Environ. Sci. Technol. 1998, 32, 987-993
Preliminary Studies for Removal of Lead from Surrogate and Real Soils Using a Water Soluble Chelator: Adsorption and Batch Extraction COLBY G. RAMPLEY† AND KIMBERLY L. OGDEN* Department of Chemical and Environmental Engineering, The University of Arizona, Tucson, Arizona 85721
A technique for removing Pb from contaminated soil is to wash excavated soil with a solution containing a chelating agent specific for the heavy metal contaminant of interest. Parameters needed to demonstrate the plausibility of this remediation method include the amount of chelator adsorption to soil and the rate of Pb extraction. This paper investigates these parameters for a newly developed watersoluble chelator, Metaset-Z, which has a high specificity for Pb. Metaset-Z chelated Pb in aqueous solutions on the time scale of seconds. This chelator has a low affinity for quartz. Its adsorption is independent of ionic strength over the range tested. Mini column experiments demonstrated that Metaset-Z adsorption is also independent of polymer, Ca, and Pb aqueous-phase concentrations. Removal of Pb to below EPA requirements was achieved using both surrogate, quartz contaminated soil and a Superfund contaminated soil. A one-site semiempirical, equilibrium reaction model fit the surrogate soil extraction data well; a two-site reaction model fit the contaminated soil data well. Overall, this study shows that Metaset-Z can be used as a batch extractant for Pb-contaminated soil.
Introduction Lead is listed as a contaminant on roughly a third of the sites on the National Priorities List (1) and is the most frequently found metal at hazardous waste sites in the United States (2). Lead contamination is most often due to industrial activities such as battery breaking and recycling, oil refining, paint manufacturing, metal molding and plating, and smelting (3). In a 1993 publication, the U.S. EPA determined that the greatest need for new remediation technologies in the Superfund program was for metals in soil (2). There are two main types of metal contaminated soil remediation: (1) technologies that leave the metal in the soil and (2) technologies that remove the metal from the soil. In the past, standard lead remediation protocol has included technologies from the first category, capping, excavation and off-site disposal, or solidification and stabilization. These methods of cleanup are not environmentally sound as they simply perpetuate cleanup, are a depletion of natural resources, and are increasingly discouraged by regulators (4). Because these methods are imperfect, meaning that * To whom correspondence should be addressed. Fax: (520) 6216048; e-mail address:
[email protected]. † Present address: Motorola, Corporation, Mesa, AZ 85202-1150. S0013-936X(97)00625-1 CCC: $15.00 Published on Web 02/24/1998
1998 American Chemical Society
entrapped metals have a small, but finite permeability, continual monitoring of the disposal sites is necessary. More recently, techniques are being investigated at the laboratory, pilot, and field scale to permanently solve the problem by removing the contaminants from the soil and provide “clean closure” to a site. Pickering (5) identified four ways in which metals are mobilized in soils: (1) changes in the acidity, (2) changes in solution ionic strength, (3) changes in the REDOX potential, and (4) formation of complexes. In practice, acid washing and chelator soil washing are the two most prevalent removal methods. Acid washing, or heap leaching, is a technology that has its roots in the precious metals mining industry. A metal is removed from a contaminated soil by increasing its solubility, driving the metal toward equilibrium in the liquid. The effectiveness of bench scale extraction has been reported, but with a final soil pH near 1, concern for the quality of the cleaned soil is significant. Such harsh treatment would lead to decreased soil productivity, increased contaminant mobility, and adverse changes in the soil’s chemical and physical structure due to mineral dissolution (2). Investigations into treatment alternatives have found chelation technology as a viable choice for lead removal. The advantageous quality of such an approach is its ability to provide “clean closure” to a site, meaning the recovery of lead as a natural resource, and the restoration and redeposition of the soil. By using a chelator with a high selectivity for lead, contaminated soil may be washed, the lead selectively