Comparison of methods for the determination of conditional stability constants of heavy metal-fulvic acid complexes

Water Res. Vo{. 18, No. 9, pp. 1149-1153, 1984 Printed in Great Britain. All rights reserved

0043-1354 84 $3.00~0.00 Copyright ~ 1984 Pergamon Press Ltd

COMPARISON OF METHODS FOR THE D E T E R M I N A T I O N OF C O N D I T I O N A L STABILITY CONSTANTS OF HEAVY M E T A L - F U L V I C ACID COMPLEXES R. M. STERRITT and J. N. LEsrER* Public Health Engineering Laboratory, Imperial College, London SW7 2BU, England

(Received August 1983) Abstract--Conditional stability constants and complexation capacities of complexes between fulvic acid and lead, cadmium and copper have been determined by ion selective electrodes, dialysis and differential pulse anodic stripping voltammetry. Most information was obtained from the ion selective electrodes which indicated that all three metals formed two types of complex with the fulvic acid. Weak cadmium complexes were undetectable when dialysis was used to separate bound and free metal. Only the stronger complexes of copper and cadmium were detected by polarography. The stability constant of the copper complex determined by polarograpby was lower than the equivalent values obtained from the other techniques, suggesting partial lability, while the lead complex was completely undetectable.

Key words~heavy metals, fulvic acid, conditional stability constants, complexation capacities, ion selective electrodes, dialysis, polarography, titration

INTRODUCTION

and the stability constant, K, is given by

There is an increasing awareness that the environmental impact of toxic heavy metals depends to a significant extent on their physicochemical forms. In natural waters, the relationship between speciation and toxicity is probably the most common reason for studying speciation (Neubecker and Allen, 1983), since the phenomena of complexation, precipitation and adsorption onto solids generally reduce the toxic effects of heavy metal ions (Allen et al., 1980). However, in other matrices, speciation studies have been conducted with greater emphasis on the partition of heavy metals into various forms which control their mobility and environmental dispersion. This is of particular importance in wastewater treatment processes since sewage effluents and sewage sludge applied to land containing high concentrations of heavy metals can have adverse effects on surface water quality and on the productivity of agricultural land (Lester, 1983; Lester et aL, 1983). In wastewater matrices, the speciation of several heavy metals is dominated by organic complexes (Bender et al., 1970; Patterson, 1979; Sterritt and Lester, 1984) and very little free metal ion is detectable (Cantwell et al., 1982; Bender et al., 1970). Thus the characterisation of organic complex formation is an important aspect of speciation in such matrices. Heavy metal complexation may be represented by the equilibrium relationship: a M + bL ~ MaL h

K = [M~Lh]/[M]a[L] h.

(2)

Values for the conditional stability constant (K'), for a particular set of defined experimental conditions of pH and ionic strength, and total ligand concentration ([L']) may be determined experimentally by titrating a ligand with metal ions using an analytical method to distinguish free metal (MF or M "+) from complexed metal (MB). For complexes where a = b = 1 the straight line relationship derived by Ruzic (1982) may be used to obtain K' and [L']: [MF]/[MB] = [Mv]/[L'] + 1/K'[L'].

(3)

This theoretical treatment has been extended to cover 2:1 complex formation and systems containing more than one ligand (Ruzic, 1982). An alternative method of interpretation (Shuman and Woodward, 1977) utilises a plot of analytical response during the titration (e.g. peak height, peak current) against added metal ([Mr]). [L'] is obtained directly from the breakpoint in the curve and the analytical response (~t) is quantified from the slope (x) of the region of the titration curve where [M-r] > [L'] such that :t = x [M v]-

(4)

For values of x and [Mr] in the region of the titration curve where [M-r] < [L'] a graphical solution for K" may be obtained from the approximation (Shuman and Woodward, 1977):

(1)

~t = [Mr]/K'([L' ] - [Mr]).

(5)

Equation (5) expands to: *To whom all correspondence should be addressed.

