Performances of stabilization/solidification process of acid mine drainage passive treatment residues: Assessment of the environmental and mechanical behaviors.

Residues from passive treatment of acid mine drainage (AMD) have variable chemical stability and could regenerate contaminated drainage. Stabilization/solidification (S/S) can prevent contaminant leaching. Residues were collected from a tri-step AMD field passive treatment system, operated for 6 years at the reclaimed Lorraine mine site, Quebec, Canada. General Use Portland cement (GU), blended binders based on GU with pozzolanic additives (ground-granulated blast-furnace slag; GGBFS and fly ash type C; FAC) were used as hydraulic binders, in proportions (w/w %) of 100GU, 20GU/80GGBFS, and 50GU/50FAC, respectively. Residues were mixed with wood ash (35%) and sand (25%), while reference samples (100% sand) were also prepared. Prior to S/S, raw materials were characterized. The S/S effectiveness was assessed mineralogically and mechanically (unconfined compressive strength; UCS). Environmental behavior assessment (static vs semi-dynamic leaching tests) was also performed. UCS results showed that strength increase with age. At 56 days, GU- (1.3 MPa) and GU/GGBFS (0.7 MPa) satisfied Quebec's strength requirements for landfill disposal (0.7 MPa), but not GU/FAC (0.6 MPa), while all samples satisfied USEPA criteria (0.35 MPa). The semi-dynamic test showed that all elements can be immobilized successfully in GU- and GU/GGBFS. The GU binder had the best stabilizing performance. Based on USEPA requirements, S/S using GU, GGBFS, and FAC can be also considered for contaminant immobilization in AMD passive treatment residues. Finally, the comparison between replicates using Student's t-test indicated good reproducibility of S/S treatment.

Geochemical behavior and stabilization of spent sulfate-reducing biofilter mixtures for treatment of acid mine drainage.

Sulfate-reducing biofilters operated in semi-passive or passive modes constitute an approach of choice for treatment of acidic mining effluents. The aim of the present study involved examining the behavior of biofilters after use based on two modes of management, namely in unsaturated and saturated media. Two acidophilic biofilters were investigated following their mixing with different alkaline industrial residues (i.e., 25% fly ash biomass or 30% aluminum red mud, or 10% kiln dust). Percolation column tests for a 330-d period indicated that aluminum red mud and lime kiln dust (to a lesser extent) are efficient materials for maintaining the pH neutrality of biofilter leachate and to reduce release of metals (i.e., Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in spent biofilters. The storage of biofilters in saturated mode also makes it possible to preserve the reducing conditions of the environment and neutrality of the pH and to limit the dissolution of the solution of cadmium, nickel and zinc. Conversely, increased iron release is noted under saturated conditions. Finally, the results indicated that a mixture of biofilters and lime kiln dust is preferable to surface addition of these to reduce the loss of metals in leachates.

Environmental behavior of metal-rich residues from the passive treatment of acid mine drainage.

In closed or abandoned mine sites, passive systems are often used for acid mine drainage (AMD) treatment. They generate metal-rich residues with variable chemical stability, which is rarely reported. The objective of the present study was to evaluate the potential mobility of contaminants (metals and sulfates) from AMD post-treatment residues to better anticipate their fate and enable their proper management. Sampling of a field tri-step passive system, consisting of two passive biochemical reactors (PBR1 and PBR2), separated by a wood ash reactor (WA), implemented in the reclaimed Lorraine mine site, QC, Canada, was carried out. Samples were collected from the inlet (In) and the outlet (Out) of each treatment unit. Physicochemical and mineralogical characterization was performed. The potential mobility of the metals was then assessed via static and kinetic leaching tests. Results showed that all residues had high metal contents (e.g. Fe content >29 g/kg in PBR1-In, > 76 g/kg in WA-In and > 80 g/kg in PBR2-Out). A high residual neutralizing potential was also found in the WA residues (inorganic carbon 6.5%). Native and organic sulfur were found in the PBR2 residues, while Fe-oxyhydroxide (hematite, goethite and magnetite), carbonate and sulfate minerals were present in all residues. According to USEPA regulations, all residues were considered non-hazardous, but Quebec's provincial regulation relative on mining effluents classifies these residues as leachable for some metals, such as Fe, Al, Ni, Zn and Mn. A potential generation of contaminated neutral mine drainage (Al, Ni, Mn and Zn concentrations exceeding criteria) could occur from PBR1 (In & Out) and WA (In & Out) residues. Moreover, the PBR2 residues (In & Out) regenerated AMD rich in Fe and sulfates, especially for PBR2-Out (1 g/L Fe and 6 g/L sulfates). Therefore, all residues were proven to require stabilization prior to their landfill (co-)disposal with municipal waste.

Treatment efficiency of iron-rich acid mine drainage in a tri-unit pilot system.

