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Evaluation of the Effectiveness of Different Acid Digestion on Sediments

Article in IOSR Journal of Applied Chemistry · January 2014

DOI: 10.9790/5736-071213947

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IOSR Journal of Applied Chemistry (IOSR-JAC)
e-ISSN: 2278-5736.Volume 7, Issue 12 Ver. I. (Dec. 2014), PP 39-47
www.iosrjournals.org

Evaluation of the Effectiveness of Different Acid Digestion on

Sediments
Fabunmi Idera1*, Olumodeji Omotola2, Uyimadu John Paul3, Adeleye Adedayo4
1
Department of Physical and Chemical Oceanography, Nigerian Institute for Oceanography and Marine
Research; Nigeria.
2
Department of Physical and Chemical Oceanography, Nigerian Institute for Oceanography and Marine
Research; Nigeria.
3
Department of Physical and Chemical Oceanography, Nigerian Institute for Oceanography and Marine
Research; Nigeria.
4
Department of Physical and Chemical Oceanography, Nigerian Institute for Oceanography and Marine
Research; Nigeria.

Abstract: The efficiency of four different acids mixtures were evaluated in the determination of total
concentration of Pb, Zn, Cu, Cd, Cr and Fe in sediments samples. The acids combination employed included
nitric- perchloric acid (NP), aqua regia (NH), sulphuric- hydrogen peroxide (SP) and nitric acid. Sediment
samples were taken from five different Lagos lagoon locations; mid lagoon (ML), University of Lagos (UL),
Oworonsoki (OW), Ikoyi (IK) and Oko-baba (OB). Validation of the method was performed by using certified
reference material IAEA-158 from the International Atomic Energy Agency analytical quality control services,
Vienna, Austria. Using Principal component Analysis (PCA), the results revealed that SP acid mixture was
efficient for the recovery of Pb, Cd and Cr while NP and NH gave good results and to some extent nitric acid for
the extraction of Zn, Fe and Cu; these procedures were recommended for the qualitative determination of Zn,
Cu and Fe in sediment. When employing the NP method, the perchloric acid should be added at the later stage
of digestion to avoid explosion.

I. Introduction
As a growing global population, there has been need to produce food, water and shelter and this has
caused focus on the ocean to help sustain some of our basic needs. The contamination of the seafloor by
chemical inputs (pollution) affects and threatens the benthic environment. These pollutants are consumed up by
bottom dwelling organisms and transferred up the food chain, adversely affecting organism's activity, growth,
metabolism and reproduction [1], killing some organisms in the process and reducing food availability in the
ocean. The formation of seawater and seafloor with relation between chemical compounds (inorganics and
organics), how chemical inputs (pollution) affects it and how the chemistry of the ocean affects or is affected by
biological geographical and physical factors has been of great concern over decades. However, the water body,
sediments and aquatic life (bio indicators) have been used as monitors to evaluate the level of pollution in the
marine environment [2].
Heavy metals have been a source of pollution to the environment over the last century. This is due to
fast growing economy and industrialization. Sources of these pollutants range from residential wastes, industrial
effluent/wastes, agricultural tools waste, mechanic village scraps and more recently electronic waste. The
receiving systems are soil, underground water, rivers/streams, lagoons and sea, which finally settle down or
accumulate in the ocean. Scientists over time have assessed the level of heavy metals in the environment as well
as investigated the level at which it could be detrimental to life [1,3,4,5,6,7,8].
Heavy metal quantitative determinations have been done majorly through two different methods; wet
digestion and dry ashing followed by acid dissolution [9]. Wet digestion involves destruction of the organic
matter and dissolution of heavy metals. Hossner [10] reported that the advantages of the dissolution of heavy
metals in sediments and soils using concentrated inorganic acids are low cost and low salt matrix in the final
solution for the determination of total heavy metal content. Acids used for wet digestion include nitric acid,
sulphuric acid, hydrochloric acid, perchloric acid, hydrogen peroxide [11]. These acids have been used either
alone or in ratios. Hydrofluoric acid (HF),hydrochloric acid (HCl), nitric acid (HNO3), perchloric acid (HClO4),
and sulphuric acid (H2SO4) have been used singly and extensively to optimize extraction from the organic
matrix [12,13].
Gorsuch [14] discovered that the methods of digestion that involves a mixture of nitric, sulphuric or
perchloric acids were satisfactory for digesting mineral elements in organic and biological materials. However,
perchloric acid is potentially hazardous during the digestion of biological materials and can cause the loss of
potassium and boron [15]. Baker and Amacher [16] recommended use of HF-HNO3-HClO4-H2SO4 for the total

