Wat. Res. Vol. 25, No. 5, pp. 617-620, 1991 0043-1354/91 $3.00 +0.00 Printed in Great Britain Pergamon Press plc

TECHNICAL NOTE

A RAPID PRECONCENTRATION METHOD FOR MULTIELEMENT ANALYSIS OF NATURAL

FRESHWATERS

PER ANDERSSON* and JOHAN INGRIt Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden

(First received June 1990; accepted in revised form October 1990)

Abstract--This study describes an inexpensive and rapid preconcentration method which can be applied directly in the field. It is based on coprecipitation with magnesium hydroxide and has been applied for A1, Fe, Mn, Zn, Cu, Ni, V, Cr, Co, Cd, Be, Y, Sc and Yb in freshwaters.

The method has been tested using inductively coupled plasma-atomic emission spectrometry (ICP-AES) by adding known amounts of metals to distilled water and natural freshwater. The detection limit for ICP-AES can be enhanced more than two orders of magnitude for Al, Y, Sc, Yb and approximately one order of magnitude for the other tested elements.

Key words--coprecipitafion, fresh water, hydrogeochemistry, metals, emission spectrometry

INTRODUCTION

The concentrations of metals in most freshwater systems are generally too low to be determined directly by inductively coupled plasma-atomic emission spectrometry (ICP-AES). In order to in- crease element concentrations various pre-treatment methods have been developed, such as ion exchange, liquid-liquid extraction, osmosis and coprecipitation. However, most of these methods are applicable for brackish- or seawater and not intended for field work.

Coprecipitation using Mg(OH)2 as a carrier has been used by Koroleff (1947) for Mn determination in brackish water. The method was further developed by Koroleff (1980) and Bostr6m and Bostr6m (1991) for metal determination in brackish water and sea- water. Buchanan and Hannaker (1984) applied the Mg(OH)e coprecipitation method for multielement determination in concentrated brines. Other reported coprecipitation methods for metal analysis of water include aluminium hydroxide (Hiraide et al., 1976), indium hydroxide (Hiraide et al., 1980) and borhy- dride reduction (Skogerboe et al., 1985).

In this paper we report experience and reliability of a simple, cheap and fast field coprecipitation method, preconcentrating metals in natural freshwater. The method is based on formation of a Mg(OH):-precipi- tate by adding MgSO4 to the sample and adjust the pH with NaOH to about 10-11. Direct addition of chemicals in the field minimize metal adsorption on

*Author to whom all correspondence should be addressed. tPresent address: Department of Economic Geology, Lule~

University of Technology, S-951 87 Lule~i, Sweden.

the walls of the sample bottle and the enrichment of elements in the precipitate reduces the contamination risk during ensuing laboratory handling.

REAGENTS AND PROCEDURE

All laboratory and field sampling equipment was acid cleaned at least 24 h in 3 M HNO 3 and rinsed several times with distilled water. The water was distilled in high quality glass facilities and deionized through a SILEX ® column. All used chemicals, 0.5 M NaOH, 0.32 M MgSO4 and 3 M HNO 3 were Merck ® analytical grade. In order to reduce contamination from the chemicals, the NaOH and MgSO4 were purified by adding 10 mi MgSO4 to 1 litre of the NaOH solution and 10 ml of the NaOH to 0.5 litres of the MgSO4 solution. The formed precipitate was allowed to settle for 24 h and the clear supernatant was decanted and transferred to clean bottles, while the Mg(OH) 2 was thrown away. This procedure was repeated twice.

Metal standard stock solutions (1.000gl -~) of A1, Fe, Mn, Zn, Cu, Ni, V, Cr, Co, Cd and Be were made from Titrisol ® ampoules, while in case of Y, Yb and Sc Alfa Products ® standard solutions were used.

