Orthophosphate and metaphosphate ion removal from aqueous solution using alum and aluminum hydroxide

Abstract

The removal of orthophosphates (10 −2 kg P m −3), condensed phosphates (10 −2 kg P m −3), and mixtures of both (5 × 10 −3 kg P m −3 as orthophosphate and 5 × 10 −3 kg P m −3 as metaphosphate) in aqueous solution is studied using alum and aluminum hydroxide. The effects of coagulant dose, pH, temperature, aging of aluminum hydroxide, and presence of different ions are investigated. On the basis of the experimental results, alum is much more efficient in phosphorus removal than aluminum hydroxide even if, in both cases, at the conditions studied, the active coagulant form is Al(OH) 3. The differences then could be due to the higher activity of the in situ formed hydroxide. Orthophosphates and metaphosphates seem to have similar behavior vs pH variation: maximum removal is achieved at pH values 5–6 in all cases. On the other hand, in the simultaneous presence of both P forms, orthophosphate and metaphosphate ions have different affinities for the surface sites of aluminum hydroxide, since for both alum and aluminum hydroxide, orthophosphates are preferentially removed compared to metaphosphates, due probably to orientation effects and the charge per P atom. The presence of sodium, potassium, magnesium, sulfate, chloride, and magnesium, at the concentrations studied and for a pH value of 6, does not influence P removal. Temperature variation, between 25 and 60 °C, does not affect alum efficiency but both P forms are increasingly removed with increasing temperature, probably due to polymer Al(OH) 3 breaking, producing new surfaces for adsorption. Aging decreases sorption capacity of Al(OH) 3, while crystallites of increasing size are formed. Finally adsorption of both P forms is best described by the Freundlich isotherm [ K F = ( 49.1 – 69.1 ) × 10 −3 ( m 3 kg −1 ) 1 / N , 1 / N : 0.14–0.19 for T = 25 – 60 ° C ] and [ K F = ( 1.58 – 2.79 ) × 10 −3 ( m 3 kg −1 ) 1 / N , 1 / N : 2.17–2.47 for T = 25 –6 0 ° C ] for orthophosphate and metaphosphate, respectively.