A Current Review of Water Pollutants in American Continent: Trends and Perspectives in Detection, Health Risks, and Treatment Technologies

Currently, water pollution represents a serious environmental threat, causing an impact not only to fauna and flora but also to human health. Among these pollutants, inorganic and organic pollutants are predominantly important representing high toxicity and persistence and being difficult to treat using current methodologies. For this reason, several research groups are searching for strategies to detect and remedy contaminated water bodies and effluents. Due to the above, a current review of the state of the situation has been carried out. The results obtained show that in the American continent a high diversity of contaminants is present in the water bodies affecting several aspects, in which in some cases, there exists alternatives to realize the remediation of contaminated water. It is concluded that the actual challenge is to establish sanitation measures at the local level based on the specific needs of the geographical area of interest. Therefore, water treatment plants must be designed according to the contaminants present in the water of the region and tailored to the needs of the population of interest.


Introduction
Water contamination represents a current crisis in human and environmental health. The presence of contaminants in the water and the lack of basic sanitation hinder the eradication of extreme poverty and diseases in the poorest countries [1]. For example, water sanitation deficiency is one of the leading causes of mortality in several countries. Due to unsafe water and a lack of sanitation, there are several diseases present in the population [2][3][4]. Therefore, the sixth global objective of the United Nations, foreseen as part of its sustainable development agent 2030, aims to guarantee the availability and sustainable management of water resources. In this sense, numerous research groups have focused on proposing alternative solutions focusing on three fundamental aspects: (a) detection of contaminants present in water for human consumption, (b) assessment of risks to public and environmental health due to the presence of contaminants in the water, and (c) the proposal of water treatment technologies. In the case of the American continent, the detection of contaminants (inorganic and organic) has been studied; the research works show alarming results in which the impact of water pollution is demonstrated, how the ecosystem is being affected, and consequently the repercussion towards human health [5][6][7][8]. This last point becomes worrying due to the fact that there are reported cases in which newborns, children, and adults consumed drinking water from various sources (such as rivers, lakes, groundwater, and wells) without the certainty that it is free of contaminants, representing a health risk factor [9][10][11]. Some of the detected contaminants have been associated with a potential health risk, such as the case of some disinfectants with cancer [12] and NO 3 − and NO 2 − as potential carcinogens in the digestive system [13].

Detection of Contaminants in Water
At present, there are various analytical techniques that have been used in the detection and quantification of inorganic and organic contaminants in aqueous matrices. Mainly, these techniques can be divided into three major groups: chromatographic, spectroscopic, and other techniques, such as electrochemical and colorimetric titration. A comparison of the advantages and disadvantages of the most commonly used analytical techniques is presented in Supplementary Table S1. From these, techniques that have been used the most are shown below.
From these techniques, it has been possible to determine the concentrations of various pollutants of interest to human health and the environment.
The compilation of information from the latest scientific reports (related to the detection of inorganic contaminants present in the water) is shown in Table 1 and Figure 1 (geographical distribution). On the other hand, the comparison of the detection limits for the limits of interest using different analytical techniques is presented in Supplementary Table S2. Among them, some works have been carried out based on water bodies in different countries, such as Canada [19], USA [20][21][22], Mexico [23], and Brazil [24], in which the presence of As, Fe, U, Zn, Na, K, Ca, Mg, HCO 3 − , and Hg with respect to interactions among water, bedrock mineralogy, and geochemical conditions of the region has been studied, so they can be classified as contamination due to a natural source. A particular case can be analyzed for U, which is present in water bodies of the southwest and west central USA, because high levels of acute exposure can be fatal for the population, and chronic exposure at low levels is associated with health problems, such as renal and cardiac risk. Although, exposure studies of surrounding communities cannot be considered conclusive, they correspond to a great advance in the field, and future studies should be carried out to assess possible damage to human health and the ecosystem. Table S2. Among them, some works have been carried out based on water bodies in different countries, such as Canada [19], USA [20][21][22], Mexico [23], and Brazil [24], in which the presence of As, Fe, U, Zn, Na, K, Ca, Mg, HCO3 − , and Hg with respect to interactions among water, bedrock mineralogy, and geochemical conditions of the region has been studied, so they can be classified as contamination due to a natural source. A particular case can be analyzed for U, which is present in water bodies of the southwest and west central USA, because high levels of acute exposure can be fatal for the population, and chronic exposure at low levels is associated with health problems, such as renal and cardiac risk. Although, exposure studies of surrounding communities cannot be considered conclusive, they correspond to a great advance in the field, and future studies should be carried out to assess possible damage to human health and the ecosystem. On the other hand, research works stand out showing that water pollution can occur due to anthropogenic activities [25], being evident that modern practices of agriculture and livestock have consequences as the indiscriminate use of fertilizers, pesticides, and hormones results in nitrates in the water, which are associated with a risk of congenital anomalies, such as heart and neural tube defects.
