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Observations on heavy metal geochemical associations in polluted and non-polluted estuarine sediments
Ent'ironmental Pollution (Series B) 6 (1983) 181-193
Observations on Heavy Metal Geochemical Associations in Polluted and Non-Polluted Estuarine Sediments
M. A. Badri* & S. R. Aston# Department of EnvironmentalSciences, University of Lancaster, Lancaster LAI 4YQ, Great Britain
ABSTRACT The total concentrations and the geochemical associations in surface sediments from three estuaries in northwest England have been studied. The estuaries represent varying degrees of pollution, and the present results indicate rather different behaviour among the metals and the sites investigated. The distribution of the metals in the 'easily or freely leachable and exchangeable', 'oxidisable-organic', 'reducible' and 'resistant' fractions of the sediments were determined by a sequential leaching technique. For all metals, the resistant fraction was dominant, especially for iron. Among the other (non-lithogenous) fractions the oxidisable organic fraction was frequently important.
INTRODUCTION Monitoring studies of heavy metal contamination in aquatic systems have utilised the chemical analysis of flowing water, biological organisms, suspended sediments, surface sediments, bottom core sediments and also pore waters. However, surface sediment analysis has received by far the * Present address: Jabatan Botani, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia. t Present address: International Laboratory of Marine Radioactivity, Mus6e Oc6anographique, Monaco, MC 98000. 181 Environ. Pollut. Ser. B. 0143-148X/83/$03-00 ~ Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
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M. A. Badri, S. R. Aston
most attention. Aside from representing the largest reservoir for trace metals within an estuarine system (Renfro, 1973), sediments can be particularly useful in detecting pollution sources and in the selection of critical sites for routine water sampling (Aston & Thornton, 1977). In addition, the ease of sampling, sample handling and chemical analysis make sediment studies preferable. Within sediments heavy metals are distributed in certain geochemical fractions, existing in several states of varying mobility, leachability and bioavailability. In general, metals in aquatic sediments can be divided into those that are relatively immobile in the long term and those whose ease of mobility in the short term poses some importance as far as exposure to the biosphere is concerned. To facilitate the understanding of metal geochemical distributions and bioavailability, geochemical fractionation studies are frequently performed, and are often in the form of sequential extractions on the sediments. This method subjects the sediments to 'selective' leachings with appropriate reagents for each of the identified geochemical fractions. In this study, sediments from three different estuaries in northwest England were collected and studied for their total metal distributions, nonlithogenous metals and four different geochemical metal fractions. In addition, the percentage organic carbon contents and the percentage less than 63/am grain size fractions were determined. The geochemical fractions identified here are: 'the easily or freely leachable and exchangeable (EFLE)', 'oxidisable-organic', 'acidreducible' and the residual or resistant fractions. The 'EFLE' fraction takes into account metals associated with the exchangeable surfaces of clays and organic matter and those easily leachable by non-oxidising and non-reducing chemical reactions. The'oxidisable-organic' fraction covers the metals which are organically bound, and which are released when oxidised by, for example, peroxides. The 'acid-reducible' fraction mainly accounts for the metals associated with the Mn and Fe oxides and hydroxides and possibly also with carbonates. The metals not released by these preceding extractions are termed 'residual' or 'resistant', and presumably are those strongly trapped within silicate minerals. The mathematical summation of the first three fractions constitutes the nonlithogenous fraction. The term 'lithogenous' is based on the sediment components primarily being derived from the breakdown of silicate rocks on the continents during weathering and soil formation (Chen & Yen, 1972). Non-lithogenous metals are correspondingly those associated with
Hea~'y metals in estuarine sediments
183
all other phases apart from the above, which in many polluted estuarine situations are of man-made as well as natural origins. The sequential extraction procedure adopted for the study was based on comparative studies of different extracting reagents and the effects of different sequences of extraction as reported by Badri & Aston (1981). E X P E R I M E N T A L METHODS Transect sediment samples from the Mersey (97 samples) and the Wyre (77 samples) estuaries in northwest England were collected by use of a hovercraft, and by scraping the top 2cm of sediment directly into previously acid-soaked polyethylene screw-cap bottles. The Mersey sediments represent a polluted situation, due to industrial and economic activities at the port of Liverpool. The Wyre estuary is also polluted but not to the same magnitude as the Mersey. For a baseline comparison, sediment samples from the Ravenglass estuary (35 samples) were collected using a Teflon spatula from the top 2 cm of the surface sediment at low tide. The wet sediments were air-dried in clean-air cabinets, homogenised, and later stored in previously acid-soaked sample bottles. Air-dried sediments were weighed into 150-ml polypropylene bottles and mixed with 1"0 M NH4CHaCOO and 0.5 M Mg(CHaCOO)z (pH 7) at the solid:reagent ratio of 1.5. They were then agitated at constant speed for 1 h and centrifuged at 3000 rpm for 30 min at 20 °C. The samples were then filtered through 0.45 ~m Millipore filters into acid-soaked 100ml glass-stoppered conical flasks. The filtrates were acidified by adding 2 ml of HNO3 and stored for AAS determination. The solid sediments in the bottles were then washed free from the first 'EFLE' reagents by shaking with 50ml of double-distilled de-ionised water (DDDW) for 1 h, centrifuged and the water decanted. The subsequent phase extractions followed the same procedure. For the 'oxidisable-organic' extraction, the solid samples were first oxidised with 3 0 ~ H 2 0 2 in a water bath at 94-97°C. After cooling the metals released from the organic complexes were extracted using 1.0 ~l NH4CH3COO acidified to pH 3.5 with HCI, and shaken at a solid:reagent ratio of 1.5 for I h, centrifuged and filtered again in the same manner. The filtrate contained the metals in the 'oxidisable-organic' fraction. The solid sediments were then washed with D D D W , and later extracted for the 'acid-reducible' fraction in the same manner using 0-25 M N H z O H . HC1 (pH 2.0) in H N O 3.
