Removal Of Humic Acid-TiO₂ Nanoparticles Composite Pollutants By Coagulation-ultrafiltration Combined Process

Nano-TiO₂ is one of the most commonly used nanometer materials in water treatment filed.With the production and usge,a large amount of nano-TiO₂ is imported into natural water directly or indirectly and has an influence on the ecological environment.Humic acid is the most common organic pollutant in natural water,it can react with nano-TiO₂ entering the water.Ultrafiltration membrane technology is widely used in water treatment field,as an effective pretreatment means,coagulation can reduce membrane fouling and extend the life of ultrafiltration membrane technology greatly.In this paper,the self-made poly ferric sulfate（PFS）was used to deal with humic acid-TiO₂-nanopaticles compound pollutants by coagulation-ultrafiltration combined process under different PFS dosage,pH value,stirring procedures and bivalent calcium ion dosage condition to investigate the effect of floc characteristic on membrane.At last,mechanism of membrane surface was explained.The main contents and results are as follows:（1）The particle size distribution and Zeta potential of the same concentration of HA and nano-TiO₂ were characterized.And UV-visible scanning,three-dimensional fluorescence spectroscopy and synchronous fluorescence spectroscopy were conducted.The results showed that the presence of HA has a very important effect on stability of TiO₂-NPs system by reducing particle size and enhancing surface negative charge of TiO₂-NPs in aqueous environment.The presence of TiO₂-NPs also has a very important effect on the structure of HA in aqueous environment.The addition of TiO₂-NPs increases the UV-VIS absorbance of HA,and quenches the quenching intensity.With the increase of TiO₂-NPs concentration,3DEEM scanning showed the same redshift trend with SSF scanning spectra.（2）Coagulant dosage,pH vaule and stirring procedures have an important effect on the treatment of HA-T compound pollutants by C-UF combined process.When PFS was used as coagulant,the dosage of PFS = 0.456 mmol / L,pH = 8（weakly alkaline）,240 rpm / min-2 min,40 rpm / min-18 min,10 rpm / min-10 min three-stage agitation procedure is the best operating condition for C-UF.Under this condition,the HA molecules were linearly stretched and had a stable complex reaction with TiO₂-NPs.At this time,the coagulation mechanism was mainly composed of adsorptive bridging and net trapping,and the floc size was large,the fractal dimension was suitable.The floc structure is loose and porous with a certain strength,the cake layer of membrane surface filled with the macromolecule skeleton structure at different angles of staggered connection,the gap between the skeleton is large and with high permeability,the end of the normalized flux was high,and membrane pollution was lighter.After the floc braskage-regeneration program,the normalized flux of the ultrafiltration membrane decreased significantly,and the membrane fouling deteriorated with the increase of the crushing strength.（3）The effect of Ca²+ on the treatment of HA-T complex pollutants by C-UF process was mainly related to the dosage of PFS.When the PFS dosage equal to 0.23 mmol/L,the addition of 2 mmol/L Ca²+ could significantly improve the effluent quality and reduce the pollution of the ultrafiltration membrane.When the PFS dosage equal to 0.46 mmol/L,the water quality of UF effluent increased slightly with the increase of Ca²+ concentration and then decreased.When the PFS dosage equal to 0.57 mmol/L,the water quality of C-UF effluent decreased with the increase of Ca²+ concentration.and normalized flux dropped sharply.At the same time,Ca²+ can significantly change the floc morphology of the cake layer on the ultrafiltration membrane.At low PFS dosing,the surface of the ultrafiltration membrane was changed from small particle floc structure to loose porous and large-sized floc with layered structure.At the optimum PFS dosage and the high PFS dosage,the surface structure of the filter cake layer becomes denser and lamellar with the addition of Ca²+.It is obviously different from the macromolecule skeleton structure without Ca²+.（4）The mechanism of surface contamination of ultrafiltration membrane is mainly related to HA-T complex pollutant properties,floc characteristics and Zeta potential.Coagulation dosage,pH,stirring procedure and Ca²+ concentration can change the properties of HA-T complex pollutants in the raw water,the floc characteristics formed during the coagulation stage,the effluent Zeta potential value and influence normalized flux at last.The higher aggregation degree of HA-T complex pollutants,the more the contaminant structure formed in the cogulation,the larger ofrthe floc size formed during the coagulation stage,the lower the fractal dimension with certain floc strength to resist breakage,and the zeta potential is negative,the end normalized flux of ultrafiltration membrane was higher,the membrane pollution was lighter.