Precipitation reduction potentials affected

The toxicological impact of chromium is dependent upon its oxidation state (chromium (III) or chromium (VI)) which, in turn, is controlled by the oxidation-reduction potential and pH. Within this framework, the hydrolytic speciation of chromium(III) is important since contamination of water by chromium is widespread (Ball and Nordstrom, 1998). Solution conditions, including ionic strength, affect the speciation of chromium and its adsorp-tion/desorption and precipitation/dissolution reactions (Ball and Nordstrom, 1998). 

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). 

The summer of 1983 provided a rehearsal for the potential doubled CO2 effect. Temperature was above normal and precipitation below normal on scales not unlike those predicted by models of the doubled CO2 effect, or of conditions typical of mid-twenty-first century if other absorbers are included. In 1983 corn yields were halved (by reference to the previous crop year). About half this reduction was probably due to the hostile climate. Winter wheat was adversely affected in some areas, but escaped the worst effects of the summer drought, because of early harvesting dates. Spring wheat was badly affected in many areas. Thus a natural rehearsal of future events appeared to confirm the pessimistic estimates of the Abrahamson forum. 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

---