| The global explosion of harmful algal blooms has had many negative impacts on water ecosystems and human health.At the same time,the continuous consumption of fossil energy with economic development has led to a crisis in fossil energy.However,algal bloom biomass energy conversion technology can solve the dual challenges of water pollution and energy shortage at the same time.Fermentation can convert algal biomass into clean fuel.However,the size of algal cells in algal bloom suspensions is in the micron range and the surface of the cells is negatively charged,making it difficult to harvest algal biomass as the cells are stably suspended in water.In addition,the dense cell wall structure of algal biomass hinders the external transfer of intracellular organic matter,resulting in a lack of easily degradable organic matter,which severely limits the use of intracellular organic matter by fermenting microorganisms and the energy conversion of algal biomass.Finally,existing techniques for harvesting algal bloom biomass contribute little to biomass C:N regulation,and low substrate C:N promotes ammonium inhibition in the anaerobic fermentation process,leading to low efficiency in its fermentation energy conversion.The paper firstly addresses the problems of low harvesting efficiency of algal biomass and difficulties in breaking the cell wall of microalgae,and designs a gradient electric treatment strategy to simultaneously harvest and pre-treat the algal biomass and break the wall.The evolution of the organic matter fraction of the electro-oxidized algal biomass system and its influence on the biochemical transformation of dark fermentation were obtained.Secondly,a strategy of using chitosan as a bifunctional medium was proposed to address the low C:N of algal biomass dark fermentation substrate,which can be used as a flocculant for efficient harvesting of algal biomass and as a C:N regulator of the substrate to promote fermentation energy conversion.The harvesting mechanism from the perspective of algal bloom biomass kinetics under different environments is described.The mechanisms associated with the use of chitosan as a carbon source supplement to promote the energy conversion of algal bloom biomass are revealed.Finally,considering the cost of supplementary carbon sources,a novel,inexpensive and environmentally friendly cationic straw flocculant was developed using wheat straw as a raw material with quaternary ammonium groups replacing the hydroxyl groups on cellulose to synergistically enhance the harvesting and fermentative energy conversion of algal bloom biomass.The preparation method and mechanism of action of the cationic straw flocculant were mastered;a mathematical model was constructed to optimize the preparation of cationic straw by the coupling of multiple factors;the thermodynamics and kinetics of the harvesting of algal biomass by cationic straw were comprehensively studied;and the mechanism of its enhanced fermentation energy conversion was mastered.The main conclusions obtained in this paper are as follows.(1)A gradient electrical treatment strategy was designed to achieve efficient algal bloom biomass harvesting and biomass cell wall breaking,with the inherent advantages of no secondary contamination and high economic feasibility.Firstly,a low current density(0.888～4.444 m A/cm~2)was used for electro-flocculation harvesting of algal biomass suspensions,with a harvesting efficiency of 98.59%;a mathematical model of algal biomass harvesting with the coupling of multiple influencing factors was developed in order to guide the large-scale application.Secondly,the electro-oxidative decomposition of harvested algal biomass was performed at a higher current density(44.44 m A/cm~2),which effectively improved the dark fermentation bioavailability of algal biomass.The results showed that under 6 min electro-oxidation conditions,the hydrogen and methane yields were 64.46m L/(g VS)and 171.82 m L/(g VS),respectively,with a maximum energy yield of 50.1 k J/L and a maximum energy conversion efficiency of 44.87%.The mechanisms of harvesting and destruction of algal bloom biomass during gradient electrical treatment and the conversion pathway from algal bloom biomass to bioenergy were revealed.In conclusion,this work provides a promising strategy for the effective treatment of algal bloom biomass with the added benefit of bioenergy generation.2)A strategy using chitosan as a bifunctional medium was proposed,i.e.chitosan was firstly used as a flocculant for efficient harvesting of algal biomass,and then chitosan was used as a carbon source supplement to regulate the fermentation substrate C:N for bioenergy production.At a concentration of 1.0 g/L of algal biomass and a chitosan dosing rate of 0.50g/L at pH 2,the harvesting efficiency was 97.58%.The harvesting mechanism of algal bloom biomass is described from a kinetic point of view.More importantly,the C:N ratio of the harvested floc was increased to 11.82 by mixing C-rich chitosan with N-rich algal biomass,resulting in better bioenergy production performance of the feedstock.The results showed that the increased C:N ratio of the chitosan-microalgae floc increased the dark fermentation hydrogen production to 49.99 m L/(g VS)and volatile fatty acid production to 1672.35 mg/L of butyric acid,1170.49 mg/L of acetic acid and 361.93 mg/L of ethanol.The highest energy yield of 70.1 k J/L was obtained at this time,with an energy conversion efficiency of 59%.In summary,the conversion mechanism of dark fermentation indicates that the conversion of active fatty acids occurred during dark fermentation.This study will provide inspiration for the simultaneous bioenergy yield of environmentally friendly algal wastewater treatment.3)A new,inexpensive and environmentally friendly cationic straw flocculant was developed,using wheat straw as raw material and employing a quaternary ammonium group to replace the hydroxyl group on the cellulose.It gives the cationic straw a high positive charge and enhances algal bloom biomass cell aggregation through charge neutralization,achieving a 93.92%algal bloom harvesting efficiency.Unlike inorganic salt flocculants,the adsorption of algal bloom biomass by cationic straw is a spontaneous exothermic process with negativeΔG°andΔH°under all adsorption conditions.Thermodynamic and kinetic analyses also indicate that the adsorption of algal biomass by cationic straw is mainly driven by physical forces.A comprehensive optimization of the cationic straw preparation process and the algal biomass harvesting process was carried out using orthogonal tests.The highest dark fermentation hydrogen yields[55.65 m L/(g VS)]and volatile fatty acid yields(ethanol11399.3 mg/L,acetic acid 413.4 mg/L and butyric acid 479.56 mg/L as the main volatile fatty acid products)were obtained for the cationic straw harvested algal biomass after hydrothermal pretreatment at 160°C for 1.5 h.Also,the highest energy yield of 64.25 k J/L was obtained,with an energy conversion efficiency of 66.97%. |
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