Environ. Sci. Technol. 1999, 33, 4278-4284
Electrochemical Recognition of Selective Mercury Adsorption on Minerals ALAIN WALCARIUS,* J EÄ R O ˆ ME DEVOY, AND JACQUES BESSIERE Laboratoire de Chimie Physique pour l’Environnement, Unite´ Mixte de Recherche UMR 7564, CNRS - Universite´ H. Poincare´ Nancy I, 405, rue de Vandoeuvre, F-54600 Villers-les-Nancy, France
Mercury species are highly toxic contaminants of the environment, and their dissemination in aquatic media is governed by sorption processes on both organic (including biological) and inorganic particles. An electrochemical method is provided for the direct monitoring of the selective binding of inorganic mercury(II) to the surface hydroxyl groups of some minerals (silica, goethite, pyrite) by using carbon paste electrodes modified by these mineral particles. After accumulation from aqueous solutions at open circuit, anodic stripping voltammetry was performed with the electrode immersed in a detection cell containing typically 0.2 M HNO3 in order to desorb and measure the amount of previously adsorbed mercury. Several experimental parameters were optimized with a silica-modified electrode to ensure quantitative and reproducible results, including the electrode composition, the accumulation time, and the voltammetric detection mode. Mercury adsorption was studied as a function of pH and chloride concentration. It was found that only the soluble mercury(II) species with hydroxyl moieties (HgOH+, Hg(OH)2, HgOHCl) were able to adsorb on these minerals by reacting with their surface hydroxyl groups. Moreover, in a rather low pH region (4-7), this reaction was extremely selective for mercury over other soluble metal species. The electrochemical monitoring, though mainly controlled by kinetics, gave significant insight in the adsorption processes as far as equilibrium states were rapidly reached.
Introduction Despite considerable work in recent years, the presence of mercury in the environment remains a serious problem, especially because the high toxicity of this element and its mobility in soils and natural waters. In aquatic environments, the concentration of dissolved trace metals is monitored by sorption processes on both organic and inorganic solid particles (1-4) as well as by biological uptake and loss processes (5, 6). Mercury may be present in natural waters in various chemical forms, mainly hydroxo- and chlorocomplexes of Hg(II) (7), and some of the species have strong affinities to binding with sedimentary minerals or organic matter (8, 9). Voluminous literature on mass transfer reactions involving mercury species at solid/liquid interfaces has appeared (928). Most studies required speciation analysis, if possible on * Corresponding author phone: (+33) 3 83 91 63 43; fax: (+33) 3 83 27 54 44; e-mail:
[email protected]. 4278
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 23, 1999
both solid and solution phases, by often costly and timeconsuming analytical methods. Several investigations were devoted to the characterization of mercury adsorption directly on complex multicomponent solid matrixes such as soils or sediments (9, 15, 23-25). For a better knowledge of the factors controlling the movement and availability of mercury species in natural systems, numerous studies were also performed using selected (sometimes model) pure solid substrates as oxide or sulfide minerals (10-14, 16, 20, 21, 28), clays and related materials (19, 22, 26), or organic compounds (17, 18, 27). These approaches on well-defined systems were directed to better understanding of adsorption (and/or desorption) mechanisms, to finding efficient scavengers for remediation purpose, or to helping in the selection of appropriate experimental conditions for treatment of mercury-contaminated sites. For a long time hydrous oxide surfaces of minerals in aquatic environments have been recognized in playing an important role in the sorption processes of metal species (29). They show propensity to adsorption of mercury(II), but it was observed in early 1974 that its accumulation from diluted solutions on goethite surface was different from that of other metal species, being considerably lower even in the presence of competing complex forming ions (30). MacNaughton and James have demonstrated the importance of the equilibrium solution species in determining the adsorption behavior of aqueous mercury(II) species at the oxide/ water interface and concluded that higher adsorption on quartz occurs when soluble Hg(OH)2 dominates over HgCl2 (10). They also suggested that , there may be other solution species not included in their analysis that adsorb less strongly than Hg(OH)2 . (10). More recently, attempts were made to model the adsorption of inorganic mercury(II) on (hydr)oxides (16, 21), and three