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Determination of organically-associated trace metals in estuarine sea-water by solvent extraction and atomic-absorption spectrometry
Talanro,Vol. 33, No. 9, pp. 154756, 1986
0039-9140/86 $3.00 + 0.00 Copyright 0 1986 Pergamon Journals Ltd
Printed in Great Britain. All rights reserved
DETERMINATION OF ORGANICALLY-ASSOCIATED TRACE METALS IN ESTUARINE SEA-WATER BY SOLVENT EXTRACTION AND ATOMICABSORPTION SPECTROMETRY KOHJI HAYASE, KIMINORI SHITASHIMAand HIROYUKI TSUBOTA Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730, Japan (Received 7 February 1986. Accepted 14 May 1986)
Summary-Chloroform extraction of trace metals (Ni, Cu, MO, Mn, Cd and Pb) in estuarine sea-water was studied at pH 8 and pH 3, on the basis that the metals would be associated with dissolved organic matter (DOM), which has recently been characterized by reversed-phase liquid chromatography. Ni, Cu, MO and Mn were extracted more at pH 8 than at pH 3. Cd and Pb were not associated with the DOM at either pH 8 or 3. The percentage of the total dissolved trace metals in sea-water associated with DOM varied from 0 to 14%. The metals extracted into chloroform at pH 8 were assumed to be associated with neutral or weakly basic DOM while at pH 3 they could be associated with either the neutral (or weakly basic) DOM or two types of acidic DOM.
Many trace metals in sea-water are partially associated with dissolved organic matter (DOM) and the nature of the organic compounds involved is of interest in geochemical, biochemical and environmental studies. Some workers’-’ have employed reversed-phase liquid chromatography (RPLC) with octadecylsilane for the isolation and characterization of organic complexes of metals in sea-water. Slowey et al.” have studied such complexes in sea-water by
made from Johnson-Matthey guaranteed materials by dissolving the metals in nitric acid and were standardized by EDTA titration. For MO, aqueous ammonium molybdate solution was prepared. The working standard solutions were prepared by diluting the stock solutions with 2M nitric acid. The water used was distilled, demineralized and further purified by the sub-boiling technique. Sample sea-waler The surface sea-water sample was the same as that reported in our previous paper,” and was collected on 11 June 1983 from Hiroshima Bay (34”21.1’N, 132”242’E) in the Seto Inland Sea by using a specially designed S-litre surface-water sampler made of polyethylene and cleaned with nitric acid. The surface sea-water was sampled from the bow with the ship moving dead slow ahead to prevent contamination from the ship itself. The sampling station was just beyond a river mouth and the sea-water at the station was rich in both metals and DOM. The salinity of the sample sea-water was 29.1%. The sample water was filtered through a nitric acid-cleaned 0.4~pm Nuclepore filter, in a Class 100 clean-room, with use of a nitrogennressurized Teflon in-line filtration apparatus. The filtration . was done within several hours of sampling. The sample was stored in a nitric acid-cleaned polyethylene bottle.
solvent extraction. They reportedq6*8 that 6-70% of the total copper in solution is organically associated Cu. Piotrowicz et a1.,9 using anodic-stripping voltammetry, found that more than 95% of the dissolved copper in surface sea-water is complexed by organic ligands. A large fraction of the organically complexed copper is not isolated by C18-SEP-PAK.” Similarly, organic solvents such as chloroform extract only a fraction of the DOM in sea-water. We have recently investigated the RPLC of DOM extracted into chloroform from estuarine sea-water, using both absorption and fluorescence detectors.” In our work three kinds of DOM were recognized, one a neutral or weakly basic material that fluoresces and the other two acidic and not fluorescent. It is likely that the three types also differ in their interactions with metal ions. The results for the-determination of trace metals associated with the chloroformextractable DOM in estuarine sea-water is reported in this paper.
