A Review on Cyclic Voltammetric Investigation of Toxic Heavy Metals

Abstract

Heavy metals are one of the toxic pollutants threatening the human kind by causing various health issues. The detection of such polutants are of important environmental concern and we need a real-time monitoring equipment. Many researchers have established a number of approaches for the detection of these heavy metals so far. But, the development of one time use sensors for the quick, and real time detection of toxic heavy metals is in great demand. The electrochemical methods like cyclic voltammetry, is proved to be one of the best and popular methods, and are preferred over other electrochemical methods because of its high sensitivity, selectivity, anti-fouling, quick and accurate detection. In the present book chapter, we will discuss the various modifiers used to detect the arsenic, cadmium, and lead heavy metals using cyclic voltammetry.

1. Introduction

Heavy metals occur either naturally by geological activities, or by agricultural and industrial wastes. Any elements exhibiting relatively high molecular weight and more than 5 g/cm³ density are considered as heavy metals [1]. They are generally non-biodegradable and toxic in nature due to their ability to transform from one oxidation state to another easily. The heavy metals can cause bioaccumulation because of absorption of them by plants and animals living in that contamination areas [2] and causes various health effects like malfunction of gastrointestinal, nervous system, kidney and immune system, followed by birth defects, skin lesions, and cancer [3]. Figure 1 depicts the different organs of humans affected by the consumption of various heavy metals.

Figure 1.

Various heavy metals affecting the different organs of human [4].

The agricultural and industrial wastes like pesticides, fungicides, refineries, fertilizers, mining, smoking, nuclear fission plants, chemical industry, paint, electroplating, welding, automobiles, batteries, are the major sources of heavy metal ions [5]. Currently, there are more than 50 heavy metals are known and some of them are metalloids, actinides, transition elements and lanthanides [6]. The elements like lead, mercury, arsenic and cadmium are considered as very toxic and their consumption in small amount can cause serious health issues. But, some of the heavy metals are human friendly and their consumption in trace amount can maintain good health. Therefore, regular determination of the heavy metals is advisable in the contamination areas. Figure 2 depicts the possible ways of heavy metal exposure, their impact on human health and their mechanism.

Figure 2.

The possible ways of heavy metal exposure, their impact on human health and their mechanism [7].

Many researchers used cold vapor atomic fluorescence, atomic absorption, and emission spectroscopies and inductively coupled plasma techniques [8] to detect heavy metals. But these methods are time consuming, chances of contamination, requires huge manpower and area. Therefore, we need some standard analytical methods to detect the presence of heavy metal ions in foods, plants, animals, water, and soils. Electrochemical methods like cyclic voltammetry [9, 10, 11, 12, 13, 14, 15, 16], stripping voltammetry, differential pulse voltammetry, and polarography are so far best and popular choice to determine the various heavy metals in the environment. These methods are cheap, highly accurate, quick, robust in nature. Among them, cyclic voltammetry is proved to be one of the better and highly advantageous electrochemical methods used to detect organic, inorganic, organometallic, and biological heavy metal ions. High sensitivity, fast response, live monitoring data acquisition, wide detection limit, and possibility of simultaneous detection of multi elements by surface functionalization has made it one of the popular choices among the environmentalists used to detect the heavy metals [6, 17, 18, 19, 20, 21]. The present book chapter focus on the recent developments, current challenges, and prospects for future research in cyclic voltammetric determination of lead, arsenic, and cadmium in the environment. We hope that this book chapter will be useful for all the environmental researchers working on heavy metals.

2. Electrochemical determination of Lead (Pb) using cyclic voltammetry

Lead is a bluish-gray metal generally found in the earth crust. Lead is very poisonous metal directly affect the human nervous system and causes severe headaches, and memory loss [22]. Lead is very dangerous woman especially during the pregnancy, when a pregnant woman consumes lead, then it reaches the fetus and causes premature childbirth, abnormal growth, and low weight of the fetus. Lead also affect the brain developments in children and causes the abnormalities [23]. Therefore, people working in lead contamination zone must regularly check the amount of lead in their body.

Mei et al. have reported the use of TC4 arene-modified screen-printed carbon electrode (SPCE) to detect lead ions in river water using cyclic voltammetry [22]. Figure 3 demonstrates the schematic representation of electrode preparation to determine Pb²⁺. They reported the 0.7982 × 10⁻² ppm detection limit for Pb²⁺ followed by the excellent reproducibility and stability. Figure 4 depicts the current response of the sensor detecting Pb²⁺ ions at different pH levels.

Figure 3.

(a) The schematic representation of electrode preparation to determine Pb2+, and (b) binding of electrode with Pb²⁺ [22].

Figure 4.

Current response of electrode during electrochemical determination of lead ions in a KCl supporting electrolyte at different pH [22].

