|A Review of Sequential Extraction Procedures for Heavy Metals Speciation
in Soil and Sediments
|Hussein K Okoro1*, Olalekan S Fatoki1, Folahan A Adekola2, Bhekumusa J Ximba1 and Reinette G Snyman3
|1Department of Chemistry, Faculty of Applied Science, Cape Peninsula University of Technology, Cape Town, Bellville Campus, South Africa
|2Department of Chemistry, Faculty of Science, University of Ilorin, Ilorin, Nigeria
|3Department of Biodiversity and Conservation, Faculty of Applied Sciences, Cape Peninsula, University of Technology, Cape Town, South Africa
||Hussein K Okoro
Department of Chemistry
of Applied Science
Cape Peninsula University of Technology
7535 South Africa
E-mail: [email protected],
|Received March 03, 2012; Published July 30, 2012
|Citation: Okoro HK, Fatoki OS, Adekola FA, Ximba BJ, Snyman RG (2012) A
Review of Sequential Extraction Procedures for Heavy Metals Speciation in Soil
and Sediments. 1: 181. doi:10.4172/scientificreports.181
|Copyright: © 2012 Okoro HK, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
|Heavy metals are stable and persistence environmental contaminant of marine sediments. The vast increase in population
growth, urbanisation and industrialisation leads to increase in of marine discharges, which results in total loads of pollutants being
delivered to the sea. Heavy metal pollution in aquatic environment and subsequent uptake in food chain by aquatic organisms and
humans put public health at risks. However, even at lower concentrations heavy metals like Cd, Hg, Cr and Pb may exhibit extreme
toxicity under certain condition. Thus, this makes regular monitoring of aquatic environment to be more imperative and necessary.
This paper therefore, review the occurrence of heavy metals and various speciation methods used for heavy speciation in soil and
|Heavy metals; Speciation; Sediments; Pollution; Marine
|Heavy metals are among the most serious environmental pollutants
due to their high toxicity, abundance and ease of accumulation by
various plant and animal organisms. Persistent increase of heavy metals
in harbour sediments can be attributed to the contribution of effluent
from waste water treatment plants, industries, mining, power stations,
agriculture  which carry run-offs to the harbour. The increase in
urbanisation and industrialisation also could lead to an increase in
marine discharges and therefore results in total loads of pollutants
discharges to the sea. These discharges may contain heavy metals among
other pollutants . In addition, ship traffic especially in and close to
the harbour and repair activities are also suspected to be indicative for
elevated concentration in the upper reaches of harbours.
|Metal concentration in sediments can be traced to high
concentration in living organisms and humans and therefore put public
health at risk. The bioavailable metal load in sediments may affect
the distribution and composition of benthic assemblages  and this
will cause increase in high concentration of these pollutants in living
organisms . High concentrations of heavy metals in living organisms
can result in morphological abnormalities, neurophysiological
disturbances, genetic alteration of cells (mutation), tetratogenesis
and carcinogenesis. Moreover, heavy metals can affect enzymatic
and hormonal activities, as well as growth rate and an increase in
mortality rate . Metals accumulates in sediments from both natural
and anthropogenic sources and sediments act as a scavenger agent as
well as an adsorptive sink for heavy metals in an aquatic environment.
Sediments can therefore be described as appropriate indicators of heavy
metal pollution .
|The accumulation of metals in sediments from both natural and
anthropogenic sources occurs in the same way, thus making it difficult
to identify and determine the origin of heavy metals present in the
sediments . Moreover, the total concentration of metals often does
not accurately represent their characteristics and toxicity. In order to
overcome the above mentioned obstacles it is helpful to evaluate the
individual fractions of the metals to fully understand their actual and
potential environmental effects . Single extractions are thus used
generally to provide a rapid evaluation of the exchangeable metal
fraction in soils and sediments [9,10]. However, various complicated
sequential extraction procedures were used to provide more detailed
information regarding different metal phase associations [8,11,12].
|A wide range of techniques is available whereby various extraction
reagents and experimental conditions are used. These techniques
involve a 5step , 4step (BCR, Bureau Commune de Reference of
the European Commission) and 6-step  extraction, and are thus
becoming popular and adopted methods used for sequential extraction
[14,15]. Several analytical methods have been used for the determination
of heavy metals contents in marine environments. These include; flame
AAS [16,17], atomic fluorescence spectrometry , anodic stripping
voltametry [19,20], ICP- AES  and ICP-MS [22,23].
|Heavy metal mobility and bioavailability depend strongly on their
chemical and mineralogical forms in which they occur . Several
speciation studies have been conducted to determine study different
forms of heavy metals rather their total metal content. These studies
reveal the level of bioavailability of metals in harbour sediments and
also confirm that sediments are bio- indicators of heavy metal pollution
in marine environment [1,7,25,26].
|Although several studies have been conducted on heavy metal
pollution of harbour sediments, this paper aims to review sources,
mobility, effects remediation and analytical methods used as well as to
compare results that have been collected around the world on heavy
|Heavy metals as marine pollutants
|Major and trace elements occur naturally in the environment .