removed to below EPA requirements, and the soil redeposited at the site. The most common chelator studied in the literature is ethylenediaminetetraacetic acid (EDTA) (2). EDTA has been used to remove Pb(NO3)2 from artificially contaminated or surrogate wastes with 40-80% efficiency. Because of the strong chelation nature of EDTA, a method for reuse, possibly electrodeposition, must be developed before such a process would be economically feasible (6). EDTA will chelate a large variety of other metal ions that are present in soils such as Ca which may limit its feasibility in some soils. Also, there are health and safety concerns in the scientific community regarding the use of EDTA (2). An alternative chelating polymer, Metaset-Z, was designed at Los Alamos National Laboratory (LANL) to have a high affinity for lead, to be soluble over a wide pH range, and to have the capability of releasing bound lead under controllable conditions. Pb is tightly bound to the chelator above pH 5.5 and can be removed from the chelator with 90% efficiency or greater below pH 2.5. The goal of this research was to better understand and quantify the behavior of Metaset-Z in both surrogate and contaminated soils. The parameters of interest are the amount of adsorption and desorption of polymer under varying solution conditions such as ionic strength and the presence of other ions (lead and calcium), the rate of lead removal from artificially contaminated soil, and any pertinent equilibrium considerations. Investigative techniques, including batch experiments, soil MARK columns (7), and radiolabeling, were utilized to complete the work.
Materials and Methods Polymer Synthesis. Polymer synthesis was done in small reaction batches of approximately 100 mL. The commercially available base polymer, BASF Polymin Water Free Polyethylenimine (PEI), MW 600-750000, was functionalized with acetic acid groups to create the desired chelating polymer. The trade name for the polymer is Metaset-Z, available from the Microset Corporation. The average MW for the chelator VOL. 32, NO. 7, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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is 100 000. The PEI and reactants were placed into a roundbottom flask with a stir bar and reacted. After reaction, the batch was decanted and the pH adjusted to 5.8 with NaOH. For experiments requiring radiolabel, a minute amount, typically 60 µL, of radiolabeled reactant, was added to the vessel. The reaction mixture was ultrafiltered in an Amicon unit using a 10 000 molecular weight cut off (MWCO) filter to remove all unused reactant, radiolabel, and polymer of low molecular weight, yielding a more homogeneous mixture. The polymer solutions were then dried under vacuum. Polymer Binding Capacity. To determine the amount of lead the polymer is able to bind, a known amount of dry polymer was dissolved in 50 mL of DI water. A known amount of lead standard was then added to the reaction vessels. The samples were ultrafiltered, and the permeate was collected for lead analysis. Using a mass balance on the lead, the amount of lead bound per mass of polymer was established. This experiment has been performed in triplicate by various researchers at LANL and the University. The loading rate of the chelator is typically 0.4 g of Pb/g of polymer. Quartz Preparation/Washing. The granulated quartz, or crystalline silica, was purchased as Unimin Granusil Silica. In the first step of the washing procedure, the quartz was acid washed in 6 M HCl for 24 h. Next, the solution was titrated back to the neutral range (pH ∼7) with NaOH. The quartz was then washed with DI water in an up flow column until the effluent conductivity equaled that of the inlet water. In the fourth step, the quartz was muffled at 800 °C for 8 h in order to remove any organic material that may have been sorbed to the quartz surface. After cooling, the quartz surface was hydrated by adding DI water to the quartz; excess water was removed by boiling (8). Quartz Contamination with Lead. For experiments requiring artificial contamination of quartz, a 100 ppm Pb solution was prepared by combining 0.16 g of Pb(NO3)2 and 0.840 g of NaNO3 in 1 L of DI water. The solution pH was adjusted to 9.0 using concentrated NaOH. In this pH range, Pb(OH)2 and PbOH+ are present in solution, but based on the IEP of the lead species, the Pb(OH)2 will not adsorb to the surface, only the positively charged species will absorb via the surface reaction (9, 10).
