Introduction

Heavy metals discharged into a marine system by natural or anthropogenic sources during their transport are distributed among the aqueous phase, suspended solids and sediments. Sediments can be, depending on the environmental conditions, either a sink or a source for heavy metals in aquatic environment. Changing environmental conditions, such as pH, redox potential or the presence of organic chelators may render the remobilization of metals from sediments (Bernhard et al. 1986; Calmano et al. 1990; 1993; Nystrom et al. 2003; Guevara-Riba et al. 2004; Lager et al. 2005).

Marine sedimentary particulate matter consists of different phases, including crystalline minerals, carbonates, hydrous metal oxides and organic matters (Szefer et al. 1995). The distribution of heavy metals in the different phases of sediment determines their mobility (Rauret et al. 1988), bioavailability (Szefer et al. 1995) and toxicity (Borovec 1996) in the aquatic environment. Thus, to assess the environmental impact of polluted sediments, the determination of the total metal concentration is not sufficient. Sequential extraction techniques have been used to examine the partitioning of sediment-associated metals among the various “operationally defined geochemical phases” in the sediment.

Since the early eighties, large amounts of different pollutants have been discharged into the coastal regions in Hong Kong with the advent of industrial and commercial activities (Environmental Protection Department 1998; Li et al. 2000). The main sources of industrial effluents include the electroplating, dying, printing, metal finishing and chemical industries (Sin et al. 2001). Increased dredging, reclamation and site formation activities have also caused a significant disturbance to the already contaminated sea bed sediments. These activities may expose anoxic sediments to oxic conditions. Changing environmental conditions, such as redox potential may render the remobilization of metals from sediments. This motivated us to assess the extent of metal contamination in dredging and reclamation sites in Hong Kong. In the present study, the characteristics and distribution of cadmium, chromium, copper, lead and zinc of marine sediments were determined using sequential extraction method (Tessier et al. 1979) in order to assess the availability of these metals in six major sampling sites. Significant dredging and reclamation will be carried out in these sites.

Methods

Sampling areas and sample treatments

Six locations were selected for this study. They were: (1) Sham Tseng; (2) Peng Chau Typhoon Shelter; (3) Kellette; (4) Tolo Harbour; (5) Cheung Sha Wan; and (6) Southeast Kowloon (Fig. 1). Fifty sediment core samples were collected from these areas and they were retrieved in thick-walled (0.25 in), 3-in inside diameter tubes (Lexan) using a vacuum-action piston corer. Sample contact with oxygen was avoided to obtain meaningful extraction results from redox-sensitive solid phases. These sediment cores were immediately packed in nitrogen-pruged delivered-boxes and stored at 4°C for transport back to the laboratory. All subsequent sediment preparation and extraction steps in the laboratory were performed in an acrylic glovebox or polyethylene glovebag purge with high-purity nitrogen gas.

Fig. 1
figure 1

Location of sampling sites (latitude/longitude)

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The sediment cores were then subsectioned into 5-cm increments using a hacksaw, and the sediment that came in contact with the hacksaw was removed with a Teflon spatula and was discarded. Each increment was then placed into 250-mL polycarbonate Sep-Cor centrifuge bottles, and the porewater was removed by centrifugation at 11,000 g for 25 min. Sediment samples were freeze-dried and were then homogenized with mortar and pestle until near uniform consistency, preferably to pass through a series of plastic sieve (1,180, 300, 150, 63 μm). All samples used in the study except the grain size effect were passed through the 63-μm ASTM sieve.

Wet sediment samples from both highly contaminated Tolo Harbour and less metal-rich sandy sediment from Sham Tseng were used to test the extraction procedure for intramethod reproducibility. In addition, intermethod variability between the sequential extraction procedure and a single-step acid digestion by aqua regia was investigated. Samples for inter- and intramethod reproducibility experiments were frozen until needed, and then thawed.

A necessary validation step for the extraction procedure was quantifying recovery of metal-bearing spikes added to sediment matrixes. Homogenized freeze-dried sediment was used as the matrix for spike recovery assessment. To minimize potential redox alterations in the freeze-dried sediment, the sediments were prepared in a nitrogen atmosphere and the samples were then loaded into a nitrogen-purged desiccator for freeze-drying. The freeze-dried matrix sediments used for the standardization were able to quantitatively determine spike recoveries.

Apparatus and reagents

All glassware and plasticware used for this experiment were previously soaked twice in 10% HNO3 (v/v) for at least 24 h and rinsed with deionized-distilled water five times before used. Volumetric flasks were of Class A. A Perkin Elmer Optima 3300 DV inductively coupled plasma-atomic emission spectrometry (ICP-AES) instrument was used to determine the metal concentrations in the sample extracts.

All reagents used were of ‘Analytical Grade’ or ‘Suprapur’ quality. Deionized-distilled water (Millipore, 18.3 Mohm/cm resistivity) was used. Stock standard solutions (1,000 mg/L) acidified with nitric acid were used. All standard and reagent solutions were stored in polypropylene bottles.

