American Journal of Anal yt ical Chemistry, 2011, 2, 116-125
doi:10.4236/ajac.2011.22013 Published Online May 2011 (
Copyright © 2011 SciRes. AJAC
Speciation of Dissolved Trace Nickel in Environmental
Waters by On-Line Sonodigestion-Flow Injection Solid
Phase Extraction Coupled to Flame Atomic Absorption
María Carmen Yebra-Biurrun1*, Jesús Manuel Castro-Romero2
1Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, University of Santiago de
Compostela, Santiago de Composte l a, Spain
2Department of Analytical Chemistry, Escuela Universitaria Politécnica, University of A Coruña, Ferrol, Spain
Received January 5, 2011; revised January 19, 2011; accepted January 20, 2011
A simple on-line sonodigestion system was successfully used for breakdown organic nickel complexes in
environmental waters acidified with diluted nitric acid prior to flow injection total dissolved nickel precon-
centration in a microcolumn containing a chelating resin (Chelite Che with iminodiacetic acid groups) and
determination by flame atomic absorption spectrometry. For the determination of the dissolved labile nickel
fraction, microcolumns packed with the chelating resin were loaded in-situ with the sample without sample
pH modification, and once in the laboratory were inserted in the flow injection device where nickel elu-
tion-detection was carried out. The performance of the chelating resin was investigated in order to elucidate
its behavior in the presence of dissolved nickel species. The results obtained reveal that the resin, at the ex-
perimental employed conditions, retained only dissolved free nickel ions and nickel bound to weak com-
plexes (labile fraction). The figures of merit for determinations in both nickel fractions are given and the ob-
tained values are discussed. The speciation scheme is applied to the analysis of nickel in river and seawater
samples collected in Galicia (Northwest, Spain). The results of fractionation showed that Ni are mainly in the
dissolved labile fraction in river water, while in seawater samples analyzed was mainly present in the organic
Keywords: Sonodigestion, Preconcentration, Flow Injection, Nickel Speciation, Flame Atomic Absorption
Spectrometry, Environmental Water
1. Introduction
Nickel is a transition metal element that is widespread in
the natural environment, and fulfils an important role in
many biochemical processes. Once dissolved in fresh or
sea water, the form of aqueous nickel is influenced by
pH, redox conditions, ionic strength, the type and con-
centration of inorganic and organic ligands, pressure and
temperature, and the surfaces available for adsorption
[1,2]. Thus, speciation studies of nickel in natural aquatic
environments such as rivers, lakes, estuaries and seas
must distinguish between dissolved and particulate nick-
el. The aqueous solubility of nickel compounds and their
chemical form and oxidation state in water strongly in-
fluences their bioavailability and therefore toxicity. The
dissolved nickel fraction in fresh and sea water forms
free hydrated Ni(II) cations and soluble nickel inorganic
and organic complexes of different stability [3]. An im-
portant group of complexing agents in natural waters,
which forms very stable complexes with transition met-
als, is called the dissolved organic matter (DOM) [4].
Humic and fulvic substances, which are made up of a
complex mixture of residues from the decomposition of
plants and animals, are the main components of DOM in
the aquatic environment, consisting of 30% - 80% dis-
solved organic carbon in waters. Several studies have
shown that binding of metallic ions with humic sub-
stances increases metal mobility in natural waters and
controls their toxicity, bioavailability and transport [5,6].
Therefore, reliable measurements of nickel fractions are
mandatory to study the behavior of this trace metal in
environmental waters.
One of the approaches for species fractionation in-
volves the use of chelating resins [7]. This operational
procedure is based on the retention of free (aquo ions)
and weakly complexed species (the labile fraction),
while the inert metal fraction (formed by stable com-
plexes formed between nickel and DOM) does not inter-
act with the chelating groups of the resin and remaining
in solution. For this, Chelex-100 resin containing chelat-
ing iminodiacetate groups is the resin most widely em-
ployed [8-10]. However, it is crucial to determine the Ni
species present before and after all sampling and storage
procedures to track any changes in species distribution
[11]. The use of on-site sampling/preconcentration de-
vices based on microcolumns is a simple alternative for
this purpose because of its relative easy to develop and
exploit automated manifolds, which allow change sam-
pling conditions (such as water level or sample flow-
rate), filter and preconcentrate the sample. As a result,
this methodology preserve the sample for further labora-
tory analysis, which can be easily carried out incorporat-
ing the sample loaded microcolumns into a flow injec-
tion manifold [12-15].
