Chitosan-silica rice husk composite membrane for removal of lead (II) ion from aqueous solution

Abstract

Heavy metal such as lead can be classified as non-biodegradable inorganic pollutants which can contaminate the ground and surface water. It cannot be breakdown or decomposed by living organism and continues to exist over a prolonged period which can generate harmful effects to the living things. Hence, lead removal is necessary in order to diminish the amount of heavy metals contaminating the water. The purpose of this study was to fabricate chitosan-silica flat sheet composite membrane for removal of lead(II) metal ions from aqueous solution via phase inversion technique by using sodium silicate from rice husk. The chemical composition of rice husk ash was determined by X-ray fluorescence (XRF) analyzer and the crystallinity of rice husk ash and sodium silicate were observed from X-ray diffraction (XRD) analyzer. The functional groups and structural morphologies were characterized using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectrometer and scanning electron microscopy (SEM), respectively. The membranes were also subjected to swelling and tensile analysis to identify their mechanical properties. The membrane performance in terms of adsorption study was conducted at various pHs, initial concentrations of lead(II) solution and contact times as well as membrane recoveries. The membrane performance in terms of ultrafiltration study was evaluated using membrane cross-flow permeation system. XRF analysis confirmed that the rice husk ash had high silica content (87.4%). XRD analysis revealed the rice husk ash was present in amorphous phase while sodium silicate was in crystalline phase. The FTIR-ATR spectra showed the presence of new adsorption peaks (Si-O-Si bond), contributed by silica interaction with hydroxyl group of chitosan. Meanwhile, SEM images revealed the presence of microvoids on the cross-section of the CHSi20 membrane which improved the morphology of pure chitosan membrane and helped in the removal of lead(II) metal ions. Although, the swelling degree of chitosan membrane declined with the addition of sodium silicate, the tensile strength of chitosan membrane increased up to 0.2 g addition of sodium silicate. The adsorption study showed that CHSi20 membrane exhibited higher efficiency of lead(II) removal at optimum pH of 7.0, initial concentration of 220 mg/L of lead(II) solution and achieved equilibrium contact time after 16 h. In addition, the composite membrane behaviour was best fitted to Langmuir isotherm model and pseudo second-order model. However, the CHSi20 membrane had low membrane recovery due to the adsorption behaviour and kinetic mechanism of the membrane. Furthermore, the rejection of lead(II) metal ions by CHSi20 membrane was 97.56% at 0.5 bar applied pressure as compared to CHSi0 which was 11.72%. Therefore, these findings indicate the potential use of silica-derived rice husk to improve chitosan membrane properties, reduce agricultural waste dumping as well as reducing lead(II) metal ions polluted in water.