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Release, Detection, and Toxicology of Heavy Metals: A Review of the Main Techniques and Their Limitations in Environmental Remediation

Dison S. P. Franco1, Jordana Georgin1, and Chandrasekaran Ramprasad2

1 Department of Civil and Environmental, Universidad de la Costa, CUC, Barranquilla, Colombia

2 School of Civil Engineering, Centre for Advanced Research in Environment (CARE), SASTRA Deemed to be University, Thanjavur, Tamil Nadu, India

1.1 Introduction to Heavy Metals: An Overview

The group of heavy metals has been the focus of much research and scientific studies for two reasons: they are released into the environment on a daily basis in different regions of the world and are highly harmful to human and animal health (Proshad et al. 2021). They are classified as heavy due to their high molecular weight being detected in biological and environmental samples in various source compositions (Proshad et al. 2021). When they are harmful to health, regardless of the concentration, they are classified as heavy metals, namely: zinc (Zn), chromium (Cr), copper (Cu), mercury (Hg), thallium (Tl), cadmium (Cd), arsenic (As), vanadium (V), nickel (Ni), cobalt (Co), iron (Fe), and lead (Pb) (Li et al. 2014). Both physiological and biological functioning depend on these elements (Lien et al. 2014; Kumar et al. 2018). Several activities in society are responsible for the release of these ions (Kim and Kim 2020). The source of release in food crops varies greatly by region of the world. In developed countries, the use of fertilizers such as sewage sludge and industrial effluents is the activity that mostly releases metals into the environment. In developing countries, irrigation with incorrectly treated sludge and effluents becomes the main input of heavy metals into the soil (Rai and Singh 2018). However, the deposition of these contaminants in the environment still involves several mechanisms. Therefore, these ions are released in different compartments present in society, namely: urban systems with industries, agricultural activities, watersheds, mining activities, public transport vehicles, and estuaries, among others (Figure 1.1). Distribution can occur on a regional, local, or global scale. Studies on the presence of heavy metals in watersheds are complex because they involve several sources such as soil, atmospheric deposition, river water, sediment, and biosphere. Physical-chemical processes, such as water-rock interactions from riverside and flood plains, atmospheric deposition (Nickel et al. 2014), and soil erosion and leaching (Li et al. 2020) are classified as carriers of contaminants. In this respect, river water acts as a sink for contaminants within the watershed. In the image, we can see that river water is the primary carrier, whereas the secondary carriers are sediments, atmospheric deposition, and soil.

Figure 1.1 Mechanisms and sources of release of heavy metals in water resources.

The degree of toxicity of these ions in food crops requires specific attention to define the real damage to health that each metal can generate. Some metals such as iron, copper, and zinc are essential in metabolic processes in biota (Zhuang et al. 2009). Uréase has nickel in its composition, and it is known that in large concentrations it can cause problems for human health (Zhuang et al. 2009). These interactions between food, soil, and plants are a model of abiotic-biotic linkages in the environment. Maintaining a balanced soil in relation to the presence of metals is essential, as sustenance and the food source depend on it. However, the unbalanced presence of these elements can cause a disturbance; an example is industrial activities such as the manufacture of chemicals (alkalis and chlorine), thermoelectric plants, energy industries, foundries, and textiles, among others, which are considered punctual sources of metal release. Agricultural runoff and soil erosion are classified as non-point sources (Figure 1.1). In addition to the human health problem, these elements alter the activities and microbial interactions of the soil, directly affecting its fertility and quality (Gadd 2010; Gall et al. 2015; Rai and Singh 2018). Other living organisms such as insects and even mammals that are needed by the soil are harmed (Gall et al. 2015; Bartrons and Peñuelas 2017; Rai and Singh 2018). Some cultures use various plants for medicinal purposes; therefore, determining the presence of metals in these plants is highly necessary since it can bring other damage to the health of these people (Shen et al. 2017). Another concern is that some species can bioaccumulate in their tissues metals such as cadmium, chromium, arsenic, lead, and iron; this situation is aggravated when these plants are close to areas of point sources (El Hamiani et al. 2015; Bolan et al. 2017; Kim et al. 2017; Kohzadi et al. 2019). Not only are species grown in the open air subject to contamination, but those grown in greenhouses are also subject to contamination; however, iron is not present in these crops due to lower lighting (Ling et al. 2017). Knowing the mechanisms that involve the release of metals in soil and food is essential for planning remediation technologies.

Due to the high toxicity of some metals, public bodies such as the World Health Organization (WHO) and the Environmental Protection Agency (EPA) have established release limits into the environment (Carolin et al. 2017). The permitted range (10-250?mg?l-1) varies depending on the metal and the regulatory agency; however, if the value is exceeded, it can cause serious damage to different regions of the human body (Vinod Kumar et al. 2014). Due to physical and chemical interactions, ions are rapidly absorbed by biological matrices through different pathways (Ray and Shipley 2015; Tan et al. 2015; Priyadarshini and Pradhan 2017; Ragab et al. 2017). The excessive release of these ions, added to consumption by the body, leads to serious health problems, making their detection and removal from the environment a major social challenge, as illustrated in Figure 1.2.

In order to perform the detection of toxic ions even in small concentrations, highly sensitive techniques were developed (Figure 1.2), with the most commonly applied being inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) (Sener et al. 2014; Oliveira et al. 2015). For removal and remediation, it is possible to observe different analytical methods that have been created and improved in recent times (chemical precipitation, electrochemical methods, ion exchange, and bioremoval); however, they are little applied due to cost (Kiatkumjorn et al. 2014; Swain et al. 2015). With the application of nanomaterials, the colorimetric technique has shown high selectivity, being efficient in environmental monitoring. Based on color change (plasmon resonance), detection by visible UV absorption spectrophotometry can also be applied. However, bringing together all the advantages such as selectivity, low cost, and environmental friendliness, there are still barriers that can be overcome with advanced technologies in optics, electronics, and electrochemistry (Zhao et al. 2017). With a greater number of desirable variables, detection in nanomaterials has been gaining ground in recent years (Hung et al. 2010; Anwar et al. 2018; Rossi et al. 2021). In this area, sensitive and efficient nanoprobes have also been developed for detecting metal ions (Liu et al. 2019). By causing a change in the surface and color plasmon resonance absorption peak, colorimetric sensors modify metal-induced particle segregation. The target ion provides the agglomeration of nanoparticles by changing the color of the solution (Wang et al. 2020).

Aiming to prevent damage to human and environmental health, calorimetric probes based on nanomaterials using plasmon resonance were applied to detect metals in low concentrations present in food samples and water resources (Shrivas and Wu 2007; Shrivas et al. 2015). For the same purpose, Khalkho et al. (2021) used Fourier transform infrared spectroscopy (FTIR) coupled with liquid/liquid solvent extraction, single drop microextraction (SDME), and cloud point microextraction techniques. By focusing on metals harmful to health, studies prove that the method of preconcentration of ions using sodium diethyldithiocarbamate immobilized in polyurethane foam (PU-NaDDC) successfully detected the target pollutant both in biological environmental samples and in liquid environmental samples (dos Santos et al. 2009). Using the same method, it was also possible to detect mercury ions in sedimented areas (dos Santos et al. 2009). Seeking to determine the presence of copper in organic food samples,Shrivas and Jaiswal (2013) used the method based on the separation and preconcentration of the target present in an extraction solvent containing PBITU together with a dispersion solvent. Shrivas and Wu (2007) jointly used SDME with atmospheric pressure mass spectrometry (AP)-MALDI to analyze the presence of metal ions in real river water samples in gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis (CE).

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