Chemical partitioning of heavy metal contaminants

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1. INTRODUCTION Sediment is a matrix of materials which is comprised of detrital, inorganic, or organic particles, and is rela- tively heterogeneous in terms of its physical, chemical, and biological characteristics (Hakanson, 1992). It is often stated that sediments have a marked ability for converting inputs of metals from various sources into sparingly soluble forms, either through precipitation as oxides or carbonates, or through formation of solid solutions with other minerals (Salomons and Förstner, 1984). Thus, aquatic sediments constitute the most important reservoir or sink of metals and other pollu- tants. However, due to various diagenetic processes, the sediment-bound metals and other pollutants may remobilize and be released back to overlying waters, and in turn impose adverse effects on aquatic organisms. In sediments, heavy metals can be present in various chemical forms, and generally exhibit different physi- cal and chemical behaviour in terms of chemical inter- actions, mobility, biological availability and potential toxicity. It is necessary to identify and quantify the forms in which a metal is present in sediment to gain a more precise understanding of the potential and actual impacts of elevated levels of metals in sediment, and to evaluate processes of downstream transport, deposi- tion and release under changing environmental condi- tions. Numerous extraction schemes for soils and sedi- ments have been described in the literature (Tessier et al., 1979; Sposito et al., 1982; Welte et al., 1983; Clevenger, 1990; Ure et al., 1993; Howard and Vandenbrink, 1999). The procedure of Tessier et al. (1979) is one of the most thoroughly researched and widely used procedures to evaluate the possible chemi- cal associations of metals in sediments and soils. Chemical Speciation and Bioavailability (2000), 12(1) 17 Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary Xiangdong Li 1* , Zhenguo Shen 1,2 , Onyx W. H. Wai 1 and Yok-sheung Li 1 1 Department of Civil & Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 2 Department of Agronomy, Nanjing Agricultural University, Nanjing 210095, China ABSTRACT Sequential extraction was used to study the operationally determined chemical forms of four heavy metals (Zn, Cu, Ni and Co) and their spatial distribution in the sediments of the Pearl River Estuary. It was found that the residual fraction was the most important phase for the four metals in these sediments. Among non-residual fractions, Zn, Ni and Co were mainly associated with the Fe–Mn oxide fraction while Cu was associated with the organic fraction. The Zn bound to the Fe–Mn oxide fraction had significant relationships with reducible Mn and reducible Fe concentrations (Fe–Mn oxides), suggesting that Fe–Mn oxides may be the main carriers of Zn from the fluvial environment to the marine body. There was a significant relationship between Cu bound to the organic fraction and sediment organic contents. The Zn bound to the Fe–Mn oxide fraction and Cu bound to the organic fraction showed general distinctive decrease from the west side to the east side of the estuary, and from upstream in the north to the sea in the south. This was in the same trend with the total Zn and Cu concentrations in these sediments. The results may reflect the anthropogenic inputs of heavy metals to the top sediments from recent rapid industrial development and urbanisation in the surrounding area. Keywords: Heavy metals; sequential extraction; chemical forms; sediments; estuary; the Pearl River; China *To whom correspondence should be addressed at: E-mail: cexdli@polyu.edu.hk; Tel: (852) 2766 6041; Fax: (852) 2334 6389 CSBLi 21/11/02 12:40 pm Page 17 However, the limitations of chemical extraction methods have also been addressed by several researchers (Jouanneau et al., 1983; Khebonian and Bauer, 1987; Rauret et al., 1989). The limitations include technical difficulties associated with achieving selective disso- lution and complete recovery of trace metals from geo- chemical phases in soils and sediments. Therefore, the chemical forms of heavy metals from the sequential extraction methods are operationally defined phases only. Some newer 3 and 4 stage BCR methods have been proposed recently in order to provide a standard procedure for metal speciation study (Quevauviller et al., 1997; Rauret et al., 1999). The Pearl River estuary is located in southern China, covering an area of about 8,000 km 2 (see Figure 1). Recent environmental monitoring results showed that there was a trend towards water and sediment quality deterioration in the Pearl River estuary (Wen and He 1985, Wen et al., 1995). In a previous study, we have shown that metal concentrations had increased over the last 20 years in the sediments of the Pearl River Estuary, and the west side of the estuary tended to be more cont- aminated than the east side (Li et al., 2000). The