Mechanisms of Phosphorus Removal by Phosphorus Sorbing Materials

Abstract

Stormwater filters are a structural best management practice designed to reduce dissolved P losses from runoff. Various industrial byproducts are suitable for use as P sorbing materials (PSMs) for the treatment of drainage water; P sorption by PSMs varies with material physical and chemical properties. Previously, P removal capacity by PSMs was estimated using chemical extractions. We determined the speciation of P when reacted with various PSMs using X‐ray absorption near edge structure (XANES) spectroscopy. Twelve PSMs were reacted with P solution in the laboratory under batch or flow‐through conditions. In addition, three slag materials were collected from working stormwater filtration structures. Phosphorus K‐edge XANES spectra were collected on each reacted PSM and compared with spectra of 22 known P standards using linear combination fitting in Athena. We found evidence of formation of a variety of Ca‐, Al‐, and/or Fe‐phosphate minerals and sorbed phases on the reacted PSMs, with the exact speciation influenced by the chemical properties of the original unreacted PSMs. We grouped PSMs into three general categories based on the dominant P removal mechanism: (i) Fe‐ and Al‐mediated removal [i.e., adsorption of P to Fe‐ or Al‐(hydro‐)oxide minerals and/or precipitation of Fe‐ or Al‐phosphate minerals]; (ii) Ca‐mediated removal (i.e., precipitation of Ca‐phosphate mineral); and (iii) both mechanisms. We recommend the use of Fe/Al sorbing PSMs for use in stormwater filtration structures where stormwater retention time is limited because reaction of P with Fe or Al generally occurs more quickly than Ca‐P precipitation. Core Ideas P speciation of PSMs was determined using XANES for stormwater filtration media. PSMs remove P via Al/Fe sorption, Ca sorption, or both mechanisms. P speciation facilitates the selection of PSMs for stormwater filtration structures. P speciation is valuable in evaluating potential disposal alternatives of spent PSMs.

Performance of Field-Scale Phosphorus Removal Structures Utilizing Steel Slag for Treatment of Subsurface Drainage

“…Not only can the formation of the calcite minerals fill the pore space, clogging the structure and reducing P removal through decreasing interaction between the slag and water, but we hypothesize that those same precipitation reactions are partly responsible for the decreased chemical potency of the slag, reducing the precipitation of calcium phosphate, the main P removal mechanism [62,63,78]. In essence, conversion of the slag calcium minerals into calcite will decrease the pH of the system and also depress the concentration of soluble calcium available for calcium phosphate precipitation.…”

Phosphorus Removal in Denitrifying Woodchip Bioreactors Varies by Wood Type and Water Chemistry

“…The poplar and hickory woodchips were relatively low in all four elements. These total metals contents allowed an initial assessment and comparison between wood types, although it is recognized that the potential reactivity of P sorbing media may be better characterized by amorphous Fe or Al or watersoluble Ca (Penn et al 2007;Qin et al 2018).…”

Utilization of Steel Slag in Blind Inlets for Dissolved Phosphorus Removal

“…Steel slag chemical properties were typical for material produced from the EAF steelmaking process ( Table 3). Slag had an elevated pH and high Ca concentration, both of which are required for the slag to be effective at dissolved P removal via precipitation of Ca phosphate [7,[15][16][17][18]. Although slag contained some trace metals, there was no environmental concern about release to treated water since EAF slag has been shown to possess negligible metal solubility [9,19] and even may remove trace metals [20].…”