Speciation of manganese in fresh water—I

Tolonra,Vol. 34, No. 3, pp. u)7-311, 1987 Printed in Great Britain. All rights rescrvcd

SPECIATION

0039-9140/87 $3.00 + 0.00 Copyright 0 1987 Pcqamon Journals Ltd

OF MANGANESE

IN FRESH WATER-I

USE OF EPR STUDIES BARRY CHISWELL and MAZLIN BIN MOKHTAR Department of Chemistry, University of Queensland, St Lucia, Queensland 4067, Australia (Received 9 July 1986. Accepted 31 October 1986) Summary-The process of standardization of the use of electron paramagnetic resonance (EPR) spectrometry in the determination of Mn(I1) species in aqueous solution by comparison with atomic absorption spectrometry (AAS) is discussed. It is shown that EPR signals obtained from standard&d aqueous solutions of manganese(I1) perchlorate and nitrate in the concentration range of 0.05-2 mg/l. do not vary significantly over the pH range 2.1-7.0. Study of the effect (on the manganese EPR signals) of the addition of inorganic ions commonly found in significant concentrations in natural waters, viz. chloride, sulphate and bicarbonate, has shown that such ions, at the levels reported to occur in surface fresh waters, will not complex manganese(H); however, evidence is obtained that humic acid/manganese complexes could be present in such waters. The EPR signals are not affected by ionic strengths of the levels found in fresh waters.

Much of the work on the speciation of metal ions in natural waters has been done by electrochemical methods.’ Although results from such work have been valuable in the study of manganese species? it is of importance to develop alternative methods of determining the speciation and to compare the results obtained by different techniques. ’ Electrochemical methods invariably require the addition of various electrolyte/buffer solutions, and these may disturb the natural equilibrium of the species in the solution under study. We have therefore applied the non-intrusive analytical technique of EPR spectrometry, which has been used by Carpenter3 and in this laboratory4 for study of the speciation of manganese in sea-water, and assessment of the manganese species present in storage-dam waters.’ We are currently assessing the results obtained by EPR studies with those from anodic-stripping voltammetry, and this paper deals with the methods of establishing EPR spectrometry standards for work to be reported at a later date. Dilute aqueous solutions of simple manganese(I1) salts give a sharp six-peak first-derivative EPR spectrum; the fourth of these peaks from the low-field side of the spectrum is typically used for quantitative work. Three different changes in the parameters of this peak have been used4 to assess manganese(I1) concentration and speciation in solution: (i) change in peak height; (ii) change in peak width; (iii) change in peak area. Earlier workers who have used EPR spectrometry to determine manganese(I1) in aqueous solution do not appear to have studied the possibility of a variation of EPR signal with change of PH. As the signal is sensitive to changes in the ligand field about the manganese(I1) ion, we thought

that a change from pH < 3 to pH 7 in solutions of a simple Mn(I1) salt might lead to EPR changes, as the species [Mn(H20)J2+, which is stable in solutions of pH < 3.5, is hydrolysed to species such as [Mn(H,O),OH]+ and the dihydroxy-bridged [(H20)4 Mn(OH)2Mn(H20)4]2+ as the pH increases to that found in dam waters. The need to clarify this situation is obvious since a particular EPR peak parameter is correlated with manganese concentration by use of solutions which have been standardized by atomic-absorption spectrometry. Such solutions, particularly if prepared by the recommended method of dissolving pure manganese metal in acid, have a very low pH value. Analysis of EPR work on manganese in sea-water has been claimed3 to indicate that the common inorganic anions, chloride, sulphate and bicarbonate, can complex Mn(I1) to a combined extent of -2O%, though there is some debate about the extent of complexation of the metal by these anions in freshwaters, in which both the manganese and anion concentrations are much lower. Work by Turekian’ and Callender and Bowsers indicates that there is some possibility of such complexation, but there is little conclusive evidence. This same situation arises in natural organic acid (humic and fulvic) complexation of manganese in fresh waters, although Gamble et aL9 claim from proton nuclear magnetic resonance work that such complexation is outer-sphere in character, even at its strongest. EXPERIMENTAL Reagents

Manganese atomic-absorption standard solution (1 g/l.) in 2% v/v nitric acid was obtained from Aldrich Chemical 307

308

BARRYCHISWELLand MAZLIN BIN MOKHTAR

Table 1. Normalized fourth EPR peak heights (mm) and AAS signals (arbitrary units) for aqueous manganese solutions at pH 2, 5 and 7