Regarding the determination of total dissolved trace
metals concentration, the high concentration of alkali and
alkaline earth metal ions, and the high saline matrix
makes very difficult direct analysis of seawater even by
the most sensitive analytical techniques [16]. Therefore,
a separation/preconcentration step is frequently neces-
sary in the analytical process. If the separation technique
chosen involves chelation, it is required to release the
trace metal from the metal-organic complex prior the
application of the separation technique. Sample acidifi-
cation could breakdown organic complexes, but as acidi-
fied samples must have their pH adjusted prior the sepa-
ration step, metals can re-complex with the dissolved
organic ligands to form again non-labile complexes.
Therefore, an adequate degradation or digestion of or-
ganic ligands is mandatory [17]. In the last years, the
removal of DOM is performed by ultraviolet radiation.
Nevertheless, there is disagreement about experimental
conditions applied for natural water digestion. Thus, wa-
ter samples were acidified with hydrochloric acid or ni-
tric (until pH about 1.7 to 2.2) with [18] and without
[19,20] the presence of diluted hydrogen peroxide, and
UV digestion time ranged between 0.5 to 8 h. By the
other hand, flow injection procedures reduced this time
to 5 - 9.3 min [21]. Development and application of so-
nolysis (ultrasound) is starting to gain an important place
in this domain. Ultrasound can degrade metal organic
complexes by means of oxidative reactions, which are
increased by the presence of nitric acid because the hy-
drogen peroxide formed as a result of water sonolysis
was found to be about twice that observed in pure water
under identical sonochemical conditions [22,23]. As a
result, this methodology is used for the treatment of
wastewater containing organic compounds to decompose
them into highly degradable products [24], for degrada-
tion of organomercurials to determine inorganic and total
mercury [25] and in a continuous mode to digest urine
samples [26] and for sonolysis of seawater samples [27].
The main objective of this work is to present an auto-
matic flow injection (FI) system coupled to flame atomic
absorption spectrometry (FAAS) for the determination of
total dissolved and labile nickel species in environmental
waters. The fractionation of labile and non-labile inert
nickel species is based on a chelating resin because the
Serdolit Chelite Che chelating resin (containing imino-
diacetic acid groups) is suitable to preconcentrate/sepa-
rate only free and labile forms of nickel. Prior to passing
through the resin-containing microcolumn, samples were
sonodigested on-line to decompose organic complexes
and to allow for the determination of total dissolved Ni.
This speciation scheme was applied to the analysis of
river surface and sea water samples collected in Galicia
(Northwest, Spain).
2. Experimental
2.1. Instrumentation
The on-line system used is shown in Figure 1. It con-
sisted of two Gilson Minipuls 3 peristaltic pumps (Gilson,
France), five Rheodyne low-pressure injection valves
(four of them adapted to work as selection valves)
(Rheodyne, USA), an ultrasonic bath (Selecta, Spain)
and a digestion chamber (three glass minicolumns con-
nected in series, 100 mm × 15 mm i.d., bed volume 12 mL,
Omnifit, UK). The laboratory-made microcolumn for the
on-line preconcentration step was prepared filling Viton
tubes (100 mm × 1.1 mm i.d.) with 50 mg of Serdolit
Chelite Che chelating resin. The experiments were per-
formed with a Perkin Elmer Model 5000 (PerkinElmer
Life and Analytical Sciences, USA) air-acetylene flame
atomic absorption spectrometer attached to a Perkin El-
mer Model 50 Servograph Recorder with a range of 5 mV.
A Ni hollow cathode lamp, operated according to the
manufacturer’s recommendations, was utilized as a ra-
diation source.
All glassware was decontaminated with 10% (v/v) ni-
tric acid for at least 48 h and washed three times with
Milli-Q water before use.
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Copyright © 2011 SciRes. AJAC
Figure 1. Schematic of the flow injection manifold used for the determination of total and labile dissolved Ni(II) in environ-
mental water samples. B: blank; BS: buffer solution; DC: digestion chamber; E: eluent; FAAS: flame atomic absorption
spectrometer; IV: injection valve; M: mixing coil; MC: microcolumn containing the resin Serdolit Chelite Che; P1 and P2:
peristaltic pumps; S/SS: sample or standard solution; SV1, SV2, SV3 and SV4: selecting valves; UB: ultrasonic bath; UW:
ultrapure w a t er; W: waste.
