A Novel Remediation Method for Arsenic Bearing Acid Mine Drainage Learnt from Natural Attenuation Process

The acid mine drainages (AMD) are treated by neutralization with the two problems: (1) an enormous expenditure; and (2) a discharge of neutralization sludge. In this context, as a case study, a novel remediation method was applied for AMD at Horobetsu mine. The schwertmannite is generated by iron-oxidizing bacteria with Fe2+ and SO42- in acidic mine waters. Its efficiency in As uptake through sorption has been documented in the natural attenuation in Nishinomaki. This study demonstrates the advantages and feasibility of utilizing schwertmannite as a novel remediation method for AMD. At first, the drainage and sludge in Horobetsu mine were characterized to investigate the current method. Secondly, the synthesis of schwertmannite and arsenic removal from the drainage were examined. Results of the characterization indicated that the sludge contained much residual calcite and arsenic by adsorption to the ferrihydrite. The calcite, however, may be dissolved with rainwater. After the reaction, pH of the sludge should be increased and then, which presumably causes the desorption of arsenic from the ferrihydrite. On the contrary, schwertmannite, successfully synthesized from Horobetsu mine drainage by pH adjustment to pH 3.5, removed As almost completely from the drainage. A novel processing method proposed based on the above results consists of the following procedure: (1) arsenic in AMD is removed by the schwertmannite, process, (2) As-free schwertmannite is spontaneously synthesized from the drainage by pH adjustment to pH 3.5 and iron-oxidizing bacteria, and supplied to (1), and (3) the drainage is neutralized in order to remove sulfate by using the waste concrete. In the above procedure, As-bearing schwertmannite, As-free schwertmannite and gypsum are produced as byproduct. This novel processing method is therefore defined as combination system including the removal of arsenic, iron, and sulfate and the resource recovery from AMD.