Process and mechanism of recovering layered double hydroxides (LDHs) from acid mine drainage (AMD) and synergetic removal of manganese

Abstract

The neutralization-precipitation method is widely used for treating acid mine drainage (AMD). However, the accumulation of sludge and the requirement for a high pH for the precipitation of Mn2+ limit the widespread application of this technology. In this work, layer double hydroxides (LDHs) were obtained by chemical neutralization of AMD, and the mechanism of LDHs for synergistic removal of Mn2+ was investigated by studying the morphological of metal elements during the neutralization process. The results showed that pH=8.5 was the critical condition for the formation of LDHs, and the sludge mainly consisted of Mg-Al-LDHs and manganese oxides. With increasing pH, Mn2+ was synergistically removed by LDHs through ion exchange (at the site 6) and complexation (with Mg-O). At pH=9.0, the concentration of Mn2+ was reduced to 1.5 mg/L, but it should be noted that high pH could destroy the structure of LDHs. Moreover, LDHs recovered from actual AMD were also found to have the ability to remove environmental pollutants. These findings not only enhance our understanding of the morphological change of metal elements during the neutralization process and the mechanism of Mn2+ removal, but also provide new insights for the recycling and utilization of neutralized sludge.