This natural occurrence of metals in the environment due to various
particle sizes for instance, complicates assessments of contaminated
marine sediments because measurable quantities of metals do not
automatically infer anthropogenic enrichment . In addition to shipping traffic especially in and close harbours Industrial activities,
vehicle emissions, agricultural activities and domestic waste can all act
as a source of heavy metal pollution in the marine environment .
|Many adverse effects have been done on human health by the
environmental pollution of heavy metals. Heavy metals condition
is problematic due to their persistence and non-degradability in the
environments . Metals distribution and association in marine
sediments occur in various ways which include ion exchange,
adsorption, precipitation and complexation. They are not permanently
fixed by sediments . Heavy metals pollution in aquatic environment
and their uptake in the food chain by aquatic organisms and humans,
put public health at risk.
|In general, heavy metals are stable and persistence environmental
contaminants of marine sediments. Interest in metals like Zn, Cu,
Fe, and Mn which are required for metabolic activities in organisms
depends on their nutritional value and their toxicity. Metals like Cd, Hg,
Cr, Pb and As may exhibit extreme toxicity even at lower concentration
under certain condition. Thus this makes regular monitoring of aquatic
environment to be imperative and necessary.
|Occurrence of heavy metals in marine sediments
|Heavy metals are stable and persistent environmental contaminants
of coastal sediments. In recent years there has been growing concern
over increased contamination of estuaries and harbours from various
anthropogenic sources . Sediments serve as the ultimate sink for
many contaminants and as a result, they pose the highest risk to the
aquatic life as a source of pollution [28,29]. Bruder- Hubscherv et al.
 worked on metal speciation in coastal marine sediments from
Singapore and confirmed that sediments are the main repository and
source of heavy metals in the marine environment and that they play a
major role in the transport and storage of potentially hazardous metals.
|A number of factors have been attributed to pollutant accumulation
in harbour sediments. The design of the harbour to minimize
hydrodynamic energy, industrial activities (ship repairs and traffic,
accidental spills, loading and unloading), agricultural activities and
urban (waste water) activities can all acts as sources of heavy metal
pollution in marine environment [1,2,5,31]. Heavy metal accumulation
in marine sediment is due to a highly dynamic nature of the marine
environment which allows rapid assimilation of these pollutants into
sediments by processes such as oxidation, degradation, dispersion,
dilution and ocean currents.
|Phytoavailability of heavy metals depends on the characteristics
of the sediment, the nature of the metal species, the interaction with
sediment matrix and the duration of the contact with the surface
binding. Heavy metal availability in marine organisms can be traceable
to sediment characteristics such as pH, organic matter content and
type, and then moisture . In general, increase in population
growth, rapid unplanned industrialization, urbanization, exploration
and exploitation of natural resources and newly introduced modern
agricultural practices are the major contributory factors responsible for
the presence of heavy metals in marine sediments.
|Heavy metals in water, soil and sediments
|Heavy metals refer to any metallic chemical element that has a
relatively high density and is toxic or poisonous at low concentration.
Heavy metals occur naturally in the ecosystem with large variations
in concentration. Nowadays, anthropogenic sources of heavy metals
i.e. pollution, have been introduced to the ecosystem. These metals are a cause of environmental pollution (heavy-metal pollution) from
a number of sources, including lead in petrol, industrial effluents and
leaching of metal ions from the soil into water bodies by acid rain.
|Toxic metals can be present in industrial, municipal and urban
runoff, and by definition they are harmful to humans and aquatic
biota. Increased urbanization and industrialization have increased
the levels of trace metals, especially heavy metals in water ways. There
are over 50 elements that can be classified as heavy metals, but only
17 that are considered to be both very toxic and relatively accessible.