One litre of freshwater was sampled in a polyethylene bottle and I0 ml MgSO 4 and I0 ml NaOH was added. The bottle was sealed and thoroughly shaken. The pH in the solution increased to about 10.5 and flocs were formed. This solution was allowed to age at least 24 h while the flocs settles. The clear supernatant was removed by careful siphoning. The precipitate in the remaining solution, about 150mi, was centrifuged (3000rpm for 15min) twice in a 100 ml polypropylene centrifuge tube and the clear super- natant solution gently removed. The precipitate was dis- solved in the tube with 5 mi 3 M HNO3.

The colour of the precipitate reflects presence of organic matter in the sample. White colour indicates low organic content and a more yeliow-brown nuance indicates a higher organic content. To oxidize the organic matter 3-5 drops 30% H202 was added. The mixture was heated on a water bath at 95°C for about 2h. After this treatment the sample was completely dissolved and usually colourless. The

617

618 Technical Note

sample was transferred to a 10ml volumetric flask and, after cooling, diluted to volume with distilled water. The hundredfold preconcentrated sample w a s r e a d y for analysis.

Blanks were made from distilled water treated as the samples, however, the distilled water was treated twice. The first precipitate was to clear the water and not used as a blank, while the second precipitate was dissolved in 3 M HNO3. Multielement calibration solutions for the ICP-AES were prepared by dilution of the stock solutions and main- taining the same HNO3 concentration as in the dissolved samples. Metal concentrations were determined by ICP- AES using an ARL ® (Applied Research Laboratories) equipment with operating conditions as shown in Table 1.

METHOD TEST

The procedure was tested by use of different types of test solutions:

(a) natural freshwater; (b) spiked natural freshwater; (c) synthetic solutions added to distilled water;

and (d) the certified reference material IAEA/W-4

(International Atomic Energy Agency).

The pH of the synthetic solutions were lower (pH 3) than most natural waters due to the acid standard stock solutions. However, when adding precipitation reagents the pH increased to 10.4 which was sufficient for formation o f Mg(OH)2 precipitate.

The natural freshwaters were sampled twice, June 1988 and August 1988 from a small oligotrophic lake (Lake Mj6sj6n) in central Sweden. Samples were taken from five depths 0 - 6 . 5 m depth). The lake water had a pH of 5.8-6.2 and an alkalinity of 0.04-4). 1 mmol 1- l as HCO~-. All samples were filtered through a 0.45 # m membrane filter (142 mm diam., Schleicher & Schiill ®) in the field. In the laboratory, the pH of the samples, after reagent addition, were recorded with a glass combinat ion electrode and was found to be around 10.5. Samples for direct measure- ment without coprecipitation were taken at each level and preserved by adding concentrated (14 M) HNO3 to pH about I.

In June 1988, after spring overturn, the concen- trations of A1, Fe and Mn in the whole lake water column were sufficiently high to be measured without preconcentration by ICP-AES. This provided oppor- tunity to compare direct determination with the

Table 1. ICP-AES instrumental operating conditions

Spectrometer ARL 3520 sequence reading; Paschen-Runge mounting; Rowland circle diameter 1 m; grating 1080 lines mm-~; primary slit 20/~m; spectral range 336-798 nm

ICP source ARL inductively coupled plasma Henry genera- tor operating power 1.2 kW, 27.1 MHz

Argon flow Coolant 121 min -I, plasma 0.81 min -t, sample l l rain-1

Nebulizer Meinhard concentric glass nebulizer uptake rate about 2 ml rain-

Spectral lines: element, wavelength in nm, spectral order in parenthe- ses. AI 167.08 (IV), Fe 259,94 (II), Mn 257.61 (II), Zn 213.86 (IIl), Cu 324.75 (II), Ni 231.60 (ID, V 290.88 (If), Cr 267.72 (II), Co 228.62 (liD, Cd 226.50 (Ill), Be 313.04 (II), Y 360.07 (II), Sc 361.38 (IX), Yb 328.94 (ll)

Table 2. Elemental analyses of spiked distilled water

Element Added Found Recovery SE n Blank (%)

