Phosphorus adsorption on iron-coated sand under reducing conditions

Area of Science:

• Environmental Science
• Water Chemistry
• Soil Science

Background:

• Agricultural runoff is a major source of phosphate pollution in surface waters.
• Iron-coated sand (ICS) is a cost-effective material for phosphate removal.
• Concerns exist about iron reduction and phosphate release from ICS filters in low-oxygen conditions.

Purpose of the Study:

• To investigate phosphate adsorption on ICS under the onset of iron reduction.
• To assess the potential for microbially mediated reduction and phosphate release from ICS filters.
• To evaluate the long-term performance of ICS filters in tile drain systems.

Main Methods:

• Field studies at two Dutch sites with ICS filters around tile drains.
• Laboratory microcosm experiments simulating reducing conditions in drainage water.
• Analysis using electron microscopy and X-ray absorption spectroscopy.

Main Results:

• ICS filters demonstrated sustained phosphorus removal in the field, even under low redox conditions.
• Laboratory experiments showed no phosphate release over 45 days, despite microbial reduction of manganese and iron oxides.
• Minimal structural changes were observed in ICS, with only minor iron sulfide formation under extreme reducing conditions.

Conclusions:

• ICS filters are effective in mitigating agricultural phosphate loads to surface waters.
• Phosphate release from ICS filters is unlikely in the short to medium term, even with microbial iron reduction.
• Detrimental effects on ICS filter performance are only expected after prolonged operation under strongly reducing conditions.