Study On Ca²⁺ Mediated Photosynthetic Hydrogen Production And Harvest Performance Regulation Of Chlamydomonas Reinhardtii

The massive use of fossil raw materials contributes to the development of human society and economy,but also pollutes the environment.Therefore,the search a clean,renewable energy source is a future energy development trend.Due to its advantages such as low density,light quality,high combustion value,clean and renewable,hydrogen is seen as a potential renewables.In this paper,Chlamydomonas reinhardtii was used as the research object.Under the condition of sulfur deficiency stress,Chlamydomonas reinhardtii was used for photosynthetic hydrogen production.Due to the rapid decrease of photosystem II（PSII）activity under sulfur deficiency stress,the hydrogen production decreased.Therefore,we increased hydrogen production by adding Ca²⁺to protect PSII activity and explored its hydrogen production mechanism.Then,the effects of different Ca²⁺dosage and alkalinity on the flocculation performance of Chlamydomonas reinhardtii after hydrogen production were studied.Finally,the harvested Chlamydomonas reinhardtii was pretreated by advanced oxidation technology,and the pretreated Chlamydomonas reinhardtii was subjected to anaerobic fermentation to produce hydrogen by dark fermentation technology.The main contents and conclusions are as follows:（1）The effects of sulfur deficiency and calcium deficiency on cell growth,hydrogen production,hydrogen production rate,chlorophyll,active oxygen（ROS）and antioxidant enzymes of Chlamydomonas reinhardtii were investigated.The experimental results show that adding Ca²⁺can effectively prolong the hydrogen production time and increase the hydrogen production.When the dosage of Ca²⁺was 150 mg/L,the hydrogen production time was extended from 8 d to 14 d,and the hydrogen production was the highest,which was 16.12 m L L^-1,which was 1.43 times that without Ca²⁺.Through comparative analysis,it can be concluded that Ca²⁺can slow down the large-scale degradation of chlorophyll within a certain range,induce antioxidant enzymes to promote ROS scavenging,protect PSII activity and provide more hydrogen-producing electrons,prompting Chlamydomonas reinhardtii to prolong hydrogen production time and increase hydrogen production.Finally,the correlation analysis of each index was carried out by’Spearman bivariate correlation analysis’,and the analysis results and the research results were mutually confirmed.（2）The effect of different Ca²⁺dosage on the flocculation effect of Chlamydomonas reinhardtii after hydrogen production was investigated.The experimental results show that with the increase of Ca²⁺dosage,adjusting alkalinity has obvious effect on the removal of algal cells.With the increase of alkalinity and Ca²⁺dosage,the effluent pH gradually increased,and the flocculation effect was better.When the alkalinity is 1.5 and the dosage of Ca²⁺is 600 mg/L,the flocculation efficiency is 93%.When the alkalinity is 1 and the dosage of Ca²⁺is 400 mg/L,the flocculation efficiency is 91.22%.Therefore,after considering the flocculation effect and cost,we determined that the optimal dosage of flocculation was 1,and the dosage of Ca²⁺was 400 mg/L.（3）The flocculation mechanism was analyzed by measuring Zeta potential,TOC,protein,polysaccharide,EEM,FIRT and SEM morphology analysis.The experimental results show that the mechanism of flocculation and algae removal is electric neutralization and net sweeping,and the net sweeping is the main flocculation mechanism.With the increase of alkalinity and Ca²⁺dosage,the TOC concentration decreased,the TOC removal rate increased,and the protein and polysaccharide content decreased significantly.It can also be seen from EEM that the removal effect of aromatic protein organic matter is significant,while the removal effect of soluble organic matter and humic acid is better.The FIRT and SEM morphology analysis further showed that it was a very effective method to collect algae cells by adding Ca²⁺and adjusting alkalinity.（4）The collected algae cells were pretreated by Ca²⁺/O₃ advanced oxidation technology to release intracellular organic matter,and then the dark fermentation technology was used to produce hydrogen by dark fermentation.The results showed that the pretreated algae cells were more conducive to fermentative hydrogen production.The cumulative hydrogen production was 9.88 ml/g DW within 18 h,which was 1.91 times that of the unbroken treatment,and the maximum hydrogen production rate was 0.905 ml/g DW/h,which was 52.8%higher than that of the unbroken treatment.