Adsorption anoxic

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). 

Brewer and Spencer [428] have described a method for the determination of manganese in anoxic seawaters based on the formulation of a chromophor with formaldoxine to produce a complex with an adsorption maximum at 450 nm. Sulfide (50 xg/l), iron, phosphate (8 ig/l), and silicate (100pg/l) do not interfere in this procedure. The detection limit is 10 pg/1 manganese. 

Other metal sulfides, such as galena (PbS) and sphalerite (ZnS), may affect the mobility of arsenic in anoxic environments. However, immobilization depends on surface complexation rather than precipitation. In contrast to iron (oxy)(hydr)oxides (discussed later), As(III) adsorption on galena and sphalerite increases with pH (Bostick, Fendorf and Manning, 2003). Surface complexation does not occur by isomorphic substitution of lead or zinc, or by a ligand exchange mechanism. Instead, multinuclear, inner-surface arsenic-thiosulfide complexes probably form on galena or sphalerite surfaces (Bostick, Fendorf and Manning, 2003). 

Krom, M.D., and Berner, R.A. (1980b) Adsorption of phosphate in anoxic marine sediments. Limnol. Oceanogr. 25, 797-806. 

Rosenfield, J.K. (1979) Amino acid diagenesis and adsorption in nearshore anoxic sediments. Limnol. Oceanogr. 24, 1014-1021. 

Based on measurements of this kind, the lifetimes of titanium nuclear waste containers under Canadian waste disposal conditions have been modeled (30). Since these conditions are expected to be anoxic, the passive corrosion of titanium will be driven by a very slow reaction with water, leading to the possibility of hydrogen adsorption into the metal ... 

As mentioned in Section IV.A.2, Leng and Pinto [3861 specifically addressed the effect of surface properties on the oxic and anoxic adsorption behavior of phenol, benzoic acid, and o-cresol. Commercial carbons were oxidized in air at 350°C, which is known [37] to introduce both CO- and C02-yielding surface groups nevertheless, from FTIR spectra, they concluded that the main differences are due to relative quantities of surface carboxylic groups. Presumably because the experiments were performed at pFl = 7.0, i.e., below the pK, of phenol, their explanation for the decrease in uptake with increasing surface oxygen was not the electrostatic repulsion but increased water cluster formation as well as increased removal of n electrons from the basal planes [450,674], which results in weaker dispersion interactions with phenol. ... 

Figure 14.2. Operational problems in size fractionation by membrane filters, (a, b, c) Size distributions of iron oxyhydroxyphosphate particles obtained by transmission electron microscopy (true distribution) and syringe filtration on Nucleopore polycarbonate filters and Schleicher-Schuell cellulose ester depth filters. Iron particles formed at the oxic-anoxic interface of eutrophic Lake Bret (Switzerland), (d) Fraction of iron particles retained on 3.0-pim membranes, as a function of flow rate (j) Nucleopore polycarbonate, and (2) Schleicher-Schuell cellulose nitrate. In the absence of coagulation or adsorption, no particle should be retained. (From Buffle et al., 1992.)...

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