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Report, 2014

Phosphorus binding by aluminium in sediment: A tool for restoring water quality in the Baltic Sea and other brackish surface waters

Huser, Brian


Lake and sea sediments act as stores for historical inputs of pollutants from both direct and diffuse sources including urbanization, agriculture, municipal and indus- trial waste waters, among others. Historical accumulation in sediments can provide a continual source of phosphorus to the water column for decades or longer after external sources have been controlled. Addition of metal salts, e.g. aluminium salts, can permanently bind this excess phosphorus in the sediment, thereby reducing internal phosphorus loading and improving water quality. Whereas this restoration method has been well studied in freshwater systems (over four decades of use), information on effectiveness in brackish and saltwater systems is limited. For this study, five sediment cores were collected from Torsbyfjärden near Lid- ingö. Sediment samples were initially analyzed for sediment phosphorus fractions and then aluminium was added to the sediment to convert mobile phosphorus to aluminum bound phosphorus. Mobile phosphorus is the pool of phosphorus that contributes directly to internal loading of phosphorus in surface waters and alumin- ium is used to convert this pool into an inert form that will remain in the sediment. Results from the initial analysis indicated that the potential for internal phosphorus loading in Torsbyfjärden is extremely high. Potential internal phosphorus loading rates ranged from 0.83 to nearly 40 mg/m2/d, with higher values generally being found in areas of greater water column depth (sediment accumulation areas). Based on the mobile phosphorus content of the sediment, aluminium was added under varying salinity (0-8) and pH (7-9) to elucidate the effects of these two vari- ables on phosphorus binding by aluminium in Baltic Sea sediment. Compared to pH, salinity had a relatively minor effect on phosphorus binding by aluminium, decreasing the effectiveness of binding by approximately 5% when comparing the 0 salinity level to a salinity of 8. pH, on the other hand, had a substantial effect on P binding by Al. The amount of mobile P inactivated by the added aluminium de- creased by approximately 22% in areas with very high mobile sediment phospho- rus and 33% in areas of moderate mobile phosphorus content in the sediment. The more important result, however, was that it may take more than twice the amount of aluminium to inactivate an equivalent amount of mobile sediment phosphorus (using pH 7 as a reference point) if pH in the treatment area is at 9 or higher. When 5 comparing pH 7 to pH 8 there was little change (approximately 5% difference) in conversion of mobile phosphorus to aluminium bound phosphorus. As part of this study, empirical models were developed to predict the effectiveness of phosphorus binding by aluminium under the above salinity and pH ranges. The overall results of this study show that the use of aluminum salts in brackish waters has good potential to limit internal phosphorus loading by inactivating the pool of mobile phosphorus via conversion to aluminium bound phosphorus. Care must be taken, however, when pH levels approach 9 or greater due to the limitation of phosphorous binding by aluminium under these conditions.


Sediment, aluminium, phosphorus, water quality, Östersjön, Baltic Sea

Published in

Rapport / Sveriges lantbruksuniversitet, Institutionen för vatten och miljö
2014, number: 2014:5
Publisher: Institutionen för vatten och miljö, SLU

Authors' information

Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment

UKÄ Subject classification

Oceanography, Hydrology, Water Resources
Geosciences, Multidisciplinary

URI (permanent link to this page)