Environ. Sci. Technol. 1999, 33, 282-287
Removal of Dissolved Heavy Metals from Acid Rock Drainage Using Iron Metal TAMARA E. SHOKES AND GREGORY MO ¨ LLER* Environmental Science Program, Department of Food Science and Toxicology, and Department of Chemical Engineering. University of Idaho, Moscow, Idaho 83844-2203
The chemical and microbial activity of corroding iron metal is examined in the acid rock drainage (ARD) resulting from pyrite oxidation to determine the effectiveness in neutralizing the ARD and reducing the load of dissolved heavy metals. ARD from Berkeley Pit, MT, is treated with iron in batch reactors and columns containing iron granules. Iron, in acidic solution, hydrolyzes water producing hydride and hydroxide ion resulting in a concomitant increase in pH and decrease in redox potential. The dissolved metals in ARD are removed by several mechanisms. Copper and cadmium cement onto the surface of the iron as zerovalent metals. Hydroxide forming metals such as aluminum, zinc, and nickel form complexes with iron and other metals precipitating from solution as the pH rises. Metalloids such as arsenic and antimony coprecipitate with iron. As metals precipitate from solution, various other mechanisms including coprecipitation, sorption, and ion exchange also enhance removal of metals from solution. Corroding iron also creates a reducing environment supportive for sulfate reducing bacteria (SRB) growth. Increases in SRB populations of 5000-fold are observed in iron metal treated ARD solutions. Although this biological process is slow, sulfidogenesis is an additional pathway to further stabilize heavy metal precipitates.
Introduction The former United States Bureau of Mines estimated that over 12 000 miles of rivers and streams and over 180 000 acres of lakes and reservoirs are adversely affected by abandoned metal and coal mines, the corresponding mine wastes and related acid rock drainage (ARD) (1). Chemical and microbial oxidation of pyritic mine wastes exposed to water produces a highly acidic, high total dissolved solids, and heavy metal laden effluent referred to as ARD (2, 3). This work studies the chemical and microbial effects in ARD test water accompanying the corrosion of iron. Current ARD management strategies are typically precipitation-based and take advantage of the insolubility of transition metal sulfides and hydroxides. ARD treatment with the alkaline compounds CaO (quicklime), Ca(OH)2 (hydrated lime), NaOH (caustic soda), or Na2CO3 (soda ash) for the production of insoluble hydroxides is a common approach for precipitation processing. In widespread use, this process produces large volume quantities of heavy metal hydroxide and calcium sulfate sludge that must be disposed in a * Corresponding author phone: (208)885-7081; fax: 885-8937; e-mail:
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regulated hazardous waste facility. ARD management by constructed wetlands and bioreactors uses compost or phytoenhanced SRB sulfidogenesis (4, 5). Although demonstrating promise in target applications, this approach is often limited by ambient conditions such as low pH, winter temperatures, or annual water availability as well as spent compost disposal. Metal sulfides demonstrate more compact volume and better dewatering characteristics than metal hydroxides; however, sulfides may require increased management of H2S and reoxidation in processing. The basic concept of using scrap bulk iron to electrochemically ameliorate ARD has been demonstrated; however, the work did not explore mechanistic or process phenomena (6). The majority of iron metal remediation has been applied to organohalide solvent contaminated sites (7-9). Iron is often a remediation reagent because it has a high reduction potential and the reaction rates are limited by mass transport (7-10). The chemical reaction steps with iron occur relatively quickly (10); however, iron does not react quickly with water near neutral pH (9). Other metals are less effective because the reaction rate is limited by the chemistry rather than diffusion (10) or because water competes for reactive sites on the metal (9). Iron is the best metallic material for environmental remediation because it is a strong reducer and is nontoxic and inexpensive (8). Examples of contaminants that have been treated with zerovalent iron are halogenated hydrocarbons, such as carbon tetrachloride and other solvents, nitro aromatics, and heavy metals, such as chromium (11-14). Remediation of metal contaminated sites with an iron metal approach is increasing (11, 13, 15, 16). Possible removal