Adsorptive hollow fibre ceramic membrane derived from palm oil fuel ash for arsenic removal

Abstract

The use of ceramic membranes as sorbents has more advantages compared to polymer filtration systems. However, their production high costs with regards to raw materials has somewhat limited further applicability. Thus, the potential of palm oil fuel ash (POFA), an agricultural waste as a low cost adsorbent for the removal of arsenite (As(III)) and arsenate (As(V)) was explored. This study comprised of two stages: (1) POFA powder: characterization and adsorption mechanism and (2) POFA adsorptive hollow fibre ceramic membrane: properties and separation performance. In stage 1 of the study, the POFA powder were characterized using nitrogen adsorption-desorption, field emission scanning electron microscopy-energydispersive X-ray spectroscopy mapping, X-ray fluorescence, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analyses while adsorptivity activity was examined by batch adsorption studies. The maximum adsorption capacities of 78.0 and 94.6 mg·g-1 for As(III) and As(V) were achieved when the smallest particle size of 30 µm POFA was used and increased from 18.75 to 99.4 mg·g-1 for both As species with increasing of calcination temperature from 900 to 1150 °C. Desorption test revealed that As-loaded POFA was stable in water. The equilibrium data was better described by the pseudo-second-order model for both As(III) and As(V) while in adsorption isotherm study, the data were better fitted to the Langmuir isotherm model. All the results were then optimized by response surface methodology which concluded that calcination temperature has a major significance in the adsorption proses. Further attempt of molecular modeling study using the density functional theory via Gaussian 09 software consequently identified optimized structure of SiO- molecule and the energy for the proposed mechanism routes between the As+ species. In stage 2, based on excellent properties and condition from stage 1 namely 30 µm particle size and calcination temperature, POFA hollow fibre ceramic membrane (PHFCM) was fabricated via phase inversion and sintering technique at three different sintering temperatures i.e. 1100, 1150 and 1200 °C, by which the samples were named as PHFCM-1100, PHFCM 1150 and PHFCM-1200 respectively. The characterization analyses clearly showed that the PHFCM was constructed of two concentric rings with rich composition of Si and Al. The highest mechanical strength of 52.84 MPa and permeation flux of 250.73 L/m2.h of PHFCM-1150 was in favor for adsorption of As species yielding maximum adsorption capacities corresponding to 95.62 and 98.34 mg·g-1 of As(III) and As(V) which then were selected for further exploration with ozonation study. The enhanced adsorption of As(III) and As(V) by the PHFCM-1150 was associated during preozonation. For post-ozonation, 3 min exposure time used had permitted satisfactory cleaning of PHFCM-1150 to mitigate fouling problem while allowing repeated usages of the adsorbent for As removal. The performance of with and without ozonated PHFCM-1150 was evaluated with real wastewater samples and showed almost total rejection of arsenic contamination which signified the possible implementation in real wastewater system. Finally, this study has demonstrated that adsorptive PHFCM was effective and its respective As removal met the maximum discharge limit of 10 µg/L set by the world health organization and the national legislation in Malaysia.