Performance and mechanism of a hydrophobic–oleophilic kapok filter for oil/water separation

A natural vegetable fiber, kapok, was evaluated for its feasibility as filter material for separating immiscible liquids such as oil/water mixtures. Two types of oils, namely diesel and hydraulic, were investigated. Various simulated oily waters were prepared by mixing tap water with one of these oils to produce binary oil/water mixtures of varying oil percentages, such as 5%, 10% and 15% by weight. The effect of filter packing density on the dynamics of oil/water separation during the filtration process and the filtrate quality were investigated. Due to the hydrophobic–oleophilic surface characteristic of the kapok fiber, water could be filtered through the filter while oils were retained, causing separation of oil from water. The filtration efficiency for removal of diesel and hydraulic oil from their respective oily waters reached 100% and more than 99.4%, respectively, which showed an excellent performance of kapok in removing oils from oily waters. The hydraulic conductivity of the kapok filter was found to decrease with the advance of the oil front. The filtration rate of both types of oily waters decreased with increasing filter packing densities and influent oil contents. The kapok filter performed equally well in terms of total filtrate collected before breakthrough for the two different oils, albeit the filtrate rate could be viscosity dependent. A packing density of 0.07 g/cm3 was the optimum one for diesel and hydraulic oil removal. At breakthrough, the oil/water/air three-phase partition within the filter bed is a function of influent oil concentration, oil type, and packing density of the filter.