Influence Of Heavy Metals On Microalgal Growth And Lipids Accumulation

Microalgae biomass growing for heavy metals biosorption and biodiesel production is nowdays a research hotspot, which is a promising mitigation for environmental pollutions and energy shortages. The microcosmic physical and chemical properties of microalgae cells with heavy metals enrichment were investigated. The dynamic microstructure changes during the microalgae cells disruption process were studied. The influences of heavy metals and cells disruption methods on biodiesel compositions from microalgae were analyzed.Four typical heavy metal ions Pb2+, As3+, Cd2+ and Hg2+ were investigated. Every country has strict restriction on the level of these four heavy metals in foods and materials. The dynamic characteristics of these four heavy metal ions adsorbed by microalgae cells were investigated. The results showed that the toxicities of four heavy metal ions on microalgae cells growth inhibition were Hg2+> Cd2+>As3+>Pb2+. Nitrogen adsorption, scanning electron microscope and transmission electron microscope were used to analyze microalgae cells both before and after heavy metals adsorption. The contents of lipid and starch in microalgae cells were increased after heavy metals biosorption. But wrinkles and ravines appeared on the surface of microalgae cell walls. The diameters of granular swelled on the cell wall surfaces increased from 12～30nm to 15～50nm. The specific surface areas of microalgae cells after heavy metals biosorption increased from 0.54m2/g to 1.10～1.80m2/g. However the inner bores superficial areas decreased from 0.44m2/g to 0.18m2/g. At the mean time, the mean pore diameters decreased from 51.6nm to 28.3～46.2nm. This is because some heavy metal ions adsorbed on the surfaces of microalgae cells which lead to the increasing roughness of cells. Meanwhile, some other heavy metal ions diffused into the inner bores of microalgae cells which lead to partial blocked of the inner bores.Infrared spectrometer, X-ray spectrometer and gene amplification were used to analyze the microalgae cells. Four heavy metal ions could combine with functional groups containing carbon, nitrigen and phosphorus to adhere to the microalgae cell surfaces. The combinations of heavy metal ions and carbonic functional groups have two mechanisms. One is the ion exchange between heavy metal ions and carboxyl (COO-). The other is the complexation between heavy metal ions and ether bonds (C-O-C). The ratio of C-C bonds and C-O bonds in the complexing units formed by Pb2+ and Hg2+ were 1.4:1 and 1.7:1 respectively. And the ratio of C-C bonds and C-O bonds in the complexing units formed by Cd2+ and As3+ were 2.5:1 and 4:1 respectively. The result of Real-Time PCR testing showed that the expression level of phosphoglycerate bisphosphoglycerate mutase family protein decreased about 10%. This lead to the accumulation of starch in microalgae cells after heavy metals biosorption.The ultrasonication can disrupt the microalgae cell walls effectively for lipid release. This is because the implosion of cavitation bubbles formed by cavitation effect of ultrasonication will produce huge mechanical shear stresses and turbulences. The increasing of ultrasonic treatment powers and times enhanced the microalgae cells disruption and at the same time increased the cell surfaces roughness. The fractal dimension of microalgae cells increased from 1.24 to 1.51 when the ultrasonic treatment power increased from 0 to 500W. The cell wall thickness was 3/5 of the initial thickness. The cell diameters decreased from 2.75μm to 1.69μm. This is because the cytomembranes of microalgae cells were destroyed more seriously than the cell wall. The cytomembranes near the vacuole were first broken which lead to the reducing of microalgae cells osmotic pressure. So the microalgae cells shrank because of dehydration.The polarization effect of microwave reversed the cellulose molecular at a very high frequency and broken cellulose molecular structures effectively. Meanwhile, the macromolecule subatances in the microalgae cells were denatured by the volumetric heating of microwave treatment. So the microwave treatment could eliminate the elasticity and tenacity of microalgae cell walls and disrupt the microalgae cells more effectively than ultrasonication. The fractal dimension of microalgae cells increased from 1.61 to 1.91 when the microwave treatment temperature increased from 80℃ to 120℃. The cell wall thichness increased from 0.11μm to 0.59μm when the microwave treatment time increased from 0 to 20min. And the pore diameters increased from 0.005μm to 0.18μm. The outer pectin layers of cell walls gradually detached and the porosity of inner cellulose layers increased when microwave treatment time increased to 26min. The initial point of disruption appeared at the maximum curvature (approximately 1.01×107 m-1) of cell walls.The lipid content of microalgae cells increased from 22.30% to 23.74%～25.41% after heavy metals biosorption. This is because the heavy metals inhibited the accumulation of phospholipid and induced the accumulation of lipids in microalgae cells. The long chain and monounsaturated fatty acids increased while the short chain and polyunsaturated fatty acids decreased in the lipids accumulated by microalgae cells after heavy metals biosorption. The highest lipid extraction efficiency was achieved with microwave treatment which was 91.7%. The total triglyceride fatty acid increased when the microalgae lipids were extracted with microwave treatment. Long-chain and unsaturated lipids disintegrated into short-chain and saturated lipids which benefitted the high-quality biodiesel production.CO2 from a coal-fired power plant flue gas after desulfurization and denitrification were pumped into an open raceway pond (the area is 1191m2). Seawater with nitrogen and phosphorus nutritive salts was used for microalgae cultivation. The heavy metal sources were analyzed industrially. The results showed the contribution of flue gas to the accumulation of four typical heavy metals (Pb,Cd,Hg and As)) was much lower than dust and seawater. So the ultilization of flue gas CO2 for microalgae cultivation is technical feasible which saves the carbon source nutrients and reduces the production cost of microalgae biomass.