GROUNDWATER DEFLUORIDATION BY A CO-PROCESS OF DOLOMITE ADSORPTION AND NANOFILTRATION

Abstract

Co-process of dolomite adsorption and nanofiltration (NF) membrane for a removal of fluoride was investigated. Dolomite with a diameter of 0.42 mm was used as adsorbent and NF-1 membrane was used for filtration. Dolomite sorbent and NF-1 membrane properties were characterized i.e. point of zero charge, specific surface area, mineral and chemical composition, surface morphology, adsorption kinetic, pure water permeability, and concentration polarization. Point of zero charge (PZC) of dolomite was observed at pH 8.5. Specific surface area was 1.17 m2/g. Average pore size was 105.7 Å. Dolomite, calcite, and quartz were major components of dolomite rock. Main chemical compositions included calcium oxide (CaO) 72.19% and magnesium oxides (MgO) 22.61%. Equilibrium contact time of dolomite adsorption was 12 hr. The suitable kinetic model was pseudo-second order kinetic, which had the rate constant of 21.07 g/mM min. An isoelectric point of NF-1 membrane was found at pH 6.0. Pure water permeability of NF-1 membrane was 3.66 m3/m2×day×MPa. The mass transfer coefficient of fluoride was 1.539 m3/m2×day. Groundwater containing high fluoride concentration (12.14-15.38 mg/L) from Pra Too Khong bottle drinking water plant, Lamphun Province, Thailand was collected. Fluoride could be removed under feed groundwater pH 7 by 91% and 99 % via a sole NF-1 and RO-1 membrane, respectively. The co-process with an adjustment of defluoridated groundwater pH after dolomite adsorption to pH 7 prior to NF-1 membrane filtration showed 78% of fluoride rejection (9% fluoride rejection via dolomite adsorption). This explains that many ions and minerals released from dolomite during adsorption process. It caused high ionic strength in the water together with possibility of membrane fouling during membrane filtration process. As the result, the co-process presented lower fluoride rejection. Although the efficiency of the co-process dolomite adsorption and nanofiltration (NF) membrane presented lower fluoride rejection than using sole NF-1 membrane, this study can be applied to improve the quality of adsorbent to enhance the efficiency for fluoride removal in groundwater combine with NF membrane in future.