Activated Hordeum vulgare L. dust as carbon paste electrode modifier for the sensitive electrochemical detection of Cd2+, Pb2+ and Hg2+ions

ABSTRACT Hordeum vulgare L. dust (HVW) chemically activated using Na2CO3 (HVW-Na2CO3) was characterised by several techniques (SEM, FTIR and TGA/DTG). The increase in the surface charge of the biomaterial induced by the activation process was demonstrated by multisweep cyclic voltammetry. The modified HVM was then exploited as carbon paste electrode (HVW-Na2CO3/CPE) modifier to prepare a sensor which was applied in the determination of Cd2+, Pb2+ and Hg2+at trace level by stripping voltammetry. Key experimental variables (nature of stripping medium, modifier mass in CPE, electrolysis time and potential and pH of preconcentration solution) were optimised to detect Cd2+, Pb2+ and Hg2+ at HVW-Na2CO3/CPE. Under optimised conditions, calibration plots were obtained for all analytes. The low limits of detection (3s/m) were found to be 1.82 nM, 0.0691 nM and 0.237 nM for Cd2+, Pb2+and Hg2+respectively. The sensor was successfully applied to the detection of trace metals in real samples.


Introduction
In most countries, the spread of industrialisation and the rise of mining activities is continuously favouring intensive discharges of heavy metals into the environment.These heavy metals often have undesirable effects on living organisms [1,2].Among these heavy metals, Cd 2+ , Pb 2+ and Hg 2+ are known to be the most widespread in the natural environment.They are indeed present in solid waste (scrap, batteries, ceramics, pipes, optical fibres, etc.) as well as in liquid wastes (paints, ink, engine oils, etc.).As most heavy metals, they are non-biodegradable and are therefore persistent in nature.They are toxic even when present at very low concentrations in ecosystems [3,4].The toxicity of these metals is enhanced through accumulation in the food chain.As a consequence on human beings for example, this contamination can lead to the damaging of the nervous system, the reproductive system, the respiratory system, cardiovascular diseases, memory loss, cancers, headache, gastric and kidney disorders [5].These harmful effects of Cd 2+ , Pb 2+ and Hg 2+ to human health show the importance of quantifying and eliminating these metals from effluents before their release into the environment.In this light, researchers have developed several techniques for their detection at trace levels in various matrices.To achieve this objective, spectrometric techniques such as atomic absorption spectrometry, mass spectrometry, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence spectrometry have been extensively used [6,7].Most of these techniques are very effective, but are very expensive since they require sophisticated equipment, and in addition tedious sample pre-treatment and long analysis times [8].Accordingly, the resort to electrochemical methods as alternatives has prompted the development of amperometric sensors based on modified electrodes.Yet, these methods are easy to handle, employ low-cost instruments and less tedious experiments.More interestingly, they are sensitive and selective if the electrode material is conveniently chosen to display chemical affinity with the target analyte [9,10].Concerning the detection of heavy metals at trace levels, a large variety of materials can be used to prepare chemically modified electrodes that include carbon derivatives [11], silica [12], clay minerals, metal oxides [9,13,14], quantum dots, enzymes and organic polymers [15].At their native state, these materials are often limited in terms of sensitivity.Additional procedures are thus needed to enhance their performance.This can be achieved by combining them with functionalised organic molecules, or with other modifiers endowed with exchange properties or chelating ability.These limitations have prompted the development of amperometric sensors based on lignocellulosic materials recognised to be friendly with the environment for their biodegradability.Also, they are widely available and are therefore of low-cost.In this light, several works have been reported in the past years on the use of natural biomass as electrode modifiers.As few examples, Yüce and co-workers exploited algal sensor for the voltammetric determination of lead [10,16].Other efficient sensing platforms have been reported for the voltammetric quantification of heavy metals using electrodes modified by Ramalinastenospora lichen and Sphagum moss [17], Aulosira sp.Alga [18], coconut shell powder [19], castor oil cake biochar [20,21] maize tassel [22] rice straw [23] and banana tissues [24].Most recent works on the topic were conducted by [25] and [26] who respectively exploited nanofibrillated cellulose and fungal soil biomass modified electrodes for the determination of trace heavy metal ions by anodic stripping voltammetry.As evidenced by research works in this field, it appears that the search of simple and cheap electrode configurations likely to display great sensitivity towards the individual and simultaneous detection of toxic heavy metals ions remains a daily and challenging task.
In this work, we report the use of Na 2 CO 3 treated Hordeum vulgare L. dust (HVW-Na 2 CO 3 ) as carbon paste electrode (CPE) modifier exploited for the electrochemical determination of Cd 2+ , Pb 2+ and Hg 2+ ions in aqueous media.Hordeum vulgare L. dust (HVW) is a largely available biomass residue released by brewing industries during the crushing of malt grains.It is essentially made up of starch, cellulose, proteins, lignin and extractables, whose percentages vary from a sample to another according to their origin and intrinsic nature [27,28] Prior to its used within the CPE, it was chemically treated in alkaline medium to increase its chelating ability towards the ionic species investigated.To evaluate its suitability for the detection of several heavy metals in real samples, the parameters of the proposed sensor were carefully optimised.It was shown to be a low cost and sensitive tool that can be used in the monitoring of toxic ions in aqueous environment.

