Surface sites reduced, dissolution

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

Both the rate and the rate constant of the photochemical reductive dissolution of lepidocrocite with oxalate as the electron donor are pH-dependent. This observation suggests that other pH effects, in addition to the pH dependence of oxalate adsorption at the lepidocrocite surface, have to be considered. Such effects are catalysis of detachment of the reduced surface iron centers by protonation of their neighboring hydroxo and oxo groups, and the pH-de-pendent readsorption of the photochemically formed Fe(II), thus blocking surface sites for the adsorption of oxalate. 

If the inhibitor simply blocks surface sites, only the dissolution rate is reduced, whereas adsorbates that intervene... 

The conditions prevailing in the these column experiments may be regarded as an extreme case since reducing conditions in natural porous media often involve the presence of significant amounts of dissolved ferrous iron due to chemical and microbial reduction of iron(hydr)oxide minerals or dissolution of Fe(II)-containing minerals. Thus, in natural anoxic media, re-adsorption of Fe(II) from solution will compete with direct microbial regeneration of reactive Fe(II) surface sites. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

Although 2-line ferrihydrite has been used for dissolution studies, 6-line ferrihydrite has, to date, not been investigated. Fischer (1976) compared the dissolution behaviour of three 2-line ferrihydrites in 0.2 M oxalate and found the slowest dissolution rate for a slowly precipitated sample and faster dissolution for rapidly precipitated samples (hydrolysed by fast addition of NH3 or by bacterial oxidation of Fe citrate). Adsorbed silicate reduced the dissolution rate in oxalate probably by blocking surface Fe sites (Schwertmann Thalmann, 1976). 

Cathodic corrosion inhibitors reduce the corrosion rate indirectly by retarding the cathodic process which is related to anodic dissolution. In this process, access to the reducible species such as protons, to electroactive site on the steel, is restricted. Reaction products of cathodic inhibitors may not be bonded to the metal surface as strongly as those used as anodic inhibitors. The effectiveness of the cathodic inhibitor is related to its molecular structure. Increased overall electron density and spatial distribution of the branch groups determine the extent of chemisorption on the metal and hence its effectiveness. Commonly used cathodic inhibitor materials are bases, such as NaOH, Na2C03, or NH4OH, which increase the pH of the medium and thereby also decrease the... 

It is very likely that micro-biofouling occurred which may have substantially reduced the surface area available for dissolution. Also, Peterson (41) pretreated the spheres with HCl which may have reduced the surface concentration of active sites. 

The etching process of silicon in HF—HNO3 solutions is an electrochemical process in which the dissolution of silicon takes place at local anodic sites while the oxidizing agent is reduced at local cathodic sites [120], The etching process may also depend on surface carrier... 

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