Elemental Precipitation and Adsorption through Mixing Experiments of Acid Mine Drainage and Seawater from Chinkuashih, Northern Taiwan

A high concentration of yellowish suspended particles in estuarine and inshore waters of Lian-Dong Bay due to contamination by acid mine drainage (AMD) from the Chinkuashih mining area, northern Taiwan triggered our great interest in carrying out mixing experiments to investigate interactions between AMD and seawater. Fresh seawater (pH 8.0) and AMD (pH 2.8) taken from Golden Falls, Chinkuashih were used in the study. Crystallization of nanocrystals of schwertmannite plus minor goethite occurred in mixed solutions of pH ＜ 4.5 and precipitation of non-crystalline nanoparticles of hydrous ferric/aluminum oxide (HFO/HAO) took place at pH ＞ 4.5, as indicated by XRD, TEM, and FTIR analyses. Analyses of mixed solutions with ICP-OES and ICP-MS showed that the non-crystalline HFO/HAO grasped almost all of the Fe, Al, As, Pb, and Cr at solution pHs of 4.5~5.5 and progressively took up significant proportions of Cu, Ni, and Zn at pH ＞ 6.3 and Si at pH ＞ 5 and measurable Mn and Cd at neutral to slightly alkaline pH. In contrast, the schwertmannite-dominated precipitates removed large amounts of only Fe, As, Cr, and Pb from the solutions at pH ＜ 4.5. The mineralogical, microstructural, and chemical variations of the solid precipitates resembled those of suspended particulates in the AMD-contaminated estuarine and inshore waters of Lian-Dong Bay. The results implied that solution pH could have controlled the mineralogical and chemical variations of suspended particulates in the mixing experiments and previous field studies. Precipitation of non-crystalline HFO/HAO is suggested to be kinetically favorable in solution at a critical pH of ＞ 4.5 with supersaturation being rapidly reached, and capable of taking up heavy metals and arsenic as an effective way of natural remediation in estuarine and inshore environments impacted by AMD.