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

analysis of Cd, Cu, Ni, and Zn in soils, but this was modified by Burau [12] by replacing H2SO4 with HCl. This
is because Pb precipitates with H2SO4 in solution. The above methods have however been shown not to
completely dissolve silicate, but they are sufficiently good to attack and dissolve heavy metals bound to soil. It
was reported by Stephen P. et al. [17] that using aqua regia and block digesters, digestion is faster, more
economical and can be very simply modified to suit different types of soil. Aqua regia (ratio 3:1 or 4:1 v/v) has
been said to extract effectively trace metals in sediments [18]. Sastre et al. [19] stated that nitric acid digestion
was an optimum method for estimating heavy metal content in soil samples with high organic matter content,
being superior to microwave-assisted and aqua regia digestions.important compost constituents [20]. Heavy
metals in the silicate mineral require digestion with hydrofluoric acid (HF) and strong acids. However, the use
of HF in routine laboratories is not recommended, as it is highly corrosive and difficult to handle.
Numerous studies have succeeded in improving methods for proper extraction of chosen elements. The
objectives of our study are twofold. First, we aim to evaluate and recommend the most appropriate acids mixture
for the dissolution of marine sediments in determination of specific heavy metals content. This has resulted in
suggesting simple, rapid, reliable and accurate method for the preparation of large numbers of marine sediment
samples. Second, we determine the total concentration of the heavy metals obtained from acid digestion
mixtures, to know the level of pollution of Lagos Lagoon. We hence discuss the extent of heavy metal pollution
around the highly populated locations of Lagos Lagoon. The overall objective of the study is to evaluate and
recommend the most appropriate digestion method for determining Pb, Zn, Cu, Cd, Cr and Fe in sediments from
various sites putting into consideration the above advantages.

II. Materials and Methods

2.1 Study Area

Figure 1: Map of the study area.

Fig. 1 above shows the sampling stations on Lagos Lagoon while Table 1 below recorded the
coordinates and approximate depth for each sampling station respectively. Lagos Lagoon has estimated area of
150.56 km². Lagos Lagoon receives a number of important large rivers, namely: Yewa, Ogun, Osun and Ona
[21]. The brackish water lagoon surrounds the Lagos Island and empties at Lagos harbour into the Atlantic
Ocean. Lagos is located on the western side of Nigeria and has in size 3,577 sq. km with a maritime shoreline of
about 180 km as its southern border. The Lagos Lagoon is generally shallow in most parts with the exception of
some dredged parts (depth >10 m), notably in the Lagos harbour. Lagos Lagoon ecosystem serves as a source of
affordable protein in term of sea foods and means of transportation for the local populates living around it. It has
been reported that the discharge of urban and industrial wastes into the Lagos Lagoon have resulted in a
significant depletion of its coastal terrain and negative impact on the brackish water [22,23].
Sediment samples were taken from five different Lagos lagoon locations; mid lagoon (ML), University
of Lagos (UL), Oworonsoki (OW), Ikoyi (IK) and Oko-baba (OB). Sediments were collected via grab sampler,
placed in nylon bags and freeze dried. The sediment samples vary from clayey, sandy and loamy textures. The

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

sediment samples were decomposed using four different acids and subsequently boiled on a hot plate of about
100oC.