Mercury, lead, arsenic, cadmium, selenium, copper, zinc, nickel and
chromium, however, should be given particular attention in terms of
water pollution and discharge effects. Toxicity levels depend on the type
of metals, its biological role, and the type of organisms that are exposed
to it .
|Zinc: Zinc is one of the numbers of trace elements considered
essential to plant growth and the physiological function of organism.
The permissible limit for zinc in portable water is 5.0ppm. At the
concentrations above, 5.0ppm, zinc can cause a bitter, astringent taste
and turbidity in alkaline waters. Zinc requirements of human vary
because individuals zinc in adults ranges from 2-3μg. The highest
concentrations are found in the urethra tract and the prostrate . It
has been found that various parts of the body contain zinc, relatively
high concentrates are present in the skin, while the visceral organs
contains approximately 30-50μg/g of fresh tissue. Most of the body zinc
is in the bones where its concentration is approximately 200 μg zn/g.
Excessive intake of Zn may lead to vomiting, dehydration, abdominal
pains, nausea, lethargy and dehydration .
|Cadmium: Cadmium is also one of the heavy metals found in soil
and water samples. It is a by-product of the mining and smelting of
lead and zinc. It is used in nickel cadmium batteries, PVC plastic and
paint pigments. It can be found in soils because insecticides, fungicides
sludge, and commercial fertilizers that use cadmium are used in
agriculture. Cadmium may be found in reservoirs containing shell fish.
Inhalation accounts for 15-20% of absorption through the respiratory
system; 2-7% of ingested cadmium is absorbed in the gastrointestinal
system. Cadmium toxicity is generally indicated when urine levels
exceed 10 μg/dl and blood levels exceed 50 μg/dl. Cadmium sulphide
and selenide are commonly used as pigments in plastics .
|Aluminium: Although aluminium is not a heavy metal (specific
gravity of 2.55 -2.80), it makes up about 8% of the surface of the earth
and is the third most abundant element. It is readily available for human
ingestion through the use of food additives, antacids, buffered aspirin,
astringents, nasal sprays and antiperspirants from drinking water .
Studies suggested that aluminium might have a possible connection
with developing Alzheimer’s and Parkinson’s disease when researchers
found what they considered to be significant amounts of aluminium in
the brain tissue of Alzheimer’s patients. Aluminium also causes senility
and presenile dementia .
|Copper: Copper is an essential substance to human life, but in high
doses it can cause anaemia, liver and kidney damage and stomach and
intestinal irritation. Copper normally occurs in drinking water from
copper pipes, as well from additives designed to control algal growth.
In humans exposure to lead can result in a wide range of biological
effects depending on the level of duration of exposure . Various
effects occur over a bound range of doses, with the developing foetus
and infant being more sensitive than the adult. High levels of exposure
may result in toxic biochemical effects in humans which in turn cause
problems in the synthesis of haemoglobin, effects on the kidneys and acute of chronic damage to the nervous system. Some studies suggest
that there may be a loss of up to 21Q points for a rise in blood lead
levels from 10 to 20 μg/dl in young children. Average daily lead intake
for adults is estimated at 1.6 μg from air, 20 μg from drinking water and
28 μg from food. Copper is generally remobilized with acid based ion
exchange or oxidation mechanism .
|Mercury: Mercury is a toxic substance which has no known
function in human biochemistry or physiology and does not naturally
in living organisms. It is a global pollutant with complex and unusual
chemical and physical properties. The major natural source of mercury
is the degassing of the Earth’s crust, emissions from volcanoes and
evaporation from natural bodies of water. The usage of mercury is
widespread in industrial processes and in various products, (e.g.
batteries, lamps and thermometers). Toxicity of mercury results mental
disturbance and impairment of speech, hearing, vision and movement
|It is also widely used in dentistry as an amalgam for fillings
and by the pharmaceutical industry. Concern over mercury in the
environment arises from the extremely toxic forms in which mercury
can occur. Natural biological processes can cause methylated forms
of mercury to form which bio-accumulate over a million fold and
concentrate in living organisms especially fish. These forms of mercury:
monomethylmercury and dimethylmercury are high toxic causing
neurotoxicological disorders. The main pathway for mercury to humans
is through the food chain and not by inhalation .
|Effects of heavy metals on public health
|Sediments house many contaminants and therefore pose the
highest risk to the aquatic environment as a source of pollution [28,29].
Environmental pollution by heavy metals impacts negatively on human
health. Their remediation proves to be problematic due to the persistence
and non degradability of heavy metals . High concentrations of
heavy metals in biota can be linked to high concentration in sediments.
