Removal of Metals and Acidity from Acid Mine Drainage Using Liquid and Dried Digested Sewage Sludge and Cattle Slurry

The metal removal and neutralization capacities of digested sewage sludges from municipal wastewater treatment plants, cattle slurry (liquid manure), and Biofert granules (dried granular anaerobic sludge) were compared under batch conditions using synthetic AMD (pH 2.8) containing high concentrations of Al, Cu, Fe, Mn, Pb, and Zn (100, 15, 270, 15, 2, and 30 mg/L, respectively). The effects of contact time and solids concentration were examined. Metal removal was variable for all materials. Contact time had a significant effect, with total removal often increasing over the experimental time interval (i.e. 30 min to 24 h). Removal efficiency (%) was generally highest for Cu, Pb, and Al, while Mn and Zn were the least removed. Cattle slurry was the best material for metal removal, with the following maximum removals at a solids concentration of 12.9 g/L: Cu >98 %, Al >98 %, Fe >60 %, Mn >18 %, Pb >96 %, and Zn >60 %. Metal removal using digested sewage sludge reached 88 % for Al, 98 % for Cu, 94 % for Pb, and 30 % for Zn. Neutralization was complete within 30 min after AMD was mixed with digested sludges or cattle slurry, with the pH reaching a maximum of 5.5 with the slurry. In contrast, neutralization by the Biofert granules only reached equilibrium after 300 min, and pH remained <4.0 except at high solids concentrations. It appears that recycled waste-derived organic materials can neutralize AMD and remove dissolved metals by adsorption and precipitation, creating a more treatable waste stream or one that could be discharged directly to surface water. Potential methods of safe disposal of metal-enriched organic materials are discussed.     

Thallium and Other Potentially Toxic Elements in the Baccatoio Stream Catchment (Northern Tuscany,Italy) Receiving Drainages from Abandoned Mines

Acid mine drainages (AMD) have adversely affected the southern Apuan Alps (northern Tuscany, Italy). The study particularly focuses on the Baccatoio stream, which receives AMD from the abandoned Pollone and M. Arsiccio mines. The mine waters have an average pH of 2.2 and contain potentially toxic elements (PTE) at concentrations that exceed the Italian regulatory threshold for Al, Fe, Mn, Cu, Zn, As, Ni, Co, Cd, Sb, Pb, and Tl. The AMD flow directly into the stream, severely contaminating it. Downstream of the mined areas, the pH increases and most PTE (especially Fe, Al, As, and Pb) are readily scavenged from the stream waters by precipitation and/or adsorption. However, Tl, which peak at 1000 µg/L in the AMD, behaves almost conservatively along the stream flow path, undergoing only dilution, and remains at or above the concentration of concern of 4 µg/L almost to the coastline, before sharply decreasing to 0.5 µg/L where seawater is encountered. Since the stream water was locally used for irrigation, these observations may have important environmental and public health consequences in such a densely populated area.     

Mineralogical and Geochemical Characterization of Mine Tailings and Pb, Zn, and Cd Mobility in a Carbonate Setting (Northern Tunisia)

Millions of tonnes of Pb–Zn ore flotation tailings and waste rock have been discharged at sites in northern Tunisia without concern for environmental issues. The tailings are dominantly fine grained (<125 μm), with high porosity and permeability. The tailings were characterized to assess base metal (Pb, Zn, and Cd) mobility. The relatively low percentage of iron sulphide and the dominance of carbonates in the matrices of the tailings indicated that only neutral mine drainage is likely. Batch sequential testing showed that the calcium and sulphate, which are the major ionic species in solution, are derived mainly from the dissolution of gypsum and not from neutralization of acidity generated by pyrite oxidation. Yet, despite the carbonate setting, the resultant neutral to slightly alkaline pH, and prolonged weathering, the studied flotation tailings maintain their capacity to release contaminants, notably Zn and Cd, into the environment. The amount of Zn that dissolves (2,400 μg L^?1, on average), though significant, is below the background concentrations in the Mejerda River and the environmental norms established for surface waters. Pb concentrations come close to the standards, but only Cd (18 μg L^?1, on average) sometimes exceeds current river water concentrations and environmental standards.     

