Mechanism analysis of membrane fouling behavior by humic acid using atomic force microscopy: Effect of solution pH and hydrophilicity of PVDF ultrafiltration membrane interface

Abstract

The adhesion forces of membrane–humic acid (membrane–HA) and HA–HA were measured by atomic force microscopy (AFM) in conjunction with self-made modified membrane-coated and HA-coated probes, respectively. Normalized flux decline rate and the irreversible fouling index of fouling experiments with HA kept good linear correlations with the AFM force measurements. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was used to explain the influence and regularity of HA solution pH and hydrophilicity of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membrane interface on membrane fouling behavior. The results of AFM force measurements illustrated that the adhesion force in acidic environment was much stronger than that in alkaline or neutral environment and the adhesion force of PVDF/polyvinylalcohol membrane–HA (PA–HA) was weaker than that of PVDF/polyvinylpyrrolidone membrane–HA (PP–HA). XDLVO theoretical calculated results demonstrated that the polar component of surface tension (γAB) of membrane–foulant could be effectively increased by improving the hydrophilicity of the PVDF UF membrane interface and reducing the solution acidity, which resulted in the decrease of absolute values of the membrane–HA interfacial free energy of adhesion (ΔGmlfAD), the mitigation of the membrane fouling rate and the irreversible fouling degree. The results preliminarily proved that the measurements of microscopic interfacial forces with modified AFM colloidal probes had potential application values in UF membrane fouling behavior.