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If this is the case, then a floc blanket consisting of precipitated coagulant could be an efficient reactor for arsenic removal.
Current Research
An important first task is to develop a reliable and sensitive measure for dissolved As (atomic absorption using graphite furnace atomization may be one option if we have a suitable lamp, inductively coupled plasma may be another option if you can get free help from someone with this instrument). There are some wet chemical methods (see Hach), but their sensitivity may not be adequate. Without an analytical tool we won't be able to verify the efficacy of any treatment scheme tested. Measuring dissolved As will require a phase separation from As adsorbed to suspended solids that does not alter the dissolved As concentration. Centrifugation is a good option (filtration probably will not work well). Uptake of dissolved As (adsorption) onto solids will likely depend on pH and ionic strength (as well as the concentration of suspended solids and dissolved As). pH control will be particularly important and it will be difficult to reproduce results if this is not part of the experimental protocol.
Big questions to begin answering
Which coagulant, Fe(Cl)3, alum, or PACl is better at removing arsenic?
Is arsenic removal limited by the mass transfer of arsenic to a precipitated coagulant surface, or by capture of the precipitated coagulant by plate settlers and filters?
Would a floc blanket formed from coagulant precipitate enhance arsenic removal?
Does addition of a small amount of clay enhance flocculation and arsenic removal?
How can we reduce the amount of coagulant loss to the reactor walls? (contact chamber, clay)
What is arsenic removal correlated with?
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