Chemical partitioning of heavy metal contaminants

Các dạng hóa học của kim loại nặng trong vùng bị ô nhiễm

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 chemiphysi-cal behaviour in terms of chemiphysi-cal 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 partitioning of heavy metal contaminants

in sediments of the Pearl River Estuary

Xiangdong Li1*, Zhenguo Shen1,2, Onyx W H Wai1and Yok-sheung Li1

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

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 km2 (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/HClO4/HNO3) (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 MgCl2, 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 NH2OHHCl in 25% (v/v) HOAc at 96C, for 6 h); (4) organic bound (oxidizable phase) (5 ml of 30% H2O2and 0.02 M HNO3for 2 h,

a second 3 ml of 30% H2O2for 3 h, at 85C); and (5) residual fractions (total digestion with a concentrated mixture of HNO3/HClO4)

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

with a concentrated mixture of HNO3/HClO4 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-Figure 1 Map of the Pearl River estuary showing locations of sampling sites.

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–1at Site 4 to 212 mg kg–1at 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

(CaCO3.ZnCO3) 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 associafrac-tion of Zn with carbonate appeared to be less pronounced due to low content of carbonates (1.8% CaCO3, 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 availexchange-able

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-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 )

ND – Not detectable

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

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)

**P<0.01; *** P<0.001; NS: not significant; (n = 26)

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

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.

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

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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