Manganese, mall. 0.050 0.100 0.150 0.200 0.400 0.600 0.800 1.400 2.000

pH 2.1 50.1

pH 5.OiO.l

pH 7.OkO.l

AAS EPR signal peak

AAS signal

AAS signal

EPR neak

0.010 6.3 0.010 6.2 0.028 12.7 0.025 12.7 0.040 20.8 0.038 20.8 0.054 25.5 0.048 25.8 0.108 51.0 0.103 51.0 0.156 73.6 0.150 73.6 0.213 104.0 0.201 105.0 0.366 184.1 0.360 185.7 0.482 252.5 0.478 257.5

EPR neak

0.010 6.3 0.023 12.7 0.033 20.8 0.044 25.0 0.097 51.0 0.143 73.6 0.213 104.0 0.353 177.8 0.479 260.0

Co. Manganese(E) perchlorate @mum p.a.) was from Fluka, Switzerland, as was the humic acid, No. 53680, molecular weight 600-1000. BDH AnalaR perchloric acid was used. Instrumental EPR work was undertaken on a Bruker ER2OOD spectrometer with a microwave frequency of 9.26 + 0.02 GHz. The spectrometer and cell were tuned by adjusting the microwave frequency and iris screw until the dip on the oscilloscope was flat at its minimum and centred on the cross-hair of the oscilloscope screen. After tuning, the microwave power was raised to 100 mW, the detector current brought to 200 PA by turning the iris screw, and the frequency error adjusted to zero. The absence of power-saturation phenomena at 100 mW power was determined. Centre field was then set at 3310 G with a scan range of 375 G each side of centre. The modulation frequency was 100 kHz, and the modulation amplitude was 8 G. The first-derivative scans were made for each sample. The first scan, which covered all six peaks, used a time constant of 200 msec and a sweep time of 200 set, and the second scan, which covered only the fourth peak from the low-field side, had a time constant of 1000 msec and a sweep time of 1000 sec. These latter scans were used for quantitative calculations. The three peak parameters measured were: (i) height of fourth peak; to allow direct comparison of peaks recorded with different gain, the peak-height was normalized by measuring the height in mm, multiplying by lo6 and dividing by the gain. (ii) width of fourth peak; this was obtained as the difference between the maximum and the minimum of the peak scan, which were read from the nmr gaussmeter probe. (iii) area of fourth peak, this was calculated from the Lorentxian equation: area = (peak width)2 x height. All EPR spectra were run at a controlled room temperature of 20 f 2”. The samples were placed in fused-silica tubes (Wilmad Glass Co., U.S.A.). Atomic-absorption measurements for manganese were made with a Varian AA 875 spectrometer, a lamp wavelength of 279.5 nm, and an air/acetylene flame. The pH-measurements were made with a Metrohm 632 pH-meter and a Metrohm 9100 combined glass electrode. Procedures Variation of EPR and AAS signals with PH. A manganese(H) perchlorate stock solution of manganese concentration N 100 mg/l. was prepared by dissolving 0.0740 g of Mn(ClO,), .6H,O in 100 ml of distilled water and filtering through a 0.45~pm membrane filter. The concentration of this stock solution was determined at pH 7 and pH 2 (after acidification with perchloric acid) by atomic-absorption with calibration by means of standards prepared from the Aldrich l-g/l. stock standard manganese solution. The

absorption signal of the perchlorate stock solution at both pH values was the same (within experimental error, see Table 1). Suitable dilution of the stock solution yielded 250-m] portions of Mn(ClO,), solutions containing 0.050, 0.100, 0.150, 0.200, 0.400, 0.600, 0.800, 1.400 and 2.000 mg/l. manganese at pH 7.0 f 0.1. These solutions were adjusted to pH 5.0 f 0.1 and 2.1 f 0.1 with perchloric acid. Atomic-absorption measurements and EPR spectra of all solutions were obtained on the day of their preparation, so that the instrumental conditions were not altered during the measurements. EPR signal and ionic strength. A manganese(I1) nitrate solution of manganese concentration 0.200 mg/l. at pH 3.0 + 0.1 was prepared from the Aldrich atomic-absorption standard solution, and a manganese(I1) perchlorate solution of the same concentration was obtained by dilution of the stock perchlorate solution and adjusted to the same pH with perchloric acid. The sodium perchlorate and nitrate solutions used for ionic strength adjustment were obtained by dilution of 100 g/l. stock solutions of the analytical grade salts in distilled water. EPR signal and anionic complexing agents. The manganese(I1) perchlorate stock solution was diluted and mixed with solutions of analytical grade sodium chloride, sulphate, or bicarbonate or of humic acid, to yield the required manganese concentrations and ionic strengths for the solutions studied by EPR. RESULTS AND

