A Method for Determining the Phosphorus Sorption Capacity and Amorphous Aluminum of Aluminum‐Based Drinking Water Treatment Residuals

A high amorphous aluminum or iron oxide content in drinking water treatment residuals (WTRs) can result in a high phosphorus (P) sorption capacity. Therefore, WTR may be used beneficially to adsorb P and reduce P loss to surface or ground water. The strong relationship between acid ammonium oxalate–extractable aluminum (Al ox ) and Langmuir phosphorus adsorption maximum (P max ) in WTR could provide a useful tool for determining P max without the onus of the multipoint batch equilibrations necessary for the Langmuir model. The objectives of this study were to evaluate and/or modify an acid ammonium oxalate extraction of Al ox and the experimental conditions used to generate P adsorption isotherms to strengthen the relationship between Al ox and P max The oxalate extraction solution to WTR ratio varied from 40:1, 100:1, and 200:1. Batch equilibration conditions were also varied. The WTR particle size was reduced from <2 mm to <150 μm, and batch equilibration was extended from 17 h to 6 d. Increasing the solution to WTR ratio to 100:1 extracted significantly greater Al ox at levels of >50 mg Al kg −1 No additional increase was found at 200:1. Reducing WTR particle size from <2 mm to <150 μm increased P max 2.46‐fold. Extending the equilibration time from 17 h to 6 d increased P max by a mean of 5.83‐fold. The resulting empirical regression equation between the optimized Al ox and P max ( r 2 = 0.91, significant at the 0.001 probability level) may provide a tool to estimate the P max of Al‐based WTR simply by measuring Al ox The accurate determination of WTR P max and Al ox is essential in using WTR effectively to reduce P loss in runoff or to reduce the solubility of P in agricultural soils or organic waste materials (biosolids, manure).