Electrochemical Sensors for the Detection of Heavy Metals in Wastewater
Authors: Pardo Diego David , Zapata Zulamita , Briceño-Gamba Nelson Oswaldo 
Published: July 2021

Heavy metals are characterized by their high density, high toxicity, non-biodegradable, and poisoning even at low concentrations1. These can be present in irrigation water, wastewater or drinking water. Their accumulation can affect the natural ecosystems, crops and mainly the health of people2
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3. The World Health Organization reported the reference values of lead 0.01 mg/l, cadmium 0.03 mg/l, mercury 0.006 mg/l, nickel 0.07 mg/l and arsenic 0.01 mg/l and showed the maximum limited of these chemicals in consumption water4.

Electrochemical sensors have assumed an importance role in analytical chemistry, for easing collection data, fast, greatly sensitive, easy to handle, cheap and versatility5. There are already analytical methods that can detect heavy metals, however, these can only detect one metal per time, so the electrochemical method, in turn, can generate advantages for measuring metal ions simultaneously6. Moreover, many sensors use mercury which generates high toxicity when replacing the electrode. For this reason, it is necessary to develop materials that are more environmentally friendly as well as, it can detect heavy metals simultaneously.

It is important that materials used as electrodes in electrochemical sensors have high surface reactivity and that their porosity can be modified, so the metal can be detected. It has been gaining momentum the of study carbon-modified electrodes in order to improve the sensitivity of electrochemical detection methods7. Carbon materials possess these characteristics and are starting to accomplish a leading role in nanotechnology field, with different structures such as carbon nanotubes, graphene, carbon dots, carbon cables, nanodiamonds8. The easy in situ data also provides an economic advantage over more expensive methods such as atomic absorption or spectroscopy. The function of carbon materials within the sensors is focused with the possibility of improving the accessibility of heavy metal atoms by modifying the surface porosity of the material and reflecting the detection the element. The carbon materials currently most studied are graphene due to its properties in electronic, mechanical, and thermal9.

There are currently studies demonstrating the efficiency of graphene oxide and iron composite materials where there is a conductive phase and an insulator phase7. Furthermore, carbon nanotubes (CNTs) are cylindrically rolled graphene structures and depending on the characteristics of winding which generate different types of carbon allotropic conformation. They are used in the development of sensors especially in composite designs, due to their high conductivity, high specific surface area, and excellent thermal stability. Conductive polymers (e.g., polyaniline, polypyrrole and polythiophene) can be used to solve the non-uniformity of the distribution of carbon nanotubes on the surface10. Motaghedifard et al.10 synthesized PANI nanocomposite and sulfated zirconium oxide nanostructures and immobilized with carbon nanotubes on the surface of a glassy carbon electrode to trace measure of Cr(VI) in aqueous solution, and found that a limit of detection for Cr(VI) was 64.3 nmol/L. Other materials such as porous carbon materials from biomass resources with heteroatoms doped into the carbon matrix or metal composites have become research interesting for electrochemical sensors due to large specific surface areas, uniform porosity, and low cost11
,
1. For example, Qin et al. studied bacterial cellulose-metal composites as electrodes of the electrochemical sensor, which showed high sensitivity and selectivity toward target metals like Cd(II) and Pb(II)11.

The techniques most used in electrochemical sensors are voltammetry, potentiometry, impedimetry, amperometry, and conductometry. The voltammetry measurement (ASV) method widely used for the detection of heavy metal ions (HMIs) due to its high selectivity, sensitivity, short detection time, and low-cost equipment12. However, the electrodes respond best depending on the metal ion to be measured (Cu+2, Cd+2, Zn+2)13. Xu et. at1 synthetized N,S co-doped porous honeycomb carbon (N,S-PHCC) as an electrode for detection of Pb(II) by DPASV. These materials showed a wider linear range (0.75 – 120 mg/L), higher sensitivity, and lower detection limit (0.064 mg/l) than those of other electrode-based carbon materials.

Figure 1