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M. A. Badri, S. R. Aston
The met~ils Fe, Mn, Pb, Cu and Cd were determined by AAS (Perkin Elmer model 305). Some of the Pb and Cd measurements were made by flameless AAS, or after pre-concentration with methyl-iso-butyl ketone (MIBK). Organic carbon analysis was performed on all the samples by the use of a chromic acid titration method (El Wakeel & Riley, 1957), and the percentage less than 63/~m grain size fraction was determined by wet sieving. Total metal concentrations in the sediments were determined by acid digestion in PTFE beakers at 170 °C, using HNO3: HF: HC10 4 at the ratio of 6:4:3 followed by AAS analysis.
RESULTS A N D DISCUSSION The concentrations of trace metals in the sediments of the three estuaries are given in Table 1, both for total metal and the non-lithogenous metal fraction, based on the average of all the sediment samples. The locations of the three estuaries are shown in Fig. 1. The concentrations of Fe and Mn in sediments from the three estuaries do not differ significantly among the locations, either in the total or the TABLE 1 Total and Non-lithogenous Elements in the Three Northwest England Estuaries (in nag kg - l) Ravenglass Mean SD Total metal Fe Mn Zn
Pb Cu Cd Non-lithogenous Fe Mn Zn
Pb Cu Cd
16783 4985 616 248 90 39 37 11 10 4.7 3.6 1.8 336 362 23 9.9 1.4 1.1
194 175 12 2.8 0.6 0-3
Wyre Mean SD
Mersey Mean SD
10555 4332 459 198 82 59 35 18 9.7 7.8 6.0 2.7
12693 8400 564 750 236 187 57 53 41 64 5-3 3.3
237 257 23 12 2.1 1.4
62 130 11 4.3 0.9 0.3
322 277 140 20 7.4 1.8
207 234 94 1! 6.7 0.9
Heat'y metals in estuarine sediments
185
Scotland
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England
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Wyre
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Fig. I.
Locations of the Mersey, Wyre and Ravenglass Estuaries in northwest England.
non-lithogenous fraction. However, the high standard deviation values indicate very high variations within and among the transects. For instance, the concentration of total Fe in the Mersey sediments varies from 5500 to 35 000 mg kg - ', Mn from 100 to 1500mg g- 1, Zn from 41 to 650mgkg -1, Pb from 12-5 to 240mgkg -1, Cu from 1 to 140mgkg -~ and Cd from 1.1 to 11 mg kg- 1 Among the heavy metals, Zn exhibits the highest percentage of the nonlithogenous fraction with 59 ~o of the total in the Mersey, 27 70 in the Wyre and 25 7o in the Ravenglass. This indicates the variations in the magnitude of the metal scavenging in the three different estuaries. For lead the non-lithogenous fraction on average accounts only for 35 ~ of the total Pb in the Mersey estuary sediments. The Wyre and Ravenglass sediments contain only 24 7o and 27 ~ non-lithogenous Pb, respectively,
M. A. Badri, S. R. Aston
186
emphasising the importance of the resistant fraction. For copper only 14 ~ on average is in the non-lithogenous fraction in the Mersey, 20 ~ in the Wyre and 14~o in the Ravenglass sediments. Non-lithogenous Mn is frequently important in the scavenging of other metals, and the sorption of heavy metals onto Mn oxides has been well documented (Forstner & Wittman, 1981). The present results indicate that non-lithogenous Mn does not show much variation among the sediments of these three estuaries. In the Mersey sediments, the nonlithogenous Mn constitutes 49 ~ of the total on average, but this increases to 56 ~ in the Wyre and 58 ~ in the Ravenglass sediments. The higher solubility and leachability of Mn as compared with Fe has also been reported (Chao, 1972). The results of this study indicate that the Fe is mainly in the resistant fraction of the sediments (about 98 ~o), and hence may not be of much significance in the sorption of the heavy metals. Chemical fractionation of Fe in Pacific pelagic sediments also showed that Fe was mainly in the resistant fraction (Forstner & Stoffers, 1981). Within the non-lithogenous fraction, the organically bound metal fraction seems to be the most dominant. Table 2 shows that the 'oxidisable-organic' fraction is the most important geochemical fraction for the metals Mn, Zn, Pb and Cu. The precision for Cd is low due to its small total concentrations, and hence it has been eliminated from the table. As may be seen, in all cases the 'oxidisable-organic' fraction constitutes more than 5 0 ~ of the non-lithogenous metals in the sediments. The affinity of Cu and other metals for sorption into organic matter has been observed in many situations (Rashid, 1974; Filipek & Owen, 1979; Fillipek et al., 1981; Forstner & Wittman, 1981). TABLE 2 'Oxidisable-organic' Metals in Percentage of Total (~oT) and Percentage of the Non-lithogenous (~oNL)
Mn Zn Pb Cu
Ravenglass %T %NL
%T
39 19 18 8
43 19 22 14
64 70 64 59
Wyre %NL 78 54 63 66
Mersey %T %NL 44 39 21 14
76 66 75 70
Heavy
metals in estuarine sediments
187
The results of the sequential extraction of metals in sediments from the three estuaries may probably be best examined in terms of their percentages of the total metal contents (Fig. 2). The percentages are based on the average values of all the analysed sediments from each of the estuaries. At a glance the Mersey sediments are seen to contain the highest proportions of non-lithogenous metals and are the most polluted. The 'EFLE' fraction is not as geochemically important here as the 'oxidisable-organic', but in most samples it is more important than the 'acid-reducible' fraction. There is no specific pattern for the affinity of the metals to the'EFLE' fraction in the sediments from the different estuaries. For instance, 11% of total Pb in the Wyre estuary sediments is in the 'EFLE' fraction, and only 4% in the Mersey, and 8 % in the Ravenglass Fe
