Sorption, ion exchange, precipitation, and coprecipitation of arsenic in water

6 Sorption, ion exchange, precipitation, and coprecipitation of arsenic in water 2.7.6.1 Introduction 

Once arsenic dissolves in natural water, it may remain in solution for an extended period of time or participate sooner in abiotic or biotic reactions that remove it from solution. Depending upon the pH, redox conditions, temperature, and other properties of an aqueous solution and its associated solids, dissolved arsenic may precipitate or coprecipitate. Arsenic may also sorb onto solid materials, usually through ion exchange. Due to their importance in understanding the behavior of arsenic in natural environments (Chapter 3) and their applications in water treatment (Chapter 7), the sorption, ion exchange, precipitation, and coprecipitation of arsenic have been the subjects of numerous investigations. 

The displaced species enters the solution as a water molecule. 

Absorption is the assimilation of a chemical species into the interior of a solid substance. Absorption may include the migration of solutes into the internal pores of a solid material (Fetter, 1993), 117 or the migration or exchange of atoms within the crystalline structure of a mineral (Krauskopf and Bird, 1995), 150. Some researchers use the generic term sorption to refer to a treatment method where both adsorption and absorption may be involved or if adsorption and absorption cannot be distinguished. Sorption and ion exchange have many important roles in immobilizing arsenic in natural environments (Chapter 3). They 

Under more alkaline conditions (Krauskopf and Bird, 1995), 138, hydrogens are removed from the hydroxides and the surfaces become negative  

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