ReviewAlgal-based removal strategies for hazardous contaminants from the environment – A review
Graphical abstract
Introduction
Emerging contaminants (ECs) are defined as naturally occurring, manufactured or manmade chemicals or materials found in the environment whose toxicity or persistence are likely to affect the metabolism of a living being significantly. This includes active ingredients from numerous industrial sectors such as pesticides, pharmaceuticals, hormones, flame retardants, nanoparticles and so on (Sauvé and Desrosiers, 2014; Bilal et al., 2019; Rasheed et al., 2019). Currently, there is a great variation in the elimination of ECs from approximately 12.5 to 100% in conventional wastewater treatment plants (WWTP) (Luo et al., 2014).
For the most part, the pharmaceutical contaminants cannot efficiently remove in conventional wastewater treatment plants (WWTPs), basically designed to remove organic matter and nutrients (Ahmed et al., 2017; Clara et al., 2005; Franka et al., 2016). The elimination of organic contaminants depends on treatment conditions of the elimination process (e.g., hydraulic residence time and the organic loading rate) and environmental conditions (sunlight, temperature, redox conditions, pH and presence of toxic compounds such as heavy metals) (Luo et al., 2014). However, the physicochemical properties of these compounds (e.g., hydrophobicity and biodegradability) are the reason why they are found in the environment (Topp et al., 2013), and this is a problem since long-term exposure of even low concentration of certain pharmaceuticals can harm water organism such as bacteria, algae, fish and plants (Klavarioti et al., 2009). Some acute and chronic damages to water organisms are: change of behavior (Stanley et al., 2007; Gaworecki and Klaine, 2008), accumulation of pharmaceuticals contaminants in tissues (Brooks et al., 2003), damaging of the reproductive system (Nentwig, 2007) and inhibition of cell proliferation (Fabbri and Franzellitti, 2016; Jennifer et al., 2017). This does not seem to be a local problem as a significant number of pharmaceuticals have been found in all over the world. This includes but is not limited to (1) Canada (Chen et al., 2015), (2) China (Xiang et al., 2018), (3) India (Rutgersson et al., 2014), (4) Mexico (Gibson et al., 2010) and (5) Scotland (Nebot et al., 2015).
There are several methods to make sure ECs do not enter the environment (Fig. 1). This paper focusses on the removal processes by biological degradation of ECs present in the industrial wastewater. Mainly microalgae responsible for the degradation process of ECs are deeply discussed. This paper focusses on the pharmaceutical-based ECs such as ciprofloxacin, metoprolol, tramadol, triclosan, salicylic acid, carbamazepine, levofloxacin and trimethoprim as most studied in recent years. Fig. 2 illustrates algal-based bioreactors/strategies to tackle or remove pharmaceutical-based ECs.
Section snippets
Strategies to remove pharmaceutical-based ECs
The list of pharmaceutical-based ECs detected in urban WWTPs include antibiotics, anti-inflammatory, antidepressants, analgesics, anti-epileptics, anxiolytics, beta-blockers, blood lipid regulators, contrast media, cytostatic and hormones, especially oral contraceptives (Choubert et al., 2009). Various contaminants along with their potential environmental risks and susceptibility to the removal process are summarized in Table 1. The limited removal of pharmaceutical contaminants through the
Bioreactors - open pond
Open ponds are artificial pools with limited depth (around 0.03–0-07 m) and used to grow microalgae without stirring (Meneses-Jácome et al., 2016). This type of bioreactors are usually used in industrial and agricultural wastewaters treatment (Christenson and Sims, 2011; Norvill et al., 2016, Norvill et al., 2017; Park et al., 2011; Pessoa, 2012; Rogers et al., 2014; Rühmland et al., 2015; Zhu et al., 2014). The advantages of open ponds system for microalgae cultivation are the low cost of
Bubble column photobioreactors
Tubular photobioreactors (PBRs) are closed systems (Fig. 4) (Singh and Sharma, 2012), mainly designed in vertical, horizontal and helical shape, this PBRs consists of transparent tubes that allow the penetration of natural or artificial light (Christenson and Sims, 2011). Closed PBRs provide a closely controlled environment with the aim to isolate the microalgal strain, guaranteeing the increase in productivity, quality of the biomass and wider choice of strains can be explored. These
Research gaps
The research gaps can be divided into three different categories. First of all, it's necessary to test more algae species on their ability to remove the contaminants. In this aspect, special attention must be paid to slow kinetics for the removal of some contaminants by microalgae. It is necessary to find a compromise solution to establish hydraulic retention times long enough to carry out the degradation of the emerging and economically feasible contaminants from the point of view of reactor
Concluding remarks and future perspectives
Pharmaceuticals as ECs show danger to our environment which calls for methods to remove them in wastewater treatments. Algae mediated removal is one of the possible ways to remove the pharmaceuticals. In laboratory scale high removal percentages have been reached for ciprofloxacin, metoprolol, tramadol, triclosan, and salicylic acid, carbamazepine and trimethoprim show fewer promising results with only moderate removal. The results collected look promising, especially the lab-scale studies
Acknowledgment
All authors are grateful to their representative institutes/universities for providing literature facilities.
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