Research Trace Metal Adsorption onto an Acid Mine Drainage Iron(III) Oxy Hydroxy Sulfate JENNY G. WEBSTER,* PETER J. SWEDLUND, AND KERRY S. WEBSTER Institute of Environmental Science and Research, Private Bag 92021, Auckland, New Zealand
The adsorption properties of Fe-rich precipitates in acid mine drainage (AMD) systems differ from those of pure hydrous iron(III) oxides, and this can lead to inaccurate predictions of trace metal adsorption and attenuation. Adsorption edges for Cu, Pb, Zn and Cd adsorption onto a poorly ordered, goethite-bearing iron(III) oxy hydroxy sulfate, precipitated in an AMD system in New Zealand, have been compared to those for adsorption onto synthetic schwertmannite and two-line ferrihydrite. Adsorption of Cu and Zn onto the AMD iron(III) oxy hydroxy sulfate was greater than onto synthetic schwertmannite, which was in turn greater than onto two-line ferrihydrite. The two factors considered most likely to enhance Cu and Zn adsorption on the AMD iron(III) oxy hydroxy sulfate were (i) the formation of ternary complexes between the oxide surface, adsorbed SO4 and the metal ion and (ii) bacterially mediated formation of the AMD precipitate. Cd adsorption was similarly enhanced on AMD iron(III) oxy hydroxy sulfate but unaffected by SO4, which did not adsorb at the relatively high pH conditions required for Cd adsorption. Although Pb did appear to form ternary complexes with SO4, Pb adsorption onto both AMD iron(III) oxy hydroxy sulfate and synthetic schwertmannite was less than adsorption onto two-line ferrihydrite.
Introduction In aquatic environments, hydrous oxides of iron(III) play an important role in the transport and attenuation of trace metals, regulating dissolved trace metal concentrations through adsorption. The degree of trace metal adsorption onto pure amorphous hydrous iron(III) oxides and onto goethite has previously been determined (1-3), and the results have been used to derive surface complexation constants. Surface complexation constants for adsorption onto pure hydrous iron(III) oxide have been compiled (4) and are incorporated into the database for the MINTEQA2 chemical speciation model (5). However, the ability of models such as MINTEQA2 to accurately predict trace metal adsorption in natural systems, even in Fe-rich acid mine drainage (AMD) systems (e.g., refs 6 and 7), can be limited by the fact that natural iron(III) oxides include variable concentrations of structural and adsorbed inorganic and organic impurities. * Corresponding author phone: 011-649-623-5600; fax: 011-649630-9619. S0013-936X(97)00439-2 CCC: $15.00 Published on Web 03/28/1998
1998 American Chemical Society
The dissolved Fe(III) ion activities in AMD systems, for example, are rarely consistent with precipitation of pure ferrihydrite or jarosite (8, 9). Instead, the ochreous oxide precipitates are poorly crystallized oxy hydroxy sulfates of iron(III) (10), containing up to 14 wt % SO4. The oxides typically comprise schwertmannite, poorly ordered goethite, and/or jarosite depending on precipitation conditions, particularly solution pH (11). As noted by Bigham et al. (10): “Because of its abundance and high surface reactivity, (iron(III) oxy hydroxy sulfate) should play an important role in regulating the solubilities of both major and trace elements in surface waters impacted by acid mine drainage”. The aim of this study was to quantify Pb, Cu, Zn, and Cd adsorption onto a natural AMD iron(III) oxy hydroxy sulfate (hereafter referred to as AMD oxide) through the determination of adsorption edges. Adsorption edges (adsorption as a function of pH) were compared to those obtained for synthetic schwertmannite and for pure hydrous iron(III) oxide. Further adsorption experiments were subsequently undertaken to identify the principal factors inhibiting or enhancing metal adsorption at the natural AMD oxide surface.
