In parallel to modernization, mother earth is confronting the emergence of hazardous pollutants. Contamination does not only affect natural resources but also raises health concerns (Yang et al.; Cabral 2010; Bwapwa and Jaiyeola 2019; Strokal et al. 2019). Water is essential for life in natural resources, but clean water worldwide is minimal. Earth has 71% water, but the maximum percentage is not drinkable. Ocean water covers 97%, while only 2.5% is freshwater, further categorized as a glacier, ground, and surface water. Most living beings rely on surface water, which gets polluted, causing abrupt shrinkage of sources.
World health organization reported that 785 million people would not have clean water, and by 2025, most populated areas will face water stress (WHO 2021). Many organic pollutants in water are from industrial waste, personal care products, pharmaceutical sewerage, textile dyes, organic fertilizers, and pesticides (Inamuddin et al. 2021; Jiang et al. 2021). Besides these organic contaminants, water is also adversely affected by microbes and favors raising pathogenic communities. However, surface water is directly polluted through the aforementioned sources, but indirectly, groundwater also receives considerable contamination.
Waterborne microbes are mainly classified under four families, i.e., bacteria, viruses, fungi, and protozoa (Gerba and Pepper 2019). Bacteria are the prevalent pathogen, which triggers many mild to acute infections. According to health organizations reports, bacteria are the leading cause of infections where water is the primary route of reliant diseases (Yang et al.; Pandey et al. 2014). Millions of deaths are associated with bacterial infections and the rising potential risk of outbreaks every year (Todar 2004).
1.1 Bacteria and contamination
Diseases related to aquatic bacteria are growing with the emergence of novel classes. Most bacterial communities belong to gram-negative and gram-positive, while the rest is non-gram stain. Bacteria are contiguous, but the severity level varies on habitation conditions and their growth rate. Another problem with bacteria is that they can form biofilm. It is an agglomerated form of bacteria that enhances complications with severe diseases in cystic fibrous patients and perturbs treatment with high drug-resistant events.
Many mediums in daily life, such as medical equipment, smooth surfaces with human interactions, water transport lines, or sewerages, experience the formation of biofilms (Davies 2003; Worthington et al. 2012; Bjarnsholt 2013; Fair and Tor 2014; Dilsaver et al. 2014; Prestinaci et al. 2015; Renwick et al. 2016). Bacterial growth depends on the medium that allows bacterial cells to utilize metabolism and energy generation by breaking nutritional bonds. Prokaryotes have versatile modes of metabolism (Embden-Meyerhof, Phosphoketolase, and Entner-Doudoroff) fermentation pathways. Anaerobic respiration includes lithotrophy, photoheterotrophy, anoxygenic photosynthesis, and methanogenesis.
The cell wall of gram-positive and gram-negative bacteria is slightly different in composition, but both work to prevent osmotic lysis of cell protoplast and confer rigidity and shape of cells. The plasma membrane is the main barrier that allows energy generation and chemical transportation and contains many enzymes. The ribosome’s main components are protein and ribonucleic acid. Inclusion contains lipids, carbohydrates, and inorganic substances used for reserved nutritional sources. Most inner sites have chromosomes, and plasmids consist of genetic material such as deoxyribonucleic acid (Parker et al. 2016; Rajagopal and Walker 2017; Kenneth Todar 2020).
1.2 Heterotopic bacterial Growth
Natural processes maintain balance in different kinds of environmental reactions. Carbon fixation is a part of such a cycle in which prokaryotic decomposers convert atmospheric carbon to organic products (Kerfeld 2016). Bacteria are categorized in the prokaryotic family, but most are heterotrophs that use organic compounds as carbon sources; for example, E. coli metabolism uses organic matter as an energy source (Görke and Stülke 2008; Chubukov et al. 2014). Catabolite repressions sense the preferred nutritional bases, and with the cooperation of signals from central carbon metabolism, bacterial cells regulate multiple tasks like inhalation, conversion, and utilization to feed intercellular organs; the metabolic process is given in Fig. 1(a).
Water bacterium is extensively exposed to organic waste that dissolves in water. These contaminants strongly influence bacterial growth, such as with preferred carbon source heterotrophs consuming persistent organic pollutants (POPs) as an energy source, which increases the doubling rate. This neglected relation of contaminants increases the risk of bacterial colonies in water. Industrial dyes are also a common source of organic contamination and in vitro study reveals that both gram stain bacteria experienced accelerated growth. E.coli and S. aureus survival in the heterotrophic condition is given in Fig. 1(b-c) (Ali et al. 2020b, 2021).
1.3 Treatment methods
Treatment of contaminants has been conventionally regulated in different steps to prevent possible routes of infections and diseases. Water cleaning techniques designed in the past solely degrade the bulk of pollutants (Sharma et al.; Crini and Lichtfouse 2019), including coagulation, precipitation, and filtration, these are less effective against dissolved organic matter and bacteria. Several cleaning procedures were intended to disinfect bacteria to prevent diseases (Saqib Ishaq et al. 2019). Some techniques with drawbacks are discussed hereafter.
