Synthesis and phosphate adsorption performance of elephant dung biochar modified with magnesium and iron

Abstract

The large production volume combined with the high lignocellulose content makes elephant dung an attractive and underutilized biomass resource, but also presents waste management problems for elephant orphanages. This study explored the conversion of elephant dung into biochars by slow pyrolysis at 500°C for the recovery of phosphate. The unmodified biochar (BC500) had a specific surface area (SBET) of 62.5 m2g-1 with point of zero charge (pHPZC) of 7.7. Biochar modification with MgCl2 (MgBC500) and FeCl3 (FeBC500) by pre-pyrolysis treatment affected the SBET (48.7 m2g-1 and 259.4 m2g-1, respectively) and pHPZC (8.7 and 3.3, respectively). In FeBC500, Fe was present as magnetite (Fe3O4) and hematite (α-Fe2O3) as confirmed by X-ray diffraction. Instead of adsorption, both BC500 and MgBC500 released phosphate at pH 3-6. Phosphate adsorption onto FeBC500 reached equilibrium within 24 h and followed pseudo-second order kinetics. The adsorption isotherm was best described with the Langmuir equation with a maximum adsorption capacity of 0.744 mg-Pg-1. The phosphate adsorption behavior was related to the pHPZC and metal content of the biochar. Conversion of elephant dung into biochar presents an environmentally friendly waste management solution that may find further applications in the adsorption of other nutrients or pollutants.