Methods Oxide Preparation or Synthesis. Natural AMD oxide samples were collected from acidic drainage below a sulfide-bearing tailings dam at the former Tui Pb-Zn mine, near Te Aroha at Coromandel, New Zealand (12). The drainage had a pH of 2.8 and a SO4 concentration of 0.017 mol kg-1 at the time of sample collection. The oxide was washed in deionized water, wet sieved through a 85 µm nylon mesh, and then stored as an acidic slurry (pH 1.5-2.0, 25 wt % oxide). Prior to use, aliquots of the slurry were placed in 0.1 mol kg-1 NaNO3 at pH 3.0-4.0 overnight to further encourage desorption of existing metals from surface sites. Synthetic two-line ferrihydrite (hereafter referred to simply as ferrihydrite) was precipitated from a solution of 1.0 × 10-3 mol kg-1 Fe(NO3)3‚9H2O and 0.1 mol kg-1 NaNO3 by gradually raising the pH from 2.0 to 8.0 by the addition of aliquots of NaOH while continuously stirring. After the pH had been raised to 8.0, the oxide was aged for a period of 24 h before adsorption experiments were undertaken. The oxide formed as a red/brown, loose gelatinous precipitate. Synthetic schwertmannite was prepared in a similar manner, but precipitated from a solution containing 0.01 or 0.02 mol kg-1 SO4. A peristaltic pump was used to slowly transfer 5.0 × 10-3 mol kg-1 NaOH into the solution, gradually raising the pH from 2.0 to 5.0 over a period of 30 h. The precipitate was aged in the SO4-rich solution at pH 5.0 for 24 h. The SO4-rich supernatant was removed with a syringe prior to adsorption experiments. Precipitates formed in this study were ochreous yellow/brown in color, adhered to the precipitation vessel, and had an average SO4 content of 11 wt %. The concentration of NO3 in the oxide was consistently low (0.01 mol kg-1. All synthetic oxides were precipitated under atmospheric conditions rather than under an inert gas to minimize the differences between natural and synthetic precipitation processes. The results of a preliminary test of Cu adsorption onto ferrihydrite in which reactants were prepared, stored, and used under N2 were identical to those for Cu adsorption under atmospheric conditions for the neutral to acid pH conditions of interest. VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1361
FIGURE 1. SEM micrographs (magnification 2.84 × 103) and XRD spectra for two-line ferrihydrite (a), synthetic schwertmannite (b), and natural AMD oxide from the Tui mine drainage (c) where G is goethite and Q is quartz peaks. Oxide Characterization. The concentration of SO4 adsorbed on the surface of the oxide was estimated by the addition of 0.05 mol kg-1 Ba(NO3)2 to oxide suspensions, which were then held at pH 3-3.5 for 24 h while BaSO4 formed with surface-adsorbed SO4, in preference to SO4 that had been incorporated into the oxide structure (10). The suspensions were filtered and rinsed, and the oxide was digested as for a standard SO4 analysis (see below), but final 1362
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
digestion pH was raised to 7.0 to reprecipitate BaSO4, which was then removed by filtration. Particle size was measured on a suspension of oxide in 0.1 mol kg-1 NaNO3 using a laser (Microtrac) ultrafine particle analyzer. The suspensions were allowed to settle for 10-15 min to remove the larger, aggregated particles that have a disproportionate light scattering effect during measurement.
FIGURE 2. Infrared spectra of ferrihydrite, synthetic schwertmannite, and natural AMD oxide. The peaks at 797 and 890 cm-1 are characteristic of goethite, and those between 950 and 1150 cm-1 are characteristic of SO4 and FeSO4. X-ray diffraction (XRD) was used to determine the mineralogy of both synthetic and natural, freeze-dried oxide phases (Figure 1). Difference XRD, comparing spectra before and after an oxalate extraction, was required to identify mineral phases that have low intensity peaks (e.g., schwertmannite and ferrihydrite) in the presence of goethite and quartz. Scanning electron photomicrographs (SEM) of airdried oxide phases are also shown in Figure 1. IR adsorption spectra were collected using a Pye Unicam SP3-200, after combining the oxide with KBr in a solid pellet (Figure 2). Oxalate-extractable Fe(III) was determined using an ammonium oxalate/oxalic acid leach at pH 3 (13). Solubility in oxalate over a 15 min reaction time has been proposed (13) as a technique to differentiate between schwertmannite (soluble) and goethite (insoluble). Surface area was measured on freeze-dried oxides using p-nitrophenol adsorption (14) and confirmed by BET-N2. Adsorption Edge Measurement. Adsorption edges for Cu, Pb, Zn, and Cd were measured in a 0.1 mol kg-1 NaNO3 medium with sorbent oxide concentrations of 1.0 (( 0.1) ×
