Modified aluminosilicates as low-cost sorbents of As(III) from anoxic groundwater
Introduction
People from many regions of India, Bangladesh and Latin American countries suffer (even nowaday) from serious diseases with symptoms typical to arsenic poisoning. A natural geologic origin of As was proved in many studies [1], [2], [3], [4], but the mechanism of its release into the groundwater has not been clarified up to now. The weathering of As-rich pyrite and arsenopyrite occured in discrete layers [5], [6] and/or the release of arsenic from As–FeOOH surface complexes under reduction conditions [4], [7], [8] are considered the most probable reasons of strong As enrichment in aquifers. The untreated groundwater is then used as a sole source of drinking and/or irrigation water in poor rural and urban areas of the above mentioned countries [1], [3], [9], [10].
Various technologies have been developed for the arsenic removal from aqueous systems. Adsorption methods fall into the most commonly used procedures due to a variability of natural and synthetic sorbents [11], [12], [13], [14]. Most of conventional and non-conventional treating methods were compared in the critical review by Mohan and Pittman Jr. [15]. Iron oxides and oxyhydroxides are the most widespread and effective sorbents for inorganic As forms both in natural environments (soils, bottom sediments, etc.) and in technological processes [16], [17], [18], [19]. However, their synthesized forms are too expensive and hardly available for wide use. Additionally, only a tiny surface fraction of Fe ions is really active in the sorption on bulk oxides, hence supporting of the iron oxides on finely dispersed, cheap, well-processable and environmentally friendly matrix is very desirable.
Recently aluminosilicates have opened new posibilities in sorption technology due to favourable surface properties, availability, environmental and economical reasons. In general, they are not selective sorbents of anionic contaminants thanks to a low value of the pHZPC. Therefore, more and more studies have been focused on the preparation of anion-selective sorbents by the surface pre-treatment of aluminosilicates such as clays and sands [12], [20], [21], [22]. Besides the sorption efficiency, a mechanism of surface processes has gained more importance in precise characterization of newly prepared sorbents [23], [24], [25].
The main aim of this work was to verify two different types of raw clays (bentonite and calcined kaolin) pre-treated with FeII, FeIII, AlIII and MnII particles as selective sorbents of AsIII from model anoxic groundwater strongly contaminated with As.
Section snippets
Raw clays
Two different types of clays, kaolin and bentonite used in this work represented second rate materials easily available at a commercial scale. Expected prices (less than 30 EUR per 1 tonne) have been only connected with handling and transporting costs. Kaolin sample contained kaolinite and lesser amounts of quartz and micas. Its quality was much decreased by the large content of Fe (Table 1). After the thermal treatment (calcination at 500 °C for 3 h to metakaolin) Fe species in kaolin were
Efficiency of sorption processes
The sorption capacity qeq (mg g−1) represents the equilibrium amount of As adsorbed on the sorbent surface, while the sorption efficiency (%) is calculated from the residual As concentration (Asres) in the solution [33].
The selectivity of modified sorbents for MOD I is demonstrated in Fig. 2 and Table 2. Except FeIII pre-treated MT, all prepared sorbents proved available for As removal; the sorption efficiency exceeded 97% at equilibrium sorption capacities about 2.0 mg g−1 for MT, and 3.0–7.0 mg g−1
Conclusions
The described methods opened new possibilities of effective and cheap decontamination of As polluted aqueous systems. It has been applicable to almost all aluminosilicates, even to low-grade or waste sources, which represent low-cost and widely available material with favourable surface properties. Aluminosilicates are not the selective sorbents of anionic contaminants, however, their simple pre-treatment with Fe, Al and Mn salts enabled to prepare anion-selective sorbents with high sorption
Acknowledgements
This work was the part of the project IAA 401250701 (Grant Agency of ASCR, CR) and the research programme MSM 6046137302 (CR).
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