Chemical activation HVW by Na 2 CO 3
The activation of HVW was performed according to the following procedure: in a flask containing 2 g of biomaterial, 50 mL of a 2 M Na 2 CO 3 were introduced.The mixture was stirred for 3 h at a speed of 200 rpm.The residue obtained after filtration was washed several times with distiled water, until a filtrate with a pH value around 6.5 was obtained.The washing was done to remove residual bicarbonate from the material.The final product obtained was dried in open air for 24 h and then in an oven at 105°C for 2 h.The resulting material is here after denoted HVW-Na 2 CO 3 .

Preparation of working electrodes
Unmodified CPE was prepared by mixing graphite powder and silicon oil (70% and 30% w/w respectively) to form a uniform paste.The obtained homogenous paste was housed into a teflon tube (3 mm diameter) with a stainless-steel wire (grade 303) electric contact.The active external part of the CPE was finally polished on a paper sheet until a smooth surface was obtained.The HVW modified CPE was prepared using the previous procedure, by keeping at 30% (w/w) the ratio of silicon oil while varying those of graphite and biomass in the following proportions: (65%/5%, 60%/10%, 55%/15% and 50%/20%) respectively.Throughout the text, the unmodified CPE, the CPE modified by the pristine and activated biomass material will be referred to UMCPE, HVW/CPE and HVW-Na 2 CO 3 /CPE, respectively.

Electrochemical procedures
Electrochemical experiments were carried out in a three-electrode cell, with Ag/AgCl (3 M KCl) reference electrode, a platinum wire as auxiliary electrode and the modified or bare CPE working electrodes.The cell was connected to a PalmSens3 potentiostat running on PS Trace 4.2 software.The voltammetric detection of heavy metals involved two steps: (i) preconcentration for a predetermined duration at opencircuit, achieved by immersing the working electrode into a stirred solution containing one or several analytes and (ii) the stripping step where the electrode was removed from the previous solution, rapidly rinsed with distiled water and placed into the electrochemical cell containing the detection solution.The voltammogram was then recorded using Differential Pulse Anodic Stripping Voltammetry (DPASV) in the potential range from −1200 to 400 mV, with the following optimised parameters: step potential 5mV, potential pulse 5 mV, pulse time 17 ms and scan rate 50 mV/s.Cyclic voltammetry (CV) was performed in the potential range from −600 to 800 mV and a scan rate of 50 mV/s.