Tailings Weathering and Arsenic Mobility at the Abandoned Zgounder Silver Mine,Morocco

The abandoned Zgounder Mine (Morocco) was exploited for Ag from 1982 to 1990 and generated nearly 490,000 t of mill tailings before it was closed without being reclaimed. The tailings contain low concentrations of sulfide (mainly as pyrite, sphalerite, and galena) and carbonates (mainly dolomite). Silicates (muscovite, albite, chlorite, labradorite, actinolite, and orthoclase) occur in high concentrations. The most abundant trace elements are As, Ti, Fe, Mn, Zn, and Pb. We studied the geochemical behavior of the mine wastes to identify the main factors controlling drainage water chemistry. Particular emphasis was put on sorption phenomena to explain the low As concentrations in the leachates despite significant As levels in the tailings. Weathering cell tests carried out on various tailings produced two types of contaminated drainage: acidic and neutral. The kinetic test leachates contained high concentrations of some contaminants, including As (0.8 mg L^?1), Co (11 mg L^?1), Cu (34 mg L^?1), Fe (70 mg L^?1), Mn (126 mg L^?1), and Zn (314 mg L^?1). Acidity and contaminants in the leachates were controlled by dissolution of soluble salts and Fe hydrolysis rather than sulfide oxidation. Batch sorption tests quantified the significance of As sorption, and sequential extraction showed that most of the As sorption was associated with the reducible fractions (Fe and Mn oxides and oxyhydroxides).     

Fate of sulphate removed during the treatment of circumneutral mine water and acid mine drainage with coal fly ash: Modelling and experimental approach

The treatment of acid mine drainage (AMD) and circumneutral mine water (CMW) with South African coal fly ash (FA) provides a low cost and alternative technique for treating mine wastes waters. The sulphate concentration in AMD can be reduced significantly when AMD was treated with the FA to pH 9. On the other hand an insignificant amount of sulphate was removed when CMW (containing a very low concentration of Fe and Al) was treated using FA to pH 9. The levels of Fe and Al, and the final solution pH in the AMD–fly ash mixture played a significant role on the level of sulphate removal in contrast to CMW–fly ash mixtures. In this study, a modelling approach using PHREEQC geochemical modelling software was combined with AMD–fly ash and/or CMW–fly ash neutralization experiments in order to predict the mineral phases involved in sulphate removal. The effects of solution pH and Fe and Al concentration in mine water on sulphate were also investigated. The results obtained showed that sulphate, Fe, Al, Mg and Mn removal from AMD and/or CMW with fly ash is a function of solution pH. The presence of Fe and Al in AMD exhibited buffering characteristic leading to more lime leaching from FA into mine water, hence increasing the concentration of Ca²⁺. This resulted in increased removal of sulphate as CaSO₄·2H₂O. In addition the sulphate removal was enhanced through the precipitation as Fe and Al oxyhydroxysulphates (as shown by geochemical modelling) in AMD–fly ash system. The low concentration of Fe and Al in CMW resulted in sulphate removal depending mainly on CaSO₄·2H₂O. The results of this study would have implications on the design of treatment methods relevant for different mine waters.     

The Chemistry of Waters Associated with Metal Mining in Macedonia

Abstract  Pollution from current and past mining is a significant problem in several parts of the former Yugoslav Republic of Macedonia. Water from six different mining areas in Macedonia was analysed to assess the effects of metalliferous mining activities. Drainage sediments at all locations show evidence of physical and chemical contamination; water compositions, however, were more variable. Low pH water associated with mining has led to the dissolution of minerals and the mobilization of metals from the ores and the host rocks. Only Sb was noted to exhibit enhanced mobility in higher pH waters. The Zletevo Pb-Zn mine discharges low pH water that has high levels of several metals, including Al, Zn, Cd, and Fe; sediment concentrations are grossly elevated for several km downstream. Toranica and Sasa Pb-Zn mines exhibit similar sediment contamination of Pb, Zn, Cd, and other ore-related metals. However, concentrations of metals in waters are far lower at both of these mines, due to less pyrite in the ore and the buffering of the acid waters by carbonate host lithologies. At the Buchim copper mine, waters are both acidic and high in dissolved solids; Cu concentrations exceed 100 mg/L. Krstov Dol and Alshar are small, disused As-Sb mines that discharge waters that exceed potable values for some contaminants (e. g. As), but this may be related to the mineralization of the bedrock rather than the mines. In general, metal concentrations decreased downstream from the source due to dilution from other rivers and coprecipitation of metals on other mineral phases (e. g. Fe-, Al- and Mn-oxides, and hydroxides).     

