Chemical partitioning of heavy metal contaminants

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: ; 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). We would like
to acknowledge Prof. Wang Wenzhi and his colleagues

in the South China Sea Institute of Oceanology,
Chinese Academy of Sciences in Guangzhou for their
help on the sampling programmes and physical para-
meter analysis of the sediment samples.
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