2.2. Reagents and Samples
Nickel stock solution (1000 g·mL–1) was purchased
from Merck (Germany). The nitric acid used to water
acidification and the hydrochloric acid used to the elu-
tion step were from Merck (Germany). The chelating
resin containing iminodiacetic acid groups was Serdolit
Chelite Che (Serva Electrophoresis, Germany). Ammo-
nia-ammonium chloride buffer solution (pH 8) was pre-
pared by dissolution of 1.1 g of ammonium chloride
(Merck, Germany) in 100 mL of ultrapure water and
adjust the pH to 8.0 by adding diluted ammonia (Merck,
Germany). Aqueous solutions of humic acid (Sigma-
Aldrich, Switzerland), ethylenediaminetetraacetic acid
(EDTA), sodium chloride and sodium sulfate (Merck,
Germany) were prepared as model natural ligands. Certi-
fied reference material Slew-3 (estuarine water) obtained
from the National Research Council of Canada. Solu-
tions (reagents and standards) were prepared using ul-
trapure water of 18.2 M cm resistivity, obtained from a
Milli-Q water purification system (Millipore, USA).
Statistical analysis of the experimental designs was
carried out by means of the Statgraphics Plus V.5.1 sta-
tistical package (Manugistic, Inc., USA).
Surface river water and seawater samples were col-
lected in Galicia (Northwest, Spain). For the determina-
tion of total dissolved nickel, samples were collected in
pre-cleaned glass bottles, filtered through a 0.45 µm
membrane filter (mixed cellulose esters, Millipore Ibéri-
ca, Spain); and immediately acidified with 10 mL of
concentrated nitric acid to avoid metal adsorption on the
container walls. The samples were stored in a refrigerator
(4˚C) for no longer than two weeks. For the determina-
tion of the nickel labile fraction, water samples were
preconcentrated on-site without prior pH modification.
After on-line filtration with a 0.45 µm filter, 200 mL of
each sample was passed through a resin-containing mi-
crocolumn at a flow rate of 4.0 mL·min–1 [13]. As a re-
sult, nickel was retained on the microcolumn only in its
labile fraction. After loading, the resin was washed with
ultrapure water and the residual internal fluid drawn off.
The microcolumns were transported in a portable refrig-
erator, and returned to the laboratory where they were
stored in a refrigerator until further analysis.
2.3. Procedure
For the determination of the concentration of the nickel
labile fraction, the in-situ loaded microcolumns were
connected to the FI manifold shown in Figure 1. The
microcolumn was located immediately before the detec-
tor. The analysis procedure consists of the injection of
110 L of 3 mol·L–1 hydrochloric acid into a carrier ul-
trapure water pumped at 5.0 mL·min–1. Thus, the labile
fraction of nickel is released directly into the nebulizer of
the FAAS spectrometer at a flow-rate of 5.0 mL·min–1.
The determination of the total dissolved concentration
of nickel takes place after decomposition of metal or-
ganic complexes by sonodigestion. Thus, water samples
(30 mL) in 0.6 mol·L–1 nitric acid medium are inserted
into the flow system at 5.0 mL·min–1 by means of a peri-
staltic pump (P2). Once all the sample volume is located
in the digestion chamber (three glass minicolumns con-
nected in series (DC)), which are immersed within the
ultrasonic bath at room temperature, the pump control-
ling the sample stream (P2) was stopped and the sample
stays under the action of ultrasound energy for a period
of 120 s. After, the selecting valve (SV3) was switched
to its other position to select the digestion chamber
downstream, and the pump P2 was again activated. In
this way, the sonodigested water sample arrives at the
part of the FI system where the preconcentration step
takes place, and converged with the buffer stream (am-
monia-ammonium chloride buffer solution, pH 8) in or-
der to obtain the optimum pH value for metal retention in
the chelating resin. Both channels are homogenized in
the mixing coil and then, the resulting stream passed
through the microcolumn containing the chelating resin
(Serdolit Chelite Che). By means of SV2, the sample
matrix is sent to waste, while ultrapure water flowing
through the detector by the nebulizer suction. Finally, the
retained nickel was subsequently eluted by injection of
110 µL of 3 mol·L–1 hydrochloric acid into a water car-
ried stream, being continuously monitored by a FAAS
instrument. In order to avoid carry-over, a washing step
was including in the analysis cycle. Thus, between each
sample analysis, the digestion chamber was washed with
ultrapure water during 60 s. For this, the selecting valve
(SV4) was switched to select the ultrapure water stream
channel, while the wash water was sent to waste through
3. Results and Discussion
In this work, was chosen a resin with iminodiacetic acid
groups (Serdolit Chelite Che) because a commercial re-
sin containing this chelating group as Chelex-100 was
proved to be an excellent medium for metal speciation.