Sequential extraction procedures

The leaching scheme used mainly followed that proposed by Tessier et al. (1979) with a change in the final step. This procedure defines the following fractions: exchangeable metal (extracted with MgCl2 solution), bound to carbonates (leached by NaOAc/HOAc), bound to Fe-Mn oxides (extracted by NH2OH·HCl), bound to organic matter/sulfides (released from sample by HNO3 and H2O2, then NH4OAc/HNO3 extraction), and residual [digested by aqua regia (3:1 HCl:HNO3)]. Then, the metal concentration in each fraction was determined by ICP-AES. Seven standard solutions were prepared for calibration of each metal concentration. Calibration curves for each heavy metal were prepared prior to every batch of sample determinations. For accuracy, triplicates were analysed per sample. A standard reference material from the National Institute for Science and Technology, USA, was used for quality control purposes.

Results and discussion

General distribution of cadmium, chromium, copper, lead and zinc

The total metal concentrations (Cd, Cr, Cu, Pb, Zn) of marine sediments collected from various sites are shown in Table 1. The results are expressed as milligrams of metal extracted per kilogram of dry sediment, and are the mean value of three replicates.

Table 1 Comparisons of heavy metal content (mg/kg) in selected estuarine and coastal surface sediments in industrial areas
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Compared with other estuarine and coastal sediment, the results show similar range of metal concentration to that in Barcelona Harbour, Spain.

Sham Tseng

Among all the selected locations, this area showed the lowest heavy metal concentration. It could be because there have not been too many industrial and commercial discharges into that area. By comparing with the selected background and other areas in this study, the pollution of this area was relatively low.

Peng Chau Typhoon Shelter

Although there are only 32 industrial sources of discharges found in that part of Hong Kong, the metal concentrations were much higher than those in Sham Tseng. This could be due to the weak tidal flushing inside the typhoon shelter. The sediment in the typhoon shelter could be vulnerable to the accumulation of heavy metals.

Kellette Bank

Sediments from this water had higher metal concentrations, especially Cu and Zn contents, than most other sites. The results indicate that significant pollution problem exists in this area. Kellette Bank is located in Victoria Harbour and some major urban areas are included in this water control zone, which received about 1.3 million m3/day of mostly untreated sewage effluents from 12 sewage outfalls during 1997. Electroplating and metal finishing factories around the area were also the likely major sources of metal contamination.

Tolo Harbour

The levels of copper, zinc and lead were relatively high. There are many industrial estates that discharge heavy metals from textiles, electroplating, galvanizing and plastic fabrication industries into the Harbour.

Cheung Sha Wan

Copper concentration was very high among all the selected locations. This high concentration is expected as there are many electroplating, textile and clothing industries in this area too.

Southeast Kowloon

As this sampling point was near Junk Bay, the results would be affected by the eastern buffer control zone to a certain extent. Relatively high concentration of copper was found in the sediment, which was mainly due to the industrial effluents discharged from Kwun Tong and Wan Chi. There are also many electroplating factories in this area.

Metal speciation

Heavy metals in sediments can be bound in different forms. The distribution of metal is mainly influenced by the competition of ligands in the sediment in a narrow range of pH, but wide range of redox potential (Oakley et al. 1981). On the surfaces of the sediments, the main ligands are, of course, Mn and Fe oxides (as Mn·MnO2), reactive Fe or Fe·MnO2, amorphous Fe oxides or Fe·Fe2O3 (Horowitz et al. 1989), clay, sulfides and organic matter (as humic acids). In a sulfide-poor environment, the adsorption could be treated as competition between metals and protons for surface sites. Table 2 shows the concentrations of Cd, Cr, Cu, Pb and Zn in the various operationally defined geochemical phases of the sediment samples obtained from six selected locations in Hong Kong.

Table 2 Chromium, copper, lead, zinc and cadmium (mg/kg) partitioning in the sediment samples
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Exchangeable (I) and carbonate phases (II)

These two phases represent the readily available fraction. The binding of metals onto these phases is vulnerable to pH changes and salinity fluctuations. Therefore, these phases are the most hazardous for the environment.

Reducible phase (III)

This phase theoretically represents the contents of each metal bound to iron and manganese oxides that would be released if the sediment is subjected to more reductive conditions (Panda et al. 1995).

Oxidable phase (IV)

This phase shows the amount of metal bound to the organic matter and sulfurs, which would be discharged into the environment if conditions change to oxidative. Such a change could occur during dredging (Förstner 1989).

Residual phase (V)

In this phase, the metals, which are associated strongly to the crystalline structures of the minerals, are extracted and are therefore the most difficult to separate from the sediments (Kersten and Förstner 1991).

