Reasearch On The Influence Of Solution Condition On Microfiltration Fouling Behavior Induced By Colloidal Polymer

Polymer-flooding technology(PFT)has been widely applied in many oil-fields at home and abroad,which ensures the continuous supply of crude oil resources.However,a serious accompanying problem is how to reasonably dispose and decontaminate the huge by-product(polymer-flooding produced wastewater,PFPW)produced during the oil exploitation process.The composition of PFPW is complex,the discharge of it without effective treatment/decontamination may cause fearful harm to water,soil,aquatic animals and plants,air environment and even human health.As one of the mature technologies in membrane separation field,microfiltration is often used in the pretreatment of PFPW before advanced desalination.However,the membrane fouling occurred during the microfiltration process seriously hinders the stable and high-efficiency operation of the microfiltration process,and the accurate elaboration of its mechanisms is vital for membrane fouling control.During the polymer-flooding oil exploitation process,anionic polyacrylamide(APAM)is one of the most widely used polymer,which largely remains in PFPW and becomes one of the most potential foulants to restrict microfiltration performance.Moreover,In PFPW,the salinity of PFPW is generally in a high level with a range of 1000–40000 mg/L.Therefore,this research took APAM as the target foulant,polytetrafluoroethylene(PTFE)membrane with a pore size of 0.1?m as microfiltration membrane,and explored the influencing mechanism of p H,monovalent cations(Na⁺and K⁺),and divalent cations(Ca²⁺and Mg²⁺)on membrane fouling based on the extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory.In this study,the p H gradients were set to 3.0,5.0,8.0,10.0,and 12.0;and the gradient of ionic strength were set to 0,1000,2000,5000,and 10000 mg/L.Moreover,the influencing mechanism of different valence cations(Na⁺and Ca²⁺)on membrane fouling at an ionic strength of 1000 mg/L was analyzed.This research aims to provide theoretical guidance for the control of membrane fouling in the process of PFPW treatment.The main contents and conclusions of the paper are as follows:1)p H can affect the physicochemical properties of the membrane and APAM,thus changing the degree and situation of membrane fouling.It was found that,the acid–base component greatly affected the membrane fouling behavior in a short range.The value of energy barrier decreased with decreasing the p H value,and there existed a critical p H(3.5)below which the energy barrier would disappear,facilitating attachment of the APAM.The APAM–APAM energy barrier was lower than that of APAM–membrane at the same p H,indicating that the membrane fouling at the later stage of filtration was more serious.Moreover,the filtration resistance of the original membrane and pore blockage decreased slightly with an increase in p H,whereas that of the cake layer decreased significantly.Furthermore,the potential for membrane fouling and the total interfacial interaction free energies had an obvious linear dependence.2)The increase in ionic strength exacerbated the reduction of relative flux(J/J₀)and the accumulation of fouling resistance,as well as making the porous APAM-induced fouling layer denser and more compact,boosting removal efficiency.The XDLVO theory agreed well with membrane fouling behavior of various ionic strengths.By contrast,K⁺had a stronger aggravating effect on membrane fouling than Na⁺,and Mg²⁺had a stronger aggravating effect on membrane fouling than Ca²⁺.Specifically,the final value of J/J₀decreased from 0.38 to 0.28/0.25,0.24/0.18,0.17/0.12,0.12/0.08,and 0.19/0.15 as the ionic strength of Na⁺/K⁺increased from 0 to 1000,2000,5000,and 10000 mg/L,and that for Ca²⁺/Mg²⁺decreased from 0.38 to 0.35/0.32,0.28/0.25,0.23/0.20.The former was mainly owing to the different hydration properties between Na⁺and K⁺.The stronger hydration effect of Na⁺made its repulsive interaction between APAM and PTFE/fouled membrane to be stronger than that for K⁺,thus causing a less severe membrane fouling.The latter was mainly owing to the different hydration properties and bridging effect between Ca²⁺and Mg²⁺.Ca²⁺had a much stronger binding affinity to APAM than Mg²⁺owing to the much weaker hydration force induced by Ca²⁺and its stronger bridging effect,which resulted in a smaller negative charge density of APAM-Ca²⁺complexes and a larger and tighter APAM-Ca²⁺particles.Therefore,Ca²⁺had a weaker aggravating effect on membrane fouling than Mg²⁺.3)The addition of Na⁺/Ca²⁺(1000 mg/L)aggravated membrane fouling during the adsorption and microfiltration process.The simulation results of adsorption kinetics and membrane fouling model were consistent with the analysis results of membrane fouling resistances,filtration efficiencies,and membrane microscopic images.Compared with Na⁺,Ca²⁺induced a stronger hydration repulsive force,which promoted the formation of loose cake layer and slowed down the membrane fouling.