An improved method for determining optical extinction characteristics of microalgae

Abstract

The urgent demand to replace traditional fossil fuels has led to emergence of biofuels as feasible alternatives. Microalgae are considered to be one promising alternative source for biofuels. Light is regarded as one of the most vital factors which affecting the photosynthetic activity. Photobioreactors are used for cultivating microalgae which absorb light as energy source for their growth. The productivity of photobioreactors are confined by light transfer. The kinetic and mathematical models for microalgal photosynthesis or growth need accurate information about light attenuation and distribution in microalgal suspensions. An improved transmission method is proposed to obtain the spectral extinction characteristics of microalgae suspensions and cells in the paper. The light attenuation in the cell mixture suspensions is considered from the combined contributions of the culture medium and the microalgae. In measuring, the microalgae suspensions are considered to be well mixed. In this method, the optical constants of the glass (cuvette) are obtained at the first step. Then, the spectral extinction coefficient of the microalgae suspensions is retrieved using the two measured normal-normal transmittance based on a rigourous theoretical model considering multiple reflections and refractions at air-glass and glass-liquid interfaces. The improved method is demonstrated to provide good results of spectral extinction characteristics over the studied spectral range 300–1800 nm. The trend of the refractive index spectra of BG11 medium and distilled water are similar, which decreases from about 1.39 to 1.32 with the increasing wavelength. The absorption coefficient between culture medium and distilled water show significant difference in the spectral range 300–700 nm while exhibit small difference in the wavelength range 700–1800 nm. The difference is attributed to the different composition between culture medium and distilled water. The effective refractive index of Chlorellasp. suspensions varies little with increasing consentration. The spectral extinction coefficient of Chlorellasp. suspensions increases significantly with cell concentrations in the spectral range 300–1300 nm, but varies little in the spectral range 1300–1800 nm. This is attributed to the photosynthetic pigments of microalgae cells being more optically active in visible spectra than in the near-infrared spectra. In spectral range 1300–1800 nm, the spectral dependence of the total extinction coefficient is very close to the spectral dependence of the absorption index of the culture medium, indicating the total extinction coefficient of mixture suspensions is dominated by culture medium. The extinction coefficient and extinction cross-sections of Chlorella sp. decrease with the increase of wavelength. The measured extinction cross-sections of the microalgae cells are independent of cell concentrations. This work provide a viable reference for the accurate measuremnt of spectral extinction characteristics of microalgae and nano-particle suspensions.