Phosphate binding by natural iron-rich colloids in streams.
Phosphorus (P) in natural waters may be bound to iron (Fe) bearing colloids. However, the natural variation in composition and P binding strength of these colloids remain unclear. We related the composition of "coarse colloids" (colloids in the 0.1-1.2 μm size range) in 47 Belgian streams to the chemical properties of the streamwater. On average, 29% of the P in filtered (<1.2 μm) samples of these streams is present in coarse colloids. The concentration of Fe-rich colloids in streams decreases with increasing water hardness and pH. The P bearing colloids in these streams mostly consist of Fe hydroxyphosphates and of Fe oxyhydroxides with surface adsorbed P, which is underpinned by geochemical speciation calculations. In waters with molar P:Fe ratios above 0.5, only a minor part of the P is bound to coarse colloids. In such waters, the colloids have molar P:Fe ratios between 0.2 and 1 and are, therefore, nearly saturated with P. Conversely, in streams with molar P:Fe ratios below 0.1, most of the P is bound to Fe-rich colloids. Equilibration of synthetic and natural Fe and P bearing colloids with a zero sink reveals that colloids with low molar P:Fe ratios contain mostly nonlabile P, whereas P-saturated colloids contain mostly labile P which can be released within 7 days. Equilibration at a fixed free orthophosphate activity shows that the Fe-rich colloids may bind only limited P through surface adsorption, in the range of 0.02-0.04 mol P (mol Fe)(-1). The P:Fe ratios measured in naturally occurring Fe and P bearing colloids is clearly higher (between 0.05 and 1). These colloids are therefore likely formed by coprecipitation of P during oxidation of Fe(II), which leads to the formation of Fe hydroxyphosphate minerals.
KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 bus 2459, 3001, Leuven, Belgium. Electronic address: stijn.baken@gmail.com.
Stijn
S
Baken
KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 bus 2459, 3001, Leuven, Belgium.
Claudia
C
Moens
Copernicus Institute of Sustainable Development, Faculty of Geosciences, Utrecht University, P.O. Box 80115, 3508 TA, Utrecht, The Netherlands; Deltares, Unit Soil and Groundwater Systems, P.O. Box 85467, 3508 AL, Utrecht, The Netherlands.
Bas
B
van der Grift
KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 bus 2459, 3001, Leuven, Belgium.
Erik
E
Smolders
D003102:Colloids / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry; D010758:Phosphorus / Q000737:chemistry; D045483:Rivers
D003102:Colloids; D010710:Phosphates; D010758:Phosphorus; D007501:Iron
Colloids; Coprecipitation; Iron oxides; Natural water; Phosphate binding; Surface adsorption