DISCUSSION

Variation of AAS and EPR signals with change in pH

The change of atomic-absorption signal with manganese concentration at pH 2.1 is linear up to about 1.4 mg/l., and then shows slight convexity up to 2.0 mg/l.; the perchlorate and nitrate solutions yield identical signals for the same concentration of manganese. The variation of the height of the fourth EPR peak with manganese concentration at pH 2.1 is linear over the range 0.05-10.0 mg/l. at pH 2.1; again there is no significant difference between the signals from the perchlorate and the nitrate for the same concentrations of manganese. Plots of the peak area and peak width of the fourth EPR signal against manganese concentration over the range 0.05-10.0 mg/l. for solutions of manganese(I1) nitrate at pH 2.1 show that the peak width is constant and that the peak area is a linear function of manganese concentration over the range tested. Carpenter’ has already indicated that for the concentrations of manganese of interest in our work on fresh waters (normally <2 mg/l.), there is no association of pet-chlorate and metal ion; the EPR spectrum at low pH values is attributable to the [Mn(H,0),12+ ion alone, with the perchlorate anion having no effect on the spectrum. This claim is substantiated by our observation (reported above), that the EPR peak heights of manganese(I1) nitrate solutions at pH 2.1 are the same as those for the same concentrations of manganese(I1) pet-chlorate. The AAS signals and EPR peak heights for various concentrations of manganese at three different pH values are summarized in Table 1. Each value is the mean of three readings. The EPR peak height varies

309

Speciation of manganese in fresh water-1 Table 2. Normalized fourth EPR peak heights (mm) for solutions containing 0.208 mg/l. manganese and varying amounts of nitrate and perchlorate ions (PH 3.0 f O.l)* Nitrate Added anion concn., g/l. 0

12.5 25.0 50.0 75.0 100.0

Perchlorate

It, M

Peak height

0, M

Peak height

1.09 x lo-’ 0.176 0.352 0.704 1.056 1.408

27.6 21.6 27.3 24.5 22.6 19.8

1.09 x 10-S 0.126 0.251 0.503 0.754 1.006

30.1 30.1 30.1 30.7 31.0 31.0

*Note that the EPR peak height for a given concentration of manganese must be obtained by standardization each time the spectrometer is turned on. The nitrate and perchlorate values were obtained on different days. TIonic strength.

Table 3. Normalized fourth EPR peak heights (mm) for various concentrations of manganese(B) perchlorate* at pH 6.9 + 0.2 with addition of various amounts of sodium chloride

Table 5. Normalized fourth EPR peak heights (mm) for various concentrations of manganese(B) perchlorate* at pH 7.0 f 0.2 with addition of various amounts of sodium bicarbonate

Peak height [Cl- I, mg/l.

A

B

0 50 500 2000 4000 6000

9.9 9.9 9.9 8.7 7.7

108.0 108.0 108.0 105.2 103.4 103.4

*[Mc”‘;‘s, 6rq/l.-A,

Peak height C

6.04 x 5.96 x 5.88 x 5.68 x 5.48 x

10’ lo3 lo3 lo3 10’

0.083; B, 0.880;

‘“GZ]~

A

B

0 50 500 1000 1900

8.9 8.9 7.2 6.2 3.6

109.9 106.8 101.2 84.9 76.4

*w;*;l;

,

7;g/l;-A,

C 6.55 x 6.50 x 5.22 x 4.85 x 3.35 x

lo3 lo3 IO3 10’ lo3

0.076; B, 0.895;

. .

3 .

by no more than 3% with change in pH from 7.0 to 2.1. On the other hand, the AAS signals vary significantly with pH for manganese concentrations below 1.4 mg/l., the greatest decrease in signal intensity being obtained for the less concentrated solutions at the highest pH. We can find no reference in the literature to such a decrease in AAS signal for manganese with increase in solution pH. However, SinghI has reported that in the concentration range 2-6 mg/l., cadmium in soil Table 4. Normalized fourth EPR Peak heights (mm) for various concentrations of manganese(R) perchlorate* at pH 6.9 + 0.2 with addition of various amounts of sodium sulphate Peak height [Z? 0 50 500 2000 4000 6000

A

B

8.8 8.7 7.9 6.4 5.5 -

122.2 120.7 113.7 96.2 76.2 62.7

*[M$+], mg/l.-A, c, 51.15.

