Buoy-bead flotation application for the harvesting of microalgae and mechanistic analysis of significant factors

A novel approach for harvesting the microalgae Chlorella vulgaris with sodium alginate microspheres using buoy-bead flotation method

In order to reduce the residue of buoy-beads and solve the problem of pollution caused by high flocculant consumption, SAMs¹(sodium alginate microspheres) with sodium alginate were used as the raw material to harvest microalgae for the first time. In addition, during the manufacturing of SAMs, the re-frying oil was used as the dispersion system, which not only reduced the cost, but also provided new ideas for the treatment of re-frying oil. Response surface methodology was used to explore the influence of different factors and the interaction of variables, and the harvesting process was optimized using the multi-objective optimization. Based upon the calculation of XDLVO (extended Derjaguin-Laudau-Verwey-Overbeek) theory and the characterization of Fourier Transform Infrared Spectroscopy, the harvesting mechanism of buoy-bead flotation method was clarified. The results showed that the combination of SAMs and a small amount of aluminum sulfate could replace air flotation and traditional buoy-bead flotation with solid particles as buoy-beads to harvest C. vulgaris (Chlorella vulgaris). For the multi-objective optimization with harvesting efficiency as the priority, the predicted pH, the concentrations of aluminum sulfate and buoy-beads and the dilution factor had values of 8.25, 56.09 mg/L, 17.46 mL/L, and 2.15, respectively. In the validation experiment, the harvesting efficiency and the enrichment ratio of C. vulgaris could reach the values of 97.51 % and 1.97 %, respectively. For the validation experiment of reverse optimization with focusing on enrichment ratio, the harvesting efficiency and the enrichment ratio of C. vulgaris had the values of 93.78 % and 2.65 %, respectively. The essence of improving the harvesting mechanism was the combination of carboxyl and hydroxyl groups between C. vulgaris and SAMs and the adsorption of positive ions by specific proteins on the surface of C. vulgaris to reduce electrostatic repulsion.

Removal of harmful algal blooms in freshwater by buoyant-bead flotation using chitosan-coated fly ash cenospheres

Harmful algal blooms (HABs) are a growing problem worldwide, damaging human and ecosystem health. In this study, a novel buoyant-bead flotation (BBF) method using chitosan-coated fly ash cenospheres (CFACs) was developed to remove HABs in freshwater. To achieve a high removal efficiency of harmful algae (Chlorella vulgaris, Scenedesmus quadricauda, and Microcystis aeruginosa), this study investigated the effects of chitosan/fly ash ratios in CFAC composite, CFAC concentration, flotation time, and pH values on the microalgae removal. The optimized ratio of CFACs is 0.1:12, and the optimized CFAC concentration is 0.3-0.7 g L^-1. However, the lower or higher ratios (0.1:4, 0.1:8, 0.1:16) result in microalgae reaching a zero-point charge too late or early, which failed to effectively remove HABs with an appropriate coal fly ash dosage. An optimized removal efficiency of 98.50% for Microcystis aeruginosa was reached at pH of 6.0. The optimized efficiency of Scenedesmus quadricauda and Chlorella vulgaris was 99.37% and 91.63%, respectively, at pH of 8.0. At neutral pH conditions, the surface charge of microalgae cells and CFACs are different, promoting aggregate formation. When CFACs were used to remove microalgae, aggregate size significantly influenced removal efficiency. Meanwhile, at the optimized pH and concentration, the removal efficiency of all three algal species exceeded 90.00% in 5 min. The study highlights an efficient and inexpensive method for removing HABs and obtains the optimized operational conditions.

Efficient microalgae harvesting using a thermal flotation method with response surface methodology

Thermal pre-flocculation to enable dispersed air flotation is an economical and ecofriendly technology for harvesting microalgae from water. However, the underlying mechanism and optimal conditions for this method remain unclear. In this study, Chlorella vulgaris (C. vulgaris) and Scenedesmus obliquus (S. obliquus) were harvested using a thermal flotation process. The surface structure and characteristics (morphology, electricity, and hydrophobicity) of the microalgae were analyzed using FT-IR (Fourier transform infrared spectroscopy), SEM (scanning electron microscopy), zeta potential, and a hydrophobic test. Further, response surface methodology (RSM) was used to optimize the flotation process. The hydrophobicity of S. obliquus exceeded that of C. vulgaris; as such, under the thermal pre-flocculation, S. obliquus (88.16%) was harvested more efficiently than C. vulgaris (47.16%). Thermal pre-flocculation denatured the lipids, carbohydrate, and proteins of microalgal cell surfaces. This resulted in a decrease in the electrostatic repulsion between the cells and air bubbles. The highest harvesting efficiency was 91.96% at 70 °C, 1,412 rpm, and 13.36 min. The results of this study demonstrate the potential for economic and ecofriendly harvesting of microalgae for biofuels and other bioproducts industries.