Risk of Metal Mobilization from Redevelopment Activities in Hyperarid Climates: A Laboratory Experiment and Discussion

The reclamation and redevelopment of abandoned mine sites into parks, golf courses, and residential communities in arid and hyperarid regions has been caused, in part, to rising land costs. A laboratory experiment using three columns was monitored for 273 days to evaluate trace metal and metalloid availability. The sediment from the Three Kids Mine that was used in this study is documented to contain high levels of trace metals and metalloids. The experiment simulated agricultural activities over time to assess whether fertilizers would mobilize trapped contaminants. Results indicate that irrigation and fertilizers can provide conditions for Pb, Mn, Zn, Al, Ba, Cu, and As to become mobile, though on a limited basis. There was an increase in Pb, Mn, Zn, Al, Ba, Cu, and As within the first 30 days followed by a decrease at 90 days. Concentrations of Pb, Mn, and Zn increased at 273 days due to application of fertilizer-fortified waters. This study shows the potential for mobilized trace metals and metalloids to enter the wider environment after developmental activities are finished.     

Mixing of Acid Rock Drainage with Alkaline Ash Leachates: Formation of Solid Precipitates and pH-Buffering

Three metal-rich, acidic mine waters (from Bersbo and Ljusnarsberg, Sweden) were mixed with alkaline fly ash leachates in various proportions, representing a pH titration. Changes in pH and the loss of metals in solution due to precipitation of solid phases were tracked. Mineral equilibria and changes in pH and alkalinity were simulated using the geochemical code PHREEQC and the MINTEQv4 database, and the measured and simulated pH responses were compared. The formation of solid precipitates corresponded to fairly well-defined pH-buffering regions, reflecting the mine water compositions (notably the levels of Fe, Al, and Mn). Zn precipitation had a distinct buffering effect at near-neutral pH for the mine waters not dominated by iron. The formation of solid Mg phases (carbonate, as well as hydroxide) was indicated at high pH (above 9), but not formation of solid Ca phases, despite high sulfate levels. The phases that precipitated were various amorphous mixtures, mostly of the metals Fe, Al, Mn, Zn, and Mg. For the Fe-rich mine water, pH was poorly simulated with a simple MIX model, while alkalinity predictions agreed reasonably well with measured data. For the Al-rich mine waters, the simulated pH responses agreed well with the measurements. In an additional step, geochemical simulations were performed where selected proxy phases for major elements were forced to precipitate; this significantly improved the pH and alkalinity predictions. This approach may be more efficient than performing mixing experiments and titrations.     

Removal of Metals and Acidity from Acid Mine Drainage Using Municipal Wastewater and Activated Sludge

Co-treatment of acid mine drainage (AMD) and municipal wastewater (MWW) using the activated sludge process is an innovative approach to AMD remediation that utilizes the alkalinity of MWW and the adsorptive properties of the wastewater particulates and activated sludge biomass to buffer acidity and remove metals. The capacity of these materials to treat AMD was investigated in batch mode metal removal tests using high-strength synthetic AMD (pH 2.8, Al 120–200 mg/L, Cu 18–30 mg/L, Fe 324–540 mg/L, Mn 18–30 mg/L, and Zn 36–60 mg/L). Using material from a range of MWW treatment plants, the performance of screened and settled MWW, activated sludges with mixed liquor suspended solids (MLSS) concentrations of 2.0 and 4.0 g/L, and return activated sludges with 6.0 and 7.4 g/L MLSS were compared. Similar trends were observed for the MWW and activated sludges, with removal efficiency generally decreasing in the order Al = Cu > Mn > Zn > Fe. Trends in Fe removal using settled MWW and activated sludges were highly variable, with removal <30 %. Using activated sludges, average removal efficiencies for Al, Cu, Mn, and Zn were 10–65 %, 20–60 %, 10–25 %, and 0–20 %, respectively. Sludge solids concentration was an important controlling factor in metal removal, with removal of Al, Cu, Mn, and Zn increasing significantly with solids concentration. Municipal wastewaters had greater neutralization capacities than activated sludges at high AMD loading ratios. Mixing AMD with screened MWW gave the highest removal efficiency for all metals, achieving average removal of 90–100 % for Al, Cu, and Fe, 65–100 % for Zn, and 60–75 % for Mn. These empirical findings are useful for developing process design parameters in co-treatment systems. Utilizing MWW and activated sludge to remediate AMD can potentially reduce materials and energy requirements and associated costs.     

