Performance of Physically and Chemically Activated Biochars in Copper Removal from Contaminated Mine Effluents

The increasing global demand for metals and minerals justifies the intensive study of treatment options for contaminated mine effluents. The present study evaluated the conversion of wood residues into physically and chemically activated biochars and their subsequent use in the treatment of Cu in synthetic and actual contaminated mine drainage. First, wood residues were converted into biochar by fast pyrolysis. Then, physical (using steam or CO2) or chemical (using KOH) activation was carried out in a homemade pilot-scale furnace. After activation, highly microporous (KOH materials) and micro/mesoporous activated biochars (CO2 and steam materials) were obtained. Batch adsorption testing was first conducted with synthetic effluents. Results showed that CO2-activated biochar was the most Cu effective adsorbent (99% removal) at low concentrations (5–20 mg L−1). The mechanisms of Cu2+ adsorption involved physical and chemisorption for biochars and CO2-activated biochar, while chemisorption for KOH-activated biochars was probably due to the high proportion of functional groups connected to their surface. In multi-metal acid mine drainage, metal adsorption capacities deteriorated for most of the materials, probably due to the effects of ion competition. However, KOH-activated biochar decreased Cu2+ concentrations to below the authorized monthly mean allowed by Canadian law (0.3 mg L−1) and decreased Co, Pb, and Mn concentrations up to 95%. These findings indicate that high porosity and oxygenated functional groups connected to the surface of activated biochars are important properties for the enhancement of interactions between carbon materials and metals from mine effluents, as well as for their performance improvement in mine drainage treatment.