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The amount of heavy metals released into the environment has significantly increased. Industrial wastewaters, e.g. from mining or battery manufacturing, are often polluted with heavy metals such as Cd, Cr or Pb. These metals threat the environment and can cause health problems even at low concentrations. Therefore, their proper removal from industrial wastewater before its disposal is of paramount importance (Javanbakht et al. [1]). Here the ability of fourteen wild Nordic microalgal strains to remove cadmium (Cd(II)) from aqueous solutions has been studied. Three of the chosen strains, namely Chlorella vulgaris (13-1), Coelastrella sp. (3-4) and Scenedesmus obliquus (13-8), demonstrated high tolerance towards Cd(II) concentrations up to 2.5 mg L⁻¹ and their sorption kinetics and equilibrium were studied. Metal sorption by Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) was described best by pseudo-second order kinetics, whereas the removal kinetics of Scenedesmus obliquus (13-8) was best fitted by the intraparticle diffusion model. Starting from an initial concentration of 2.5 mg L⁻¹ Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) removed 72% and 82%, respectively, of the Cd(II) within only 24 h. Modeling their Cd(II) sorption equilibria revealed that the SIPS- and Dubinin-Radushkevich models were best suited for living microalgae, and the maximum adsorption capacity (q_max) was calculated. While Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) were able to remove about 49 mg g⁻¹ and 65 mg g⁻¹ Cd(II), respectively, Scenedesmus obliquus (13-8) only removed around 25 mg g⁻¹. Fourier-Transform Infrared Spectroscopy (FTIR) analyses of the biomass revealed the carboxylic moieties of the cell wall to be the key player in Cd(II) removal. This study demonstrates the high potential of Nordic microalgae to remove heavy metals at conditions relevant for an industrial tertiary wastewater treatment unit and will support the development of new, biobased, innovative technologies for the bioremediation of heavy metal polluted streams.

Microalgae are promising candidates for sustainable wastewater treatment coupled to the production of biofuel, bioplastic and/or bio-fertilizers. In Nordic countries, however, light is a limiting factor for photosynthesis and biomass production during the winter season. Compared to municipal wastewater, industrial wastewater streams from the pulp and paper industry contain lower amounts of nitrogen, but high concentrations of carbon sources, which could be utilized by microalgae to enhance biomass production in limiting light. This study focused on the utilization of methanol, glycerol and xylose by five different Nordic microalgae [Chlorella vulgaris (13–1), Coelastrella sp. (3–4), Desmodesmus sp. (2–6), Chlorococcum sp. (MC1) and Scotiellopsis reticulata (UFA-2)] grown under mixotrophic conditions. Two of these strains, i.e., Chlorococcum sp. (MC1) and Scotiellopsis reticulata (UFA-2) were able to grow in the presence of xylose or methanol at concentrations of 6 g L^–1, or 3%, respectively, in a 12/12 h day/night cycle. HPLC analysis confirmed the consumption of those substrates. Glycerol (2.3 g L^–1) was tolerated by all strains and increased growth for Chlorella vulgaris (13–1), while higher concentrations (20 g L^–1) were only tolerated by Chlorococcum sp. (MC-1). Fourier-transform infrared spectroscopy, performed after growth in presence of the dedicated carbon source, indicated an increase in the fingerprint region of the carbohydrate fraction. This was particularly the case for Chlorococcum sp. (MC1), when grown in presence of glycerol, and Scotiellopsis reticulata (UFA-2), when grown in presence of xylose. Therefore, these strains could be potential candidates for the production of biofuels, e.g., bioethanol or biogas. We could show that Nordic microalgae are able to grow on various carbon sources; the actual uptake rates are low during a 12/12 h day/night cycle requesting additional optimization of the cultivation conditions. Nonetheless, their potential to use pulp and paper waste-streams for cheap and sustainable biomass production is high and will support the development of new technologies, turning waste-streams into resources in a circular economy concept.

Biomass from four different Nordic microalgal species, grown in BG-11 medium or synthetic wastewater (SWW), was explored as inexpensive carbohydrate-rich feedstock for polyhydroxybutyrate (PHB) production via microbial fermentation. Thermochemical pre-treatment (acid treatment followed by autoclavation) with 2% hydrochloric acid or 1% sulphuric acid (v/v) was used to maximize sugar yield prior to fermentation. Pre-treatment resulted in ∼5-fold higher sugar yield compared to the control. The sugar-rich hydrolysate was used as carbon source for the PHB-producing extremophilic bacterium Halomonas halophila. Maximal PHB production was achieved with hydrolysate of Chlorococcum sp. (MC-1) grown on BG-11 medium (0.27 ± 0.05 g PHB/ g DW), followed by hydrolysate derived from Desmodesmus sp. (RUC-2) grown on SWW (0.24 ± 0.05 g PHB/ g DW). Nordic microalgal biomass grown on wastewater therefore can be used as cheap feedstock for sustainable bioplastic production. This research highlights the potential of Nordic microalgae to develop a biobased economy.

Heavy metals in industrial wastewaters are posing a serious threat to the environment and to human health. Microalgae are increasingly being seen as potential solutions to this problem as they can remove pollutants through biosorption. This process offers certain advantages over other more traditional metal removal techniques as it is simple, inexpensive, eco-friendly, and can be performed over a wide range of experimental conditions. Biosorption is possible due to the unique and complex structure of the microalgal cell wall. The variety of functional groups on the surface of the cell wall (such as carboxyl or amino groups) can act as binding sites for the heavy metals, thus removing them from the environment. This review focuses on the cell wall composition and structure of the most commonly used microalgae in heavy metal removal and shows the role of their cell wall in the biosorption process. This review also aims to report the most commonly used models to predict the velocity of microalgal biosorption and the removal capacities.

The growth of the world's population increases the demand for fresh water, food, energy, and technology, which in turn leads to increasing amount of wastewater, produced both by domestic and industrial sources. These different wastewaters contain a wide variety of organic and inorganic compounds which can cause tremendous environmental problems if released untreated. Traditional treatment systems are usually expensive, energy demanding and are often still incapable of solving all challenges presented by the produced wastewaters. Microalgae are promising candidates for wastewater reclamation as they are capable of reducing the amount of nitrogen and phosphate as well as other toxic compounds including heavy metals or pharmaceuticals. Compared to the traditional systems, photosynthetic microalgae require less energy input since they use sunlight as their energy source, and at the same time lower the carbon footprint of the overall reclamation process. This mini-review focuses on recent advances in wastewater reclamation using microalgae. The most common microalgal strains used for this purpose are described as well as the challenges of using wastewater from different origins. We also describe the impact of climate with a particular focus on a Nordic climate.