Apparatus Atomic-absorption measurements were made with a Perkin-Elmer Model 500 instrument, HGA 5000 graphite furnace and AS 40 auto-sampler. Procedure The sea-water sample was used for the extraction of DOM after the filtration. Every step other than evaporation was performed in the clean room. The sample was divided into eight I-litre aliquots, and each aliquot was extracted with 20 ml of chloroform in a Teflon separating funnel. Four aliquots were extracted at natural pH (8) and the other four at pH 3 (obtained by addition of distilled nitric acid). The four extracts (at each pH) were combined in a quartz dish and gently evaporated to dryness on a hot-plate inside a box flushed with nitrogen. Each residue was treated with
EXPERIMENTAL Reagents Chloroform of ultrafine grade (Nakarai Co. Ltd.) was used. The stock metal ion solutions, except for MO, were 754
755
SHORT COhUWNlCATlONS
Table 1. Metal concentrations Total dissolved metal concentration,
in sea-water
Extracted by CHCl, at pH8
Extracted by CHCl, at pH 3,
Element
nmolelkg
nmolelkg
%*
nmole /kg
%*
Ni CU MO Mn Cd Pb
12.0 9.1 96 70 0.42 0.18
1.64 0.71 0.89 0.25 N.D.t N.D.
14.0 7.8 0.9 0.4 0.0
0.07 0.60 0.81 0.04 N.D. N.D.
0.6 6.6 0.8 0.06 0.0 0.0
0.0
*Percentage of total dissolved metal. tN.D. = Not detected ( < 0.01 nmole/kg).
about 10 ml of 2M nitric acid, followed by evaporation to dryness and heating at 140”. The final residues were each dissolved in 10 ml of 2M nitric acid and stored in Teflon bottles cleaned with hot nitric acid, prior to determination of the metal concentrations by graphite-furnace atomicabsorption spectrometry (GFAAS). For determination of total dissolved metals in the sample, the following procedure was applied in the clean room. To 1 litre of the filtered sea-water, 5 ml of concentrated nitric acid were added. A few months later, the acidified sample was adjusted to pH 5.3-5.5 with aqueous ammonia and acetic acid and passed through a Chelex 100 column. To elute the major elements, the column was washed with 200 ml of 1M ammonium acetate solution @H 5.5), followed by 50 ml of water. The trace metals sorbed on the column were eluted with 2M nitric acid. The eluate was evaporated to dryness and then heated at 140” inside a box flushed with nitrogen, the residue finally being dissolved in 10 ml of 2M nitric acid. This solution was analysed by GFAAS.
RESULTS
AND DISCUSSION
The concentrations found for the trace metals (total in solution, and extracted by chloroform at pH 8 and 3) are shown in Table 1. In the previous study,” it was found that chloroform extracted a neutral or weakly basic DOM from the sea-water at pH 8, and two types of acidic DOM in addition to the neutral or weakly basic DOM at pH 3. Although the neutral DOM fluoresced (excitation and emission wavelengths were 320 and 420 nm, respectively), the acidic types of DOM did not. The trace metals extracted by chloroform at pH 8 are considered to be associated with the neutral or weakly basic DOM (peaks 1-X and 2-X in Figs. 1 and 2 of the previous paper”). Nickel and copper were obviously associated with the neutral or weakly basic DOM, the fractions being 14 and 8% of the total dissolved nickel and copper, respectively. Kremling et al.'*reported that in Baltic waters about 5% of the total copper was organically associated. The fraction of organically associated copper in sea-waters off Port Hacking in Australia has also been reported as about 5%.5 Negishi and Matsunaga” showed that the ratios of organicallybound to total copper ranged from 10 to 70% in lake and river water at Hokkaido in Japan. Mills and Quinn6 reported that in the Narragansett Bay estuary the ratio of copper retained by SEP-PAK to total
dissolved copper ranged from 14 to 70%. The corresponding ratio found for Hiroshima Bay water is rather low and is consistent with the values found by Kremling et ~1.'~ and Mackey.5 Molybdenum and manganese were little associated with the neutral or weakly basic DOM. NO detectable levels of cadmium and lead were found to be associated with the neutral or weakly basic DOM. MackeyS reported that cadmium was not detected (by atomic fluorescence) in chromatography of material extracted from sea-water by a SEP-PAK cartridge. The metals extracted with chloroform at pH 3 are associated with both the neutral or weakly basic DOM and the acidic types of DOM. Since cadmium and lead were still not detected in the pH-3 extract, they may not be associated with any type of DOM. For the other four metals examined, the concentrations extracted at pH 3 are lower than those at pH 8. Mills et ~1.~ studied the effect of pH on retention of organic copper complexes in sea-water. Their results showed that there was a reduction in the relative amount of organic copper as the pH of the sea-water was decreased, and our result is similar. This effect is attributed to protonation of the functional groups on the DOM when the pH is decreased. The decrease in the concentration of nickel and manganese extracted when the pH is decreased from 8 to 3 indicates that most of these two metals was associated with the neutral or weakly basic DOM. Sequential extraction of DOM from sea-water at pH 8 and 3 will be interesting for further investigation of the metals associated with DOM. Acknowledgements-The
authors deeply thank Professor S. Hayano for his valuable suggestions and discussion. They also appreciate the assistance of the captain, officers and crew of the Toyoshio Maru, Hiroshima University. This work was financially supported by a grant from the Ministry of Education, Science and Culture, Japan.
