Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 22, 1997
Publication Date: N/A
Interpretive Summary: Arsenic (As) is a potentially toxic trace element that can simultaneously exist in more than one chemical form or species' in the environment. This paper primarily investigates the adsorption of arsenite (As(III)) on clay minerals, but also compares As(III) adsorption with that of arsenate (As(V)). A chemical model (the constant capacitance model) was used to describe As(III) and As(V) adsorption on the minerals as a function of pH. We also confirmed the speciation (chemical identity) of the surface-bound portion of arsenic using high performance liquid chromatography-hydride generation atomic absorption spectrophotometry (HPLC-HGAAS) to detect As(III) and As(V) in phosphate extractions of the As(III) treated mineral solids. The As(III) species was found to be partially oxidized at the mineral-water interface of kaolinite and illite and may have been due to low level impurities of TiO2 and MnO2.
Technical Abstract: Redox transformations between arsenite (As(III)) and arsenate (As(V)) and adsorption reactions at the mineral-water interface are two important factors governing the fate and transport of arsenic in the environment. In this investigation, As(III) adsorption on kaolinite (KGa-1), illite (IMt-2), montmorillonite (SWy-1), and amorphous aluminum hydroxide (am-Al(OH)3) was studied as a function of pH and ionic strength and was compared with As(V) adsorption under identical conditions. In addition, surface complexation modeling was used to describe As(III) and As(V) adsorption on the four minerals. After the initial equilibrium adsorption period (16 h), the supernatants were analyzed for As(III)/As(V) speciation to detect conversion of As(III) to As(V). In As(III)-clay suspensions, the As(V) species was only detected in the KGa-1 supernatant, however extraction of clay solids with 1 mM phosphate following As(III) adsorption experiments revealed that a portion of As(III) on solid surfaces had converted to As(V) for all materials except am-Al(OH)3. This heterogeneous oxidation of As(III) at the clay-water interface may have been due to the presence of trace impurities in the solids such as TiO2 and MnO2. Adsorption of As(V) on clays under identical experimental conditions as As(III) confirmed that As(V) was adsorbed more strongly than As(III).