Phosphorus adsorption and desorption characteristics of constructed wetland gravels and steelworks by-products

Laboratory phosphorus (P) adsorption and desorption experiments were conducted on 9 substrata to evaluate their potential to remove P from sewage effluent. The substrata comprised 2 gravels used in constructed wetlands, Hawkesbury sandstone, and 6 steelworks by-products: granulated blast furnace slag, blast furnace slag, steel slag, fly ash, bottom ash, and coal wash. The studies involved ion-exchange experiments and calculation of Langmuir and Freundlich adsorption isotherms and column adsorption/desorption trials. The ability to adsorb P was then correlated to the physico-chemical attributes including X-ray fluorescence analyses of each substratum. High P adsorption capacities (>380 mg/kg) were shown for granulated blast furnace slag, blast furnace slag, and steel slag, as well as fly ash. All steelworks by-products had adsorption capacities greater than the constructed wetland gravels and Hawkesbury sandstone. The P adsorption capacities of the substrata were significantly correlated with Ca (r(2) = 0.9206), Mg (r(2) = 0.8681), pH (r(2) = 0.7009), S (r(2) = 0.6696), and Si (r(2) = 0.6438) when fly ash was omitted from the analyses. Further research is recommended to evaluate the sustainability of using slags for P removal (as well as other contaminants present in wastewater), using full-scale constructed wetlands. Research should include an evaluation of any likely environmental impacts using leachability and toxicity studies.