REFERENCES
1. J. B. Derenbach, M. Ehrhardt, C. Osterroht and G. Petrick, Mar. Gem., 1978, 6, 351. 2. G. L. Mills and J. G. Quinn, ibid., 1981, 10, 93.
756
SHORT COMMlJNlCATIONS
3. J. Lee, Water Res., 1981, 15, 507. 4. G. L. Mills, A. K. Hanson, J. G. Quinn, W. R. Lammela and N. D. Chasteen, Mar. C/rem., 1982, 11, 355. 5. D. J. Mackey, ibid., 1983, 13, 169. 6. G. L. Mills and J. G. Quinn, ibid., 1984, 15, 151. 7. D. J. Mackev. ibid.. 1985. 16. 105. 8. J. F. Slowey: .L. MI Jeffrey and D. W. Hood, Nature, 1967, 214, 377. 9. S. R. Piotrowicz, G. R. Harvey, M. Springer-Young, R. A. Courant and D. A. Boran, in Trace Metals in Sea
10. Il. 12. 13.
wurer, C. S. Wong, J. D. Burton, E. A. Boyle, K. W. Bruland and E. D. Goldberg @IS.), pp. 699-717. Plenum Press, New York, 1983. J. R. Donat, P. J. Statham and K. W. Bruland, Mar. Chem., 1986, 18, 85. K. Hayase, K. Shitashima and H. Tsubota, J. Chromatog., 1985, 322, 358. K. Kremhng, A. Wenck and C. Osterroht, Mar. Chem., 1981, 10, 209. M. Negishi and K. Matsunaga, Wafer Res., 1983, 17, 91.
0039-9140/86 $3.00 + 0.00 Copyright 0 1986 Pergamon Journals Ltd
Printed in Great Britain. All rights reserved
DETERMINATION OF ORGANICALLY-ASSOCIATED TRACE METALS IN ESTUARINE SEA-WATER BY SOLVENT EXTRACTION AND ATOMICABSORPTION SPECTROMETRY KOHJI HAYASE, KIMINORI SHITASHIMAand HIROYUKI TSUBOTA Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730, Japan (Received 7 February 1986. Accepted 14 May 1986)
Summary-Chloroform extraction of trace metals (Ni, Cu, MO, Mn, Cd and Pb) in estuarine sea-water was studied at pH 8 and pH 3, on the basis that the metals would be associated with dissolved organic matter (DOM), which has recently been characterized by reversed-phase liquid chromatography. Ni, Cu, MO and Mn were extracted more at pH 8 than at pH 3. Cd and Pb were not associated with the DOM at either pH 8 or 3. The percentage of the total dissolved trace metals in sea-water associated with DOM varied from 0 to 14%. The metals extracted into chloroform at pH 8 were assumed to be associated with neutral or weakly basic DOM while at pH 3 they could be associated with either the neutral (or weakly basic) DOM or two types of acidic DOM.
Many trace metals in sea-water are partially associated with dissolved organic matter (DOM) and the nature of the organic compounds involved is of interest in geochemical, biochemical and environmental studies. Some workers’-’ have employed reversed-phase liquid chromatography (RPLC) with octadecylsilane for the isolation and characterization of organic complexes of metals in sea-water. Slowey et al.” have studied such complexes in sea-water by
made from Johnson-Matthey guaranteed materials by dissolving the metals in nitric acid and were standardized by EDTA titration. For MO, aqueous ammonium molybdate solution was prepared. The working standard solutions were prepared by diluting the stock solutions with 2M nitric acid. The water used was distilled, demineralized and further purified by the sub-boiling technique. Sample sea-waler The surface sea-water sample was the same as that reported in our previous paper,” and was collected on 11 June 1983 from Hiroshima Bay (34”21.1’N, 132”242’E) in the Seto Inland Sea by using a specially designed S-litre surface-water sampler made of polyethylene and cleaned with nitric acid. The surface sea-water was sampled from the bow with the ship moving dead slow ahead to prevent contamination from the ship itself. The sampling station was just beyond a river mouth and the sea-water at the station was rich in both metals and DOM. The salinity of the sample sea-water was 29.1%. The sample water was filtered through a nitric acid-cleaned 0.4~pm Nuclepore filter, in a Class 100 clean-room, with use of a nitrogennressurized Teflon in-line filtration apparatus. The filtration . was done within several hours of sampling. The sample was stored in a nitric acid-cleaned polyethylene bottle.