The bioavailable metal load in sediments may affect the distribution
and composition of benthic assemblages , and this can be linked to
high concentration recorded in living organisms .
|The most obvious effect of pollution is to reduce diversity of
biological species that are not able to tolerate the toxicants. Most resistant
organisms are often undesirable in human terms. Example is the bluegreen
algae or sewage fungus that forms slime or scum. Heavy metals
are dangerous because they tend to bio-accumulate. Bioaccumulation
means an increase in the concentration of a chemical in a biological
organism over time, compared to the chemical’s concentration in the
environment. Heavy metals can cause serious health effects with varied
symptoms depending on the nature and the quantity of the metal
|Antimony is a metal used in the compound antimony trioxide, a
flame retardant. There is a little information on the effect of long term
antimony exposure, both lead and antimony are suspected human
carcinogen . Cadmium derives its toxicological properties from
its chemical similarity to zinc an essential micronutrient for plants,
animals and human. In human, long term exposure is associated with
renal dysfunction. High exposure can lead to obstructive lung disease
and has been linked to lung cancer. Cadmium may also produce bone
defects (osteomalacia, osteoporosis) in human and animals. This is an
intensely painful disease leading to deformity of bone .
|The biological activity of selenium has been of interest since it is
needed by humans and other animals in small amounts, but in larger amounts can cause damage to the nervous system, fatigue and irritability.
Selenium accumulates in living tissue, causing high selenium content
in fish and other organisms, and causing greater health problems in
human over a lifetime of over exposure. Acute exposure to lead is also
more likely to occur in the work place, particularly in manufacturing
processes that include the use of lead symptoms include abdominal
pain, convulsion, hypertension, renal dysfunction. Etc. Chronic
exposure and accumulation of lead may result in birth defects, mental
retardation, and autism. Lead also depresses sperm count .
|Arsenic is a highly toxic metalloid element. It is a key additive in
rat poison, and with constant exposure, it is thought that arsenic may
affect the chromosomes of humans and their health. However, very
small amounts of arsenic could be good for humans to live and even be
able to breathe. The inorganic form of arsenic found in contaminated
meats, weed killers and insecticides, however can be very toxic .
Chromium is used in metal alloys and pigments for plants, cement,
paper, rubber and other materials. Low level exposure can irritate the
skin and cause ulceration. Long term exposure can cause kidney and
liver damage, and damage to circulatory and nerve tissue. Chromium
often accumulates in aquatic life, adding to the danger of eating fish
that may have been exposed to high levels of chromium. However,
under certain environmental conditions and certain metabolic
transformations, chromium (III) may readily be oxidized to chromium
(VI) compounds that are toxic to human health [44,45].
|The vast increase in environmental pollution by heavy metals puts
public health at risk. Various effects of heavy metal pollution in humans
are morphological abnormalities, neurophysiological disturbances,
genetic alteration of cells (mutation), tetratogenesis and carcinogenesis.
The presence of heavy metals affects enzymes and hormonal activities
as well as growth and in mortality rate .
|The influence of salinity on results of heavy metal mobility of
|Trace metals are among the most common contaminants bound
to estuarine sediments. The bioavailability and toxicity of these metals
to aquatic organisms depend on the physical and chemical forms
of the metal as well as several physicochemical parameters such as
temperature, pH, salinity, dissolved oxygen and particulates matter
composition. In fresh water, pH is the controlling factors while salinity
is stated as one of the controlling factors affecting the partitioning of
contaminants between sediments and water in sediments in marine or
estuarine environment due to the great variability of this parameter in
|Several studies relating the effects of salinity and pH on heavy
metals mobility in estuarine and marsh sediments are reported [47-
49]. A decrease in the salinity of dredged harbour sediments may
lead to a different partitioning coefficients of (ratio between metal in
sediment and the interstitial water, Kd) heavy metals but depends on
several predominant processes such as mobilisation of metals through
complexation with seawater anions (Cl- and SO4
2-) [46Changes in
salinity play a major role in metal distribution in dredged harbour
sediments, especially when washing procedure is applied as a
remediation technique or when dredged harbour sediments are
deposited in the open air.
|In related studies,  investigated the influence of pH, and
salinity on the toxicity of heavy metals in sediments to the estuarine
calm Ruditape Phillippinarium. They found out that heavy metals tend
to be more bioavailable at lower salinity than at higher salinity value and this may be more toxic to the exposed organisms. They were able
to establish that the effect of the salinity varies from metal to metal
depending on the relative important of two counteracting processes,
desorption from sediments to water or coagulation, flocculation and
precipitation. From their results, sediments collected in area affected
by chronic heavy metal contamination tend to be more efficient in
trapping Zn, Cu and Pb at low salinity values. They found out that Cd
tends to be more mobile as salinity increases.
