Effect Mechanism Of Rice Straw Biochar On Mn(Ⅱ) Enrichment In Acidic Water By Scenedesmus Quadricauda

Manganese ore resources are abundant in China,acidic high manganese wastewater has not been properly treated and discharged into rivers,resulting in pollution of surrounding water bodies.Microalgae in natural water can regulate the pH and dissolved oxygen of the environment through photosynthesis,which will promote the oxidation of Mn（Ⅱ）and play an important role in the remediation of manganese pollution in water.In addition,biochar,as a new type of environmental functional substance,has been widely used in various fields.Biochar will enter the water body in different ways in the process of production,transportation and application,which will inevitably affect the enrichment of Mn（Ⅱ）by microalgae.However,there are not many related researches.Based on the above discussion,this studies selected biochar made from common agricultural waste rice straw as raw material,and selected Scenedesmus quadricauda with strong vitality and strong acid resistance as the research object,to further study the manganese removal characteristics and mechanism of Scenedesmus quadricauda in acidic water,and the influence mechanism of rice straw biochar on Mn（Ⅱ）enrichment of Scenedesmus quadricauda.The main research contents and conclusions were summarized as follows:（1）The growth characteristics and environmental regulation performance of Scenedesmus quadricauda under acidic conditions were studied.The results showed that the growth adaptation period of Scenedesmus quadricauda was 4 days,and the rapid growth period was 6days.There was a significant linear relationship between cell density and light density,between cell density and chlorophyll,mass concentration and light density.Scenedesmus quadricauda has strong ability to adjust the pH and dissolved oxygen of the solution.When the pH value is greater than or equal to 4,it can quickly raise the pH value of the solution.（2）The characteristics and mechanism of manganese removal by Scenedesmus quadricauda under acidic conditions were studied.The results show that Scenedesmus quadricauda has strong manganese tolerance and manganese removal ability.When the initial Mn（Ⅱ）concentration was 50mg/L,the removal rate can still reach about 40%after 7 days of reaction,and the production of biological manganese oxide was 11.14mg/L.The optimal conditions for removing Mn（Ⅱ）from acidic water were as follows:temperature 25℃,pH=6.0,initial algae density OD₆₈₀0.2,and illumination period 16:8.The mechanism of manganese removal is that photosynthesis of algae cells increases dissolved oxygen and pH,which is beneficial to the production of BioMnO_x.Moreover,the surface of algae cells contains a large number of proteins,amino acids and polysaccharides,which can remove manganese through surface adsorption oxidation and intracellular absorption oxidation.The algae indirectly oxidized Mn（Ⅱ）to produce Mn（Ⅲ）or Mn（Ⅳ）biological manganese oxides,mainly composed of MnO₂,MnO（OH）,FeMnO₃ and Mn₂O₃.（3）The adsorption capacity of rice straw biochar for manganese was studied by batch experiment.The results showed that the adsorption rate of straw charcoal was the fastest in the first hour,and reached the adsorption equilibrium after 8 hours.Under acidic conditions,the adsorption rate increases with the increase of pH value;With the increase of initial concentration of Mn（Ⅱ）,the adsorption capacity increases and the adsorption rate decreases.The characterization showed that straw carbon has large specific surface area and pore volume,and its surface contains many functional groups,such as hydroxyl,amine and carboxyl.Mn（Ⅱ）in the solution can be removed by electrostatic adsorption,surface complexation or ion exchange.（4）The sequential batch tests was used to explore the manganese removal characteristics of Scenedesmus quadricauda in algae-carbon coexistence system,and compared the manganese removal of Scenedesmus quadricauda alone,revealing the influence of rice straw charcoal on the mechanism of manganese enrichment of Scenedesmus quadricauda in acidic water bodies.The results showed that the chlorophyll,pH and dissolved oxygen of the solution increased first and then decreased gradually with the increase of biochar content,which was always higher than that of Scenedesmus quadricauda alone.This may be because the competitive adsorption of straw charcoal at the initial stage of the experiment reduced the toxicity of Mn（Ⅱ）to Scenedesmus quadricauda,and the soluble organic matter in biochar could provide more nutrients for microalgae and promote the growth of microalgae.However,the biochar prepared by pyrolysis will produce toxic and harmful organic substances such as polycyclic aromatic hydrocarbons,persistent free radicals in the environment,etc.,and the biochar also has certain shading property,so excessive biochar will adversely affect the reproduction of microalgae.No matter whether Scenedesmus quadricauda alone or algae-carbon coexisting system removes manganese,the oxidation adsorption of Mn（Ⅱ）by algae cells is much larger than its intracellular absorption,and the enrichment of Mn（Ⅱ）by Scenedesmus quadricauda mainly depends on the oxidation adsorption of algae cell surface.When the concentration of Mn（Ⅱ）in the solution is20mg/L-50 mg/L,the straw charcoal promotes the adsorption and oxidation of Mn（Ⅱ）on the surface of algae cells,and the straw charcoal always inhibits the intracellular absorption of Mn（Ⅱ）by Scenedesmus quadricauda.With the increase of straw charcoal concentration,the extracellular adsorption oxidation amount of Scenedesmus quadricauda first increased and then decreased,and the intracellular absorption decreased continuously.The oxidation adsorption and absorption of Mn（Ⅱ）on Scenedesmus quadricauda increased with the increase of pH（3-7）.According to kinetic fitting analysis,the constant of manganese enrichment rate of Scenedesmus quadricauda was greater than that of tetrasporous alone.Characterization analysis showed that the surface groups of Scenedesmus quadricauda cells in the coexistence system were more compatible with Mn（Ⅱ）,and Mn（Ⅱ）in the indirect oxidation solution of algal cells generated manganese oxide particles,which were the same as the manganese removal products of Scenedesmus quadricauda.