Interaction between Monovalent and Multivalent

Interaction between Monovalent and Multivalent M etals and Fulvic Acids A Streaming Current Detection Study Michael Weis and Fritz H. Frimmel* Engler-Bunte-Institut, Universität Karlsruhe, Richard-W illstätter-Allee 5, D -7500 Karlsruhe D edicated to Prof. Dr. H. P. Fritz on the occasion o f his 60th birthday Z. Naturforsch. 45b, 8 8 7 -8 9 1 (1990); received December 15, 1989 Fulvic Acids, Metal Com plexation Capacity, Streaming Current D etection The complexation reaction between metal cations o f different valence states (Al, Bi(III), Cr(III), Fe(III); Ca, Co(II), Cu(II), M n(II), N i(II), Pb(II), Zn; N a) and fulvic acids is investi­ gated by the streaming current detection technique. It can be shown that the metal com plexa­ tion capacity does not only depend on the nature o f the cation but also on the origin o f fulvic acids, which were isolated from anaerobic and aerobic landfill leachates.

Introduction

Fulvic acids (FA) are defined as the hydrophilic fraction of highmolecular refractory organic substances, present in most aquatic systems. Ac­ cording to their great significance for ecological processes and their role in water supply there is much interest in their properties. Despite the great effort to investigate the composition of these com ­ pounds very little is known about the chemical structure itself. The sophisticated problem and the state of analytical methods suggest the character­ ization and quantification o f the functionality of the polydisperse organic acids and especially of their reactivity with defined water constituents. Research in the field of the complexation reactions with metal cations has focused on hydrochemical [1], geochemical [2], environmental [3], and ecolog­ ical [4] questions in the past twenty years. Several experimental techniques to investigate the metalFA interaction have been developed for laborato­ ry and field studies [5], Because o f their physicochemical nature most of these m ethods (e. g. voltammetry [6], potentiometry [7], resonance spectroscopy [8 , 9], and fluo­ rescence quench measurement [ 10]) can only be applied to a limited num ber of metal cations. Some techniques (e.g. chrom atography [11], ul­ trafiltration [ 12] and dialysis [13]) are suited for the separation o f complexed and uncomplexed metal

* Reprint requests to Prof. Dr. F. H. Frimmel. Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen 0932-0776/90/0600-0887/$ 01.00/0

ions. However, these m ethods are in general use­ less for studying the metal com plexation reactions. It is beyond doubt that the modeling of environ­ m ental reactions has to include all significant m et­ als present. The objective o f this work includes the applica­ tion of streaming current detection (SCD) [14, 15] for the determ ination of m etal-FA interactions be­ tween metals with different valence state and or­ ganic acid fractions isolated from landfill leach­ ates. Results and Discussion Fulvic material

Fulvic acids were isolated by XAD -adsorption [17] from an anaerobic (BR 6 A IN; April 1987) and an aerobic (BR7 B IN; August 1987) leachate of two independent landfills in Braunschweig, F.R .G . [16]. A description of the isolation procedure and the characteristic data o f the FA samples are re­ ported in refs. [18, 19]. The FA samples in the protonated form were nearly metal free. Table I gives their metal content as DOC(dissolved organic carbon)-normalized metal concentrations determined by atomic ab­ sorption spectrometry. The higher valent metals (Al, Cr, Fe, Mn) and the industrial pollutants (Cr, Cu, Sn, Zn) are dom inant. Size exclusion chrom atography (according to refs. [20-22]) shows that the FA samples have quite a different m olecular weight (MW) distribu­ tion. In Fig. 1 the D O C -track o f the gel permea­ tion chrom atogram is com pared with the UV-absorbance at wavelength X = 254 nm.

Unauthenticated Download Date | 11/17/15 1:40 AM

888

M. W e is -F . H. Frim m el • In teractio n betw een M onovalent an d M ultivalent M etals and Fulvic A cids

Metal concentration [/'g/gD O C ]

Ag

Al

Au

Be

Cd

BR 6 A IN BR 7 B IN

0.20 0.27

19.59 0.11 285.00 0.53

0.01 0.93 9.50 0.04 0.25 3.38

11.00 4.76 25.14 49.67

Metal concentration [//g/g DOC]

Fe

Me

Mn

Ni

Sr

BR 6 A IN BR 7 B IN

70.00 82.00 205.80 22.00

25.25 95.83

9.20 0.79 42.70 0.50 7.01 0.75 69.52 4.00

Pb

Co

Sn

Cr

Cu

Table I. Metal concentration o f FA samples BR 6 A IN and BR 7 B IN normalized to DOC.

