Environ. Sci. Technol. 2006, 40, 1792-1798
The Effect of Aging and pH on Pb(II) Sorption Processes at the Calcite-Water Interface A S H A K I A . R O U F F , * ,† EVERT J. ELZINGA, AND RICHARD J. REEDER Department of Geosciences and Center for Environmental Molecular Science, Stony Brook University, Stony Brook, New York 11794 NICHOLAS S. FISHER Marine Sciences Research Center and Center for Environmental Molecular Science, Stony Brook University, Stony Brook, New York 11794
The effect of aging on Pb(II) retention in 1 µM Pb, calcite suspensions at pH 7.3, 8.2, and 9.4, under roomtemperature conditions, was explored via a combination of batch sorption-desorption experiments and X-ray absorption spectroscopy (XAS). Short-term experiments, up to 12 days, reveal the predominance of an adsorption mechanism at pH 8.2, as confirmed by XAS analysis. Linearcombination fitting of XANES spectra indicates a dual sorption mechanism, with ∼95% adsorbed and ∼5% coprecipitated, and ∼75% adsorbed and ∼25% coprecipitated Pb at pH 7.3 and 9.4, respectively. For long-term sorption, 60-270 days, slow continuous uptake occurs at pH 7.3 and 8.2, determined by EXAFS to be due to an adsorption mechanism. At pH 9.4, no further uptake occurs with aging, and the solid-phase distribution of Pb is commensurate with that for short-term experiments, suggesting that coprecipitated metal may alter the calcite surface precluding further Pb sorption. Desorption experiments indicate that at pH 7.3 and 8.2 long-term sorption productssconstituted primarily of Pb inner-sphere adsorption complexessare reversibly bound. For aged pH 9.4 samples, significant sorption irreversibility indicates that the coprecipitated component is not readily exchangeable with the aqueous phase, and thus coprecipitation may be effective for long-term metal sequestration.
However, short-term studies, up to several days, under variable conditions, have implemented spectroscopic techniques to identify relevant Pb sorption mechanisms, including the local coordination environment of adsorption complexes and precipitates (1), as well as Pb in the calcite structure (2, 3). Increased sorption time can reduce trace metal desorbability by enhancing the stability of surface complexes (4) or via mechanisms such as microporous diffusion (5), recrystallization-induced incorporation (6), and formation and stabilization of surface precipitates (7). For Pb sorption with hydrous iron oxide (6), goethite (8), and Pb-contaminated soils (9), results for aged samples indicate the presence of reversibly bound Pb. This suggests metal association with the sorbent surface, and thus the dominance of adsorption with insignificant effects of aging on this process for these substrates. For Pb sorption with calcite, though, aging effects may come into play due to the observance of multiple sorption mechanisms even for short sorption times. For calcite, divalent metals such as Mn (10), Co (11), and Cd (10, 12, 13) display a continuum from adsorption to solidsolution formation for sorption times of one to several days. The influence of aging on these processes, however, has been primarily explored for calcite crystals initially sorbed with metal, removed from the aqueous phase and subsequently stored for months to several years (13, 14). The current study differs from the aforementioned work in that the effects of aging on Pb sorption were examined in situ: in Pb-calcite aqueous suspensions under conditions of ambient temperature, atmospheric PCO2(g), and controlled solution pH. Short-term experiments have elucidated dominant Pb sorption mechanisms with calcite substrate, at ambient temperature and atmospheric PCO2(g), in pH 7.3, 8.2, and 9.4 solutions, covering the pH range of the Pb sorption edge (15), for sorption times up to 12 days (4). Results from macroscopic experiments combined with X-ray absorption spectroscopy (XAS) indicate the presence of Pb inner-sphere adsorption complexes at pH 8.2. At pH 7.3 and 9.4, in addition to adsorbed Pb, a coprecipitated component was detected, accounting for ∼5% and ∼25% total sorbed Pb, respectively. In the current study, both macroscopic and spectroscopic techniques were implemented to determine the speciation of sorbed Pb for pH 7.3, 8.2, and 9.4 samples reacted for 60 to 270 days. Results are compared to those previously obtained for shorter sorption times, and are used to assess the efficacy of long sorption times for Pb sequestration by calcite as a function of pH.
Experimental Section Introduction Elucidating Pb(II) interactions with calcite is fundamental for predicting Pb behavior in soils and other near-surface geochemical systems in which calcite may be a prominent mineralogical constituent. The efficacy of Pb sequestration is contingent upon the existent physical and chemical conditions of the system, including solution pH and contact time, which can dictate Pb speciation in both the solid and aqueous phases. Long-term Pb-calcite interactions, on the order of months to years, have not been documented. * Corresponding author phone: (312) 413-8271; fax: (312) 4132279; e-mail:
[email protected]. † Current address: Department of Earth and Environmental Sciences, University of Illinois at Chicago, MC 186, 845 W Taylor Street, Chicago IL 60607-7059.. 1792
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 6, 2006
Macroscopic Sorption-Desorption Experiments. For macroscopic sorption experiments, pH 7.3, 8.2, and 9.4 solutions were prepared using the following solution chemistry. pH 7.3: 2.3 × 10-4 M NaHCO3, 3.8 × 10-2 M Ca(NO3)2; pH 8.2: 1.4 × 10-1 M NaNO3; pH 9.4: 2.3 × 10-2 M NaHCO3, 1.1 × 10-1 M NaNO3. Solutions were equilibrated with calcite at atmospheric PCO2(g) (10-3.5 atm) and ambient temperature (∼20-22 °C) for two weeks, during which acid as HNO3 (∼1.79 × 10-2 M total) or base as NaOH (∼8.3 × 10-3 M total) was added to pH 7.3 and 9.4 solutions, respectively, to maintain the desired pH. Filtered solutions were treated with 10-6 M Pb as Pb(NO3)2 containing 0.3% 210Pb tracer to yield a total activity of 14.6 kBq. A reagent calcium carbonate powder, confirmed as pure calcite by X-ray diffraction with a specific surface area of 0.2 m2g-1 and average particle size of ∼5 µm, was used as the sorbent. The calcite was suspended in deionized water and 10.1021/es051523f CCC: $33.50
2006 American Chemical Society Published on Web 02/07/2006
equilibrated with atmospheric PCO2(g) for a two-week period, with pH stabilizing at ∼8.2. Suspensions were filtered, and residual calcite dried and added to the pH solutions at a loading of 0.5 gL-1. For each pH, duplicate samples as well as calcite-free blanks were prepared. Samples were reacted for 180 days, after which aliquots were removed to assess total metal concentrations, and residue from filtered aliquots was used to determine the concentration of sorbed Pb. Total radioactivity of the aliquots and that on the filters were measured at the 46.5 keV emission energy of 210Pb via gamma spectrometry using an LKB Compugamma counter with a NaI (Tl) well detector. The fraction of sorbed Pb was quantified as the quotient of the counts on the filter (minus blank values) and the counts obtained in the total suspension, modified by the percentage of radioactive Pb in the system (16). Desorption was initiated by filtering the remaining volume of suspension, rinsing with preequilibrated solution, and resuspending collected solid in identical, but metal-free solution at the same particle loading. Upon resuspension, pH varied