10-3 mol kg-1 Fe, in 500 mL capacity acid-washed HDPE bottles. Trace metals were added from nitrate salt stock solutions to achieve solution concentrations of 0.5 or 5.0 mg kg-1 as required. Each adsorption edge was measured in triplicate, adjusting pH in ca. 0.5 unit increments from 3.0 to ca. 9.0 using dilute NaOH. Equilibrium tests indicated that there was little change in solution metal concentrations after 1 h for Pb and Cu and after 2 h for Zn and Cd. Consequently, for all adsorption edges, reaction times of at least 2 h were allowed at each pH, and two measurements on the steeper part of the adsorption edge were repeated after 24 h to ensure that there had been no further change in solution metal concentrations. Aliquots of 10 mL were extracted, filtered through a 0.45 µm membrane, and acidified to pH ca. 2.0 with HNO3 to prevent metal loss prior to analysis. The acidified solution was also analyzed for Fe (see below) to ensure that oxide colloids had not passed through the filter membrane. No detectable Fe concentrations were observed in the filtrate (detection limit ) 5.0 × 10-6 mol kg-1). As has been noted in previous adsorption studies of this nature, the adsorption edges determined were not reversible. This may be due to ongoing changes in the oxide surface and/or to metal ions moving to sites with slower desorption reactions; a trend that increases with time after initial adsorption (15). Control experiments without an oxide sorbent present were also undertaken to determine the degree of metal adsorption onto container walls and of metal hydroxide or carbonate salt precipitation. Analytical Methods. Concentrations of Cu, Pb, Zn, and Cd in solution were measured by atomic absorption spectrometry (AAS) or, for low-level Pb, by ICP-mass spectrometry. Sorbent concentrations were measured as Fe on an unfiltered, acidified aliquot of the suspension after addition of KSCN to form the red Fe(SCN)3 complex and using UV/ VIS spectrometry (450 nm; 16). Total Fe, Si, Al, and Mn concentrations in the AMD oxide were determined by XRF analysis of a pressed powder disk (detection limit ) 0.001 wt %). Total organic carbon (TOC) was determined by combustion, using a PE 2400 CHN analyzer. Trace metals were determined by AAS, and Fe was determined by UV/VIS spectrometry, following a hot, concentrated HNO3 acid digestion of the sample. The acid digestion left a residue of fine-grained quartz (and minor feldspar) fragments. The concentration of Cl was also determined on this digest after Hg(SCN)2 had been added to form HgCl2 and release SCN, which was then measured as the red Fe(SCN)3 complex by UV/VIS spectrometry (17). The concentration of SO4 and NO3 in the synthetic and natural oxides was determined by high performance ion chromatography (HPIC), following a hydrochloric acid digestion (0.005 g oxide: 2 mL 1:1 HCl) from which Fe had been removed by extraction into diethyl ether (three sequential extractions into 3 mL of diethyl ether) and Cl had been removed by boiling to near dryness.
Results Natural AMD Oxide Characterization. The synthetic oxides used in this study were pure, well-defined mineral phases: two-line ferrihydrite (Figure 1a) and schwertmannite (Figure 1b). The natural AMD oxide sample was a mixed phase, the principal mineral components of which were poorly ordered goethite and quartz (Figure 1c). Only ca. 10% of the Fe in the AMD oxide was soluble in oxalate after 15 min. This indicates that goethite, which unlike schwertmannite and ferrihydrite is not readily soluble in oxalate (13), is a major constituent. The following observations support this premise: (a) difference XRD, which compared oxide spectra before and after a 4 h oxalate extraction, failed to indicate the presence of significant schwertmannite or ferrihydrite, and (b) IR adVOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1363
sorption spectra of the AMD oxide showed distinctive goethite peaks at 890 and 797 cm-1 (Figure 2). The surface area of freeze-dried natural AMD oxide was 90 ((13) m2 g-1, similar to that typically measured for goethite (18). Freeze-dried synthetic schwertmannite has a surface area of 55 ((6) m2 g-1, somewhat less than the range of 100200 m2 g-1 specified for schwertmannite when it was first recognized as a mineral (19). The ferrihydrite surface area of 237 ((5) m2 g-1 falls within the range of values previously determined by BET-N2 for hydrous iron(III) oxide (4). The suspensions of natural AMD oxide used in the adsorption experiments had a mean aggregate size of 3-4 µm, slightly greater than that of synthetic schwertmannite (ca. 2 µm) and ferrihydrite (1-2 µm) suspensions. After the suspension had been allowed to settle for 24 h, leaving only the finest material in solution, the mean particle size in all cases was 5 nm, which is more indicative of primary particle dimensions. Although the SO4 content of AMD oxides collected from the tailings dam site generally ranged from 5.6 to 11.3 wt %, those used for experimental adsorption edge determination contained 11.1-11.3 wt % SO4. The natural AMD oxide contained very low levels of major ion impurities such as Cl and NO3 (