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samples. For zinc, the Mersey sediments exhibit the highest proportion of this fraction with 13 ~ of 'EFLE' Zn, while in the Wyre and Ravenglass sediments this fraction constitutes only 4 9/o and 2 ~ respectively. The different pattern of Pb and Zn sorption in this very loosely bound phase suggests dissimilar chemical associations of the metals in the surface coatings of the sediments. The 'EFLE' Cu is almost the same in all three estuaries' sediments. Even though Mn is usually believed to be easily leachable, the 'EFLE' reagents only extracted 6 9/ooof the total Mn from the Mersey and Ravenglass sediments, and only 2~o from the Wyre samples. However, Mn constitutes the highest percentage in the 'acid-reducible' fraction compared with the rest of the metals. In the Ravenglass sediments, the 'acid-reducible' Mn is 13 ~ of the total, with the Wyre and Mersey sediments having l0 9/o and 8 9/0, respectively. The metals Pb, Cu and Cd have the smallest proportion contained in the 'acid-reducible' fraction, and only Zn has a substantial contribution. The distribution of the 'acidreducible' Zn is not dependent on the locations or the degree of total metal pollution. Aside from Mn, it is very obvious that the resistant fraction constitutes the highest percentage of metal accumulation in the sediments from all three estuaries. The above data show the resistant fraction as the most dominant, but these conclusions are based on average values. A more detailed analysis of all the sampling points shows a very high variation in the distribution of the metals in sediments throughout the estuaries. This high variability highlights two important points: (1) the deposition of the metals (by natural and man-made sources) is not homogeneous with respect to the recently deposited sediments, and (2) there is a danger of misleading interpretation of monitoring studies of heavy metal pollution which are based on sediment analysis if samples are few or unrepresentative in their locations. The results show good correlations for the influence of Fe and Mn deposition on the distribution of heavy metals Zn, Pb, Cu and Cd. This means that sediments with high Fe content tend to be also high in the concentrations of Mn, Zn, Pb, Cu and Cd (and probably other metals). The table of correlation coefficients (Table 3) shows the relationships between the total metals and the 'oxidisable-organic' fractions with the non-lithogenous Fe and Mn, organic carbon and grain size (percentage less than 63 #m) for the three estuaries. With the exception of Cu and Mn in Mersey sediments, total metals correlate well with the non-lithogenous
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Fe, and similar correlations are also seen between total metals and the non-lithogenous Mn. Such good correlations are not quite so evident for the 'oxidisable-organic' fraction. While the 'oxidisable-organic' Fe and Mn are dependent on the local Fe and Mn, the other metals in this fraction do not show this similarity, especially in the relatively unpolluted Ravenglass sediments. It is apparent that 'oxidisable-organic' Zn, Pb and Cu are not dependent on either the non-lithogenous Fe or Mn. While a comparison of the affinity of metals for the organic fraction of sediments is made difficult due to different sequential extraction techniques employed by different workers, it is quite evident at least in theory that the nature of the organic matter available in sediments serves as a possible reservoir for metal sorption. For example, Gibbs (1977) found that the solid organic materials in sediments account for 5-19 ~o of the total metals transported in the Amazon estuary. The relationships between the metals and organic carbon in the present sediments are most noticeable in terms of the total metal concentrations for all three estuaries, and for the 'oxidisable-organic' fractions of metals of the Mersey. Percentage less than 63 #m grain size correlations with metals both in the total and the 'oxidisable-organic' fraction are highly variable. In the 'oxidisable-organic' fraction, only Mn shows good correlations with the percentage less than 63 #m grain size. This indicates that ~ organic carbon and percentage less than 63 #m grain size only show good relationships with total metal concentrations, and the geochemical fractions do not in general depend so much on the percentage of organic carbon present, and especially not the percentage less than 63 #m grain size. The effects of grain size on total metal concentrations in sediments are related to the surface area of coatings on the grains which act as adsorption sites of heavy metals (Forstner et al., 1979). Analyses of polluted sediments from the lower Rhine indicated that there tends to be a distinct reduction of Fe, Mn and Zn contents as the grain size increases for most of the geochemical phases extracted (Forstner & Patchineelam, 1980). Gupta & Chen (1975) concluded that there was an increase in the non-residual fraction of the heavy metals with decreasing sand content of near-shore sediments in Los Angeles harbour. Metal concentrations in sediments were also found to increase with decreasing particle size in the Severn estuary, UK (Thorne & Nickless, 1981), and in Little Traverse, Lake Michigan, USA (Filipek & Owen, 1979). These other studies show similar trends to those found in the present estuarine sediments.
Hea W metals in estuarine sediments
191
The present results have shown correlations between percentage organic carbon and total metals and the non-lithogenousmetals and most of the other geochemical phases. The importance of organic carbon as a heavy metal reservoir and the tendency for metals to be dominant in the organically bound sediment phase is quite well established for river and estuarine sediments (Gardiner, 1974; Serne & Mercer, 1975; Nissenbaum & Swaine, 1976; Filipek & Owen, 1979), and also for soils and street dusts (Harrison et al., 1981). While the humic acid component of the organic matter represents the main component in marine and estuarine sediments (Nissenbaum & Kaplan, 1972; MacFarlane, 1978), the affinity of metals for humic acid may vary between the individual metals. For instance, Cu is much preferred by humic acids than is Cd (Oakley et a l., 1981). The total organic component has also been shown to have higher preference for Cu than Cd (Stumm & Brauner, 1975). The present study indicates that percentage organic carbon correlates much better with Cu and Cd than for other metals in the Wyre sediments for total element, non-lithogenous fractions, and the rest of the geochemical fractions. However, in the Mersey sediments with equally as much organic carbon but higher heavy metal concentrations this feature is only true for total metals, 'oxidisableorganic' metals and the resistant fraction. For the other fractions no significant difference was observed between the estuaries. In relatively unpolluted sediments (Ravenglass) the strong correlation of organic carbon and Cu is observed only for the total metal and the 'EFLE' fraction.
