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Application of Square Wave Adsorptive Stripping Voltammetry in the Temporal Variation of Trace Co(II), Ni(II), Cr(VI), and Mo(VI) Ions in Drinking Water Samples at Al-Baha City, Saudi Arabia
*Corresponding author: Dr. Ali Y. Alzahrani Department of Chemistry, Al-Baha University, King Fahad street, Al-Aqiq, 65779, Al-Baha, Saudi Arabia. alzahrani.ay@bu.edu.sa Tel: 0552777816
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Received: ,
Accepted: ,
How to cite this article: Alzahrani AY. Application of Square Wave Adsorptive Stripping Voltammetry in the Temporal Variation of Trace Co(II), Ni(II), Cr(VI), and Mo(VI) Ions in Drinking Water Samples at Al-Baha City, Saudi Arabia. J Qassim Univ Sci. 2026;1:75-84. doi: 10.25259/JQUS_27_2025
Abstract
Objectives
Trace Co(II), Ni(II), Cr(VI) and Mo(VI) ions in drinking water samples taken from eight sampling regions at Al–Baha city, Saudi Arabia, were assessed.
Material and Methods
square wave adsorptive stripping voltammetry (SWAdSV) with a domestic carbon paste electrode and graphite furnce atomic absorption spectrophotometric(GFAAS) techniques, were used. Samples of drinking water were taken from eight sampling regions at Al–Baha city, Saudi Arabia. The influence of various parametres were studied by means of the SWAdSV technique.
Results
The results obtained showed that the detection limits of the examined cations at 30 s deposition time (n = 5) are: 0.15 ng L-1 for Co(II), 0.05 ng L-1 for Ni (II), 0.2 ng L-1 for Cr(VI) and 3.2 ng L-1 for Mo(VI), respectively. The assesment of the examined cations in drinking water ranging from : for Co(II) (0.42–68.5 µg L-1, n = 5), Ni(II) (0.29–0.87 µg L-1, n = 5), Cr(VI) (1.5–22.35 µg L-1, n = 5) and for Mo(VI) (0.58–2.58 µg L-1, n = 5), respectively. The resulting data for studied cations are compared with those assessed by the graphite furnce atomic absorption spectrophotometric technique(GFAAS). Interfering between cations under analysis have been examined. Moreover, the storage of the studied samples was verified, and the results were agreement with the other processes. The accuracy of the proposed technique was attained via statistical analysis for the gained data.
Conclusion
The study concluded that most of the drinking water samples used were a very good agreement with the other processes.
Keywords
Drinking water samples at Al-Baha city
Graphite furnace atomic absorption spectroscopy
Real environmental analysis
Square wave adsorptive stripping voltammetry
INTRODUCTION
Trace cations, especially heavy metal ions, are a major source of the chief bases of contamination in the environment, as they have a considerable influence on its environmental quality. Human activities often mobilize and rearrange natural materials in the environment, thus greatly affecting them and causing adverse effects.[1] Heavy metallic ions are common poisons of pronounced preservation concern since they are non-degradable, toxic, and permanent.[2] Heavy metal ions and trace metals exist as biologically poisonous substances and can impact human existence due to their presence in the nutrition series. It is well known that humans are in contact with approximately 35 elements either by occupational or domestic contact. However, 23 of these are heavy metal ions. Specific heavy metal ions are required for good health, and considerable quantities of some of them could cause critical or chronic harm. Heavy metal ions may enter the human body through food, water, and the atmosphere. Heavy metal ions turn poisonous once they are not consumed in the body and stored in the soft tissues.[3] The aquatic chemistry for heavy metal ions is clearly identical, a challenging study area with an extensive spectrum of features to be examined for specific marine systems. From an operational perspective, such examinations continually require the procedures and employed methodologies of progressive and often motivated trace chemistry. Metal ions are important for human existence at small concentrations, but they are considered contaminants or even carcinogenic at high concentrations. Assessment of human health risk comprises qualitative and quantitative evaluation of the human’s exposure to contamination originating from the environment and food, particularly in drinking water. The quality of water consumed has a strong impact on our health and well-being. Human life strongly depends on the quality of drinking water intake required to prevent any risks to human health. The consumption of water containing a certain number of certain metals may lead to health problems such as cancer in humans.[4,5] Some metals and metalloids, such as zinc, iron, selenium, cobalt, copper, chromium, vanadium, or molybdenum, are essential elements for growth and reproduction. However, their accumulation in excess in the human body is undesirable.
