Surface chemistry of ferrihydrite: Part 2. Kinetics of arsenate adsorption and coprecipitation

Journal Article · Sat May 01 00:00:00 EDT 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90568-H· OSTI ID:6200464

Fuller, C C; Davis, J A ^[1]; Waychunas, G A ^[2]

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1. Geological Survey, Menlo Park, CA (United States)
2. Stanford Univ., CA (United States)

The kinetics of As(V) adsorption by ferrihydrite was investigated in coprecipitation and post-synthesis adsorption experiments conducted in the pH range 7.5--9.0. In coprecipitation experiments, As(V) was present in solution during the hydrolysis and precipitation of iron. In adsorption experiments, a period of rapid (< 5 min) As(V) uptake from solution was followed by continued uptake for at least eight days, as As(V) diffused to adsorption sites on ferrihydrite surfaces within aggregates of colloidal particles. The time dependence of As(V) adsorption is well described by a general model for diffusion into a sphere if a subset of surface sites located near the exterior of aggregates is assumed to attain adsorptive equilibrium rapidly. The kinetics of As(V) desorption after an increase in pH were also consistent with diffusion as a rate-limiting process. Aging of pure ferrihydrite prior to As(V) adsorption caused a decrease in adsorption sites on the precipitate owing to crystallite growth. In coprecipitation experiments, the initial As(V) uptake was significantly greater than in post-synthesis adsorption experiments, and the rate of uptake was not diffusion limited because As(V) was coordinated by surface sites before crystallite growth and coagulation processes could proceed. After the initial adsorption, As(V) was slowly released from coprecipitates for at least one month, as crystallite growth caused desorption of As(V). Adsorption densities as high as 0.7 mole As(V) per mole of Fe were measured in coprecipitates, in comparison to 0.25 mole As(V) per mole of Fe in post-synthesis adsorption experiments. EXAFS spectroscopy showed that neither ferric arsenate nor any other As-bearing surface precipitate or solid solution was formed. The results suggest that the solid solution model proposed by Fox (1989, 1992) for control of arsenate and phosphate concentrations in natural waters may be invalid. 61 refs., 10 figs., 1 tab.

• OSTI ID: 6200464
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:10; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ARSENATES
ADSORPTION
REACTION KINETICS
IRON OXIDES
CHEMICAL REACTIONS
SURFACE PROPERTIES
SURFACE WATERS
ARSENIC COMPOUNDS
CHALCOGENIDES
IRON COMPOUNDS
KINETICS
OXIDES
OXYGEN COMPOUNDS
SORPTION
TRANSITION ELEMENT COMPOUNDS
400201* - Chemical &amp; Physicochemical Properties
400102 - Chemical &amp; Spectral Procedures
540320 - Environment
Aquatic- Chemicals Monitoring &amp; Transport- (1990-)