[My] = ([My] + [Me])/K'[L']- ([MF] + [MB]) 1149

(6)

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R . M . SrERRrrT and J. N. LESTER

Taking reciprocals, adding K ' to each side and rearranging yields: [My] ([My] + [Ms]) = [My] [L'] + 1 K ' [ L ' ]

(7)

which approximates to equation (3) if [Mv] '~ [Ms]. A number of physical separation, charge specific or electrochemical techniques exist for the differentiation of [MB] or [Mv] (Neubecker and Allen, 1983; Florence. 1982) but in complex matrices poor analytical selectivity for the species of interest matrix interferences or complex lability due to the analytical conditions used may lead to errors in the determination of the complexation parameters (Sterritt and Lester, 1984: Neubecker and Allen, 1983). The work reported here was undertaken in order to compare values of K ' and [L'] for complex formation between fulvic acid and cadmium, lead and copper, in order to assess the influence of interference and selectivity of the analytical methods employed on the results obtained. MATERIALS AND METHODS

Fuh'ic acid Fulvic acid was extracted from a sample of anaerobically digested sewage sludge using the method of Holtzclaw et al. (1976). Heavy metals were removed from the preparation by batching with Chelex-100 in the sodium lbrm (Biorad Laboratories. U.K.). The high molecular weight fraction was isolated by dialysis in cellophane visking tubing (Medicell International, London) for 48h against four changes of distilled water.

Complexation titrations Rather than use a single sample to be titrated against increasing concentrations of heavy metal, which would have involved withdrawal of multiple aliquots for analysis at each successive addition, or more importantly in the case of polarographic analysis, repeated plating and stripping on the same sample with the attendant risks of compromising the sample integrity or the recurrence of irreversible adsorption, 15 subsamples of a fulvic acid solution were used, each containing a single metal addition, to give a range of total metal concentrations from 0.0l to 25 x 10 -5 M. The samples were adjusted to pH 6.5 although no pH buffers were used. Concentrated, unacidified solutions of copper, cadmium and lead as the nitrate salts were used to make small additions to the fulvic acid solutions, which after checking the pH, were allowed to equilibrate for 24 h prior to analysis. All samples contained 0.01 M potassium nitrate to maintain constant ionic strength. All experiments were conducted at 22-25~C.

Ion selectice electrode (ISE) determinations All ISE determinations were conducted using sample solutions of 100ml which were stirred at a constant rate using a magnetic stirrer with a PTFE-coated follower. Free C u " , Cd-'" and Pb:" were determined using Orion electrodes models 94-29A, 94-48A and 94-82 respectively in conjunction with an Orion model 90-02 double junction reference electrode and an Orion 701A digital millivolt meter. Calibration was performed with 10 -5, 10 -b and 10-TM unacidified metal solutions. The logarithmic response of the electrodes was taken into account when plotting analytical response against [M-r].

Dialysis The dialysis technique used was similar to that of Truitt and Webber ( 1981 ). Cellophane visking tubing dialysis bags containing 10 ml of 0.01 M potassium nitrate were placed in

150ml metal fulvic acid solutions. After equilibration the internal and external solutions were taken and analysed by flameless atomic absorption spectrometr? for [MF] and [MT] respectively. The flameless atomic absorption conditions used were those ~ven by Carrondo et al. (1979) for sewage effluent samples.

Polarographic analysis Differential pulse stripping was conducted using a Princeton Applied Research Corporation (PAR) model 303 Static Mercury Drop Electrode incorporating a silver/silver chloride reference electrode and a PTFE-coated platinum counter electrode. This was used in conjunction with a PAR model 384 polarographic Analyser and a Houston Instruments model DMP-2G Digital Plotter. The analysis was conducted using argon as a purge gas with an initial potential of -0.950 V. a final potential of +0.100 V, at a scan rate of 2 mV s -~ and a pulse height of 50 inV. Purge, deposition and equilibration times were 500, 60 and 30 s respectively. Automatic blank subtraction and tangent fit were employed.

