Iron, aluminum, and manganese oxy hydr oxides

Over time, two-line ferrihydrite normally transforms into goethite or hematite in laboratory or natural environments (Rancourt et al., 2001, 839). However, extensive sorption of As(V) could delay the transformation (Ford, 2002). The crystallization of arsenic-bearing amorphous iron compounds often releases arsenic from the compounds (Welch et al., 2000, 599). In particular, while aging in seawater from Ambitle Island near Papua New Guinea, two-line ferrihydrites transformed into less arsenic-rich six-fine varieties. The arsenic released by the transformation of the ferrihydrites produced distinct crystals of claudetite (As203) (Rancourt et al., 2001, 838-839). 

Although arsenic-bearing ferrihydrites are mostly amorphous, Rancourt et al. (2001, 849) concluded that As(V) is generally tetrahedrally coordinated with Fe(III) in the compounds. The chemical properties of the marine ferrihydrites described in Rancourt et al. (2001, 848) more closely resemble synthetic ferrihydrites formed by the coprecipitation of arsenic and Fe(III) rather than compounds that had sorbed arsenic sometime after their precipitation. 

As further discussed in Chapters 2 and 7, the sorption of arsenic on iron (oxy)(hydr)oxides is very sensitive to pH and competing anions, such as phosphate and sulfate (Goh and Lim, 2004). In general, the sorption of inorganic As(V) decreases as pH values rise from 3 to 10 (Su and Puls, 2001, 1489). H2As04 is the dominant As(V) ion at pH 3-6. At pH 6, the surfaces of iron (oxy)(hydr)oxides usually have net positive charges (i.e. they are below their zero points of charge (ZPCs) and isoelectric points  

Chapter 2). The positively charged surfaces would readily attract and sorb H2ASO4- and other As(V) oxyanions (Su and Puls, 2001, 1489). At pH 6-7, the surface charges of most iron (oxy)(hydr)oxides become dominantly negative and the sorption of As(V) oxyanions diminishes because of charge repulsion. 

The sorption of inorganic As(III) on iron (oxy)(hydr)oxides generally increases as pH rises from 3 to 9. Maximum As(III) sorption generally occurs around pH 9 (Su and Puls, 2001, 1489). At this point, As(III) oxyanions start to become prominent (Chapter 2), but yet iron (oxy)(hydr)oxide surfaces still have some residual positive charges. 

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Additional information on adsorption mechanisms and models is in Stollenwerk (2003), 93-99 and Prasad (1994). Foster (2003) also discusses in considerable detail how As(III) and As(V) may adsorb and coordinate on the surfaces of various iron, aluminum, and manganese (oxy)(hydr)oxides. In adsorption studies, relevant laboratory parameters include arsenic and adsorbent concentrations, adsorbent chemistry and surface area, surface site densities, and the equilibrium constants of the relevant reactions (Stollenwerk, 2003), 95. Once laboratory data are available, MINTEQA2 (Allison, Brown and Novo-Gradac, 1991), PHREEQC (Parkhurst and Appelo, 1999), and other geochemical computer programs may be used to derive the adsorption models. 

Adsorption of arsenic Iron, aluminum, and manganese (oxy)(hydr)oxides widely occur as sorbents and coatings on other solid materials in nature. They are often important in adsorbing arsenic from water ((Stollenwerk, 2003), 73 Chapter 3). Below the ZPCs of the (oxy)(hydr)oxides, the presence of... 

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