Complex and colloid formation

Mobilization of arsenic can be promoted by the presence of other solutes and their interaction with arsenic to form complexes, as detailed above for NOM. As(V) has been documented to form complexes with calcium and magnesium in brackish water and seawater (Cullen and Reimer, 1989). Arsenic commonly forms complexes with sulfide, such as As3S3(SH)3° (Bostick, Fendorf and Manning, 2003) (Chapter 3). These complexes may exist in slightly sulfidic environments. Arsenic-fluoride complexes (e.g. AsC F2-and HAsC F-) can also form in high-fluoride waters ((Apambire and Hess, 2000 Bundschuh, Bonorino and Viero, 2000), Chapter 3). 

Decomposition of organic matter and release of bound arsenic 

Oxidation of arsenic-bearing pyrite with adsorption onto iron oxides and/or other metal (oxy)(hydr)oxides Nitrate reduction by pyrite oxidation (note that Appelo and Postma, 1999 referred to pure rather than arsenian pyrite) Manganese oxide reduction and release of sorbed arsenic Fe(lll) reduction on oxide surfaces changes net charge leading to arsenic desorption Iron oxide reductive dissolution and release of sorbed arsenic catalyzed by NOM degradation 

As(V) reduction to As(lll) and change in sorption properties Precipitation of insoluble sulfides (e.g. pyrite) and sequestration 

---