Treatment efficiency of iron-rich acid mine drainage (AMD; pH 3, and 2 and 4 g/L Fe) was tested in a laboratory tri-unit pilot-scale reactor (2.65 m3) for 1 year. The first unit consisted of a passive biochemical reactor (PBR1), filled with reactive mixture (50% of manure, sawdust, maple chips, compost, urea, sediment, and sand; 50% of calcite), with the aim to neutralize acidity and to partially remove metals. The second unit contained wood ash and acted as neutralizer and iron retention filter (by sorption and precipitation). The last unit was a second polishing PBR2, filled with reactive mixture (98% of manure, sawdust, maple chips, compost, urea, sediment, and sand; 2% of calcite), which aim was to remove the residual metals. The results showed that pH increased to about 6 and redox potential decreased significantly (from 550 mV to -100 mV). Iron, the most challenging metal in the AMD, decreased from 4 g/L (the highest tested concentration) to approximately 100 mg/L. The performance of the multistep treatment system was controlled by the capacity of the wood ash to immobilize iron.

Stability of metal-rich residues from laboratory multi-step treatment system for ferriferous acid mine drainage.

Passive systems are often used for the treatment of acid mine drainage (AMD) on closed and abandoned mine sites. Metal-rich residues (solid precipitates) with variable chemical composition and physical properties can be generated. Their characterization is required to better anticipate the potential fate, including stability for disposal, potential recovery, or reuse. The present study evaluated the leaching potential of solids from a laboratory passive multi-step treatment for Fe-rich AMD (2350 ± 330 mg/L Fetot, 0.7 ± 0.4 mg/L Ni, 0.2 ± 3.0 mg/L Zn, and 5073 ± 407 mg/L SO42-, at pH 3.04 ± 0.45). To do so, post-treatment solids from three units (Fe-pretreatment reactor (50% wood chips and 50% wood ash, WA50), passive biochemical reactor, PBR for SO42- treatment (30% inorganic materials, 70% organic substrate), and polishing reactor (50% calcite and 50% wood chips, C50)) of a pilot laboratory treatment system were sampled. Physicochemical and mineralogical characterization, as well as static leaching tests were then performed. Results showed that all solids had high neutralizing potential, while high inorganic carbon was found in C50. Moreover, high metal concentrations were found in WA50. Metals and sulfates in all solids precipitated in the form of oxyhydroxides, oxy-hydroxy-sulfates, carbonates, sulfides, sulfate, and native sulfur. The Fe was not found as problematic contaminant in solids, but it was in AMD. However, a probable generation of contaminated neutral drainage by Ni and Zn could occur from WA50. The C50 had the highest acid neutralizing capacity and could better resist to acid aggression relative to solids from PBR and WA50. The PBR and C50 solids were considered as non-hazardous towards regulation's limits and a potential co-disposal with municipal wastes could be a storage option. Further studies should be undertaken by testing other leaching and kinetic tests to assess long-term metal stability.

Removal efficiency of As(V) and Sb(III) in contaminated neutral drainage by Fe-loaded biochar.

Performance of raw and two Fe-loaded biochars, produced either by evaporation (E-product, 26.9% Fe) or precipitation (P-product, 12.6% Fe), was evaluated in batch and column testing for As(V) and Sb(III) removal from contaminated neutral drainage (CND). Batch testing results showed that sorption capacity of the E-product tripled for As(V) and quintupled for Sb(III), whereas for the P-product, it doubled for both contaminants, relative to the raw biochar. Moreover, As(V) removal by the E-product reached 90% in less than 8 h, for initial concentrations up to 50 mg/L. In column testing, the E-product efficiently treated the influent [pH 6; 1 mg/L As(V)] for more than 286 days. The pH of the final effluent was within the legally allowed limits (6-9.5) while less than 0.3 mg/L Fe leached out. Based on these findings, Fe-loaded biochar by evaporation (E-product) seems promising for As(V) treatment in CND.

Performance assessment of laboratory and field-scale multi-step passive treatment of iron-rich acid mine drainage for design improvement.

Multi-step passive systems for the treatment of iron-rich acid mine drainage (Fe-rich AMD) perform satisfactorily at the laboratory scale. However, their field-scale application has revealed dissimilarities in performance, particularly with respect to hydraulic parameters. In this study, the assessment of factors potentially responsible for the variations in performance of laboratory and field-scale multi-step systems was undertaken. Three laboratory multi-step treatment scenarios, involving a combination of dispersed alkaline substrate (DAS) units, anoxic dolomitic drains, and passive biochemical reactors (PBRs), were set up in 10.7-L columns. The field-scale treatment consisted of two PBRs separated by a wood ash (WA) reactor. The parameters identified as possibly influencing the performances of the laboratory and field-scale experiments were the following: AMD chemistry (electrical conductivity and Fe and SO42- concentrations), flow rate (Q), and saturated hydraulic conductivity (ksat). Based on these findings, the design of an efficient passive multi-step treatment system is suggested to consider the following: (1) Fe pretreatment, using materials with high ksat and low HRT. If a PBR is to be used, the Fe load should be < 26 g/m3 substrate/day (Fe < 200 mg/L) and SO42- < 110 g/m3 substrate/day; (2) PBR/DAS filled with a mixture with at least 20% of neutralizing agent; (3) include Q and ksat (> 10-3 cm/s) in the long-term prediction. Finally, mesocosm testing is strongly recommended prior to construction of full-scale systems for the treatment of Fe-rich AMD.