Apparatus
Scanning electron microscopy (SEM) images were recorded on a FEI-NOVA nanolab 600 FIB/SEM microscope, as well as EDX analysis.
Powder XRD patterns of dried raw and treated materials were recorded at room temperature using a Bruker D2 Phaser diffractometer operating with Cu-Kα radiation (λ = 1.54056Å) using a generator with a voltage of 45 kV and a current of 40 mA.The functional groups present on the material were identified using Fourier Transform Infrared Spectroscopy (FTIR) performed on a FT-IR spectrometer (model Bruker Vector 22) in the range from 4000 to 500 cm −1 .Thermogravimetric analysis measurements were performed on a Perkin Elmer Pyris 1 TGA analyser.These physico-chemical characterisations were carried out in the Centre of Nanomaterials Research at the University of Johannesburg (South Africa).

Morphological and structural analysis
The SEM images on Figure 1(a-d) showed the morphology or external texture of HVW samples before and after activation, as a function of the different magnifications.The micrographs show irregular and relatively heterogeneous porous surfaces made up of agglomerates of small particles of variable shapes and dimensions.The particles of HVW-Na 2 CO 3 are uniformly distributed (Figure 1(a)) compared to HVW (Figure 1(c)).The uniform structure of HVW-Na 2 CO 3 makes available the active sites present on its surface and therefore indicates that HVW activated by Na 2 CO 3 would have a specific surface area larger than raw HVW.
Structurally, the pristine HVW and its activated counterpart were characterised using EDX analysis coupled to elemental analysis, and X-Ray diffraction.
The mean percentage of cellulose, hemicellulose and lignin in HVW and HVW-Na 2 CO 3 (determined in triplicate) are shown in Table 1.Cellulose, lignin and hemicellulose accounted for 10.17%, 3.16% and 2.08% respectively, of the total dry weight.In HVW-Na 2 CO 3 material, a decrease in these percentages was observed.This observation is probably due to the chemical activation by the alkaline treatment using Na 2 CO 3 which led to the elimination of extractables from HWV [31,32].
The results of EDX are given in Figure SI 1 (Supplemental file) while elemental analysis data obtained for C, H, N and O are gathered in Table 2.These results show a slight decrease in the percentages of most elements (e.g.C, H, N and O for elemental analysis; O, K, Ca and Si for EDX), as observed previously when HVW was treated in alkaline medium.The XRD patterns of pristine and modified HVW are shown in Figure SI 2 (Supplemental file).On both curves, five main diffraction peaks (appearing at 2θ = 15.02°,17.04°, 17.98°, 19.95°and 22.86°) were recorded at the same 2θ position and location, that were attributed to crystalline cellulose and lignin, as identified from previous studies on the XRD analysis of lignocellulosic materials [33][34][35].This indicated that the crystalline structure of HWV did not change upon treatment by Na 2 CO 3.

