An integrated process incorporating pH-controlled biomineralization and sulfate bioreduction to facilitate recovery of schwertmannite and sulfated polysaccharides from acid mine drainage

Acid mine drainage (AMD) poses an environmental challenge to the global mining industry. However, it is also a potential source of iron and sulfate for recovery. In this work, we proposed an integrated process of pH-controlled biomineralization of iron and biological sulfate reduction to facilitate the concurrent purification of AMD and selective recovery of iron and sulfate as valuable products. Compared with the biomineralization controlling pH at 2.8, the modified biomineralization controlling pH at 3.2 improved the iron precipitation efficiency from 9.7 % to 95.3 %, leading to a 53-fold increase in pure-grade schwertmannite Fe8O8(OH)(4.08)(SO4)(1.96) production in AMD. This improvement may be related to the relatively low Gibbs free energy value of the Fe3+ mineralization reaction at pH 3.2. The solution equilibrium calculation using Visual MINTEQ indicated that keeping the system pH at an appropriate level is crucial for the spontaneous and effective biomineralization of Fe to schwertmannite at room temperature. The specific pH required for Fe biomineralization is slightly affected by the content of metal cations (e.g., Al3+, Ca2+, Mg2+, Cu2+, and Mn2+) and oxoanion (AsO33-/AsO43-) in AMD but greatly affected by sulfate content (mM) (pH = 0.149 ln c(SO42-) + 2.551). For the effluent resulting from AMD biomineralization, sulfate reduction at pH 5.0 removed residual Zn2+ (99.9 %), Mn2+ (97 %), Cu2+ (100 %), and sulfate (87.1 %) while recovering sulfated polysaccharides with antiangiogenic bioactivity (as confirmed by the chorioallantoic membrane tests). The pilot-scale continuous flow operation experiments demonstrated that this integrated method has the potential to recover valuable iron/sulfate products from AMD.