Riyanto has determined lead ions by cyclic voltammetry method using platinum wire as a working electrode in wastewater [24]. Authors reported that, they successfully detected the lead from wastewater with correlation of determination (R² = 0.999), LOD of 0.9029 mg/L, limit of quantification (LOQ) of 3.0098 mg/L and recovery of 100.67% respectively. They found that, the electro-oxidation of lead on Pt wire electrode occurs in a reversible system and reported that the analytical parameter from cathodic peak is better compared to anodic peak for analyzing of Pb using CV method. They claimed that, the fabricated electrode is simple, economic and showed an excellent selectivity, accuracy, sensitivity, reproducibility [24].

Khodari et al. İnvestigated the cyclic voltammetric behavior of lead ions well water samples using glassy carbon electrode [25]. Cyclic voltammogram showed an anodic peak at −520 mV for lead ions. Authors reported that, the glassy carbon electrode showcased an excellent linear response to a good linear response to Pb²⁺ in the concentration range from 8 × 10⁻⁶ M to 1 × 10⁻⁴ M with a detection limits of 2 × 10⁻⁷ M. They also studied the effect of scan rate, deposition time, deposition potential, and the pH of the supporting electrolyte on lead ions. They claimed that, the fabricated electrode is accurate, precise, highly selective in determining the lead ions in well water.

Honeychurch used carbon rod electrode extracted from zinc-carbon batteries to determine the traces of lead ions present in tap water sample using cyclic voltammetry [26]. Author studied the electrochemical behavior of Pb at different supporting electrolytes like ortho-phosphoric acid, HNO₃, HCl, CH₃COOH, KCl and malonic acid. They found the optimum electrochemical condition of a supporting electrolyte of 4% v/v acetic acid, with a deposition potential and time of −1.5 V (vs. SCE) and 1100 seconds as per the Figure 5. They reported the linear range of 2.8 μg/L to 110 μg/L and a detection limit of 2.8 μg/L with 95.6% mean recovery.

Figure 5.

Effect of acetic acid supporting electrolyte concentration on the cyclic voltammetric behavior of 116 μM Pb: (a) 0.0 M, (b) 0.03 M, (c) 0.1 M, (d): 0.66 M, and (e) 3.0 M [26].

3. Electrochemical determination of arsenic (As) using cyclic voltammetry

Arsenic is the 20th most abundant and high mobile element found in earth crust and exists as dust in air and dissolves in rain water and other water sources easily. The World health organization standard for As concentration must be 10 μg/L, but some parts of the World it exceeds the standard concentration. The excessive intake of As causes lung and skin cancer along with negative impacts on cognitive development and increased deaths [27]. Therefore, detection of arsenic in the environment is important for maintaining public health.

Du et al. fabricated a novel Au-stained Au nanoparticles/pyridine/carboxylated multiwalled carbon nanotubes/glassy carbon electrode for the detection of Arsenic (III) traces in real water samples using cyclic voltammetry [28]. The fabrication of the electrode is schematically represnted in Figure 6.

Figure 6.

Preparation of the electrode [28].

Authors observed the peak currents for oxidation of As (0) to As (III) are linear with a concentration of As (III) from 0.01 to 8 μM with a sensitivity of 0.741 mA μM⁻¹ and an LOD of 3.3 nM respectively. Similarly, for the peak currents fort he oxidation of As (III) to As (V) are found to be linear from 0.01 to 8.0 μM with a sensitivity of 0.175 mA μM⁻¹ and an LOD of 16.7 nM respectively. Figure 7 depicts the cyclic voltammogramms of prepared and gassy carbon electrode (GCE) during the electrochemical behavior of As (III).

Figure 7.

Cyclic voltammogramms GCE and the fabricated Aus/Py/C-MWCNTs/GCE electrode in 0.1 M aqueous H₂SO₄ with 1 μM As(III) and without As (III) [28].

Ismail et al. fabricated low cost silica nanoparticles modified SPCE for the detection of As (III) using cyclic voltammetry as shown in the Figure 8 [29]. They performed optimzation of the experimental conditions and reported that, the anodic peak current exhibited a linear range of 5 to 30 μg/L to the As(III) concentration, with a LOD of 6.2 μg/L. Authors claimed that, the fabricated electrode is very economic with high accuracy, high selectivity, sensitivity, stability with good reproducebility.

Figure 8.

Fabrication of silica nanoparticles modified SPCE [29].

Trachioti et al. fabricated a sparked gold nanoparticles from eutectic Au/Si alloy for the determination of arsenic ın drinking water using cyclic voltammetry [30]. They reported that the anodic peak current was proportional to the arsenic concentration over a linear range of 0.5 to 12 5 ppb, with a LOD of 6.2 ppb. The fabricated electrode exhibited excellent detection capability, high selectivity and reproducibility. Authors claimed that, the fabrication method of electrode is extremely simple and economic with wide scope of applicability. Table 1 depicts the various electrodes used to detect arsenic by cyclic voltammetric method.

Modified electrode	Linear range (μg/L)/ppb	LOD (μg/L)/ppb	Reference
Au/SPE	0.5–12	0.22	[30]
Au-Cu/GCE	30–130	5.64	[31]
Si/SPCE	5–30	6.2	[29]
Au/GCE	0.01–8	0.0033	[28]
Au/GCE	0.01–8	0.0167	[28]

Table 1.

