Treatment of Wastewater Phosphate by Reductive Dissolution of Iron: Use of Ferric Oxyhydroxide Media

Abstract

In smaller wastewater treatment systems such as septic systems, there is an interest in the development of passive phosphorus (P) removal methods. This study tested fixed-bed filters containing ferric oxyhydroxide media for wastewater P removal in a laboratory column test and in a full-scale domestic septic system. In the column test, during 30 mo of dosing with domestic wastewater, reductive iron dissolution reactions delivered consistent moderate concentrations of Fe into solution (2.9 ± 1.6 mg L), and influent PO-P of 3.7 ± 1.0 mg L was attenuated to 0.09 + 0.04 mg L in the column effluent (98% removal). Phosphorus breakthrough at successive locations along the column indicated that in addition to sorption, mineral precipitation reactions probably also played an important role in the observed P attenuation. This was supported by electron microprobe analyses, which showed the presence of thick (20 μm) secondary Fe-rich coatings containing P on the primary ferric media grains. Assays of NaHCO-leachable and acid-extractable P on the column solids showed accumulation of up to 5.4 mg g acid-extractable P near the column inlet, but <5% of this amount was easily desorbable, further indicating P attenuation from processes other than sorption. Over 19 mo of operation, the domestic septic system also showed generally consistent increased Fe in the filter effluent (2.6 ± 1.7 mg L) and achieved 99% P removal to 0.03 ± 0.02 mg L when the effluent was subsequently oxidized in a sand filter. Ferric iron filters could be attractive options for P removal in smaller wastewater systems because of their passive nature.

Laboratory Comparison of Four Iron‐Based Filter Materials for Drainage Water Phosphate Treatment

“…Iron-based materials, such as elemental iron (i.e., zero-valent iron) and iron oxides/hydroxides, have been evaluated for PO 4 3water treatment (McCobb et al, 2009;Robertson and Lombardo, 2011). There are three possible mechanisms by which zerovalent iron (ZVI) can remove PO 4 3-.…”

“…Similar to ZVI, there are three possible mechanisms by which IOH can remove PO 4 3-. Dissolution of IOH can release ferrous/ ferric iron (Fe 2þ /Fe 3þ ) ions (Robertson and Lombardo, 2011), which can combine with PO 4 3to form low solubility ironphosphate precipitates (Bohn et al, 1985;Stumm and Morgan, 1981). Also, IOH particle surfaces have functional groups sites where PO 4 3can become attached via ligand exchange (Bohn et al, 1985;Goldberg, 1985;McBride, 1994).…”

Batch Test Screening of Industrial Product/Byproduct Filter Materials for Agricultural Drainage Water Treatment

“…The sulfur modified iron filter materials (SMI1, SMI2, and SMI3) probably removed NO3 − via oxidation/reduction reactions that convert NO3 − to ammonia/ammonium, NH3/NH4 + , or nitrogen gas, N2 [22][23][24][25][26]. Phosphate removal by high iron content media, high aluminum content media, coal combustion residuals, and spent foundry sands could be the result of either the formation of low solubility chemical precipitates (i.e., aluminum, calcium, iron, and magnesium-phosphate compounds) [28][29][30][31] or direct PO4 3− adsorption via ligand Again, the purpose the study was to delineate potential industrial product/byproduct filter materials warranting further investigation for agricultural drainage water treatment. The processes by which filter materials can remove NO 3 − , PO 4 3− , and/or atrazine were previously described in the Introduction section.…”

“…The sulfur modified iron filter materials (SMI1, SMI2, and SMI3) probably removed NO 3 − via oxidation/reduction reactions that convert NO 3 − to ammonia/ammonium, NH 3 /NH 4 + , or nitrogen gas, N 2 [22][23][24][25][26]. Phosphate removal by high iron content media, high aluminum content media, coal combustion residuals, and spent foundry sands could be the result of either the formation of low solubility chemical precipitates (i.e., aluminum, calcium, iron, and magnesium-phosphate compounds) [28][29][30][31] or direct PO 4 3− adsorption via ligand exchange at oxygen containing functional group sites present along surfaces of filter material particles [29][30][31][32][33]. Phosphate removal by the high carbon content media and surfactant modified clay/zeolite is probably due to ligand exchange adsorption [29,33,57,58] and/or electrostatic attraction between negatively charged PO 4 3− ions and positively charged filter material surfaces [34][35][36]41].…”

“…Nevertheless, cations (e.g., Al 3+ , Ca 2+ , Fe 3+ , Mg 2+ , etc.) dissolved from filter materials can combine with PO 4 3− to form low solubility chemical precipitates [28][29][30][31]. Direct PO 4 3− adsorption typically involves ligand exchange at oxygen containing functional group sites present along surfaces of filter material particles [29][30][31][32][33].…”