Assessment of Soil and Water Contamination at the Tab-Simco Coal Mine: A Case Study

In 1996, the Tab-Simco site, an abandoned coal mine 10 km southeast of Carbondale, Illinois, was listed as one of the most highly contaminated AMD sites in the mid-continent region. A suite of impacted soil and water samples were collected from various locations to characterize the current extent of AMD pollution, following standard U.S. EPA protocols. The mean pH of soil and water samples were found to be 2.69 and 2.07, respectively. The mean sulfur content of the soil samples was 0.5 %. The AMD-impacted soils contained high concentrations of Fe, Zn, Ni, Cr, Cu, Pb, and As. The AMD also contained high concentrations of Fe, As, Zn, Pb, Cr, Al, Cd, Cu, and Ni, as well as \({\text{SO}}_{4}^{2 - }\), all of which were significantly above their U.S. EPA permissible limits for surface water.     

Evaluation of Metal Attenuation from Mine Tailings in SE Spain (Sierra Almagrera): A Soil-Leaching Column Study

A laboratory study was undertaken using mine tailings and soil columns to evaluate some of the natural processes that can control the mobility of metals at Pb–Ag mine tailings impoundments. The effects of buffering, pH, and salinity were examined with tailings from the El Arteal deposit. Al, Ba, Cd, Cu, Fe, Mn, Ni, Pb, Sr, and Zn were mobilized when the tailings were leached. However, when the mine tailings were placed above alluvial soils, Al, Ba, Cd, Cu, Mn, Pb, and Zn were retained, although Fe and Sr clearly remained mobile. Most of the metal retention appears to be associated with the increase in pH caused by calcite dissolution. The sorption of some metals (Cu, Pb, and Zn) onto oxyhydroxides of Fe and Mn, sulphates, clay materials, and organic matter may also explain the removal of these metals from the leachate.     

Thallium Contamination in the Raibl Mine Site Stream Drainage System (Eastern Alps,Italy)

The Raibl mine (Cave del Predil village, northern Italy) belongs to the Pb–Zn minerogenetic district in the southeastern Alps, hosted in Middle Triassic carbonates. The drainage water quality reflects the high acid-buffering capacity of the carbonate rocks, which controls the mobility of most metals. In particular, Fe is non-detectable in solution, having formed hydrous-oxides precipitates. Molybdenum, Ni, Zn, Cd, Pb, and Tl are present, and the Pb, Tl, and Zn concentrations sometimes exceed the Italian regulatory thresholds. Thallium concentrations substantially exceed the 2 µg/L limit at some sampling stations, ranging between 12 and 30 µg/L in the mine drainage, and reaching 5 µg/L downstream of the mine site, despite strong dilution. The data indicate that Tl behaves almost conservatively and is not significantly scavenged by the Fe precipitates. The elevated Tl represents a potential risk for the stream ecosystem. Although Tl is not regulated in drinking water in Italy or the European Community, its distribution in natural waters may help to determine if health actions should be taken.     

Statistical Evaluation of Dependence Between pH,Metal Contaminants,and Flow Rate in the AMD-Affected Smolnik Creek

Acid mine drainage (AMD) with a pH of 3.7–4.1 seeps from an abandoned sulphide mine in Smolnik, Slovakia at a flow rate of 5–10 L/s. Metals precipitate as the AMD mixes with the higher pH Smolnik Creek, adversely affecting the stream’s water quality and ecology. Multivariate statistics were used to interpret surface water and sediment quality effects. Factor analysis generated three significant factors that explained 79.9 % of the variance in the data: the pH is indirectly proportional to the concentration of dissolved metals; Fe precipitation is associated with a decrease in Al in the sediment; and increased Cu concentrations are associated with more Zn in the sediment. High rainfall events increase the flow of Smolnik Creek, which ranges from 0.3 to 2.0 m³/s (monitored 2000–2012). Increased flow is associated with a pH increase and precipitation of metals (Fe, Al, Cu, and Zn). The dependence of pH on flow in Smolnik Creek was evaluated using regression analysis, which confirmed the significance of the exponential relationship between pH and flow rate.     

