Fe(III)-modified bamboo biochar for the removal of phosphate from synthetic and field stormwater runoff

Application of fertilizers over agricultural fields, domestic sewage, and livestock are major point and non-point sources of nutrients such as phosphate and nitrate in stormwater runoff. These nutrients, specifically phosphate, is the major pollutant triggering eutrophication in freshwater bodies, thus damaging both- the aquatic ecosystem and the environment. The current study investigates the removal of phosphate using biochar-based treatment units, which can be an efficient and cost-effective removal method. The biochar used in this study was produced by slow pyrolysis of bamboo and it was further chemically modified with 1%, 5%, and 10% FeCl3 solutions. The iron modification transformed the biochar surface to be electro-positive. Characterization of both biochar and modified biochars was carried out using proximate analysis, elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. Furthermore, a series of batch experiments were conducted to evaluate the phosphate and nitrate removal efficiencies from the synthetic stormwater runoff. The removal efficiencies were found to be 29%, 49%, and 61%, with an initial phosphate concentration of 5 mg/L, using 1%, 5% and 10% FeCl3 treated biochar respectively, while the nitrate removal efficiencies (with an initial concentration of 50 mg/L) were found to be 12.4%, 14.6%, and 17.0% respectively. The phosphate removal kinetics by 10% FeCl3 modified biochar indicated that the phosphate removal followed pseudo-second-order kinetics. Isotherm modelling indicated both Langmuir and Freundlich isotherms as best fit for phosphate adsorption using 10% FeCl3 modified biochar thus representing mono-layer and multi-layer adsorption, respectively. Langmuir isotherm reported the maximum phosphate adsorption capacity of 11.57 mg/g by 10% FeCl3 modified biochar. Additional experiments were performed using real urban stormwater runoff from various regions of Delhi, which revealed similar phosphate removal efficiencies. The treated stormwater runoff can be used for water capacity building and non-potable applications in water-scarce urban areas. Pyrolysis of bamboo has 40% yield.Phosphates and nitrates in stormwater runoff can be effectively removed by biochar.Biochar requires its treatment with electro-negative chemical species for effective removal of nutrients.