objec- tive of the present study is to identify and compare different chemical forms of heavy metals and their spatial distribution in the sediments of the Pearl River Estuary using sequential chemical extraction procedure. In order to assess the impacts of different factors on the metal accumulation and transportation in the estuary, rela- tionships between selected heavy metal contaminants and sediment characteristics have also been investigated. MATERIALS AND METHODS 2.1. Study area The Pearl River is the largest river system flowing into the South China Sea. The main Pearl River estuary (also called Lingdingyang) is a north-south bell-shape area, with a N–S distance averaging about 49 km and the E–W width varying from 4 to 58 km (see Figure 1). The whole study area is within the sub-tidal zone with strong fresh water and marine water inter-reactions and circulation currents along the west coast (Zheng, 1992; Wong et al., 1995). The rapid industrial development and urbanisation in the Pearl River Delta region in the last two decades has put great pressure in the estuarine environment. The main sources of heavy metal conta- minants in the river system have been reported to be industrial waste water discharge, domestic sewage effluent, marine traffic and runoff from upstream mining sites (Zheng, 1992). 2.2. Sediment Sampling In the present study, 21 sediment cores were collected in the Pearl River estuary (see Figure 1). The sampling programmes were carried out in the summer of 1997, with the assistance of the South China Sea Institute of Oceanology, Chinese Academy of Sciences. Core sam- ples were taken with a gravity corer with automatic clutch and reverse catcher. Most of the cores are more than 2 metres for in depth except the three cores (Core A–C) collected in the shallow water area. Sediment cores collected at each sampling station were stored at 4–6°C immediately after collection until the laboratory analysis. 2.3. Analysis methods About 15 samples (at 10 cm intervals between 0 and 1 m and at 20 cm intervals between 1 and 2 m) were taken from each core for further physical and chemical analysis. The physical parameter testing programmes included total organic matter (loss on ignition), mois- ture content and particle size analysis, according to the methods described by Mudroch et al. (1996). The total metal concentrations in sediments were determined by ICP-AES after acid digestion (HF/HClO 4 /HNO 3 ) (Li and Thornton, 1992). The details of the sampling locations and total metal concentration analysis were described by Li et al. (2000). The top two layers (0–5 cm and 10–15 cm) of each sediment core were selected to study the chemical partitioning of heavy metals using the sequential extraction procedure suggested by Tessier et al. (1979). The scheme consisted of sequential extractions in the following order and associated reagents, and opera- tionally defined geochemical forms: (1) exchangeable fraction (1 M MgCl 2 , pH 7.0, for 20 min); (2) carbon- ate bound fraction (1 M NaOAc adjusted to pH 5.0 with acetic acid, for 6 h); (3) Fe–Mn oxide bound fraction (reducible phase) (0.04M NH 2 OHHCl in 25% (v/v) HOAc at 96C, for 6 h); (4) organic bound (oxidizable phase) (5 ml of 30% H 2 O 2 and 0.02 M HNO 3 for 2 h, a second 3 ml of 30% H 2 O 2 for 3 h, at 85C); and (5) residual fractions (total digestion with a concentrated mixture of HNO 3 /HClO 4 ). After each successive extraction, separation was done by centrifuging at 2000 rpm for 15 min. The supernatants were separated with a pipette. The sedi- ment was washed in 10 ml of deionized water and again centrifuged. The wash water was discarded. Metal con- centrations were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin-Elmer, 3300 DV). The details of the sequential extraction method and ICP analysis were reported by Li et al. (1995). A standard reference material (IC-HRM2) was used to verify the accuracy of metal determination in the sequential extraction analysis (Ramsey and Thompson, 1985; Li et al., 1995). The recovery rates for heavy metals in the standard reference material were around 85–110%. Moreover, cumulative concentrations of the metals in sediments were compared with the indepen- dent total concentrations by digesting the same sample Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary18 CSBLi 21/11/02 12:40 pm Page 18 with a concentrated mixture of HNO 3 /HClO 4 . The total recovery rates for metals in sediment samples were around 82–104%. Blanks were also used for back- ground correction and random error calculation. At least one duplicate was run for every six samples to verify the precision of the sequential extraction method. The precision and bias were generally <10%. RESULTS AND DISCUSSION Sequential extraction results can provide information on possible chemical forms of heavy metals in sedi- Xiangdong Li, Zhenguo Shen, Onyx W. H. Wai and Yok-scheung Li 19 Figure 1 Map of the Pearl River estuary showing locations of sampling sites. CSBLi 21/11/02 12:40 pm Page 19 ments. The extraction scheme used in the present study is based on operationally defined fractions: exchange- able, carbonate, Fe–Mn oxides, organic, and residual. Assuming that bioavailability is related to solubility, then metal bioavailability decreases in the order: exchangeable > carbonate > Fe–Mn oxide > organic > residual (Tessier et al., 1979; Ma and Rao, 1997). The residual fraction could be considered as an inert phase corresponding to the part of metal that cannot be mobilised and as the geochemical background values for the elements in the sediments (Tessier et al., 1979). 