Several metal ions are present in the environment in various forms, which are distinguished not only by their physicochemical properties but also by their varying toxicity to active organisms.[6] A review of water quality evaluation for energy, drinking, and irrigation determinations in the Al Jouf area in Saudi Arabia was presented.[7]
On the other hand, nickel (Ni), cobalt (Co), chromium (Cr), and molybdenum (Mo) are trace elements with varying degrees of biological importance and potential toxicity in drinking water, which is heavily dependent on the concentration and the specific chemical form (oxidation state). Nickel’s functional role as a trace element for humans has not been definitively established. However, a few studies suggest it may be a micronutrient involved in lipid metabolism and enhancing hormonal activity. Natural deficiency is rare due to its abundance in the environment and food sources. Nickel is a common sensitizer, and exposure (especially dermal contact) can lead to allergic contact dermatitis. Ingestion of high doses of soluble nickel salts can cause acute symptoms such as nausea, vomiting, diarrhea, and abdominal pain. Chronic exposure, particularly through inhalation in occupational settings, has been linked to respiratory issues (rhinitis, sinusitis, and asthma) and an increased risk of lung and nasal cancers. The World Health Organization (WHO) has a guideline value of 70 µg L-1 for nickel in drinking water, set to protect sensitized individuals. Cobalt is an essential trace element and a vital component of vitamin B12 (cobalamin), a coenzyme necessary for DNA synthesis, amino acid metabolism, and the formation of the myelin sheath in nerve cells. While essential in trace amounts, high concentrations of inorganic cobalt ions are toxic. Acute ingestion can cause gastrointestinal distress. The allowed limit of Cobalt concentration in drinking water is typically less than 1-2 mg L-1. Trivalent chromium (Cr(III)) is considered a non-essential element. However, it is often included in nutritional supplements and found in food, with some evidence suggesting a role in carbohydrate, lipid, and protein metabolism (potentiating insulin action). The toxicity of chromium is highly dependent on its oxidation state. Cr(III) is considered less toxic and poorly absorbed through the gut. However, Cr(VI) (hexavalent chromium) is highly toxic and a known Group 1 carcinogen via inhalation exposure. In drinking water, ingested Cr(VI) can be partly reduced to Cr(III) in the acidic stomach environment, which acts as a detoxification process. However, Cr(VI) can escape reduction and cause DNA damage and tumors in the gastrointestinal tract (specifically, small intestinal and oral cavity tumors in animal studies). The U.S. Environmental Protection Agency (EPA) sets a maximum contaminant level for total chromium at 100 µg L-1 in drinking water. Mo is an essential trace element for all life forms, including humans. Mo has low toxicity in humans compared to other heavy metals. The body efficiently excretes excess Mo in the urine. High exposure (10-15 mg day-1) has been associated with elevated uric acid levels and gout-like symptoms in some populations. Extremely high doses can lead to copper deficiency because Mo interferes with copper metabolism.[8]
Graphite furnace atomic absorption spectrometry (GFAAS) is a quantitative technique of metal analysis that is appropriate for the estimation of seventy metals. AAS measures the concentration of the elements via transient light (wavelength released) on a radioactivity basis of a particular metal ion, such as mercury, arsenic, and lead.[9] Numerous methods have been useful for assessing heavy metal ions in different water samples, like electrothermal atomic absorption spectrometry (ET-AAS),[10,11] flame atomic absorption spectrometry (FAAS),[12,13] and cold vapor-atomic absorption spectrometry (CV-AAS).[14] The detection of metal ions in the ecological trials via capillary electrophoresis is an essential area for ecological chemists. Several metal ions are studied through capillary electrophoresis under different conditions.[15-17] Separation of an 8-constituent combination of transitional metal ions has been premeditated on pure and permeated silica gel films.[17] Direct and simultaneous voltammetric analysis of heavy metals in tap water samples at Assiut city: an approach to improve the analysis time for nickel and cobalt determination at mercury film electrode was reported.[18] Improved Amberlite XAD-4 resin was utilized for the determination of selected heavy metals by selective solid-phase extraction.[19] Most of these methods are sophisticated. Therefore, simple, accurate, precise, and selective methods such as electrochemical techniques were valuable for the simultaneous detection of metal ions in drinking water samples.
Electrochemical techniques can be developed as compact and efficient methods for trace determination of heavy metal ions in solution. Among these, adsorptive stripping voltammetry (AdSV) is one of the most used approaches for environmental samples. This method ensures high accuracy for various metal ions, enables cost-effective multi-cation analysis, and is suitable for automation.
