Soil amendments to enhance phosphorus sorption
Principal mechanisms for phosphorus (P) removal in bioretention are the filtration of particulate-bound P and chemical sorption of dissolved P (e.g. summarized in Hunt et al 2012). Most stormwater control measures (SCMs) capture particulate P by settling or filtration, but leave dissolved P (typically phosphates), which is most bio-available, untreated. This untreated P accounts on average for 45 percent of total phosphorus in stormwater runoff and can be up to 95% of the total phosphorus, depending, for example, on the storm event (Erickson et al 2012). Phosphorus sorbing materials contain a metal cation (typically di or trivalent) that reacts with dissolved phosphorus to create an insoluble compound by adsorption or precipitation or both (Buda et al 2012). Soil components and amendments that have been shown to be effective to increase chemical sorption of dissolved P include:*Iron filings (Erickson et al 2012)
- Steel wool (Erickson et al 2007)
- Native iron rich soils such as those in the Piedmont of the Mid and Southern Atlantic USA (Hunt et al 2012), or Krasnozem soil in Australia (Lucas and Greenway 2011)
- Water treatment residuals (WTRs), which are a by-product of drinking water treatment and a source of aluminum and iron hydroxides (O’Neill and Davis 2012 a and b, Hinman and Wulkan 2012, Lucas and Greenway 2011, Lucas and Greenway 2010)
- Sorptive media (Imbrium) (Balch et al 2013)Buda et al (2012) provide a literature review of P-sorption amendments. Characteristics of ideal P-sorption amendments include low cost, high availability, low toxicity to soil and water resources, potential for reuse as a soil amendment once fully saturated (Buda et al 2012), and no toxicity to plants, wildlife, or children. It is also crucial that soil amendments do not negatively impact soil infiltration rate and ability to grow vigorous plants. Some P sorptive amendments, such as water treatment residuals (WTRs), are waste products turned into a resource to reduce P in bioretention (or agricultural) soils.Results from much of the research to date on use of P-sorbing materials to reduce nutrients in stormwater effluent are promising so far, but much remains to be learned about lifespan and long term effects of P sorbing materials on soils and plants.