Research On Harvesting Performance And Mechanisms Of Chitosan-based Functionalized Flocculants On Oil-producing Microalgae

Microalgae are considered to be one of the most promising biomass resources for the energy industry because of their high photosynthetic efficiency and rich content of oil,carbohydrates,and proteins.However,the small cell size and low biomass of microalgae and the expensive harvesting process limited its application as a biomass energy source on a large scale.Polymeric chitosan flocculants have great application potential in microalgae harvesting due to its non-toxic,non-hazardous and biodegradable characteristics.However,the use of chitosan as a flocculant requires high dosage and is not economically feasible.In addition,the harvesting mechanism between microalgae and flocculants has not been clarified,resulting in low harvesting efficiency and lack of specificity,and the impact on downstream production processes needs to be further explored.Therefore,this work focused on the harvesting performance and interaction mechanism of chitosan-based functionalized flocculants.Aiming at the flocculation properties and the utilization of microalgae biomass energy,the effectiveness of chitosan-based functionalized flocculants in microalgae harvesting and the impact on downstream production processes was investigated,and the changes of the interaction mechanism between the composite flocculant and microalgae was revealed at the microscopic level.The main research contents and conclusions are as follows:（1）Analysis of the performance of Fe₃O₄-chitosan cross-linked composite flocculant for efficient harvesting of microalgae.A magnetic chitosan polymer flocculant（Fe₃O₄-chitosan）with high collection capacity and fast separation rate was successfully prepared by ionic gelation method using sodium tripolyphosphate（Sodium tripolyphosphate,STPP）as a cross-linking agent and applied to microalgae harvesting.The results showed that the harvesting efficiency of C.vulgaris reached97.81%at p H=3.0,the mass ratio of chitosan to Fe₃O₄ was 3:1,the settling time was 3min,the dosage was 0.5 g/L,and the microalgae concentration was 1.09 g/L.The adsorption energy between Fe₃O₄-chitosan and microalgae cells was analyzed by Langmuir and Freundlich isothermal model.Fe₃O₄-chitosan harvesting of C.vulgaris is a non-homogeneous multilayer process,and the theoretical maximum adsorption capacity can reach 75.43 g/g.A quadratic polynomial model revealed that the effect of flocculant dosage on the harvesting process was significantly higher than that of microalgae concentration and p H of microalgae solution.The harvesting efficiency of Fe₃O₄-chitosan composite flocculant was maintained at 85.10%after four cycles.It was shown that the harvesting mechanism between Fe₃O₄-chitosan and C.vulgaris was mainly adsorption bridging and charge neutralization.（2）Study of chitosan-clay mineral inorganic composite flocculants to enhance microalgae harvesting and settling performance.In order to further improve the harvesting efficiency and settling performance of microalgae,three different inorganic clay minerals（kaolin,montmorillonite and diatomaceous）were selected as negative carriers,and chitosan-kaolin（CTS/KAL）,chitosan-montmorillonite（CTS/MML）and chitosan-diatomaceous（CTS/DTE）composite flocculants were prepared by solution blending method,and their effects on the harvesting and settling characteristics of microalgae were investigated.The results showed that the harvesting efficiency of CTS/KAL,CTS/MML and CTS/DTE reached 97.37%,97.81%and 98.34%,respectively,at p H=10.0,dosage of 60 mg/L,settling time of 30 min,and microalgae concentration of 0.923 g/L.Second-order polynomial model fitting showed that the effect of microalgal concentration on harvesting efficiency was higher than that of settling time and flocculants dosage.The number and density of flocs after CTS/DTE harvesting were higher than those of CTS/KAL and CTS/MML,with equivalent diameters>0.27 mm.The fractal dimension of the flocs after CTS/KAL harvesting was 1.78,indicating a tightly connected internal structure.The chitosan-clay mineral inorganic composite flocculant has high specific surface area and surface charge,which provided more adsorption sites for C.vulgaris harvesting and promoted charge neutralization,sweeping and bridging,thus improved the harvesting efficiency of microalgae.The study integrated the clay mineral resources and realized the resourceization of clay minerals and the preparation of chitosan-based functional flocculants,and improved the harvesting performance and settling properties of microalgae.