3.1. Heavy metal concentrations and chemical partitioning in sediments The results of sequential chemical extraction of the top sediments are summarised in Table 1. The concen- tration of total Zn in the top layers of sediments was the highest among the trace metals studied, ranging from 40 mg kg –1 at Site 4 to 212 mg kg –1 at Site B. The top layers of sediments (0–5 cm) generally had higher con- centrations of total Zn than the second layers of sedi- ments (10–15 cm) in the west side of the estuary. Results of the sequential extraction showed that the residual fraction dominated the Zn distribution in sediments, accounting for over 41% of the total Zn con- centration (Table 1). This result is in agreement with observations of Gupta and Chen (1975) and Ma and Rao (1997). Among the nonresidual fractions, the Fe–Mn oxide fraction was much more important than other fractions in all sediments, which accounted for 18–42% of total Zn. The soils in most of the Pearl River basin are highly weathered and rich in Fe- and Mn- oxyhydroxides (Wen and He, 1985). Several other workers have also reported Zn to be associated with Fe–Mn oxides of soils and sediments (Fernandes, 1997; Ma and Rao, 1997; Ramos et al., 1999). The Zn adsorp- tion onto Fe–Mn oxides has higher stability constants than onto carbonates. Zhou et al. (1998) found that Zn was mainly associated with Fe–Mn oxide, carbonate and residual fractions in sediments from inland rivers of Hong Kong. Calcium carbonate is a strong absorbent to form complexes with Zn as double salts (CaCO 3 . ZnCO 3 ) in the sediments. For some metals such as Zn, coprecipitation with carbonates may become an important chemical form, especially when hydrous iron oxide and organic matter are less abun- dant in the sediment (Förstner and Wittmann, 1979). In the Pearl River sediments, the percentage of Zn bound to carbonate ranged from 1.9 to 7.8%, and was lower than that of the zinc associated with the organic frac- tion at most sampling sites. The association of Zn with carbonate appeared to be less pronounced due to low content of carbonates (1.8% CaCO 3 , on the average) in these sediments of the estuary. The exchangeable Zn was very low (<0.2% of total Zn) in these sediments. The average total concentration of Cu in the sedi- ments was 45.6 mg kg -1 (see Table 1). Sediments from Sites A, B and D had higher total Cu concentration than those from other sites. Most of the Cu was present in the residual (52–75%) and organic (7–26%) fractions in the sediments (Table 1). On the average, the per- centage of Cu associated with different fractions in the top two layers of sediment cores from all sites was in the order of: residual (64.4%)> organic (19.8%) > Fe–Mn oxide (10.2%) > carbonate (5.3%) > exchange- able (0.4%). These results are consistent with available data in the literature (Tessier et al., 1979; Ramos et al., 1999). Rapin et al. (1983) reported that Cu was mostly bound to the organic matter/sulfide fraction (70–80%) in marine sediment in highly polluted area of Villefranche Bay. Copper can easily complex with organic matters because of the high formation constants of organic-Cu compounds (Stumm and Morgan, 1981). In aquatic systems, the distribution of Cu is mainly affected by natural organic matter such as humic materials and amino acids. When content of organic matter is low, Fe–Mn oxides might become more sig- nificant for binding Cu. Han et al. (1996) found that the Cu carbonates might dominate as the available form of Cu to marine bivalves (Hiatula diphos) under natural physicochemical conditions. The first two fractions, i.e., the exchangeable and carbonate fractions were found to be minor contributors for Cu. Low Cu content in the carbonate fraction of Cu in the present study indi- Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary20 Table 1 Means and ranges of heavy metal concentrations in various operationally defined geochemical fractions and cumulative total concentrations in the top sediments from 15 sampling sites (mg kg –1 ) Zn Cu Ni Co Mean Range Mean Range Mean Range Mean Range Exchangeable 0.14 0.06–0.23 0.19 ND–0.34 0.41 0.30–0.49 0.08 0.03–0.11 Carbonate 5.75 2.79–11.7 2.40 1.41–3.42 1.84 1.04–2.31 1.20 0.52–1.72 Fe–Mn oxide 36.2 16.0–89.1 4.99 1.89–8.48 6.52 3.03–9.52 5.86 3.58–8.11 Organic 6.82 2.48–18.4 8.84 0.68–14.7 0.77 ND–1.3 0.82 0.22–1.15 Residual 70.9 16.4–95.6 29.5 