CONCLUSIONS Geochemical fractionation studies of sediments from the three English estuaries varying in their degree of heavy metal pollution suggest rather different behaviour among the heavy metals. In all cases Fe was mainly resistant to sequential extractions. There are high variations within transects and among transects, which call for large sample numbers and sampling sites for monitoring purposes. The metals, as a general rule, have different affinities depending on the nature of metal scavengers that are available in sediments. The dominance of the organically held fraction was very conspicuous in the non-lithogenous metal contents. The presence of metals in the 'EFLE' and organic fractions suggests that a portion of the metals may be easily available for biological uptake. In a
192
M. A. Badri, S. R. Aston
polluted estuary, such as the Mersey, with a high percentage of metals such as Zn and Cd in the non-lithogenous fractions this could be of significance. H u m a n activities such as dredging, deepening of navigation channels or extension of harbour areas may expose the potentially available metals to the biosphere, and the continuing study of the problem of polluted sediments is of importance. ACKNOWLEDGEMENT One of us (M.A.B.) wishes to thank the National University of Malaysia, Bangi, Salangor, Malaysia for financial support. REFERENCES Aston, S. R. & Thornton, I. (1977). Regional geochemical data in relation to seasonal variations in water quality. Sci. Total Environ., 7, 247-60. Badri, M. A. & Aston, S. R. (1981). A comparative study of geochemical fractionation of heavy metals in estuarine sediments. In Proe. int. Conf. Heavy Metals in the Environment, 15-18 September, 705-8. Amsterdam, WHO. Chao, T. T. (1972). Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Proc. Soil Sei. Soc., 36, 764-8. Chen, K. Y. & Yen, T. F. (1972). Models for the fate of heavy metals in sediments. ACS Division of Water, Air and Waste Chemistry Reports, 12, 165-79. El Wakeel, S. K. & Riley, J. P. (1957). Determination of organic carbon in marine muds. J. Cons. int. Explor. Met., 22, 180-3. Filipek, U H., Chao, T. T. & Carpenter, R. H. (1981). Factors affecting the partitioning of Cu, Zn, Pb in boulder coatings and stream sediments in the vicinity of a polymetallic sulphide deposit. Chem. Geol., 26, 105-17. Filipek, L. H. & Owen, R. M. (1979). Geochemical associations and grainsize partitioning of heavy metals in lacustrine sediments. Chem. Geol., 26, 105-17. Forstner, U., Patchineelam, S. R. & Schmoll, G. (1979). In Proc. int. Conf. on Management and Control of Heavy Metals in the Environ., September, 316-19. London. Forstner, U. & Patchineelam, S. R. (1980). Chemical associations of heavy metals in polluted sediments from the lower Rhine river. Am. Chem. Soe. Adv. Chem. Ser., 189, 177-93. Forstner, U. & Stoffers, P. (1981). Chemical fractionation of transition elements in Pacific pelagic sediments. Geoehim. Cosmoehim. Aeta, 45, 1141--6.
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Forstner, U. & Whittman, G. T. W. (1981). Metal pollution in the aquatic environment. Berlin, Springer-Verlag. Gardiner, J. (1974). The chemistry of cadmium in natural waters, II. The adsorption of cadmium on river muds and naturally occurring solids. Water Resour. Res., 8, 157-64. Gibbs, R. J. (1977). Transport phases of transition metals in the Amazon and Yukon rivers. Bull. Geol. Soc. Am., 88, 829-43. Gupta, S. K. & Chen, Y. K. (1975). Partitioning of trace metals in selective chemical fractions of nearshore sediments. Environ. Lett., 10, 129-58. Harrison, R. M., Laxen, D. P. H. & Wilson, S. J. (1981). Chemical associations of lead, cadmium, copper and zinc in street dusts and roadside soils. Environ. Sci. Technol., 15, 1378-82. MacFarlane, R. B. (1978). Molecular weight distribution of humic and fulvic acids of sediments from a north Florida estuary. Geochim. Cosmochim. Acta, 42, 1579-82. Nissenbaum, A. & Kaplan, J. R. (1972). Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnol. Oceanogr., 17, 570-82. Nissenbaum, A. & Swaine, D. J. (1976). Organic matter metal interactions of recent sediments: the role of humic substances. Geochim. Cosmochim. A cta, 40, 809-16. Oakley, S. M., Nelson, P. D. & Williamson, K. J. (1981). Model of trace metal partition in marine sediments. Environ. Sci. Technol., 15, 474-80. Rashid, M. A. (1974). Adsorption of metals on sedimentary and peat humic acids. Chem. Geol., 13, 115-23. Renfro, W. C. (1973). Transfer of 65Zn from sediments by marine polychaete worms. Mar. Biol., 21, 305-16. Serne, R. J. & Mercer, B. W. (1975). Dredge disposal study, San Francisco Bay and Estuary. In Appendix F, Crystalline Matrix Study. San Francisco, US Army Engineer District. Stumm, W. & Brauner, P. A. (1975). Chemical speciation. In Chemical oceanography, ed. by J.P. Riley and G. Skirrow, 173-239. London, Academic Press. Thorne, L. T. & Nickless, G. (1981). The relation between heavy metals and particle size fractions within the Severn estuary (UK) intertidal sediments. Sci. Total Environ., 19, 207-13.