|In another study,  worked on the effect of chloride on heavy
metal mobility of harbour sediments. Modified BCR- SEP was applied
to harbour mobility in order to assess the extent trace element mobility
(Cd, Cr, Cu, Ni, Pb and Zn) could be influenced by chloride content
in sediments. Washed and non-washed sediment were compared
respectively. The relative mobility order found for the six trace metals
studied was not seen to be influenced by the presence of chloride in the
sediments. An increase in mobility was observed for Cd and Zn (the
most mobile metals) when chloride was present in the sediments. This
was in agreement with findings from Riba et al. .
|Therefore, further studies on the combine effects of pH and salinity
on heavy metal mobility in marine harbour are recommended to be
able to compare smaller difference in salinity values and in order to
ascertain the major influence of chloride on results of heavy metals
|Tremendous amounts of toxic pollutants have been discarded
into coastal environment and the sediments of harbours represented
large sink of heavy metals [51-54]. The sea and more particularly the
aquatic systems are the ultimate respiratory of man’s waste. Due to the
dynamic nature of marine environment there is rapid assimilation of
these materials by processes such as dilution, dispersion, oxidation,
degradation or sequestration into sediments . The release of heavy
metals from sediments to water and organisms can be accelerated by
processes which alter redox potentials of sediments and chemical forms
of heavy metals. Toxic heavy metals are adsorbed onto organic matter
and mineral surfaces in inorganic and organic forms [54,55].
|Heavy metal mobility and bioavailability in sediments depend
strongly on the mineralogical and chemical forms in which they occur
. Therefore, measurement of total metal concentrations is useful
to estimate the heavy metal burden since their mobility depends on
ways of binding. In other words, determination of specific chemical
species or binding focus is very complex and hardily possible often. It
is very imperative to study different forms of heavy metal mobility and
bioavailability rather than the total concentration in order to obtain an
indication of the bioavailability of metals. For this reason, sequential
extraction procedures are commonly applied because they provide
information on the fractionation of metals in the different lattices of the
solid sample which serves as a good compromise to give information on
environmental contamination risk [56,57].
|Metal accumulation in sediments from both natural and
anthropogenic sources is thus making it difficult to identify and
determine the origin of heavy metals present in the sediment . Since
the early 1980s and 1990s sequential extraction methodology has been
developed to determine speciation of metals in sediments [8,58] due to
the fact that the total concentration of metals often does not accurately
represent their characteristics and toxicity. In order to overcome the
above mentioned obstacles it is helpful to evaluate the individual
fractions of the metals to fully understand their actual and potential environmental effects . Heavy metal pollution is a serious and widely
environmental problem due to the persistent and non- biodegradable
properties of these contaminants . Sediments serve as the ultimate
sink of heavy metals in the marine environment and they play an
important role in the transport and storage of potentially hazardous
|To date, strong acid digestion is used often for the determination
of total heavy metals in the sediments. However, this method can be
misleading when assessing environmental effects due to the potential
for an overestimation of exposure risk. Moreover, in order to eliminate
the mobility of heavy metals in sediments, various sequential extraction
procedures have been developed [59-63]. However, the number of steps
in this extraction varies from 3 to 6 steps: 3,  5  to 6 .
|Sequential extraction procedures (SEP) are operationally defined
methodologies that are widely applied for assessing heavy metal
mobility in sediments [66,67], soils  and waste materials . Single
extractions are thus used generally to provide a rapid evaluation of the
exchangeable metal fraction in soils and sediments [9,10]. Various
complicated sequential extraction procedures were used to provide
more detailed information regarding different metal phase associations
|In addition, heavy metal speciation in environmental media using
sequential extraction is based on the selective extraction of heavy
metals in different physicochemical fractions of material using specific
solvents . These methods have been used widely in determining
specific chemical forms of heavy metals in a range of environmental
media which include sediments [1,27,66,67] soils [68,70,71] and waste
|Among a range of available techniques using various extraction
reagents and experimental conditions to investigate the distribution of
heavy metals in sediments and soils, the 5-step Tessier et al.  and the
6-step extraction method, Kerstin and Fronstier  were mostly widely
used. Following these two basic schemes, some modified procedures
with different sequences of reagents or experimental conditions have
been developed [72-75]. Considering the diversity of procedures and
lack of uniformity in different protocols, a BCR, Bureau Commun
de Recherche (now called the European Community (EC) Standards
Measurement and Testing Programme) method was proposed . It
harmonized differential extraction schemes for sediment analysis. The
method has been validated using a sediment certified reference material
BCR-701 with certified and indicative extractractable concentration of
Cd, Cr, Cu, Ni, Pb and Zn . This method was applied and accepted
by a large group of specialists [77-82] despite some shortcoming in the
sequential extraction steps [83,84].
