Download Eutrophication of lakes and streams, due to phosphorus (P) coming

Abstract
Eutrophication of lakes and streams, due to phosphorus (P) coming from drained farmlands, is a
serious problem in areas with intensive agriculture as in Denmark. Installation of P sorbing filters at
drain outlets has been proposed as a solution. Efficient sorbents to be used as such filters must
possess high affinity to retain orthophosphate (phosphate) at low concentrations. In addition, high
phosphate sorption capacity, fast bonding and low desorption are necessary. This PhD study
therefore seeks to identify phosphate sorbing materials (PSMs) that are capable of removing and
retaining phosphate at low concentrations and with short reaction times. Additionally, the aim of
this thesis is to get a better understanding of the sorption reactions, e.g. surface complexation and/or
precipitation, related to different types of commercially available PSMs.
Five potential filter materials (Filtralite®P, limestone, calcinated diatomaceous earth, shell-sand and
iron-oxide based CFH) fractionated in four particle size intervals were investigated under field
relevant phosphate concentrations (0-161 µM) and retention times of 0-24 min. For the same
particle size sorption decreased on the order: CFH ≈ Filtralite®P > limestone > shell-sand ≈
calcinated diatomaceous earth. The finest CFH and Filtralite®P fractions (0.05-0.5 mm) were found
to be the best sorbents with a phosphate retention of ≥ 90% from an initial concentration of 161 µM
corresponding to 14.5 µmol/kg sorbed within 24 min. Both PSMs were also capable of retaining
≥90% of phosphate from a 16 µM solution within 1.5 min. However, only the finest CFH fraction
was also able to retain ≥90% of phosphate sorbed from the 16 µM solution when subjected to 4
times desorption sequences with 6 mM KNO3.
Filtralite®P and CFH was also tested in a flow-through setting. The flow-through investigation
showed that retention times between 0.5-9 min and inlet phosphate concentrations between 1.6 and
32 µM influenced the materials phosphate sorption capacity and affinity. CFH shows up to 10 times
higher phosphate sorption capacity and affinity compared Filtralite®P. The difference is especially
pronounced at the low phosphate concentrations (1.6 and 3.2 µM). CFH released less than 10% of
previously sorbed phosphate compared to Filtralite®P, which released ≥ 35% in the desorption
experiments. Both materials’ phosphate sorption capacity and affinity are highly dependent on the
phosphate inlet concentration, which illustrates how important it is to test a PSM using field
relevant concentrations. Furthermore, phosphate sorption by Filtralite®P was also positively
correlated to retention time. This was not the case for CFH, indicating that CFH is capable of
removing phosphate even at high flow rates.
In order to improve the understanding of the phosphate sorption reactions and kinetics for different
types of commercial available PSMs, CFH, Filtralite®P and Limestone were studied by means of
isothermal titration calorimetry (ITC), sorption isotherms, sequential extractions and SEM-EDS.
For CFH, the results indicate formation of strong Fe-P bonding under formation of surface
complexes on outer surfaces followed by slow migration into interior sorption sites. For Filtralite-P
and Limestone, the phosphate sorption strongly depended on reaction time and pH, probably
because the phosphate retention was due to formation of Ca-phosphate precipitates.
In conclusion, of the five materials investigated the results from phosphate sorption and desorption
studies clearly demonstrate that regarding phosphate sorption affinity, capacity and reactivity, CFH
is superior as a filter material compared to the other tested materials.