Screening of Ca- and Fe-rich materials for their applicability as phosphate-retaining filters

Abstract

Ecology of a large number of lakes and estuaries is affected by external phosphorus (P) loading. Many studies have investigated the applicability of different materials as P-filters for treating runoff or wastewaters. We performed flow-through retention tests and water extractions to estimate the P retention and release properties of potential Ca- and Fe-rich filter materials. In addition to the fresh Ca-rich materials, the P retention tests involved their weathered counterparts. During weathering, the materials were immersed into water to leach out soluble species (e.g., Ca2+ and OH−), thus mimicking aged filters. For the most efficient P-retainers (Filtra P®, steel slag, Sachtofer PR® and mine drainage residual), the cumulative P retentions ranged between 12 and 24mg P/g. The weathering reduced the P retentions of Filtra P® and of slag by 70–80%, but did not affect the abilities of other materials to retain P. No clear relationship existed between the P retentions and water-extractable Ca or oxalate-extractable Fe and Al amounts in the materials. During one month of extraction with a large volume of water, the maximum release of P from the P-saturated materials was 35% of the total P. To recognize the P retention mechanism (precipitation of Ca-phosphates or sorption onto metal hydroxides) and thus predict possible P release, we performed equilibration of P-saturated materials in solutions with variable pH. We suggest incorporation of weathering and P release tests in the screening scheme for identifying potential P-retaining filters.