Removal of CrO4 anions from waters using surfactant enhanced hybrid PAC/MF process

Abstract

The removal of CrO42- anions from aqueous solution using surfactant added hybrid powdered activated carbon (PAC) and crossflow microfiltration (CFMF) process was performed. Experimental runs were studied at various amounts of cetyl trimethyl ammonium bromide (CTAB) as cationic surfactant, PAC as adsorbent and CrO42- ions in feed solution. CFMF unit was operated at constant values of the transmembrane pressure (150 kPa), crossflow velocity (1.18 m/s) and temperature (30 degrees C). Hybrid PAC/MF process performance was investigated in terms of CTAB and CrO42- rejections and, the flux decline. It was shown that the flux declined with time and the rejections varied for various properties of feed solution. CTAB and CrO42- rejections could be achieved as 91 and 97.2% at the conditions of 0.5 g PAC/L, 5 mM CTAB, 0.2 mM CrO42- and 120-min process time. It was found that over the critic micelle concentration (CMC), both CTAB and CrO42- rejections were in the higher values according to the values under the CMC. In addition, membrane blocking mechanisms were investigated to clarify the cake formation mechanism due to PAC. For all experimental conditions, the best results of blocking models obtained for cake filtration, intermediate blocking and standard pore blocking, respectively. It was understood that these blocking mechanisms occurred simultaneously on membrane surface in terms of high r(2) values. A cake layer formed on membrane surface (cake filtration). Pore entrance blocked partially due to particles bridging over pore opening (intermediate blocking). Free CTAB aggregates with or without CrO42- anions, entered into the membrane pores, caused to a layer formation by adsorption and entrapment on the membrane's pore walls (standard pore blocking). Besides, cake resistance (R-c), specific cake resistance (alpha), modified fouling index (MFI) and total dried solid mass of cake per unit membrane area (omega) were also determined to comprehend the reasons of clogging within the membrane and on the surface due to both CTAB aggregates and PAC, respectively. Membrane fouling (MFI) increased with the increasing of PAC, CTAB and CrO42- amounts in feed solution. As a result, flux, rejections and fouling in membrane were shown as a dynamic function of PAC, CTAB and CrO42- as the properties of the feed solution. (c) 2005 Elsevier B.V. All rights reserved.