On the other hand, over the last thirty years, electrochemical methods have been widely applied to the analysis of trace metal ions in various types of water.[2,20-23] In addition, carbon paste electrodes (CPE) have been effectively used to determine both cationic and anionic metal species, as well as organic compounds.[24-29]
The first aim of this research is to simultaneously deaminate Co(II), Ni(II), Cr(VI), and Mo(VI) metal ions in tap water samples collected from Al-Baha city, Saudi Arabia, (between March and May, 2025), by Square wave adsorptive stripping voltammetry (SWAdSV). The interference among these metal ions has been investigated. The other aim of the study was to evaluate the stability and storage capability of the samples at pH = 2 and -4 °C. The study extended the application of SWAdSV to the detection of ultrarace metal ions in 8 of the drinking water samples located in Al-Baha city, Saudi Arabia (between March and May 2025). The results obtained for all metal ions were compared with those determined by the GFAAS method to validate the accuracy and reliability of the proposed technique.
MATERIALS & METHODS
Instruments
All voltametric experiments were performed by the EG&G PAR Modeling 273 A potentiostat, produced by Princeton Applied Research, NJ, USA. The potentiostat is connected to a 3-electrode system, which was made up of an unpolarized reference electrode (Ag Ag-1 Cl-1 electrode), an auxiliary electrode (Pt wire), and the working electrode, which is a carbon past electrode (CPE) prepared as previously mentioned.[20,21,30] A magnetic stirrer (KIKA Labortechinik, Germany), at approximately 400 rpm with a Teflon-coated bar, was utilized to stir the solution. A Teflon cell was used to prevent the contamination of H2O at a pH ≈ 2. GFAAS measurements were carried out via an instrument (Buck Scientific Model 220- GF, USA). All pH assessments were performed with an Orion model 601 digital pH meter.
Reagents and solutions
Standard solutions of 1.0 M HNO3 were prepared by diluting a certain volume of concentrated HNO3(BDH) with bi-distilled water. Once prepared, the solution was neutralized with the stock solution of NaOH(BDH), free sodium carbonate.[31] Furthermore, HCLO4 and H3PO4 acids, sodium monohyderogen phosphate, Na2HPO4, NH4Cl, NH4OH, NaNO3, and borax (Merck) are used. The supported electrolyte compulsory for the assessed Ni(II) & Co(II) metal ions was 2×10-2 M ammonium buffer (HCl +NH3) with 0.2×10-4 M dimethylglyoxime (DMG) (Analar Grade) is used.[32] A mixture of 0.5×10-2 M phosphate buffer (Na2HPO4 and NaH2PO4) and 0.01M NaNO3 was prepared for the measurement of Mo(VI).[33] Though a 0.01 M sodium nitrate and 0.001 M mixture of mono and dihydrogen phosphate buffer, respectively, was prepared for the particular assessment for Cr(VI) as suggested by Ghandour et al.[34] Bi-distilled water was used for the preparation of the standard solution. Stock solution (0.003 M) from the individual cation under investigation was prepared using their Analar grade salt.
Procedures
The peak current obtained from square wave voltammetry (SWV) is dependent on several apparatus parameters, including square wave frequency, square wave amplitude, and scan increments. These parameters were interrelated, and they influenced the response; however, only the common trend will be detected. It was established that this parameter had a slight influence on the peak potential. The data of this optimization study displays that the set of square wave amplitudes of 25 mV, frequency of 60 Hz, and scan increment of 2 mV is best suited for routine analysis of these metals. Since the scan increment, together with the frequency, defines the effective scan rate. An increase of either the frequency or the scan increment results in an increase in the effective scan rate. By maintaining both frequency and scan increment at 60 Hz and 2 mV, respectively, the operative scan rate is 120 mVs-1
For Ni(II) and Co(II) determination by (SWAdSV), a mixture of 2×10-2 M ammonium buffer (HCl + NH3), pH ∼ 9, was used as a supported electrolyte, and 0.2×10-4 M dimethylglyoxime (DMG) was used as a complex reagent. The deposition potential was -0.85 and -0.7 V for Co(II) and Ni(II) at 10 and 30 s. preconcentration period, respectively.
