (Characterization of the transport of cadmium, lead and copper in selected mineral phases from natural aquifers). Vom Wasser 83: 9-22; 1994. Johannsen, K.; Worth, E. Eine mathematische Methode zur Durchfiihrung von Adsorptionsanalysen (A mathematical method for evaluation of adsorption analysis). Acta hydrochim. hydrobiol. 22(5): 225-230; 1994. KoD, V. Zur Modellierung der Metalladsorption im natiirlichen Sediment-Grundwasser-System (Metal adsorption modelling in natural sediment-groundwater systems). Habilitation; TU Berlin; Fachbereich 2 1; 1992. Marinsky, J.A.; Ephraim, J. A unified physicochemical description of the protonation and metal ion complexation equilibria of natural organic acids (humic and fulvic acids): Analysis of the influence of polyelectrolyte properties on protonation equilibria in ionic media: Fundamental concepts. Environ. Sci. Technol. 20: 345-354; 1986. Perdue, E.M. Modelling the acid-base chemistry of organic acids in laboratory experiments and in freshwaters. In: Perdue, E.M.; Gjessing, E.T., eds. Organic acids in aquatic ecosystems. Chichester: John Wiley & Sons; 1990: 111-126. Radke, C.J.; Prausnitz, J.M. Adsorption of organic solutes from dilute aqueous solution on activated carbon. Int. Eng. Chem. Fund. 11: 445-451; 1972. Sontheimer, H.; Crittenden, J.C.; Summers, R.S. Activated carbon for water treatment. DVGW-Forschungsstelle, Engler-BunteInstitut, Universitgt Karlsruhe; 1988. Stbver, T.; Roennefahrt, K. Arsenentfemung aus Trinkwasser (Arsenic removal from drinking water). Vom Wasser 78: 363376; 1992. Stumm, W. Chemistry of the solid-water interface. New York: John Wiley & Sons; 1992. Tipping, E. Some aspects of the interactions between particulate oxides and aquatic humic substances. Marine Chem. 18: 161169; 1986. Zasoski, R.J.; Burau, R.G. Sorption and sorptive interaction of cadmium and zink on hydrous manganese oxide. Soil Sci. SOC. Am. J. 52: 81-87; 1988.