INTRODUCTION

The presence of arsenic in water, especially groundwater, is a major problem in different areas of the world. It is considered a potent human carcinogen, associated with increased risk of cancer. Moreover, arsenic toxicity is closely related to its oxidation state: the most common forms of As in natural water are the inorganic forms of arsenate

[As(V)] and arsenite [As(III)], the latter occurring at much lower concentrations in natural waters but being more toxic [1]. Due to its high toxicity, it is of great importance to develop separation systems for the treatment of arsenic polluted waters as well as to help in their monitoring in terms of As(III)/As(V) speciation. Broad technology options of water purification have been established to deal with As contamination, such as adsorption, chemical coagulation-precipitation, ion-exchange and membrane separation [2]. In the case of membrane technology reverse osmosis, nanofiltration and membrane distillation have been identified as the most efficient technologies in terms of As removal. However, fouling as well as transmembrane pressure problems can accompany these processes [3].

Arsenic removal based on ion-exchange processes takes benefit of the different species formed depending on the pH. It is important to note that in neutral conditions, the As(V) species are completely in ionic forms (H₂AsO₄^- and HAsO₄^2-), while As(III) is in molecular forms (H3AsO3 or HAsO2). This fact has been exploited by Ben Issa et al. for the determination of inorganic species in natural waters using ion exchange and hybrid resins [4], as well as in our previous study where the anion-exchanger Aliquat 336 (a quaternary ammonium salt) was used as extractant in liquid-liquid studies [5].

From the solvent extraction results, it could be stated that when dealing with solutions at pH=13, the extraction of both As(III) and As(V) species took place via the formation of two species in the organic phase with 1:2 and 1:3 stoichiometries (As:Aliquat) in both cases. However, it is worth pointing out that the rate of extraction was different depending on the oxidation state of As, since in the case of As(V) the extraction equilibrium was attained in less that 5 minutes, whereas in the case of As(III) it took more than 2 hours. Aliquat 336 was also used as a carrier in a supported liquid membrane system (SLM).

A typical supported liquid membrane consists of a microporous polymeric support impregnated with an organic solution [6-9] containing the carrier, thus contacting both the feed phase and the stripping phase. The extraction and stripping reactions, which take place at the interfaces of the aqueous solutions and the organic solution, originate a chemical pumping that allows the transport of the species through the liquid membrane.

In our previous study, Aliquat effectively transported As(V) from the feed phase at pH=13 to a 0.1 M HCl solution [5].

Donnan dialysis (DD) is an ion-exchange membrane-based process used to exchange counter-ions between two solutions, the feed (A) and receiver (R) solution,

separated by the membrane. The difference in the electrochemical potential on both sides of the membrane acts as the driving force. Thus, fluxes of permeable counter-ions present in the two solutions through the membrane occur in opposite directions until the Donnan equilibrium is reached [10]. The transfer mechanism involves both ionic diffusion and an exchange reaction, being generally the first the slowest and therefore the one determining the rate of transport [11]. Ion exchange membranes have successfully been applied for the removal of specific charged inorganic pollutants such as fluoride [12], aluminium [13, 14] or metal ions [15]. Moreover, a process called ion exchange membrane bioreactor (IEMB) has been used to remove mono-valent oxyanions from water [16-18]. This process uses a mono-anion permselective membrane as a barrier between a water stream, containing target polluting monovalent oxyanion(s), and a biocompartment, containing a suitable driving counter-ion (e.g.

chloride) and a microbial culture capable of their bioreduction to harmless products.

A number of commercially available anion exchange membranes (from Ionics, Sybron Chemicals, Tokuyama Soda, Fumatech and PCA) were tested in preliminary experiments under Donnan dialysis operating conditions for the bi-ionic system Na₂HAsO₄/NaCl in order to determine their suitability for arsenate transport and separation from aqueous streams. By assessing the arsenate flux through these membranes, as well as comparing their relative cost and mechanical stability, it was found that several membranes would make suitable candidates for application, particularly Ionics AR204-UZRA, Fumatech FTAM and PCA PC SA [19].

Recently, Zhao et al. performed studies at different pH values and using two types of anion-exchange membranes, one homogeneous and another heterogeneous [20].

HAsO42-

was believed to be the most mobile species in both membranes. The same authors evaluated the effect of accompanying components such as chloride, nitrate, sulphate, bicarbonate, silicate and phosphate in the arsenate removal by Donnan dialysis [21]. Among the anions tested, nitrate, sulphate and phosphate were the ones that more significantly decreased the overall removal efficiency of As(V) mainly due to competition for the positively charged fixed functional groups of the membranes.

In the present study, two different types of functionalized membranes are investigated for arsenic transport at neutral pH. On one hand, SLM containing Aliquat 336, which proved its efficiency at basic pHs, and, on the other hand, two anion- exchange membranes with different permselectivities, a multi-valent and a mono-valent

investigated, with special emphasis on the pH effect. Finally, the applicability of these membranes in As(V)/As(III) separation as well as their selectivity towards As(V) in the presence of interfering anions has been investigated.