The Research On Ammonium-rich Ionic Rare Earth Tails Wastewater Treatment By Chlamydomonas Sp. YC

Rare earth elements（REEs）naturally found in the environment are regarded as essential resources by various countries in the 21st century.REEs play an indispensable role in energy,environmental protection,digital technology,nuclear industry,and medical treatment.Currently,the global demand for REEs is about 170,000 tons annually and continues on the upward trend.China,America,Australia,and India are the major REEs producing countries.And China provided for more than 70%of the world’s REEs production.Recently,in the REEs production,the devolved processes mediated with ammonium sulphate（（NH₄）₂SO₄）for REEs extraction from ores have caused many environmental problems in China,such as causing the eutrophication and generating cancerogenic substances（e.g.,nitrite）of fresh water in the surroundings.According to the survey,over 2,000 million tons of REEs wastewater containing 300-5,000 mg/L NH₄⁺-N is produced in rare earth industry every year.About 1 tonne of REEs production generates 20,000 tons of REEs wastewater.For this situation,the National Development and Reform Commission has proposed accelerating the mine environment’s remediation and promoting the industry’s green and sustainable development.And the environmental protection department has formulated a standard that the NH₄⁺-N concentration in REEs tails water must be lower than 15 mg/L before discharge to rivers.Thus,exploiting an economical and efficient process to treat the NH₄⁺-N-rich REEs wastewater is urgent.To date,some existing approaches,including physical methods,chemical methods,and biological methods have been developed to deal with REEs wastewater.However,for the plenty REEs wastewater treatment,physical methods are expensive processes mediated by some special equipment with high energy consumption;chemical methods are leading some new environmental problems easily;biological methods are time-consuming,lower performance in NH₄⁺-N resistant and high cost from the unitization of organic sugar.Therefore,this work focus on isolating an indigenous single-celled phototrophic microalga from the real REEs wastewater and establishing an effective and economical manner to treat the REEs wastewater.Firstly,the isolated microalga was identified by the18s RNA and ITS gene sequencing and the morphological characteristics.Then the kinetic parameters of the NH₄⁺-N removal ability of the selected microalga were analyzed.To efficiently treat real REEs wastewater,we established some promising approaches mediated with the isolated microalga on laboratory scale.Further,three photobioreactors were used to evaluate the feasibility of REEs wastewater treatment on pilot scale by this microalga under outdoor conditions.Moreover,genomic and transcriptome sequencing techniques were used to analyze the carbon fixation,NH₄⁺-N utilization and tolerance mechanisms of this microalga.More details of this study were listed as follows:1.A microalgal strain was successfully isolated from the REEs wastewater by enrichment and purification methods and identified as Chlamydomonas sp.（namely Chlamydomonas sp.YC）.The NH₄⁺-N tolerance of this microalga was studied by the modified BG11 medium（NH₄⁺-N concentrations,318-5000 mg/L）.The results showed the growth ability of Chlamydomonas sp.YC decreased with the increasing NH₄⁺-N concentrations in the medium,and the EC₅₀ of NH₄⁺-N to microalga reached 3250 mg/L.Further,the N/P of Chlamydomonas sp.YC for REEs wastewater treatment was optimized.Chlamydomonas sp.YC could consume 15 mg/L TP completely and obtained the highest NH₄⁺-N removal efficacy（17.24%）.Moreover,the NH₄⁺-N removal ability of Chlamydomonas sp.YC for different REEs wastewater concentrations was evaluated.Chlamydomonas sp.YC could almost utilize NH₄⁺-N of 10%REEs wastewater and attained the highest NH₄⁺-N removal rate of undiluted REEs wastewater for 10 days of cultivation.Meantime,the K_m of NH₄⁺-N removal by Chlamydomonas sp.YC was calculated.The obtained K_m value of 2528 mg/L suggested that Chlamydomonas sp.YC is a promising strain to treat NH₄⁺-N-rich REEs wastewater.2.The process of diluted（10%）and undiluted REEs