solvent extraction. They reportedq6*8 that 6-70% of the total copper in solution is organically associated Cu. Piotrowicz et a1.,9 using anodic-stripping voltammetry, found that more than 95% of the dissolved copper in surface sea-water is complexed by organic ligands. A large fraction of the organically complexed copper is not isolated by C18-SEP-PAK.” Similarly, organic solvents such as chloroform extract only a fraction of the DOM in sea-water. We have recently investigated the RPLC of DOM extracted into chloroform from estuarine sea-water, using both absorption and fluorescence detectors.” In our work three kinds of DOM were recognized, one a neutral or weakly basic material that fluoresces and the other two acidic and not fluorescent. It is likely that the three types also differ in their interactions with metal ions. The results for the-determination of trace metals associated with the chloroformextractable DOM in estuarine sea-water is reported in this paper.
Apparatus Atomic-absorption measurements were made with a Perkin-Elmer Model 500 instrument, HGA 5000 graphite furnace and AS 40 auto-sampler. Procedure The sea-water sample was used for the extraction of DOM after the filtration. Every step other than evaporation was performed in the clean room. The sample was divided into eight I-litre aliquots, and each aliquot was extracted with 20 ml of chloroform in a Teflon separating funnel. Four aliquots were extracted at natural pH (8) and the other four at pH 3 (obtained by addition of distilled nitric acid). The four extracts (at each pH) were combined in a quartz dish and gently evaporated to dryness on a hot-plate inside a box flushed with nitrogen. Each residue was treated with
EXPERIMENTAL Reagents Chloroform of ultrafine grade (Nakarai Co. Ltd.) was used. The stock metal ion solutions, except for MO, were 754
755
SHORT COhUWNlCATlONS
Table 1. Metal concentrations Total dissolved metal concentration,
in sea-water
Extracted by CHCl, at pH8
Extracted by CHCl, at pH 3,
Element
nmolelkg
nmolelkg
%*
nmole /kg
%*
Ni CU MO Mn Cd Pb
12.0 9.1 96 70 0.42 0.18
1.64 0.71 0.89 0.25 N.D.t N.D.
14.0 7.8 0.9 0.4 0.0
0.07 0.60 0.81 0.04 N.D. N.D.
0.6 6.6 0.8 0.06 0.0 0.0
0.0
*Percentage of total dissolved metal. tN.D. = Not detected ( < 0.01 nmole/kg).
about 10 ml of 2M nitric acid, followed by evaporation to dryness and heating at 140”. The final residues were each dissolved in 10 ml of 2M nitric acid and stored in Teflon bottles cleaned with hot nitric acid, prior to determination of the metal concentrations by graphite-furnace atomicabsorption spectrometry (GFAAS). For determination of total dissolved metals in the sample, the following procedure was applied in the clean room. To 1 litre of the filtered sea-water, 5 ml of concentrated nitric acid were added. A few months later, the acidified sample was adjusted to pH 5.3-5.5 with aqueous ammonia and acetic acid and passed through a Chelex 100 column. To elute the major elements, the column was washed with 200 ml of 1M ammonium acetate solution @H 5.5), followed by 50 ml of water. The trace metals sorbed on the column were eluted with 2M nitric acid. The eluate was evaporated to dryness and then heated at 140” inside a box flushed with nitrogen, the residue finally being dissolved in 10 ml of 2M nitric acid. This solution was analysed by GFAAS.