Zn

5.59 134.30

nm: 0.72 (0.06) [1/mg DOC-m ]) in comparison to sample BR 7 B IN (absorbance at / = 254 (436) nm: 3.49 (0.21) [1/mg DOC-m ]) parallels the values of the elemental composition (BR 6 A IN: C: 57.88; H: 5.87; N: 2.31; O: 30.40; S: 2.34; ash: 1.20% dry weight [18]; BR N B IN (n = 9) C: 50.49; H: 4.40; N: 3.45; O: 35.60; S: 1.68; ash: 4.33% dry weight; mean value of nine B IN sam ­ ples [23]). All data indicate a higher degree of oxidation and condensation in case o f the sample from the aerobic landfill leachate (BR 7 B IN). M ethod

The operationally defined parameter aPC (an­ ionic Particle Charge) [24] is used for the control of the complexation reaction between metal cations M n+ and macromolecular fulvic acid anions F A X_ (eq. (1)). From the volume o f a cationic polyelec­ trolyte Rm+ consumed for neutralization o f FA species (eq. (2) and (4)) aPC values can be calculat­ ed. The complex formation between fulvic acids and metals is given by eq. (3).

Fig. 1. Nom inal molecular weight distribution o f FA samples BR 6 A IN and BR 7 B IN by synchronous de­ tection o f DOC and UV-absorbance.

Sample BR 6 A IN shows a high portion o f lowmolecular weight substances (82% of the DOC has a MW < 800) with a strong decrease in UV-absorbance (42% with MW < 800), whereas sample BR 7 B IN has a fairly similar molecular weight distribution with a MW > 800 for 78% of the DOC and 79% for the UV-absorbance. The relatively low spectral absorbance o f the sample BR 6 A IN (absorbance at X = 254 (436)

F A -H ^ F A x~ + x H + F A X“ + R m+ =f^ [R -F A ]° with m = x FA*- + yM n+ ^ [ F A - M v](ny-X) [ F A - M J ny~x + Rm+ ^ [ R - F A - M y]° with m = n y - x

(1) (2) (3) (4)

Thus the complexation of metal cations by ful­ vic acids can be determined by a titration with streaming current detection (equivalence point = isoelectrical point). Variations of the FA/M ratio leads to the determination of metal complexation capacities as outlined for Cu(II) in refs. [25, 26]. Complexation reactions

The complexation reaction between a metal ca­ tion and a macromolecular FA cannot be simply

Unauthenticated Download Date | 11/17/15 1:40 AM

M . W e is -F . H . Frim m el ■In teractio n betw een M o n o v alen t an d M ultiv alen t M etals a n d Fulvic A cids

described in classical terms by a single equilibrium constant because of the polyfunctionality of the li­ gand [27, 28]. In addition complexing ligand sites are not discrete as a consequence of the high mo­ lecular weight o f FAs [29]. Therefore titration of FA solutions by metal cations and detection of the m etal-FA interaction lead to plots presenting re­ gions o f complexed metals and of pseudofree ones

aPC

t^ e q /m g

[30-32]. The results of complexation reactions be­ tween BR 6 A IN and BR 7 B IN as polyfunctional ligands and twelve metal cations o f different va­ lence states are shown in Fig. 2 and Fig. 3. The lines are calculated by a twofold linear re­ gression o f the aPC readings in the low and high level o f metal concentration according to the m ethod in ref. [25]. The m arked breakpoint of the

DOC =

DOC]

M "*

889

10 m g /l

[p.m ol/1]

Fig. 2. Titration o f sample BR 6 A IN (D O C = 10 mg/1) by twelve metal cations (total M"+ concen­ tration (/miol/1) plotted vs. results o f aPC determination).

Fig. 3. Titration o f sample BR 7 B IN (D O C = 10 mg/1) by twelve metal cations (total M n+ concen­ tration (/ Cr(III), Pb(II) > Zn, Ni(II), Al > Ca, M n(II) > Na. M CC

Al, Fe(III) > Cu(II), Bi(III) > Cr(III) > Ni(II), Pb(II), Co(II), Ca, Zn > M n(II) => Na. There seem to be two striking differences. Co(II) possesses a surprisingly high sensitivity for FA li­ gand sites, but is poorly complexed. The behav­ iour of Al deduced from these experiments is vice

Fig. 4. Individual aPCcrjt values [/^eq/mg DOC] plotted vs. metal complexation capacities MCC [umol M/mg DOC] o f FA samples.

Unauthenticated Download Date | 11/17/15 1:40 AM

M. W eis—F. H. Frim m el • In teractio n betw een M onovalent an d M ultivalent M etals an d Fulvic Acids

versa. F urther investigations are needed to clarify this phenom enon, especially the different results for Al and Fe(III), which are of special interest for water treatm ent [34, 35]. In this context it has to be mentioned, that streaming current detection is a probe for dissolved as well as colloidal and sus­ pended m etal-FA complexes. Experimental The DO C-concentration of the FA solutions was determined by a UV/DO C-Analyzer DC 80 (D O H R M A N N ). Atomic absorption spectrometry was done with an Atomic Absorption Spectrophotom eter 4000 (PER K IN ELM ER ) using the flame and graphite furnace (H G A 400) modus. For aPC measurements of FA samples (DOC = 10 mg/1) a Streaming Current Detector PCD 02

