Impacts of greenwaste biochar on ammonia volatilisation from bauxite processing residue sand

Abstract

Background and Aims The objective of this study was to test the suitability of greenwaste biochar to aid nitrogen (N) retention in rehabilitated bauxite-processing residue sand (BRS). Methods Bauxite residue sand was collected from the Alcoa of Australia Pinjarra refinery. The pH of BRS was adjusted to values of 5, 7, 8 and 9 and subsequently amended with different rates (1, 5, 10 and 20 %, w/w) of greenwaste biochar. The loss of N via NH3 volatilization following addition of di-ammonium phosphate (DAP) was determined using an acid trapping method. Results At low pH (5), increasing pH rather than adsorption capacity, resulting from biochar addition, caused greater losses of N through volatilization from BRS. In BRS with medium pH (7, 8), increasing adsorption capacity, induced by biochar addition, played the more dominant role in enhancing adsorption of NH 4 + -N /NH3-N and lowering NH3 volatilization. In the BRS with high pH (9), the majority of NH 4 + -N /NH3-N pools was lost via NH3 volatilization due to the strong acid-base reaction at this pH. Conclusions It is concluded that the interaction of changes in pH and adsorption capacity induced by greenwaste biochar addition affects the availability and dynamics of NH 4 + -N/ NH3-N in BRS amended with DAP.