Table 1: The study stations, their coordinates and approximate depth

LOCATION COORDINATES APPROXIMATE DEPTH (m)

OWORONSOKI N 06°32.961” 0.40

E 003°24.532”
OKO BABA N 06°28.800” 1.25
E 003°23.468”
UNILAG N06o30’479” 1.50
E003o24’024”
MID LAGOON N 06°30.770” 4.20
E 003°28.212”
IKOYI N0628.209 2.40
E00323.872

2.2 Reagents
All reagents and acids used were of high purity and analytical grade supplied by BDH laboratory
supplies England.
Stock standard solutions of the elements were obtained from standard inorganic ventures, USA. Serial
dilutions were made with de-ionized double-distilled water in order to prepare working solution.

2.2.1 Nitric acid digestion method (N)

One gram each of sample was placed in a 250ml conical flask and 10ml HNO3 was added, the sample
was heated for about 45mins. 10ml of HNO3 was then added and heated at a constant temperature of about
120oc until a clear solution was obtained and the volume was reduced by evaporation to about 5mls. The flask
was cooled at room temperature and the mixture was filtered through a 0.45-μ Millipore membrane filter paper
transferred quantitatively to a 50ml volumetric flask by adding de-ionized and double-distilled water.

2.2.2 Aqua-regia digestion method (NH)

This method was partly modified from that of ISO 11466 [24]. 1.0 g each of sample was treated with
15 ml HCl and 5 ml HNO3 (ratio 3:1). The sample was then heated on a hot plate with the temperature gradually
increased until decomposition was complete and volume reduced by evaporation to about 5ml. The samples
were filtered, washed with de-ionized and double-distilled water, transferred quantitatively to a 50 ml
volumetric flask.

2.2.3 Sulphuric acid- Hydrogen peroxide digestion method (SP)

Jones and Case [25]’s approach was adopted. 1g each of sediment sample was weighed into a conical
flask. 7ml of concentrated H2SO4 was added. The mixture was allowed to stand for 45mins at room temperature.
7ml of 30% H2O2 was added to the sample and heated on a hot plate for about 45mins. The mixture was allowed
to cool at room temperature and 2ml H2O2 was added and the mixture further heated gradually. The mixture was
removed when the solution was clear and volume was about 5ml. Following cooling, the mixture was filtered
through a 0.45-μ Millipore membrane filter paper transferred quantitatively to a 50ml volumetric flask by
adding de-ionized and double-distilled water.

2.2.4 Nitric-perchloric acid digestion method (NP)

One gram each of sample was weighed into a conical flask and 10ml concentrated HNO3 was added.
The mixture was boiled at a constant temperature for about 45mins. After cooling, 5ml of 70% HClO 4 was
added and the mixture further boiled to release white fumes. After cooling 20ml distilled water was added and
heated until a clear solution was obtained [26]. At room temperature, the mixture was filtered through a 0.45-μ
Millipore membrane filter paper transferred quantitatively to a 50ml volumetric flask by adding de-ionized and
double-distilled water.

2.3 Quality control

Validation of the method presented in this study was performed by standard reference material from the
International Atomic Energy Agency (IAEA-158), analytical quality control services; Vienna, Austria was
digested using the four different digestion methods. This was done as a reference for quality control and
assurance of the different methods; nitric acid method (SN), aqua regia method (SNH), nitric-perchloric acid
method (SNP) and sulphuric acid- hydrogen peroxide method (SSP).

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

2.4 Instrumentation
An Agilent Atomic Absorption Spectrometer model 240A was used for the determination of all the
elements after successive dissolution, decomposition and made up to the 50ml mark in a volumetric flask.
Simultaneous background corrections were made using a deuterium lamp.

