Biochar as a sustainable sorbent for the removal of lanthanides from acid mine drainage

The extraction and processing of ores containing lanthanides (Ln) generate significant amounts of waste that may lead to Ln-rich leachates and acid mine drainage (AMD). AMD is characterised by an acidic pH and high iron (Fe) concentrations. Due to the environmental risk of these leachates, there is a need to apply effective remediation strategies to decrease Ln concentrations in leachate-affected waters. In this scenario, biochar was investigated as a sustainable and cost-effective alternative for the treatment of these waters. This study explored the viability of two biochar materials (derived from pine branches (PB) and garden wastes (GaW)) for the removal of Ln (La, Sm and Lu) from contaminated waters, including AMD. A continuous-flow sorption technique was used to simulate a real scenario of water filtration using a biochar barrier. Initially, two Sm-spiked water matrices, double-deionised water (DD) and river freshwater (FW) were tested, revealing no statistically significant differences in the biochar sorption capacities obtained (220 and 216 meq kg⁻¹ for PB, 459 and 392 meq kg⁻¹ for GaW in DD vs. FW scenarios, respectively), despite the slight differences in pH, dissolved organic matter, and water-soluble cation concentrations between the matrices. For mixed Ln contamination (La + Sm + Lu), Ln appeared to be equally distributed at the biochar sorption sites, whereas the biochar showed a similar overall sorption capacity when comparing mixed Ln contamination to only Sm contamination. After the initial tests, the efficiency of the biochar materials to sorb Ln from AMD was tested in a simulated AMD matrix. Biochar materials with high buffering capacity, such as those tested in this work, were considered a suitable option for the treatment of Ln-contaminated AMD, provided that the operation times are adapted so as not to exceed the acid neutralising capacity of the material.