Changes in Efficiency and Hydraulic Parameters During the Passive Treatment of Ferriferous Acid Mine Drainage in Biochemical Reactors

Abstract

The objective of this study was to evaluate the effects of different reactive mixtures and hydraulic retention times (HRTs) on hydraulic parameters (hydraulic conductivity, k sat , and porosity) and the efficiency of passive biochemical reactors (PBRs) for treatment of ferriferous acid mine drainage (AMD). Five 10.7 L PBRs were filled with three reactive mixtures, containing either a carbon-rich substrate (60% w/w) or an inert/neutralizing agent (50% w/w). The PBRs were tested over a 450 day period using two qualities of iron-rich AMD (4 and 1 g L−1 Fe in AMD1 and AMD2, respectively), and two HRTs, of 5 and 7 days. During the last week of the columns’ operation, a tracer test (5 g L−1 of NaCl) was also performed, in addition to monthly measurements using the falling head method. Changes in HRT and ksat were evaluated throughout the experiment. The PBRs increased the pH of AMD influents from 3.5 to 6 and efficiently removed Al, Cd, Cr, Ni, Pb, and Zn (> 90%), whereas Fe was only partially and inconsistently treated. No significant differences were observed among the three tested mixtures, regardless of the HRT or the AMD quality. Results from the tracer test and k sat measurements showed no significant decrease in the initial values of the hydraulic parameters with time except for column 3, where a slight decrease was observed. Although sorption could have been important during the start-up of the PBRs, post-testing characterization of the spent reactive mixtures showed that the Fe was mainly retained as oxy-hydroxides and sulfides. Given the PBRs’ marginal effectiveness for Fe-rich AMD, pre-treatment removal of the iron is recommended.