Thermodynamic Analysis Of The Effcet Of Draw Solution And Calcium And Magnesium Ions In Feed Solution On Forward Osmosis Filtration Behavior

Water scarcity and resource depletion have spurred the development of water treatment technologies.The driving force of the forward osmosis（FO）technology for water molecules to pass through the membrane is the osmotic pressure difference between the feed solution and draw solution.FO technology has attracted more attention owing to its lower energy consumption and lower fouling tendency.However,FO technology still faces the problem of membrane fouling which limits its wide application.Both the property of the feed solution and the reverse solute diffusion of draw solution have an important influence on the fouling of FO membrane.This paper is devoted to studying the influence of the thermodynamic mechanism of membrane fouling influenced by the three draw solutions of NaCl,Mg Cl₂,Ca Cl₂ and Ca²⁺,Mg²⁺concentration in feed solution.Details as follows:（1）Humic acid（HA）,bovine serum albumin（BSA）,and sodium alginate（SA）were used to simulate the foulants of humus,proteins and polysaccharides respectively,and the effects of inorganic draw solutions NaCl,Mg Cl₂ and Ca Cl₂ on the filtration behaviors of the above foulants were investigated.Results showed that,there were obvious differences among NaCl,Mg Cl₂ and Ca Cl₂ draw solutions of the effects on the filtration behavior of SA feed solutions.The percent of flux decline followed an order of Ca Cl₂（64.9%）>Mg Cl₂（47.7%）>NaCl（8.5%）when 500 m L SA solution were filtrated.Reverse-diffused Ca²⁺,Mg²⁺induced the formation and cross-linking of long SA chains,and the binding ability of bound water to gel chains were enhanced.Based on Flory-Huggins theory,the different SA filtration behaviors caused by the reverse solute diffusion of three draw solutions was explained from the perspective of water chemical potential.Compared with SA,there were no obvious differences among NaCl,Mg Cl₂ and Ca Cl₂ draw solutions of the effects on the filtration behavior of HA and BSA.The flux decline caused by filtering 500 m L HA and BSA were less than 15%and 21%,respectively.（2）The effects of different concentrations（0,1,5,10,15 m M）of Ca²⁺,Mg²⁺on the filtration behavior of SA solution were investigated when NaCl were used as draw solution.Results showed that FO membrane fouling first intensifies and then slows down with the increase of Ca²⁺concentration（from 0 to 15 m M）.SA solution added with 1 m M Ca²⁺formed a uniform and dense gel layer with the greatest flux decline（84.2%）.Increase in Ca²⁺concentration（5,10,15 m M）resulted in the increase in size of SA-Ca²⁺complex,resulting in the flocculation effect and the formation of a structure with large pores and defects,and the percent of flux decline decreased（<25%）.Mg²⁺had the property of inducing gel formation.The increase of Mg²⁺concentration（from 0 to 15 m M）aggravated membrane fouling.The percent of flux decline increased from 54.1%when 1m M Mg²⁺was added to 79.7%when 15m M Mg²⁺was added by filtrating 500 m L SA solutioms.There were no significant changes in the morphology and molecular size of SA solution,which promoted the formation of a uniform and dense gel layer.