Mercury removal from water by ion exchange resins adsorption
Introduction
Many authors, in the past, studied the best method, both from the economical and technological point of view, to remove mercury and heavy metals in general from superficial waters, with the aim of the preservation of the entire ecosystem from the damages due to their accumulation (see e.g. Humenick and Schoor, 1974, Logsdon and Symons, 1975, Knocke and Hemphill, 1981, Deshkar et al., 1990, Barkley, 1991, Ritter and Bibler, 1992, Mazidji et al., 1992, Zouboulis et al., 1993, Couillard and Mercier, 1994, Pazirandeh et al., 1998).
Mercury is commonly transferred from water by the following methods:
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conventional coagulation;
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chemical precipitation;
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adsorption (both chemical and physical);
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reverse osmosis.
All the above mentioned materials present good adsorption capacity, but in general they are not able to reduce mercury concentration below discharge limits established in environment legislation. Discharge limits are, e.g. 10 ppb in USA (Ritter and Bibler, 1992), 5 ppb (Italian legislation), whereas for drinking water the limit is 1 ppb for the Italian legislation.
Such discharge requirements can be matched using specific ion exchange resins, characterised by a very high selectivity with respect to mercury: see e.g. Ritter and Bibler, 1992, Mazidji et al., 1992.
In particular, Ritter and Bibler (1992) studied the performance of Duolite GT-73 ion exchange resin in a large-scale process, discussing some operational problems encountered and their remedy. This work aims to perform an experimental and theoretical study of the removal of mercury from water by ion applying the same exchange resin as Ritter and Bibler (1992) process and to evaluate:
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adsorption efficiency;
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effect of pH on the uptake of mercury;
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adsorption kinetics.
Section snippets
Experimental
All experimental analysis has been carried out in a batch mechanically stirred reactor (volume 1.5 dm3). The resin used in this study was Duolite™ GT-73, a polystyrene/divinylbenzene resin with thiol (S-H) functional groups, which has been already studied by other authors (e.g. see Ritter and Bibler, 1992). Before each experiment, the resin was dried by air, weighted and wetted by adding distilled, deionised water as described in the manual provided by Duolite™ supplier. Since the resin was
The model
As in previous works (Deshkar et al., 1990, Orhan and Büyükgüngör, 1993, Ho et al., 1995), experiments show, that sorption kinetics changes with the initial mercury concentration in the liquid. In such previous studies, the mathematical description of the system was addressed to the identification of a first order reaction rate constant, in order to fit the experimental data of the removal extent vs time. This kind of description was justified by the chelating properties of such resins, by the
Conclusion
In this work a detailed experimental and theoretical analysis of mercury sorption on ion exchange resins is presented. On the basis of the present study, the following conclusions can be drawn:
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Sorption capacity of mercury on this resin is very high, with the possibility to obtain extremely purified water.
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The degree of removal strongly dependent on the initial pH of the solution: it decreases as the pH increases. Furthermore pH decreases with time during adsorption. This is due to the exchange
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