To determine Cr(VI) in the sample under attention, 0.01M NaNO3 and 0.01 M of a mixed NaH₂PO₄ (Na₂HPO₄)-1 buffer were used. A sample of 5 mL was added, and the volume was diluted to 10 mL in the voltammetry cell. The accumulation potential was set at +0.1 V for 30 s, and the potential was terminated at -0.5 V.
For the determination of Mo(VI) of the sample, the previously described method was repeated with 5×10-3 M phosphate buffer and 0.01 M sodium nitrate. The accumulation potential was set at +0.1 V, and the potential was ended at -0.5 V. Instantly, the voltammogram was verified (preconcentration with the equilibrium time 15 s was enough to acquire an improved peak current). The same procedure was repeated after adding 5 mL of the drinking water sample. All measurements were carried out at 25°C.
Real environmental water random sample
Drinking water samples were collected from 8 regions (Bany Sar, Bany Farwa, Al Hawey, Al Zarga, Al Aqiq, Al Dapheer, Al Soug region, and Al Swad region), which were distributed at Al-Baha city, Saudi Arabia (between March and May, 2025) in polyethylene flasks with a volume of 1.0-1.5 L. Prior to sampling, the flasks were rinsed several times with tap water and then filled with water, which was allowed to stand for approximately 30 min before being emptied and refilled with fresh samples. To avoid the contact of air with the sample, the flasks were opened and closed under running tap water. Water samples were collected after allowing the tap to run for 15 min to ensure a representative sample from the distribution system.
Water sample preparations
Suspended materials were removed from the water sample by filtration[35] utilizing Whatman number 1 filter paper. After that, the filtrate solution is acidified to pH = 2 by adding high-purity HNO3.[36] Acidification prevents the adsorption of solute ions onto the walls of the container[37] and minimizes potential interactions that could lead to the loss or transformation of metal ions, thereby ensuring their availability for analysis. The acidified sample can be kept at -4°C. For analysis by the GFAAS method, the water samples were concentrated by gentle evaporation, transferred to polyethylene flasks, and stored in a refrigerator at -4°C until needed.[20]
RESULTS AND DISCUSSION
Ni(II) and Co(II) were determined using SWAdCSV. This technique includes complexation of both metal ions with DMG, followed by adsorption of the resulting complexes onto the electrode surface. The preconcentration step includes the interfacial deposition of an adsorbed layer of Ni and Co bis(dimethylglyoximate) complexes on the electrode surface. The adsorbed metal ion complexes are then electrochemically reduced during the potential scan in the cathodic direction. The optimum operating parameters, such as supporting electrolyte, pH, deposition period, and deposition potential, have been summarized in Table 1.
| Metal ions | Supporting electrolyte (molar) | pH | Accumulation potential (V) | Final potential (V) | Preconcentration time (s) |
|---|---|---|---|---|---|
| Co(II) | 1×10-2 M (NH4OH - NH4Cl + 0.2×10-3 M DMG) | 9.2 | -0.85 | -1.3 | 10 |
| Ni(II) | 2×10-2 M (NH4OH - NH4Cl + 0.2×10-3 M DMG) | 9.0 | -0.7 | -1.2 | 30 |
| Mo(VI) | 5×10-3 M (NaH2PO4 - Na2HPO4 + 1×10-2 M NaNO3) | 2.3 | +0.1 | -0.5 | 10 |
| Cr(VI) | 1×10-3 molar (NaH2PO4 -Na2HPO4 + 1×10-2 M NaNO3) | 7.2 | +0.1 | -0.5 | 30 |
AdSWV: Adsorption square wave voltammetry, DMG: Dimethyl glayoxime
In the case of Cr(VI) assessment, the peak height reached its maximum in the presence of 0.01 M of phosphate buffer NaH2PO4 + Na2HPO4 and 0.01M sodium nitrate at pH ≈ 7.2. A sharp cathodic peak was observed at -0.15 V, which corresponds to the reduction of Cr(VI) to Cr(III). The resulting Cr(III) phosphate hydrolyzed species are adsorbed onto the electrode surface, forming a thin layer that contributes to the observed voltametric signal.[35]
The adsorptive accumulation of Mo(VI) on a carbon paste electrode in the presence of 0.005 M phosphate buffer (NaH2PO4 (Na2HPO4)-1) and 0.01 M sodium nitrate (pH ≈ 2.3) was effectively used for the determination of trace levels of Mo (VI).[34] This behavior is attributed to the formation of a twelve-molybdophosphate complex, which exhibits stronger adsorption onto the carbon paste electrode surface than molybdic acid.[38] A cathodic peak observed at -0.1 V corresponds to the electrochemical reduction of Mo(VI) to Mo(V).[39,40]
The effect of deposition potential and preconcentration period on the peak current was studied, and the optimum conditions have been established for the measurement of Mo(VI), as summarized in Table 1. Figure 1 shows SWAdCS voltammograms obtained for Ni, Co, Cr, and Mo in drinking water samples.