Within the works carried out, one of the most concurrent techniques used in the evaluation of contaminants has been performed via ICP (MS or OES) due to its high precision, low cost, low detection limits, and the advantage of analyzing a large number of elements simultaneously in a short time [26]. However, in some cases, the detection limits of the technique are above the maximum permissible limits proposed by the WHO (World Health Organization), such is the case of Hg, for which the detection limit is of 0.0025 mg L −1 and the maximum detection limit recommended by the WHO is 0.002 mg L −1 . Therefore, it is concluded that one of the challenges to be dealt with for metal detection in water is based in the fact that current techniques must be complemented by advanced analytical techniques, such as electrochemical tests [27]. These techniques are of great interest for On the other hand, research works stand out showing that water pollution can occur due to anthropogenic activities [25], being evident that modern practices of agriculture and livestock have consequences as the indiscriminate use of fertilizers, pesticides, and hormones results in nitrates in the water, which are associated with a risk of congenital anomalies, such as heart and neural tube defects.
Within the works carried out, one of the most concurrent techniques used in the evaluation of contaminants has been performed via ICP (MS or OES) due to its high precision, low cost, low detection limits, and the advantage of analyzing a large number of elements simultaneously in a short time [26]. However, in some cases, the detection limits of the technique are above the maximum permissible limits proposed by the WHO (World Health Organization), such is the case of Hg, for which the detection limit is of 0.0025 mg L −1 and the maximum detection limit recommended by the WHO is 0.002 mg L −1 . Therefore, it is concluded that one of the challenges to be dealt with for metal detection in water is based in the fact that current techniques must be complemented by advanced analytical techniques, such as electrochemical tests [27]. These techniques are of great interest for their study due to the benefits they have, such as improvements in detection limits, low operating costs, short analysis times, and mobility, being able to perform analytical determinations in situ [27]. It is concluded that the contaminants with the greatest presence in the continent are As, U, Pb, Mn, Se, and Hg, mainly related to the mineralogy of the analyzed site and anthropogenic activities in the analysis areas. However, in some cases, the source of contamination is natural and occurs periodically due to seasonal changes, with the rainy season being the period with the greatest presence due to the mobility of metals contained in the rock and soil of the region [28,29]. Moreover, the presence of ions in solution related to the use of fertilizers and agrochemicals in crop fields has also been documented [30]. It is important to denote that the origin of the contamination source is not accurately concluded, providing a current challenge for the exact determination of the source to propose containment and sanitation actions to solve the problem. Research studies presented in Table 1 demonstrated the potential human health risks that metal presence can have in water bodies, being important to highlight that there is still a need to evaluate the impact that inorganic contaminants have on human health. Furthermore, several research groups in different countries have detected the presence of contaminants not only in the supply sources, such as water bodies, but also in aquatic environments, such as flora and fauna being affected and representing economic importance since certain species can be traded, based on great demand to satisfy local and international markets.
On the other hand, organic contaminants can be divided into several groups; nevertheless, the principal groups are the ones denominated as persistent organic pollutants (POPs). These pollutants have an important impact on the environment and human health. Some examples are per-and polyfluoroalkyl substances (PFAS), personal care products, pharmaceutical compounds, pesticides, phenolic compounds, dyes, hormones, sweeteners, surfactants, and others.
Their detection has been primarily necessary to assess the effects that these pollutants have. Most of them are primarily obtained from industrial activities having different uses, such as flame retardants, coolants, cement, and others. Their presence represents an important contribution to water ecotoxicity (Ecuador, Argentina, Mexico) that affects the integrity of the species that inhabit that ecosystem [53][54][55].
Important issues have been detected in aquatic environments. The bioaccumulation of several organic compounds, such as polychlorinated biphenyl compounds (PBCs) and polybrominated diphenyl ethers (PBDEs), in important water bodies, such as Lake Chapala (Mexico), has been reported, through the analysis of samples recollected from water, fish, and sediments from two local seasonal periods. In this case, the fish analyzed were Cyprinus carpio, Oreochromis aureus, and Chirostoma spp., establishing that these chemical substances can reach the lake via industrial activities and strong winds and enter from the Lerma River (Mexico) [55].
For this reason, analytical assays must be performed to establish the concentrations of these pollutants using techniques that are capable of studying a complex matrix and if it is possible, in situ. In Table 2, the description of several studies that were able to detect organic compounds in environmental samples and the technique that was employed are provided.  As it can be appreciated in Table 2, a variety of organic compounds have been identified as being associated with several disorders and diseases. Nevertheless, most of the studies analyzed correlated its contaminant of interest with previous research that evaluated its potential human health risk effect. For this reason, it is important to detect the contaminant and correlate it with its health impact in the environment (population and biota).