Observations on Heavy Metal Geochemical Associations in Polluted and Non-Polluted Estuarine Sediments
M. A. Badri* & S. R. Aston# Department of EnvironmentalSciences, University of Lancaster, Lancaster LAI 4YQ, Great Britain
ABSTRACT The total concentrations and the geochemical associations in surface sediments from three estuaries in northwest England have been studied. The estuaries represent varying degrees of pollution, and the present results indicate rather different behaviour among the metals and the sites investigated. The distribution of the metals in the 'easily or freely leachable and exchangeable', 'oxidisable-organic', 'reducible' and 'resistant' fractions of the sediments were determined by a sequential leaching technique. For all metals, the resistant fraction was dominant, especially for iron. Among the other (non-lithogenous) fractions the oxidisable organic fraction was frequently important.
INTRODUCTION Monitoring studies of heavy metal contamination in aquatic systems have utilised the chemical analysis of flowing water, biological organisms, suspended sediments, surface sediments, bottom core sediments and also pore waters. However, surface sediment analysis has received by far the * Present address: Jabatan Botani, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia. t Present address: International Laboratory of Marine Radioactivity, Mus6e Oc6anographique, Monaco, MC 98000. 181 Environ. Pollut. Ser. B. 0143-148X/83/$03-00 ~ Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
182
M. A. Badri, S. R. Aston
most attention. Aside from representing the largest reservoir for trace metals within an estuarine system (Renfro, 1973), sediments can be particularly useful in detecting pollution sources and in the selection of critical sites for routine water sampling (Aston & Thornton, 1977). In addition, the ease of sampling, sample handling and chemical analysis make sediment studies preferable. Within sediments heavy metals are distributed in certain geochemical fractions, existing in several states of varying mobility, leachability and bioavailability. In general, metals in aquatic sediments can be divided into those that are relatively immobile in the long term and those whose ease of mobility in the short term poses some importance as far as exposure to the biosphere is concerned. To facilitate the understanding of metal geochemical distributions and bioavailability, geochemical fractionation studies are frequently performed, and are often in the form of sequential extractions on the sediments. This method subjects the sediments to 'selective' leachings with appropriate reagents for each of the identified geochemical fractions. In this study, sediments from three different estuaries in northwest England were collected and studied for their total metal distributions, nonlithogenous metals and four different geochemical metal fractions. In addition, the percentage organic carbon contents and the percentage less than 63/am grain size fractions were determined. The geochemical fractions identified here are: 'the easily or freely leachable and exchangeable (EFLE)', 'oxidisable-organic', 'acidreducible' and the residual or resistant fractions. The 'EFLE' fraction takes into account metals associated with the exchangeable surfaces of clays and organic matter and those easily leachable by non-oxidising and non-reducing chemical reactions. The'oxidisable-organic' fraction covers the metals which are organically bound, and which are released when oxidised by, for example, peroxides. The 'acid-reducible' fraction mainly accounts for the metals associated with the Mn and Fe oxides and hydroxides and possibly also with carbonates. The metals not released by these preceding extractions are termed 'residual' or 'resistant', and presumably are those strongly trapped within silicate minerals. The mathematical summation of the first three fractions constitutes the nonlithogenous fraction. The term 'lithogenous' is based on the sediment components primarily being derived from the breakdown of silicate rocks on the continents during weathering and soil formation (Chen & Yen, 1972). Non-lithogenous metals are correspondingly those associated with
Hea~'y metals in estuarine sediments
183
all other phases apart from the above, which in many polluted estuarine situations are of man-made as well as natural origins. The sequential extraction procedure adopted for the study was based on comparative studies of different extracting reagents and the effects of different sequences of extraction as reported by Badri & Aston (1981). E X P E R I M E N T A L METHODS Transect sediment samples from the Mersey (97 samples) and the Wyre (77 samples) estuaries in northwest England were collected by use of a hovercraft, and by scraping the top 2cm of sediment directly into previously acid-soaked polyethylene screw-cap bottles. The Mersey sediments represent a polluted situation, due to industrial and economic activities at the port of Liverpool. The Wyre estuary is also polluted but not to the same magnitude as the Mersey. For a baseline comparison, sediment samples from the Ravenglass estuary (35 samples) were collected using a Teflon spatula from the top 2 cm of the surface sediment at low tide. The wet sediments were air-dried in clean-air cabinets, homogenised, and later stored in previously acid-soaked sample bottles. Air-dried sediments were weighed into 150-ml polypropylene bottles and mixed with 1"0 M NH4CHaCOO and 0.5 M Mg(CHaCOO)z (pH 7) at the solid:reagent ratio of 1.5. They were then agitated at constant speed for 1 h and centrifuged at 3000 rpm for 30 min at 20 °C. The samples were then filtered through 0.45 ~m Millipore filters into acid-soaked 100ml glass-stoppered conical flasks. The filtrates were acidified by adding 2 ml of HNO3 and stored for AAS determination. The solid sediments in the bottles were then washed free from the first 'EFLE' reagents by shaking with 50ml of double-distilled de-ionised water (DDDW) for 1 h, centrifuged and the water decanted. The subsequent phase extractions followed the same procedure. For the 'oxidisable-organic' extraction, the solid samples were first oxidised with 3 0 ~ H 2 0 2 in a water bath at 94-97°C. After cooling the metals released from the organic complexes were extracted using 1.0 ~l NH4CH3COO acidified to pH 3.5 with HCI, and shaken at a solid:reagent ratio of 1.5 for I h, centrifuged and filtered again in the same manner. The filtrate contained the metals in the 'oxidisable-organic' fraction. The solid sediments were then washed with D D D W , and later extracted for the 'acid-reducible' fraction in the same manner using 0-25 M N H z O H . HC1 (pH 2.0) in H N O 3.