PbOH+ + Si-O-H w Si-O-Pb + H2O. An initial sample of lead contaminating solution was taken for later analysis by atomic absorption. Prepared quartz, 100 g, was added to the solution, and after 24 h of mixing, a supernatant sample was taken and the solution discarded, leaving the contaminated quartz behind. The contaminated quartz was washed with DI water a minimum of 10 times to remove any unbound or precipitated lead and then air-dried. The typical amount of Pb adsorbed to quartz was 300 mg/kg. No detectable Pb was present on the uncontaminated quartz. Lead Contamination Quantification. Quantification of the amount of adsorbed lead was done by acid digestion. Approximately 2 g of surrogate waste or contaminated soil and 10.0 mL of concentrated HNO3 were put into a sealed vial. The vial was agitated on a shaker table for 24 h, and then the supernatant was tested for lead concentration by atomic absorption. The lead mass balances always closed. Lead Contaminated Soil. The contaminated soil was obtained in cooperation with the EPA from the Cal-West Superfund site. This site is in Lemitar, NM, and contains 44 acres including a lead-acid battery recycling facility and secondary-lead smelter, which were operated between 1979 and 1981. From 1982 to 1984, the site was used for Research and Development related to lead recovery. The soil is classified as a poorly graded gravely silt (11). The soil contains on average 0.38 mequiv/L sodium, 5.51 mequiv/L calcium, 0.74 mequiv/L magnesium, and 1.9 mequiv/L bicarbonate. 988
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FIGURE 1. Schematic of the MARK column; x is the length of each slice. The sodim adsorption ratio is 0.21. The percent organics is 0.38%. The cation exchange capacity is 4.71 mequiv/100 g. The clay fraction is 6% (10% kaolinite, 20% illite, 10% chlorite, 20% smectite, and 40% is a mixed layer of illite and smectite). MARK Column Experiments. MARK (microbe and radiolabled kinesis) column experiments have been used effectively to investigate factors affecting the transport of biocolloids through saturated porous media (7). This technique of using radiolabeled material to measure the fraction of material retained as opposed to the effluent concentration as a function of time was adapted for this work due to its high sensitivity. The column itself was the bottom barrel of a 3 mL syringe with 0.8 cm ID. The syringes were prepared by inserting a 0.8 cm GF/D filter (Whatman, 2.7 µm nominal pore size) into the bottom of the syringe barrel. The syringe was then packed with 2 g of soil that was preequilibrated with the test solution minus polymer for 24 h. The medium was typically stirred to remove trapped air, ensure homogeneity and provide an even surface at the top of the barrel. The barrel was then rinsed with water of the same ionic composition as the test solution (DI water to 3 M NaNO3 dependent on experimental objectives) until there was no pH change across the column. Figure 1 shows the column when it was fully prepared. Chelator transport experiments were initiated by adding a known volume (between 0.4 and 40 mL) of the test solution, typically containing 0.5% radiolabeled polymer, to the top of the packed mini-column. Using a flow rate of 4 mL/min, the flow conditions were manipulated to ensure laminar flow (Re ≈ 0.1). The column was then rinsed with 8 mL of solution of the same ionic composition to remove residual, unattached label from the column pores. The procedure permitted tightly controlled application of soluble radiolabeled chelator. After rinsing, the column bottom was cut off and the soil pushed upward with the plunger, allowing cross-sectional layers of soil to be sliced (∆x in Figure 1), weighed, and placed into scintillation vials. This technique provided a measurement of retained labeled chelator in the column media as a function of bed depth. If only the effluent concentration was measured the curve would jump from C/C0 value of 0 to values of 1 rapidly because of the small amount of adsorption to the column matrix. The measurement of retained material as a function of depth for small columns at high flow rates is more accurate. Radiolabel counts of soil slices and column effluent were made on a Beckman liquid scintillation counter. The number of counts observed were converted to dry mass of polymer. The conversion factor was experimentally determined to be 5 × 10-5 CPM/mg of polymer. The pore volume for the column is 0.8 mL. Batch Equilibrium Experiments. An important parameter key to understanding and modeling lead removal from
