The Influence Of Surface Modification Of Carbon Fibers On Bacterial Cells Immobilization Based On DLVO Theory

The biofilm is an accumulated biomass of microorganisms and extracellular polymeric substances.The biofilm wastewater treatment is an economical and high-effective wastewater treatment method,which utilizes microorganisms in biofilm to absorb organic pollutants as nutrients for degradation.The surface properties of biofilm supports have an effect on microorganisms immobilization,thereby affecting wastewater purification efficiency.Carbon fibers（CF）are widely used as biofilm supports in biofilm wastewater treatment because of its good biocompatibility.Nevertheless,untreated CF are characterized with smooth surface,great inertia,low surface energy and low reactivity,which barely meet the growing demand of wastewater treatment.Accordingly,it is critical to improve biocompatibility of CF supports and wastewater treatment efficiency.The DLVO theory is named after Boris Derjaguin,Lev Landau,Evert Verwey and Theodor Overbeek,which is used to describe the charged colloid solution.It also could be used to describe the attachment of microorganisms to a surface.Based on experimental results and DLVO theory,this thesis aimed to systematically analyze the effects of different surface preparation methods on the biocompatibility of CF supports.It went deeply into the effects of surface properties on the biocompatibility of CF supports.The main conclusions can be summarized as follows:（1）Three kinds of CF hybrid materials（Hydroxyapatite/CF,Polymethyl methacrylate/CF,Carbon nanotubes/CF hybrid materials）were prepared by electrochemical deposition（ECD）.The biofilm colonization and bacteria cells immobilization assays indicated that carbon fiber hybrid materials had the higher capacity to immobilze bacteria cells than untreated CF.DLVO theory illustrated that CF hybrid materials had a lower total interaction energy than untreated CF.It was owning to the lower diiodomethane contact angles of CF hybrid mateials.And the lower diiodomethane contact angles resulted in the lower Lifshitz-van der Waals interaction energy.In addition,hydroxyapatite and polymethyl methacrylate,acting as a glue,could combine CF with bacterial cells,and carbon nanotubes wound around the curved surface of bacterial cells,which enabled CF to capture more bacterial cells.Both the findings and the theory demonstrated that CF hybrid materials had the high capacity to immobilize bacterial cells and could be used as excellent biofilm supports.（2）Nitric acid oxidation and urea modification were used to optimize the capacity of CF supports to immobilize bacterial cells.The biofilm colonization and bacteria cells immobilization assays revealed that CF after 2h oxidation immobilized the most bacaterial cells among oxidation CF.And urea modification could enhance the capacity of oxidation CF to immobilize bacterial cells.DLVO theory indicated that CF after 2h oxidation had a lowest interaction energy among oxidation CF supports,and urea modification could decrease the interaction energy of oxidation CF.Compared with untreated CF,oxidation CF had a lower diiodomethane contact angle and zeta potential because of oxygen containing groups.The lower diiodomethane contact angle and zeta potential resulted in lower Lifshitz-van der Waals interaction energy and higher electrostatic interaction energy,respectively.These two factors resulted in CF after 2h oxidation had a lowest interaction energy among oxidation CF.Urea modification increased the zeta potential without changing the diiodomethane contact angles,resulting in the lower interaction energy.Experimental results and DLVO theory revealed that a combination of nitric acid oxidation and urea modification could increase the capacity of CF supports to immobilize bacterial cells.（3）Graphite nanopetals（GNPs）containing a few layers of graphene were prepared on CF surface by microwave plasma chemical vapor deposition（MPCVD）.Vertically aligned graphenes with sharp edges grew on CF surface after 30 min reaction（CF-GP30）.And parallel aligned graphenes with curly edges grew on CF surface after 60 min reaction（CF-GP60）.The biofilm colonization and bacteria cells immobilization assays indicated that CF with GNPs coating could only immobilize few bacterial cells,while CF with GNPs after nitric acid oxidation(CF-OGP₆₀)could immobilize a larger number of bacterial cells.Bacterial cells viability assays illustrated that CF-GP₃₀0 had a strong bactericidal performance（13.6%E.coli cells alive,6.5%S.aureus cells alive）,while CF-GP60 and CF-OGP60 had nontoxicity for bacterial cells.The toxicity of CF-GP₃₀0 was attributed to the sharp edges of vertically aligned graphenes which could penetrate bacterial cells membrane and rupture the bacterial cells.According to DLVO theory,a big energy barrier appeared between CF-GP₃₀and bacterial cells.And bacterial cells could hardly break the energy barrier to attach the surface of CF-GP₃₀.There was not energy barrier between CF-OGP₆₀ and bacterial cells,and CF-OGP60 had a low interaction energy.Experimental results and DLVO theory concluded that CF-GP₃₀ was an excellent antibacterial material,as it had the capacity for prevention of bacterial adhesion and bactericidal function,while CF-OGP₆₀ could be a good biofilm support with high biocompatibility.（4）We comparative analyzed the biocompatibility of these five CF supports.The biofilm colonization and bacteria cells immobilization assays were used to evaluate the capacity of CF supports to immobilize bacterial cells.DLVO theory was used to calculate Lifshitz-van der waals interaction energy,electrostatic interaction energy and total interaction energy between CF supports and bacterial cells.Experimental findings and DLVO theory indicated that the capacity of these five CF supports to immobilize bacterial cells increases as follows:CF-OGP₆₀、CF-U5、CF-CNTs、CF-P20、CF-HA₁₈₀.The maximum biomass of these five biofilm supports is 3.21 g/g,3.43 g/g,3.73 g/g,3.85 g/g,4.14 g/g,respectively.Thus,CF-HA₁₈₀ is the best biofilm support among these five CF supports for wastewater treatment.