However, few reports appear in the bibliography in
which other resins with these chelating groups are em-
ployed for speciation schemes [8,9]. Furthermore, the
chelating resin Serdolit Chelite Che is a macroporous
(macroreticular) polystyrene based resin, which effec-
tively size excludes colloids from binding sites. More-
over, this resin has as advantage a wide pH range in-
cluding unpolluted river and sea water [28]. Thus, the
labile nickel fraction can be retained by the resin without
pH modification, which prevent of changes in the ana-
lytical composition of the different nickel fractions.
3.1. Effect of Complexing Agents
Competition for metals from ligands in solution will de-
termine the distribution among labile (free (aquo-ions)
and weakly complexed species) and inert fractions (metal
complexes unable to be dissociated by interaction with
the active groups of the resin). In this study were selected
as representative ligands chloride, sulfate, EDTA and
humic acid at a concentration of 19000, 905, 30 and
30 mg·L–1, respectively. The inorganic ligands chosen
are the major anionic components of river and sea water,
and regarding the organic ligands, humic acids are the
very important. EDTA is an organic ligand present in
environmental waters, which is frequently used as or-
ganic ligand model because has high affinity for metals,
with which forms strong complexes [29]. The influence
of these ligands on the complexation and retention of
nickel by the Serdolit Chelite Che resin was investigated
at pH 6 - 8 by using a solution containing 1 µg·L–1 of
Ni(II) and a volume of 200 mL. The results of this study,
shown in Figure 2, demonstrate that for both pH studied,
more than 95% Ni(II) in chloride and sulfate solutions
were retained by the resin. It is due to the formation of
labile complexes with these anions. Nevertheless, when
the solution contains humic acid or EDTA, for both
tested pH values the nickel recovery was less than 10%.
This behavior can be explained by the formation of non-
labile complexes, which are not retained on the chelating
resin. Thus, in view of the obtained values, it was estab-
lished that the Serdolite Chelite Che resin can be used for
separation of the free and the labile nickel fraction from
their inert complexes formed with organic ligands.
3.2. Optimization of the Experimental
Conditions for the FI Preconcentration
of Ni(II)
Experimental work was initially focused on the optimi-
zation of the variables implied on the utilization of the
chelating resin Serdolite Chelite Che in an on-line mode
because the preconcentration step determines the effi-
ciency of the final method. By applying the Plackett-
Burman statistical method, it was possible to select the
variables which had the largest influence on percentage
of nickel recovery (analytical response). The selected
Figure 2. Influence of several ligands on the complexation
and retention of Ni(II).
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factors and their levels are presented in Table 1. These
factors were selected according to available data and
experience gathered in previous screening experiments.
Thus, a screening Plackett-Burman 2 ^ 6 × 3/16 experi-
mental design with one centerpoint, which studied the
effects of the factors in 13 experiments was run. All ex-
periments were performed using a sample volume of 30
mL and introducing a standard solution of 1 g·L–1 of
Ni(II) into the FI manifold. The analysis of the results
produced the main effect Pareto chart (P = 95%) (Figure
3). In these charts, bar lengths are proportional to the
absolute value of the estimated effect, which helps when
comparing the relative importance of effects. The mini-
mum ‘‘t’’ value at the 95% confidence interval was 2.4.
The t value is plotted as a vertical black line in Figure 3.
We considered as significant any variable that offered a
value higher than ±t. Pareto chart (Figure 3) shows the
most important factors affecting the on-line preconcen-
tration of nickel. The sample pH (A), the eluent concen-
tration (diluted hydrochloric acid) (C) and the eluent
volume (E) were statistically significant with positive
sign. The sample flow-rate, (B) was a variable with a
great influence on the experimental response with nega-
tive sign. The other variables considered, elution flow-
rate and minicolumn diameter did not produce any sig-
nificant effect. Therefore, these statistically insignificant
variables were fixed at certain values taking into account
the best operational, flow and analytical conditions.
