Alum (aluminum sulfate) is increasingly being used in lake management to control internal recycling of phosphorus from bottom sediments. Alum added to water undergoes rapid hydrolysis reactions, forming an amorphous Al(OH)3 floc with a high capacity for sorption of phosphorus. While it is known that the Al(OH)3 floc transforms over time to more ordered microcrystalline and crystalline gibbsite phases, there remains an incomplete understanding of the forms of Al present immediately following alum addition to lake water. A laboratory study was thus undertaken to evaluate the forms of Al present following alum addition using ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid) timed-colorimetric and 27Al-NMR measurements. A polymeric Al species with moderate reactivity with ferron (Alb2) was initially formed, although it rapidly transformed to a less ferron-reactive colloidal form (Alc) and also decomposed at low alum doses to monomeric Al (Ala) in response to pH increases associated with outgassing of CO2. The Ala fraction in these solutions could be adequately estimated based upon measured pH assuming Al solubility was controlled by an amorphous Al(OH)3 phase. Al13 was inferred from ferron measurements to be present, but only at quite low concentrations in the alum-treated waters.
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