reaction mechanisms are similar in that they depend on the surface characteristics of a metal. Direct reaction with iron results in electrochemical reduction of a dissolved metal species to the zerovalent metal onto the iron surface or cementation (17, 18). This cementation process can be predicted by the standard reduction potentials of the metals (17, 18). As iron metal corrodes in water, ferrous iron is produced at local anodic sites, and acid is consumed by forming hydrogen gas at local cathodic sites (19). As protons are being consumed from iron corrosion, the concentration of hydroxide increases. Metal hydroxides and hydroxide complexes precipitate when dissolved metals react with hydroxide ions (17, 18). Metallic atoms and compounds may adsorb to the surface armoring iron oxides and hydroxides. Dissolved metals may also ion exchange with the iron ions in iron oxides and hydroxides. The ability of a crystal lattice to undergo ion exchange is related to the ionic radius rather than the respective charge of the ions (19). Microbial oxidation of ores, tailings, and overburden contribute to ARD. Some oxidation reactions, such as pyrite to ferrous sulfate and iron monosulfide to sulfur and hematite, do take place spontaneously under sterile conditions, but the rate of the chemical reaction is usually slower than biological oxidation by Thiobacilli ferrooxidans which are commonly found at sources of ARD (20-22). To remediate ARD, it would be desirable to induce the reduction of sulfate to sulfide resulting in the formation of metal sulfides. Chemical reactions spontaneously, but slowly, reduce oxidized sulfur to H2S under reduced, anaerobic conditions. Microbial processes, involving sulfate reducing bacteria (SRB) such as Desulfovibrio desulfuricans or Clostridium desulfuricans, rapidly induce corrosion of metals in anaerobic conditions and use sulfate as the terminal electron acceptor in respiration, reducing it to sulfide (20, 23, 24). Therefore, a viable SRB population may yield an end-result of precipitated metal sulfides as a way of remediating ARD. 10.1021/es980543x CCC: $18.00
1999 American Chemical Society Published on Web 12/03/1998
ARD from Berkeley Pit is the test material for this study. The Berkeley Pit, located above Butte, MT, is a 77 ha inactive open pit copper mine with a depth of about 545 m (25). The pit is filling at a rate of about five million gallons per day from ground and surface water and is rising about 10 m/yr. It is the most severely contaminated large water body in the United States. It presently contains over 30 billion gallons of pH 2.3, Ca-Fe sulfate solution with high concentrations of aluminum, arsenic, cadmium, copper, iron, manganese, and zinc (25). The purpose of batch reactor and column experiments with Berkeley Pit water is to determine the capacity of iron metal to neutralize acidic water resulting from pyrite oxidation and to reduce the mobility of its dissolved heavy metals (26). The change in the concentration of dissolved metals in ARD is examined for an iron treatment in both a batch reactor and a column. The resulting change in pH in the batch reactor is compared to the iron surface area. The role of the iron surface area in the rates of heavy metal removal is examined. We also examine the capacity of iron metal to establish and maintain an environment that will promote favorable SRB growth conditions.
Experimental Section Materials. All chemicals were reagent grade or better and used as received. Metal salts used include reagent grade anhydrous ferric sulfate (Allied Chemical Co.) and reagent grade ferrous sulfate (J. T. Baker Inc.). Nitrogen gas was continuously bubbled into batch reactors to inhibit oxygen diffusion into the solution. ARD from 200 ft depth from the Berkeley Pit (BP) in Butte, MT, was provided by the Montana Bureau of Mines and Geology at Montana Tech of the University of Montana. BP ARD was stored at room temperature and used as received. Colloidal iron (Micropowder iron, 1-3 µm, grade S-3700, ISP Technologies, Inc.) and granular iron (Fluka, turnings) were used as the iron metal in experiments in the batch reactor. Industrial mixed mesh scrap iron (100% passing 8 U.S. Sieve; Master Builders, Inc.) was used in column experiments as an effluent treatment. Eh measurements were taken with an Orion Model 9616 internal reference Eh electrode (Analytical Technology, Inc.). The response of the electrode was calibrated using the 475 ( 30 mV potential of a ferrous-ferric reference solution as described in ASTM Method D 1498. The pH was measured with a pH electrode and the