The main conclusions regarding the sorption properties of diatomite are as follows:
Diatomaceous silica (also known as opal-AN) is the principal constituent of diatomite. Diatomite appears to function by providing an inert, rigid substrate for other mineral phases involved in the sorption of arsenic.
Diatomaceous silica was not directly implicated in the sorption of arsenic. An antipathetic trend was observed between arsenate sorption and the opal-AN content of samples. Opal-AN also possesses a negative surface potential between pH 12 and 1 which precludes the physical adsorption of negatively-charged arsenate species. Under acid conditions, co-precipitation of arsenic with silica is also considered unlikely as it was demonstrated that the solubility of opal-AN is minimal below pH 4.
There is some evidence that arsenate undergoes physical adsorption onto iron oxide coatings bonded to the opal-AN substrate. Uptake of arsenate correlates with the iron oxide content of samples and also exhibits a pH-dependency characteristic of physical adsorption.
The sorption of arsenate by iron oxide involves electrostatic attraction and the formation of weakly-bonded, outer-sphere complexes. Arsenate adsorbed onto iron oxide will be re-mobilised under anaerobic conditions. The iron oxide content of the diatomite may also initiate co-precipitation of arsenic by raising iron concentration of the solute above saturation.
For diatomite sample D027, used in the project's field trials, the sorption of arsenate closely-matched the Langmuir isotherm. This behaviour suggests that the sorption of arsenate involves a single type of surface functional group. From the Langmuir equation, an adsorption capacity for arsenate of 0.23 mg/g was derived for sample D027.
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