|Wang et al.  used a modified Tessier sequential extraction
method to investigate the distribution and speciation of Cd, Cu, Pb,
Fe, and Mn in the shallow sediments of Jinzhou Bay, Northeast China.
This site was heavily contaminated by nonferrous smelting activities.
They found out that the concentrations of Cd, Cu and Pb in sediments
was to be 100, 73, 13and 7times, respectively, higher than the National
guidelines (GB 18668-2002). The sequential extraction tests revealed
that 39% -61% of Cd was found in exchangeable fractions. This shows
that Cd in the sediments posed a high risk to the local environment.
Cu and Pb were found to be at moderate risk levels. According to the
relationships between percentage of metal speciation and total metal
concentration, it was concluded that the distributions of Cd, Cu and
Pb in some geochemical fractions were dynamic in the process of
pollutants migration and stability of metals in marine sediments from Jinzhor Bay decrease in the order Pb > Cu >Cd. Capalat et al. 
developed a modified three-step sequential extraction procedure to
examine the heavy metal mobility in harbour- contaminated sediments
of Port-en-Basin, France. It was revealed that metallic contaminants
associated with sediments showed various behaviours depending on
physicochemical conditions. In the studied core, anoxic condition was
developed a depth of 15cm. A 3-step sequential extraction procedure
was applied to the anoxic sediment in order to evaluate the potential
mobility of fixed metals. According to their findings zinc was the most
labile metal recovered in the first extraction stages, and was associated
with the non- residual fraction of sediment. Lead was found to be
the least labile, with up to 70% associated with the residual fraction
of the sediments. Copper was associated with organic matter, and its
mobility was controlled by the concentration and degradation of the
organic fraction. They finally concluded that discharge of organic
rich dredged sediments at sea results in degradation of contaminated
organic matter and these may affect the environmental impact of these
|In similar studies, Yuan et al.  applied BCR-sequential
extraction protocol to obtain metal distribution patterns in marine
sediments from the East China Sea. The results showed that both the
total contents and the most dangerous non-residual fractions of Cd and
Pb were extremely high. More than 90% of the total concentration of
V, Cr, Mo and Sn existed in the residual fraction while more than 60%
of Fe, Co, Ni, Cu, and Zn were mainly present in the residual fraction.
Mn, Pb, and Cd were dominantly present in the non- residual fractions
in the top sediments.
|Jones and Turki  worked on distribution and speciation of heavy
metals in surface sediments from the Tees estuary, North East England.
Tessier et al.  metal speciation scheme modified by Ajay and van
Loon  was used for the study. They found out that the sediments
were largely organic- rich clayey silts in which metal concentrations
exceed background levels, and which attain peak values in the upper
and middle reaches of the estuary. Cr, Pb and Zn were associated with
the reducible, residual, and oxidizable fractions. Co and Ni were not
highly enriched while Cu is associated with the oxidizable and residual
fractions. Cd is associated with the exchangeable fractions.
|Pempkowlak et al.  investigated the speciation of heavy
metals in sediments and their bioaccumulation by mussels. They
used a 4-step sequential extraction procedure adapted from Forstner
and Watmann . Their investigation which was characterized by
varying metal bioavailability was aimed at revealing differences in the
accumulation pattern of heavy metals in mussel inhabiting that inhabit
in sediments. The bioavailabilities of metals were measured using the
contents of metals adsorbed to sediments and associated with iron
and manganese hydroxides. The biovailable fraction of heavy metals
contents in sediments collected from Spitsbergen represented a small
proportion (0.37% adsorbed metals and 0.11%, are associated with
metals hydroxides). It was also revealed that the percentages of metals
adsorbed and bound to hydroxides of the sediments ranged from 1 to
46% and 1 to 13%, respectively.
