3.1. EFFICIENT HOLLOW FIBER SUPPORTED LIQUID MEMBRANE SYSTEM FOR THE REMOVAL AND PRECONCENTRATION OF Cr(VI) AT TRACE LEVELS
3.1.1. ABSTRACT
A hollow fiber supported liquid membrane (HFSLM) system for the removal and preconcentration of Cr(VI) has been developed and characterized using Aliquat 336 as carrier. The influence of the chemical composition on the efficiency of the membrane system has been investigated, such as the organic solvent and the stripping composition.
Among the stripping reagents tested, a solution of 0.5 M HNO3 was found to be the most effective to strip Cr(VI) from the loaded organic phase. Moreover, physical parameters such as the stability of the membrane and the operation mode of the module were also evaluated. The results demonstrated the effectiveness of the HFSLM system during 8 non-stop days’ operation. This membrane-based separation system has effectively been used to remove Cr(VI) from different aqueous samples, such as industrial waters and spiked natural waters at µg L-1 levels. Moreover, the system has allowed both the separation and enrichment of the metal and, thus, facilitating the detection of chromate contained in aqueous samples and reducing the volume of polluted water to be treated.
3.1.2. INTRODUCTION
The removal of metal ions from dilute or concentrated solutions has received a great deal of attention for the recovery of valuable metals or decontamination of effluents. Among the heavy metals, chromium, especially in its hexavalent form, is considered one of the most toxic elements despite the broad use of this metal in industries such as leather tanning, metallurgy, electroplating and textile manufacturing [1]. Even at low concentrations, Cr(VI) has mutagenic and carcinogenic effects, and, for this, the World Health Organization (WHO) establishes the toxic limits of Cr(VI) in waste water at a level of 0.005 mg L-1 [2]. Moreover, many countries have regulations
of the maximum permissible concentration of Cr(VI) and total chromium in natural or drinking water. In Spain the permissible concentration of total chromium in drinking water is 0.05 mg L-1 [3]. Taking into account the large quantities of Cr discharged into the environment due to the wastewaters produced by chromate-using industries, there is an urgent need for efficient separation techniques which may reduce the concentration of this metal to recommended values.
In recent years, several techniques have been developed to remove Cr(VI) from industrial effluents. The most common method is the chemical precipitation, where Cr(VI) is first reduced to Cr(III). However, this process consumes a large amount of reducing agents [4]. Some other methods, such as solvent extraction [5], ion-exchange [6] and membrane technologies [2,7-11] have also been developed for Cr(VI) removal.
Among membrane technologies, the supported liquid membrane (SLM) technique has gained a prominent importance. In this separation method, an organic solvent alone or containing a complexing agent is immobilized in a thin, macroporous, hydrophobic support, and interposed between two aqueous phases: the source solution containing the metal ion to be transported (feed) and the stripping solution into which metal ions will be released [12]. The more extended configurations used are flat-sheet and hollow fiber (HF). HF modules are of particular interest because of their versatility: they can be connected in series or in parallel and the length and diameter of the fibers and modules can be varied to provide the required interfacial area. In addition, this configuration allows not only the removal of the required compounds from the stream but also the concentration of these species for possible further processing and exploitation.
Application of microporous hollow fiber modules to the separation-concentration of Cr(VI) from wastewaters have appeared in the literature under different configurations varying from one single module (working as a supported liquid membrane), two modules in non-dispersive solvent extraction (NDSX) and the combination of one membrane module plus an emulsion phase (emulsion pertraction systems) [13-18]. In these works, tertiary amines are widely used to extract the metal from highly polluted ground waters or wastewaters from galvanic processes, as well as the quaternary ammonium salt Aliquat 336 (see Table 1 in ref [13]). It is assumed that the extraction with the quaternary ammonium salt occurs via a reversible ion-exchange equilibrium involving an anionic species of Cr(VI) and the chloride counter-ion present in Aliquat 336 [11].
The aim of this work is to investigate the applicability of a hollow fiber supported liquid membrane using Aliquat 336 as extractant for the removal of Cr(VI), in particular from polluted waters containing low levels of the metal. The study is focused on the selection of the most suitable chemical conditions of the system and the evaluation of the physical parameters that affect its efficiency, such as the stability of the membrane and the operation mode of the module. Special emphasis is given in the preconcentration of Cr(VI) in a clean matrix to facilitate the analytical determination of the metal with conventional atomic spectroscopic techniques, as well as to reduce the volume of polluted water to treat.
3.1.3. EXPERIMENTAL
3.1.3.1. REAGENTS AND SOLUTIONS
The extractant, tricaprylmethylammonium chloride (Aliquat 336), was supplied by Fluka Chemie. The organic solvents were dodecane modified with 1-dodecanol (both analytical reagent grade, Merck) and decaline (decahydronaphtalene, cis + trans) (Aldrich, ACS reagent). A stock solution (1000 mg L-1) of Cr(VI) was obtained from the solid K2Cr2O7 (Panreac, ACS reagent) and was used to prepare the feed solutions.
The pH was adjusted by using HCl. The reagents tested as stripping solution were NaNO3, HNO3 and hydrazine sulphate (Panreac, ACS reagent). Calibration standards of Cr were prepared using Reagecon Chromium ICP standard solution. All solutions were prepared using analytical reagent grade chemicals and distilled water, purified through a MilliQ Plus system (Millipore).
To evaluate the performance of the developed separation system, several samples containing Cr(VI) were used, such as an industrial sample obtained from an electroplating plant as well as spiked river water or spiked tap water. The samples were chemically characterized prior to their use.
3.1.3.2. APPARATUS
An Inductively Coupled Plasma emission spectrometer (ICP-AES) (Liberty RL, Varian Australia, Mulgrave, Vic., Australia) was used for the analysis of metal concentrations in the feed and stripping phases at =267.7 nm.