RESULTS

K" and [L'] o f fidvic acid-heavy metal complexes Determinations of [My] and [MB] obtained from dialysis and ISE determinations were used to obtain K ' and [L'] from the application of equation (3) which was solved graphically from plots of [Mv]/[M~] vs [Mv], the resultant straight lines having slopes of I/[L'] and intercepts of I/K[L']. Values o f [ U ] for the polarographic analysis were obtained from plots of peak current (ip) against [Mv]. Plots of ip against [ M r ] / ( [ L ' ] - [MT]) gave straight lines of slope I/K'. Additionally, values of K ' and [L'] from dialysis and ISE determinations were obtained from similar plots of atomic absorption peak height for [Mv] or ISE response against [Mr] and from the substitution of these values into equation (5). Values of K ' and [L'] for each metal, utilising each analytical approach interpreted by equations (3) and (5) are shown in Table 1. In general, these results suggested that two distinct types of complex were formed by each metal, the second having a conditional stability constant approximately one order of magnitude lower than the first. This was particularly evident from the ISE determinations which indicated that copper and lead formed complexes of similar stability and that values for the two distinct binding sites ([L'd and [L',]) were similar in each case, possibly suggesting that these two metals were associated with the same binding sites. Cadmium, however, appeared to form weaker complexes and the concentrations of binding sites were significantly larger than those which complexed lead and copper. Equations (3) and (5) yielded similar results for the ISE determinations, but not for the dialysis titrations. In the case o f copper the application of equation (3) to the dialysis titration yielded similar results to the ISE determinations, but no complexation was detectable from the plot of analytical response against [Mr]. No complex was detectable from the dialysis titration of cadmium. The

Metal-fulvic acid stability constants

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Table 1. Conditionalstabilityconstantsand complexationcapacitiesof heavymetal-fulvic acid complexes determined by ion-selectiveelectrodes, dialysis and differential pulse stripping Analytical [LI] [LJ Metal method log K~ log K'., ( x 10-s M) Cu ISE Equation (3) 6.5 5. I 0.48 2.2 Equation (5) 7.0 5.4 0.25 ".2 Dialysis Equation (3) 6.3 4.9 0.38 7.4 Equation (5) ND ND ND ND DPASV Equation (5) 5.5 ND 0.35 ND Cd ISE Equation (3) 4.7 3.8 2.5 10.0 Equation (5) 5.2 ND 1.5 ND Dialysis Equation (3) ND ND ND ND Equation (5) ND ND ND ND DPASV Equation (5) 5.6 ND 1.0 ND Pb ISE Equation (3) 6.3 5.4 0.23 2.9 Equation (5) 6.3 4.9 0.55 5.3 Dialysis Equation (3) 6.9 5.6 0.64 4.4 Equation (5) 5.4 ND 0.84 ND DPASV Equation (5) ND ND ND ND

dialysis titration gave a greater scatter of data than the other techniques, and because of the low stability constants of the cadmium-fulvic acid complexes, a breakpoint in the plot of analytical response against [Cdr] could not reliably be observed. There are several significant phenomena associated with the polarographic analysis. In the case of cadmium, the data for K~ and [L~] were similar to the values obtained by ISE. No secondary complex was detected, but this was probably too weak to produce a recognisable breakpoint. In the case of copper, the value of [L~] obtained by DPASV was very close to the equivalent data generated by the other techniques, but the magnitude of K ' was significantly lower, suggesting that although the same binding site was being titrated, in effect, partial dissociation of the complex occurred as a result of the analytical conditions used. Again, no secondary complex was detected. The DPASV analysis of lead complexation revealed no complex formation at all, and in this respect, the complex would normally be described as "labile".

DISCUSSION The approximation in equation (5) used by Shuman and Woodward (1977) when applied to ISE data gave results comparable with those obtained by the application of equation (3), but was not applicable to data from the dialysis determinations. Equation (5) is only applicable to the determination of [L'] if the product K'[L'] is greater than unity (Shuman and Woodward, 1977). However, if K'[L'] > 103, [L'] is easily determined, but K" is not. In the work reported here, although an appropriate fulvic acid concentration was chosen at the outset, values of K'[L'] ranged from approx. 0.6 to 20. Since the artificial preconcentration of samples to obtain a practicable working range is generally considered undesirable (Means et al., 1980) the application of a technique which has an intrinsically narrow effective "window" of K'[L'] values may be difficult in heterogenous samples where the range of such values may be wider than that reported here.