FTIR analysis
Figure 2 presents the IR spectra of HVW before and after activation.One can observe that similar bands are visible on both spectra, proving that the treatment of HVW did not alter its main functional groups.The main bands due to the functional groups encountered in lignocellulosic materials were observed.The band at 3280 cm −1 is assigned to the vibration of the O-H groups of lignin and polysaccharides while that at 2921 cm −1 is due to aliphatic C-H stretching.At 1730 cm −1 appears the C = O stretching band of aldehydes, esters and carboxylic acids.The same band attributed to the C = O of carboxylates functions can be seen at 1642 cm −1 .The signals appearing The strong band at 1016 cm −1 is assigned to C-O stretching in cellulose, hemicellulose and lignin.At 1544 cm −1 are the lignin aromatic ring C = C stretching vibrations [36,37].Comparing the spectra of the starting biomass and its activated counterpart also showed that the treatment with Na 2 CO 3 resulted in the disappearance of the peak corresponding to the carbonyl group of ether and carboxylic functions (1730 cm −1 ).On the other hand, the peak of the carbonyl of carboxylate functions (1642 cm −1 ) increased.This probably means that the treatment with Na 2 CO 3 tends to hydrolyse the ester functions and convert the carboxylic functions to carboxylate [38,39].
3.1.4.Electrochemical behaviour of [Ru(NH 3 ) 6 ] 3+ at HVW/CPE and HVW-Na 2 CO 3 /CPE Upon activation, the surface charge of HVW was evaluated using [Ru(NH 3 ) 6 ] 3+ as redox probes, when the biomaterial was inserted in the CPE.Multisweep cyclic voltammograms were recorded for the probe using HVW/CPE and HVW-Na 2 CO 3 /CPE as working electrodes.By continuous cycling, the electrode response gradually increased as a result of [Ru(NH 3 ) 6 ] 3+ cations progressive uptake from the bulk supporting electrolyte.However, the phenomenon was less pronounced for unmodified biomass CPE compared to HVW-Na 2 CO 3 /CPE (See Figure SI 3,Supplemental data).Yet, the current obtained upon saturation (after 25 cyclic scans) was ca 30 µA on HVW/CPE while the same parameter was more than 150 µA on activated HVW CPE.The observed best affinity between the electrode material and the positively charged [Ru(NH 3 ) 6 ] 3+ ions on HVW-Na 2 CO 3 /CPE could be explained by the chemical treatment of HVW dust in alkaline solution which increased the adsorbing sites by reducing the amount of hemicellulose and lignin.Consequently, the negatively charged surface due to carboxylate ions from cellulose was more important, favouring the electrostatic interactions between this surface and the cationic probe [38,45].This result was the relevant aspect exploited for the building of the sensor proposed in this work for the detection of metal ions as revealed by the investigations in the next section.2+ , Pb 2+ and Hg 2+ on various CPEs 3.2.1.Preliminary investigation on the determination of Cd 2+ , Pb 2+ and Hg 2+ Before using the HVW modified CPE for the detection of Cd 2+ , Pb 2+ and Hg 2+ ions, the change in electrode response observed by cyclic voltammetry was also chequed by investigating electrochemical preliminary tests by differential pulse voltammetry (DPV).Figure 4(a) compares the DPV curves of Cd 2+ , Pb 2+ and Hg 2+ recorded on the CPE before, and after its modification in turn by pristine and activated HVW.For this first trial, the accumulation of heavy metals was performed in 3 different aqueous solutions containing each species alone, at the same concentration.The results obtained clearly showed well define peaks for all analytes on the investigated electrodes.One can also observe that these electrodes are more sensitive to lead detection, followed by mercury and cadmium, the highest response being recorded on HVW-Na 2 CO 3 /CPE as a result of more chelating groups at the electrode/electrolyte interface.Yet, it has been shown that, apart from the number of adsorbing sites, the uptake of ionic species by biomass materials is also affected by the size of ions [46].Since the ionic radius of Pb 2+ ion (1.21 Å), is larger than those of Hg 2+ (1.10 Å) and Cd 2+ (0.90 Å), a stronger physical affinity for Pb 2+ was expected at the adsorption sites on the studied HMV biomaterial.

Electrochemical response of Cd
In a second trial, the electrodes were tested for the simultaneous detection of Cd 2+ , Pb 2+ and Hg 2+ ions, that is when the three analytes were together present in the same preconcentration medium.To clearly appreciate the signal due to each of them, the concentration of Cd 2+ and Hg 2+ were increased to 5 × 10 −5 M and 2 × 10 −5 M respectively, compared to the case of individual detection (Figure 4(b)).One can notice that the peak potentials of the analytes shifted slightly due to some mutual interactions between these analytes, the three corresponding peaks remained meanwhile clearly distinct.
Generally, in electrochemical experiments exploiting stripping voltammetry for the determination of heavy metals, the detection step is performed in acidic media recognised to favour the formation of complexes between chelating groups and cationic species [13,47].Thus, several acidic solutions were evaluated as detection solution media, upon accumulation of the target analytes.The voltammograms recorded in HNO 3 , HClO 4 , HCl and H 2 SO 4 all at the same concentration (0.1 M) showed that the peak current recorded in 0.1 M HCl is the most intense (See Figure SI 4, E-Supplemental data).This indicates much easier desorption of the Cd 2+ , Pb 2+ and Hg 2+ ions from the binding sites in this medium.The favourable affinity between the investigated cations and Cl − ions that leads to the formation of stable complexes could probably explain such a result which is in perfect agreement with previous studies [13,24,48].HCl was then selected hereafter as stripping medium.
The last step was reserved to study the effect of its concentration which was varied from 0.1 to 0.6 M (See Figure SI 5, E-supplemental data).The desorption of all heavy metals was shown to be more efficient in 0.5 M HCl used in further investigations as stripping medium.