（3）Study of chitosan-cyclodextrin composite flocculants to improve the harvesting and dewatering performance of microalgae.To further improve the harvesting efficiency and dewatering performance of highly concentrated microalgae,a series of chitosan-cyclodextrin composite flocculants（CTS/β-CDs）with different graft chain length（L）and graft chain density（D）distributions were prepared by graft copolymerization method by regulating different ratios of chitosan,β-cyclodextrin（β-CD）and 2-methacryloyloxyethyl trimethyl ammonium chloride（DMC）,and ammonium nitrate（CAN）,the effects on the harvesting efficiency and dewatering performance of highly concentrated microalgae were investigated.The results showed that the harvesting efficiency of CTS/β-CDs composite flocculant on high concentration of microalgae could reach more than 94%when p H=8.0,the dosage was 100 mg/L and the microalgae concentration was 2.19 g/L,and the harvesting performance of CTS/β-CD-1 reached 99.91%.CTS/β-CDs with medium graft chain length and high-density distribution of graft chains could be effectively enhanced the water solubility and surface charge of flocculants,and improved the adsorption bridging and charge neutralization effects.The use of CTS/β-CDs promoted the dewatering performance of subsequent microalgal biomass by analyzing the moisture content（MC）,specific resistance to filtration（SRF）,particle size distribution and floc fractal dimension of microalgal flocs.The dual flocculant system of CTS/DTE+CTS/β-CDs was used to harvest microalgae,and the results showed that the dual flocculant harvesting system was significantly higher than the single flocculant harvesting system,and the harvesting efficiency was 94.86%when CTS/β-CD-1（7.14mg/L）was added first and then CTS/DTE（28.5 mg/L）was added.The dual flocculant harvesting system was able to reduce both the amount of flocculant added and the residual concentration of flocculant.This study further enhances the harvesting and dehydration properties of high concentration microalgae by increasing the water soluble,cationic characteristics and amorphous structure of chitosan,and provides theoretical reference for the application of high concentration microalgae.（4）Study on the effects of chitosan-based functionalized flocculants on downstream processes and water recycling.The effects of chitosan-based functionalized materials on downstream processes and water recycling were investigated by analyzing the contents of oil,carbohydrates,proteins,and fatty acids in the post-harvest biomass.The results showed that the lipids,carbohydrate,and protein as well as fatty acid contents in the Fe₃O₄-chitosan collected biomass were reduced,the lipids content was less affected.After the harvesting of chitosan-clay mineral inorganic composite flocculant,the lipids and carbohydrate contents of microalgae biomass were slightly reduced,and the protein content did not change significantly.CTS/β-CDs had no significant inhibitory effect on the extraction of biochemical components from biomass,which proved that CTS/β-CDs were biocompatible when applied to the harvesting process of microalgae.Dissolved organic matters（DOMs）were analyzed by EEM coupled with PARAFAC combined with fluorescence index.The results showed that DOMs mainly contained four components,which were humic acid substances represented by huminic acid,humic acid-like substances represented by fulvic acid,protein-like tryptophan substances and natural xanthic acid compounds,respectively.There was a positive correlation between changes in Biotic index（BIX）and Humus index（HIX）and the harvesting performance of chitosan-based functionalized flocculants.The harvesting water of CTS-clays and CTS/β-CDs can be used in combination with 50%fresh BG11 media to support biomass growth of C.vulgaris and reduce harvesting cost.This study presents the idea of integrated production and application of microalgal biomass energy,which practices the concept of green and sustainable development and is expected to realize the industrial scale application of microalgal biomass energy.（5）Study on the interaction mechanism between chitosan-based functionalized flocculants and harvested microalgae.The DLVO and E-DLVO models were used to further reveal the differences in the interaction mechanisms between chitosan-based functionalized flocculants and microalgae at the microscopic level.The results show that E-DLVO could be more accurately explained the interaction mechanism between microalgae and different chitosan-based functionalized flocculants.The₍-（1）interaction potential energy between microalgae is positive and exhibits repulsive potential energy,mainly contributed by the_Edl interaction force.The addition of Fe₃O₄-chitosan,CTS/DTE,and CTS/β-CD-1 all resulted in a higher gravitational interaction than the repulsive potential energy between microalgae,which led to the destabilization and sedimentation of microalgae.When p H=3.0,the₍-（1）interaction between Fe₃O₄-chitosan and C.vulgaris was mainly expressed as gravitational potential energy,and the contribution of_Edl interaction was higher than that of_AB and_Vdwinteraction.At p H=10.0,the₍-（1）interaction between CTS/DTE and C.vulgaris exhibited repulsive interaction but with high harvesting efficiency,indicating that the flocculation mechanism between CTS/DTE and C.vulgaris was mainly dominated by adsorption bridging.The harvesting performance of CTS/β-CD-1 on C.vulgaris was mainly a combination of electrostatic and acid-base forces.At higher p H,the harvesting performance of CTS/β-CD-1 on C.vulgaris decreased,the₍-（1）effect showed repulsive force,and electrostatic force was the main harvesting mechanism between CTS/β-CD-1 and C.vulgaris.This study systematically reveals the mechanism of interaction between different chitosan-based functionalized flocculants and microalgae,and further demonstrates that chitosan-based functionalized flocculants can effectively improve the harvesting performance of microalgae.