While the significant ones (sample pH (A), sample
flow-rate (B), eluent concentration (C) and eluent vol-
ume (E)) were optimized by using a central composite 24
+ star design with 11 error degree of freedom and 26
runs. Table 1 lists the upper and lower values given for
each variable together with the fixed values for insig-
nificant variables studied in the screening design. By use
of the standardized Pareto chart is found that the sample
pH and eluent concentration are statistically significant
factors in the ranges under study. Thus, maximum values
tested for these variables were selected as optimum. The
values chosen for sample flow-rate and eluent volume
were a compromise between reasonable recovery, rea-
sonable analysis time, and adequate concentration factor
achieved. The choice was inferred from predictions pro-
vided by the Statgraphics software. The values selected,
on the basis of results obtained by use of the response
surface (Figure 4), are listed in Table 1. As the recovery
in the optimum conditions for pH values studied ranged
between 85.7% to 96.7%, it was verify the optimum pH
range for nickel preconcentration. Thus, recovery was
calculated under the selected experimental conditions
and modifying the sample pH (between 4 and 8). The
results obtained demonstrated that nickel was recovered
quantitatively in the pH range 5 - 8. This pH range in-
cludes the normal pH range of river and sea water, which
implies that the pH does not have to be modified when
the labile nickel species are on-site preconcentrated.
Table 1. Variables and their experimental levels adopted for the selection of variables involving the FI preconcentration step.
Plackett-Burman 2 ^ 6 × 3/16 design 2 ^ 4 + star central composite design
Level Level
Key Low Upper
Optimum Low Upper
Sample pH A 1 8 - 2 8 8
Sample flow-rate (mL·min–1) B 1 5 - 1 4 4
HCl concentration (eluent solution, mol·L–1) C 0.1 3 - 1 3 3
Elution flow-rate (mL·min–1) D 3 5 5 - - -
Eluent volume (µL) E 70 110 - 90 110 110
Minicolumn diameter (mm) F 1.1 2.3 1.1 - - -
Figure 4. Cubic response surface estimated for the central
composite design in the optimization of the FI preconcen-
tration step.
Figure 3. Pareto chart for the Plackett-Burman design ap-
plied to optimize the FI preconcentration step.
3.3. Optimization of the Experimental
Conditions for the On-Line Sonodigestion of
Environmental Water Samples
The optimization of the variables affecting the on-line
sonodigestion procedure of environmental water samples:
concentration of nitric acid (acid sample medium) (A),
sonication time (B) and sonodigestion temperature (C),
was carried out by building a central composite design
(23 + star with 11 error degree of freedom, 16 random-
ized experiments and three replicates). The tested values
for each variable are shown in Table 2. The response
variable was the recovery calculated according to the
following equation: % R = (C1/C2) × 100, where C1 is
the concentration of Ni(II) obtained by the proposed
procedure and C2 is the concentration obtained for the
same water sample, but performing the sample digestion
procedure by traditional way using ammonium peroxy-
disulfate at elevated temperature [30]. The experiments
were performing by using a sample volume of 30 mL,
and as digestion chamber two possible operational modes
were studied: 1) a glass minicolumn (100 mm × 25 mm
i.d., bed volume 35 mL), 2) three glass minicolumns
connected in series (100 mm × 15 mm i.d., bed volume
12 mL). After performing the experiments, it is proven
that for both operational modes tested, sonodigestion
temperature is not a significant variable, resulting in low
estimated effects. In fact, some experiments result in
quantitative recoveries when the sonodigestion tempera-
ture is 20˚C. Therefore, 20˚C (room temperature) was the
value chosen as optimum for this variable. Thus, only the
two statistically significant factors (with positive sign):
acid sample medium concentration (A) and sonication
time (B) were taken into account to analyze the design.
The values chosen for these variables were a compro-
mise between, reasonable analysis time and the lowest
nitric acid concentration with which a quantitative re-
covery is obtained. The choice was inferred from predic-
tions provided by the Statgraphics software. The values
selected, on the basis of results obtained by use of the
response surface (Figure 5) are listed in Table 2. The
analysis of the response surface plot indicates that for the
same sonodigestion acid medium (0.6 mol·L–1 nitric ac-
id), the sonication time required for quantitative recovery
when a single minicolumn was used as digestion cham-
ber is higher than that need when three minicolumns
were used. This can be explained by the fact that the sin-
gle minicolumn has a diameter (25 mm) higher than that
of the three minicolumns connected in series (15 mm),
which generates a high attenuation of ultrasound waves,
reducing the effects of ultrasound energy in the sample
within the minicolumn. Consequently, in order to in-
crease the analysis time, for subsequent work we decided
to use minicolumns with the minor diameter (15 mm).
