Carbon Source Regulation Scenedesmus Obliquus Treatment Of Rare Earth Wastewater And The Influence Mechanism Of Rare Earth Ions

In recent years,rare earths have been widely used in industry,agriculture,national defense,medicine and new materials,etc.At present,China’s rare earth production accounts for more than 85% of the global total,and rare earth industry is an advantageous industry in China,but at the same time,rare earth mining produces a large amount of mine wastewater with high ammonia nitrogen,low carbon to nitrogen ratio and containing low concentration of rare earth ions.It has been confirmed that microalgae treatment technology is a new method for the treatment of ammonia and nitrogen wastewater from rare earth mines,but in the actual treatment process,there are problems such as slow growth of microalgae,low efficiency of ammonia and nitrogen removal,and high cost of carbon source added.In order to solve the above problems,this paper selects the algae as the experimental species to treat the rare earth mine wastewater,and optimizes the culture parameters and selects the suitable carbon source to promote the growth of microalgae and improve the ammonia nitrogen removal effect,and studies the effect of different rare earth ion concentration on the growth and denitrification performance of microalgae,in order to provide some theoretical basis and technical support for the application of microalgae treatment technology to the actual rare earth mine wastewater treatment.The specific studies are as follows:The optimal initial inoculum concentration and the optimum growth pH of the algae in rare earth wastewater were firstly carried out.The study showed that the domesticated Scenedesmus obliquus had good growth ability under acidic conditions,and the growth could be sustained under acidic conditions at pH=4.0.The optimum growth conditions of pH=6.0and the optimal initial inoculum concentration was 0.3 g/L.The dry weight of microalgae was0.67 g/L,the biomass yield of microalgae was 0.047 mg/L/d,the removal rate of ammonia nitrogen was 28.77% and the removal rate of total nitrogen was 27.06%.The lack of carbon source is an important reason for the difficulty of microalgae growth in rare earth wastewater and the low removal rate of pollutants.Secondly,studies on the effects of different carbon sources on the growth,self-flocculation performance and removal of pollutants from wastewater were conducted to further improve the effect of microalgae treatment on rare earth wastewater.The study showed that among the organic carbon sources with added carbon content of 200 mg/L,glucose had the most significant effect on the growth promotion of P.aeruginosa,and the highest biomass yield of microalgae was 101.5 mg/L/d,and the removal rate of ammonia nitrogen was 81.11% and TN was 89.86%.Among the inorganic carbon sources,sodium carbonate was more effective than sodium bicarbonate in promoting the growth of microalgae and wastewater treatment,and the removal rate of ammonia nitrogen was the highest among all carbon source groups,up to 86.13%.Taking into account the growth of the slanting algae,the removal rate of ammonia nitrogen and the cost of carbon source injection,crude glycerol was selected as the optimal carbon source for the treatment of rare earth wastewater by microalgae.The crude glycerol group showed comparable biomass conversion ability with the glucose group,and the biomass yield of microalgae was 91.08 mg/L/d,and the removal rates of ammonia nitrogen and total nitrogen were 76.35% and 83.86%,respectively,and the removal rate of ammonia nitrogen was 3.08 times higher than that of the control group.The removal rate of ammonia nitrogen and total nitrogen was 76.35% and 83.86%,respectively,and the removal rate of ammonia nitrogen was 3.08 times that of the control group.Finally,the physiological response mechanisms of microalgae under different rare earth ion concentration stress were analyzed by measuring algal biomass,chlorophyll a/b content and superoxide dismutase(SOD)and catalase(CAT).It was shown that the rare earth ions exhibited a promoting effect on the growth of microalgae at low concentrations of 1 mg/L and5 mg/L,which could increase the chlorophyll a content and promote the photosynthetic capacity of algal cells,and the biomass of microalgae reached the maximum at the rare earth ion concentration of 5 mg/L,with a biomass yield of 100.85 mg/L/d,which was 1.12 times higher than that of the blank control.The addition of rare-earth ions enhanced the denitrification performance of the algae,with the best ammonia removal effect of 87.37% at the concentration of 5 mg/L,which was 11.04% higher than that of the control group,but the denitrification performance began to decrease when the concentration of rare-earth ions exceeded 10 mg/L.The SOD and CAT contents increased with the increase of rare-earth ion concentration and reached the maximum at 10 mg/L.When the algal cells were stressed by rare earth ions,the microalgae produced a large amount of reactive oxygen species(ROS),which caused an increase in the activity of antioxidant enzymes in the algal cells to resist oxidative damage and maintain the normal physiological metabolism of the algal cells,but the antioxidant defense system of the algal cells was damaged when the rare earth ions were added at 15 mg/L,and the algal cells were damaged and the biomass of the algal cells decreased sharply.