- SWAdS voltammogram of Cr, Ni, MO, and Co in a drinking water sample at 30 s accumulation time. (a) Drinking water sample for Mo, Cr, Ni, and Co cations. (b) 5, 2, 0.6, and 20 µg L-1 for Mo, Cr, Ni, and Co cations. (c) 15, 4, 2, and 40 µg L-1 for Mo, Cr, Ni, and Co cations. (d) 24, 9.5, 4, and 70 µg L-1 for Mo, Cr, Ni, and Co cations.
The use of computer-controlled instrumentation for flexible parameter selection made it possible to eliminate residual currents effectively. The process of residual current adjustment is straightforward, and background correction often results in a significant improvement in analytical accuracy. Accordingly, in the determination of Co(II) and Ni(II) in drinking water samples, background correction helps to remove the nickel signal associated with trace nickel contamination in the supporting electrolyte, thereby improving the accuracy of cobalt quantification.[41] A major advantage of SWV is the elimination of all current components related to slow electrode processes. In voltammetry trace analysis, oxygen reduction often yields unsatisfactory and unbalanced background current. Though with SWV, the current oxygen reduction is almost completely suppressed.[42]
The limits of detection (LOD) and quantification (LOQ) were defined as the concentrations of the analyte (in ng L-1) producing current responses statistically distinct from those of the blank. These values were computed by dividing three, ten times the standard deviation for the current reading of the blank by the slope for the methodical curve, respectively. Taking into consideration the dilution and size of the sample, the detection and quantification limit in ng L-1 of the sample has been computed.[43] The values obtained have been presented in Table 2. The accuracy of this technique has been assessed from the standard deviation and correlation coefficient for 5 repeated analyses of the sample solution, and it provides values larger than 0.02 and 0.996 for all cations, respectively.
| Element | Lower limit of detection (ng L-1) | Lower limit of quantitation (ng L-1) | Intercept ± SD | Slope ± SD | Correlation coefficient (R) |
|---|---|---|---|---|---|
| Ni | 0.05 | 0.18 | 15.9 ± 0.03 | 1.1 ± 0.05 | 0.9992 |
| Co | 0.15 | 0.55 | 21 ± 0.02 | 3.8 ± 0.04 | 0.9889 |
| Cr | 0.2 | 0.68 | 22 ± 1.2 | 14 ± 0.07 | 0.9898 |
| Mo | 3.2 | 10.11 | 20.5 ± 1.7 | 1.5 ± 0.5 | 0.9988 |
The tolerance levels for various foreign ions were evaluated using a concentration of 0.5×106 M of each target cation individually. It was established that two-hundred-fold molar ratio of Mg(II), Fe(III), Mn(II), and Al(III), hundred-fold ratio of W(V), Bi(II), Ta(V) & Fe(II), and fifty-fold ratio of UO2(II) and Ag(I), can be tolerated. In contrast, surfactants, such as Triton X-100 (0.2 mg L-1) and tetramethyl ammonium bromide (0.1×10-3 M), were found to reduce the peak currents of the individual metal ions slightly.
The standard addition technique determined the concentrations of trace cations in all examined water samples. This approach was selected because it minimizes matrix effects and provides greater analytical reliability. By plotting the added metal ion concentration against the corresponding peak current, the concentration of each metal ion in the sample was obtained, as illustrated in Figure 2.
- Representative current-concentration plots of metal ions in drinking water, under their optimal conditions. (a) Ni(II), (b) Co(II), (c) Mo(VI), (d) Cr(VI).
