| At present,the municipal water supply mainly use iron pipelines.Due to the corrosion of iron,the water usually contains a certain amount of iron ions.In the process of using nanofiltration to treat municipal tap water,the pollution of the nanofiltration membrane will be affectd by iron ions.The effect of the pollution of nanofiltration membrane organic matter/calcium ion composite membranes,when the water contains iron ions,was studied in this paper.Humic acid(HA)is used as the main source of organic pollution in simulated water,and it is mixed with inorganic pollutants such as calcium chloride and ferric chloride to make simulated wastewater for experiment.The DF 400 nanofiltration membrane is tested with a small cross-flow filtration flat membrane device at 25℃and0.3 Mpa.The effects of different experimental conditions on permeation flux,rejection rate and steady-state flux of nanofiltration membrane is studied.And according to the change tendency of membrane anti-fouling property and membrane fouling rate in the process of membrane filtration,combined with scanning electron microscope(SEM)and optical microscope to observe the structure of pollutants and the mechanism of pollutant formation.It is showed that:(1)With the increase of the concentration of iron ions,the membrane specific flux shows a trend of decreasing first and then increasing.The pollution reach a peak at the iron concentration is 3 mg/L.And the rejection rate keep stable at 70%-73%.(2)With the increase of calcium ion concentration,the membrane specific flux also showed a trend of decreasing first and then increasing.It reached its lowest point when the dosage of calcium ion was 120 mg/L,and the rejection rate fluctuated between 60%-75%.(3)By comparing the decreasing trend of the membrane specific flux,the reduction rate of the membrane specific flux caused by the unit concentration of iron and calcium ion is calculated as 11.67%and 0.42%,indicating that the pollution ability of iron ion is stronger than calcium ion.(4)Comparing the trends of membrane specific flux with calcium ion concentration at pH 3 and 7.It is found that the membrane fouling was lighter and the rejection rate was smaller at pH 3.Combined with the structure of the contaminated layer,the membrane fouling esistance and membrane fouling rate,the pollution mechanism of the pollution phenomenon can be studied.This study draws the following conclusions:(1)The reason for the effect of iron ion concentration on the membrane specific flux is that calcium ions in the solution will have a bridging effect with the pollutants,and iron ions will cause flocculation.At low iron ion concentration,the bridge effect is greater than the flocculation effect.Therefore,the specific flux of the membrane will decrease.As the iron ion concentration increases,the effect of flocculation becomes more and more serious,resulting in a reduction in pollution.(2)The reason for the influence of calcium ion concentration on membrane pollution is mainly due to the bridging effect.Under the condition of low calcium ion concentration,nanofiltration membrane pollution is mainly caused by the surface bridging effect between the pollutants and calcium ions.However,as the calcium ion concentration increasing,the bridging effect in solution begins to manifest,resulting in the reducing of pollution.(3)The reason for the influence of pH on the membrane specific flux may be:(1)The nanofiltration membrane is suitable for use in a neutral environment,an over-acid environment may affect the function of the nanofiltration membrane,which leads to the destruction of the retention capacity of the nanofiltration membrane.(2)H~+in an acidic environment occupies the effective binding site,resulting in the reduction of the effective binding site and neutralization the surface potential of nanofiltration membrane.Thus the Donan effect is reduced,which leads to a reduction in the adhesion rate of pollutants on the surface,and then reducing the degree of pollution.(3)Iron is in ionic state in acid environment,so the flocculation effect will be weakened which lead to bridging are dominant. |
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