Mercury species adsorption

Figure 2. Selective adsorption traps for mercury species in gases...

Since the pH p of the copolymer is 4.9, at the experimental pH (pH= 6), the copolymer has negative surface sites that are responsible for the adsorption of positive mercury species (Figure 10.11). 

Only a few analytical techniques have been developed so far for the reliable determination of organo-mercury species in water samples. In many studies, mercury compounds in aquatic environments are speciated according to their ability to be reduced to the elemental state. Dimethylmercury and Hg° (the most volatile mercury compounds) can be isolated by aeration and adsorption on a suitable adsorbent (dimethylmercury on Carbotrap or Tenax and Hg° on gold trap) or by cryotrapping, or coupled with noble metal amalgamation (for Hg°), immediately after sampling. Alternatively, they can be directly analyzed by separation on GC columns and detected by one or more suitable mercury detectors. Samples should not be acidified prior to such separations since dimethylmercury and Hg° are transformed into... 

Revis NW, Osborne TR, Holdsworth G, Hadden C (1989) Distribution of mercury species in soil from a mercury-contaminated site. Water Air Soil Poll, 45 105-113 Schuster E (1991) The behaviour of mercury in the soil with special emphasis on complex-ation and adsorption processes - a review of the literature. Water Air Soil Poll, 56 667-680... 

Bench-scale experiments were performed in a 5-cm ID fluidized-bed reactor (FBR). These experiments were intended to 1) prepare carbon products with varying ash content, sulfur content, surface area, pore volume and production yield, 2) identify optimum conditions for producing activated carbon samples with desired properties for removal of various mercury species from utility flue gas, 3) evaluate the influence of inherent sulfur of the coal precursors on the mercury adsorption reactivity and capacity of the resultant activated carbon products, and 4) obtain scale-up date for producing larger quantities of carbon products in a pilot-scale FBR. 

For the preservation of mercury species, some authors recommend acidification of the water sample with HNOs. Hintelmann" reports that MeHg in water samples is stable for at least 6 months when acidified to 1% HCl, or when deep-frozen immediately after sampling and storing in the dark. No special container material was recommended, but in an earUer pubUcation it was stated that Teflon and glass are appropriate, while plastics like pol3 propylene and polyethylene cause loss due to adsorption on the container walls. 

Characterization. Ceramic bodies are characterized by density, mass, and physical dimensions. Other common techniques employed in characterizing include x-ray diffraction (XRD) and electron or petrographic microscopy to determine crystal species, stmcture, and size (100). Microscopy (qv) can be used to determine chemical constitution, crystal morphology, and pore size and morphology as well. Mercury porosknetry and gas adsorption are used to characterize pore size, pore size distribution, and surface area (100). A variety of techniques can be employed to characterize bulk chemical composition and the physical characteristics of a powder (100,101). 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams. 

Fig. 4.8 compares data on the adsorption of lauric acid (C12) and caprylic acid (Cs) at a hydrophobic surface (mercury) as a function of the total bulk concentration for different pH-values. As is to be expected the molecular species becomes adsorbed at much lower concentrations than the carboxylate anions. The latter cannot penetrate into the adsorption layer without being accompanied by positively charged counterions (Na+). As was shown in Fig. 4.4, the adsorption data of pH = 4 can be plotted in the form of a Frumkin (FFG) equation. Fig. 4.9 compares the adsorption of fatty acids on a hydrophobic model surface (Hg) with that of the adsorption on Y-AI2O3. 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

The enhancement of SWV net peak current caused by the reactant adsorption on the working electrode surface was utilized for detection of chloride, bromide and iodide induced adsorption of bismuth(III), cadmium(II) and lead(II) ions on mercury electrodes [236-243]. An example is shown in Fig. 3.13. The SWV net peak currents of lead(II) ions in bromide media are enhanced in the range of bromide concentrations in which the nentral complex PbBr2 is formed in the solntion [239]. If the simple electrode reaction is electrochemically reversible, the net peak cnnent is independent of the composition of supporting electrolyte. So, its enhancement is an indication that one of the complex species is adsorbed at the electrode snrface. 

In Ref 169, some peculiarities associated with adsorption of alkyne peroxides from DM F-water solutions onto the mercury electrode in the presence of tetraethylammonium cations have been described. Polarography and electrocapillary measurements were employed as the experimental techniques. It has been shown that interfacial activity of these peroxides was determined by the species generated as a result of associative interactions between peroxides and DMF and tetraethylammonium cations. 

More than at mercury, it makes a difference whether the electrode is inert or not. In the first case, the electrode reaction is of the type Fe3+/ Fe2+ etc. and the modelling of processes is the same as with mercury. However, if the electrode reaction is of the type Zn2+/Zn, e.g. at a gold electrode, at least the electrode surface will be modified by the deposited zinc, Frequently, it is observed that the first monolayer of the foreign metal is deposited at a potential substantially positive to its standard potential. This phenomenon is named underpotential deposition and bears some resemblance to an electrode reaction that involves adsorption of the reacting species (see Sect. 6). 

Current spikes that are attributable to rapid adsorption or desorption of an adsorbate are sometimes observable for strongly adsorbing but electroinactive species such as camphor at a mercury electrode. The spike is a nonfaradaic current caused by the change in capacitance resulting from the sudden alteration in double-layer structure when the molecule adsorbs or desorbs. 

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