Effect of the mass of activated HVW in the CPE
In order to select suitable conditions for the determination of heavy metals using DPV, various parameters were scrutinised.The influence of the activated biomass in the bulk of CPE was the first to be investigated.Thus, CPEs were modified using HVW-Na 2 CO 3 with mass varying from 2 to 20 mg.Thus, in this study, CPEs were prepared by keeping the silicon oil weight percentage at 30% while varying those of graphite (from 50 to 65%) and biomass (5 to 20%), as shown by Table SI 1 (E-supplemental data).The detection was performed in 0.5 M HCl upon 3 min preconcentration.The peak current of Cd 2+ , Pb 2+ and Hg 2+ ions initially increased with the amount of biomass from 0 to 10 mg, due to an increase of binding sites on the electrode which facilitates the accumulation of Cd 2+ , Pb 2+ and Hg 2+ ions at the surface of this electrode.Then, a continuous increase in the amount of the modifier causes a decrease of the peak current, probably as a consequence of the reduction of the conductivity of the electrode (see Figure SI 6, E-supplemental data).A CPE containing 10 mg of HVW-Na 2 CO 3 was selected in further experiments.

Influence of accumulation time
The duration of the open-circuit accumulation expected to influence the performance of the sensor was optimised in the range from 1 to 6 min.As shown in Figure 5(a), the peak current increases almost linearly with accumulation time, which indicates a gradual accumulation of Cd 2+ , Pb 2+ and Hg 2+ ions at the binding sites at the electrode/solution interface.For accumulation time above 3 min, a small discrepancy in the electrode signal was observed, which tended to remain constant between 4 and 6 min.The optimal accumulation time used for other experiments was set to 5 min.

Influence of electrolysis potential and time
The effect of deposition potential on the voltammetric response of Cd 2+ , Pb 2+ and Hg 2+ was studied to determine the potential at which their reduction is maximal.The evolution of the peak intensities recorded on HVW-Na 2 CO 3 /CPE after 3 min accumulation for potentials ranging from −1.4 to −0.5V is presented in Figure 5(b).The deposition of cadmium and mercury was observed to be optimal for potentials between −1.4 and −1.0 V.For lead, the peak current increased when the electrolysis potential was varied from −1.4 V to −1.0 V, then decreased from -1.0 V to −0.5 V.For all studied metals, a maximum current was obtained for −1.0 V taken as the optimum electrolysis potential.Also, as illustrated in Figure 5(c), the electrode signal gradually increased with an increase in the electrolysis duration, reaching its optimal values from 60 s.

Influence of the pH of the accumulation medium
The last parameter expected to affect the peak current during the detection of heavy metals was the acidity of the accumulation medium.From relatively high to moderate acidic media (2 ≤ pH ≤ 5), the peak current increased, and reached a maximum value at pH 6 and started to decrease Figure 5(d).The weak electrode response in the acidic accumulation media could be explained by the protonation of the main functional groups of the biomaterial (e.g.-NH 2 , -OH and COOH) which reduce their complexing ability due to unfavourable electrostatic repulsion [20,49].The decrease of the electrode response from pH 7 to pH 9 could be attributed to the complexation of cationic species by OH − as previously observed in previous studies [20,50,51].The optimisation of the previous parameters for the use of HVW-Na 2 CO 3 /CPE sensor for the individual determination of Cd 2+ , Pb 2+ and Hg 2+ ions was also performed, and no significant difference was observed for optimal values achieved in this section.Table 3 summarises the optimised conditions for the detection of Cd 2+ , Pb 2+ and Hg 2+ .