chemical forms of stable surface complexes were found to occur from the reaction between surface hydroxyls (, tS-OH .) and mercury(II), tS-OHg+, tS-O-Hg-OH, and tS-O-Hg-Cl, depending on pH and chloride concentration. The adsorption of methylmercury(II) was also investigated (20). However, most of these previous investigations of mercury(II) transfer across the mineral/solution interface were based on one and only analysis of the solution phase at the equilibrium, without any quantitative analysis of the solid phase. From batch experiments, such quantitative measurements would require the separation of the solid and liquid phases before their respective analysis, preventing therefore and in situ approach under nondestructive conditions. It will be shown hereafter how electrochemistry at modified electrodes can contribute in the field. Opportunities for electrochemical technology in the environmental sciences are continuously growing (31-33). Recent examples are available on the following: cleaner generation of energy by means of fuel cells (34); cleaner and more selective synthesis, or on-site generation of chemicals, by electrolysis (35, 36); improving water quality (37) or atmospheres (38), recycling process streams (39), remediation (40, 41), or effluent and waste treatments (42-44); and electrochemical devices (sensors) for online monitoring and analysis (45-50). However, the implication of electrochemistry for the direct monitoring of mass transfer reaction at the mineral/water interface remains uncommon. Despite electrodes modified with nonelectronically conductive minerals as clays (51, 52), zeolites (53, 54), or other inorganic (55, 56) solids are widespread and largely used; for example, in electroanalysis (52, 54, 56), the exploitation of their voltammetric response 10.1021/es990525v CCC: $18.00
1999 American Chemical Society Published on Web 10/23/1999
for the in situ characterization of transport of toxic compounds by these mineral particles was quite rare and restricted to solute transport in clay media (57-59) or to the exchange of herbicide within zeolites (60). In this paper, we propose a new approach to study inorganic mercury sorption on surface hydroxyl groups of minerals, based on carbon paste electrodes modified with these mineral particles. Special attention will be given to a high surface area silica sample (with consequently a large number of surface hydroxyl groups), because of the lack of experimental data for mercury adsorption on this kind of solid, and the optimized procedure will be then applied to goethite and pyrite. Adsorption isotherms will be drawn from electrochemical analysis of the adsorbed species as a function of pH and chloride concentration, and comparison will be made with isotherms obtained using conventional methods applied to similar minerals. Uncommon with previous studies, chloride concentrations of the same order of magnitude than those of mercury will be investigated, and adsorption processes will be discussed by taking into account the speciation of mercury in solution.
Experimental Section Apparatus. Electrochemical experiments were carried out with a Model 283 potentiostat/galvanostat model 362 (EG&G Instruments, Princeton Applied Research) equipped with an undivided three-electrode system mounted on a 50-mL cell. Both cyclic and anodic stripping square wave voltammetry measurements were performed at room temperature and monitored by the Model M270 research electrochemistry software (EG&G Instruments, Princeton Applied Research). The working electrode was a carbon paste modified with either silica gel, goethite, or pyrite (see preparation below), the counter electrode was a Pt wire, and the reference was a saturated calomel electrode. Unless specified otherwise, all square wave voltammograms were recorded after a preelectrolysis time of typically 60 s at a constant potential of -0.5 V, by scanning potentials up to +0.5 V, with a 100 Hz frequency, a 5 mV step height, and a 50 mV modulation amplitude. Inductively coupled plasma atomic emission spectroscopy (plasma 2000, Perkin-Elmer) was used for monitoring Hg(II) concentration in solution. Reagents. All chemicals were analytical grade reagents and used without further purification. The certified standard solution of mercury(II), at 1.001 ( 0.002 g L-1, was purchased from Merck. Mercury solutions were prepared daily by diluting a stock solution of 1.00 × 10-4 M mercury nitrate (Merck) in high purity water (18 MΩ cm) obtained from a Millipore Milli-Q water purification system. Adjustment of pH was made by addition of either HNO3 or NaOH; no buffer was used to avoid any eventual complexation of mercury by one of the buffer components. All other solutions were prepared with high purity water. High purity Ultra F carbon graphite (
