Effects of redox potential and pH changes on phosphorus retention by melter slag filters treating wastewater.
The release of phosphorus (P) and iron (Fe) from a melter slag filter in solutions of varying Eh and pH was examined. The filter had been removing P from waste stabilization pond effluent for several years. The study revealed that the highest P (95% of total P) and Fe (25% of total Fe) release from the slag occurred in the solution with the lowest Eh (-400 mV, relative to the standard hydrogen electrode, SHE) and lowest pH (4.9). Solutions with high pH (9.1) also proved favorable for P release (20 to 40% of total P) from the slag, at both reducing (-400 mV) and oxidizing (+300 mV) Eh. By contrast, solutions with pH 4.9 and 6.7 and oxidizing Eh (+300 mV) liberated the lowest P and Fe contents into the aqueous phase (<1% for both elements). The findings showed that Eh and pH are important parameters affecting P release from slag filters. At low Eh and low pH, P is released due to the dissolution of Fe oxides/oxyhydroxides, as supported by scanning electron microscopy (SEM)/energy dispersive spectrometry (EDS). At high pH, P is desorbed from negatively charged Fe oxide/oxyhydroxide surfaces. The results of this investigation are pertinentto the design and operation of melter slag filters that treat all forms of P-rich waters, such as wastewater, stormwater, and farm runoff. The study demonstrated that P retention by melter slag filters is optimal in water bodies characterized by near-neutral pH and oxidizing Eh because these conditions favor P adsorption onto Fe oxides/oxyhydroxides.
Pratt
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C
Centre for Environmental Technology and Engineering, Massey University, Palmerston North, Turitea Campus, New Zealand. C.C.Pratt@massey.ac.nz
Shilton
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A
Pratt
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S
Haverkamp
R G
RG
Elmetri
I
I
D005374:Filtration / Q000295:instrumentation / Q000379:methods; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D008855:Microscopy, Electron, Scanning; D010084:Oxidation-Reduction; D010758:Phosphorus / Q000737:chemistry*; D014865:Waste Disposal, Fluid / Q000379:methods*; D018508:Water Purification / Q000379:methods*
D010758:Phosphorus; D007501:Iron