III. Results and Discussion

Fig. 2 shows percentage recoveries of heavy metals in standard reference material IAEA-158 by four
digestion methods described above. The standard deviation of replicated standard samples was less than 2%.
The concentrations of heavy metals are expressed as milligram/Kilogram (mg/Kg) of dried sediment. The
recoveries of Pb, Zn, Cd, Cu, Cr and Fe ranged from 74-104% for Pb, 88-99% for Zn, 87-123% for Cd, 78-90%
for Cu, 95-108% for Cr and 65-108% for Fe. The recovery of Zn and Cu were similar as they are said to have
similar behaviours in soil [27] with the lowest percentage recovery of 78-99%. The best recoveries were found
in Cd which ranged from 87-123% for the digestion methods. The measured values were within 1% of accepted
values.

Figure 2: Percentage (%) recovery analysis of heavy metals content in standard reference material

The mean values and standard deviations of the total content of heavy metals in sediment determined
by the different digestion methods are shown in Table 2. The data showed levels of heavy metals in sediment at
different points of the Lagos lagoon; Mid-lagoon (ML), Unilag (UL), Oworonsoki (OW), Ikoyi (IK) and Oko-
Baba (OB).
For Pb, it was observed that the SP acid mixture gave the highest extraction for the entire sampled
sites. Although H2SO4 has been said to reduce the sensitivity of metal detection since it precipitates metals
[12,28,29], the reverse was the case for this study. Simpson et al. [30] reported that in anthropogenically metal
contaminated sulphide enriched estuarine sediments, Pb and Zn contaminants might not occur as metal
sulphides but metal powder oxides. Stum and Brauner [31] also explained that in oxygenated seawater, Pb
occurred predominantly as carbonates of inorganic species. The relative affinity of soil for various heavy metals
was said to increase in order Ni<Zn<Cd<Cu<Cr<Pb [27]. This emphasises the effectiveness of the strongest
oxidizing acid mixture SP used in this study for the extraction of Pb. NP extraction was the next preferred acid
mixture, although the values were lower compared to that of SP acid mixture, its values were higher than that of
nitric acid and NH. This shows that for effective release of Pb, the use of a strong oxidizing acid mixture must
be employed.
In this study, all sediments decomposed with acid mixture sulphuric acid and hydrogen peroxide (SP)
gave good extraction of Cd concentration. Earlier investigation supports the efficiency of SP acid mixture in the
determination of Cd, which reveals that Cd bond to acid volatile sulphur can be liberated rapidly as a result of
aeration of sediment and elevated oxygen [32,33]. Due to the high oxidizing quality of hydrogen peroxide, Cd is
oxidized to soluble CdSO4 during aerobic extraction [34]. Based on this result, aqua regia (NH) predominantly
gave a lower extraction in the determination of Cd followed by nitric acid (N) extraction. Zeng et al. [28],
presented similar results in the digestion methods for total heavy metals in sediment and soil, which revealed
that the total mean Cd content of sediments digested by NH method was lower compared to other acid mixture
digestion.

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

From the plot below (Fig. 3) nitric acid, NH and NP acid mixtures were good for the extraction of Zn.
Although all the acid mixtures showed high extraction of Zn concentration from the sampled sites, however, it
cannot be clearly determined the method that best determines Zn. Thalita [35] showed that EPA 3050 and EPA
3051A presented the highest recovery of Pb and Zn for most residues. Also Nemati et al. [36] reported that
microwave assisted EPA 3015A is most recommended for determination of Zn. Adsorption density of heavy
metals is said to increase in order of Pb<Cu<Zn which shows that adsorption behaviour of Cu and Zn is very
much similar in terms of maximum adsorption capacity [27]. Morillo et al. [37] reported that Zn metal has the
greatest mobility. Large amount of these element are present in the acid soluble fraction.

