Wettability studies of filter media using capillary rise test

Filtration process is a typical tertiary treatment method for oily wastewater, suitable for the lower oil concentration and oil-in-water emulsion system. Therein the wettability of oil-in-water emulsions to filter media probably has some significant influences on the oil removal efficiency, namely a lipophilic filter medium have a better performance in oil droplets coalescence and attachment than a hydrophilic one. In this paper, a Lipophilic to Hydrophilic Ratio (LHR) concept was defined on the basis of Washburn's equation and a test equipment was correspondingly designed, which were used to compare quantificationally the wettable selectivities of three filter media to oil and water. The selected filters were anthracite, manganese ore and quartz sand particles with a size fraction of +0.9 to 1.2 mm and the wetting liquids were apolar cyclohexane and polar deionized water. At the same time, the effect of filter particle size on the LHR value was also explored. Linear least-square fits for all wetting rates gave regression coefficients of more than 0.9991, confirming the suitability of the experimental method for filter particles and further validating Washburn's theory. The determined LHR values of anthracite, manganese ore and quartz sand are 1.93, 0.75 and 0.69, respectively, which means anthracite is lipophilic while manganese ore and quartz sand are hydrophilic. Moreover, for three selected particle size fractions the LHR values of anthracite particles are always the greatest and that of manganese ore are a little bit greater than quartz sand particles. Therefore, it can be deduced that the wettable differences probably be attributed to the differences of surface chemistries of filter samples. Namely, anthracite surface contains principally organic functional groups composed of carbon and oxygen elements and therefore presents apolar and lipophilic characterization, while manganese ore and quartz sand surfaces have SiO₂ species and make them polar and hydrophilic. This conclusion is approximatively confirmed by the analysis results of X-ray photoelectron spectroscopy (XPS).