wastewater treatment by Chlamydomonas sp.YC was established via batch,semi-continuous,and continuous culture modes.It was found that,the NH₄⁺-N removal rate of Chlamydomonas sp.YC treated 10%and undiluted REEs wastewater by semi-continuous and continuous culture modes（32.75-61.05 mg/（L·d））were superior to batch modes（19.50-30.38 mg/（L·d））.Interestingly,compared to the 10%REEs wastewater,Chlamydomonas sp.YC treated undiluted wastewater obtained the greater NH₄⁺-N removal rate by semi-continuous（61.05 mg/L·d）)and continuous modes（57.10 mg/L·d）).In term of the macromolecules of microalgal biomass of Chlamydomonas sp.YC obtained from three culture modes,the carbohydrate and total fatty acids（TFAs）were 35.40-44.40%and 4.97-6.03%,respectively.These biomasses derived from REEs wastewater could develop for the sustainable and cleaner biofuels.In addition,the highest productivities of carbohydrates and TFAs attained 226.36 mg/（L·d）and 32.98 mg/（L·d）via cultivated Chlamydomonas sp.YC in continuous modes.Taken together,Chlamydomonas sp.YC treated REEs wastewater in continuous mode was a promising manner to remediate NH₄⁺-N and produce valuable biomass for renewable biofuels production.3.The effects of Ca CO₃automatic regulation of p H value of REEs wastewater treated by Chlamydomonas sp.YC was evaluated.To save cost,the Ca CO₃-rich eggshell piece（ESP）and oyster shell piece（OSP）were used to replace the commercial Ca CO₃.Based on this,the best CO₂ concentrations of the REEs wastewater treatment by Chlamydomonas sp.YC was comprehensively studied.And the obtained process was verified in 50 L airlift photobioreactors（AL-PBRs）under indoor conditions.The results were listed as follows:（1）Chlamydomonas sp.YC treated by 1 g/L Ca CO₃ performed the best ability for the growth and NH₄⁺-N removal,showing the biomass of 1.64 g/L and NH₄⁺-N removal rate of 46.5 mg/（L·d）.（2）The p H value of the culture could be stable at7-8 treated by ESP and OSP.In terms of the growth and NH₄⁺-N removal performance of Chlamydomonas sp.YC,OSP exhibited more suitability for application in REEs wastewater treatment than ESP.（3）Chlamydomonas sp.YC treated with 3%CO₂ showed1.90 g/L biomass,34.1%NH₄⁺-N removal efficacy,and 255.7 mg/（L·d）CO₂sequestration rate that was superior to other CO₂concentration.（4）The obtained process of REEs wastewater treatment by Chlamydomonas sp.YC could be applied to 50 L AL-PBRs under indoor condition.4.The AL-PBRs,tubular photobioreactors（TB-PBRs）,and open raceway ponds（ORWPs）were used for the REEs wastewater treatment by Chlamydomonas sp.YC under outdoor conditions on a pilot scale.Based on the primary and secondary data,the input costs of the three photobioreactors were analyzed.The obtained results were shown as follows:（1）Chlamydomonas sp.YC obtained the highest biomass and NH₄⁺-N removal efficacy in the TB-PBRs,which were 2.34 g/L and 42.86%,respectively.（2）The developed kinetic models based on Logistic and Ludeking-Piret formula could be well fitted to the experimental data of microalgal growth,protein formation,and NH₄⁺-N utilization.（3）REEs wastewater treatment by Chlamydomonas sp.YC needs the minimum cost in TB-PBRs.Showing that TB-PBRs could be an optional photobioreactor for Chlamydomonas sp.YC to treat rare earth wastewater.5.Under the different CO₂ concentrations,the mechanism of Chlamydomonas sp.YC tolerance and utilization of NH₄⁺-N-rich REEs wastewater were comprehensively analyzed.And the obtained results were shown as follows:（1）To increase the NH₄⁺-N concentration in the cell,Chlamydomonas sp.YC under 3%CO₂ condition has upregulated the genes coding the NH₄⁺-N membrane transporter（AMT）proteins.Then,the process of intracellular NH₄⁺-N assimilation was promoted by up-regulating two genes encoding glutamine synthetase（GS）and two genes encoding glutamate synthase（GOGAT）.（2）Under high NH₄⁺-N concentrations,Chlamydomonas sp.YC maintained the high GS and GOGAT activities,and could translate the intracellular NH₄⁺-N into amino acid timely,helping the tolerance of cells to high NH₄⁺-N conditions.（2）The GS1protein of Chlamydomonas sp.YC possesses a blocked ammonia channel that could improve the activity and prevent the cytotoxicity caused by ammonia leakage.