As each column has a bed volume of 12 mL, are neces-
sary three to contain a water sample volume of 30 mL. In
these conditions, nickel was quantitatively recovered
when the sonication time was 120 s.
Table 2. Optimization of experimental parameters for continuous sonodigestion.
2 ^ 3 + star central composite design
Level Optimum
Variable Key Low Upper M1a M2b
Concentration of acid sample medium (nitric acid, mol·L–1) A 0.2 1 0.6 0.6
Sonication time (min) B 60 240 180 120
Sonodigestion temperature (˚C) C 20 50 20 20
aM1: a glass minicolumn (100 mm × 25 mm i.d., bed volume 35 mL); bM2: three glass minicolumns connected in series (100 mm × 15 mm i.d., bed volume 12 mL).
Total bed volume 36 mL.
Figure 5. Response surfaces estimated for the central composite design in the optimization of the on-line sonodigestion. (a)
Digestion chamber: one minicolumn (25 mm i.d.); (b) Digestion chamber: three minicolumns connected in series (15 mm i.d).
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3.4. Analytical Data and Validation of the
The sample volume used for the determination of the
total dissolved concentration of nickel was 30 mL, and
the sample volume used for the determination of the dis-
solved labile nickel fraction was 200 mL. Therefore, for
each one of these sample volumes calibration plots were
established by use standard solutions at eight concentra-
tions levels (0 - 7.5 µg·L–1 Ni and 0 - 1.1 µg·L–1 Ni, for
30 and 200 mL, respectively). These standard solutions
were treated in accordance with the sample-treatment
procedure using the described optimized system de-
scribed above.
The regression equations of the calibration graph ob-
tained for both sample volumes are shown in Table 3.
The limits of detection (LOD) and the limits of quantifi-
cation (LOQ) for the different nickel fractions were cal-
culated by use of the IUPAC criterion [31]. As a result of
use a minor sample volume, the detection limit for the
determination of the dissolved labile nickel fraction is
lower than that for the determination of its total dissolved
Analytical and statistical data for each nickel fractions
are summarized in Table 3. The precision of the method
(within day), expressed as relative standard deviation,
was evaluated by eleven independent measurements of a
water sample. The preconcentration factors calculated as
the ratio of the slope of the calibration graph with and
without preconcentration were also shown in Table 3.
Analytical recovery was assessed after spiking aliquots
of seawater and river water samples with metals (Table 3).
It can be concluded that a complete analytical recovery
(within the 95.5% - 98.6% range) was reached for both
nickel fractions. A certified reference material (Slew-3)
with a certificate value of (1.23 ± 0.07) µg·L–1 Ni was
used to verify the accuracy of the proposed methodology.
The concentrations found was (1.25 ± 0.07) µg·L–1,
which is in concordance with the certified value. To
evaluate the possible sample matrix interferences in the
nickel determination, a standard addition method was
performed. In this way, the certified reference material
(Slew-3) was spiked with several nickel standard solu-
tions (0 - 5 μg·L–1). In these conditions, an addition cali-
bration graph was run (n = 6) under the optimal chemical
and flow conditions for the whole process. The equation
was: absorbance = 8.8 × 10–3 C + 0.011 (r2 = 0.9996),
where C is Ni concentration, expressed as μg·L–1. This
equation and the calibration graph have the same slope,
demonstrating that Ni determination is free of matrix
With the aim to establish the time that, once in situ
loaded with water samples, the minicolumns could be
stored before nickel determination, stability studies at
optimum conditions were performed. For this, the nickel
retained on minicolumns was determined at the same day
of the sampling, and after 5 and 10 days stored at 4˚C
(refrigerator). The obtained results indicated that mini-
columns were stable for at least 10 days and could be
stored during this time without any significant losses.
3.5. Determination of Labile and Total Nickel in
Environmental Water Samples
In order to evaluate the analytical applicability of the
proposed method, it was applied to the determination of
labile and total nickel in several environmental water
samples from Galicia (north-western Spain) and the re-
sults obtained are summarized in Table 4 (mean ± stan-
dard deviation, n = 3). The percentages for the labile
fraction of each metal are also presented in Table 4.