Table 2: Heavy metal content (mg/kg) in sediment samples by four digestion methods
Pb Zn Cd Cu Cr Fe
MLN 14.035±0.001 104.845±0.0010 3.55±0.0004 22.25±0.0001 7.95±0.0002 1182.5±0.0006
MLNH 2.86±0.0001 39.125±0.0006 2.6±0.0007 38.75±0.0001 7.5±0.0004 1443±0.0009
MLNP 21.87±0.0004 33.66±0.0004 4.3±0.0023 28.5±0.0001 12.35±0.0001 1465.5±0.0004
MLSP 67.535±0.0058 30.38±0.0009 9.1±0.0004 10±0.0001 26.05±0.0001 2601±0.0006
ULN 32.005±0.0005 130.07±0.0014 3.75±0.0003 17.95±0.0002 53.35±0.0001 39010±0.0030
ULNH 30.24±0.0005 140.935±0.0007 4.5±0.0004 19.25±0.0001 62.4±0.0004 40458.5±0.0029
ULNP 46.035±0.0003 148.07±0.0002 5.55±0.0007 21.55±0.0001 68.35±0.0004 41564±0.0024
ULSP 67.94±0.0004 70.43±0.0002 8.45±0.0004 10.15±0.0001 59±0.0002 10091±0.0006
OWN 19.85±0.0006 63.815±0.0005 5.65±0.0005 6.7±0.0001 34.8±0.0003 26047±0.0023
OWNH 19.125±0.0003 113.69±0.0013 4.55±0.0004 13.65±0.0001 48.85±0.0004 36478±0.0019
OWNP 35.435±0.0007 110.455±0.0008 7.05±0.0002 13.35±0.0001 57.2±0.0001 38036±0.0029
OWSP 94.625±0.0002 75.825±0.0008 11.3±0.0006 9.4±0.0001 96.15±0.0002 34510.5±0.0008
IKN 14.83±0.0003 36.135±0.0003 5.25±0.0002 2.2±0.0001 37.4±0.0001 13997±0.0016
IKNH 6.55±0.0013 35.635±0.0003 2.85±0.0037 2.35±0.0001 42.5±0.0001 17091.5±0.0016
IKNP 11.145±0.0022 25.485±0.0002 4.8±0.0001 1.45±0.0001 36.55±0.0003 12555±0.0031
IKSP 79.81±0.0005 30.695±0.0002 9.1±0.0003 2.95±0.0001 57.45±0.0004 10831.5±0.0014
OBN 16.27±0.0005 34.84±0.0007 6±0.0003 1.6±0.0001 39.7±0.0003 19797±0.0018
OBNH 11.255±0.0005 51.78±0.0009 4.05±0.0003 2.15±0.0001 47.35±0.0001 25276.5±0.0017
OBNP 19.525±0.0007 37.33±0.0001 5.75±0.0001 1.7±0.0001 49.95±0.0002 23578±0.0005
OBSP 93.21±0.0003 39.74±0.0003 5.6±0.0015 2.55±0.0001 56.5±0.0003 24297±0.0011

It was observed that the NH acid mixture showed good extraction of Cu for the entire sampled sites.
Tayab [27] reported that aqua regia gave higher results because of the solubility of Cl - and NO3- species. Zeng
[38] presented results which correlate to those obtained as he verified that NP and SP acid mixtures were more
efficient in the recovery of Cu. Sco [39] explained further that Cu can be leached to achieve 80% recovery using
H2SO4 which can further be improved to <90% by the addition H2O2.
Very high levels of Fe concentration were recorded in all the samples site. Metals are said to be mainly
bond in silicate such as feldspar and Fe-Mn micas by replacing Ca2+, Fe2+ and other cations in the structure
[40.41]. This process liberates Fe ions which can easily be extracted. Nitric acid (N) more often showed the least
extraction of Fe. WAS [42] reported that Fe may not be fully recovered from some matrix type (example-grains)
using nitric acid. Investigation on digestion of organic matrix with nitric acid found that the digestion did not
oxidise the organic matrix completely [43,44]. NP has been used for the determination of heavy metals in
sediments [37,41]. Higher Fe content was reported by Thalita et al. [35] using nitric- perchloric acid mixture and
EPA 3050 (HCl + HNO3 + H2O2). However, to overcome safety issues associated with the use of perchloric acid
and lack of recovery of Fe and Al using nitric acid, WAS [42] stated that the use of aqua regia is preferred for
total recovery of Fe in plants, however when soil is a part of the sample matrix (contaminant), this method does
not recover all the Fe. From the result given, NH predominantly gave a better result for the extraction of Fe in
sediments from all the sampled sites.
Due to the relative stable nature of Cr in sediment [27], the use of SP acid mixture was predominantly
the most effective for extraction of Cr followed by the NP acid mixture. This confirms results from Thalita et al.
[35] work where higher contents of Cr and Fe were recovered when nitric-perchloric acid and EPA 3050 were
used in the opening of organic matrices. From the result shown, nitric acid showed the lowest extraction for Cr
for most of the sampled sites. WAS [42] stated that nitric acid cannot be used for total recovery of Cr from plant
matrix. Schlieckman et al. [45] reported that the use of chloric acid in combination with HF and HNO 3 in the
digestion of dust sample at different temperatures lead to loss of Cr through the formation of chromyl chloride at
higher temperature.