Application of coal fly ash to circumneutral mine waters for the removal of sulphates as gypsum and ettringite

South African power stations generate large amounts of highly alkaline fly ash (FA). This waste product has a serious impact on the environment. Acid mine drainage (AMD) is another environmental problem associated with mining. AMD has high heavy metal content in addition to high sulphate concentrations. Several studies have shown that 80–90% of sulphates can be removed when FA is co-disposed with AMD rich in Fe and Al. In South Africa, sources of contaminated mine waters, unlike AMD have circumneutral pH and much lower concentrations of Fe and Al, but rich in Ca and Mg. Treatment of such waters with FA resulted in no significant removal of sulphates when treated to pH less than 10. Subsequent treatment of circumneutral mine water to pH greater than 11 resulted in more than 60% sulphate removal. Treatment of circumneutral mine water to pH greater than 11 with FA followed by seeding with gypsum crystals and the addition of amorphous Al(OH)₃ resulted in removal of sulphate to levels below the Department of Water Affairs and Forestry (DWAF) water quality effluent limit (500 ppm).     

Treatment of Acid Mine Drainage Using a Fly Ash Zeolite Column

Acid mine drainage (AMD) and fly ash from thermal power plants both pose substantial environmental problems in India. Fly ash from the Talcher super thermal power plant was converted into zeolite and used in a column to treat AMD from the abandoned Gorbi opencast mines (Singrauli coalfields, NCL). The pH of the mine water increased, and 100 % of the total hardness, Ca hardness, Mg hardness, Mn, Zn, Pb, Cd, Ni, and acidity were removed, along with 99 % of the Fe and 90 % of the Cu.     

煤矿酸性矿井水中有害元素的迁移特性

利用电感耦合等离子质谱（ICP-MS）、离子色谱（IC）和X射线衍射（XRD）等方法研究了马兰煤矿酸性矿井水及其沉淀物的化学成分和物相组成,并通过吸附解吸实验和PHREEQC水化学模拟计算研究了典型酸性矿井水样品中Pb,Th,U,Be,Zn,Ni,Co,Cd,Cu,As,Cr,V,Ba等有害元素的迁移特性.研究表明：① 煤矿酸性矿井水中SO^2-₄,Fe,Mn,Al,Pb,Th,U,Be,Zn,Ni,Co,Cu等离子含量较高,对环境存在潜在危害;② 酸性矿井水中有害元素的迁移主要受pH,Fe-Al-Mn含量和水体颗粒物矿物组成的控制;③ Fe,Al和Mn的含量随pH上升而迅速下降,并控制着Pb,Th,U,Be,Zn,Ni,Co,Cu等潜在有害微量离子的迁移行为; ④ 各离子随pH上升被去除的先后顺序为: Th>Fe>Pb >Cr>Al>Cu>Be>U>Zn>As>Cd>Mn>Co>Ni>Ba;⑤ 酸性矿井水中V不能够随pH的升高而去除,反而会有更多的V溶解在水中.     

Use of Steel Slag Leach Beds for the Treatment of Acid Mine Drainage

Steel slag from the Waylite steel-making plant in Bethlehem, Pennsylvania was leached with acidic mine drainage (AMD) of a known quality using an established laboratory procedure. Leaching continued for 60 cycles and leachates were collected after each cycle. Results indicated that the slag was very effective at neutralizing acidity. The AMD/slag leachates contained higher average concentrations of Ba, V, Mn, Cr, As, Ag, and Se and lower average concentrations of Sb, Fe, Zn, Be, Cd, Tl, Ni, Al, Cu, and Pb than the untreated AMD. Based on these tests, slag leach beds were constructed at the abandoned McCarty mine site in Preston County, West Virginia. The leach beds were constructed as slag check dams below limestone-lined settling basins. Acid water was captured in limestone channels and directed into basins to leach through the slag dams and discharge into a tributary of Beaver Creek. Since installation in October 2000, the system has been consistently producing net alkaline, pH 9 water. The treated water is still net alkaline and has a neutral pH after it encounters several other acidic seeps downstream.     