Thus, it has demonstrated that in the analyzed seawater
samples, nickel is principally present in the inert fraction
forming strong organic complexes ((10.8 ± 0.4)%) in the
Table 3. Analytical and statistical data of the determination of total and labile dissolved nickel in environmental water samples.
Labile Ni Total Ni
Sample volume, mL 200 30
Calibration equation (C, µg·L–1) 5.9 × 10–2 C + 8.0 × 10–5 8.8 × 10–3 C + 3.1 × 10–4
Linear range, µg·L–1 0.065-1.17 0.45 - 7.52
Correlation coefficient, r2 0.999 0.999
Preconcentration factor 1792.7 265.9
Detection limit, µg·L–1 0.021 0.13
Precision (RSD, %) 2.7 1.8
Recovery(%)a 95.5 96.6
Recovery(%)b 96.9 98.5
Recovery(%)c 97.4 97.2
Recovery(%)d 98.6 97.7
aTotal dissolved Ni: seawater sample spiked with 2 µg·L–1. Labile dissolved Ni: seawater sample spiked with 0.3 µg·L–1 (n = 3); bTotal dissolved Ni: seawater
sample spiked with 3 µg·L–1. Labile dissolved Ni: seawater sample spiked with 0.4 µg·L–1 (n = 3); cTotal dissolved Ni: river water sample spiked with 3 µg·L–1.
Labile dissolved Ni: river water sample spiked with 0.3 µg·L–1 (n = 3); dTotal dissolved Ni: river water sample spiked with 5 µg·L–1. Labile dissolved Ni: river
water sample spiked with 0.5 µg·L–1 (n = 3).
Table 4. Determination of labile and total nickel in environmental water samples from Galicia (north-western Spain).
Nickel concentration, µg·L–1
Sample Labile Total % LFa
Seawater 1 0.48 ± 0.02 4.62 ± 0.11 10.4
Seawater 2 0.34 ± 0.02 3.22 ± 0.17 10.6
Seawater 3 0.19 ± 0.01 1.71 ± 0.07 11.1
Seawater 4 0.11 ± 0.01 1.03 ± 0.07 10.7
Seawater 5 0.26 ± 0.02 2.24 ± 0.11 11.6
Seawater 6 0.21 ± 0.01 1.97 ± 0.07 10.7
Seawater 7 0.42 ± 0.02 3.83 ± 0.11 11.0
Seawater 8 0.50 ± 0.03 4.55 ± 0.07 11.0
Seawater 9 0.63 ± 0.03 6.02 ± 0.17 10.5
Seawater 10 0.34 ± 0.02 3.26 ± 0.23 10.4
River water 1 1.15 ± 0.01 1.29 ± 0.10 89.1
River water 2 0.62 ± 0.01 0.68 ± 0.03 91.2
River water 3 1.06 ± 0.01 1.18 ± 0.07 89.8
River water 4 0.55 ± 0.02 0.61 ± 0.03 90.2
River water 5 0.82 ± 0.01 0.91 ± 0.07 90.1
River water 6 0.99 ± 0.02 1.14 ± 0.09 86.8
River water 7 0.90 ± 0.01 1.03 ± 0.07 87.4
River water 8 0.81 ± 0.02 0.96 ± 0.03 84.4
a LF: Percentage of labile fraction
labile dissolved fraction). By the other hand, in river wa-
ter samples nickel is found mainly in the labile dissolved
fraction ((88.6 ± 2.3)%).
4. Conclusions
A combination of continuous sonodigestion and solid-
phase extraction by using a chelating resin was exploited
for dissolved nickel speciation in environmental water
samples, determining labile (comprising the free ion,
inorganic complexes and weak organic complexes) and
total dissolved nickel (mainly comprising, additionally,
strong organic complexes) with a high sensitivity and
precision. Sonodigestion has been used for first time for
decomposition of organic complexes of nickel from en-
vironmental waters, which allowed simplify sample
preparation and speed up and the analysis time for the
determination of total dissolved Ni concentration. On the
other hand, for the determination of the labile fraction, it
is demonstrated for the first time that Chelite Che resin
can be used as an effective material for the in-situ sepa-
ration/preconcentration of free and labile dissolved Ni(II)
species. Furthermore, the proposed methodology could
be applied with detection by inductively coupled plasma
atomic emission spectrometry and inductively coupled
plasma-mass spectrometry to improve analytical sensi-
5. Acknowledgements
This work has been funded by the Spain's Ministry of
Science and Innovation, within the framework of Project
CTQ 2009-12282.
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