|Wepener and Vermeulen  worked on the concentration
and bioavailability of selected metals in sediments of Richards Bay
harbour, South Africa Sequential extraction of sediments was carried
out according to Tessier et al.  methods. The following metals were
investigated: Al, Cr, Fe, Mn, and Zn respectively. Their studies revealed
that metals concentrations in sediments samples varied only slightly
between seasons, but showed significant spatial variation, which was
significantly corrected to sediment particle size composition. Highest metal concentration was recorded in sites with substrates dominated
by fine mud. Mn and Zn had more than 50% of this concentration
in reducible fraction while more than 70% of the Cr was associated
with the inert fractions and the concentration recorded at some sites
were still above action levels when considering only the bioavailable
fractions. They also concluded that the concentration of zinc recorded
was not elevated their results were compared with the historic data.
Coung and Obbard  used a modified 3-step sequential extraction
procedure to investigate metal speciation in coastal marine sediments
from Singapore as described by the European Community Bureau of
Reference (ECBR). Highest percentages of Cr, Ni, and Pb were found in
residual fractions in both Kranji (78.9%, 54.7% and 55.9% respectively)
and Pulang Tokong (82.8%, 77.3% and 62.2% respectively). This means
that these metals were strongly bound to sediments. In sediments from
Kranji, the mobility order of heavy metals studied were Cd > Ni > Zn
> Cu > Pb > Cr while sediments from Pulan Tekong showed the same
order for Cd, Ni, Pb and Cr. but had a reverse order for Cu and Zn (Cu
> Zn). The sum of the 4-step s (acid soluble, + reducible + oxidizable
+ residual) was in good agreement with the total metal content, which
confirmed the accuracy of the microwave extraction procedure in
conjunction with the GFASS analytical method. Analytical methods
used for heavy metal speciation are summarized in Table 1 and Table
||Table 1: Analytical methods used for speciation of heavy metals.
||Table 2: Speciation of the analyzed heavy metals in sediment of different coastal
||Table 3: Other Analytical techniques applied for metal speciation Track et al. (1995)
|A critical appraisal of different sequential fractions
|Exchangeable fraction: This fraction involves weakly adsorbed
metals retained on the solid surface by relatively weak electrostatic
interaction, metals that can be released by ion-exchangeable processes
etc. Remobilisation of metals can occur in this fraction due to adsorptiondesorption
reactions and lowering of pH [96,97]. Exchangeable metals
are a measure of those traces metals which are released most readily to the environment. Corresponding metals in the exchangeable fraction
represent a small fraction of the total metal content in soil, sewage
sludges and sediments and can be replaced by neutral salts . This
fraction generally accounted for less than 2% of the total metals in
soils presents, the exceptions to this microelement, K, Ca, and Mn
. Exchangeable fraction is also known as non-specifically adsorbed
fraction, it can be released by the action of cations such as K, Ca, Mg
or (NH4) displacing metals weakly bond electrostatistically organic or
inorganic sites . The common reagents used for the extraction of
metals in this fraction are MgCl2, sodium acetate (pH 5.4) by acetic acid
. Reagents used for this purpose are electrolytes in aqueous solution,
such as salts of strong acids and bases or salts of weak acids and bases
at pH 7. Other reagents showing similar properties have seldom been
used, such as nitrate salts (to avoid complexation that is too strong)
or calcium salts (Ca 2+ being more effective than Mg2+ or NH4+ in
removing exchangeable ions). Results obtained with these reagents give
good correlation with plant uptake .
|The carbonate phase: Carbonate tends to be a major adsorbent
for many metals when there is reduction of Fe-Mn oxides and
organic matter in the aquatic system. The most popular use reagent
for the extraction of trace metals from carbonates phases in soils and
sediments is 1M sodium acetate adjusted to pH 5.0 with acetic acid .
The carbonate fraction is a loosely bound phase and bound to changes
with environmental factors such as pH . The time lag for the
complete solubilisation of carbonates depends on some factors such as
the type and amount of the carbonate in the sample, particle size of
the solid . Extraction of metals from carbonates phases enhances
the leaching of metals specifically sorbed to organic and inorganic
substrates . In general, this fraction is sensitive to pH changes, and
metal release is achieved through dissolution of a fraction of the solid
material at pH close to 5.0 .
