Performance evaluation of different chitosan-clay composite materials for efficient harvesting of Chlorella vulgaris and impacts on downstream bioproduct processing and water reusability

Abstract As an environmentally friendly bioflocculant, chitosan has a great application potential in the field of microalgae harvesting due to its biocompatibility. In this work, three different inorganic clay minerals (kaolin, montmorillonite, and diatomite) were selected to prepare chitosan-clay composite materials (CTS/KAL, CTS/MML and CTS/DTE) as flocculants to harvest microalgae cells. The harvesting capacities of CTS/KAL, CTS/MML and CTS/DTE were accordingly 97.37%, 97.81% and 98.34% for microalgae cells under the dosage of 60 mg/L, microalgae concentration of 0.923 g/L, settling time of 30 min and pH of 10. CTS/DTE showed excellent harvesting performance for microalgae cells, mainly due to the enhancement of charge neutralization, sweeping and bridging effects. Based on the phenomenological theory, the second-order polynomial model was used to analyze the influences of different external factors on the microalgae harvesting efficiency. A reasonable performance relationship between CTS-clays for harvesting of microalgae cells and external factors was established. The analysis of carbohydrates, proteins, and fatty acids of microalgae biomass collected by CTS/KAL, CTS/MML, CTS/DTE and centrifugation showed that the harvesting of CTS-clay did not influence the properties of biomass. The experimental results of water resource recycling and re-cultivation after microalgae harvesting by CTS/KAL, CTS/MML and CTS/DTE exhibited that the growth rates of 50% circulating medium + 50% fresh BG11 culture medium were accordingly 2.512, 2.466 and 2.572 times higher than those of 100% circulating medium. It is concluded that chitosan-clay composite materials are efficient and environmentally friendly flocculants with a great applicable potential in microalgae harvesting.