184
M. A. Badri, S. R. Aston
The met~ils Fe, Mn, Pb, Cu and Cd were determined by AAS (Perkin Elmer model 305). Some of the Pb and Cd measurements were made by flameless AAS, or after pre-concentration with methyl-iso-butyl ketone (MIBK). Organic carbon analysis was performed on all the samples by the use of a chromic acid titration method (El Wakeel & Riley, 1957), and the percentage less than 63/~m grain size fraction was determined by wet sieving. Total metal concentrations in the sediments were determined by acid digestion in PTFE beakers at 170 °C, using HNO3: HF: HC10 4 at the ratio of 6:4:3 followed by AAS analysis.
RESULTS A N D DISCUSSION The concentrations of trace metals in the sediments of the three estuaries are given in Table 1, both for total metal and the non-lithogenous metal fraction, based on the average of all the sediment samples. The locations of the three estuaries are shown in Fig. 1. The concentrations of Fe and Mn in sediments from the three estuaries do not differ significantly among the locations, either in the total or the TABLE 1 Total and Non-lithogenous Elements in the Three Northwest England Estuaries (in nag kg - l) Ravenglass Mean SD Total metal Fe Mn Zn
Pb Cu Cd Non-lithogenous Fe Mn Zn
Pb Cu Cd
16783 4985 616 248 90 39 37 11 10 4.7 3.6 1.8 336 362 23 9.9 1.4 1.1
194 175 12 2.8 0.6 0-3
Wyre Mean SD
Mersey Mean SD
10555 4332 459 198 82 59 35 18 9.7 7.8 6.0 2.7
12693 8400 564 750 236 187 57 53 41 64 5-3 3.3
237 257 23 12 2.1 1.4
62 130 11 4.3 0.9 0.3
322 277 140 20 7.4 1.8
207 234 94 1! 6.7 0.9
Heat'y metals in estuarine sediments
185
Scotland
Solway Firth
England
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tavenglass 1 : 10 s Irish Sea
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Wyre
Liverpool ,,~ Bay
Fig. I.
Locations of the Mersey, Wyre and Ravenglass Estuaries in northwest England.
non-lithogenous fraction. However, the high standard deviation values indicate very high variations within and among the transects. For instance, the concentration of total Fe in the Mersey sediments varies from 5500 to 35 000 mg kg - ', Mn from 100 to 1500mg g- 1, Zn from 41 to 650mgkg -1, Pb from 12-5 to 240mgkg -1, Cu from 1 to 140mgkg -~ and Cd from 1.1 to 11 mg kg- 1 Among the heavy metals, Zn exhibits the highest percentage of the nonlithogenous fraction with 59 ~o of the total in the Mersey, 27 70 in the Wyre and 25 7o in the Ravenglass. This indicates the variations in the magnitude of the metal scavenging in the three different estuaries. For lead the non-lithogenous fraction on average accounts only for 35 ~ of the total Pb in the Mersey estuary sediments. The Wyre and Ravenglass sediments contain only 24 7o and 27 ~ non-lithogenous Pb, respectively,
M. A. Badri, S. R. Aston
186
emphasising the importance of the resistant fraction. For copper only 14 ~ on average is in the non-lithogenous fraction in the Mersey, 20 ~ in the Wyre and 14~o in the Ravenglass sediments. Non-lithogenous Mn is frequently important in the scavenging of other metals, and the sorption of heavy metals onto Mn oxides has been well documented (Forstner & Wittman, 1981). The present results indicate that non-lithogenous Mn does not show much variation among the sediments of these three estuaries. In the Mersey sediments, the nonlithogenous Mn constitutes 49 ~ of the total on average, but this increases to 56 ~ in the Wyre and 58 ~ in the Ravenglass sediments. The higher solubility and leachability of Mn as compared with Fe has also been reported (Chao, 1972). The results of this study indicate that the Fe is mainly in the resistant fraction of the sediments (about 98 ~o), and hence may not be of much significance in the sorption of the heavy metals. Chemical fractionation of Fe in Pacific pelagic sediments also showed that Fe was mainly in the resistant fraction (Forstner & Stoffers, 1981). Within the non-lithogenous fraction, the organically bound metal fraction seems to be the most dominant. Table 2 shows that the 'oxidisable-organic' fraction is the most important geochemical fraction for the metals Mn, Zn, Pb and Cu. The precision for Cd is low due to its small total concentrations, and hence it has been eliminated from the table. As may be seen, in all cases the 'oxidisable-organic' fraction constitutes more than 5 0 ~ of the non-lithogenous metals in the sediments. The affinity of Cu and other metals for sorption into organic matter has been observed in many situations (Rashid, 1974; Filipek & Owen, 1979; Fillipek et al., 1981; Forstner & Wittman, 1981). TABLE 2 'Oxidisable-organic' Metals in Percentage of Total (~oT) and Percentage of the Non-lithogenous (~oNL)
Mn Zn Pb Cu
Ravenglass %T %NL
%T
39 19 18 8
43 19 22 14
64 70 64 59
Wyre %NL 78 54 63 66
Mersey %T %NL 44 39 21 14
76 66 75 70
Heavy
metals in estuarine sediments
187
The results of the sequential extraction of metals in sediments from the three estuaries may probably be best examined in terms of their percentages of the total metal contents (Fig. 2). The percentages are based on the average values of all the analysed sediments from each of the estuaries. At a glance the Mersey sediments are seen to contain the highest proportions of non-lithogenous metals and are the most polluted. The 'EFLE' fraction is not as geochemically important here as the 'oxidisable-organic', but in most samples it is more important than the 'acid-reducible' fraction. There is no specific pattern for the affinity of the metals to the'EFLE' fraction in the sediments from the different estuaries. For instance, 11% of total Pb in the Wyre estuary sediments is in the 'EFLE' fraction, and only 4% in the Mersey, and 8 % in the Ravenglass Fe
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188
M. A. Badri, S. R. Aston