AI 100 92 92 1 5 0 5.0 4.6 92 0.3 5 0

Fe 100 93 93 1 5 1 Mn 50.0 43.0 86 0.3 5 0.2

0.5 0.43 86 0.01 5 0.2 Zn 10.0 8.2 82 0.3 5 0.2

3.0 2.7 90 0.1 5 0.2 Cu 10.0 7.8 78 0.2 5 0.3 Ni 10.0 7.5 75 0.1 5 1 V 10.0 7.2 72 0.1 5 1.5 Cr 10.0 7.8 78 0.1 5 1.5 Co 10.0 7.6 76 0.1 5 0.5 Cd 10.0 8.0 80 0.1 5 0.2 Be 0.10 0.08 80 0.001 5 0.01 Y 0.50 0.46 92 0.01 5 0.03 Sc 0.10 0.09 90 0.002 5 0.0! Yb 0.05 0.05 100 0.001 5 0.01

Concentrations in #g 1 -~. n, number of measured samples. SE, standard error of the arithmetic mean.

coprecipitation method. In August 1988, when sum- mer stratification was established, the surface waters were depleted of most elements. Two 1 litre bottles were therefore sampled at each level for the precon- centration procedure. One of the bottles was spiked with the elements listed in Table 2.

The certified reference material IAEA/W-4 is a simulated freshwater with certified values for trace elements. However, the metal concentrations in IAEA/W-4 are on a higher level than most natural freshwaters in Sweden. The IAEA/W-4 was analysed with ICP-AES by the coprecipitation method using 1 litre and also by direct measurement.

RESULTS AND DISCUSSION

Data for the preconcentration procedure of the spiked distilled water are given in Table 2. Added and found concentrations are reported to show the recov- ery for each element. Recovery is defined as found concentration divided by added concentration [see Guidelines for data acquisition and data quality evaluation in environmental chemistry (1980)]. Blank values were obtained from ten separate samples of 1 litre distilled water with precipitated Mg(OH)2. The results (Table 2) indicate that Al, Fe, Mn, Zn, Cd, Be, Y, Sc and Yb can be precipitated with high efficiency (80-100%) whereas the remaining five elements show a slightly lower recovery (70-80%).

Table 3 shows the recovery of AI, Fe, Mn, Zn, Cu, Be, Y, Sc and Yb added to natural freshwater. The added amounts were approximately the same as the concentrations in natural waters excepting A1 for which the added amount was an order of magnitude higher than its natural concentration. The concen- trations of Ni, V, Cr, Co and Cd in lakewater were spiked with very small amounts and accurate analyses were impossible to obtain. The data in Table 3 clearly show the ability of the method for increasing the metal concentrations to levels at which they can be reliably determined by ICP-AES.

Technical Note 619

~ d d o d

o

g

~ o d 6 d ~

~ . . . . u

o

N

~ g

8

- g

~ = ~ = ~ ~

~ o ~ . .

.N

klJ < 13. _o

E

O.

• Fe I~g/I / -

1 0 0 0 - :

100

10 . . . . . . . . , . . . . . . . . , . . . . . . . .

10 1 0 0 1 0 0 0 1 0 0 0 0

Di rect de te rm ina t i on IGP-AES

Fig. I. Comparison of preconcentration method and direct measurement of lake water using ICP-AES.

In Fig. 1 the results of direct ICP-AES measure- ments of AI, Fe and Mn in lake water sampled in June 1988 are plotted vs data for these elements obtained by the magnesium hydroxide precipitation. The data in Fig. 1 reveal that the coprecipitation method combined with ICP-AES are valid in a concentration interval spanning at least 3--4 orders of magnitude, which is of major importance.

Analyses of the IAEA reference sample are re- ported in Table 4. Most of the studied elements fall within the given confidence interval for IAEA/W-4, although there are slight discrepancies for A1, Fe, Mn and V.

Detection limit, defined as the concentration required to give a signal equal to three times the standard deviation of the blank [see, Guidelines for data acquisition and data quality evaluation in environmental chemistry (1980)], was used to reveal the practical utility of the precipitation method for multielement determination of natural freshwater. In Fig. 2 detection limits for the studied elements are given. The detection limits for direct measurements with ICP-AES are based on literature data (Winge et al., 1979) which agrees fairly well with our own measurements and experiences. For each element direct measurement and preconcentration detection limit are plotted. For AI, Be, Y, Sc and Yb the detection limit could be enhanced by two order of magnitude and for the other elements by one order.

