Sustainable nexus for treating acid mine drainage: How does bio-alkali matrix impact acidity neutralization and heavy metal removal?

Acid mine drainage (AMD), characterized by high acidity and elevated concentrations of heavy metals, poses a persistent threat to ecological systems. Conventional neutralizers (NaOH, Na₂CO₃, Ca(OH)₂) raise pH effectively but generate large sludge volumes and incur high operating costs. Here, three urease-positive microbial consortia, enriched from soil and municipal activated sludge, were cultivated with urea to produce bio-alkali matrices (BAM-A/B/C, final pH 9.3). Abundant –CONH₂, –NH₂, –OH, and –COOH groups capable of chelating metal ions were found in BAM, suggesting alkaline buffering is complemented by ligand-mediated metal sequestration. When each BAM was mixed with AMD at a 3:10 vol ratio, the effluent pH stabilized at 7 and removal efficiencies reached ∼100 % for Al, Fe, Cr, Cd; > 90 % for As, V, Co, Ni; 79–80 % for Cu; and 60 – 62 % for Mn. Geochemical analysis identified Fe/Al hydroxysulfates and organo-metal complexes as dominant precipitates. A full cost comparison showed BAM-A lowered treatment expenses to 35.5 RMB per m³, which is 12 % below NaOH and 26 % below Ca(OH)₂, while reducing sludge generation. Coupling BAM with sulfate-reducing bacterial systems is advisable to enhance Mn and SO₄^2- removal. Overall, BAM provides an economical and environmentally sustainable alternative for AMD neutralization by uniting alkaline buffering with organic complexation.