Collision-adhesion-based Study On The Transition Mechanism Of Interaction In The Initial Stage Of Membrane Filtration

Colloid fouling restricts the promotion and application of membrane technology.In the membrane filtration process,the initial fouling is determined by foulant-clean membrane interaction（F-M）,and the long-term fouling is determined by foulant-fouled membrane interaction（F-F）.Although the interaction transition（from F-M to F-F）has important influence and practical significance in the preparation of anti-fouling membranes,optimization of operating conditions,and raw water pretreatment,there is still a lack of a theoretical framework to explore its mechanism in depth.Based on this,this article uses Collision-Attachment（CA）theory in the field of coagulation and Monte Carlo method in the field of statistics to establish a membrane fouling theoretical model of Collision-Attachment-based Monte Carlo（CA-MC）.Use the model to systematically study the transitional behavior of the interaction in the initial stage of membrane filtration,and provide new directions and theoretical guiding significance for the formulation of pollution control strategies.The main research work and conclusions are as follows:（1）According to the CA theory,the membrane fouling is regarded as the two processes of colloid transporting and colliding to membrane surface,and then adhering to membrane surface;the rate of membrane fouling is determined by the collision frequency and adhesion probability;the random transport and collision process is determined by the MC method for simulation,the random adhesion behavior is simulated by Boltzmann energy level distribution;a random CA-MC theoretical model is established from the perspective of microscopic particle transport and adhesion.The simulation results are in good agreement with the concentration polarization theoretical curve and the experimental water flux reduction,which proves the reliability and accuracy of the model.（2）Using the model to study the change behavior of water flux under different energy barrier conditions,the results show that:under the effect of a higher F-F repulsive energy barrier E_f（11 k _BT）,even if the F-M repulsive energy barrier E_m is very low（0 k _BT）,the flux is still at higher level for long filtration duration（1000 min）;severe fouling occurs when the repulsive interactions of F-M and F-F are weak（for example,E_m=E_f=3 k _BT）,and flux decreases rapidly from the beginning.In the case of a higher E_m value and a lower E_f,the flux is in a metastable state in initial stage of filtration,and after a period of time,a rapid transition of interaction and water flux reduction begin to appear.Further analysis shows that the metastable flux duration t_mp is positively correlated with energy barrier（E_mand E_f）,but negatively correlated with the initial water flux（J₀）.The linear relationship is:ln t_mp=0.8E_m/k _BT+0.2E_f/k _BT-0.174J₀–ln（0.00159 J₀）-4.79.In order to reduce foouling and maintain a relatively long metastable state,membrane surface modification and raw water pretreatment should increase the repulsion of F-M and F-F,and the operation process should be kept at a lower flux.（3）Studying the evolution of particle deposition distribution on the membrnae surface shows that in the case of higher E_f,the initially deposited particles make the surface still keep a higher repulsive interaction,which is not conducive to the deposition of subsequent particles,and the transition of interaction is very slow.Under the action of higher E_m and lower E_f,particles are not easy to deposit in the initial stage;however,accidental particle deposition（due to Brownian motion）will reduce the local energy barrier on the membrane surface,acting as a“seed”of fouling and inducing the expansion of the fouling area.This will lead to a rapid transition of interaction;once there are obvious stains on the membrane surface,timely membrane cleaning is very important;cleaning can prevent the interaction from transitioning to weaker F-F interaction.（4）Study the influence of membrane surface’s inherent energy barrier defect E_md on the interaction transition and the duration of metastable flux.It can be seen that in the case of higher E_f,the membrane surface defects will"self-heal"with the deposition of the initial particles.At this time,the membrane defect does not affect the membrane flux behavior.In the case of E_m>>E_f,the defect area on the membrane surface will act as a fouling source,inducing the transport and aggregation of colloidal particles,which will accelerate the interaction transition and the occurrence of fouling,and the duration of the metastable flux will increase and increase with the density of membrane defects so membrane defects have a significant impact on fouling.（5）By analyzing the relationship between flux,fouling rate,and energy barrier,it can be seen that for a certain initial flux,there is a critical energy barrier value corresponding to a given fouling rate.When the energy barrier value E_m is greater than critical energy barrier E_c,there will be a metastable flux;further research found that the duration of metastable flux(ln t_mp)and energy barrier（E_m-E_c）are linear;for membranes with energy barrier defects,ln t_mp and energy barrier E_m-E_c is also linear.In order to make the membrane have a metastable state,the selection of operating flux should ensure that E_c is lower than E_m and E_md.