Arsenic surface complexation reactions

In REACT, we prepare the calculation by disenabling the redox couple between trivalent and pentavalent arsenic (arsenite and arsenate, respectively). As well, we disenable the couples for ferric iron and cupric copper, since we will not consider either ferrous or cupric species. We load dataset FeOH+.dat , which contains the reactions from the Dzombak and Morel (1990) surface complexation model, including those for which binding constants have only been estimated. The procedure is... 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Iron and manganese have a strong influence on the availability of trace metal pollutants through precipitation-dissolution reactions (Burdige, 1993 Cornell and Schwertmann, 1996). Trace metals form surface complexes or co-precipitate with Fe(III) and Mn(IV) oxides, and they are released upon Fe(III) and Mn(IV) reduction (Zachara et al., 2001). For example, processes that oxidize Fe(II) retain arsenic in sediments... 

Sun and Doner (31) examined the OH stretching modes of deuterated goethite in the presence of arsenate using FTIR. Based on transmission and ATR modes in dry systems, the authors concluded that arsenate oxyanions replaced singly coordinated surface hydroxyl groups to form bidentate binuclear complexes. Reactions presented by Sun and Doner (31) indicate HAs04 as the surface species, bound to metal centers via two O bonds. 

Adsorption in Reactions 2.47-2.52 also involves ion exchange (Eby, 2004), 345. During the formation of inner-sphere complexes, adsorbing arsenic commonly replaces hydroxides or other chemical species on the surface of the adsorbent. Complexes in Stern outer-sphere and Gouy layers are also susceptible to ion exchange, especially because they are weakly adsorbed (Krauskopf and Bird, 1995), 150. 

Gao, Y. and Mucci, A. (2001) Acid base reaction, phosphate and arsenate complexation, and their competitive adsorption at the surface of goethite in 0.7M NaCl solution. Geochimica et Cosmochimica Acta, 65(14), 2361-78. 

Many studies on the direct reaction of methyl chloride with silicon-copper contact mass and other metal promoters added to the silicon-copper contact mass have focused on the reaction mechanisms.7,8 The reaction rate and the selectivity for dimethyldichlorosilane in this direct synthesis are influenced by metal additives, known as promoters, in low concentration. Aluminum, antimony, arsenic, bismuth, mercury, phosphorus, phosphine compounds34 and their metal complexes,35,36 Zinc,37 39 tin38-40 etc. are known to have beneficial effects as promoters for dimethyldichlorosilane formation.7,8 Promoters are not themselves good catalysts for the direct reaction at temperatures < 350 °C,6,8 but require the presence of copper to be effective. When zinc metal or zinc compounds (0.03-0.75 wt%) were added to silicon-copper contact mass, the reaction rate was potentiated and the selectivity of dimethyldichlorosilane was enhanced further.34 These materials are described as structural promoters because they alter the surface enrichment of silicon, increase the electron density of the surface of the catalyst modify the crystal structure of the copper-silicon solid phase, and affect the absorption of methyl chloride on the catalyst surface and the activation energy for the formation of dimethyldichlorosilane.38,39 Cadmium is also a structural promoter for this reaction, but cadmium presents serious toxicity problems in industrial use on a large scale.41,42 Other metals such as arsenic, mercury, etc. are also restricted because of such toxicity problems. In the direct reaction of methyl chloride, tin in... 

There is no pH dependence to the reaction for As(OH)3, which sorbs strongly when pH is near neutral. Under acidic conditions, however, high H+ activities drive the reactions for protonation and the sorption of SO4-, which displace arsenic from the mineral surface. Arsenic sorbs poorly under alkaline conditions because low H+ activities work against the complexation of As(OH)4, or AsO2OH, as shown in the reactions for these species,... 

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