C O N C L U S I O N S

The described preconcentration method enhanced detection limits for a number of elements with one or two orders of magnitude for ICP-AES multielement determination. The recovery was accurate for a wide range of element concentrations in natural fresh- water. Simple handling and inexpensive equipment

620 Technical Note

Table 4. Reference sample (IAEA/W-4) analysed with the preconcentration method and direct measurement using ICP-AES

AI Fe Mn Zn Be Cu Co Cr V Ni Cd

IAEA/W-4 Preconc. 55 87 22 46 1.0 23 2.2 9. I 4.2 2.3 4.4 IAEA/W-4 Direct 46 100 27 49 1.4 25 . . . . .

IAEA report Given concentration 50 100 25 50 1.25 25 2.5 10 5.1 2.5 5 Certified value 48 97 25 48 1.1 25 2.2 9.9 5.8 2.2 4.6 Confidence interval 40-53 91-102 23-27 43-54 0.6-1.6 22-26 2.1-2.7 9.0-10.5 4.9-10 2.0--4.0 3.9-5.0

Concentrations in #g 1 -l. --, Below detection limit. Given concentration, concentration given by IAEA. Certified value, obtained from the intercomparison organized by IAEA. Confidence Interval, obtained from the intercomparison, significance level 0.05.

100

10

,$ t~

.01

.001 AI Fo Ida ~ Oa Ni V Gr Co ~ Bo Y $c Yb

Fig. 2. Detection limits for the studied elements using ICP-AES direct measurements and with the preconcentration method.

are ma jo r advantages of the me thod and the possi- bility o f direct precipi ta t ion of the sample in the field reduces the ri~k of con tamina t ion and provides a preserved sample.

Acknowledgements--This study was made possible thanks to financial support from Professor Kurt Bostr6m and grants from the Hierta-Retzius Foundation. We thank S. Blomqvist, K. Bostr6m, R. Lffvendahl, C. Pont6r and L. Sjfberg for valuable comments on the manuscript. B. Bostr6m kindly shared her experiences in ICP-AES analysis.

REFERENCES

Bostr6m B. and Bostr6m K. (1991) Magnesium hydroxide precipitation as preenrichment procedure for inductively coupled plasma -atomic emission spectrometric environ- mental analyses. Geol. F~reningens Stockholms F~rhand- lingar 113. In press.

Buchanan A. S. and Hannaker P. (1984) Inductively coupled plasma spectrometric determination of minor elements in concentrated brines following precipitation. Anal. Chem. 56, 1379-1382.

Guidelines for data acquisition and data quality evaluation in environmental chemistry (1980) Anal. Chem. 52, 2242-2249.

Hiraide M., Yoshida Y. and Mizuike A. (1976) Flotation of traces of heavy metals coprecipitated with aluminium hydroxide from water and sea water. Anal. chim. Acta 81, 185-189.

Hiraide M., Ito T., Baba M., Kawaguchi H. and Mizuike A. (1980) Multielement preconcentration of trace heavy metals in water by coprecipitation and flotation with indium hydroxide for inductively coupled plasma-atomic emission spectrometry. Anal. Chem. 52, 804-807.

Koroleff F. (1947) Determination of manganese in natural waters. Acta chem. scand. 1, 503-506.

Koroleff F. (1980) Determination of traces of heavy metals in natural waters by AAS after concentration by co- precipitation. In Proc. Conf. Baltic Oceanographers, 12th Leningrad Meeting, 1980.

Skogerboe R. K., Hanagan W. A. and Taylor H. E. (1985) Concentration of trace elements in water samples by reductive precipitation. Anal. Chem. 57, 2815-2818.

Winge R. K., Peterson V. J. and Fassel V. A. (1979) Inductively coupled plasma-atomic emission spec- troscopy: Prominent lines. Appl. Spectrosc. 33, 206-219.