However, the results of the measured Ni(II), Co(II), Cr(VI), and Mo(VI) in drinking water samples have been shown in Table 3.
| Metal ions | Deposition times (s) | Linearity ranges (µg L-1) | Detection limit (µg L-1) at (30 s) | Corr. Coeff. |
|---|---|---|---|---|
| Ni(II) | 10 | 1.2-70.00 | 0.05 | 0.9889 |
| 30 | 0.42-68.50 | 0.9992 | ||
| Co(II) | 10 | 1.1-11.0 | 0.15 | 0.9898 |
| 30 | 0.29-0.87 | 0.9889 | ||
| Cr(VI) | 10 | 10.2-20 | 0.2 | 0.9898 |
| 30 | 1.5-22.35 | 0.9898 | ||
| Mo(VI) | 10 | 6.25-25.5 | 3.2 | 0.9895 |
| 30 | 0.58-2.58 | 0.9988 |
SWAdSV: Square wave adsorptive stripping voltammetry, CPE: Carbon paste electrodes
Table 4 presents the results of metal ion determinations in drinking water samples. Since no certified reference material was available, the GFAAS technique was employed to confirm the stripping voltammetry results for the four studied metal ions (Cr, Ni, Mo, and Co). The precision of the method was further evaluated through recovery tests conducted at μg L-1 concentration levels (n = 5). The obtained standard deviations, correlation coefficients, and confidence limits for all investigated metal ions showed satisfactory analytical precision. However, Additional data have been summarized in Table 5. Table 4 also compares the results obtained for the eight analyzed cations in drinking water samples using both SWADSV and GFAAS methods. For these techniques, the standard deviation ranged between 0.02% and 3.23% for all metal ions analyzed.
| Metals | Period | Concentration* (µg L-1) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Bany Sar | Bany Farwa | Al Hawey | Al Zarga | Al Aqiq | Al Dapheer | Al Soug region | Al Swad region | ||
| Ni(II) |
March April May |
14.11 (14.00) 14.25 (13.50) 14.12 (14.00) |
10.12 (10.15) 10.33 (10.14) 10.65 (10.16) |
10.32 (10.79) 10.12 (10.82) 10.41 (10.86) |
9.12 (9.72) 9.13 (9.70) 9.11 (9.78) |
0.44 (0.44) 0.42(0.43) 0.42(0.41) |
10.35 (10.30) 10.41 (10.20) 10.55 (10.29) |
68.50 (67.00) 67.30 (68.00) 68.15 (67.00) |
10.51 (10.85) 10.51 (10.70) 10.32 (10.72) |
| Co(II) |
March April May |
0.51 (0.50) 0.55 (0.50) 0.53 (0.53) |
0.32 (0.33) 0.33 (0.34) 0.31 (0.34) |
0.43 (0.40) 0.45 (0.42) 0.46 (0.45) |
0.63 (0.64) 0.63 (0.62) 0.62 (0.63) |
0.75 (0.74) 0.74 (0.80) 0.77 (0.80) |
0.81 (0.88) 0.83 (0.82) 0.79 (0.87) |
0.31 (0.35) 0.29 (0.32) 0.32 (0.32) |
0.52 (0.55) 0.41 (0.53) 0.50 (0.52) |
| Cr(VI) |
March April May |
2.11 (2.51) 2.16 (2.55) 2.23 (2.53) |
10.05 (9.90) 10.10 (9.70) 10.10 (9.80) |
22.10 (22.34) 21.20 (22.34) 22.00 (22.35) |
2.92 (2.58) 2.91 (2.58) 3.03 (2.50) |
15.10 (14.70) 14.9 (14.56) 14.1 (14.2) |
2.91 (3.00) 2.85 (3.23) 2.78 (3.30) |
3.51 (1.56) 3.49 (1.55) 3.39 (1.50) |
20.00 (18.60) 21.10 (18.90) 20.00 (19.00) |
| Mo (VI) |
March April May |
1.53 (1.56) 1.52 (1.58) 1.55 (1.54) |
2.42 (2.35) 2.41 (2.35) 2.35 (2.36) |
1.51 (1.50) 1.52 (1.51) 1.52 (1.52) |
1.45 (1.48) 1.43 (1.47) 1.47 (1.49) |
2.50 (2.50) 2.52 (2.50) 2.53 (2.58) |
0.58 (0.58) 0.63 (0.62) 0.58 (0.57) |
1.46 (1.50) 1.45 (1.46) 1.50 (1.52) |
1.52 (1.53) 1.42 (1.42) 1.55 (1.56) |
*Average of 5 replicates. Data between brackets is from GFAAS technique. SWAdSV: Square wave adsorptive stripping voltammetry, GFAAS: Graphite furnace atomic absorption spectrophotometric technique, CPE: Carbon paste electrodes
| Metals |
Type of Statistics (n = 5) |
Site of Samples | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Al Aqiq | Al Hawey | Al Zarga | Bany Sar | Al Dapheer | Al Soug region | Al Swad region | Bany Farwa | ||