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

Plot of extracted Cd, Cr, Cu, Pb, Zn concentrations against the acid mixtures
Bars are 95% CI for the Mean
160

140

120
Concentration (mg/Kg)
100

80

60

40

20

Acid mixture N NH NP SP N NH NP SP N NH NP SP N NH NP SP N NH NP SP
Pb Zn Cd Cu Cr

Figure 3: Plot of Pb, Zn, Cd, Cu, Cr extracted concentrations against acid mixtures

Table 3 below shows past works on the efficiency of different acid mixtures for determination of metal
concentrations in organic samples using standard reference samples. Abreu et al. [46] showed that NH gave the
best recoveries for all metals when compared with the use of NP and N. It was clearly shown that nitric acid
containing mixtures gave good results for the recovery of Cu with preference to NH which gave higher
percentage recoveries. Fe recovery was seen to follow the same trend with NH being the preferred method
followed by NP and N. Sulphuric acid containing mixtures gave good results for recoveries of Cr and Cd,
alternatively, NH was shown to also give good percentage recoveries. For the recoveries of Zn and Pb, there
were variations in results shown below which ranged from NH to NP with optimal results of recovery.

Table 3: Comparison of acid mixtures for recovery of heavy metals in standard materials
NH-Aqua regia, N-Nitric acid, NP- Nitric-Perchloric acid, SP- Sulphuric-Hydrogen peroxide, NS-Nitric-
Sulphuric acid
Abreu et al.[46] Oyeyiola et al.[47] Badran et al.[48] Present Study
Cd NH>NP= N NH>NS>NP>N N>NS>NP SP>NP=NH>N
Cr NH>NP=N NS>NH>N>NP N>NS>NP SP>NH>NP>N
Cu NH>NP=N NH>NS>NP>N NP>N>NS NH>NP>N>SP
Pb NH>NP=N NP>NH>NS=N NP>N>NS SP>NP>N>NH
Zn NH>NP=N NH>NS>NP>N NP>NS>N NP>NH>N>SP
Fe NH>NP=N - NP>N=NS NH>NP>N>SP

Score plot of acid mixture versus locations

MLSP IKSP
2

IKNP ULSP OBSP

1 IKN OBN
PCA-2 (31.3%)

OBNP OWSP
MLNP
IKNH
OBNH
0 OWN
MLNH
MLN

-1 OWNP

OWNH

-2 ULN
ULNH ULNP

-3
-4 -3 -2 -1 0 1 2 3 4
PCA-1 (45.6%)
Figure 4: Score Plot of Acid Mixtures versus Locations

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 Evaluation of the Effectiveness of Different Acid Digestion on Sediments