samples. For zinc, the Mersey sediments exhibit the highest proportion of this fraction with 13 ~ of 'EFLE' Zn, while in the Wyre and Ravenglass sediments this fraction constitutes only 4 9/o and 2 ~ respectively. The different pattern of Pb and Zn sorption in this very loosely bound phase suggests dissimilar chemical associations of the metals in the surface coatings of the sediments. The 'EFLE' Cu is almost the same in all three estuaries' sediments. Even though Mn is usually believed to be easily leachable, the 'EFLE' reagents only extracted 6 9/ooof the total Mn from the Mersey and Ravenglass sediments, and only 2~o from the Wyre samples. However, Mn constitutes the highest percentage in the 'acid-reducible' fraction compared with the rest of the metals. In the Ravenglass sediments, the 'acid-reducible' Mn is 13 ~ of the total, with the Wyre and Mersey sediments having l0 9/o and 8 9/0, respectively. The metals Pb, Cu and Cd have the smallest proportion contained in the 'acid-reducible' fraction, and only Zn has a substantial contribution. The distribution of the 'acidreducible' Zn is not dependent on the locations or the degree of total metal pollution. Aside from Mn, it is very obvious that the resistant fraction constitutes the highest percentage of metal accumulation in the sediments from all three estuaries. The above data show the resistant fraction as the most dominant, but these conclusions are based on average values. A more detailed analysis of all the sampling points shows a very high variation in the distribution of the metals in sediments throughout the estuaries. This high variability highlights two important points: (1) the deposition of the metals (by natural and man-made sources) is not homogeneous with respect to the recently deposited sediments, and (2) there is a danger of misleading interpretation of monitoring studies of heavy metal pollution which are based on sediment analysis if samples are few or unrepresentative in their locations. The results show good correlations for the influence of Fe and Mn deposition on the distribution of heavy metals Zn, Pb, Cu and Cd. This means that sediments with high Fe content tend to be also high in the concentrations of Mn, Zn, Pb, Cu and Cd (and probably other metals). The table of correlation coefficients (Table 3) shows the relationships between the total metals and the 'oxidisable-organic' fractions with the non-lithogenous Fe and Mn, organic carbon and grain size (percentage less than 63 #m) for the three estuaries. With the exception of Cu and Mn in Mersey sediments, total metals correlate well with the non-lithogenous
Heavy metals in estuarine sediments
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190
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Fe, and similar correlations are also seen between total metals and the non-lithogenous Mn. Such good correlations are not quite so evident for the 'oxidisable-organic' fraction. While the 'oxidisable-organic' Fe and Mn are dependent on the local Fe and Mn, the other metals in this fraction do not show this similarity, especially in the relatively unpolluted Ravenglass sediments. It is apparent that 'oxidisable-organic' Zn, Pb and Cu are not dependent on either the non-lithogenous Fe or Mn. While a comparison of the affinity of metals for the organic fraction of sediments is made difficult due to different sequential extraction techniques employed by different workers, it is quite evident at least in theory that the nature of the organic matter available in sediments serves as a possible reservoir for metal sorption. For example, Gibbs (1977) found that the solid organic materials in sediments account for 5-19 ~o of the total metals transported in the Amazon estuary. The relationships between the metals and organic carbon in the present sediments are most noticeable in terms of the total metal concentrations for all three estuaries, and for the 'oxidisable-organic' fractions of metals of the Mersey. Percentage less than 63 #m grain size correlations with metals both in the total and the 'oxidisable-organic' fraction are highly variable. In the 'oxidisable-organic' fraction, only Mn shows good correlations with the percentage less than 63 #m grain size. This indicates that ~ organic carbon and percentage less than 63 #m grain size only show good relationships with total metal concentrations, and the geochemical fractions do not in general depend so much on the percentage of organic carbon present, and especially not the percentage less than 63 #m grain size. The effects of grain size on total metal concentrations in sediments are related to the surface area of coatings on the grains which act as adsorption sites of heavy metals (Forstner et al., 1979). Analyses of polluted sediments from the lower Rhine indicated that there tends to be a distinct reduction of Fe, Mn and Zn contents as the grain size increases for most of the geochemical phases extracted (Forstner & Patchineelam, 1980). Gupta & Chen (1975) concluded that there was an increase in the non-residual fraction of the heavy metals with decreasing sand content of near-shore sediments in Los Angeles harbour. Metal concentrations in sediments were also found to increase with decreasing particle size in the Severn estuary, UK (Thorne & Nickless, 1981), and in Little Traverse, Lake Michigan, USA (Filipek & Owen, 1979). These other studies show similar trends to those found in the present estuarine sediments.
Hea W metals in estuarine sediments
191
The present results have shown correlations between percentage organic carbon and total metals and the non-lithogenousmetals and most of the other geochemical phases. The importance of organic carbon as a heavy metal reservoir and the tendency for metals to be dominant in the organically bound sediment phase is quite well established for river and estuarine sediments (Gardiner, 1974; Serne & Mercer, 1975; Nissenbaum & Swaine, 1976; Filipek & Owen, 1979), and also for soils and street dusts (Harrison et al., 1981). While the humic acid component of the organic matter represents the main component in marine and estuarine sediments (Nissenbaum & Kaplan, 1972; MacFarlane, 1978), the affinity of metals for humic acid may vary between the individual metals. For instance, Cu is much preferred by humic acids than is Cd (Oakley et a l., 1981). The total organic component has also been shown to have higher preference for Cu than Cd (Stumm & Brauner, 1975). The present study indicates that percentage organic carbon correlates much better with Cu and Cd than for other metals in the Wyre sediments for total element, non-lithogenous fractions, and the rest of the geochemical fractions. However, in the Mersey sediments with equally as much organic carbon but higher heavy metal concentrations this feature is only true for total metals, 'oxidisableorganic' metals and the resistant fraction. For the other fractions no significant difference was observed between the estuaries. In relatively unpolluted sediments (Ravenglass) the strong correlation of organic carbon and Cu is observed only for the total metal and the 'EFLE' fraction.