Metals and metalloids treatment in contaminated neutral effluents using modified materials.

Circumneutral surface water and groundwater can contain hazardous concentrations of metals and metalloids that can threaten organisms in surrounding ecosystems. Extensive research has been conducted over the past two decades to prevent, limit, and treat water pollution. Among the currently available treatment options is the use of natural and residual materials, which is generally regarded as effective and inexpensive. The modification of such materials enhances the removal capacity of metals and metalloids, as well as the physical and chemical stability of the materials and resulting sludge (after treatment). This paper reviews several modified materials that have produced and evaluated in the past twenty years to treat various contaminants in water under specific conditions. Important factors on performance improvement following the modifications are emphasized. Sorption capacity and kinetics, and element removal mechanisms are also discussed. Element recovery, material regeneration, water reuse, evaluation of treatment efficiency for real effluents are also considered, as well as the applicability of these materials in both active and passive treatment systems. Modified natural and residual materials are a promising option for the treatment of metals and metalloids in circumneutral contaminated waters. However, further research is necessary to evaluate their field-scale performance and to properly assess treatment costs.

Iron removal in highly contaminated acid mine drainage using passive biochemical reactors.

Passive biochemical reactors (PBRs) are a viable alternative to neutralization plants for the treatment of acid mine drainage (AMD) because they require lower investment costs and use residual materials. However, high iron (Fe) concentrations (≥0.5 g/L) in AMD are challenging for their long-term efficiency. Sorption and precipitation are the main Fe removal mechanisms, but the relative importance of each is mostly unknown. In this study, locally available natural materials (organic and inorganic) were characterized and tested for their performance in Fe removal from highly contaminated AMD (pH 3.5, 4 g/L of Fe, and 9 g/L of sulfate). Iron retention capacity of the materials was then evaluated and the efficiency of eight mixtures of materials was compared through 40-day laboratory batch tests. All batch-type PBRs increased the pH up to 6.5 and decreased dissolved metals concentrations, including Fe, up to 99%. Results showed that organic residual materials (manures, municipal wastewater sludge, and compost) were the best substrates for Fe removal.These findings allowed for the selection of three reactive mixtures with distinct characteristics (mixture #1 - 30% organic wastes; mixture #4 - 50% calcite; and mixture #7 - 50% sand) to be further evaluated in column type PBRs.

Removal of Ni and Zn in contaminated neutral drainage by raw and modified wood ash.

In the present study, wood ash was modified by alkaline fusion, prior to hydrothermal synthesis, for potential application in the treatment of mine drainage impacted water. With this objective, two types of wood ash (both raw and modified) were evaluated for the treatment of Ni and Zn in contaminated neutral drainage (CND). Batch adsorption experiments were initially conducted on synthetic CND, and then on two real CND, sampled on two active mine sites, contaminated by either Ni (3.7 mg/L) or Zn (9.1 mg/L). Leaching of Zn was observed during the kinetic tests for the raw wood ash, whereas its modification suppressed the leaching. The cation exchange capacity acquired by modification of the two samples of wood ash exceeded 300 meq/100 g (which is two to fourfold higher than those of the raw ash), while sorption capacity for Ni and Zn tripled relative to the raw material. The Langmuir model best described the sorption process for all materials, while potential mechanisms of metal removal include adsorption, precipitation and ion exchange, following pseudo second-order kinetics. Results also showed that within 2 h of contact of mine effluents with one modified wood ash, Ni and Zn concentrations decreased below the maximum authorized monthly mean concentration allowed by the Canadian law (0.5 mg/L), whereas the other modified wood ash allowed reaching the regulatory conformity after 2 h for Ni but 7 days for Zn (although 93% removed after 2 h). Nonetheless, the pH was raised (10.9-11.8) above the legally allowed limits (6-9.5). Based on these findings, modified wood ash could be considered as a promising option for the treatment of Ni and Zn in CND, but the pH correction of final effluent might be necessary.

Performance of thermally activated dolomite for the treatment of Ni and Zn in contaminated neutral drainage.

Intensive research is ongoing for developing low-cost and highly efficient materials in metal removal from contaminated effluents. The present study evaluated dolomite [CaMg(CO3)2], both raw and modified by thermal activation (charring), for Ni and Zn treatment in contaminated neutral drainage (CND). Batch adsorption testing (equilibrium and kinetics) were conducted at pH 6, to evaluate the performance of initial vs. modified dolomite, and to assess potential mechanisms of metal removal. Charring of dolomite led to a rigid and porous material, mainly consisting of CaCO3 and MgO, which showed a sorption capacity increased sevenfold for Zn and doubled for Ni, relative to the raw material. In addition, Freundlich model best described the sorption of the both metals by dolomite, whereas the Langmuir model best described their sorption on charred dolomite. Plausible mechanisms of metal removal include cation exchange, surface precipitation and sorption processes, with carbonate ions and magnesium oxides acting as active centers. Based on these results, charred dolomite seems a promising option for the efficient treatment of Ni and Zn in CND.