Calibration graphs for individual and simultaneous detection of heavy metals
By using the optimum conditions in Table 3, the concentrations of Cd 2+ , Pb 2+ and Hg 2+ ions were individually varied, and as illustrated in Figure 6(a-c), the peak intensity increases linearly with the concentration of studied ions in the range from 10 to 85nM for Cd 2+ , 0.2 to 14 nM for Pb 2+ and 1 to 11.5 nM for Hg 2+ as shown by the insets in  respectively.The detection limits calculated as 3s/m (where s is the blank standard deviation) and m is the slope of the calibration curve, were found to be 1.82 × 10 −9 M, 6.96 × 10 −11 M and 2.37 × 10 −10 M for Cd 2+ , Pb 2+ and Hg 2+ respectively.The potential of using HVW-Na 2 CO 3 /CPE sensor for the simultaneous detection of the heavy metals was also demonstrated in this work.For this to be achieved, the preconcentration step was performed in a solution containing the three ions with concentrations set from the individual detection investigations.As a matter of fact, the peak current also increased linearly with the concentration of analytes (Figure 6(d)) in the range from 200 to 1200 nM, 2.5 to 20.6 nM and 70 to 101 nM for Cd 2+ , Pb 2+ and Hg 2+ respectively.These variations followed the equations I P (µA) = 0.004607 [Cd 2+ ] (nM) -0.28162,I P (µA) = 0.174625 [Pb 2+ ] (nM) + 0.59361 and I P (µA) = 0.097049 [Hg 2+ ] (nM) -6.27716 with correlation coefficients of 0.998.The detection limits were 2.83 × 10 −8 M, 4.3 × 10 −10 M and 3.3 × 10 −9 M for Cd 2+ , Pb 2+ and Hg 2+ respectively, slightly different from values obtained in the individual detection.It was observed that the elaborated sensor presented a better affinity with respect to lead, then mercury and finally cadmium.By comparing individual to simultaneous analysis, it was noticed a decrease in the HVW-Na 2 CO 3 /CPE sensor sensitivity, of 6.43%, 16.19% and 7.18% for Cd 2+ , Pb 2+ and Hg 2+ respectively from individual to simultaneous calibrations.This observation is probably due to the mutual interactions between the ions when they are present at the same time in the same solution.However, the proposed method remains useful for the determination of these metals in the same matrix.Yet, the results achieved with Na 2 CO 3 /CPE sensor in terms of performance are comparable to data reported for other modified electrodes [52][53][54] or even better [20,55].

Stability and analytical application to real samples
Prior to analytical application, the long-term stability and reproducibility of the HVW-Na 2 CO 3 /CPE sensor were evaluated.Four electrodes prepared, and their signals towards the detection of studied metal ions (in a solution containing 2 × 10 −8 , 5 × 10 −7 and 8 × 10 −8 M of Pb 2+ , Hg 2+ and Cd 2+ respectively) were regularly recorded within days and for a total period of four weeks.The results showed a good reproducibility since a relative standard deviation less than 5% (2.89%, 3.34% and 3.85% for Pb 2+ , Hg 2+ and Cd 2+ respectively) was obtained within electrodes.Additionally, upon 4 weeks, the peak currents were observed to be quite stable since only a decrease of 3.49%, 4.24% and 4.95% in the signals of Pb 2+ , Hg 2+ and Cd 2+ respectively was observed.Finally, the developed analytical method was applied to river and tap water.No sample pre-treatment was carried out.Each sample was spiked with a known amount of Cd 2+ , Pb 2+ and Hg 2+ .The results obtained are summarised in Table 5.The activated biomaterial modified electrode produced satisfactory recovery rates, with mean values of 96.8% for Pb 2+ , 97.2% for Cd 2+ and 96.4% for Hg 2+ in river water.
In tap water, relative standard deviations less than 5% were also obtained.These findings indicated that the proposed HVW-Na 2 CO 3 /CPE sensor is suitable for the determination of Cd 2+ , Pb 2+ and Hg 2+ ions in real samples.

