There is a growing demand for exploiting cost-effective carbon sources for mixotrophic cultivation of microalgae. Fucoidan and alginate were extracted from the brown seaweed Cystoseira trinodis and were subjected to hydrolysis using H2O, HCl, or NaOH for the mixotrophic cultivation of Dunaliella salina. The algal biomass was enhanced to 0.46 g L−1 and biomass productivity reached 41.83 mg L−1 day−1 using alkali hydrolyzed alginate, which was ∼1.6-fold higher than the values under the photoautotrophic conditions. Lipid content was increased to 21.81% w/w using alkali hydrolyzed fucoidan which was 1.78-times higher than the control. Several biodiesel quality parameters were calculated and indicated that mixotrophic cultivation enhanced the biodiesel properties. The biochemical composition showed that amino acid and insoluble and soluble protein productivities were promoted 2-3-folds in relation to the control. Similarly, glycerol and soluble carbohydrate (starch) productivities were increased ∼ 1.4-times, while insoluble carbohydrate productivity was 3.3-times higher than the control. Principal component analysis demonstrated that the starch content of D. salina was reduced in compensatory of lipid accumulation. In addition, the algal growth behavior was statistically fitted using modified logistic model which indicated that lag-phase and maximum growth rate were 0.75–3.89 day and 0.019–0.066 day−1, respectively.

There is a growing demand for exploiting cost-effective carbon sources for mixotrophic cultivation of microalgae. Fucoidan and alginate were extracted from the brown seaweed Cystoseira trinodis and were subjected to hydrolysis using H2O, HCl, or NaOH for the mixotrophic cultivation of Dunaliella salina. The algal biomass was enhanced to 0.46 g L−1 and biomass productivity reached 41.83 mg L−1 day−1 using alkali hydrolyzed alginate, which was ∼1.6-fold higher than the values under the photoautotrophic conditions. Lipid content was increased to 21.81% w/w using alkali hydrolyzed fucoidan which was 1.78-times higher than the control. Several biodiesel quality parameters were calculated and indicated that mixotrophic cultivation enhanced the biodiesel properties. The biochemical composition showed that amino acid and insoluble and soluble protein productivities were promoted 2-3-folds in relation to the control. Similarly, glycerol and soluble carbohydrate (starch) productivities were increased ∼ 1.4-times, while insoluble carbohydrate productivity was 3.3-times higher than the control. Principal component analysis demonstrated that the starch content of D. salina was reduced in compensatory of lipid accumulation. In addition, the algal growth behavior was statistically fitted using modified logistic model which indicated that lag-phase and maximum growth rate were 0.75–3.89 day and 0.019–0.066 day−1, respectively.

The present study investigated the use of algae biorefinery waste and wastepaper in the preparation of cost-effective and eco-friendly xerogels for the removal of congo red (CR) and Fe2+. The xerogel properties such as density, swelling degree and porosity were modified by incorporating alginate extracted from the brown seaweed Cystoseira trinodis. The developed biosorbents exhibited a light and porous network structure and were characterized by a fast uptake of CR and Fe2+ and adsorption efficiency was increased at pH 6–8. The equilibrium adsorption capacity was found to be 6.20–7.28 mg CR g−1 biosorbent and 8.08–8.39 mg Fe2+ g−1 biosorbent using different xerogels. The adsorption of CR obeyed first-order kinetics, while, Fe2+ followed second-order kinetics. Intraparticle diffusion model suggested a boundary layer effect. The adsorption capacity was maximally obtained as 41.15 mg g−1 and 169.49 mg g−1 for CR and Fe2+ using wastepaper/Spirulina and wastepaper/alginate/Spirulina xerogel, respectively. Temkin isotherm fitted better to the equilibrium data of CR adsorption than Langmuir and Freundlich models. While, equilibrium data of Fe2+ exhibited a best fit to both Langmuir and Freundlich models. Additionally, the Dubinin-Radushkevich isotherm suggested that adsorption mechanism of CR or Fe2+ is predominately physisorption. Investigation of thermodynamic parameters such as ΔH° and ΔS° and ΔG° confirmed the feasibility, spontaneity, randomness and endothermic nature of the adsorption process. Electrostatic attraction, H-bonding and n-π interactions were mainly involved in the biosorption process of CR. The results of this study showed that the developed xerogels could be effectively applied for dye and heavy metal removal at low concentrations.

The present study investigated the use of algae biorefinery waste and wastepaper in the preparation of cost-effective and eco-friendly xerogels for the removal of congo red (CR) and Fe2+. The xerogel properties such as density, swelling degree and porosity were modified by incorporating alginate extracted from the brown seaweed Cystoseira trinodis. The developed biosorbents exhibited a light and porous network structure and were characterized by a fast uptake of CR and Fe2+ and adsorption efficiency was increased at pH 6–8. The equilibrium adsorption capacity was found to be 6.20–7.28 mg CR g−1 biosorbent and 8.08–8.39 mg Fe2+ g−1 biosorbent using different xerogels. The adsorption of CR obeyed first-order kinetics, while, Fe2+ followed second-order kinetics. Intraparticle diffusion model suggested a boundary layer effect. The adsorption capacity was maximally obtained as 41.15 mg g−1 and 169.49 mg g−1 for CR and Fe2+ using wastepaper/Spirulina and wastepaper/alginate/Spirulina xerogel, respectively. Temkin isotherm fitted better to the equilibrium data of CR adsorption than Langmuir and Freundlich models. While, equilibrium data of Fe2+ exhibited a best fit to both Langmuir and Freundlich models. Additionally, the Dubinin-Radushkevich isotherm suggested that adsorption mechanism of CR or Fe2+ is predominately physisorption. Investigation of thermodynamic parameters such as ΔH° and ΔS° and ΔG° confirmed the feasibility, spontaneity, randomness and endothermic nature of the adsorption process. Electrostatic attraction, H-bonding and n-π interactions were mainly involved in the biosorption process of CR. The results of this study showed that the developed xerogels could be effectively applied for dye and heavy metal removal at low concentrations.

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