Relationships between redox-sensitive phosphorus concentrations in sediment and the aluminum:phosphorus binding ratio

Abstract

Aluminum (Al) sulfate dosage to control internal phosphorus (P) loading in lakes can be estimated by determining the mass of Al required to bind redox-sensitive P (loosely bound and iron-bound P fractions, redox-P) in sediment (Al:P binding ratio). Recent research found that the Al:P binding ratio varied negatively with redox-P, suggesting competition for binding sites between P and other constituents. We examined relationships between redox-P and the Al:P binding ratio over a broad range of redox-P to delineate general patterns that might be used to improve Al dosing calculations. Surface sediments collected from lakes in eastern Minnesota and western Wisconsin were subjected to a range of precipitated Al(OH)3 concentrations to determine the Al required to bind redox-P (Rydin and Welch 1999). Although sediments exhibited similar physical-textural characteristics, redox-P, dominated by iron-bound P, ranged widely between <0.1 and 8.8 mg/g. The Al:P binding ratio exceeded 100:1 as the redox-P concentration declined to <0.15 mg/g and approached 10:1 for redox-P concentrations exceeding 5 mg/g. These relationships were attributed to (1) competition for binding sites by other constituents at lower redox-P and (2) increased competition for binding sites by PO43− relative to other constituents at higher redox-P. Regression relationships between redox-P concentration and the Al:P binding ratio may be used with vertical sediment P profiles to estimate Al dosage required to reduce internal P loading, particularly for stratified or polymictic lakes where iron-bound P is the dominant fraction.