| Ni(II) |
SD Corr. Coef Confide.* |
0.52-0.88 0.9989 0.69-1.09 |
0.24-0.65 -0.8871 0.375-0.824 |
0.250.63 0.3563 0.387-0.816 |
0.13-0.62 0.9894 0.115-0.539 |
0.22-0.83 0.8980 0.25-1.08 |
0.41-0.91 0.6654 0.47-1.05 |
0.33-0.96 -0.120.22-1.0 1 |
0.33-0.81 0.121 0.45-1.05 |
| Co(II) |
SD Corr. Coef. Confide.* |
0.03-0.14 -0.8780 0.032-0.146 |
0.03-0.07 0.9919 0.022-0.07 |
0.03-0.06 -0.887 0.025-0.055 |
0.25-0.35 0.8790 0.15-0.279 |
0.33-0.95 0.123 0.35-1.18 |
0.28-1.02 -0.120 0.28-1.16 |
0.33-0.99 0.8750 0.25-1.09 |
0.15-0.31 -0.8880 0.171-0.342 |
| Cr(VI) |
SD Corr. Coef. Confide.* |
0.09-0.42 -0.9989 0.038-0.121 |
0.15-0.72 -0.7980 0.16-0.69 |
0.14-0.73 -0.796 0.137-0.697 |
0.13-0.42 0.7860 0.13-0.38 |
0.08-0.13 0.750 0.078-0.16 |
0.132-0.352 0.866 0.15-0.38 |
0.13-0.41 0.1230 0.16-0.345 |
0.15-0.28 0.510 0.135-0.278 |
| Mo(VI) |
SD Corr. Coef. Confide.* |
0.035-0.085 0.798 0.32-0.122 |
0.03-0.08 0.786 0.019-0.08 |
0.03-0.075 0.758 0.025-0.084 |
0.03-0.06 0.4327 0.03-0.07 |
0.025-0.05 0.0125 0.03-0.075 |
0.04-0.11 -0.5409 0.05-0.15 |
0.02-0.06 0.124 0.021-0.058 |
0.021-0.039 -0.5865 0.13-0.35 |
*Confidence interval for standard deviation. SWAdSV: Square wave adsorptive stripping voltammetry, GFAAS: Graphite furnace atomic absorption spectrophotometric technique
SWAdSV offers a significant advantage over conventional techniques, particularly in terms of analysis speed. Whereas conventional differential pulse procedures typically run at scan rates of 1-10 mV s-1, SWV allows scan rates up to 1 V s-1 or higher, thereby greatly reducing the total analysis time. The comparison of measurement times on SWV and GFAAS revealed that the time required for multi-assessment of four cations (i.e., Cr, Ni, Mo, and Co) by SW voltammetry is approximately equivalent to the time required for the assessment of two cations using GFAAS.
The influence of storage
Before the assessment of dissolved metal ions under optimal conditions, water samples were filtered and acidified (by adding a small quantity of concentrated HNO3, 2 mL L-1) to adjust the pH to 2 and then stored at -4°C. The increase in heavy metal ion concentration caused by the addition of such insignificant amounts of ultrapure acid is negligible. Filtration should be performed prior to acidification when the sample contains significant suspended matter, as otherwise metal ions may adsorb onto the solids and be lost from the solution during acidification. However, the acidic medium also helps prevent microbial contamination and reduces the adsorption of metal ions onto the container walls. As a result, acidified samples can be stored without significant changes in metal ion content for extended periods, especially if kept in darkness at -20°C.[30,35,37]
The stability of Co(II), Ni(II), Cr(VI), and Mo(VI) ions in drinking tap water samples from Al-Baha City, Saudi Arabia (between March and May 2025) was evaluated using the SWADSV technique, and changes in cation concentrations during storage were monitored. The results indicated:
-
1.
The concentration of Ni(II) and Co(II) ions has a slight effect, especially in the first month.
-
2.
Mo(VI) ions showed slight adsorption onto the container walls, resulting in a 2-5% decrease in concentration over three months.
-
3.
Cr(VI) ions exhibited more pronounced losses, with concentrations decreasing by 5.5-15.5% during the three-month storage period.
