Metal ions accumulation

Accumulation in soil solutions Altered microflora Reduced litter decomposion Metal ions accumulate Reduced water/mineral uptake Reduction in mycorrhizae Reduced vigor/resistance... 

Simine DD, FinolilC, Vecchio A, Andreoni V (1998) Metal ion accumulation by immobilised cells of Brevibacterium sp. J Ind Microbiol Biotechnol 20 116-120. 

Fig. 10.6. Baseline subtracted OSW voltammograms of the difference between the voltammograms obtained before and after metal ion accumulation in test sample number 1 for 10 min at 25°C. Test sample 1 contained 0.100 pM Cu2+, 3.00 pM Cd2+ and 0.600 pM Pb2+ in 50 mM ammonium acetate (pH 7.0). The electrodes were modified with TA-Gly-Gly-His, TA-glutathione, TA-angioten-sin I and TA. OSW voltammograms were measured in 50 mM ammonium acetate (pH 7.0) and 50 mM NaCl at 25°C at a pulse amplitude of 0.025 V, a step of 0.004 V and frequency of 25 Hz.

At the present time, considerable insight into the biochemistry of vanadium in tuni-cates is associated with the mechanism of vanadium uptake. We shall concentrate on this topic, first presenting the biological context in which to view this remarkable feat of metal ion accumulation. 

In order to avoid this undesirable effect and to promote metal ion accumulation in the liquid phase, Ryss and Goldberg (1973) developed a special element-collector. This consists of a vessel containing a metallic electrode and a semi-permeable membrane, on one side of which is a solution of nitric acid (Fig. 2-17). The semi-permeable membrane prevents egress of the acid solution and allows ionic exchange between the element-collector and the surrounding environment. The acid neutralises the hydroxyl ions and thereby maintains the solubility of metal ions in the vicinity of the cathode. 

Uranium selectivity of hydrous titanium oxide can be derived from the concentration factors of several metal ions accumulated from sea water103) (Table 5). 

Armstrong, R.A., 1997. Are metal ions accumulated by saxicolous lichens growing in a rural environment . Environ. Exp. Bot. 38, 73-79. 

The tunichromes are a group of reducing blood pigments from tunicates which were initially implicated in metal ion accumulation (particularly vanadium and iron), and have been recently reviewed [34]. They are dipeptides constructed from 2P(3,4,5-trihydoxyphenyl)alanine (Topa) or 2p(3,4-dihydoxyphenyl)aIanine (Dopa) and a C-terminally bound 3,4-dihydroxystyrylamine or 3,4,5-trihydroxy-styrylamine unit, and while it is now postulated that small proteins which are rich in Dopa and/or Topa units accumulate the metal ions rather than the tunichromes themselves,... 

Examples of the latter are chromates, which are reduced to Cr(III) hydroxide or oxyhydroxide on the metal surface, or polyphosphates, in which decomposition and subsequent precipitation of Ca phosphate has been suggested [8]. The precipitation reactions will depend on the local solution composition (pH, metal ion concentration) in the near-surface region of the corroding metal, which may pronouncedly deviate from that in the bulk. For instance, the production of OH in the cathodic partial reaction will raise the surface pH and thus promote the precipitation of compounds, such as Zn hydroxides, even in noticeable acidic solution. In a similar way, the pore-plugging ability of anodic inhibitors may be enhanced by reactions with local metal ion accumulations in the vicinity of active pores in a passive film. 

The positive charge on the complex is probably critical for myocardial cell uptake. Indeed, this was the rationale of Deutsch and coworkers who noted that cationic species such as ammonium salts and alkali metal ions accumulate in normal heart muscle... 

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

As metal ion concentration increases in the crevice, a net positive charge accumulates in the crevice electrolyte. This attracts negatively charged ions dissolved in the water. Chloride, sulfate, and other anions spontaneously concentrate in the crevice (Figs. 2.4 and 2.5). Hydrolysis produces acids in the crevice, accelerating attack (Reactions 2.5 and 2.6). Studies have shown that the crevice pH can decrease to 2 or less in salt solutions having a neutral pH. 

The corrosion rates of nickel and nickel alloys in pure water and steam at elevated temperatures are generally extremely low, typically of the order of 1 m/year. The metal and its alloys are therefore often selected for service in these environments in circumstances where contamination of the water by metal ions is to be avoided. It should be noted, however, that the possibility of stress corrosion may need to be taken into account in certain circumstances (see below). Additionally where phosphate water treatment has been used in PWR secondary heat exchangers, severe localised corrosion has occurred when alkaline phosphates have been permitted to accumulate . 

FIGURE 3-16 Accumulation and stripping steps in adsorptive stripping measurements of a metal ion (M I+) in the presence of an appropriate chelating agent (L). 

Damage to epicuticular waxes Altered photosynthesis Increased water loss Accumulation of acidic anions Leaching of ions, sugars, etc. Mineral imbalances Altered metabolism Increased susceptibility to winter freezing injury Death of fine roots Destabilization of trees Reduced water/mineral uptake Reduced water uptake Cations leached below roots Accumulation of acidic anions Altered structure/texture Altered microflora Reduced litter decomposition Altered N transformations Solubilization of metal ions... 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

Norberg, A.B. and Persson, H., Accumulation of heavy-metal ions by zooglea ramigere, Biotechnol. Bioeng., 26, 239-246, 1984. 

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