Presence of Pollutants in Water: Impact on Human Health and Its Possible Sources
The inorganic contaminants with the greatest presence in water bodies correspond to heavy metals. At the moment, the potential damage to health due to heavy metals has been reported as listed below: As(III) (skin damage, circulatory system issues), Cd(II) (kidney damage, carcinogenic, cardiovascular damage, hematological, and skeletal changes), Cr(III) (allergic dermatitis, diarrhea, nausea, and vomiting), Cu(II) (gastrointestinal, liver or kidney damage), Pb(II) (kidney damage, reduced neural development, behavioral disorders), Hg(II) (kidney damage, nervous system).
According to the scientific reports analyzed, it is concluded that there are two main risk factors in public health: (i) the intake of contaminated water, being the main factor due to direct exposure to the contaminant, which can produce different anomalies as those described in the previous paragraph. However, the studies presented cannot be considered conclusive, since the reports show that the impact on health is directly related to the clinical history of the exposed population [20]. (ii) The consumption of contaminated food, such as in the case of the report of da-Silva et al. (2019) [24], which reported Hg migration in water from the Western Amazon Basin (Amazon Triple Frontier: Brazil, Peru, and Colombia) to fish; being that if they are intended for human consumption, this can cause mercury intoxication (mercurialism). While the intake of contaminated food is the most likely action to occur, there are other special factors that particularly attract attention, such as the report presented by Oliveira et al. (2021) [87] studying a potential health risk in terms of a cognitive deficit due to soil intake by pre-school children aged 1 to 4 years, which presents high levels of Pb and Cd due to contact with contaminated wastewater from industries in the region of São Paulo (Brazil).
On the other hand, for organic contaminants, data analysis and comparison has been performed in different countries evidencing the necessity of establishing strategies to remediate water pollution (Figure 1). These strategies are urgent, based on the potential risk that these contaminants can have on human health [88][89][90]. Although there are currently certain reports, guidance values or standards that allow establishing criteria based on the presence of these contaminants and their potential toxic effect are needed [43,91]. Efforts have been performed to establish international regulations since the majority of organic compounds are not quality controls [92].
For this reason, several research groups have tried to determine the impact a chemical compound has on human health. For example, atrazine, an artificial herbicide that was detected in surface water, has been associated with an impact on human health and aquatic biota [93], upon evaluating endocrine-disrupting compounds that can affect human health via cell-based assays [94]. Moreover, per and polyfluoroalkyl substances have been determined, but there are no reference points that establish a water quality criterion for its impact on human health [91]. Based on this, there is a need to establish scientific studies in a human population and evaluate the impact of water pollution on its health. Some studies have been performed (see Table 3) to correlate the exposure of contaminants in people's life and if possible, establish the impact that water sources and body contamination have. Table 3. Scientific studies on the correlation between a water source and the presence of certain pollutants in a human population.  Taking into consideration the environmental and public health risk represented by effluents and water bodies contaminated with metals, numerous research groups have focused on proposing remediation alternatives, highlighting the adsorption process [104,105], coagulation/flocculation [106], chemical precipitation [107], ion exchange [108], electro-chemical treatments [109,110], membrane use (ultrafiltration, osmosis, and nanofiltration) [111,112], and other alternative treatments based on the use of biopolyelectrolytes and coupled adsorption processes with electrochemical regeneration [113,114]. In all cases, the actual challenge consists of evaluating the scale-up process, for which studies have been performed on a small scale under controlled conditions.
Although, scientific reports have demonstrated great efficiencies in the removal of heavy metals, there has been certain problems documented for each technology, which must be addressed to present advanced remediation technologies. For the ion exchange process, it has been documented that those present with low efficiencies for the removal of high concentrations of metals [115]. For example, Malik et al. (2019) reported removal efficiencies of 55% for Pb and 30-40% for Hg [116]. In the case of membrane filtration, good removal efficiencies have been reported (around 90% for Cu and Cd) [116], however, it requires high installation costs and maintenance [117]. Likewise, it has been reported that the electrochemical, catalysis, and coagulation/flocculation processes present high metal removal efficiencies (around 85-99% for Cd, Zn, and Mn) [118]. On the other hand, the main drawbacks are high installation costs and extra operational costs, as well as the generation of unwanted by-products (sludge) [119]. These drawbacks significantly reduce the effectiveness of water treatment processes, so a second challenge to deal with is process optimization.
Finally, the third challenge is the design of environmentally and economically sustainable treatment processes. The current paradigm of water treatment of metal contamination must be broken; the importance is not only in water sanitation, but also in recovering the metal in order to obtain valuable products and not only change the pollutant phase [120]. For all the above, adsorption and chemical precipitation have turned out to be the most used methods. However, the removal results obtained depend on each matrix used, so the materials and experimental conditions must be proposed based on the needs and the type of effluent to be treated [121].

