Acid rock discharge (ARD) or acid mine drainage (AMD) formed through the oxidation of sulfide minerals (usually pyrite) is a major source of water contamination impairment in both coal and hard rock mining regions throughout the worldwide. Aluminum, as an environmentally toxic contaminant, is found in some acid rock discharge (ARD) and acid mine drainage (AMD) locations. The composition of ARD/AMD is complex and lead to the effluent Al concentration varied in different conditions. In this study, the possible effluent aluminum concentrations and species of precipitates likely to be formed in Al-bearing ARD/AMD active treatment system was measured and modeled. Accurate models and calibrated are necessary in predicting an expected discharge Al concentration from the Al-bearing ARD/AMD active treatment systems.A series of experiments were conducted to evaluate the effect of sulfate and silica on both suspended and soluble Al concentrations in a laboratory synthesized Al-bearing ARD. The experimental results showed that the presence of sulfate could reduce the soluble Al concentrations due to the formation of Al-sulfate precipitates, basaluminite. The simulation with Mineql+ model shows that solution was in approximate equilibrium with amorphous Al(OH)3 when sulfate was absent. In the cases when sulfate was present, the solution was in equilibrium with amorphous Al(OH)3 solids after 30 minutes of settling, subsequently it became respect to unsaturated with amorphous Al(OH)3, following with the reduction of soluble Al concentration due to the formation of basaluminite. Although most of sludge are in form of amorphous Al(OH)3(s) after short settling time, the molar ratio of Al:S in sludge showed that the fraction of Al-sulfate precipitates increased with settling time and the fraction of Al-sulfate precipitates in ¡°new¡± produced precipitates increased with settling time after 30 minutes. After 48 hours of settling, the molar ratio of Al:SO4 in suspended particles size was close to the theoretical molar ratio of Al:SO4 of basaluminite. Suspended size distribution revealed that the presence of sulfate reduced the particles size in the supernatant, and these small particles remained in the supernatant , following with a relative higher suspended Al concentration. The presence of silica can reduce the soluble Al concentration immediately after pH adjustment due to the formation of Al-silica precipitates. The experimental results also showed that there was interaction between sulfate and silica in experiments. The presence ulfate can enhance the sedimentation of silica in form of Si(OH)4(s) not as Al-silica precipitates. Either sulfate or silica and combination can reduce the suspended particles size. The high level of silica could increase the effluent Al concentration since it can reduce the settleable proportion of particulate Al solids
