Phosphate binding by natural iron-rich colloids in streams

Area of Science:

• Environmental Chemistry
• Geochemistry
• Aquatic Chemistry

Background:

• Phosphorus (P) in natural waters can associate with iron (Fe) bearing colloids.
• Understanding the variability in colloid composition and P binding strength is crucial for aquatic ecosystems.
• Coarse colloids (0.1-1.2 μm) play a significant role in P transport and fate.

Purpose of the Study:

• To investigate the relationship between the composition of coarse colloids in Belgian streams and streamwater chemical properties.
• To determine the forms of P bound to these colloids and their lability.
• To elucidate the formation mechanisms of Fe-P colloids in natural waters.

Main Methods:

• Analysis of coarse colloid composition in 47 Belgian streams.
• Correlation of colloid composition with streamwater parameters like hardness and pH.
• Geochemical speciation calculations and colloid equilibration experiments to assess P binding and lability.

Main Results:

• Fe-rich colloids decrease with increasing water hardness and pH.
• P-bearing colloids are primarily Fe hydroxyphosphates or Fe oxyhydroxides with adsorbed P.
• Colloid P:Fe ratios indicate P saturation in some waters, affecting P lability; low P:Fe colloids contain non-labile P, while saturated colloids release labile P.
• Surface adsorption limits P binding to 0.02-0.04 mol P (mol Fe)-1, suggesting coprecipitation is key.

Conclusions:

• Fe-rich colloids are significant P carriers in streams, with their abundance influenced by water chemistry.
• The P binding strength and lability are strongly related to the colloid's P:Fe ratio and saturation state.
• Coprecipitation of P during Fe(II) oxidation, forming Fe hydroxyphosphate minerals, is the likely mechanism for Fe-P colloid formation.