Organic Contaminants
In the previous sections, the detection of these pollutants is only the first step to evaluate the environmental risk that communities and countries have in their respective water sources. The next step is to determine technologies that can establish an efficiency in the removal of these contaminants in a complex matrix without affecting the environment using novel systems [122][123][124]. In this regard, an actual challenge is the development of technologies capable of treating specific organic compounds and if it is possible, to use these treatment technologies with the current systems that governments have implemented. Some technologies that have been investigated are the use of continuous flow supercritical water (SCW) for the removal of hormones from the wastewater of a pharmaceutical industry. In their results, the technology was demonstrated to reduce 88.4% of the initial total organic carbon (TOC) value, and the presence in gas phase of H 2 , CH 4 , CO, CO 2 , C 2 H 6 , and C 2 H 4 , which could be used to produce renewable energy. Moreover, phytotoxicity assays demonstrated that there was no risk of the treated samples with respect to the germination of Cucumis sativus seeds [125]. Another technology that has been used is direct contact membrane distillation, which can be used to treat raw surface water contaminated with phenolic compounds [126]. In this case, water samples were spiked with 15 phenolic compounds. An important parameter evaluated was the recovery rate (RR) to demonstrate the stability of the direct membrane distillation, being up to a 30%. Pollutant removal reached 94.3 ± 1.9% and 95.0 ± 2.2% for 30% and 70% RR, respectively. A consideration for this technology is to work at a recovery rate in which flux does not decay (RR < 30%) to avoid performing loss and fouling.
Different approaches have been used for the removal of contaminants, such as the use of a photocatalytic paint based on TiO 2 nanoparticles and acrylate-based photopolymer resin for the removal of dyes in different water matrices [127]. Another strategy was subsurface horizontal flow-constructed wetlands (planted in polyculture and unplanted) as secondary domestic wastewater treatment to demonstrate the removal of personal care and pharmaceutical products [128].
Considering the above mentioned content, among all technologies evaluated currently to eliminate organic contaminants present in water, Advanced Oxidation Processes (AOPs) stand out, since they generate highly reactive and non-selective radicals capable of almost completely mineralizing the contaminant of interest, generating mainly CO 2 and H 2 O as an oxidation product. In this sense, the most widely studied AOPs correspond to catalytic wet peroxide oxidation, catalytic wet air oxidation, homogeneous catalyst, photo-Fenton, Fenton process, photocatalysis, Fenton-like, electro-Fenton, heterogeneous catalyst, ultrasound, and microwave [129]. Although the results show the potential use of technologies for water treatment, there are still challenges to address. The current challenge of this technology must be aimed at scaling the process, optimizing operational parameters, and designing a sustainable technology to have a low cost and be environmentally friendly, achieving the lowest generation of by-products. In this sense, two recently published research articles stand out in which AOPs have been evaluated for the treatment of contaminated water effluents in the Latin American region. Mejía-Morales et al. (2020) [130] presented the use of an AOP based on UV/H 2 O 2 /O 3 for the remediation of residual water from a hospital in Puebla (Mexico), showing the feasibility of its use to remediate effluents contaminated with a high organic load. On the other hand, Zárate-Guzmán et al. (2021) [131] presented the scale-up of a Fenton and Photo-Fenton process for the treatment of piggery wastewater in Guanajuato (Mexico). The results show that these two AOPs have great application potential for the remediation of effluents contaminated with a high organic load due to their high removal percentages (COD, TOC, and Color) and low operating costs.

Conclusions
The presence of contaminants in the water is a severe environmental and public health problem in the American continent. The presence of inorganic (As, Cd, Cr, Pb, Cu, Hg, and U) and organic pollutants (dyes, phenolic compounds, hormones, pesticides, and pharmaceuticals compounds) in effluents and water bodies is due to anthropogenic activities and natural factors in the region. The health risks associated with these contaminants primarily encompass skin damage, carcinogenic effects, nervous system damage, circulatory system issues, kidney damage, gastrointestinal damage, and impacts on the food chain. The critical review of the reports presented in this document identifies the following as the main challenges: (i) Implement advanced analytical detection techniques, such as those based on electrochemical tests, to achieve improvements in detection limits, low operating costs, short analysis times, and mobility to perform in situ determinations. (ii) Accurately determine the source of contamination in each geographic site of interest to propose containment and sanitation actions to solve the problem. (iii) Evaluate water treatment technologies on a large scale and under real conditions to optimize the treatment processes. (iv) Design and/or conditioning of specific water treatment plants according to the pollutant of interest in the region. The universal design paradigm of a water treatment plant must be broken; the pertinent modifications must be made according to the needs of the population of interest. (v) Design environmentally and economically sustainable treatment processes. Future water treatment processes will need to integrate circular economy concepts to obtain high-quality water and valuable products, such as precious metals, and/or produce biofuels.