Real environmental sample analysis
Drinking water samples from the eight locations (Bany Sar, Bany Farwa, Al Hawey, Al Zarga, Al Aqiq, Al Dapheer, Al Soug region, and Al Swad region) located in Al-Baha city, Saudi Arabia, were collected on three different dates of the year (between March and May, 2025). The concentrations of four metal ions Ni(II), Co(II), Cr(VI), and Mo(VI) were determined using the SWADSV method in an HNO3 medium at pH ≈ 2, following deposition on a CPE at an initial potential of +0.1 V. Well-resolved voltametric peaks for all four metal ions were observed, allowing their simultaneous determination from a single sample.[44] The analysis was performed using the standard addition method of individual cations in the sample and extrapolation of the linear relationship between the peak current and the concentration of the added cations (illustrative curves have been shown in Figure 3)
- Plot of current against concentration in the study of Ni(II) ions in eight of the drinking water samples were collected from Al-Baha city, Saudi Arabia (between March and May 2025) at 0.01 M HNO3 & pH = 2.2: 1-Bany Sar, 2-Bany Farwa, 3-Al Hawey, 4-Al Zarga, 5-Al Aqiq, 6-Al Dapheer, 7-Al Soug region, and 8-Al Swad region).
Comparison between SWAdSV and GFAAS techniques
The results obtained using both methods (SWAdSV & GFAAS) are in good agreement in the majority of the tap water samples, demonstrating that both techniques are suitable for the determination of metal ions at µg L-1 levels. Minor differences observed between the two methods in some samples are primarily attributed to contamination in the GFAAS analyses. In these cases, the laboratory personnel did not strictly follow standard protocols recommended by the U.S. EPA or the American Public Health Association (APHA). According to these protocols, trace cations should be preconcentrated through chelation with ammonium pyrolidine dithiocarbamate (APDC) and extraction with methyl isobutyl ketone (MIBK) to ensure that the analyte concentrations fall within the linear range of the instrument. The analytical parameters of Ni(II), Co(II), Cr(VI), and Mo(VI) determined by GFAAS and SWAdSV in various tap water samples have been presented in Table 4. For all cases, the standard deviation (n = 5) was calculated at 95% confidence, confirming the reliability of the proposed methods [Table 5]. The negative or minor values of the correlation coefficient in some samples might be due to seasonal variations, biological activity, or minor contamination affecting the measured concentrations over time.
Results indicated that the concentration of the investigated cations in drinking water at Al-Baha city, Saudi Arabia, has been within the allowed limits established by American,[45] European,[46] and World[47] chemical standards. These findings confirm that the proposed method (SWAdSV) could be used for the rapid quantitative determination of the four metallic ions in drinking water samples.
The quality of drinking water is regulated at international levels.[44,46] Limit values for selected metals recommended by the WHO and the U.S. EPA have been given in Table 6
| Metal ion | WHO (ppm) | US EPA (ppm) | Average values of our work (ppm) |
|---|---|---|---|
| Ni | 0.07 | 0.1 | 0.008 |
| Co | -- | 0.07 | 0.001 |
| Cr | 0.05 | 0.1 | 0.009 |
| Mo | -- | 0.07 | 0.002 |
US EPA: United Stat of Environmental Protection Agency
CONCLUSION
The present study demonstrates that both SWADSV and GFAAS are accurate and rapid analytical techniques for the determination of trace metal ions. SWADSV, in particular, offers high sensitivity due to its fast scan rate and the adsorptive preconcentration of analytes onto the electrode surface. The measurement of Ni(II), Co(II), Cr(VI), and Mo(VI) in all of the drinking water samples ranged for ions as follows: Ni(II) (0.42–68.5 µg L-1), Co(II) (0.29–0.87 µg L-1), Cr(VI) (1.5–22.35 µg L-1), and Mo(VI) (0.58–2.58 µg L-1). The obtained data are within permissible limits established by local and international standards. The short analysis period in SWAdSV made this procedure extremely attractive for routine assessments of the designated cations in drinking water samples. The short analysis time of SWADSV makes it particularly suitable for routine monitoring of these cations in drinking water. Its key advantages include high sensitivity, selectivity, simplicity, and reduced analysis time compared to GFAAS. Notably, the time required for simultaneous determination of four cations by SWADSV is comparable to that needed for analyzing only two cations by GFAAS. Additionally, storage conditions were observed to influence metal ion concentrations, highlighting the importance of proper sample handling.
Acknowledgement
The author would like to express their heartfelt appreciation to the department of Chemistry, college of Science, Al-Baha University, Al- Baha, KSA, for their encouragement through this study.
Financial support and sponsorship
Nil.
Conflicts of interest
The authors declare no conflict of interest.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
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