soil is the amount of polymer adsorbed. To investigate this, a known amount of radiolabeled polymer solution along with a saline solution (e.g., NaNO3) was added to a sealable centrifuge tube. A 1 mL sample was taken to find the initial concentration. Quartz sand or contaminated soil, 5 g, was added to the solution. The mixture was then agitated for 24 h on a shaker table. After agitation, the experiment continued much the same as the MARK columns. Soil samples were poured into the syringe columns with GF/D frits in the bottom to retain the soil, and were rinsed with 10 pore volumes of the test without polymer solution (water at the pH and salt concentration as the solution containing the polymer). The liquid was drained and the damp quartz or soil was placed into preweighed scintillation vials for drying. The vials were reweighed and then analyzed for adsorbed polymer by liquid scintillation. This method of removing excess polymer was used because the amount adsorbed was so small that measuring differences in the liquid solution was not feasible. Therefore, the amount of polymer adsorbed to the solid was measured. Determination of Pb Removal Rates. Batch experiments were done by agitating 2-10 g of contaminated or surrogate soil and 0.03-0.5% chelator in 0.05 M NaNO3 for 24 h. Between 3 and 15 times the minimum amount of polymer required (based on the binding capacity) was used to ensure an adequate amount of polymer was present. Samples of the solution were taken periodically for Pb analysis. The Cal-West samples were centrifuged to settle any soil particles, and 1 mL of liquid was taken for analysis. Both atomic and inductively coupled plasma were used to analyze the solution for lead. Control experiments containing 0.05 M NaNO3 and soil were run in parallel for comparison. Lead Analysis. Lead analysis was performed two different ways. The Cal-West soil was analyzed using a Varian Liberty 220 Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) unit at LANL. The ICP was calibrated using standards containing lead and any appropriate ionic species, such as NaNO3. Prepared samples were injected into a chamber and were exposed to argon plasma at temperatures up to 10 000 K; the resulting atomic emission spectra were analyzed. The detection limit for Pb is 1 ppm. The surrogate soil analysis was done using a Perkin-Elmer 2380 Flame Atomic Absorption (FLAA) at the University of Arizona. Using an air/acetylene flame, atomic absorption of acidified (5% HNO3) samples were analyzed against standards. The detection limit is 10 ppm. All standards were prepared to contain the same ionic species as the samples, using Fisher brand lead standard as the lead source. For both systems, standards were retested after every five samples and the system recalibrated as necessary. Toxicity Characteristic Leaching Procedure (TCLP). The TCLP is a standardized EPA procedure, Method W1650, used to test the amount of leachable metal in contaminated soils. It was used in this work to determine the leachable lead in surrogate and actual contaminated soils before and after chelation experiments. The first step was to add 11.4 mL of glacial acetic acid and 12.9 mL of 10 M NaOH to 1975 mL of DI H2O/100 g of soil sample, yielding a pH equal to 4.93 ( 0.05. This mixture was rotated at 30 rpm for 18 ( 2 h. After the samples settled, they were filtered through a 0.8 µm filter or were centrifuged. Two milliliters of concentrated nitric acid/500 mL of filtered sample were added for storage. For ICP-AES analysis, EPA Methods 3010 and 6010 were utilized in preparing the sample. EPA Method 7000 is used for FLAA analysis.
