Research On Treatment Of Desulfurization Wastewater By Forward Osmosis Method And Surface Modification Of Membrane

Due to the characteristics of easy scaling and high salt content,flue gas desulfurization（FGD）wastewater has become one of the most difficult wastewater in the industry.At present,the membrane separation technology has been widely used in the field of FGD wastewater treatment.The forward osmosis technology driven by the osmotic pressure difference has some characteristics,such as low energy consumption,low membrane fouling tendency and high effluent water quality,which makes forward osmosis membrane separation technology have great potential in the field of desulfurization wastewater treatment.This paper studies the membrane fouling mechanism of the forward osmosis process in the treatment of desulfurization wastewater,the influence of operating parameters,membrane cleaning and membrane surface functionalization on the separation effect.The specific content and main conclusions are as follows:Firstly,this paper studies the influence of parameters such as permeation time,membrane orientation,osmotic pressure of the draw solution,experimental temperature,and rotating speed in the forward osmosis process on the treatment of FGD wastewater,which includes the water molar flux,the membrane to Mg²⁺,Ca²⁺,SO₄^2-and Cl^-separation and fouling index on the membrane surface.The results show that as the permeation time increases,the water molar flux and ion separation gradually decrease,and the membrane fouling index increases.Membrane fouling is lower when the separation layer faces the feed solution.The increase in the osmotic pressure of the draw solution is beneficial to promote the water molar flux and ion separation,but it also produces a higher membrane fouling index.The rise of the experimental temperature makes the water molar flux and membrane fouling index increase,and the separation of ions decreases.Increasing the speed has a positive effect on the increase of water molar flux and ion separation and the decrease of the membrane fouling index.In the forward osmosis process,the degree of separation of Mg²⁺,Ca²⁺,SO₄^2-and Cl^-ions by the membrane is as follows:SO₄^2->Mg²⁺>Ca²⁺>Cl^-.Next,this article uses SEM,XRD,TGA,XPS,ATR-FTIR and other characterization methods to determine the composition of membrane fouling,which include gypsum（CaSO₄·2H₂O）and calcite(Mg_0.03Ca_0.97（CO₃）,Ca CO₃).The fouling mechanism is that the concentration of SO₄^2-,Mg²⁺,Ca²⁺,and CO₃^2-ions on the separation layer are supersaturated and precipitated,the specific interaction between Ca²⁺and the carboxyl functional group promotes the pre-nucleation of CaSO₄·2H₂O crystals,inducing gypsum fouling is formed on the surface of the TFC membrane.The membrane cleaning was explored for inorganic fouling,the results show that the cleaning effect of acid washing followed by alkaline cleaning>acid cleaning>alkaline cleaning>DI water cleaning,the water molar flux recovery rate of the membranes after acid washing followed by alkaline washing and acid washing both exceeded95.0%.At the same time,the recovery rates of the separation of Mg²⁺,Ca²⁺,SO₄^2-and Cl^-ions of the membrane after pickling first and then alkaline cleaning were 99.73%,97.96%,97.54%,and 99.30%,respectively.Secondly,this article uses the method of oxidative polymerization of dopamine in Tris-HCl buffer solution to form a hydrophilic polydopamine（PDA）layer,which modifies the TFC membrane separation layer.The changes in the chemical structure,morphology and hydrophilicity of the modified membrane separation layer were analyzed by SEM,XPS,AFM,ATR-FTIR and contact angle measuring instrument.The results showed that the hydrophilic groups（-C=O,-NH₂,-OH）on the separation layer of the modified membrane increased and the roughness decreased,and the PDA layer was successfully coated on the surface of the membrane separation layer.Among them,the coating membrane（TFC-3 membrane）with the dopamine（DA）monomer concentration of 0.08 mmol/L has the best forward osmosis performance,the water molar flux of the FO process is 52 mol/（m²·h）higher than that of the original membrane,and the selectivity of the modified membrane is 3 times that of the original membrane.FGD wastewater was used as the penetrant to carry out the membrane surface fouling and physical cleaning experiments after modification.The results show that the TFC-3membrane exhibits a higher water molar flux and a lower membrane fouling index（10%）than the original membrane,the separation of SO₄^2-,Mg²⁺,Ca²⁺,Cl^-ions by TFC-3 membrane is twice that of TFC-0 membrane in the treatment of FGD wastewater,the water flux recovery rate of the TFC-3 membrane is 12.52%higher than that of the original membrane.The stability of the PDA layer under different pH environments was found to exist stably in acidic and neutral environments,while the stability of the PDA layer in a strongly alkaline solution（pH≥9）was poor.