Surface Modification Of Basalt Fiber Bio-carrier And Evaluation Of Its Application In Wastewater Treatment

Biological contact oxidation process is a kind of technology that integrates the advantages of activated sludge process and biofilm process,being widely used in wastewater treatment.As a major component of biological contact oxidation process,the type,geometry and surface properties of bio-carrier are vital for biomass aggregation,the following biofilm formation,structure and bacterial viability.Therefore,the development of high-performance bio-carrier has become a hotspot in the study of biological contact oxidation process.At present,most of the commercial bio-carriers are polymeric,and of which the production and disposal will cause environmental pollution.Basalt fiber(BF)is an environmental-friendly and high-performance inorganic fiber,produced by melting and extruding natural volcanic rocks.BF has excellent hydraulic property,high mechanical strength and corrosion resistance,and its potential in the application of bio-carrier has been primarily demonstrated.However,the capacity of commercial BF for microorganism adhesion is still limited by the surface properties such as hydrophobicity and electronegativity.In order to facilitate its extensive applications in environmental engineering,high performance BF bio-carrier is urgently in need.In this study,the surface properties of BF were improved by surface modification to enhance the bio-affinity.The mechanism of the adhesion behavior between bacteria and MBF bio-carrier was interpreted by surface thermodynamics and extended Derjaguin-Landau-Verwey-Overbeek(DLVO)theory,which could provide theoretical implications for the improvement of the bio-affinity of MBF bio-carrier.Furthermore,the wastewater treatment efficiency of modified basalt fiber(MBF)was evaluated in biological contact oxidation process.The main contents and results of this work were as follows:(1)Preparation and characterization of MBFBF was modified by surface coating,chemical grafting and liquid-phase deposition of iron compounds methods to obtain six types of MBF.The results showed that the hydrophilicity and surface charge of all MBF had been significantly improved compared to BF(water contact angle of 117.46°,Zeta potential of-17.64 mV).The water contact angle of CTAC-MBF,CPAM-MBF,CMBF,DEA-MBF,Fe-MBF-1 and Fe-MBF-2 was 61.64°,68.65°,44.06°,63.08o,56.74°and 64.85°,respectively.The surface charge of CTAC-MBF,CPAM-MBF,CMBF,DEA-MBF,Fe-MBF-1,and Fe-MBF-2 was+20.99 mV,+12.59 mV,+3.82 mV,+10.58 mV,-5.03 mV,and-8.03 mV,respectively.(2)Bio-affinity of MBF bio-carriersThe bio-affinity of MBF bio-carriers was evaluated by microbial immobilization tests.The structure of biofilm formed on MBF surface was characterized by SEM and the optical microscope.The results showed that the improvement of surface properties of MBF bio-carriers could enhance the biomass attachment and promote the subsequent formation and development of biofilm.The immobilization ratio of microorganisms of CTAC-MBF and CMBF bio-carriers was 224.74%and 218.40%,increased by 56.77%and 52.34%than BF,respectively.The amount of immobilized sludge on CPAM-MBF and DEA-MBF bio-carriers was 34.53 g/m~2 and 28.07 g/m~2 at 12 h,being about 2.5times and twice as that of BF bio-carrier(13.92 g/m~2).The results indicated that the surface charge of bio-carrier could determine the microorganism immobilization capability when the hydrophilicity of bio-carrier was comparable.Furthermore,the amount of immobilized sludge with Fe-MBF-1 and Fe-MBF-2 was 27.91 g/m~2 and26.48 g/m~2 at 12 h,both remarkably higher than that of BF bio-carrier.The total extracellular polymeric substance(EPS)content extracted from activated sludge adhered on BF,Fe-MBF-1 and Fe-MBF-2 bio-carriers surface was 14.55 mg/g?VSS,27.16 mg/g?VSS and 26.63 mg/g?VSS,respectively.The analysis of EPS content showed that the deposition of iron compounds can not only accelerate the bacteria adhesion to both Fe-MBF bio-carriers surface,but also enhance the microbial activity and increase the EPS secretion of microorganism.(3)Mechanism analysis of adhesion behavior between bacteria and MBF bio-carrier with extended DLVO theoryWith CPAM-MBF as representative bio-carrier,and Escherichia coli(E.coli)and Bacillus subtilis(B.subtilis)as representative bacteria,the adhesion behavior and interaction between bacteria and MBF was evaluated by extended DLVO theory.The results showed a strong repulsion between BF and E.coli in close contact,and thus irreversible adhesion of E.coli would not take place on BF.However,there was a secondary energy minimum at the long distance,which could allow E.coli adhering reversibly on BF surface.The interaction between CPAM-MBF and E.coli was turned to be strong attractive forces with no interaction energy barrier,and thus E.coli strains could irreversibly adhere on CPAM-MBF surface.As B.subtilis strain was more hydrophilic than E.coli strain,Lewis-acid-base interactions between B.subtilis and BF was attractive.Therefore,reversible adhesion of B.subtilis on BF surface at the long distance and possibly irreversible adhesion at the close distance would take place.Furthermore,the total interaction energy between CPAM-MBF and B.subtilis exhibited strong attractive forces,indicating that B.subtilis adhered irreversibly on CPAM-MBF surface.The improvement of hydrophilicity of basalt fiber can enhance the Lewis-acid-base interaction between bacteria and bio-carrier,while the improvement of the surface charge of basalt fiber would enhance the electrostatic interaction between bacteria and bio-carrier.Therefore,the more hydrophilic and positively charged CPAM-MBF bio-carrier enhanced the Lewis-acid-base and electrostatic attraction between the CPAM-MBF and bacteria,thus showed improved bio-affinity.(4)Wastewater treatment efficiency of MBF bio-carriersWith the synthetic municipal wastewater,the prepared MBF and BF bio-carriers was applied in bioreactors to verify its effectiveness in practical application.The results showed that prominent wastewater treatment efficiency was achieved by CPAM-MBF bio-carrier,followed by Fe-MBF-1,DEA-MBF,CTAC-MBF,CMBF and BF under stable operation conditions.For CPAM-MBF,the chemical oxygen demand(COD),ammonia nitrogen and total nitrogen removal efficiencies increased by 4.0%,0.5%and20.5%,respectively,compared with BF.Therefore,CPAM-MBF bio-carrier could have be a high-performance bio-carrier material in future application.