Publication: Superhydrophobic polyvinylidene fluoride-halloysite membranes for oxytetracycline treatment via membrane distillation
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Date
2023-01-01
Authors
Wan Aisyah Fadilah Binti Wae Abdulkadir Usin
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Abstract
The indiscriminate use of Oxytetracycline (OTC) contributes to water pollution, which adversely affects aquatic microorganism and to some extent harmed public health. Recently, membrane distillation (MD) is being introduced as a promising separation process for antibiotics (i.e., OTC) removal. However, the limitation of MD membrane is wetting and fouling issues that affect the efficiency of separation process. Therefore, in this study, superhydrophobic polymeric flat sheet membrane made from polyvinylidene fluoride (PVDF) and clay fillers (montmorillonite (MMT) and halloysite nanotube (HNT)) were developed via phase inversion with investigation of solvent, different types of clay fillers and fabrication parameters and modified through carnauba wax spray-coating. Clay fillers was selected due to its unique structure which helped to improve membrane permeability, mechanical properties and biofouling resistance. Meanwhile, the carnauba wax could provide a durable superhydrophobic surface by forming hierarchical structure on the membrane and minimize the wetting and fouling issue. The selected membrane was investigated on different HNT loading, immersion time, nominal thickness, addition of different types and concentrations of additives (polyethylene glycol 400 (PEG400) and Tween 80 (Tw80)). The composite membrane was characterized for its wetting properties, morphological structure, pore size and porosity. The prepared membrane had formed symmetrical structure in all formulations as shown by scanning electron microscopy (SEM) images. 0.5% HNT loading achieved 100% of OTC rejection with low flux permeation. This membrane maintained LEP >1 bar with hydrophobic contact angle (CA). The modification by optimal fabrications parameters (1 %PEG, 5 s of immersion time, 250 μm of nominal thickness) had improved flux permeation (11.06 ± 0.42) with rejection >97%. The optimum superhydrophobic membrane was obtained for 3 ml Ti(OBu)4 in carnauba wax solution at 2 min and 3 h of spraying and drying time, respectively. The CA had achieved to 157° with an improvement of surface roughness obtained by atomic force microscopy (AFM). The performance of superhydrophobic membrane using direct contact membrane distillation (DCMD) has achieved more than 99% rejection with average flux at different feed temperature, OTC concentration, OTC pH and different feed solutions. Regeneration and long-term operation studies revealed that superhydrophobic PVDF/HNT/PEG showed a positive impact on CA after DCMD, stable permeate flux and high OTC rejection with average value of 156°, 10.75 ± 0.34 L/m2h and 100% within 30 h of DCMD. Fouling mechanism was best fitted to pore constriction model with reversible fouling layer. The OTC separation for superhydrophobic PVDF/HNT/PEG membrane was observed to sustain when exposed to long-term operation. Thus, the modification approach of PVDF/clay membrane using HNT, and carnauba wax can be as an alternative strategy to develop MD membrane for organic pollutant treatment.