Abstract
Phosphate barriers may mitigate dissolved P losses from critical source areas. We studied P retention of industrially produced Ca–Fe oxide as potential P barrier material. In batch tests with 1 mg l−1 P solution, P retention was 85% efficient in 5 min. In a flow-through system, the granule phosphate-retention capacity was 6–7 mg g−1, being largely unaffected by pre-leaching. Phosphate release from P-saturated granules was pH-dependant and suggested P association with Fe oxides, and as Ca-phosphate precipitates. In a sequential extraction of P-saturated granules, about 25% of retained P was released, whereas a separate anaerobic incubation resulted in negligible release of P. Immersion of unleached, P-saturated granules for 16 days in a low-P–concentration lake resulted in more than 80% Ca loss, but no loss of metals, and about 25% loss of P accumulated earlier in granules. These granules are promising for P retention and merit a field-scale study.
Highlights
Of the Finnish coastal sea areas, the Archipelago Sea (SW Finland) is especially affected by nutrient losses from agriculture
The capacity of P retention was assessed in a flow-through system that is less likely to overestimate retention in field conditions when dealing with a material that contains abundant soluble Ca and elevates the pH of the solution
Batch tests with variable contact times suggested that P retention by the Ca–Fe oxide granules is rapid at relatively low P concentrations (1 mg l−1) but that with increasing P concentration, a longer contact time between solution and the granules is required
Summary
Of the Finnish coastal sea areas, the Archipelago Sea (SW Finland) is especially affected by nutrient losses from agriculture. From an outer Archipelago Sea monitoring station, Bonsdorff et al (1997) reported an average annual 0.5 μg l-1 increase in wintertime total P concentration in the productive water layer during 1968–1993. They referred to other studies that showed decreasing trends in N:P ratio, transparency, and oxygen concentrations, all indicating a decline in quality of the coastal waters of the northern Baltic Sea. according to a recent ecosystem modelling study by Pitkänen et al (2007), a 30–40% reduction in N and P loads would significantly decrease the algal biomasses of the Archipelago Sea in 5 years. A 30–40% reduction in P losses in a few years, calls for novel, more effective measures to complement the water protection operations currently supported in the Finnish Agri-Environmental Programme
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