Nutrients Adsorption onto Biochar and Alum Sludge for Treating Stormwater

Abstract

In this study, Eucalyptus wandoo (EW) biochar and alum sludge and their mixture are used in batches to remove nutrients (ammonium-nitrogen (NH₃-N), nitrate-nitrogen (NO₃-N), nitrite-nitrogen (NO₂-N), and phosphate-phosphorus (PO₄-P)) from synthetic stormwater. First, batch tests were conducted using biochar and alum sludge separately with varying concentration (0.5–5 mg/L); dosage (2–10 g); pH (4–9); and contact time (0–24 h). The results revealed that the EW biochar alone could remove 100% of NO₂-N and NH₃-N while the alum sludge alone could remove 100% of PO₄-P. Next batch tests were carried out with the mixture of EW biochar and alum sludge of different proportions and the results revealed that the mixture of 8 g biochar and 2 g alum sludge gave the best combination for removing all nutrients (NH₃-N = 98.2%; NO₂-N = 99.4%; PO₄-P = 99.8%) except NO₃-N. The adsorption kinetics of mix-medium were studied for Intraparticle diffusion, liquid film diffusion and Lagergren pseudo first and second-order models. The nutrient adsorptions onto mix-medium show two-stage adsorption process following Intraparticle diffusion and liquid film diffusion. The results revealed that the pseudo-second order kinetic models fitted better with high R² (0.98–1.00) and small normalized standard deviation, Δq (0.00–0.62). The isotherm results revealed that the NH₃-N adsorption followed both Langmuir and Freundlich model while NO₂-N and PO₄-P adsorption followed Langmuir model better.