Results and Discussion The goal of this research is to understand the behavior of the chelator, Metaset-Z, during contaminated soil remediation
processes. To that end, batch and MARK column experiments were completed using radiolabeled polymer. Batch experiments without solids present determined the binding capacity of the polymer for lead and the rate of lead chelation. Batch experiments with solids quantified polymer adsorption under static conditions and measured lead release kinetics. MARK column experiments measured the rate of polymer adsorption and the effects of solution chemistry. All experiments were run at pH 5.8, which was the pH of the final polymer solution. Aqueous-Phase Pb Chelation. The first step for characterizing the polymer was to determine the binding capacity for Pb. In liquid studies using the polymer and lead dissolved in DI water, Metaset-Z was found to bind 0.4 mg of Pb for every 1 mg of polymer over a pH range of 5.5 to 7. The second step was to determine the rate at which Pb in liquid solution was bound by the polymer. To determine this, liquid cultures containing a known amount of lead and excess polymer were added to test tubes at pH 6. Immediately (less than 10 s) after mixing, the solution was filtered though a 10 000 MW filter. The filtrate was analyzed for Pb and none was detected. Thus, it was concluded that Metaset-Z chelates Pb in solution on the time scale of seconds or less, and therefore, this will not be the rate-limiting step for Pb removal from soil; processes will either be limited by desorption of the polymer into solution or by mass transfer. Chelator Adsorption to Soil. To establish an equilibrium isotherm for polymer adsorption to quartz and understand the effects of solution chemistry, batch adsorption experiments were run under a variety of solution conditions. The tested ionic concentrations were DI water (0%), 0.05 M, 1.3 M (1%), 2.6 M (2%), and 3.9 M (3%) NaNO3. The results for adsorption of Metaset-Z are shown in Figure 2. Polymer adsorption was not affected by the presence of NaNO3 over the concentration range investigated. A statistical analysis of the data showed no differences within a standard deviation. Polymer concentrations of less than 0.05 mg/mL were not performed because of detection limit problems, and because applied soil washing extraction would be done at polymer concentrations on the order of 1% polymer (10 mg/mL). These results show that only a finite amount of polymer will adsorb onto the quartz surface. In similar work, effects of solution chemistry on hydrolyzed polyacrylamide (HPAM) adsorption to silica were studied by Szabo (12). He observed an increase of polymer adsorption with increasing NaCl concentration when the NaCl was increased from 0 to 2%. However, when the NaCl concentration was increased from 2 to 10%, the same amount of adsorption was observed. This result was explained by looking at the conformational changes that occur during the addition of ionic species. HPAM is an ionic polymer, meaning that it has charge centers located intermittently on the polymer. These charge centers repel each other when no ions are present and the polymer is fully extended. However, when ionic species are added, the expansion of the polymer is suppressed by the screening effect the ionic species have on the electrostatic repulsion between carboxylate groups, this phenomenon is known as the Debye-Huckel effect (13). The polymer studied in this research is an anionic polymer, with carboxylate groups; and thus, based on literature, its adsorption was expected to vary with salt concentration. However, adsorption of Metaset-Z did not vary with ionic strength. One possible explanation is that the polymer is sufficiently shielded prior to the addition of any ionic species, meaning that an adequate number of cations are present in solution from polymer synthesis to shield the polymer’s electrostatic repulsion. To test this hypothesis, the ionic strength of the DI water (6 µS/cm) and the DI water containing the polymer (650 µS/cm) were determined. When the VOL. 32, NO. 7, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Batch adsorption of Metaset-Z to quartz at various ionic strengths. (0) DI water; (]) 0.05 M NaNO3; (O) 1.3 M NaNO3; (4) 2.6 M NaNO3; (9) 3.9 M NaNO3.
FIGURE 3. Retention of Metaset-Z as a function of bed depth in MARK columns. ([) 5 pore volumes; (0) 50 pore volumes. polymer is present, the ionic strength is significantly increased. Stuart et al. (14) termed this lack of dependence of polymer adsorption on ionic strength a “pseudo-plateau” characteristic. The plateau is observed as the osmotic forces become greater than the attractive forces over a very small range of surface coverage, making the amount of adsorption constant over a wide range of solution concentrations. The MARK column allows for the measurement of polymer retention in a flow-through situation for comparison with batch experiments. Also, these experiments test what type of adsorption was taking place; i.e., whether the polymer 990
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exhibits some type of first-order adsorption characteristics (7). In Figure 3, the amount of polymer retention is graphed as a function of bed depth. Polymer retention is graphed in units of mass polymer retained/total mass polymer passed through column (M/M0). Polymer concentration did not vary with depth and did not exhibit the expected first-order adsorption characteristic. Instead, only a finite amount of polymer adsorbed to the quartz. For example, when M0 was increased by a factor of 10, the amount of polymer adsorbed decreased by a factor of 10 resulting in the same amount of total polymer adsorbed (Table 1). This result indicated that polymer adsorption was independent of concentration.
