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dental care, paints, and fertilizers, but landfill leachate must also be considered mercury‐containing waste (Hargreaves et al., 2016).

      There are other toxic metal compounds of great concern, such as hexavalent chromium and copper. However, these chemicals were not included in the WHO list due to fewer cases compared to the ones previously mentioned. Many of them are also genotoxic gastrointestinal disruptors and are found in fertilizers and biocide compositions. Consequently, industrial wastewater containing toxic metals must be properly treated in order to not expose humans and other animals to harmful health impacts.

      Toxic heavy metals have been part of human life for quite some time; hence there have been several attempts to find the best method to effectively remove them from industrial wastewater. Conventional methods including chemical, physical, and biological mechanisms have been adopted. For inorganic wastewater, chemical precipitation and ion exchange have been most widely used, but more recent advances have shown promising results using adsorption, electrodialysis, membrane filtration, and photocatalysis (Barakat, 2011). It is noteworthy that biosorption has also shown high efficiency for removing and recovering metals from wastewater, in addition to being a much cleaner and more cost‐effective alternative (Kurniawan et al., 2006; Calderón et al., 2020).

      Taking note of the facts and trends regarding toxic heavy metals, proper management of industrial wastewater must be taken into consideration. Each type of industry brings its own complexity and selected metals due to its manufacturing processes, as well as other constituents that must be treated before effluent discharge.

Dyes

Dyes are strongly related to toxic metals, given that they may contain such chemicals, and must be addressed. Some of their negative effects are inhibiting photosynthesis and plant growth, toxicity, carcinogenicity, mutagenicity, persistence, and recalcitrance. In addition, they increase biochemical oxygen demand (BOD) and chemical oxygen demand (COD) in industrial wastewaters (Lellis et al., 2019), which leads to further treatment expenditures. These wastewater contaminants are soluble organic compounds; because of the presence of chromophoric groups (azo, carbonyl, nitro, methine, etc.) in their molecules, it is hard to remove them from solution by conventional methods due to the presence of hydrophilic groups such as auxochromes.

      Since such substances are used to color solid structures, when disposed in water bodies, they block the penetration of solar rays and reduce photosynthesis activity, which leads to lower dissolved oxygen levels and compromises aquatic life and self‐purification processes. As a result, dye‐containing industrial wastewater requires efficient methods to eliminate this chain of negative impacts throughout ecosystems.

      Dyes can be manufactured from artificial and organic materials. Synthetic dyes are usually derived from petroleum and contain various toxic chemicals. Despite natural dyes being much more biodegradable and environmentally friendly, they must be mixed with a mordant, a kind of binding agent that helps attach dyes to fabrics. According to Katheresan et al. (2018), such binding agents may be more dangerous than synthetic dyes alone.

Over 800,000 tons of synthetic dyes are produced annually worldwide and generate around 1 trillion US dollars (Hassaan and El Nemr, 2017; Lellis et al., 2019). However, the manufacturing processes make them recalcitrant (resistant to biological degradation), and this is one of the most water‐consuming sectors. For example, 1 kg of dye produced normally requires 0.15 m³ of water. Unfortunately, over 200,000 tons of these dyes are lost during coloring processes and are carried in wastewater (Chequer et al., 2012), implying an unsustainable situation. The primary use of dyes is in the textile industries, but they are also present in the paper, tannery, food, pharmaceutical, and cosmetic industries, as shown in Figure 1.1.

In the textile industry, most dyes are used for fabric coloring, especially synthetic fabrics such as polyester, polyamide, and other polymers. In this case, dyes may impact both the environment and consumers because some dyes cause allergic reactions and skin and respiratory tract irritation. When dyes are inhaled, they attack the immune system and may cause itchy, blocked noses, sneezing, and sore eyes. Furthermore, textile dyes are usually made of azo compounds, which are strong and complex structures made of nitrogen‐nitrogen bonds that can be harmful even at concentrations as low as 1 part per million (ppm). To destroy such components, advanced oxidation processes (AOPs) have shown promising results, despite their high cost and sophisticated equipment requirements.

Figure 1.1 Main industries dealing with dyes.

      Source: Modified from Katheresan et al., 2018.

      In the food industry, dyes are divided into two groups: those that require certification and those that do not. Based on the previous explanation, synthetic dyes such as azo, xanthene, triphenylmethane, and indigoid dyes are subject to a certification process (Barrows et al., 2014) due to their greater risk of toxicity. On the other hand, natural‐based dyes commonly made from plants, insects, or minerals may be exempt from licensing, but their usage still needs concentration limits and attendance to levels of purity. It is worth noting that such licensing practices vary from country to country; hence, deeper understanding is necessary for any individual case. Besides the usual adverse effects listed so far, in rare cases, consuming colored foods may trigger anaphylactic shock that can cause death in minutes if proper treatment is not received, due to hypersensitivity to a dye. Any harmful effect of dye‐containing food may occur due to an individual’s intolerance or overdose of such chemicals when the food was not carefully manufactured.

In a completely different approach from how they are usually applied, dyes can be used with drugs in the pharmaceutical industry for the same purpose. Dyes are gaining credence in therapy due to their ability to engage with drug‐resistant organisms, creating intolerable toxicity for microbes but not humans. A well‐known example is Prontosil Rubrum® from Bayer (IG Farben Conglomerate, Germany), which behaves like a pro‐drug since its reduction products are highly effective against bacterial illness, as shown in Figure 1.2. Consequently, pharmaceutical wastewaters contain, among a variety of complex, toxic, and recalcitrant compounds, dye waste from manufacturing processes and must be treated adequately before discharge.

      There is a lack of complete understanding about the most effective technique for removing dyes from wastewater, and some approaches can generate by‐products of secondary pollution. One of the first methods was an activated sludge process. However, although this biological process is relatively cheap and widely implemented worldwide, it is insufficient to remove such hazardous compounds (Katheresan et al., 2018). Another biological method that has achieved better removal is adsorption in biomass simultaneously with biodegradation by enzymes. Further research must take place since enzyme degradation seems to be a cheaper and safer method for dye wastewater treatment (Katheresan et al., 2018).

      More expensive methods are available to treat dye wastewater using chemical and physical processes. Among the chemical methods are AOP, electrochemical destruction, ozone oxidation, ozone plus ultraviolet (UV) irradiation, etc. Unfortunately, such methods remain unattractive due to their higher investment and operational costs. On the flip side, physical methods are straightforward approaches relying on mass‐transfer phenomena. These methods encompass adsorption, precipitation, cementation, coagulation, filtration, ion exchange, and reverse osmosis. Such technologies also have advantages and disadvantages and must be carefully analyzed before implementation.

After this brief review of dye wastewater and its hazardous impacts, it should be noted that the structure of dyes turns them into significant threats to the environment. With hydrophilic groups and properties that allow them to add or change color, they can seriously modify the environment and affect all dynamics of ecosystems. In light of this, every industry using dyes during manufacturing processes must be aware of their negative environmental

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