RESULTS
AND DISCUSSION
The concentrations found for the trace metals (total in solution, and extracted by chloroform at pH 8 and 3) are shown in Table 1. In the previous study,” it was found that chloroform extracted a neutral or weakly basic DOM from the sea-water at pH 8, and two types of acidic DOM in addition to the neutral or weakly basic DOM at pH 3. Although the neutral DOM fluoresced (excitation and emission wavelengths were 320 and 420 nm, respectively), the acidic types of DOM did not. The trace metals extracted by chloroform at pH 8 are considered to be associated with the neutral or weakly basic DOM (peaks 1-X and 2-X in Figs. 1 and 2 of the previous paper”). Nickel and copper were obviously associated with the neutral or weakly basic DOM, the fractions being 14 and 8% of the total dissolved nickel and copper, respectively. Kremling et al.'*reported that in Baltic waters about 5% of the total copper was organically associated. The fraction of organically associated copper in sea-waters off Port Hacking in Australia has also been reported as about 5%.5 Negishi and Matsunaga” showed that the ratios of organicallybound to total copper ranged from 10 to 70% in lake and river water at Hokkaido in Japan. Mills and Quinn6 reported that in the Narragansett Bay estuary the ratio of copper retained by SEP-PAK to total
dissolved copper ranged from 14 to 70%. The corresponding ratio found for Hiroshima Bay water is rather low and is consistent with the values found by Kremling et ~1.'~ and Mackey.5 Molybdenum and manganese were little associated with the neutral or weakly basic DOM. NO detectable levels of cadmium and lead were found to be associated with the neutral or weakly basic DOM. MackeyS reported that cadmium was not detected (by atomic fluorescence) in chromatography of material extracted from sea-water by a SEP-PAK cartridge. The metals extracted with chloroform at pH 3 are associated with both the neutral or weakly basic DOM and the acidic types of DOM. Since cadmium and lead were still not detected in the pH-3 extract, they may not be associated with any type of DOM. For the other four metals examined, the concentrations extracted at pH 3 are lower than those at pH 8. Mills et ~1.~ studied the effect of pH on retention of organic copper complexes in sea-water. Their results showed that there was a reduction in the relative amount of organic copper as the pH of the sea-water was decreased, and our result is similar. This effect is attributed to protonation of the functional groups on the DOM when the pH is decreased. The decrease in the concentration of nickel and manganese extracted when the pH is decreased from 8 to 3 indicates that most of these two metals was associated with the neutral or weakly basic DOM. Sequential extraction of DOM from sea-water at pH 8 and 3 will be interesting for further investigation of the metals associated with DOM. Acknowledgements-The
authors deeply thank Professor S. Hayano for his valuable suggestions and discussion. They also appreciate the assistance of the captain, officers and crew of the Toyoshio Maru, Hiroshima University. This work was financially supported by a grant from the Ministry of Education, Science and Culture, Japan.
REFERENCES
1. J. B. Derenbach, M. Ehrhardt, C. Osterroht and G. Petrick, Mar. Gem., 1978, 6, 351. 2. G. L. Mills and J. G. Quinn, ibid., 1981, 10, 93.
756
SHORT COMMlJNlCATIONS
3. J. Lee, Water Res., 1981, 15, 507. 4. G. L. Mills, A. K. Hanson, J. G. Quinn, W. R. Lammela and N. D. Chasteen, Mar. C/rem., 1982, 11, 355. 5. D. J. Mackey, ibid., 1983, 13, 169. 6. G. L. Mills and J. G. Quinn, ibid., 1984, 15, 151. 7. D. J. Mackev. ibid.. 1985. 16. 105. 8. J. F. Slowey: .L. MI Jeffrey and D. W. Hood, Nature, 1967, 214, 377. 9. S. R. Piotrowicz, G. R. Harvey, M. Springer-Young, R. A. Courant and D. A. Boran, in Trace Metals in Sea
10. Il. 12. 13.
wurer, C. S. Wong, J. D. Burton, E. A. Boyle, K. W. Bruland and E. D. Goldberg @IS.), pp. 699-717. Plenum Press, New York, 1983. J. R. Donat, P. J. Statham and K. W. Bruland, Mar. Chem., 1986, 18, 85. K. Hayase, K. Shitashima and H. Tsubota, J. Chromatog., 1985, 322, 358. K. Kremhng, A. Wenck and C. Osterroht, Mar. Chem., 1981, 10, 209. M. Negishi and K. Matsunaga, Wafer Res., 1983, 17, 91.