Phosphate adsorption by lanthanum modified bentonite clay in fresh and brackish water

Effects of pH, alkalinity and conductivity on the adsorption of soluble reactive phosphorus (SRP) onto lanthanum (La) modified bentonite clay (Phoslock®) were investigated in laboratory experiments using eight different types of filtered water representing freshwater with low and normal alkalinity and brackish water with high alkalinity. Different dose ratios (0–200; w/w) of Phoslock®:P were applied to determine the maximum P binding capacity of Phoslock® at SRP concentrations typical of those of sediment pore water. The 100:1 Phoslock®:P dose ratio, recommended by the manufacturer, was tested with 12 days exposure time and generally found to be insufficient at binding whole target SRP pool. The ratio performed best in the soft water from Danish Lake Hampen and less good in the hard water from Danish Lake Langesø and in brackish water. The explanation may be an observed negative relationship between alkalinity and the SRP binding capacity of Phoslock®. A comparative study of Lake Hampen and Lake Langesø suggested that the recorded differences in P adsorption between the two lakes could be attributed to a more pronounced dispersion of Phoslock® in the soft water of Lake Hampen, leading to higher fractions of dissolved (<0.2 μm) La and of La in fine particles. In the same two lakes, pH affected the SRP binding of Phoslock® negatively at a pH level above 8.1, the effect being reversible, however. The negative pH effect was most significant in hard water Lake Langesø, most likely because of higher CO32−CO32− concentrations.

► Phoslock® was able to reduce SRP below 10 μg L−1 when the Phoslock® to SRP dose was 200:1.
► SRP adsorption by Phoslock® was negatively affected by increasing pH and alkalinity.
► The pH effects were reversible.
► P binding by Phoslock® was negatively correlated with alkalinity.
► A 100:1 Phoslock®:SRP dose was generally too low to bind all SRP in solution.