4.75–41.6 26.7 4.61–34.8 9.69 2.64–11.8 Total 119 39.9–211 45.9 9.21–68.2 36.2 9.66–45.6 17.6 7.61–21.0 ND – Not detectable CSBLi 21/11/02 12:40 pm Page 20 cates that Cu may be less bioavailable in these sedi- ments. There were no significant differences of the total Cu concentration and chemical fractions between the top layers and the second layers of the sediments. Nickel was mostly concentrated in the residual frac- tion, although it was present in small amount in other fractions (see Table 1). The percentage of Ni in the residual fraction ranged from 63 to 80% in the top two layers of sediments at most of the sampling sites. These results are in agreement with the observations of Tessier et al. (1980), who suggested that a majority of the Ni in sediments was detrital in nature. Adamo et al. (1996) demonstrated that Ni in contaminated soils often occurs as inclusions within the silicate spheres rather than as separate grains using scanning electron microscopy and enery dispersive X-ray analysis (SEM/EDX). The Ni inclusions are protected against natural decomposition as well as reagent alteration, and only the dissolution of the silicates would ensure their extraction. Generally, the Ni associated with different fractions followed the order: residual > Fe–Mn oxide > carbonate > organic > exchangeable. The concentration of Co was the lowest when com- pared with other trace elements studied (Table 1). The total concentrations of Co were in the range of 7.61–21.0 mg kg –1 . In general, Co was mainly associ- ated with the residual fraction (35–63%) and Fe–Mn oxide fraction (26-47%), with all other forms making up less than 10% in all sites. There was a trend of higher percentage of Co bound to the Fe–Mn oxide fraction in top layers than that in second layers of the sediments. These findings may indicate that Fe–Mn oxides can be the major carriers of Co in top sediments. 3.2 Heavy metal associations with Fe–Mn oxides and organic matter The relationships among trace and major elements often give information on the geochemical associations and possible sources of trace metals. The sequential extraction results of the major elements can provide some information on the chemical forms of heavy metals in sediments. The relationships between Zn and Cu concentrations in the Fe–Mn oxide fraction, reducible Fe and Mn (Fe–Mn oxides), and organic matter (L.O.I.) are given in Table 2. The concentrations of the Zn bound to the Fe–Mn oxides had significant relation- ships with reducible Mn, reducible Fe, and reducible Fe + Mn (Fe–Mn oxides). The Cu in the Fe–Mn oxide fraction is significantly related to reducible Mn. There was a significant relationship between Cu bound to organic fraction and the sediment organic content (L.O.I.). No significant relationship between organic bound Zn and sediment organic matter content was found. These results are in agreement with the fact that non-residual Zn is mostly concentrated in the Fe-Mn oxide fraction, and non-residual Cu is mainly present in the organic fraction (see Table 1). Moreover, reducible Mn concentration gave higher correlation coefficient values than reducible Fe, indicating that reducible Mn might play a major role in binding heavy metals in these sediments. 3.3 Spatial distribution of heavy metals and their chemical partitioning in sediments Both natural processes and human activities influence trace metals deposition in coastal sediments (Förstner and Wittmann, 1979). The previous and present results have showed that heavy metals exhibited higher concen- trations in the top layer of sediments located in the west side than in the east side of the estuary (Figure 2) (Li et al., 2000). These observations can be attributed to the direct input of pollutants from the major tributaries and higher sedimentation rates from circulation currents in the west coast of the estuary (Chen and Luo, 1991; Huang, 1995). Figure 3 shows the distribution of various opera- tionally defined chemical fractions for Zn along the west- east transect. As can be seen, the percentages of Zn in the Fe–Mn oxide, carbonate and organic fractions decreased from western sites to eastern sites of the estuary. In con- trast, Zn in the residual fraction increased markedly. The result suggested that the decrease of total Zn concen- tration could be attributed to the decrease of Zn in non- residual fractions (e.g. the Fe–Mn oxide, carbonate and organic phases). Similarly, the decrease of Cu in the organic fraction mainly contributed to the decrease of total Cu concentration from west to east of the estuary (see Figure 4). Although there were similar distribution patterns of total Ni and total Co to that of total Zn, spatial patterns in various fractions of Ni and Co were less apparent. This may be due to the fact that these two ele- ments were derived from natural geological sources and generally present in the residual fractions. Xiangdong Li, Zhenguo Shen, Onyx W. H. Wai and Yok-scheung Li 21 Table 2 Relationship between Zn and