Arsenic redox process

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

The arsenic and antimony pentahalides EX5 (E group 15 element As or Sb X = F or Cl) are strong, irreversible oxidants the gas AsFs has little been used, but SbCF and SbFs are commercially available, very air-sensitive liquids which are used in dry and deoxygenated dichloromethane and liquid sulfur dioxide respectively. SbCls is easier to handle than SbFs which gives the dangerous HF by reaction with moist air. Moreover, SbCls is conveniently used in dichloromethane whereas SbFs is best used in liquid SO2. On the other hand, the side products (halogenation) are more frequently encountered with SbCls than with SbFs. The redox process follows ... 

Kuhn A. and Sigg E. (1993) Arsenic cycling in eutrophic Eake Greifen, Switzerland influence of seasonal redox processes. Limnol. Oceanogr. 38, 1052-1059. 

The similarities between ITIES and conventional electrode electrochemistry provide an arsenal of electrochemical techniques that have been previously tested in the more common electroanalytical chemistry and physical electrochemistry. To understand the similarities between ITIES and electrode electrochemistry, it is more useful to look at the differences first. Faradaic current flow through an electrochemical cell is associated with redox processes that occur at the electrode surface. The functional analog of an electrode surface in ITIES is the interface itself. However, the net current observed when the interface is polarized from an outside electric source is not a result of a redox process at the interface rather, it is an effect that is caused by an ion transport through the interface, from one phase to another. 

A Kuhn, L Sigg. Arsenic cycling in eutrophic Lake Greifen, Switzerland Influence of seasonal redox processes. Limnol Oceanogr 38 1052-1059, 1993. 

The Thylox process is no longer used commercially, while the arsenic-based version of the Giammarco-Vetrocoke process is still supported by the licensor in countries where the use of arsenical solutions is permitted. A non-arsenic, evolutionary modification of the G-V process is currently being offered for CO2 removal, and is claimed to be very competitive. This form of the process can not be classified as a liquid redox process, and is discussed in Chapter 5. 

As in all liquid-redox processes, part of the sulfur is converted to thiosulfate, although the rate of formation is appreciably lower in the essentially neutral Thylox solution than in more alkaline solutions used in other processes. Hydrogen cyanide, which is absorbed in the absorber, reacts readily with the sulfur formed in the thionizer to yield sodium thiocyanate. Because of these side reactions, the active thioarsenate has to be replenished continuously by addition of arsenic oxide and sodium carbonate. 

Chromium(IV) is stabilized in the reduction of [HCr04] by H3ASO3 in the presence of the ligand (CH3CH2)2C(0H)C02" (L"). The mechanism of the redox process involves formation of a complex between chromium(IV) and the arsenic(III) species followed by two pathways in which and L are required. The chromium(IV) product, formulated as [CrOL2], comproportionates with excess [HCr04] to give the more stable chromium(V) derivate. A number of reactions of chromium(V) derivates have been reported. ... 

The first time the body realizes that arsenic has been incorporated is when the redox activity (as above) proceeds at potentials when nitrogen or phosphorus are inert. By the time we detect the arsenic poisoning (i.e. we feel unwell), it is generally too late, since atoms of arsenic are covalently bound within body tissue and cannot just be flushed out or treated with an antidote. The arsenic sequesters electrons that might otherwise be involved in other relay cycles, which is a concurrent kinetic process see Figure 8.16. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

In general, the direct-oxidation processes employ a redox couple that has sufficient oxidation potential to convert H2S into elemental sulfur but insufficient potential to oxidize sulfur to higher states. Examples of materials that have this redox potential are vanadium compounds, arsenic compounds, iron compounds, and certain organic species. Typically, the redox materials, dissolved in a hot potassium carbonate solution with the species in its oxidized form, contacts the I S-laden gas and the H2S dissolves as the hydrosulfide. This sulfur reacts with the redox couple, forming elemental sulfur and the reduced state of the couple. Airblowing of the solution reoxidizes the couple and removes the elemental sulfur from solution as a product froth. 

Table 6.2 Processes involved in the liberation of sorbed arsenic or sequestering with changes in the redox state of the hydrologic system (after (O Shea, 2006))... 

Arsenic speciation is important in surface waters because acute toxicity of As" is greater than that of As or the organic forms of As (e.g., Jain and Ali, (2000). For human chronic toxicity the redox form of arsenic may not matter because arsenic is largely reduced to As(III) and methylated (National Research Council, 1999). Another issue is the processes by which As interacts with sediments, organic and biotic substrates in freshwaters, as this... 

There are two schools of thought about the time of the initial As mobilization either (i) it is recent and has been induced by man s activities [there are proponents of this who support both the pyrite oxidation hypothesis and the iron oxide reduction hypothesis (Acharyya et al., 2000)], or (ii) it occurred much earlier and is therefore dominantly a natural process. While we believe that an early release date, (ii) above, is the more likely, this is not to imply that man s recent activities have not had, or will not have, any impact on the extent of the groundwater arsenic problem. For example, recent changes in land use such as irrigation will not only alter the groundwater flow patterns but could also affect the boundary conditions for oxygen diffusion into the aquifer and so could also affect its redox status (Bhattacharya et al., 1997). 

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