Phosphorus sequestration by chemical amendments to reduce leaching from wastewater applications

Phosphorus-immobilizing amendments can be useful in minimizing P leaching from high P soils that may be irrigated with wastewater. This study tested the P-binding ability of various amendment materials in a laboratory incubation experiment and then tested the best amendment in a field setup using drainage lysimeters. The laboratory experiment involved incubating 100-g samples of soil (72 mg kg^-1 water-extractable phosphorus, WEP) with various amendments at different rates for 63 d at field moisture capacity and 25°C. The amendments tested were alum [Al₂(SO₄) ₃·14H₂O], ferric chloride (FeCl₃), calcium carbonate (CaCO₃), water treatment residual (WTR), and sugarbeet lime (SBL). Ferric chloride and alum at rates of 1.5 and 3.9 g kg ^-1, respectively, were the most effective amendments that decreased WEP to 20 mg kg^-1, below which leaching has previously been shown to be low. Alum (1.3 kg m^-2), which is less sensitive to redox conditions, was subsequently tested under field conditions, where it reduced WEP concentration in the 0- to 0.15-m layer from 119 mg kg^-1 on Day 0 to 36.1 mg kg^-1 (85% decrease) on Day 41. Lysimeter breakthrough tests using tertiary-treated potato-processing waste-water (mean total phosphorus [TP] = 3.4 mg L^-1) showed that alum application reduced leachate TP and soluble reactive phosphorus (SRP) concentrations by 27 and 25%, respectively. These results indicate that alum application may be an effective strategy to immobilize P in high P coarse-textured soils. The relatively smaller decreases in TP and SRP in the leachate compared to WEP suggest some of the P may be coming from depths below 0.2 m. Thus, to achieve higher P sequestration, deeper incorporation of the alum may be necessary.