Microalgae in aquatic environs: A sustainable approach for remediation of heavy metals and emerging contaminants
Graphical abstract
Introduction
Water is one of the most vital natural resource necessary for the sustenance of life on the earth (Gleick, 2000, Mushtaq et al., 2020). However, unplanned establishment of industries and industrialization (Kollmuss and Agyeman, 2002, Walker et al., 2019), urbanization (Chan, 2006), higher living standards and population explosion deteriorate the quality as well as quantity of water (Goel, 2006). Pollution of freshwater resource is currently the burning issue worldwide (Abdel-Raouf et al., 2012). Discharge of untreated wastewater into fresh water environs from different point and non-point sources viz., cities, industries and agricultural units etc., degrade the quality of concerned water bodies (Lim et al., 2010, Bhat et al., 2018, Mushtaq et al., 2020). Due to improper discharge of wastewater, the available sources of water are shrinking in the major part of the world (Edokpayi et al., 2017). Generally, wastewater is rich in nitrogen and phosphorus (Bhat et al., 2017, Solovchenko, 2019), heavy metals, pesticides, endocrine disruptors and pathogens (Ahmad et al., 2019, Sarkar et al., 2019). Besides, it contains some emerging concerned contaminants (ECC) and organic contaminants (polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAHs)). These contaminants have detrimental effects on recipient aquatic environs (Okoh et al., 2007) viz., high nutrient load (eutrophication), hyphoxia (Al-Gheethi et al., 2013) and loss of aquatic biota (Paerl et al., 2018, Nagarajan et al., 2019). Therefore, treatment of wastewater prior to discharge into aquatic environs is crucial for the safeguard of precious life on earth (Oh et al., 2018, Khattiyavong and Lee, 2019). With the advancement, various conventional and new techniques have been employed but still have certain drawbacks like low efficiency, high maintenance and operational cost. Nowadays, microalgae are gaining rapid interest owing to dual role of wastewater remediation and useful biomass production (Kadir et al., 2018). Growing microalgae in wastewater can play an important role in reducing the production cost of biofuel by using wastewater as a source of nutrients (Abinandan et al., 2018). Biofuel extracted from microalgal biomass could be valuable substitute of energy in lieu of fossil fuels (Randolph and Masters, 2018).
Microalgae are the oxygen evolving organisms that can assimilate large amount of nutrients (carbon, nitrogen and phosphorus) from wastewater for their growth and development (Supeng et al., 2012). They are also capable to grow in varied climatic and environmental conditions (Vernes et al., 2019), thus providing an alternative option for wastewater treatment (Kadir et al., 2018). Remediation of wastewater with the help of algae is economically efficient and environmentally proficient for wastewater treatment (Cheah et al., 2016). Phycoremediation approach of wastewater treatment can enhance the availability of dissolved oxygen, promote extraction of nutrients as well as toxic pollutants (Tibbetts et al., 2015). Micro-organisms (microalgae) are known as “biological purifiers” because of their intrinsic phenomenon of not only extracting and accumulating nutrient but also sorption of various toxic and persistent pollutants from the wastewater (De-Bashan and Bashan, 2010). Microalgae can also easily fix carbon dioxide from the atmosphere thus help in the sequestration of carbon dioxide (Qiu, 2019) with simultaneous remediation of wastewater and biofuel production (Bhatia et al., 2019).
The aim of an effective and sustainable wastewater treatment can be fulfilled by not only focusing on removal of macronutrients and heavy metals from the contaminated environment (Bilal et al., 2018; Wang et al., 2017) but also the remediation of several other toxic pollutants like pesticides, emerging contaminants which even in minute quantity can have serious impacts on human health and freshwater ecosystems (Shahid et al., 2019). Thus, holistic approach is the necessity of current time in order to enhance the freshwater availability for the swiftly growing global population. The main of this review is to focus on effective role of microalgae not only in stabilizing the physico-chemical characteristics of wastewater but also on the exclusion of myriad of pollutants (heavy metals, pesticides and emerging contaminants) which are commonly discharged from several industries. Exploiting microalgae for the treatment of wastewater of different origin is beneficial paradigm in each way that can boost the effectiveness of treatment plants through tapping nutrient load by microalgae and biomass obtained after culturing can become the feedstock for multiple products have multifarious roles in diverse industries.
Section snippets
Wonders of microalgae
Microalgae are microscopic, photosynthetic, fast growing aquatic organisms (Suthers et al., 2019). These organisms inhibit wide range of habitats; freshwater, brackish and saline water (Neto and Pinto, 2019) and can be cultured without competing with arable land resources (Bhagea et al., 2019). The most important feature of microalgae is to produce oxygen by the process of photosynthesis and is known to be responsible for more than 50% of the total oxygen production on the biosphere (Chew et
Colour
Wastewater generated from various industries contain the toxic colouring agents (Benit and Roslin, 2015, Libralato et al., 2012). Release of such kind of pollutants has significant impact on the physico-chemical and biological properties of the recipient water environs (Amin et al., 2008). Effluents containing high organic load and dark coloured water shield the sunlight necessary for phytoplankton and other photosynthetic organisms. Microalgae have great ability to detoxify phenolic and
Advantages of microalgae based wastewater treatment over other techniques
Ultraviolet disinfection (Gibson et al., 2017), flocculation(Wang and Chen, 2009), ultrafiltration(Te Poele and Van der Graaf, 2005), chemical precipitation (Rajasulochana and Preethy, 2016a, Rajasulochana and Preethy, 2016b), electro-coagulation(Tahreen et al., 2020), activated charcoal (Wang and Chen, 2009), ion exchange (Rajasulochana and Preethy, 2016a, Rajasulochana and Preethy, 2016b), reverse osmosis (Vinardell et al., 2020) are the major techniques employed for the treatment of
Challenges and future prospects
Although, several benefits ranging from stabilization of physico-chemical attributes to biofuel production are associated with microalgal cultivation in wastewater but numerous risks in the form of pathogenic contamination, highly coloured water, exposure to heavy metals and emerging contaminants and high variable nutrient concentration linked with wastewater condition can obliterate the functioning of microalgae. The essential attributes of microalgae must be able to effectively adapt under
Conclusion
Wastewater contains abundant nutrients, heavy metals and recalcitrant persistent pollutants. Conventional treatment approaches are not capable of eliminating the contaminants effectively in wastewater. The complexity and composition of wastewater are putting more pressure in conservation of aquatic systems. Thus causing imbalance in the functioning of concerned aquatic ecosystems. Therefore, need of an hour is to stabilize the perturbed aquatic ecosystems by employing the techniques based on
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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