Effect o f successire reductions on the polarographic response to copper Repetitions of the analysis undertaken on the same sample indicated that during plating and stripping the integrity of the sample speciation was possibly compromised. An example of this phenomenon is shown in Fig. 1, which shows the polarograms obtained from two successive analyses of a fulvic acid solution containing 5 x 10-rM copper. The peak at - 0 . 2 8 V may have been due to a copper species, in addition to the normal copper peak at - 0 . 0 6 V, since the former peak was not evident from the polarographic analyses of cadmium and lead complexes. Repetition of the analysis resulted in a 20% decrease in the peak at - 0 . 2 8 V and a 45% decrease in the normal copper peak. While this phenomenon remained unexplained, it may have been a manifestation of a combination of irreversible adsorption and partial lability of the complex.

~2

'.,3

-0.4

-0.2

0

Potential (V)

Fig. 1. Effect of successive polarographic analyses on the response to copper in a fulvic acid solution. First reduction .; second reduction - - -

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R.M. S'rERRITTand J. N. LESTER

The use of equation (5) places constraints on the derivation of K ' and [L'], whereas, because it contains no approximations and selection of the most appropriate region of the titration curve may be made (Ruzic, 1982), the precision of data obtained from equation (3) is dependent only on the precision of the analytical technique itself. As far as can be ascertained in a study where no standard technique exists, the ISE analysis appeared to give the most reliable results in terms of applicability and sensitivity. However, in the application of ISEs to matrices of a different composition, interferences due to enhanced electrode response to charged complexes (Barica, 1978) or the development of electrode potentials due to eomplexation of the electrode membrane itself by ligands in solution (EI-Taras and Pungor, 1976) have been observed. Interferences of this type have not, however, been fully characterised in environmental matrices. The dialysis technique, which may be expected to be less susceptible to interferences due to its physical separation characteristics, provided values of K ' and [L'] for the copper complex which were similar to equivalent values found by ISE. However, no cadmium-fulvic acid complex was detectable by this method, indicating that [MF] and [Mr] were effectively the same. The limitations on the determination of conditional stability constants for weak complexes are illustrated in the following example. For an arbitrary value of [L'] of 10-~M and [Mr] = 5 x 10-6M, if only a 5~o difference between MF- and Mr were detected then substitution of these values into equation (3) would yield a value of K ' of approx. 5 x 103, which probably represents the lower limit of accurate determination for many analytical methods. Several workers have observed the phenomenon of polarographic lability of heavy metal complexes (Tuschall and Brezonik, 1981; Nurnberg et aL, 1976). However, the factors which control lability have not been evaluated fully, although the kinetics of complex dissociation and steric factors (Shuman and Woodward, 1974) are thought to be important. For routine analysis, the use of a single plating potential for the determination of the labile responses of several metals may be considered advantageous, but may affect the lability of each of them to a different extent. Although this can be avoided to an extent by selecting a reduction potential just sufficient to reduce only the free metal (Nurnberg et al., 1976), if partial lability of the complex occurs, then it may not be possible to detect this. It seems probable that the polar0graphic response to copper was due to a single, partially labile complex, rather than a labile and a non labile complex, because the value of [L'] was close to the equivalent values determined by the other two methods. The lead complex was effectively completely labile, while the cadmium complex appeared non labile, despite it being much weaker. In the interpretation of the polarographic data for

the copper complex, the peaks at - 0 . 2 8 V were ignored, since these were not typical of copper stripping peaks, and the analytical response was taken to be the peak current at - 0 , 0 6 V obtained during the first analysis of the sample. Peak current measurements as a function of [Cur] taken from subsequent analyses would have generated a significantly different titration curve: it would appear therefore that single analyses of separate aliquots of a sample each containing a different concentration of metal could yield more reliable results than repeated stripping analysis of a single sample to which successive additions of titrant are made.

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