pH meter of the batch reactor apparatus. The pH meter was calibrated ((0.1) with pH 4.0, 7.0, or 10.0 standards as appropriate. Elemental and Dissolved Ion Analysis Method. Chemical analyses were conducted by the University of Idaho Analytical Sciences Laboratory (UI-ASL). UI-ASL is a U.S. EPA Drinking Water Program certified laboratory facility operating in compliance with Good Laboratory Practice standards (40 CFR §160). All aqueous samples were filtered with a 0.2 µm nylon syringe filter with prefilter (Titan, Scientific Resources, Inc.) and acidified with concentrated nitric acid (trace metal grade, GFS Chemicals, Inc.). Aqueous samples were analyzed for dissolved metals by U.S. Environmental Protection Agency Method 200.7 Dissolved Metal Screen (27). A sequential inductively coupled argon plasma emission spectrometer (Leeman PS1000 ICAP) was used for elemental analysis. The calibration solutions covered the range of the expected concentrations in the samples. Quality control samples used laboratory performance check solutions at known concentrations run periodically to verify the validity of the calibration curve. Recoveries of trace metals from standard reference water (APG 4873, APG 7878; Analytical Products Group, Inc.) were used for quality control. Manufacturer published data acceptance criteria at their recommended statistical performance limits were used as the analytical batch data quality acceptance criteria. All elemental analysis data are traceable
to the U.S. National Institutes of Standards and Technology through the use of certified standards. Product Analysis. Solids were stored in open containers in a vacuum desiccator until the day of analysis. Samples for microbial analysis were analyzed immediately upon removal from the experimental matrix. Solids and precipitates from experiments were analyzed by scanning electron microscopy with X-ray fluorescence (SEM-XRF). Particle size and surface composition were estimated by an Energy Dispersive X-ray System with micro-z detector and ultrathin window (Noran Instruments) and Amray Model 1830 Scanning Electron Microscope with an acceleration potential of 20.0 kV. Batch Reactor Experiments. Batch reactor experiments were performed in a modified 2 L glass Biostat reactor (B. Braun) at 25 ( 2 °C. The reactor was modified by placing a nylon sleeve and paddle on the stainless steel stirring shaft. BP water (1.25 L) was placed in the batch reactor, and colloidal iron (3% w/v) was injected (stirred at 400 rpm) (n ) 2) or granular iron (3% (w/v) Fluka) (n ) 1) was added (stirred at 600 rpm) after equilibration at 25 °C. At timed intervals, Eh and pH were measured, and samples for elemental analysis were pipetted out of the reactor vessel under positive N2 pressure. The Eh and pH probes were cleaned and calibrated daily. The samples were immediately syringe filtered at 0.2 µm and acidified prior to elemental analysis. Two sets of replicate (n ) 3 each) samples of BP water (untreated) and ∼200 h colloidal iron treatment samples were transferred under Ar to 100 mL Teflon bombs. Fresh cow manure, 5% (w/v), was added to one set of the untreated BP water samples and to one set of the colloidal iron treated samples. The samples were placed in a 25 °C shaker bath for 27 days. The samples were examined for SRB activity by terminal dilution using the agar shake method (28). This method was designed to provide an estimate of the number of viable cells that have a specific physiological capability. In the case of SRB, that capacity is the reduction of sulfate and the consequent generation of sulfide. The progress of the reaction was visually monitored by a ratable change in the appearance of the media by the formation of a black precipitate of iron sulfide. Samples were diluted to extinction in complex SRBspecific media containing 1.5% agar. A triplicate set of 10fold serial dilutions starting from a known volume allowed estimation of SRB number within an order of magnitude. Column Reactor Experiment. Industrial mixed mesh scrap iron was loosely packed in 80% (w/w) iron/ sand mixture into 75 mL polypropylene columns fitted with 20 µm plastic frits (n ) 3) with an average porosity of 40% (Bond Elut; Varian, Inc.). The columns were attached to a reservoir bottle that contained BP water. The pressure head was adjusted for slow flow, about 2 mL/h. The columns were allowed to activate for 1 day before sample collection by filling with BP water. Two 50 mL quantities of the column effluent were collected successively following flushing. The pH of the effluents and reservoir solution was measured.
