PASSIVE TREATMENT OF LOW-PH, FERRIC IRON-DOMINATED ACID ROCK DRAINAGE IN A VERTICAL FLOW WETLAND II: METAL REMOVAL 1

A limestone-buffered organic substrate (LBOS) is used to passively remove iron (140 mg·L -1 ) and aluminum (85 mg·L -1 ) from low pH (<3), ferric iron-dominated acid rock drainage (ARD). Iron and aluminum removal is attributed to an increase in pH with precipitates of both elements segregated along a pH gradient. The increase in pH is rapid and occurs across a limestone dissolution front. Three distinct mineralogical zones demarcate the limestone dissolution front. The oxide zone overlies the dissolution front and contains organic material, iron oxyhydroxides, and quartz; all of the original limestone has been dissolved. Below the limestone dissolution front, the limestone is pristine and largely unreacted. Iron sulfides predominate. The zone of transition between iron oxyhydroxide and sulfide is also the zone of active limestone dissolution and the focus of aluminum precipitation.