Assessing Zeolite Amended Bioretention Media for Removal of Nutrients and Metals from Stormwater

Abstract

Bioretention cells are installed to treat stormwater through physical, biological, and chemical processes facilitated by a permeable soil media. The soil media is commonly composed of sand, silt, clay, and organic matter, but alternative materials may improve cell performance. Zeolites, an aluminosilicate mineral group, have properties including high hydraulic conductivity and cation exchange capacity (CEC) that may increase pollutant removal while maintaining appropriate hydrologic conditions. The objectives of this study were to design and characterize three bioretention soil mixtures that incorporate a zeolite mineral, perform column studies to compare the nutrient and metal removal capabilities of the mixtures, and evaluate the longevity of the mixtures. A standard bioretention mixture of 85% sand, 11% fines, and 4% organic matter by volume (ALMIX) was altered by replacing sand with Ecolite, a commercially available zeolite product. Mixtures were created with 2% (AUMIX), 10% (AUMIX10), and 20% (AUMIX20) volume of Ecolite, and a control of 100% sand was included. The addition of Ecolite decreased maximum bulk density and particle density, F(3,4)= 25.38, p= 0.005. Estimated CEC increased with increasing Ecolite addition, F(4,5)= 100.97, p< 0.001. Saturated hydraulic conductivities of mixtures containing Ecolite were significantly higher than or similar to ALMIX, F(4,55)= 319.03, p< 0.001. All mixtures were placed in columns, and four storm events were simulated by running synthetic stormwater containing copper, zinc, phosphorus, ammonium, and nitrate through the columns. Pollutant concentrations in collected effluent were measured by inductively coupled plasma mass spectrometry and catalytic reduction with colorimetric readings. There was no significant difference in effluent concentrations among mixtures or storm events for zinc, F(12,57)= 0.90, p= 0.65, or copper, F(12,57)= 0.76, p=0.68. All non-control mixtures had statistically lower effluent phosphorus than the control, F(12,57)= 1.9, p= 0.04. Mixtures containing Ecolite yielded significantly lower ammonium concentrations than ALMIX, but this difference decreased over time, F(12,60)= 5.18, p< 0.001. All non-control mixtures had increases in effluent nitrate concentration, but mixtures containing Ecolite had significantly lower effluent nitrate than ALMIX, F(12,60)= 2.38, p= 0.014. Amending bioretention media with Ecolite did not appear to affect longevity as compared to ALMIX.