The Potential Use of Phosphatic Limestone Wastes in the Passive Treatment of AMD: A Laboratory Study

In Morocco, there are many sedimentary phosphate mines that produce large quantities of phosphatic limestone wastes (PLW) that contain calcite (46 wt%) and dolomite (16 wt%). These mines are located near contaminated sites, such as the abandoned Kettara pyrrhotite mine. The surface drainage water at the Kettara mine site has a pH of 2.9–4.2 and elevated concentrations of SO₄ (from 47 to 5,000 mg/L) and Fe (from 1 to 1,200 mg/L). The efficiency of PLW was assessed in the laboratory as an alternative alkaline material for passive acidic mine drainage (AMD) treatment. A series of experiments were carried out using a synthetic AMD (pH 3) containing Fe (500 mg/L), SO₄ (3.4 g/L), Ca (220 mg/L), Al (160 mg/L), Mn (20 mg/L), Zn (15 mg/L), Cu (23 mg/L), and trace amounts of Co, Cr, and Ni. Experiments were done in both anoxic and oxic conditions, in batch and column tests, with hydraulic retention times of 24 and 15 h, respectively. The PLW efficiently increased the alkalinity and pH, inducing precipitation of most metals. The neutralizing capacity of PLW prepared at different particle sizes (0.8 mm–0.5 cm, 0.5–1, 1–2, and 2–3 cm) was found to be similar in batch tests. The initial AMD value increased from 3 to 5–6.5 during the batch tests and 6.5–8 in the columns. In batch tests under anoxic and oxic conditions, there was a significant decrease in concentrations of Fe (500–120 mg/L), Al (160–1.7 mg/L), and Cu (23–0.002 mg/L). In the column tests, Al and Cu decreased (177–2.5 and 26–0.002 mg/L, respectively), while Fe decreased less significantly (618–300 mg/L). The availability and low cost of the PLW make its use in passive AMD treatment potentially feasible.     

Passive Treatment of Circumneutral Mine Drainage from the St. Louis Mine Tunnel,Rico, CO: Part 1—Case Study: Characteristics of the Mine Drainage

This publication is a case study of the seasonal variability of mine water drainage from the Saint Louis Tunnel (SLT) at the inactive Rico-Argentine mine site located in southwestern Colorado. It is an introductory paper for the two passive water treatment system technology evaluations contained in this issue. Mine water chemistry changes from baseflow to a snowmelt runoff event (SMRE) where snowmelt runoff follows preferential migration pathways to flush acidic weathering products from the upper mine workings to the SLT. Baseflow mine drainage is characterized as circumneutral, with Zn, Cd, Mn, and Ni concentrations primarily in the dissolved form. Dissolved Zn, Mn, Fe, and potentially Cd illustrate equilibrium with carbonate minerals. Total concentrations of Fe, Cu, Pb, and As are primarily in the suspended form and suggest sorption to Fe oxides. Mine water chemistry during the SMRE reflects mixing of circumneutral baseflow waters with more acidic waters flushing the upper mine workings. Geothermal activity provides for a consistently warm mine water discharge from the SLT. The two seasons that provide the most challenge to passive water treatment of SLT mine drainage are the SMRE period and the low flow stage of the Dolores River. Mine water flow and chemistry during SMRE are highly correlated with Dolores River flow and this site conceptual model was and will be used to assist in pilot project evaluation, water treatment system design, monitoring system design, a seasonal compliance approach, and water management.

     

International Trials of Vertical Flow Reactors for Coal Mine Water Treatment

Vertical flow reactors (VFRs) were tested at coal mine sites in New Zealand, South Korea, and the USA. The objective was to evaluate the iron removal efficiency and iron removal mechanisms during field trials at low pH and circumneutral pH, and to evaluate the potential use of VFRs as stand-alone systems or in combination with other passive treatment technologies. Total iron and manganese removal efficiencies at circumneutral pH (6–8) often exceeded 90%, with effluent concentrations less than 1 mg/L. This is attributed to both homogeneous and heterogenous Fe(II) oxidation and filtration of the precipitated ferrihydrite. Iron removal efficiencies at moderately acidic conditions (pH 3–4.5) averaged close to 40%, with an average 71.0% removal in one of the trials after iron removal capacity was stabilized. Microbial Fe(II) oxidation and precipitation as schwertmannite together with aggregation of colloidal and nano-particulate Fe(III) are suspected to be the main removal mechanisms. Iron solubility limited removal under very acidic conditions (pH < 3). The reproducibility of the results with respect to previous research confirmed that VFRs can be used as stand-alone passive treatment systems for iron removal from mine waters with a footprint less than half of the area required by a conventional aerobic wetland. A VFR can also provide useful iron pretreatment for other passive treatment systems under circumneutral conditions, but would have to be combined with alkaline generating systems to achieve full iron removal from acidic mine waters.