Investigations of aluminum hydroxyphosphates and activated sludge by 27Al and 31P MAS NMR

High-resolution solid-state 27Al and 31P MAS NMR has been used to investigate the nature of aluminium- and phosphorus-containing solids formed during wastewater treatment. Although difficult to characterize by conventional techniques, these amorphous solids can be characterized by MAS NMR, providing information about their short-range ordering. In the present study, most of the solids were precipitated under conditions of high alkalinity, similar to those encountered during wastewater treatment. The ageing time of the aluminum hydroxide prior to the addition of phosphate (t1), the ageing time of the aluminum hydroxyphosphates after the addition of phosphate (t2), and the phosphorus to aluminum molar ratio were controlled while the effects of changing the parameters were examined. It was found that the aluminum MAS NMR chemical shift was related to the amount of phosphate present in the solid, which in turn was related to t1, t2, and the P:Al molar ratio. The results also lend support to the hypothesis that phosphate removal occurs through an adsorption process onto amorphous aluminum hydroxide, rather than through direct precipitation of aluminum phosphate. The increased understanding of aluminum and phosphate chemistry and the species formed during wastewater treatment will be useful in optimizing wastewater treatment processes. Keywords: aluminum, phosphate, NMR, activated sludge, wastewater treatment.