Kulak, Renewable Materials and Nanotechnology Research Group
Abstract Bachelor project 2020-2021: Modification of cellulose with amino acids and derivatives to flocculate microalgae
Microalgae are a new source of biomass with many possibilities such as the synthesis of chemicals, nutrients, and biofuels. Currently many applications of microalgae are not exploited since it is too expensive to grow and harvest the microalgae. It is possible to reduce the cost of harvesting of microalgae by using flocculants. These flocculants can aggregate several microalgae in a suspension so a bigger mass that will sink to the bottom is formed. When using flocculants, the need for expensive separation methods during the harvesting of microalgae is lowered.
This research attempts to synthesize new flocculants for microalgae which can make the harvesting of the microalgae cheaper.
The flocculants are produced by first hydrolyzing cellulose with an acid. After the hydrolysis, the produced cellulose nanocrystals (CNCs) are modified with amino acids or derivatives so that they are given a positive charge. During the modification, an ester should be formed between the CNCs and the amino acids or derivatives. The positive charge given to the CNCs enables them to interact with the negatively charged cell wall of the microalgae. In this research the modification is attempted in HCl vapors, dry sulfolane and dry pyridine for several amino acids and derivatives.
Neither the modification in dry sulfolane nor in HCl vapors was successful, but there was some success with the modification in dry pyridine. In a pyridine solution the CNCs were successfully modified with betaine HCl and N,N-dimethylglycine HCl. Unfortunately, only the CNCs modified with betaine HCl were able to flocculate the tested microalgae. The modifications in pyridine with other amino acids and derivatives such as N-methylglycine, arginine and histidine did not result in an ester. The CNCs modified with these compounds are not given a positive charge. During the modification with glycine and lysine amides were formed. It is possible that along with the amides esters were formed, but in such a small quantity that they are not easily detected. Because of this modification it is possible to build chains of amino acids on the CNCs. There is a chance that this modification also occurs when using histidine however these results are not yet clear.
Currently only CNCs modified with betaine HCl in a pyridine solution can be used to flocculate microalgae. This method might also prove useful with the glycine derivative N,N-dimethylglycine after some more research. When using this method with the amino acids glycine and lysine it is possible to build polymers of amino acids on the CNCs. This modification might also be possible when using histidine.
Abstract Bachelor project 2017-2018: Synthesis and characterization of functionalized nanocellulose for the flocculation of microalgae
With the world population growing exponentially, the global demand for biomass is expected to increase in the future. Several studies have shown that microalgae could be considered as a promising new sources of biomass. The biomass of algae can been seen as a possible way to solve the food and energy crisis. But a major challenge is separating the microalgae from their growth medium. Methods available today are still not on point, because they need too much energy, contaminate the harvested biomass and/or are very expensive. Therefore new low-cost methods are needed.
The aim of this study is the production of positive charged nanocellulose as new kind of flocculants, which can be used to harvest microalgae. Acid hydrolysis treatment of cotton wool results in cellulose nanocrystals (CNCs). Nanocellulose, covered with hydroxyl groups, provide an ideal platform for modification. During modification, hydroxyl groups are being replaced by positive chemical groups. The resulting positive charged nanocellulose is able to interact with the negative surface of microalgae to form flocs and are allowed to sediment. In this work, modification is performed by adding betaine (HCl) or imidazole on the surfaces of nanocellulose.
To achieve optimal flocculants, modification protocols needs optimization, by experimenting with parameters such as the temperature, time and stoichiometry of the chemicals. The flocculants are analyzed with several techniques like the Fourier-Transform infrared spectroscopy, thermal gravimetric analysis, element analyzer and zeta-potential. The quantification of modification is presented in terms of degree of substitution (DS). A flocculant is expected better when modified CNCs have a large DS, without damaging the structure. Finally, all flocculants were tested for flocculation of the fresh water microalgae Chlorella vulgaris.
CNCs with betaine (HCl) on the surface were positive charged and were independent of the pH. Imidazole grafted CNCs were shown to have a pH dependent surface charge which was in general positive below pH 7.5 and negative above pH 9. The best betaine grafted CNC had a maximum flocculation of 95.12 % with a dose of 40 mg/L. The flocculation efficiency of imidazole grafted CNCS was also tested, but the maximum flocculation was very low.
Both types of modified CNCs were successfully synthesized, but didn’t have a large DS. It seems that highly charged CNCs are excellent flocculants for Chlorella vulgaris. The DS turns out to have an influence on the flocculation. An increase of DS results in a decrease in dosage. The main of future experiments is the improvement in level of modification, repeat the flocculation test of imidazole grafted CNCs, remove imidazole grafted CNCs out of the harvested biomass.