Conclusion
This study presents a new electrochemical sensor for individual and simultaneous trace determination of Cd 2+, Pb 2+ and Hg 2+ ions.The new sensor is based on a carbon paste electrode modified with activated Hordeum vulgare L. as sensing bulk electrode material.
The proposed sensor was shown to be a promising analytical tool for the detection of heavy metal ions by differential pulse voltammetry.It exhibited a high sensitivity and great selectivity and was successfully applied to real samples.The biomaterial was highly efficient and could be used as cheap and non-toxic electrode modifier, an alternative to mercury and other expensive material.

Figure 3 .
Figure 3. TG curves of (a) HVM and (b) HVW-Na 2 CO 3 materials recorded under nitrogen atmosphere from room temperature to 900°C.Traces (a') and (b') are the DTG curves of HVW and HVW-Na 2 CO 3, respectively.

Figure 6 (
Figure6(a).These variations followed the equations I P (µA) = 6.6078 [Cd 2+ ] (nM) -0.10398,I P (µA) = 0.44151 [Pb 2+ ] (nM) -0.01285 and I P (µA) = 0.46161 [Hg 2+ ] (nM) -0.0093 all together with a correlation coefficient of 0.999 for Cd 2+ , Pb2+ and Hg 2+ respectively.The detection limits calculated as 3s/m (where s is the blank standard deviation) and m is the slope of the calibration curve, were found to be 1.82 × 10 −9 M, 6.96 × 10 −11 M and 2.37 × 10 −10 M for Cd 2+ , Pb 2+ and Hg 2+ respectively.The potential of using HVW-Na 2 CO 3 /CPE sensor for the simultaneous detection of the heavy metals was also demonstrated in this work.For this to be achieved, the preconcentration step was performed in a solution containing the three ions with concentrations set from the individual detection investigations.As a matter of fact, the peak current also increased linearly with the concentration of analytes (Figure6(d)) in the range from 200 to 1200 nM, 2.5 to 20.6 nM and 70 to 101 nM for Cd 2+ , Pb 2+ and Hg 2+ respectively.These variations followed the equations I P (µA) = 0.004607 [Cd 2+ ] (nM) -0.28162,I P (µA) = 0.174625 [Pb 2+ ] (nM) + 0.59361 and I P (µA) = 0.097049 [Hg 2+ ] (nM) -6.27716 with correlation coefficients of 0.998.The detection limits were 2.83 × 10 −8 M, 4.3 × 10 −10 M and 3.3 × 10 −9 M for Cd 2+ , Pb 2+ and Hg 2+ respectively, slightly different from values obtained in the individual detection.It was observed that the elaborated sensor presented a better affinity with respect to lead, then mercury and finally cadmium.By comparing individual to simultaneous analysis, it was noticed a decrease in the HVW-Na 2 CO 3 /CPE sensor sensitivity, of 6.43%, 16.19% and 7.18% for Cd 2+ , Pb 2+ and Hg 2+ respectively from individual to simultaneous calibrations.This observation is probably due to the mutual interactions between the ions when they are present at the same time in the same solution.However, the proposed method remains useful for the determination of these metals in the same matrix.Yet, the results achieved with Na 2 CO 3 /CPE sensor in terms of performance are comparable to data reported for other modified electrodes[52][53][54] or even better[20,55].

Table 1 .
Chemical composition of HVW before and after activation using Na 2 CO 3.
Figure 2. FTIR spectra of (a) HVW and (b) HVW-Na 2 CO 3 materials.at1375 and 1252 cm −1 are attributed to absorption by C-H and C-O in acetyl group in hemicelluloses and aryl group in lignin, respectively.

Table 5 .
Results obtained for the quantification of Cd 2+, Pb 2+ and Hg 2+ ions in two real samples and recovery rates.