Scavenging of metals

The most likely sources of metal ions for marine organisms are the colloidal or particulate moieties such as plankton, organic detritus, and clay minerals. The latter may be coated with iron or manganese oxides which are efficient scavengers of metals in slightly alkaline media. 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

Edhaie, S., Fleischmann, M. and Jansson, R.E.W. (1982) Application of the trickle tower to problems of pollution control. Part I. The scavenging of metal ions. Journal of Applied... 

As already mentioned, PVA is a water soluble polymer used in applications such as packaging films where water solubility is desired. It is the most readily biodegradable of the vinyl polymers, which makes it a potentially useful material in biomedical, agricultural, and water treatment areas, for example, as a flocculant, or scavenger of metal ions. Moreover, due to its water solubility, PVA can also be used as a model for particle dispersion in aqueous suspensions, especially those from CNWs and some clays. As a consequence, PVA has been largely used to produce nanocomposites with clays, cellulose, and chitin whiskers, silver nanoparticles, graphite oxide, and carbon nanotubes. 

Johnson Johnson accomplished a robust and multi kilogram scale (250 Kg) synthesis of intermediate 36, en route to Rilpivirine 37, a NNRTl [17], It involves Heck coupling of iodoaniline 34 and acrylonitrile 35 under optimized eonditions. Subsequent scavenging of metal residual and treatment with eoncentrated hydrochloric acid provides HCl salt 36 as an E Z (98 2) mixture in 60-70% yield (Scheme 9.9). 

Chemical Applications. Cesium metal is used in carbon dioxide purification as an adsorbent of impurities in ferrous and nonferrous metallurgy (qv) it can be used as a scavenger of gases and other impurities. 

When the ore contains a large amount of clay minerals, these form difficult to separate slimes, which hinder the recovery of the minerals (see Clays). The tailing from the scavenger cells can be cycloned to remove the slimes before the coarse material is floated in a tailings retreatment plant. The flotation product from the rougher cells of this plant can be reground and cleaned. This additional treatment of the tailings from the main copper flotation plant may improve the recovery of metal values by 1—3%. 

You may have noticed Ca-EDTA on the list of ingredients of many prepared foods, ranging from beer to mayonnaise. EDTA acts as a scavenger to pick up traces of metal ions that catalyze the chemical reactions responsible for flavor deterioration, loss of color, or rancidity. Typically, Ca-EDTA is added at a level of 30 to 800 ppm. 

Here, the use of inhibitor formulations having a less dramatic effect on TDS (such as certain tannins) may be extremely beneficial. Formulations are available that are based on tannin chemistry and contain blends that act as oxygen scavengers and metal passivators, with additional sludge dispersant and antifoam properties. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

Thermal reactions of the second type, which do not involve adding ligands toward formation of metal organic product compounds, are less widely recognized as chemical phenomena. There may be here a simple failure of reaction, as in Eq. (16), because (99) of the diffusion of CO from the reaction site there may be stabilization of an intermediate stage by scavenger action (26, 27, 56) ... 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

Initial tests of Brownian pumping required the measurement of Th in colloids separated from seawater samples. " Th proved to be an especially useful tracer of colloidal uptake of metal species because of its constant source and relative abundance. Baskaran et al. (1992) and Moran and Buesseler (1992) used cross-flow filtration to separate the colloidal fraction and both studies reported significant (up to 78% of total) " Th in this fraction. Subsequent work largely supported these observations (Moran and Buesseler 1993 Huh and Prahl 1995) and suggested the importance of colloidal organic matter in scavenging Th (Niven et al. 1995). 

Chung Y, Finkel R, Bacon MP, Cochran JK, Krishnaswami S (1983) Intercomparison of °Pb measurements at GEOSECS station 500 in the northeast Pacific. Earth Planet Sci Lett 65 393-405 Chung Y, Finkel R (1988) Po-210 in the Western Indian-ocean - distributions, disequilibria and partitioning between the dissolved and particulate phases. Earth Planet Sci Lett 88 232-240 Clegg SL, Whitfield M (1991) A generalized model for the scavenging of trace metals in the open ocean -... 

Clegg SL, Whitfield M (1992) A generalized model for the scavenging of trace metals in the open ocean -... 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

A common form of EDTA used as a preservative is calcium disodium EDTA (CaNa2EDTA). What metals will this form of the sequestrant scavenge effectively The dissolution of the solid will yield calcium ions, sodium ions, and the EDTA anion. Any metal more effectively complexed than calcium will be readily scavenged, including all ions listed in Table 9.1 except silver (Ag+) and magnesium (Mg2+). (In the absence of the calcium counterion, as in the case of the acid form of EDTA, chelation of calcium in the body can occur. In fact, EDTA administered orally is an FDA-approved treatment for calcium deposits in the bloodstream that lead to cardiovascular disease.) Citric acid (Fig. 9.3.3) is another sequestrant of metal ions in foodstuffs. 

As might be expected, 02 impurities were very effective scavengers of Cr(CO)5, but more unexpectedly C02 had no effect. Reactions carried out under pressures of C02 (97) were indistinguishable from those carried out under Ar. This contrasts sharply with matrix studies (101) where C02 was found to photooxidize metal carbonyls. 

---