The Efficiency And Mechanism Study On Hg(?) Removal In Industrial Wastewater By Microalgae Biomaterials

Since the 20th century,along with the rapid development of heavy industry such as metallurgy,coal,chlor-alkali and battery,the large amount of mercury-containing wastewater discharge seriously threatens the natural ecological environment and human health.Although varies of traditional methods have been developed to conquer this problem,each method has advantages and disadvantages,they can't completely satisfy increasingly mercury pollution and stringent emission standards.Therefore,to develop more effective and cheaper environmentally friendly processing technology for mercury wastewater is necessary.In recent years,more attentions have been paid on the resource utilization of microalgae such as the microalgae biodiesel,the health food product an so on,obtaining more economic benefits and high value-added products by sewage microalgae cultivation to reduce costs has become the research hospot.The coupling process of microalgae utilization and biosorption has gradually become a new alternative on environmental protection and efficient processing method of heavy metals.Microalgae has become one of the most concerned adsorbents meatrials for heavy metals removal because of its rapid growth,remarkable environmental tolerance ability and abundant source.Based on the previous research,this work proposes an approach based on the coupling process of microalgae resource utilization and industrial wastewater Hg???treatment technology.Unlike agricultural sewage and municipal sewage,heavy metals in industrial wastewater can have a fatal effect on algae activity and oil yield.However,a series of problems exits,such as the Hg???adsorption efficiency by microalgae is reported to be much lower than other heavy metals,the structure stability of the microalgae based biosorbent materials inferior to that of activated carbon and other inorganic materials,the problem such as micro algae utilization is expensive.Before commercialization,the above key technical difficulties should be focused on in the early relevant basic research.Based on these problems,this work mainly carries out research work in the following aspects.First,three small organic acids-formic acid,acetic acid and propionic acid were used to pretreat microalgae and its lipid residue.Compared with the original algal powder,the adsorption capacity of the three modified samples increased by 2.71,1.47 and 0.82 times,respectively.Compared with the residue,the increase was 1.52,0.68 and 0.24 times,respectively.The characterization of material structure indicated that the specific surface area of modified residue increased from 3 m²/g to 58.2 cm³/g,49.9 cm³/g and 42.8 m²/g,respectively,by 18.4,15.6 and 13.3 times.It was found that the adsorption process of microalgae and its residues to Hg???was mainly through the surface carboxylic acid site binding in the form of single bond.Organic acid modification not only significantly improved the pore structure of the material,but also loaded more acidic sites by binding with small and medium molecular alkaline groups of microalgae.Secondly,based on the low efficiency of adsorbent recovery in current sewage treatment process,calcium alginate microspheres,another derivative product of microalgae resource utilization,were selected as the absorbent material.The spherical particles of calcium alginate were modified by NH₄HCO₃,and the modified solid particles presented a porous hollow shell structure with a dense carbonated substance on the surface.The adsorption efficiency of the modified materials was significantly improved,and the adsorption capacity increased from 18±1 mg/g to 48±4 mg/g.It was found that the material had the best adsorption performance when the solid-liquid ratio was 1g/L and pH=5.After modification,the problem of material quality wear and adsorption efficiency decline has been significantly improved.The 10-hour continuous adsorption study found that the adsorption efficiency of modified materials decreased by up to 26%,far less than 90%of raw materials.The results show that this method can improve the stability of calcium alginate microspheres in the liquid phase adsorption environment.Thirdly,in combination with micro algae cultivation technology for biodiesel production,the Hg???sewage cultivate chlorella coupling experiment was designed to explore the mercury biological stress on microalgae Chlorella vulgaris cultivation,adsorption ability,physiological activity and the effect on the lipid yield.Several experiments results implied that although low concentration of mercury could inhibits the increasing of cell biomass,but the hormesis effect will slightly promote the increase of the lipid content of micro algae at the same time.This study also found that mercury removal ability of living algae is greatly influenced by environment and culture conditions change,but the deactivation dead algae in the process of mercury adsorption showed a greater resistance to environmental interference characteristics,this may be related to the increased cell membrane permeability because of cell inactivation.Based on living algae adsorption process of Hg???established a NICA-DONNAN heterogeneous isothermal adsorption model,which proves that the surface of living algal Gemini site area heterogeneous adsorption phenomenon.At the same time,according to the model data,it can be concluded that the adsorption process of Hg???combined with carboxyl site adsorption method for single tooth,and combine with chelating carrier protein as bidentate mode,thus proving the living algal heavy metal residue rate can be significantly higher than the dead algae experimental phenomena.Finally,in view of a series of questions appeared in the process of C.vulgaris cultivation in Hg???contained sewage in the previously study,the pretreatment based on the functionalized biomimetic mineralization was conducted on the microalgae cell surface.On the account of the strong interaction with Hg ions,the sulfur atoms were in situ doped into phosphate,to form a layer of functional mineralization by the Layer-by-Layer self-assemable of polyelectrolytes on the outer of cells.Studies showed that after modification of biomimetic mineralization,better tolerability,higher adsorption efficiency and lipid yield were observed for modified microalge cultivated in the mercury contaminated wastewater,which proposes a new method for for coupling of wastewater treatment and microalgae cultivation.moreover,the Hg???content in both lipid and residue of mineralized microalgae were significantly lower than that of unpretreated cells,which can inhibit secondary emissions of mercury in the following resource utilization,such as bio-oil production and biomass pyrolysis.