CONCLUSIONS Geochemical fractionation studies of sediments from the three English estuaries varying in their degree of heavy metal pollution suggest rather different behaviour among the heavy metals. In all cases Fe was mainly resistant to sequential extractions. There are high variations within transects and among transects, which call for large sample numbers and sampling sites for monitoring purposes. The metals, as a general rule, have different affinities depending on the nature of metal scavengers that are available in sediments. The dominance of the organically held fraction was very conspicuous in the non-lithogenous metal contents. The presence of metals in the 'EFLE' and organic fractions suggests that a portion of the metals may be easily available for biological uptake. In a
192
M. A. Badri, S. R. Aston
polluted estuary, such as the Mersey, with a high percentage of metals such as Zn and Cd in the non-lithogenous fractions this could be of significance. H u m a n activities such as dredging, deepening of navigation channels or extension of harbour areas may expose the potentially available metals to the biosphere, and the continuing study of the problem of polluted sediments is of importance. ACKNOWLEDGEMENT One of us (M.A.B.) wishes to thank the National University of Malaysia, Bangi, Salangor, Malaysia for financial support. REFERENCES Aston, S. R. & Thornton, I. (1977). Regional geochemical data in relation to seasonal variations in water quality. Sci. Total Environ., 7, 247-60. Badri, M. A. & Aston, S. R. (1981). A comparative study of geochemical fractionation of heavy metals in estuarine sediments. In Proe. int. Conf. Heavy Metals in the Environment, 15-18 September, 705-8. Amsterdam, WHO. Chao, T. T. (1972). Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Proc. Soil Sei. Soc., 36, 764-8. Chen, K. Y. & Yen, T. F. (1972). Models for the fate of heavy metals in sediments. ACS Division of Water, Air and Waste Chemistry Reports, 12, 165-79. El Wakeel, S. K. & Riley, J. P. (1957). Determination of organic carbon in marine muds. J. Cons. int. Explor. Met., 22, 180-3. Filipek, U H., Chao, T. T. & Carpenter, R. H. (1981). Factors affecting the partitioning of Cu, Zn, Pb in boulder coatings and stream sediments in the vicinity of a polymetallic sulphide deposit. Chem. Geol., 26, 105-17. Filipek, L. H. & Owen, R. M. (1979). Geochemical associations and grainsize partitioning of heavy metals in lacustrine sediments. Chem. Geol., 26, 105-17. Forstner, U., Patchineelam, S. R. & Schmoll, G. (1979). In Proc. int. Conf. on Management and Control of Heavy Metals in the Environ., September, 316-19. London. Forstner, U. & Patchineelam, S. R. (1980). Chemical associations of heavy metals in polluted sediments from the lower Rhine river. Am. Chem. Soe. Adv. Chem. Ser., 189, 177-93. Forstner, U. & Stoffers, P. (1981). Chemical fractionation of transition elements in Pacific pelagic sediments. Geoehim. Cosmoehim. Aeta, 45, 1141--6.
Heat')' metals in estuarine sediments
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Forstner, U. & Whittman, G. T. W. (1981). Metal pollution in the aquatic environment. Berlin, Springer-Verlag. Gardiner, J. (1974). The chemistry of cadmium in natural waters, II. The adsorption of cadmium on river muds and naturally occurring solids. Water Resour. Res., 8, 157-64. Gibbs, R. J. (1977). Transport phases of transition metals in the Amazon and Yukon rivers. Bull. Geol. Soc. Am., 88, 829-43. Gupta, S. K. & Chen, Y. K. (1975). Partitioning of trace metals in selective chemical fractions of nearshore sediments. Environ. Lett., 10, 129-58. Harrison, R. M., Laxen, D. P. H. & Wilson, S. J. (1981). Chemical associations of lead, cadmium, copper and zinc in street dusts and roadside soils. Environ. Sci. Technol., 15, 1378-82. MacFarlane, R. B. (1978). Molecular weight distribution of humic and fulvic acids of sediments from a north Florida estuary. Geochim. Cosmochim. Acta, 42, 1579-82. Nissenbaum, A. & Kaplan, J. R. (1972). Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnol. Oceanogr., 17, 570-82. Nissenbaum, A. & Swaine, D. J. (1976). Organic matter metal interactions of recent sediments: the role of humic substances. Geochim. Cosmochim. A cta, 40, 809-16. Oakley, S. M., Nelson, P. D. & Williamson, K. J. (1981). Model of trace metal partition in marine sediments. Environ. Sci. Technol., 15, 474-80. Rashid, M. A. (1974). Adsorption of metals on sedimentary and peat humic acids. Chem. Geol., 13, 115-23. Renfro, W. C. (1973). Transfer of 65Zn from sediments by marine polychaete worms. Mar. Biol., 21, 305-16. Serne, R. J. & Mercer, B. W. (1975). Dredge disposal study, San Francisco Bay and Estuary. In Appendix F, Crystalline Matrix Study. San Francisco, US Army Engineer District. Stumm, W. & Brauner, P. A. (1975). Chemical speciation. In Chemical oceanography, ed. by J.P. Riley and G. Skirrow, 173-239. London, Academic Press. Thorne, L. T. & Nickless, G. (1981). The relation between heavy metals and particle size fractions within the Severn estuary (UK) intertidal sediments. Sci. Total Environ., 19, 207-13.