Cu in Fe–Mn oxide and organic fractions, and reducible Fe, reducible Mn and organic matter (L.O.I) Reducible Fe Reducible Mn Reducible Fe+Mn Organic matter Zn bound to Fe-Mn oxide 0.628*** 0.935*** 0.672*** Cu bound to Fe-Mn oxide 0.566** 0.782*** 0.595** Zn bound to organic 0.308 NS Cu bound to organic 0.782*** **P<0.01; *** P<0.001; NS: not significant; (n = 26) CSBLi 21/11/02 12:40 pm Page 21 In estuaries, river water velocity decreases, relative to the river channel areas, as fresh water mixes with seawater. This process would result in deposition of sediments with associated heavy metals (Salomons and Förstner, 1984). The concentrations of total Zn in the top sediments showed a slight decrease from the upstream of the estuary in the north to the sea boundary in the south (Li et al., 2000). This pattern was much evident for Zn at the western sites, i.e. along Sites A–B –D–12 (Figure 5). Like total Zn, the percentage of Zn bound to the Fe-Mn oxide fraction showed a general distinctive decrease from north to south in the transect (Figure 6). But the residual fraction showed the increas- ing trend in the same direction. There were no signifi- cant variations of other Zn fractions in the transect. The same decreasing trends of total Cu concentration and percentage of Cu in the organic fraction were found along the transect (Figure 7). The percentage of Cu in the residual fraction tended to increase in the transect. Metals in Fe–Mn oxide or organic fractions may become soluble under the changing environmental con- ditions (e.g. pH and Eh changes). Hydrous oxides of Fe and Mn on particulate surface are significant carriers for Zn in aquatic systems. It has been reported that metals adsorbed to Fe–Mn oxides decrease in the order Cr > Zn > Ni > Cu (Badarudeen et al., 1996). The sequential extraction results of the current study sug- gest that Fe-Mn oxides may be the main carriers of Zn from the fluvial environment to the marine body in the estuary. Sediment organic matter is important for Cu in these sediments. The present results indicate that Zn has higher potentials for mobilization from the sediments than Cu because of its higher concentration in the Fe–Mn oxide fraction. Spatial distribution patterns of Zn and Cu in various fractions also indicate a higher potential for mobilization of these metals from the Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary22 Figure 2 The total heavy metal concentrations in top sediments along the west–east transect of the Pear River estuary. Figure 3 The chemical partitioning (operationally defined geochemical phases) of Zn in top sediments along the west–east transect. CSBLi 21/11/02 12:40 pm Page 22 Xiangdong Li, Zhenguo Shen, Onyx W. H. Wai and Yok-scheung Li 23 Figure 4 The chemical partitioning (operationally defined geochemical phases) of Cu in top sediments along the west–east transect. Figure 5 The total metal concentrations in top sediments along the North-South transect of the Pearl River estuary. Figure 6 The chemical partitioning (operationally defined geochemical phases) of Zn in top sediments along the north–south transect. CSBLi 21/11/02 12:40 pm Page 23 sediments of western sites than those of eastern sites, and northern sites than southern sites. The higher total metal concentrations and higher percentages of metals in non-residual fractions indicate the anthropogenic inputs to surface sediments from the recent industrial development and urbanisation in the surrounding areas. 4. CONCLUSIONS The sequential extraction results showed that Zn, Ni and Co in the top sediments were mainly associated with the residual and Fe–Mn oxide fractions. The Zn bound to the Fe–Mn oxide fraction had significant rela- tionships with reducible Mn and reducible Fe concen- trations (Fe–Mn oxides), suggesting that Fe–Mn oxides may be the main carriers of Zn from the fluvial environ- ment to the marine body. The major geochemical phases for Cu were the organic and residual fractions. There was a significant relationship between Cu bound to the organic fraction and sediment organic contents. The metals in the non-residual fractions (Zn in the Fe–Mn oxide fraction and Cu in the organic fraction) showed general distinctive decrease from the western sites to the eastern sites of the estuary, and from upstream in the north to the sea in the south. The results may reflect the anthropogenic inputs of heavy metals to the sediments from recent rapid industrial develop- ment and urbanisation in the surrounding area. ACKNOWLEDGEMENT This research project was funded by the Hong Kong Polytechnic University (PolyU PA41). 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Wai and Yok-scheung Li 25 CSBLi 21/11/02 12:40 pm Page 25 . mobilization of these metals from the Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary22 Figure 2 The total heavy metal. of Cu in the present study indi- Chemical partitioning of heavy metal contaminants in sediments of the Pearl River Estuary20 Table 1 Means and ranges of