Nanocomposite ultrafiltration membrane incorporating zinc/iron oxide photocatalyst for decolourisation of treated palm oil mill effluent

Abstract

Palm oil mill effluent (POME) is brownish, high organic loading effluent produced by palm oil industry. Conventionally used biological treatment has successfully reduced the biochemical oxygen demand of POME below 25 ppm, which is considered as clean effluent. However, chemical oxygen demand (COD) and colour of the aerobically treated palm oil mill effluent (AT-POME) are still high. Therefore, tertiary treatment such as membrane processes are used to further polish AT-POME in order to reduce its COD and colour to meet discharge standard. However, fouling becomes severe issue in restricting the membrane lifespan and usage in this application. This work aimed to develop hybrid photocatalytic membranes with self-cleaning properties to mitigate the membrane fouling and also photodegrades the chemical compounds in AT-POME. First, coupled zinc-iron oxide (ZIO) was synthesized from its precursor, i.e. zinc nitrate and iron (III) nitrate through solution combustion by varying the molar ratio between zinc and iron (ranging from 1:1 to 1:4 with respect to zinc to iron ratio). Second, the optimum molar ratio of ZIO was calcined at temperature ranging from 400?C to 800?C. Third, the self-synthesised ZIO was incorporated into polyvinylidene fluoride (PVDF) polymer matrix to produce mixed matrix photocatalytic ultrafiltration membrane (MMMs) for decolourisation of AT-POME. Five membranes were formulated by varying ZIO from 0.0 wt % to 2.0 wt%. The fabricated membranes were subjected to physico-chemical analysis, i.e. field emission scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller, ultraviolet-visible-near infrared, absorption test, filtration process, photodegradation test to identify the self-cleaning properties, separation performance and fouling mitigation properties. Based on the experimental results, ZIO with 1:4 zinc to iron ratio was the optimum ZIO which provided large surface area (30.9130 m2/g), lowest band gap energy (2.07 eV), high photocatalytic activity (achieved 35% of mineralisation in 6.5 hours and 100% degradation in 3.5 hours). On the other hand, as calcination temperature increased, the particle size of ZIO increased gradually. This phenomenon led to the decrease of surface area of ZIO that reduced its performance in absorption and photodegradation of organic compounds. The results demonstrated that calcination temperature of 500?C was the optimum temperature to provide the highest photodegradation. For the MMMs, as the ZIO loading increased, the porosity decreased, and surface negativity increased. However, when higher loading of ZIO was used, the mechanical strength of the membrane structure deteriorated and cannot withstand long-term operation. Therefore, the optimum loading was identified as 0.5 wt% ZIO in which the membrane achieved 75% colour removal efficiency with flux of 20 – 25 LMH (L.m-2.hr-1). Furthermore, the self-fabricated MMMs photocatalytic ultrafiltration (UF) membrane possess the high flux recovery after ultraviolet and visible light cleaning, i.e. 92.3% and 90.3%, respectively. The addition of ZIO in polymeric matrix enabled the photodegradation of colour pigments in AT-POME. After 16 hours of operation, the colour of AT-POME reduced by 8%. In a nutshell, PVDF/ZIO photocatalytic UF membrane had successfully decolourised AT-POME and reduced its COD.