C 6.35 6.26 6.10 4.86 3.90 3.42

x x x x x x

10’ lo3 10’ 10’ 10) 10’

0.075; B, 0.992;

samples produces a lower atomic-absorption signal in non-acidified solution than it does in acid solution, the reduction in signal varying from 18.2% at the lower concentration to 3.6% at the higher. He sug-

gests that, at higher pH, hydrous cadmium oxides adhere to the atomizer surface, thus yielding low analytical results. Cresser and Ha&t” have reported a similar effect for chromium(II1) solutions, which at

Table 6. Normalized fourth EPR peak heights (mm) for various concentrations of manganese(I1) perchlorate’ at neutral pH with addition of various amounts of humic acid Peak height [Humic acid], mgll. 0 1 5 10 20 50 100 200 500

A

B

5.9 106.8 4.2 l.O0.0 67.8 47.4 16.6 5.7 ---

C 6.68 x 6.15 x 5.84 x 5.55 x 3.52 x

lo3

10’ lo3 10’ lo3

*mn2+], mg/l.-A, 0.051 (pH 7.5 kO.3); B, 0.853 @H 6.8 fO.l); C, 53.79 @H 6.9 f 0.1).

310

BARRYCHISWELLand MAZLIN BIN MOKHTAR

,K

6000 k

I

I 40

Ethanol (Xv/v)

80

120

Acetone (%v/v)

Fig. 1. Normalized fourth EPR peak height vs. percentage volume of ethanol for manganese concentration of 50 mg/l.

Fig. 2. Normalized fourth EPR peak height vs. percentage volume of acetone for manganese concentration of 50 mg/l.

pH >6 give low absorbance readings if allowed to stand. They attribute this to deposition of the hydrous metal oxide. Other workers’2~13 have connected reduction in absorbance-signal for zinc, as the pH is increased, with adsorption of hydrous zinc oxides on the container walls, but some of these results have been disputed. I4 Lowering of the absorbance signal with increase in pH has also been noted by Danielson et ~1.‘~ in their work on calcium and magnesium in serum. If it is accepted that the lowering of the absorbance signal with increase in pH, in our work, is due to the production of hydrous metal oxide adsorbed on the vessel walls, it is of interest that the EPR signal does not change with increase in pH. This suggests that no oxidation occurs, as this would lower the EPR signal, which derives from the Mn(I1) species.

such anions are found in fresh waters, such as those in the water storage dams which we are studying in the vicinity of Brisbane, which have chloride and sulphate concentrations of -30 and 5 mg/l., respectively. The results also indicate that the presence of large amounts of chloride, sulphate and bicarbonate are required at neutral pH to change the EPR spectrum of manganese. As might be expected from the work of Callender and Bowser,* sulphate is the anion most likely to form complexes (Table 4). By comparison, the addition of humic acid to aqueous manganese perchlorate solutions produces a major change in EPR peak height (Table 6). Although our studies on dam storage waters to date support the contention of Gamble9 that humic acid complexation of manganese in fresh waters does not appear to be strong, its possibility is clearly demonstrated by these results. The disappearance of all six EPR peaks of manganese solutions on addition of sufficient humic acid is similar to the results obtained when aqueous solutions of manganese perchlorate are made up with progressively increasing amounts of ethanol or acetone. Figures 1 and 2 demonstrate the decrease in signal as the water molecules around the manganese(I1) ion are replaced by organic solvent molecules.

Variation of EPR signal with change in ionic strength Previous workers3 studying speciation of manganese in sea-water by use of EPR spectrometry have found it necessary to adjust the ionic strength of the test solutions. However, Miller0 and Schreiber’6 have estimated that river water has an ionic strength of 2.09 x 10m3m whereas that of sea-water’ is 7.23 x lo-‘m, and we have found that for solutions of manganese(I1) nitrate and perchlorate containing 0.200 mg/l. manganese at pH 3.0 + 0.1, the addition of up to 25 g/l. nitrate (as sodium nitrate) or perchlorate (as sodium perchlorate) produces no change in the EPR signal (Table 2). Although higher concentrations of either salt do lead to a change in the EPR peak signal, such concentrations are vastly in excess of those in freshwaters, and we conclude that ionic strength adjustments are not required for use of the EPR spectrum of [Mn(H,0)6]2+ as a manganese concentration probe for fresh waters. Variation of EPR signal with addition of possible complexing anions Tables 3-5 indicate that the presence of inorganic anions capable of forming complexes with manganese has no appreciable effect on the EPR peak height (or width) at neutral pH, and at the levels at which

Acknowledgement--The authors would like to thank the Queensland Department of Local Government for financial support of this work. REFERENCES

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12. B. V. L’vov, N. A. Orlov and E. K. Mandrazhi, Zh. Analit. Khim., 1980, 35, 894. 13. A. E. Dong, Appl. Spectrosc., 1973, 27, 124. 13. G. E. Bentley and M. L. Parsons, ibid., 1974, 28, 71. 14. B. G. Danielson, E. Pallin and M. Sohtell, Upsafa .I. Med. Sci., 1982, 87, 43. 16. F. J. Miller0 and D. R. Schreiber, Am. J. Sci., 1982,282, 1508.