Removal of arsenic(V) onto chitosan: From sorption mechanism explanation to dynamic water treatment process
Introduction
Arsenic contamination in natural waters is a world wide problem and due to its established toxicity and its presence in overcrowded areas [1], [2], the guideline concentration limit value recommended by WHO in drinking waters has been 10 μg L−1 since 1993. The European Union, USA, Canada, Japan, and Vietnam have accepted this value in their regulatory systems, but other countries (Bangladesh, Bolivia, India, etc.) still operate at present to the 50 μg L−1 standard [3], [4]. Arsenic is introduced in water through natural and anthropogenic sources: release from mineral ores, probably due to long term geochemical changes [2], [3], and from various industrial effluents like metallurgical industries, ceramic industries, dye and pesticides manufacturing industries and wood preservatives. Two predominant species found in natural waters are inorganic forms of arsenic namely, arsenate As(V) and arsenite As(III) and their presence depends on the pH and redox conditions. As(V) which is the thermodynamically stable form, is found in oxic surface waters, rivers and lakes [5]. It is also reminded that it presents three pKa (2.2, 7 and 11.6): it means in most of natural waters, arsenic V is mainly within the H2AsO4− form (at 3 < pH < 6) or associated with HAsO42− form (at 6.5 < pH < 7.5). After pH 8, HAsO42− is the predominant species. Different technologies based on sorption mechanisms have been reported in literature in order to remove arsenic from industrial effluents or natural waters and include sorption/precipitation on iron and aluminum by-products [6], [7], [8]. Other adsorbents have also been investigated like carbonaceous adsorbents [9], [10], low-cost mineral materials [11] and biosorbents [12], [13]. These last ones can be compared as wastes, coming from food or seafood industries. Ion exchange has also been performed for As(V) removal, with strong base anion exchange resins like protonated amine functions or quaternary amines [14], [15], [16]. Among the biosorbents, chitosan and its derivatives are obtained from the deacetylation of chitin, the major component of the crustacean shells. It has been widely used for the removal of metals from waters [17] and has also shown efficiency to adsorb arsenate ions [18], [19] or similar anions like perrhenate [20].
Preliminary results conducted with low concentrations (C < 500 μg L−1) of arsenate indicated that the adsorption reaction was fast and followed a pseudo first order model [19]. The authors also showed that the sorption mechanism was an ion exchange reaction. The aim of this study is firstly to clarify the sorption mechanism with an extension of the arsenate concentration in batch reactor. Two approaches are proposed: the use of different models like Freundlich, Langmuir, Redlich–Peterson and Tempkin to describe the sorption isotherm and the investigation of the pH effect in synthetic and natural waters. In order to complete these batch results, a feasibility study is conducted to propose a continuous reactor for arsenate removal. Classically fixed bed columns are often considered but it has been chosen to test an original membrane process. It consists in a continuous stirred tank reactor coupled with a microfiltration immersed-membrane. The breakthrough curves are obtained in deionised and natural spring waters and they are simulated by a mass balance model. These data confirm the potential use of chitosan for the removal of arsenic from contaminated waters.
Section snippets
Chemicals and apparatus
The chitosan, an amino-polysaccharide, was provided by France-Chitine (La Ciotat-France) and raw flakes have been ground and sieved for a size range between 0.25 and 0.35 mm. It has been characterised [21] and the main properties are summarized in Table 1. All chemical reagents were of analytical grade and the arsenic concentrations were measured by a Perkin Elmer graphite furnace atomic absorption spectrometer (GF-AAS AAnalyst 600). Each determination was realised in 3 replicates, the
Batch adsorption studies
Isotherm curves have been plotted as a function of the temperature (Fig. 2) and the different adsorption models described in Section 2.2 were tested. An example using the Langmuir equation is given in Fig. 2. A comparison of the distribution coefficient indicates that the Langmuir model seems to be the best to describe the experimental data. Maximum fixation capacities, deduced from the equation, are slightly overestimated compared to the experimental ones. However, the variation shows that at
Conclusions
This work has showed that chitosan, a biopolymer extracted from the wastes of the seafood industries, could be potentially used for the removal of arsenate from natural waters. The sorption mechanism proposed for the fixation of arsenate ions onto chitosan, could be described by the following steps: (i) the increase of amine protonation and zeta potential by a decrease of the pH, at the surface of the chitosan, (ii) the fixation by electrostatic attraction between a positive surface charge
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