Micellar Enhanced Ultrafiltration for Arsenic(V) Removal: Effect of Main Operating Conditions and Dynamic Modelling F. BEOLCHINI,* ,† F. PAGNANELLI, ‡ I. DE MICHELIS, § AND F. VEGLIO Å § Dipartimento di Scienze del Mare, Universita ` Politecnica delle Marche, Via Brecce Bianche, Ancona, Italy, Dipartimento di Chimica, Facolta ` di S.M.F.N., Universita ` degli Studi “La Sapienza”, P.le A. Moro, 5, 00185 Roma, Italy, and Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita ` degli Studi di L’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy In this work arsenic removal by micellar enhanced ultrafiltration (MEUF) was investigated using cetylpyridinium chloride (CPC) in ceramic membrane apparatus. Permeability tests and discontinuous diafiltration tests were performed in different operating conditions to evaluate the effect of membrane pore size (20 and 50 nm), transmembrane pressure, pH, surfactant concentration (1-3 mM), and arsenic concentration (10-40 mg/L) on permeate flux decline, arsenic, and CPC rejections. These preliminary experimental results showed that a ceramic membrane with large pore size allows treament of high fluxes of concentrated arsenic-bearing solutions even by using low surfactant concentrations. Arsenic concentration in the permeate was at the 1 ppm level, with feed As concentrations (10 ppm) that are larger than those generally used in MEUF studies and with CPC amounts that are lower than the usual ones. In addition, operating conditions adopted in these tests obtained CPC concentrations in the permeate always lower than its critical micellar concentration (0.9 mM). Dynamic simulations of discontinuous two-step diafiltration tests allowed a simple and adequate representation of the performance of the process especially for 1 mM CPC, while discrepancies for 2.5 mM CPC level denoted complex interactions between CPC and As. Introduction Membrane processes can be used as cost-effective and environmentally safe separation techniques as an alternative to sorption onto active carbon and synthetic resins (1). The attractive low-energy characteristics of ultrafiltration can be exploited also for the removal of low dimension ionic species (such as heavy metals) by using surfactant-based separation processes such as micellar-enhanced ultrafiltration (MEUF) (2-10). The effectiveness of MEUF processes is strictly related to the achievement of high permeate fluxes and high rejections of both heavy metals and surfactant. Low permeate fluxes are observed working at high pressure, low retentate flow velocity, and high viscosity of feed solution (11). Surfactant rejection is increased by high retentate flow velocity due to the attenuation of concentration polarization produced by fast retentate flux (reduced surfactant gradient across the membrane) (11). On the other hand, opposite effects on surfactant rejections were observed by increasing transmembrane pressure: the increase of surfactant in the permeate as pressure increases was explained by taking into account micelle deformation, micelle decomposition, and high surfactant concentration near the membrane (increased surfactant gradient across the membrane), while the opposite effect (diminution of surfactant in permeate as pressure increases) was explained by a presieving effect sometimes observed for concentration polarization (11, 12). The im- provement of surfactant rejection can be then approached by the modification of membrane module or material in order to reduce the concentration polarization (13). Nevertheless, concentration polarization itself can be also exploited to design MEUF working below CMC and reducing surfactant release and consumption. In fact, the concentration polar- ization can result in the formation of micelles near the membrane surface even below the surfactant CMC (14). In addition, the presieving of gel layer associated with con- centration polarization can have a crucial effect on metal rejection when the surfactant concentration is lower than its CMC (2, 6, 15). Membrane pore size also plays an important role in this context (12). An increase of the molecular weight cutoff (MWCO) of the membrane can cause an earlier development of concentration-polarization regime and consequently reduce the surfactant release in the permeate due to presieving effect. As a consequence, even for a very large pore size (50 KDa MWCO), the vast majority of micelles can be rejected (12). According to this finding, high MWCO membranes present extremely good rejection characteristics with minimum area requirement and capital cost. In this work arsenic removal by MEUF was investigated using cetylpyridinium chloride (CPC) and a cross-flow ceramic membrane apparatus. The effects of some operating factors on permeate flux, arsenic, and CPC rejections were investigated, in particular transmembrane pressure, pH, CPC concentration, and As concentration. The novelty was in evaluating the possible advantages of using large-pore membranes (20 and 50 nm) and reduced surfactant con- centrations (1-3 mM) for treating high fluxes of concentrated arsenic-bearing solutions (10-40 mg/L). An example of such concentrated solutions is given by acid mine drainage (AMD), which is a common problem in different areas with a past history of mining activities. Oxidation of pyritic ores is the major cause of acidic drainage which is characterized by low pH (2-4) and toxic metal concentrations often well above maximum contaminant levels for drinking water (16). In particular arsenic is one element that is common in AMD being generally associated with metal sulfide deposits that are responsible for acid drainage (17-19). AMD waters can be characterized by a wide range of As concentration depending on the specific site geochemistry and the type of past mining and metallurgical activities: literature data denoted As concentrations in AMD ranging from 2-4 ppm (20-24) up to 8 (19), 14 (20), 64, and 95 ppm (24, 25). Furthermore AMD is generally characterized by oxidizing redox conditions in which As(V) species prevail over As(III) ones (4, 24). Another example of concentrated arsenic-bearing waste can be found in biohydrometallurgical processes. In fact, biooxidation of arsenic-bearing refractory gold ores is performed as a pretreatment in order to enhance gold * Corresponding author phone: +39 071 2204225; fax: +39 071 2204650; e-mail: f.beolchini@univpm.it. † Universita ` Politecnica delle Marche. ‡ Universita ` degli Studi “La Sapienza”. § Universita ` degli Studi di L’Aquila,. Environ. Sci. Technol. 2006, 40, 2746-2752 2746 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 8, 2006 10.1021/es052114m CCC: $33.50  2006 American Chemical Society Published on Web 03/14/2006