|Iron and Manganese oxides phases: This is referred to as sink
for heavy metals. Scavenging by these secondary oxides, present as
coating on mineral surfaces or as fine discrete particles. This can occur
as a combination of the precipitation, adsorption, surface complex
formation and ion exchange . Extraction of metals in Fe-Mn oxides
phases with 0.1M hydroxylamine when compared with the extraction
with 0.5M hydroxylamine. There is a variation accounted, 0.1M release
metal mainly from amorphous manganese oxide phases with less attack
on the iron oxide phase . Extract with 0.5M gives effective attack
on the iron oxide phase while still release metals from manganese
oxide phase. Different reagent has been used for metal extraction in
Fe-Mg oxide phases amongst are sodiumdithionate in combination
with sodium citrate and sodium bicarbonate in a varying concentration
range . Extraction with ascorbic acid / ammonium oxalate reagent
offers great merits over others because high purity degree is achieved
and does not attack silicates. However, the most successful reagents for
evaluating the total amount of metal ion associated with these minerals
contain both a reducing reagent and a liable ligand able to retain released
ions in a soluble form, the efficiency of the reagent being determined by
its reduction potential and its ability to attack the different crystalline
forms of Fe and Mn oxyhydroxides. Hydroxylamine, oxalic acid and
dithionite are the most commonly used reagents .
|Organic phases: The bioaccumulation or complexation process
being the primary source in which trace metal get associated with
organic material such as living organisms, detritus etc. In aquatic
systems, organic substances tends to have high degree selectivity for
individuals ions compared to monovalents ions into organic matter
being Hg > Cu > Pb > Zn > Ni > Co . In organic phase, metallic
pollutant bound to this phase are assumed to stay in the soil for longer
periods but may be immobilized by decomposition process .
Under oxidizing conditions, degradation of organic matter can lead to
a release of soluble trace metals bound to this component. The extracts
obtained during this step are metals bound to sulphides . The
organic fraction released in the oxidisable step is considered not to be
bioavailable due to the fact that it is thought to be associated with stable
high molecular weight humic substances that release small amount of
metals in a slow manner . The most commonly used reagent for
the extraction of metals in organic phases is hydrogen peroxide with
ammonium acetate readsorption or precipitation of released metals
. Other reagents such as H2O2 / ascorbic acid or HNO3 + HCl
have been used which can dissolve sulphides with enhanced selectivity,
but on the other hand, silicates are attacked to some extent .
Oxidation with sodium hypochlorite has also been recommended 
but fraction of organically bond metals released showed considerable
variability in different soil horizons .
|Strong acid-Extractable fraction: Residual phase: Residual phase
serve as a useful tool in the assessment of the long-term potential
risk of heavy metal or toxic metals entering the biosphere. Digestion
in strong acid such as nitric acid, hydrochloric acid or mixture
such as aqua regia that do not dissolve the silicate matrix have been
commonly used to leach out the recasistrant metals that are bound to
the sediment in the residual phase. Residual phase give an estimate
of the maximum amount of elements that are potentially mobilisable
with changes environmental conditions. ISO 11466  aqua regia
digestion procedure is the known well procedure with a legal back in
some European countries and had been used for the standard reference
material of soil and sediments. Moreover, primary and secondary
minerals containing metals in the crystalline lattice constitute the bulk
of this fraction. Its destruction is achieved by digestion with strong
acids, such as HF, HClO4, HCl and HNO3. The amounts of associated
metals are also associated by some authors as the difference between the
total concentration and the sum of the fractions of the metals extracted
during the previous steps . Results of heavy metal speciation in
sediment in different fractions collected from different coastal system
worldwide are presented on Table 2.
|In this paper, we have reviewed the analytical methods for chemical
speciation of heavy metals in marine harbour sediments. Heavy metal
is one of the most serious environmental pollutants because of its
high toxicity, abundance and ease of accumulation by various plant
and animals. This review highlighted various effects, occurrence and
different speciation methods to determine concentration of heavy
metals and their mobility in different fractions. Also, we were able to
highlight different activities that take place in and around harbours
such as loading and offloading of goods, cleaning, ballasting, fuelling,
maintenance practise which include painting of ships, motors and
engines emission, agricultural activities, urban runoff to be contributing
factors to the dumping of pronounced amounts of wastes directly into
the seas. Although, a considerable amount of work has been conducted
on the heavy metal pollution in marine water and sediments and their
effects on aquatic life’s. Very limited data are available on the effects of
these heavy metals on human.
|The authors wish to thank the management of Cape Peninsular University
of Technology, Cape Town, South Africa for financial support. The authors also
acknowledged University of Ilorin, Ilorin, Nigeria for supplementation staff
development award offered to H. K Okoro throughout the period of his Doctoral
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