Apparatus for the Sampling and Concentration of Trace Metals from

3 Apparatus for the Sampling and Concentration of Trace Metals from Seawater

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E A R L W . D A V E Y and A L B E R T E . SOPER National Marine Water Quality Laboratory, Environmental Protection Agency, South Ferry Rd., Narragansett, R. I. 02882

Apparatus and techniques for the sampling and concentration of particulate and dissolved trace metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) from seawater were used to monitor ambient trace metal concentrations and heavy metal discharges. Particulate trace metals were collected in 0.4 µm Nuclepore filter bags, and dissolved metals were concentrated on Chelex-100 chelating resin. The apparatus is constructed of polyethylene and polypropylene plastics to reduce possible sample contamination.

*Tphe capabilities and limitations of analyses of water for trace metals have been described b y Hume ( J ) . F r o m his discussion it can be inferred that few reliable methods have been available to analyze directly for particulate and dissolved trace metals i n seawater. Therefore, i t has been necessary to pre-concentrate seawater samples to reduce the high salt content and to increase trace metal concentrations to within the range of existing conventional instrumentation, such as flame atomic absorption spectrometry ( A A S ) or neutron activation analysis ( N A A ) . Another problem, according to Robertson (2), is that seawater samples can be contaminated b y trace metals during sampling and subsequent storage. Consequently, this paper describes apparatus which is designed to concentrate trace metals from seawater, thereby reducing some poten tial sources of sampling contamination errors. A

Analysis Methods In 1970 Davey et al. (3) reported on a modification of the technique of Riley and Taylor (4) which selectively removed trace metals from 16

Gibb; Analytical Methods in Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

3.

Sampling and Concentration of Trace Metals

D A V E Y A N D SOPER

17


marine culture media. Radioactive trace elements were used to verify that the chelating resin, Chelex-100 (Bio-Rad, Richmond, C a l i f . ) , effec tively removed certain trace elements from natural or artificial seawater (Table I ) . Essentially identical results were obtained i n these experi ments for either seawater collected from the Sargasso Sea or artificial seawater prepared according to Kester et al. ( 5 ) . The water was radio labeled with specific trace elements and passed through 10 m l sodium Chelex at 5 m l / m i n . However, trace metals were not completely released (3) from Chelex-100 b y using the technique of Riley and Taylor (4). This problem was circumvented by the following modifications: 1. The ammonium rather than the sodium form of the resin is used as was later suggested by Riley and Taylor ( 6 ) . 2. The volume of resin is reduced to 5 m l for a 4-1. seawater sample. 3. T h e flow rate is decreased to approximately 1 m l / m i n to compen sate for the reduced resin volume. 4. Concentrated nitric acid is used for extraction and total release of the metals from the resin. Table I.

Adsorption Efficiency of Purified Sodium Chelex-100 for Trace Metals in Seawater Isotope

Adsorption (%)

Zn U5m Mn Cu

>99 >99 >99 >99 95 92 92 33

65

Cd

6 4

64

210p

b

63

N i

69

F e

110 m ^ g

The acid-extracted metals are usually analyzed by conventional flame A A S . However, the Chelex-100 can also be analyzed directly for cad mium, cobalt, manganese, and zinc b y N A A if the resin is first rinsed with deionized water after sample passage to reduce salt content and then dried. The carefully purified ammonium form of Chelex-100 after N A A was essentially free from trace metal contamination. Chelex-100 also competes against the added chelators glycine, histidine, and ethylenediaminetetraacetic acid ( E D T A ) (Table I I ) . I n these experiments, various concentrations of chelators or phytoplankton cultures with cells removed were mixed i n seawater with C u , and the radio activity was counted before and after the water was passed through Chelex columns. Since the only competition observed was with 10" M E D T A , the resin should be able to compete against natural complexes 64

6


18

ANALYTICAL METHODS IN OCEANOGRAPHY

Table II. Chelator

Competition for C u Between Chelex-100 and Various Chelators 64

Concentration (M)

Competition (%)

Glycine Histidine


EDTA Phytoplankton

0

having stability constants less than the 10" M E D T A - C u complex i n seawater. 6

Apparatus The apparatus i n Figure 1 is arranged to sample 0.1 m from the sediment-water interface when the polyethylene sampling tube is at tached directly to the anchoring weight and 0.5 m below the air-water interface. However, intermediate depths could also be sampled by ad justing the bottom sampling tube to other desired depths. The depth at which the water collection bottles are placed provides the hydrostatic driving force to fill the bottles with seawater through the connecting polyethylene tubing. The bottle is designed to fill with sea water from the bottom to the top to carefully exclude air. Thus, the collected seawater can be analyzed for oxygen, p H , salinity, and trace metals. The apparatus is constructed of polyethylene or polypropylene materials to reduce potential trace metal contamination to the sample. The only items not made from plastics are the weights. The seawater from the sampler is poured into a 1- or 2-1. acid-cleaned and deionized water-rinsed linear polyethylene storage container. This can be converted later into a trace metal processing bottle by changing to a modified cap as shown in Figure 2. The processing bottles work i n the following sequence. The sample passes through a ΟΑ-μτη Nuclepore (Pleasanton, Calif.) filter bag placed i n a tubulated polyethylene container. The filter bags are made from 142-mm filters or, if a larger surface area is needed, by heat-sealing rectangles cut from 8 χ 10-in. Nuclepore filter sheets. A small bead of silicone rubber is applied to the filter holder cap to make the filter assembly leak-tight. The first Chelex column removes the trace metals from the seawater. The second Chelex column is used as an analytical blank to correct for any salt matrix effects from the first column. After collection, the samples are processed by the following method.