The purpose of Principal Component Analysis (PCA) is to reduce a number of observed variables into
a relatively smaller number of components. In this present study, PCA was used with the aim of analyzing the
extraction efficiency of acid mixtures within the sediments sampled at different locations on Lagos Lagoon. The
score plot of the first two components allows for the characterization of the sampling areas according to acid
mixtures concentrations. The three principal components (PCs) explained 77.0% of the total variance with the
values for PC1, and PC2 of 45.6% and 31.3% respectively. SP acid mixtures exhibited high extraction
efficiency of the metals in all the stations, Oko-Baba, Unilag, Mid-lagoon, Ikoyi and Oworonsoki (Fig. 3) and
were well differentiated by the higher scores on the first component in the positive part. Sediment samples from
Ikoyi and Oko-Baba were separated from other stations in the positive part of the second component suggesting
relative good extraction efficiency of acid mixtures NH, NP and N for the regions. Oworonsoki and Unilag
sediment samples decomposed by acid mixtures NH, NP and N were clearly separated from other stations in the
negative part of the first component suggesting poor/low decomposition (extraction) of metals from sediment of
these regions. PC-3 (16.3%) represented NH and N acid mixture in the negative part and NP in the positive part
due to geochemical characteristics of sediment from Mid-Lagoon.
The variation of the effectiveness of the different acid mixture used at the different sites could be
explained using the soil texture. McLean and Bledsof [49] reported that sandy soil did not retain metal
effectively, this correlates with the result given where a less oxidizing reagent NH was gave good results for
clayed soil of Mid-lagoon and Oworonsoki which stronger oxidizing mixtures of NP and SP gave better results
for clay-loamy soils of Unilag, Ikoyi and Oko-baba.

IV. Recommendation
The SP acid mixture gave the greatest extraction for Pb, Cd and Cr. H2O2 and H2SO4 are powerful
oxidizers and dehydrating agent. The combination of these two have been said to be the most powerful
oxidizing reagent for organic matter in sediment [50] (Polley and Miller, 1955). It was shown that the
dissolution of Pb, Cd and Cr using NH acid mixture and nitric acid method were mostly lower compared to the
SP and NP acid mixture. This was also shown by Stoeppler et al. [43] who found that digestion of organic
matrix using nitric acid did not completely digest the organic matrix and also Monika [51] who stated that aqua
regia digestion does not obtain the true content of species. NP and NH gave good results and to some extent
nitric acid for the extraction of Zn, Fe and Cu. This is in agreement with Griepink [52] work who reported that
addition of strong oxidants HClO3 and HClO4 to HNO3 increases the power of oxidation.
The recoveries of Zn, Cu and Fe in the standard reference material IAEA-158 which ranged from 19-
54% were highest using NP for Zn extraction; NH for Fe and Cu extraction. This indicates that these acid
digestion mixtures were not efficient for the extraction of Zn, Cu and Fe. Studies have shown that HF has been
used for extraction of heavy metals in sediments and soil because it bonds with silicate to form SiF4 enabling
total digestion [28,29,53,54], however Atgin et al. [55] pointed out that HF can result in damage of the FAAS
apparatus. According to Garcia et al. [54], microwave digestion with HF can decrease the recovery of some
chemical element due to the formation of calcium fluoro-aluminates or precipitate calcium. Also the PTFE
material used in HF digestion is expensive and when used repeatedly, the seemly stable inert vessel may
gradually increase its surface area (cracks) into which part of the digest diffuses losses and memory effect [52].

V. Conclusion
The choice of extractants should depend on the aim of the study, type of contaminants, properties of the
extractants and experimental conditions. Unfortunately, there was no digestion method which was widely
efficient in the extraction of all the metals. This is due to the differing characteristic of each metal in sediment.
The advantages of safety, short digestion time, less acid consumption, and high extraction efficiencies should be
put into consideration in heavy metal determination. For every metal a possible optimal digestion technique has
been recommended. Most of the sediment samples analysed showed that SP acid mixture was efficient for the
determination of Pb, Cd and Cr. NP, NH and also nitric acid mixture are recommended for the qualitative
determination of Zn, Cu and Fe. However perchloric acid may cause severe explosion if used with large amount
of oxidizable matter. It is recommended to use this acid to complete digestion by adding in later stage of
digestion.

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