Study On Mechanism Of Alginate Ultrafiltration Membrane Fouling Based On Analysis Of Transparent Exopolymer Particles Properties

In recent years,membrane technology has been widely used to treat surface water,domestic sewage and industrial wastewater in order to meet increasingly stringent water quality standards.However,membrane fouling is still the bottleneck of membrane technology application.Membrane fouling can reduce water yield,worsen effluent quality and increase operating cost.At present,natural organic matter is considered to be one of the main substances causing membrane pollution,including humic acid,protein and polysaccharide.Compared with protein and humic acid,polysaccharide with gel properties can cause more serious membrane fouling behavior.In addition,the binding of carboxyl groups on the polysaccharide chain with divalent or trivalent ions can enhance its gelatinization properties,which aggravate membrane fouling.The complexation of metal ions and polysaccharides is the transparent extracellular polymeric particles.In the membrane filtration process,TEP increases the removal rate of other natural organic matter by using the gel property and forms a dense pollution layer on the membrane surface.It can also provide growth energy for bacteria,resulting in a large number of bacteria on the membrane surface reproduction,finally leading to membrane biological pollution.Therefore,TEP is widely used in membrane fouling analysis as a characterization substance.The effect of different kinds and species of metal ions on the filtration behavior of sodium alginate fouling layer was studied.Firstly,based on the changes of TEP concentration of different fraction,the effects of Ca²⁺,Fe³⁺and Al³⁺on the membrane fouling behavior of sodium alginate were analyzed and the cross-linking structure of single ion with sodium alginate and the structure of sodium alginate when divalent and trivalent ions was coexisted was discussed.Subsequent studies found that hydrolysis products could affect the fouling behavior of sodium alginate membrane.Therefore,based on the changes of the colloid TEP and particle TEP concentration,the effects of Al（Ⅲ）and Fe（Ⅲ）with different polymerization species on the fouling layer of sodium alginate and the removal mechanism of sodium alginate were analyzed.In this work,a serious of filtration experiments were designed and implemented to compare the effects of different metal cations(Ca²⁺,Fe³⁺,and Al³⁺)on fouling by sodium alginate.The experimental results indicated that these three types of cations all induced biphasic filtration behavior by 50 mg/L sodium alginate with the maximumα_v value increased in the following order:Al(34.5±1.2×10¹⁴ mg/kg)<Ca(52.4±2.5×10¹⁴ mg/kg)≈Fe(54.1±1.2×10¹⁴ mg/kg),but with the critical value increased in the order:Al³⁺（0.02 m M）=Fe³⁺（0.02 m M）<Ca²⁺（0.05 m M）.The transparent exopolymer particle^0.05-0.1/transparent exopolymer particle ^Total ratio was a good indicator for the fouling by sodium alginate with these metal cations.Increasing the metal cation concentration induced a higher transparent exopolymer particle concentration with their sizes being initially decreased but then gradually increased mainly via the cation bridging effect,until the cation concentration reached up to a high value（2 m M）with the formation of metal hydroxides.Similar biphasic fouling behavior also observed in the coexistence of Ca²⁺and Fe³⁺/Al³⁺,but the comparable maximumα_v values were obtained at much lower ion concentrations.The synergistic fouling was owing to the transparent exopolymer particles^0.05-0.1 formation,hindered back transport of different sodium alginate molecules,and steric effect.Furthermore,there was little competition between the binding sites for divalent and trivalent ions,but the coiling behavior could be limited by changing the dosing sequence of the cations,leading to a weakened synergistic effect.In the study of Al（Ⅲ）and Fe（Ⅲ）polymerization on membrane fouling behavior of sodium alginate,Na OH and Na₂CO₃ was selected as the alkalizing agent to prepare PACl and PFC with different basicity values,respectively.Four coagulants(Al Cl₃,Fe Cl₃,PACl_2.5 and PFC₅)can induce biphasic filtration behavior of 1 g/L sodium alginate behavior at the B values≤0.5.PACl₂₃ and PFC₂₃ can only induce single phase filtration behavior when the B values was 2.3.The maximum a_av values and the maximum cake resistance increased in the following order:PACl₂₃<PFC₂₃<Al Cl₃<PFC₅<Fe Cl₃<PACl_2.5.The removal rate of polysaccharides increased in the following order:PACl₂₃<PFC₂₃<PACl_2.5<PFC₅<Fe Cl₃≈Al Cl₃.Fe_a can effectively promote the formation of small polymer of sodium alginate than Al_a,which leads to more serious membrane fouling behavior of sodium alginate.Al_a,Al_b,Fea and Feb in PACl_2.5 and PFC₅ can promote the formation of small molecule polymers and slow down the descent rate of membrane fouling.The adsorption capacity of Al_c and Fe_c in PACl₂₃ and PFC₂₃ for sodium alginate is weak,but due to their large particle size,which can be captured on membrane and form cake layer to slightly improve membrane fouling.Effects of three mechanism for the removal of sodium alginate decreased in the following order:complexation（Al_a and Fe_a）>charge neutralization（Al_b and Fe_b）>adsorption（Al_c and Fe_c）.The change of TEP concentration was consistent with that of membrane fouling resistance in the presence of PACl₂₃ and PFC₂₃.With the increase of coagulants(Al Cl₃,Fe Cl₃,PACl_2.5 and PFC₅),the increasing trend of TEP was significantly different from the change of first increasing and then decreasing resistance of sodium alginate.The results showed that Al_a,Al_b,Fe_a and Fe_b all promoted the formation of colloidal TEP,while only Al_a and Fe_a promoted the formation of particulate TEP.Al_c and Fe_c could not promote the formation of TEP,in spite to colloidal TEP or particulate TEP.