| The main objectives of the thesis were to quantify the relation between the composition of a natural organic material and its potential for biodegradation under sulfate-reducing conditions, to evaluate the ecotoxicological potential of treated effluent, and to identify and quantify metal removal mechanisms by sulfate-reducing passive bioreactors from a highly contaminated AMD.;In the first Chapter, several studies conducted to find the best mixture of natural organic substrates for SRB are reviewed and critical parameters for design and long-term operation are discussed. The main conclusion of this critical review of the available literature on sulfate-reducing passive bioreactors is that additional work needs to be done to properly assess the long-term efficiency of reactive mixtures and to assess metal removal mechanisms. Furthermore, metal speciation and ecotoxicological assessment of treated effluent from passive bioreactors have yet to be performed.;In the second Chapter of the present study, four natural organic materials (maple wood chips, maple sawdust, composted poultry manure, and leaf compost) were thoroughly characterized to assess their ability to serve as substrates and to find a parameter that links organic carbon sources with their biodegradability. Three reactive mixtures using the four organic materials were then comparatively evaluated for their performance to treat a highly contaminated AMD (13-19 mg/L Mn, Cd, Ni, and Zn; 1700 mg/L Fe; pH 5.5) in long-term (120-152 days) batch experiments. All three mixtures were successful for sulfate reduction as well as metal removal efficiency which reached 91.8-99.8% for Fe, Ni, Cd, Zn, and Mn.;In the third Chapter of the present study, the effect of two hydraulic retention times (HRTs) of 7.3 days and 10 days on the performance of 3.5L passive bioreactors was evaluated over an 11-month period for the treatment of a highly contaminated AMD (10-15 mg/L Mn, Cd, Ni, and Zn; 500 mg/L Fe; pH 2.9-5.7). Evolution of the porosity and hydraulic conductivity of the reactive mixture was also evaluated during the 15-month operation of two bioreactors. Results indicated that the bioreactors were effective at both HRTs for increasing the pH and alkalinity of the contaminated water and for sulfate and metal removal (60-82% for Fe and up to 99.9% for Cd, Ni, and Zn).;In Chapter 4 of the study, acute and sublethal toxicity was tested on effluent from column bioreactors filled with the mixture of four natural organic carbon sources and operated at the two different hydraulic retention times (HRTs) for treatment of the highly contaminated AMD. This part of the study was justified based on the fact that in addition to discharge limits based on physicochemical parameters, treated effluent is also required to be nontoxic. Effluent was first tested for acute (Daphnia magna and Oncorhynchus mykiss) and sublethal (Pseudokirchneriella subcapitata, Ceriodaphnia dubia, and Lemna minor) toxicity. Acute toxicity was observed for D. magna and a toxicity identification evaluation (TIE) procedure was then performed to identify potential toxicant(s). Finally, metal speciation in the effluent was determined using ultrafiltration and geochemical modelling for the interpretation of the toxicity results.;In Chapter 5, column bioreactors were used for studying mechanisms of metal removal, assessment of long-term stability of spent reactive mixtures, as well as potential metal mobility after treating highly contaminated acid mine drainage. Several physicochemical, microbiological, and mineralogical analyses were performed on spent reactive mixtures collected from four bioreactors, after operation for over an 11-month period. Consistent with the high metal concentrations in the AMD feed, and with low metal concentrations measured in the treated effluent, the physicochemical analyses indicated very high concentrations of metals (Fe, Mn, Cd, Ni, and Zn) in the top and bottom layers of the reactive mixtures from all columns. Besides identifying (oxy)hydroxide and carbonate minerals, the mineralogical analyses identified metal sulphides containing Fe, Cd, Ni, and Zn. Metal removal mechanisms were, therefore, mainly adsorption and other binding mechanisms with organic matter (for Cd, Ni, and Zn), and precipitation as (oxy)hydroxide minerals (for Fe and Mn). After 15 months, the column bioreactors did not lose their capacity for removing metals from the AMD that are immobile during operation of the bioreactors. Moreover, metal mobility in spent reactive mixtures can be increased. This could be an economically viable alternative for metal recuperation, which should be evaluated in the future. (Abstract shortened by UMI.)... |
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