Results and Discussion Batch Reactor Experiments. The changes in pH and Eh for the two iron treatments are shown in Figure 1. The BP reactor pH increases to 6.5 within 4 h after addition of colloidal iron but only achieves pH 5 upon addition of granular iron. The colloidal iron is capable of raising and maintaining the pH near neutral while establishing a reducing environment. Colloidal iron reduces the Eh from 1200 to -400 mV and sustains this level for the term of the experiment; however, the Eh in the granular iron treatment is unstable and fluctuates between 0 and -300 mV resulting from the buildup and shedding of surface armoring compounds. The before and after treatment concentrations as well as the percent change of dissolved elements, pH, and Eh for the VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Percent Decrease [Increase] and Reaction Half-Life of Elements in BP Water Treated with Colloidal Iron (n ) 2) or Granular Iron (n ) 1) in the Batch Reactor Experiments colloidal iron treatment
Al As Cd Co Cr Cu Fe Ni Pb S Sb Zn pH Eh (mV) a
before (µM)
after (µM)
10600 29.1 22.0 26.8 17.7 2170 3430 22.0 4.18 65000 113 6180 2.3 760
47.2 10.4 0.0649 0.781 90%) in about 5 h with the high surface area colloidal iron but remains largely unchanged in the granular iron reactor. Chromium and nickel appear to follow first-order kinetics in the initial reaction with colloidal iron, whereas antimony and zinc appear to follow zero-order kinetics in the initial reaction. Copper, cadmium, and aluminum are most rapidly removed from colloidal iron solution each with a reaction half-life less than 1 min. As shown in Figure 2, the granular iron treatment removes metals more slowly and less effectively. However, 284
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in both cases, copper and cadmium are removed most rapidly as shown by the reaction half-life (Table 1). The concentration of dissolved iron quickly increases in the colloidal iron treatment compared to the granular iron treatment (Figure 3a). The greater surface area of colloidal iron facilitates rapid hydrolysis of water and formation of hydroxide ions. Dissolved sulfate decreases about 10 mM and is the same order of magnitude as the increase in dissolved iron (Figure 3b). Sulfate may be removed from solution by precipitation of metal sulfur compounds, e.g., metal sulfate/ hydroxide complexes. Introducing iron to corrosive ARD initiates several reaction mechanisms that remove dissolved metals from solution. Iron corrodes in acidic sulfate solution forming hydroxide, hydride, and iron oxides and hydroxides (29). An uneven iron surface leads to an increase in the initial reaction rate (30). A controlling factor is the diffusion of atoms or molecules to the surface of the iron (31). Depending on the metal ions, the two possible mechanisms are reactions with hydrolysis products or direct reaction with the surface (32). Colloidal iron reduces the concentration of dissolved heavy metals of concern in BP ARD. Aluminum, copper, and cadmium are eliminated from solution within minutes as shown by their reaction half-life (Table 1). Cadmium and copper cement on the surface of the iron by reduction to the elemental metal (30). Aluminum precipitates out of solution as Al(OH)3 (Ksp ) 3 × 10-34) (31) or an aluminum hydroxide/ sulfate complex (26) as iron dissociates water to hydroxide ion and the pH rises over 5.5. Chromium is reduced to the less soluble Cr(III) form as observed in other work (33). According to the thermodynamic stability diagram for lead (26), it can form soluble Pb4(OH)44+ and Pb6(OH)84+ at pH 7 as well as PbCO3; however, it is amenable to further reaction to the insoluble sulfide. The solubility product constant of Pb(OH)2 is 3 × 10-28 (31); therefore, this solid may also be formed as the pH rises. Lead (Pb2+) can also complex with inorganic ligands, e.g. OH- (34, 35). Iron metal has been shown to reduce carbon dioxide, forming hydrocarbons; hence, the previous mechanisms are more likely than formation of lead carbonate (36). Nickel and zinc seem to be removed from solution as the pH increases and may be forming complexes with hydroxide and/ or sulfate (35). Under reducing conditions, nickel and zinc form sulfides, but as previously discussed, this is not likely without a microbial process. Arsenic complexes with iron oxide and may be removed from solution by coprecipitation with sulfides, iron oxides, and iron oxyhydroxides (35, 37-39). As iron corrodes in acid,
FIGURE 2. Changes in dissolved (a) aluminum, (b) zinc, (c) cadmium, (d) antimony, (e) nickel, and (f) copper concentrations following 3% (w/v) colloidal [ and granular 0 iron treatment of acid rock drainage in the batch reactor experiments. surface sites on granular iron. Copper and cadmium cementation occurs very rapidly, coating the iron surface and further reducing the exposed surface area of the iron by chemical armoring (30). The granular iron has an estimated surface area of 0.005 m2/g determined by gas adsorption using BET analysis (42), whereas the estimated surface area of colloidal iron is 0.3 m2/g of iron (26). Since the available surface area kinetically mediates the removal of metals from solution, the granular iron does not have sufficient surface area to raise the pH to about 7 or maintain a reducing Eh steadily within the term of this experiment (26). Therefore, removal reactions that may require a hydroxyl ion in a step, such as aluminum, nickel, and zinc, do not proceed significantly to completion in this experimental configuration. The small amount of removal for these elements is probably due to a combination of precipitation with and sorption on the iron oxide/hydroxide surfaces. However, lead is removed to about 3 µM with both iron types. Apparently, the reduction in the number of surface sites does not significantly interfere with this mechanism.