3.



19


The water collection bottles are weighed before and after sample passage to determine the mass of water processed. The drained filter bags are removed from the plastic filter containers, placed into quartz vials, and digested with Ultrex (J. T . Baker Co., Phillipsburg, N.J.) concen trated nitric acid. T h e solution is taken to dryness, the residue is eluted with 0.5 m l of both Ultrex concentrated nitric and hydrochloric acids, and the eluate is diluted to 5 m l with deionized water. T h e Chelex material is removed from the columns, placed into quartz vials, rinsed w i t h deionized water, and dried. T h e resin is digested i n 3 m l Ultrex concentrated nitric acid for at least 1 hr before dilution to 10 m l with deionized water. The resin particles are removed b y filtering through a porous plastic

^STYRENE FLOAT

1/4" POLYETHYLENE FILLING TUBE

,1/4" POLYETHYLENE VENT TUBING 1/4" POLYETHYLENE

FILLING TUBE

LPE 4 LITER ^ COLLECTION BOTTLE

-POLYPROPYLENE

ANCHORING LINE

CINDER BLOCK WEIGHTS

7 Figure 1. Apparatus for sampling water for trace metals analysis. It is designed to sample 0.1 m from the sediment-water interface and 0.5 m below the air-water interface.


20

ANALYTICAL METHODS IN OCEANOGRAPHY

LINEAR POLYETHYLENE 2 LITER SAMPLE STORAGE β PROCESSING BOTTLE


1/4" POLYETHYLENE VENT TUBING

1/2" POLYPROPYLENE FITTING WELDED TO BOTTLE CAP

FILTER

ASSEMBLY

1/2" POLYPROPYLENE UNION l/2 X 4"

POLYETHYLENE

M

COLUMN FILLED WITH CHELEX

IOO> POLYETHYLENE POROUS FRITS FOR RESIN SUPPORT

(

1/2" X 4" POLYETHELENE COLUMN TAPERED TO 1/4"FILLED WITH CHELEX CLAMP

Figure 2.

Sample storage and processing bottle with modified cap, filter assembly, and columns

filter. Both the acid-extracted filters and resin are analyzed by A A S for C d , C r , C u , Fe, M n , N i , P b , and Z n . Results Table I I I presents results obtained from the sampling and concen tration apparatus for both particulate and dissolved trace metals. T h e percentage of the ratio of dissolved-to-total trace metals also was com puted. These results compare with values reported b y others for Narragansett Bay ( 7 ) .


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21


Table III. Concentration of Trace Metals from Narragansett Bay (25 October 1974) Metal

Particulate

Dissolved

Dissolved (%)

Cd Cr Cu Fe Mn Ni Pb Zn

not detected 0.30 ± 0.07 0.38 ± 0.07 55.12 ± 3.39 3.70 ± 0.32 0.18 ± 0.06 0.65 ± 0.07 0.94 ± 0.26

0.08 ± 0.03 not detected 0.74 ± 0.05 0.30 ± 0.22 11.36 ± 0.20 4.08 db 0.22 0.51 ± 0.13 5.28 ± 0.18

100.0 not detected 66 0.5 78.7 95.8 44.0 84.9

0

All concentrations are expressed in Mg/kg (ppb) as a mean (x) of four replicates (n = 4) at the 95% confidence limit. α

Discussion The apparatus can theoretically sample to any depth. However, at depths greater than 100 m, it is logistically difficult to anchor and to retrieve. The apparatus could be used to monitor baseline trace metal levels, sewage and industrial metal outfalls, offshore dumping sites, and diffusion of heavy metals from polluted sediments. The concentration apparatus could also be adapted to monitor other compounds such as methylated mercury, chlorinated hydrocarbons, amino acids, etc. by replacing the Chelex-100 i n the columns with other resins specific for the compounds to be monitored.

Literature Cited 1. Hume, D. N., ADVAN. CHEM. SER. (1967) 67, 30. 2. Robertson, D. E.,Anal.Chem. (1968) 40, 1067. 3. Davey, E. W., Gentile, J. H., Erickson, S. J., Betzer, P., Limnol. Oceanog. (1970) 15, 486. 4. Riley, J. P., Taylor, D., Anal. Chim. Acta (1968) 40, 479. 5. Kester, D. R., Duedall, I. W., Connors, D. N., Pytkowicz, R. M., Limnol. Oceanog. (1967) 12, 176. 6. Riley, J. P., Taylor, D., Anal. Chim. Acta (1968) 41, 175. 7. Bender, M. L., University of Rhode Island, Graduate School of Oceanog raphy, Narragansett, R. I., personal communication. RECEIVED January 3, 1975


Apparatus for the Sampling and Concentration of Trace Metals from

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