[1] E. T. Gjessing, “Physical and Chemical Characteris­ tics o f Aquatic H um us”, Ann Arbor Science, Ann Arbor, Michigan (1976). [2] J. H. Reuter and E. M. Perdue, G eochim . Cosmochim. Acta 41, 325 (1977). [3] U . Förstner and G. T. W. W ittmann, Metal Pollu­ tion in the Aquatic Environment. Springer Verlag, Berlin (1981). [4] F. H. Frimmel and R. F. Christman (eds.): Humic Substances and their Role in the Environment, John W iley, Chichester (1988). [5] R. A. Saar and J. H. Weber, Environ. Sei. Technol. 16, 510 A (1982). [6] T. M. Florence, Analyst 111,489(1986). [7] W. T. Bresnahan, C. L. Grant, and J. H. Weber, Anal. Chem. 50, 1675 (1978). [8] J. R. Tuschall and P. L. Brezonik, in R. F. Christ­ man and E. T. Gjessing, Aquatic and Terrestrial Humic Materials, Ann Arbor Science, Ann Arbor, Michigan (1983). [9] N . Senesi, D. F. Bocian, and G. Sposito, J. Am. Soil. Sei. Soc. 49, 114(1985). [10] F. H. Frimmel and W. H opp, Fresenius Z. Anal. Chem. 68, 325(1986). [11] J. Harmsen, Wat. Res. 17, 699(1983). [12] M. R. H offm ann, E. C. Y ost, S. J. Eisenreich, and W. J. Maier, Environ. Sei. Technol. 15, 655 (1981). [13] R. E. Truitt and J. H. Weber, Environ. Sei. Technol. 15, 1204(1981). [14] S. K. Dentel, A. V. Thom as, and K. M. Kingery, Wat. Res. 2 3 ,413(198 9 ). [15] S. K. D entel, A. V. Thom as, and K .M . Kingery, Wat. Res. 2 3 ,423(198 9 ). [16] P. Spillmann, Int. Symp. on Groundwater M anage­ ment, Quantity and Quality, Benidorm, Spain (1989).

891

(M ÜTEK), an A utotitrator DL 25 (M ETTLER) and a Plotter FX 800 (EPSON) were used. Metal solutions were commercially available, acidified standard solutions (M ER C K ) based on chloride anions except of Pb(II) and Bi(III) (ni­ trate anions). Size exclusion chrom atogram s were recorded by on line-detection of DOC (UV/DOC-Analyzer; G R Ä N TZEL) and UV-absorbance at 254 nm (Optical and Control Units; PH A R M A C IA ). The nominal molecular weight units were calibrated by non-humic substances like polyethylene glycols, carbohydrates and alcohols. We thank Jasmin Taherivand and Frieder Hetzel for the aPC determ inations, Brigitte Raue for metal analysis and the Deutsche Forschungsge­ meinschaft, B onn/FR G , for a research grant (FR 536/9).

[17] R. F. C. Mantoura and J. P. Riley, Anal. Chem. 76, 97(1975). [18] F. H. Frimmel and M. Weis, Vom Wasser 71, 255 (1988). [19] M. Weis, G. Abbt-Braun, and F. H. Frimmel, Sei. Total Environ. 81/82, 343 (1989). [20] F. Fuchs, Vom Wasser 64, 129(1985). [21] F. Fuchs, Vom Wasser 65, 93 (1985). [22] F. Fuchs, Vom Wasser 66, 127 (1986). [23] F. H. Frimmel and M. Weis, Int. Assoc. Wat. Poll. Res. Control Conf. K yoto (1990), accepted. [24] M. Weis and F. H. Frimmel, Fresenius Z. Anal. Chem. 334, 664 (1989); 335, 927 (1989). [25] M. Weis, F. S. Valera, and F. H. Frimmel, Z. W as­ ser Abwasser Forsch. 22, 253 (1989). [26] M. Weis and F. H. Frimmel, in M etalspeeiation in the Environment, N ato Advanced Study Institute (1990), in press. [27] D. A. D zom bak, W. Fish, and F. M. M. M orel, En­ viron. Sei. Technol. 20, 669 (1986). [28] W. Fish, D. A. D zom bak, and F. M. M. M orel, En­ viron. Sei. Technol. 20, 676 (1986). [29] E. M. Perdue and C. R. Lytle, Environ. Sei. Tech­ nol. 17,654(1983). [30] D. S. Gam ble, W. A. U nderdown, and C. H. Lang­ ford, Anal. Chem. 52, 1901 (1980). [31] T. A. Neubecker and H. E. Allen, Wat. Res. 17, 1 (1983). [32] I. M. Klotz, Sei. 217, 1247 (1982). [33] M. A. Rashid, “Geochem istry o f Marine Humic Substances”, Springer Verlag, N ew York (1985). [34] T. R. Hundt and C. R. O ’M elia, J. Am. Wat. Works Assoc. 80, 176(1988). [35] S. J. Randtke, J. Am. Wat. Works A ssoc. 80, 40 (1988).

Unauthenticated Download Date | 11/17/15 1:40 AM