FIGURE 3. Changes in dissolved (a) iron and (b) sulfur concentrations following 3% (w/v) colloidal [ and granular 0 iron treatment of acid rock drainage in the batch reactor experiments. hydride is produced that can react with arsenate (AsO33-) to form arsine, AsH3. A similar mechanism uses zinc granules to form arsine for the determination of arsenic poisoning; this is called the Marsh Test (40). Hydrogen may also react with arsenate to produce water and elemental arsenic (41). Antimony is in the same periodic group as arsenic and can be removed by the same mechanisms. Sorption onto complex solids formed in these reactions also plays a role in dissolved metal removal. The dissolved BP elements are not removed as quickly nor to the same levels with granular iron as with colloidal iron. This is due to a smaller quantity of available reactive
Therefore, we conclude that iron metal is effective in changing the chemistry of acidic systems toward conditions promoting immobilization of dissolved heavy metals, pH increase, and redox potential decrease. Addition of iron initiates reactions in acidic water that result in the removal of dissolved heavy metals by several mechanisms. These pathways include cementation, precipitation of metal hydroxides, and adsorption. Many of the dissolved metal removal reactions in iron treatment experiments are rapid and effective. Kinetic studies show that treatments of 3% colloidal iron effectively removed many heavy metals of concern from acid rock drainage water greater than 90% within nine days. A primary removal mechanism is identified for some of the heavy metals; however, the metals are not removed by a single mechanism. It is the combined effect of multiple reactions that makes this treatment so effective in removing metals with a variety of chemical characteristics. The resulting sludge is insoluble while anoxic conditions are maintained. Sulfate-Reducing Bacteria. Most probable number estimates of the SRB populations in BP ARD that are treated VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Sulfate Reducing Bacteria (SRB) Most Probable Number, by Terminal Dilution Enrichment, for Berkeley Pit Water, Treated and Untreateda
treatment pH Eh (mV) replicate A replicate B replicate A replicate B
untreated
5% (w/v) fresh cow manure
3% (w/v) colloidal iron
3.0 3.4 7.3 766 525 -247 SRB Estimates (Cells/mL Inoculum) 10 106 104 0 104 103 SRB-Prevalent Morphotype CMR CMR CMR NA rods and CMR bacilli
3% (w/v) colloidal iron + 5% (w/v) fresh cow manure 7.2 -200 108 108 CMR CMR
a CMR ) curved, motile rods; NA ) not applicable; colloidal Fe batch reactor treatment alone ∼ 9 days; manure treatment ∼ 27 days at 25 °C.
FIGURE 4. SEM micrograph (3140 X) of scrap iron surface that had been in Berkeley Pit water for 30 days. Smooth spherical and oblong rod shaped objects are identified as sulfate-reducing bacteria. with iron metal, manure, and iron and manure are shown in Table 2. Colloidal iron treatment of BP ARD increases the SRB population 5000-fold indicating the presentation of favorable iron biocorrosion conditions, i.e., anoxic, reducing, near-neutral pH waters. Addition of fresh cow manure to colloidal iron treated BP ARD increases the SRB population 100 million-fold over untreated BP water and is 1000 times greater than treatment with manure alone. Figure 4 shows the smooth spherical and oblong shape of SRB bacteria that grew on the surface of scrap iron turnings immersed in BP ARD about 30 days. These data demonstrate iron metal with and without cow manure is effective in oxidized acidic solutions for establishing sulfate-reducing bacteria populations. It is also apparent that the combined effect of the colloidal iron pretreatment and cow manure create a microenvironment at the corroding metal surface inducing vigorous SRB growth in an originally inhospitable solution. The related sulfidogenesis may allow for the removal of dissolved iron and heavy metals in this reaction system. Therefore, high concentrations of corroding metallic iron increase pH to near-neutral values and develop reductive conditions to create an environment supporting SRB growth and activity. Although heavy metal removal from solution by formation of sulfides by SRB is slow, the advantage of enhancing SRB populations is the presentation of an additional pathway that further stabilizes the precipitates. In anoxic domains, these sulfides are more insoluble than the corresponding heavy metal hydroxides and may allow for in situ repyritization approaches to ARD control. 286
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TABLE 3. Concentration Changes and Percent Decrease [Increase] of Dissolved Elements in Mixed-Mesh Scrap Iron, Column Treatment of Berkeley Pit Water
Al As Be Cd Co Cr Cu Fe Ni Pb S Sb Zn pH a
before (µM)
after (µM)
11300 40.4 7.35 19.4 24.7 17.6 1980 2950 20.7 4.08 66600 136 6330 2.7
70.1 14.5 0.370