Chlorination is a widely adopted technique due to less cost and effectiveness against a wide range of microbes. But this method faces many limitations; it makes water taste bad with an unpleasant smell, can’t be applicable at one personal level, and produces reaction byproducts with other water contaminants, which could be toxic (Du et al. 2017; Luo et al. 2021). Sodium hypochlorite (NaOCl) is similar to chlorination, but the handling is relatively tricky. For on-site production of sodium hypochlorite brine gets dilute for electrocatalysis where it produces hydrogen in the electrolytic cell, which is necessary to exhaust to prevent the explosion (Lantagne 2008). Solid calcium hypochlorite (Ca(OCl)2) is more stable, but improper dosage could cause a fire and release toxic gases. Chloramine is another chemical approach for disinfection and is more durable than chlorine and its derivatives. However, its byproducts with organic contaminants may result in health risks.
Ozonation is a well-known decontamination method; this technique uses triplet-oxygen (O3), an unstable form of oxygen, and has rapid decomposition and reaction with water. When gas passes through water, it produces reactive species like hydroxyl radicals that disinfect bacteria via oxidative stress. It has an advantage over other chemical processes because it is easy to handle, and no water tastes change (Saqib Ishaq et al. 2019). But high ozone concentrations are toxic and not stable at normal atmospheric pressure.
Ultraviolet light is widely used as an antimicrobial tool due to no chemical involvement. Short-range wavelengths break the deoxyribonucleic acid and ribonucleic acid (DNA/RNA) of bacteria. Its limitations are high energy consumption and ineffective underwater with deep inside shields because irradiation can’t reach the contaminant across an unclear stream (Black & Veatch Corporation 2009). Using antibiotics, prolonged treatment of water bacteria gradually reduces inactivation efficiency (Arakawa 2020). Gram-positive and gram-negative bacterial species exhibit strong drug resistance (Roberts and Buikstra 2019; Broes et al. 2019) and accelerate the risk of bacterial infections. Antimicrobial resistance diminishes clinical efficacy with increased cost and mortality (Huijbers et al. 2015; Chatzopoulou and Reynolds 2020; Oliver et al. 2020).
New techniques based on advanced oxidation processes proved beneficial due to lower cost, sustainability, and high efficiency against contaminants. The generation of highly reactive oxygen species in response to applied advanced oxidation processes promotes the inactivation of water pollutants (Stefan 2017; Miklos et al. 2018). Nanomaterials have revolutionized new technologies and exhibit significant development in the control and degeneration of emerging contaminants. Modular design and control physiochemical approach made nanomaterials resistant to microbial growth (Zhu et al. 2014).
The catalytic process of newly designed nanomaterials with different morphologies significantly controlled pollutants and restricted drug-resistance development. Many degradation assessments using nanomaterials are advanced oxidation techniques such as photocatalysis. Inorganic materials are investigated widely for the inactivation of both gram-positive and gram-negative bacteria. Metal oxide, metal or sulfides such as silver (Ag), zinc oxide (ZnO) (Ansari et al. 2012), copper oxide (CuO) (Karlsson et al. 2013; Hans et al. 2013; Meghana et al. 2015), magnesium oxide (MgO), titanium dioxide (TiO2) (Allahverdiyev et al. 2011), aluminum oxide (Al2O3) (Ansari et al. 2013), and iron oxide (Fe2O3) (Ismail et al. 2015) revealed antibacterial properties. However, different textured designs that make the surface rough at micron levels achieved surface killing and microbial resistance, which proved beneficial (Cui et al. 2012; Zare et al. 2012; Kim et al. 2016; Ogunsona et al. 2020).
Transition metal sulfides, due to narrow bandgap and better electronic states, exhibit significant reactive oxygen species-oriented degradation of contaminants using the visible spectrum (Ali et al. 2020a; Han et al. 2021). Even though photocatalysis is a renewable and straightforward technique, catalytic materials have selective light absorption and stability limits. It is essential for light irradiation on the semiconductor material's surface to begin the redox reaction (Khare et al. 2021). Advance oxidation process-based decontamination proved beneficial and urged the development of new catalysts that are more functional and reliable.
Piezocatalysis is an emerging technique based on the specific category of materials, i.e., piezo-materials that generate electron-hole pairs in response to applied stress to trigger redox reactions (Bell et al. 2021; Kraśny and Bowen 2021). It is more efficient than other advanced oxidation processes because it could degrade contaminants in the shielded/dark areas using different vibrational sources or tiny external stimuli. Moreover, hybridization significantly enhances the deactivation rate in combination with other primary techniques.
Piezocatalyst is based on a mechanism that can use as a home water remediation tool. It does not produce any odor or taste in water and is safe to use in various conditions. This approach reveals significant degradation efficiencies against water contaminants and bacteria (Joseph et al. 2009; Kanakaraju et al. 2018; Garrido-Cardenas et al. 2019; Tu et al. 2020). Piezocatalytic inactivation of microbes and decontamination of dissolved organic matters significantly contribute to water treatments. Owing to unique structural properties with novel designs could potentially enhance their efficiencies. Therefore, this paper briefly studies recently practiced piezocatalysis to eliminate bacterial water contaminants.
In conclusion, the first section includes water statistics around the world. Water crises are becoming a threat to living beings due to the continuous increase in pollution. Predictions show that the rate of clean water sources drowning will cause most of the world to stress in the coming years. Although many substances pollute water, emerging micro contaminants like organic waste and bacteria are growing threats that trigger diseases. Previously developed methods could not deal with these contaminants. Advanced oxidation techniques such as photocatalysis and piezocatalysis built a new platform to restrain micropollutants sustainably. Moreover, the discussion brings an unidentified relation of bacteria with organic sources into focus to understand the mechanism of heterotrophic growth that could increase the degradation efficiencies.