| Oleaginous microalgae are viewed as renewable biomass energy,which have attracted more and more attentions recent years.Due to the growth characteristics of microalgae,harvesting of microalgae has become the main factor that limiting the high efficient utilization of microalgae.Based on the conventional methods of microalgae harvesting,magnetic harvesting is applied as a new microalgae harvesting method,which has attracted more and more attentions due to its high-speed and high-efficient characteristics.In this study,based on the magnetic flocculation separation technology,three cationic polymers named as polyaluminium chloride(PACl),amino-methylated modified plant polyphenol(APP),quaternary ammonium modified plant polyphenol(QPP)were employed to functionalize Fe3O4,and prepared the magnetic composites Fe3O4/PACl,Fe3O4/APP,Fe3O4/QPP,respectively.The physicochemical properties and harvesting characteristics of the functionalized magnetic composites were studied.By employing the DLVO(Derjaguin Landau Verwey Overbeek,DLVO)theory,and acid-base modified E-DLVO(Extended-DLVO,E-DLVO)theory,the interactions between functionalized magnetic composites and oleaginous microalgae were studied.Also,the magnetic force modified M-DLVO(Magnetic force modified DLVO,M-DLVO)was employed to study the interactions between magnetic aggregates.By using PACl coated Fe3O4 particles,the Fe3O4/PACl magnetic composite was obtained,and the isoelectric point of Fe3O4/PACl was 13.1.96%harvesting of C.vulgaris was realized when Fe3O4 and PACl were added as "Fe3O4/PACl" strategy,and the concentration of Fe3O4/PACl was((10.0 g/L)/(0.625 mmol Al/L)).It showed that the alkaline solution was more favorable for Fe3O4/PACl harvesting.When the PAM and Fe3O4/PACl were added as the "Fe3O4/PACl+PAM" strategy((10.0 g/L)/(0.625 mmol Al/L)+3 mg/L),99%harvesting efficiency of C.vulgaris could be realized.Also,the "Fe3O4/PACl+PAM" strategy could overcome the obstacles that induced by the pH variation and algal organic matter(AOM).The mechanism of microalgae harvesting by using "Fe3O4/PACl+PAM" strategy,which was mainly due to the charge neutralization between Fe3O4/PACl and microalgal cells first,then the trap bridging and sweeping that induced by PAM further.By employing APP coated Fe3O4 particles,the obtained Fe3O4/APP magnetic composites could achieve 77%harvesting efficiency of C.vulgaris cells.The FTIR(Fourier Transform infrared spectroscopy,FTIR)results showed that the functional group-N-of APP was coated onto Fe3O4 surface successfully.The XPS(X-ray photoelectron spectroscopy,XPS)results showed that the functional group of APP was coated onto Fe3O4 surface by the N-O bond.After the functionalization of APP,the isoelectric point of Fe3O4/APP was improved to 7.3.The results showed that the mixture solution of Fe3O4 and APP could achieve higher harvesting efficiency than that Fe3O4 and APP dosage one by one.The optimal coating ratio between Fe3O4 and APP in Fe3O4/APP were(20.0 g/L)/(200 mg/L).The Fe3O4/APP composite was suitable for acid solution(pH<7.0),the AOM secreted by algal cells,high cell density can disturb the harvesting process,increasing the adding dosage of Fe3O4/APP.The Fe3O4 particles could be regenerated and reused after ten cycles,and could still achieve 72%harvesting efficiency after recoated with APP after the tenth cycleBased on the APP polymer,the quaternary ammonium modified reaction was employed to modify the APP further and obtained QPP polymer.The obtained QPP polymer was employed to functionalize Fe3O4 and obtained Fe3O4/QPP magnetic composite.The isoelectric point of Fe3O4/QPP was improved from 7.3 to 13.5.The FTIR results showed that the-N+-(CH3)2 group of QPP was coated onto Fe3O4 surface successfully.XPS results showed that the functional group of QPP was coated with Fe3O4 by the N-0 bond.By the functionalization of QPP,the dosage of Fe3O4/QPP was decreased from 20.0 g/L to 5.0 g/L,and the harvesting efficiency was improve from 77.0%of Fe304/APP to 91.0%.Fe3O4/QPP could be applied in the full pH ranges(pH 6.0-12.0),and could overcome the obstacles of AOM and high cell density of algal broth.Fe3O4 can be recovered from algal aggregates by ultrasounds,and the recoated composites can also achieve 76.3%harvesting efficiency after tenth cycle.The Fe3O4/QPP composite was more competitive in harvesting oleaginous microalgae from the original culture media.The DLVO theory was employed to study the interactions between two colloidal particles,the results showed that the interaction forces between microalgal cells have intimate connections with the harvesting effects.The acid-base interaction modified E-DLVO theory explained the harvesting of Chlorella vulgaris by using Fe3O4/APP in the alkaline solutions,the reason was that the "Hydrogen bond bridging" induced by the hydrogen bond between two colloidal particles,thus causing the violent collision between microalgae cells.The E-DLVO theory could describe the interactions between magnetic composites and Chlorella vulgaris cells more accurately.The magnetic force modified M-DLVO theory interpreted the reason that magnetic aggregates could be captured more quickly during the magnetic force interaction,for that the main energy of movement and collision of magnetic aggregates is the magnetic potential energy.The UM-DLVOo of Fe3O4/QPP-cell aggregates was more negative than that of Fe3O4/APP-cell aggregates,meaning that the Fe3O4/QPP-cell aggregates could realize the magnetic capturing process more quickly. |
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