Research articlePhosphorus and metal removal combined with lipid production by the green microalga Desmodesmus sp.: An integrated approach
Introduction
Even in regions where specific integrated policies for the protection of water environments have been established, such as the EU's Water Framework Directive (WFD) (2000/60/EC), the quality of surface water bodies is still hampered by anthropogenic activities causing diffuse and/or point-source emissions of both organic and inorganic pollutants. Indeed, almost twenty years after the adoption of the WFD, 47% of EU surface waters did not reach a “good ecological status” (Voulvoulis et al., 2017). The presence in the water of various organic, as well as inorganic nutrients such as nitrogen (N) and phosphorus (P), can lead to eutrophication, while the presence of metals and metalloids, due for example to mining operations, both for energy production and consumer goods (Torres et al., 2017), may lead to potential risks to human and ecosystem health. Some metals are micronutrients necessary for living organisms (e.g. Zn, Cu, Mn, Ni, and Co), while others have unknown biological functions (e.g. Cd, Pb, and Hg). Conventional physico-chemical methods for the treatment of wastewater containing high concentrations of nutrients and metals prove often ineffective or require high energy input, capital investment and operational costs (Chan et al., 2013). Phycoremediation is a process defined as the utilization of microalgae in the treatment of polluted wastewater (Jais et al., 2017). This process contributes significantly to the removal of nutrients and metals from wastewater, especially for pollutant concentrations between 1 and 100 mg L−1, where chemical and physical methods such as chemical precipitation, electrolytic recovery, adsorption/ion exchange, solvent extraction and membranes are not fully satisfactory (Jais et al., 2017). Green microalgae remove N and P from wastewater through assimilation, while metal ions are removed through ‘biosorption’ (involving both adsorption and absorption) as defined by Gadd (2008). Phycoremediation technologies present an additional asset besides wastewater treatment, i.e. they lead to the production of microalgal biomass (Gupta et al., 2016, Gupta et al., 2017, Yang et al., 2015). Such biomass has gained an increasing interest due to its great potential for different biotechnological applications in the fields of energy, nanotechnology and environment (Bruno et al., 2012, De Angelis et al., 2016, Di Pippo et al., 2013, Gismondi et al., 2016). In particular, microalgal biomass produced during wastewater treatment can be used as feedstock for the production of a variety of biofuels such as biodiesel, bio-methane, ethanol, hydrogen, etc. (Chisti, 2007, Gupta et al., 2016). During the past few decades, biofuels have attracted tremendous attention due to limited stock of fossil fuels, and the necessity to reduce the continuously increasing greenhouse gas emissions contributing to climate change (Gupta et al., 2017). The development of carbon-neutral biofuel is generally based on two primary concerns: environmental sustainability and economic viability. Only algal biodiesel has been estimated to present the potential to fulfil the global requirement of biofuels for transport (Chisti, 2007, Chisti, 2008) with little impact on the carbon footprint. Wastewaters provide a sustainable means for microalgal biofuel production; however, not all algae can survive in these harsh and extreme environments. Even if suitable, different environmental stresses related to polluted wastewater, especially the toxicity caused by metals, would significantly affect the growth of the algae (Torricelli et al., 2004, Yang et al., 2015, Kumar et al., 2015) biomass production and lipid yield, as well as high production costs. Only a few works tried to combine lipid production with both P and metal removal, but these studies were limited to the construction of system dynamics models and the prediction of lipid production (Richards and Mullins, 2013). Based on a literature survey there is a lack of studies that address and quantify the impact of individual contaminants on microalgae grown in wastewater with the aim of identifying promising feedstock candidates for biofuel production (Yang et al., 2015, Jais et al., 2017). Thus, this work focused on the potential of the green microalga Desmodesmus sp. to be used for bioremediation of wastewater laden with P, copper (Cu) and nickel (Ni). Moreover, the effects of Cu and Ni on lipid accumulation for biodiesel production were evaluated. This approach could reduce the cost of algal biofuel by increasing the intrinsic algal biomass value with the ultimate purpose to find a cost-effective and eco-friendly method for biofuel production and wastewater bioremediation.
Section snippets
Microalgal strain
The strain of Desmodesmus sp. was isolated by serial dilutions and plate streaking from the outflow of a secondary sedimentation tank of a municipal wastewater treatment plant (WWTP) located south of Rome (Italy), where P concentration of the treated water was between 1 and 6 mg L−1. The sludge of the secondary sedimentation tank where Desmodesmus sp. was isolated from contained the following concentrations of copper (509 mg kg−1), nickel (21 mg kg−1), chromium (29 mg kg−1), cadmium (3 mg kg−1)
Growth curves and P removal
Several studies demonstrated the potential of using microalgae isolates from WWTPs and other hypereutrophic systems to remove contaminants (Rugnini et al., 2017, Samorì et al., 2013). This may be related to the fact that microalgal isolates obtained from polluted environments (such as the strain of Desmodesmus sp. employed in this study) might be more suitable for nutrients and metal removal than those grown in otherwise clean environments, exhibiting a great potential to tolerate and remove
Conclusions
The results reported in this study suggest that Desmodesmus sp. could be successfully used for wastewater bioremediation of phosphorus and metal removal. Biomass obtained as by-product could be further employed as metal adsorbent and as feedstock for biofuel production. In these growth systems, Desmodesmus sp. was able to remove more than 90% of total phosphorus and demonstrated a good biosorption ability for both Cu and Ni by living biomass, with a removal efficiency around 90% in less than 2
Contributions
RL performed the experiments and was the primary author involved in writing the original draft of the paper; CG and SA carried out the metal investigation and lipid analyses, respectively; RL, CG, and BL were involved in the conceptualization of the paper; CR, LSdT and BL were responsible of funding acquisition. All authors contributed to the review and editing of the paper, providing helpful comments and discussion.
Acknowledgement
This research was partially supported by Italian Ministry of University and Scientific Research (Miur, PRIN 2015, Prot. 20158HTL58).
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2022, Journal of Environmental Chemical EngineeringCitation Excerpt :In this context, heavy metals induced a stress protein production that would shift starch production to lipid production, making this a viable option for biodiesel production [40,41,96]. Microalgae exposed to heavy metal samples yielded higher biodiesel quantity and quality than that grown without metals [81]. Given the importance of lipids in economically feasible biodiesel production, it is important to define the most suitable heavy metal concentration and exposure time to achieve high lipid yields in microalgae [40].