Different electrodes used to detect arsenic by cyclic voltammetric method.

4. Electrochemical determination of cadmium (Cd2+) using cyclic voltammetry

Cadmium ion (Cd²⁺) are one of most toxic heavy metals that contaminates water and causes deadliest diseases like cancer, kidney dysfunction, cardiovascular disease, bone degeneration, lung, and liver damage to humans [32]. Cadmium ions are considered to have a greater solubility in water than any other heavy metals and offering a great threat to the biosystem. The standard level of Cd²⁺ in drinking water is 0.003 mg/L as per the World Health Organization (WHO). Therefore, the detection of Cd²⁺ in drinking water samples is of utmost important.

Attaallah and Amine reported the use of enzymic membrane as a electrode without any pre-treatment to detect cadmium ions in drinking water using cyclic voltammetric method [32]. Figure 9 depicts the fabrication of enzymic membrane electrode to detect cadmium ions. They also modified the fabricated electrode with screen printed electrodes to further increase the selectivity, sensitivity, reproducibility. They modified electrode has showed the linear calibration range between 0.02–100 ppb (R² = 0.990) and a detection limit of 50 ppt respectively. Figure 10a depicts the cyclic voltammogram of TMB/H₂O₂, HRP/TMB/H₂O₂, and HRP/Cd²⁺/TMB/ H₂O₂. [TMB = 3,3′,5,5′-Tetramethylbenzidine, HRP = horseradish peroxidase] and Figure 10b represents the cyclic voltammograms of variation in the cadmium ions from 0.02 to 100 ppb, in 0.1 M acetate buffer.

Figure 9.

Schematic representation of preparing enzymic membrane electrode to detect cadmium ions [32].

Figure 10.

(a) Cyclic voltammogram of TMB/H₂O₂, HRP/TMB/H₂O₂, and HRP/Cd²⁺/TMB/ H₂O₂, and (b) cyclic voltammograms of variation in the cadmium ions from 0.02 to 100 ppb, in 0.1 M acetate buffer [32].

Wang et al. electropolymerized ion imprinted poly (o-phenylenediamine) PoPD/electrochemical reduced graphene (ERGO) composite on glass carbon electrode (GCE) to detect Cd²⁺ in water using cyclic voltammetry [33]. Figure 11 depicts the procedure diagrams for fabrication of the electrode.

Figure 11.

The procedure diagrams for fabrication of the electrode [33].

The prepared electrode exhibited a excellent selectivity toward the target Cd(II) ions in the presence multi heavy metal ions. Under optimized conditions, the electrochemical sensor showed a good linear relationship between Cd (II) concentration in the range of 1 to 50 ng/mL, with the limit of detection of 0.13 ng/mL respectively. Figure 12 depicts the cyclic voltammetric curves of bare GCE, ERGO/GCE, Cd(II)-IIP/ERGO/GCE and IIP/ERGO/GCE.

Figure 12.

CV curves on bare GCE (curve a), ERGO/GCE (curve b), Cd(II)-IIP/ERGO/GCE (curve c), and IIP/ERGO/GCE (curve d) between −0.2 V and 0.6 V at a scan rate of 50 mV/s [33].

Liu et al. prepared Ti-modified Co₃O₄-based electrochemical aptasensor to detect Cd (II) as shown in Figure 13 [34]. Authors studied the effect of aptamer concentration, incubation time and pH of the solution to optimize the experimental condition to get good results. Under these conditions, peak current was proportional to the Cd (II) concentration over a wide linear range of 0.20 to 15 ng/mL, with a detection limit of 0.49 ng/mL respectively. Authors reported that, they have used cyclic voltammetry method not only to characterize each preparation and optimization step, but also to profile the bindings of aptamer to Cd²⁺. Figure 14 depicts the CV of bare electrode, Co₂Ti₁ modified electrode, Co₂Ti₁/aptamer modified electrode, and Co₂Ti₁/aptamer/Cd²⁺ modified electrode respectively.

Figure 13.

Schematic representation of fabricting the electrochemical aptasensor used to determine Cd²⁺ [34].

Figure 14.

The CV of (a) bare electrode, (b) Co₂Ti₁ modified electrode, (c) Co₂Ti₁/aptamer modified electrode, and (d) Co₂Ti₁/aptamer/Cd²⁺ modified electrode respectively at 100 mV/s scan rate [34].

5. Conclusion

The heavy metals like cadmium, arsenic and lead results in the bioaccumulation and causes adverse effect on gastrointestinal, nervous system, kidney and immune system, and also causes cancer. The detection of heavy metals and their ions is of utmost important in this world. As we discussed, cyclic voltammetry is one of the potential electrochemical method used to detect the heavy metals and their ions easily and quickly. The discussed method is one of the better method and exhibit excellent sensitivity, selectivity, good current response, and possible detection of multi heavy metals simultaneously compared to other electrochemical methods. The recent developments, challenges and future prospectus of the electrochemical detection of heavy metals by cyclic voltammetry is successfully discussed in this chapter. The electrochemical methods are more accurate, simple, robust methods compared to other traditional methods.