A three-step test of phosphate sorption efficiency of potential agricultural drainage filter materials

Phosphorus (P) eutrophication of lakes and streams, coming from drained farmlands, is a serious problem in areas with intensive agriculture. Installation of P sorbing filters at drain outlets may be a solution. Efficient sorbents to be used for such filters must possess high P bonding affinity to retain ortho-phosphate (Pi) at low concentrations. In addition high P sorption capacity, fast bonding and low desorption is necessary. In this study five potential filter materials (Filtralite-P(®), limestone, calcinated diatomaceous earth, shell-sand and iron-oxide based CFH) in four particle size intervals were investigated under field relevant P concentrations (0-161 μM) and retentions times of 0-24 min. Of the five materials examined, the results from P sorption and desorption studies clearly demonstrate that the iron based CFH is superior as a filter material compared to calcium based materials when tested against criteria for sorption affinity, capacity and stability. The finest CFH and Filtralite-P(®) fractions (0.05-0.5 mm) were best with P retention of ≥90% of Pi from an initial concentration of 161 μM corresponding to 14.5 mmol/kg sorbed within 24 min. They were further capable to retain ≥90% of Pi from an initially 16 μM solution within 1½ min. However, only the finest CFH fraction was also able to retain ≥90% of Pi sorbed from the 16 μM solution against 4 times desorption sequences with 6 mM KNO3. Among the materials investigated, the finest CFH fraction is therefore the only suitable filter material, when very fast and strong bonding of high Pi concentrations is needed, e.g. in drains under P rich soils during extreme weather conditions.