Study On Behavior And Control Of Gypsum Scaling On Nanofiltration Membranes

Nanofiltration membranes have an excellent rejection efficiency of calcium ions and sulfate in brackish water,resulting in serious inorganic fouling.Meanwhile,the effects of other coexisting pollutants,including inorganic pollutants,organic pollutants and inorganic-organic complexation,on inorganic fouling on nanofiltration membranes are still unclear.Therefore,the present study started with the investigation about the effects of coexisting pollutants on gypsum scaling on nanofiltration membranes,and then the control of membrane fouling during nanofiltration of brackish water was further studied.Firstly,ferric ions and aluminum ions were chosen as the representatives of coexisting inorganic pollutants which could be introduced in the traditional municipal water treatment process.The effects of coexisting inorganic pre-deposition layer and solution coexisting inorganic pollutants on gypsum scaling on nanofiltration membranes were investigated at the same time.The results of pre-deposition experiments indicated that coexisting inorganic pollutants directly promoted the formation of dense gypsum scaling layer on nanofiltration membranes.The larger size made it easier for ferric ions to act as nuclei to enhance crystallization process and even membrane fouling.After prolonging the pre-deposition time properly,the flux decline caused by scaling became more obvious.However,excessively increasing the concentration of coexisting inorganic pollutants in pre-deposition solution could alleviate membrane fouling to a certain extent.The ability of solution coexisting inorganic pollutants to deteriorate gypsum scaling on nanofiltration membranes was weaker than that of coexisting inorganic pre-deposition layer,because the former indirectly induce loose membrane fouling by strengthen the formation and deposition of bulk crystallization on membrane surface.Adding coexisting ferric ions and increasing the p H of nanofiltration influent had a synergistic effect on aggravating long-term nanofiltration membrane fouling.The results of correlation analysis indicated that Fe（OH）₃ was a kind of important element for ferric ions to enhance gypsum scaling on nanofiltration membranes.Based on the effects of coexisting inorganic pollutants on gypsum scaling on nanofiltration membranes,humic acid（HA）,bovine serum albumin（BSA）and sodium alginate（SA）were selected as the representatives of different organic pollutants in the actual water for investigating the effects of coexisting organic pollutants and also coexisting inorganic-organic complexation on gypsum scaling on nanofiltration membranes.The experimental results showed that HA could enhance gypsum scaling on nanofiltration membranes by complexing with calcium ions and also had a synergistic effect on aggravating membrane flux decline with ferric ions,because the electrostatic force between HA and ferric ions strengthened the deposition of them and also the formation,deposition and overlapping of gypsum crystallization,which had a comparable size with membrane pores,on membrane surface,because they were difficult to freely complex with each other on membranes.Both BSA and SA could induce the formation of smooth organic fouling layer on nanofiltration membranes and the adhesion between calcium ions and them was weak.Therefore,they could alleviate membrane scaling by reducing the gypsum crystallization with a bigger size than membrane pores.Besides,the experiment results confirmed that ferric ions could weaken the atrazine rejection efficiency of nanofiltration membranes by means of weakening the steric hindrance effects.On the basis of the effects of coexisting pollutants on gypsum scaling on nanofiltration membranes,sodium hypochlorite（Na Cl O）,potassium ferrate（K₂Fe O₄）and potassium permanganate（KMn O₄）were chosen as the commonly-used oxidants for the investigation of the effects of pre-oxidation on controlling membrane fouling during nanofiltration of brackish water.The experimental results showed that with the increasing Na Cl O concentration,the short-term membrane fouling continuously increased,while the long-term membrane fouling increased firstly and then decreased.There is a positive correlation between the degree of Ca²⁺-HA complexation and the degree of nanofiltration membrane fouling.Although K₂Fe O₄ could aggravate the nanofiltration membrane fouling by introducing ferric ions when its concentration was low,it alleviated the membrane fouling when its concentration was high,probably because the large-sized flocs were easily detached from nanofiltration membrane surface under electrostatic repulsion.On the contrary,KMn O₄ remarkably alleviated nanofiltration membrane fouling when its concentration was low,because the soluble organic pollutants in the brackish water were effectively mineralized.However,it significantly increased nanofiltration membrane fouling when its concentration was high,because manganese oxides,the reduction product,could adsorb HA and then strengthen its complexation with calcium ions.Therefore,the bulk crystallization was enhanced and it could deposit on nanofiltration membranes more easily under the electrostatic attraction between pollutants and membranes,forming dense membrane fouling layer.What’s more,the experimental results verified the reliability of differential log-transformed absorbance spectroscopy in predicting short-term membrane which was influenced by the complexation between the carboxyl groups of humic acid and calcium ions,when it was applied in the wavelength range of 260-280 nm.The present study started with the investigation about the effects of coexisting inorganic pollutants,organic pollutants and inorganic-organic complexation on gypsum scaling on nanofiltration membranes,and on the basis of these results,the comprehensive effects of sodium Na Cl O,KFe₂O₄ and KMn O₄ pre-oxidation on membrane fouling during the nanofiltration of brackish water were further studied.The research results could provide theoretical basis and guidance for controlling inorganic fouling on nanofiltration membranes.