Ca–Fe oxide granules as potential phosphate barrier material for critical source areas: a laboratory study of P retention and release

Abstract

Phosphate barriers may mitigate dissolved P losses from critical source areas. We studied P retention of industrially produced Ca–Fe oxide as potential P barrier material. In batch tests with 1 mg l−1 P solution, P retention was 85% efficient in 5 min. In a flow-through system, the granule phosphate-retention capacity was 6–7 mg g−1, being largely unaffected by pre-leaching. Phosphate release from P-saturated granules was pH-dependant and suggested P association with Fe oxides, and as Ca-phosphate precipitates. In a sequential extraction of P-saturated granules, about 25% of retained P was released, whereas a separate anaerobic incubation resulted in negligible release of P. Immersion of unleached, P-saturated granules for 16 days in a low-P–concentration lake resulted in more than 80% Ca loss, but no loss of metals, and about 25% loss of P accumulated earlier in granules. These granules are promising for P retention and merit a field-scale study.

Potential and limitations of phosphate retention media in water protection: A process-based review of laboratory and field-scale tests

“…close to that reported by Chardon et al (2012) for their P-saturated iron sludge sample]. Uusitalo et al 2012 also measured a similar rate of P release (around 20%) to that in lake immersion when they extracted P-saturated Ca-Fe oxide granules sequentially with water, a mixture of anion and cation exchange resins, and dithionite solution (at a pH of 6.9 and reaching a redox potential < -300 mV). Pratt et al (2007) conducted tests with steel slag saturated with P and observed that at pH 6.7 under oxidising conditions, only negligible (< 1%) amounts of Fe and P were released into the solution.…”

“…However, the authors reported that the peak in P sorption by red mud was recorded at pH 7 (in the pH range 1-11), suggesting that other minerals, such as hematite and maghemite, in the material strongly influenced P sorption. Another example is Ca-Fe oxide granules produced by mixing Fe 2 (SO 4 ) 3 and CaO, described by Uusitalo et al (2012). The authors hypothesised that the Ca-Fe oxide granules initially retained P mainly as Ca-phosphates, but because the content of soluble Ca in the system decreased as a result of continued leaching of the material, Fe hydroxides became the principal P retention component.…”

“…The retention-release curve was strongly hysteretic, thus suggesting that most of the release occurred at full P-saturation of Fe oxides, but then rapidly decreased such that small amounts of P was released after the gradient of the original P retention curve begun to decrease (at a P content of about 30-40 mg g -1 Fe). Uusitalo et al (2012) reported 25% and 80% reductions in the total content of P and Ca, respectively, after immersing P-saturated Ca-Fe oxide granules in an oligotrophic lake for 16 days. The P/Fe ratio was then about 0.12 [i.e.…”

“…Chardon et al (2012) concluded that, due to the hydraulic properties of the materials, iron sludge would be more suitable as a soil amendment for highly P-saturated soils, but that iron-coated sand has potential in landscape P barriers or backfill material around field drains. Uusitalo et al (2012) also investigated Ca-Fe oxide granules, made by mixing Fe 2 (SO 4 ) 3 , CaO and water. This initially alkaline (pH about 9.5) product consisted mainly of gypsum (CaSO 4 × 2H 2 O, by about 70%) with an Fe content of about 10%.…”

Evaluation of a universal flow-through model for predicting and designing phosphorus removal structures

“…Conventional BMPs focus on reducing particulate P through erosion prevention, but do little to reduce transport of dissolved P and potentially increase losses of dissolved P (Darch et al, 2015;Fox and Penn, 2013;Sharpley and Smith, 1994). Materials with a high affinity for P have been used to reduce solubility of soil P in efforts to reduce dissolved P transport in runoff, tile drainage, and other effluents (Ippolito, 2015;Dunets et al, 2015;Callery et al, 2015;Karczmarczyk and Bus, 2014;Claveau-Mallet et al, 2013;Uusitalo et al, 2012;McDowell et al, 2008;. These P sorbing materials (PSMs) are able to reduce dissolved P concentrations in water, but have a finite ability to sorb P, requiring additional material for continued reductions.…”