The reduction of dissolved phosphorus (P) transport to water systems is of critical importance for water quality. Phosphorus sorption materials (PSMs) are media with high affinity for dissolved P, and therefore serve as the core components of P removal structures. These structures can intercept dissolved P in surface and subsurface flows, before discharge into water bodies. While the P removal ability of PSMs has been extensively studied, lesser is known about the capacity to regenerate and recover P from P-saturated PSMs. This article evaluates a methodology to recover the P removal ability of aluminum- and iron-rich P-saturated PSMs. A series of flow-through experiments were conducted, alternating between P sorption (0.5 and 50 mg L − 1 P) and desorption with potassium hydroxide (KOH; 5 or 20 pore volumes [PV]), varying residence times (0.5 min and 10 min), and number of recirculations (0, 6 and 24). Across two cycles of sorption-desorption, Alcan, Biomax and PhosRedeem showed an average P recovery of 81%, 79%, and 7%, with standard deviation of 10%, 21% and 6%, respectively. The most effective regeneration treatment was characterized by the largest KOH volume (20 PV) and no recirculation, with up to 100% reported P recovery. Although KOH at 5 PV was less effective, the use of recirculation did increase P recovery. The lifetime of Al/Fe-dominated PSMs in P removal structures can be extended through feasible regeneration techniques demonstrated in this study, for both high and low P concentration scenarios.
Controlling phosphorus (P) losses from intensive agricultural areas to water bodies is an ongoing challenge. A critical component of mitigating P losses lies in accurately predicting dissolved P loss from soils, which often includes estimating the amount of soluble P extracted with a laboratory-based extraction, i.e., water-extractable P (WEP). A standard extraction method to determine the WEP pool in soils is critical to accurately quantify and assess the risk of P loss from soils to receiving waters. We hypothesized that narrower soil-to-water ratios (1:10 or 1:20) used in current methods underestimate the pool of WEP in high or legacy P soils due to the equilibrium constraints that limit the further release of P from the solid-to-solution phase. To investigate P release and develop a more exhaustive and robust method for measuring WEP, soils from eight legacy P fields (Mehlich 3–P of 502 to 1127 mg kg−1; total P of 692 to 2235 mg kg−1) were used for WEP extractions by varying soil-to-water ratios from 1:10 to 1:100 (weight:volume) and in eight sequential extractions (equivalent to 1:800 soil-to-water ratio). Extracts were analyzed for total (WEPt) and inorganic (WEPi) pools, and organic (WEPo) pool was calculated. As the ratios widened, mean WEPi increased from 23.7 mg kg−1 (at 1:10) to 58.5 mg kg−1 (at 1:100). Further, WEPi became the dominant form, encompassing 92.9% of WEPt at 1:100 in comparison to 79.0% of WEPt at 1:10. Four of the eight selected soils were extracted using a 1:100 ratio in eight sequential extractions to fully exhaust WEP, which removed a cumulative WEPt of 125 to 549 mg kg−1, equivalent to 276–416% increase from the first 1:100 extraction. Although WEP concentrations significantly declined after the first sequential extraction, WEP was not exhausted during the subsequent extractions, indicating a sizeable pool of soluble P in legacy P soils. We conclude that (i) legacy P soils are long-term sources of soluble P in agricultural landscapes and (ii) the use of a 1:100 soil-to-water ratio can improve quantification and risk assessment of WEP loss in legacy P soils.
<p>Preventing dissolved phosphorus (P) accumulation in soils and its transport to water bodies has been subject of many studies. However, despite the continuous efforts and advances, excessive P is still a concern and it is especially problematic in freshwater systems: excessive dissolved P leads to eutrophic conditions, a threat for water quality and aquatic life. P removal structures are a novel technology used in urban and rural settings to intercept dissolved P in surface and subsurface flows. P sorption materials (PSMs), active media with high affinity for dissolved P, are the core components of these structures. Once the PSMs reach service life, replacing the spent media can be costly. The objective of this research is to assess potential regeneration techniques that will extend the lifetime of Aluminum (Al)/Iron (Fe)-rich PSMs. We are proposing a regeneration involving a continuous circulation of 1M KOH aiming to restore unavailable sorption sites on the PSMs. A series of flow-through experiments was conducted alternating between P sorption (0.5 and 50 mg/L input solution) and desorption with KOH (5 or 20 pore volumes), varying residence times (0.5 min and 10 min) and number of recirculations (0, 6 and 24). We tested the treatments in 3 manufactured PSMs, Alcan, Biomax and PhosRedeem. Across two cycles of sorption-desorption, Alcan, Biomax and PhosRedeem showed an average P recovery of 81%, 79% and 7%, respectively.&#160; The comparative investigation of the tested treatments revealed that the most effective regeneration treatment is characterized by a larger KOH volume (20 pore volumes) and no recirculation, with up to 100% reported P recovery. This research demonstrates the ability of Al/Fe-rich PSMs regeneration to contribute to a circular economy of P, as P recovery enables a more sustainable P cycle in both terrestrial and aquatic environments.</p>
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