The idea of metal ‘availability’ to plants and animals is well known, and there are many empirical chemical extractants used on a routine basis to estimate the metal components in sediment and soil available for plant as well as animal uptake under varying environmental conditions (Förstner 1983, 1985; Usero et al. 1998). Figure 2 gives the metal speciation of the sediment collected from the various sites. The readily available fraction includes the exchangeable phase (I), carbonate phase (II) and the reducible phase (III). The binding of metal onto these three fractions is vulnerable to pH changes, salinity fluctuations and variations in redox potential.

Fig. 2
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Chromium, copper, lead, zinc and cadmium partitioning in the collected samples

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Slightly over 20% of total Cu and Cr existed as readily available forms in Peng Chau and Kellette Bank. Cu and Cr present in the phase would remobilize rather easily in the seawater. At some sampling sites, over 15% of the Cu existed as the exchangeable form. This fraction would be readily released if pH is lowered in the aqueous phase. Significant amounts of Cu and Cr were associated with the oxidable phase at most sampling sites. This agrees with the result of other studies, which reported that a high portion of Cu became bound to the organic matter in the sediment (Pardo et al. 1990). Stumm and Morgan (1981) proposed that copper can easily form complexes with organic compounds that make them rather stable in the aquatic environment. The complex binding with the organic matter and the high affinity of the metal-sulfide bond provide a strong and stable binding under anoxic conditions. However, a change of oxidation state will convert organic-bound metals into mineralized metals that could cause a significant effect on the environment. Such a change could occur during dredging and thus metals would remobilize from sediments.

A significantly higher percentage, over 50%, of Pb and Zn, were associated with the three non-residual fractions at most sampling sites. Zn and Pb were mainly associated with Fe-Mn oxides, as they formed stable complexes with Fe-Mn oxides. A typical example is with Zn, which could be adsorbed on Mn/Fe-oxy-hydroxide: Zn2+ + 2 Fe3+ + 4H2O → ZnFe2O4 + 8H+ (Pulford and Bakhsh 1989). A substantial amount of zinc was found in carbonate phase (20.5 and 20.1%) at Tolo Harbour and Kellette Bank, as calcium carbonate is a strong absorbent to form complexes with zinc as double salts CaCO3·ZnCO3 in the sediment (Zhou et al. 1998). The zinc associated with the carbonate phase would remobilize and redissolve into the seawater if the pH is changed. Hence, a greater environmental concern is warranted for the remobilization of Pb and Zn compared with Cr.

The percentage of metal extracted in each step of the sequential extraction procedure is represented as bar diagrams (Fig. 2). More than 50% of Cd existed in residual form, except for samples collected from Tolo Harbour, which were dominated by the oxidable phase. The high amount of residual Cd and the relatively lower Cd content indicate that little environmental concern is warranted for the remobilization of Cd.

Effect of depth

Figure 3 shows the distribution of metals in sediments collected from different depths at the Kellette Bank. In general, it shows that metal contents in the surface sediments were generally higher than those in the deeper layers. However, the difference in concentration between top and bottom sediment layers was not significant for Cu, Cr and Cd. The levels of Pb and Zn associated with phases II and III in the surface sediment increased sharply. These metals represented the pollutants that were introduced into the area not too long time ago as a result of rapid economic and industrial development. As these metals were bound to the readily available phases of the surface sediments, metal remobilization could be a concern. The metal distribution in the profile could be affected by many factors including the water input, sediment properties and dredging activities. Hence, this can also reflect the natural dynamic conditions and the disturbance by dredging activities in this area.

Fig. 3
figure 3

Effect of depth on metal partitioning demonstrated by sediments at the Kellette Bank

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Conclusions

The results obtained in this project showed that the behaviours of heavy-metal partitioning onto the sediments were quite different at various locations in the Hong Kong waters. This reflected different degrees of pollution among the areas studied. Sediments from the Kellette Bank had higher metal concentrations, especially Cu (202 mg/kg) and Zn (225 mg/kg) contents, compared with most other sites in Hong Kong. Tolo Harbour and Cheung Sha Wan also had high levels of Pb and Cu in the sediments. Slightly over 20% of total Cu and Cr existed as readily available forms in Peng Chau and Kellette Bank. At most sampling sites, over 15% of the Cu existed as the exchangeable form indicating that this fraction may be readily released into the aqueous phase. A significant higher percentage of Pb and Zn were associated with the three non-residual fractions. These metals, which were bound to the first three phases, were relatively more mobile than the others, thus increasing their bioavailability. More than 50% of Cd existed in the residual form, except for samples from Tolo Harbor, which were dominated by the organic fraction. The high amount of residual Cd and the relatively lower Cd content indicate that little environmental concern is warranted for the remobilization of Cd. Distribution of metals in sediments collected from different depths at the Kellette Bank shows that metal concentrations decreased with profile depth. The levels of Pb and Zn associated with phases II and III increased sharply in the surface sediment compared with the deeper layer. These metals represented the pollutants that were introduced into the area in the mid-eighties through the early nineties as a result of rapid economic and industrial development. As these metals were bound to the readily available phases of the surface sediments, metal remobilization could be a concern.