TABLE 1. Effects of Concentration on Polymer Adsorption volume of polymer polymer added (mL) concentration amount retained (pore volume) (mg of polymer/mL) (mg of polymer/g of quartz) 40 (50) 4 (5) 0.8 (1) 0.4 (0.5)
7.6 4.9 4.7 4.7
0.09 0.08 0.06 0.06
TABLE 2. Effects of Ca and Pb on Polymer Adsorption solution chemistry 0.0006 M Ca(NO3)2 0.006 M Ca(NO3)2 0.06 M Ca(NO3)2 Pb bound polymer in DI
polymer concentration amount retained (mg of polymer/mL) (mg of polymer/g of quartz) 5.3 5.3 5.3 3.4
0.08 0.07 0.07 0.07
Experiments were then performed at lower M0 values to further investigate polymer adsorption. As shown in Table 1, the amount of polymer retained did not significantly change even when only 1 pore volume of material was passed through the column, again suggesting polymer retention is independent of concentration over the range tested. The amount of adsorption of polymer in the MARK columns agrees well with the adsorption of the polymer in the static adsorption experiments. This result suggests that the polymer reaches its equilibrium value quickly, as the MARK column experiments have a contact time on the order of 10 s, compared to 24 h of contact time for the batch experiments. Thus, MARK experiments were used to study a variety of soil conditions and water chemistries since the experimental procedure is simple and fast and therefore a useful tool for comparison studies. Previous work done in collaboration with LANL showed that Metaset-Z, although specific for Pb, will bind some Ca (15). For example, in a 24 h batch extraction experiment of soil contaminated with Pb, Mg, Fe, Mn, and Ca, 97% of the Pb was removed and 2% or less of the Mg, Fe, and Mn were removed, but 10% of the Ca was removed. Thus, the effect of Ca on polymer adsorption was examined in this study. The results of the MARK experiments for varying Ca concentration are shown in Table 2. The presence of calcium over a range of ionic strength of 0.0006-0.06 M was not shown to have any effect on polymer adsorption. It was also desirable to determine if Metaset-Z fully chelated with Pb (0.4 g of Pb/g of polymer) would behave similarly to unchelated polymer or if it would bind more readily to quartz. To determine this, a known amount of lead was added to the polymer solution to fully bind the chelator. The results showed (Table 2) that the amount of polymer retained per gram of sand is approximately the same (0.08 mg of polymer/g of quartz), whether the polymer is fully bound with lead. Changing the solution composition in MARK columns made no measurable effect on the amount of polymer adsorption, even when bound with lead or calcium. Conclusions made in the discussion of the batch experiments regarding the addition of ionic species apply here as well. The polymer is believed to be sufficiently shielded by ions already present in the solution. Thus, the addition of any cationic species, whether bound by the chelator or not, does not affect the polymer’s conformation in solution and, therefore, the degree of adsorption. Because only 0.08 mg of Metaset-Z adsorb to 1 g of quartz under a variety of solution conditions, the chelator has a great potential for Pb remediation of quartz-like contaminated soils.
Uncontaminated soil taken from nearby the Cal-West Superfund site was tested in MARK columns. For the entire column, 0.242 mg of polymer/g of soil was adsorbed. The increased amount of adsorption is attributed to either increased surface area and/or a higher affinity of the polymer to other soil compounds. A systematic study of Metaset-Z adsorption to a variety of minerals and soil types would be needed in order to fully understand its adhesion. However, these initial studies with actual contaminated soil demonstrate that Metaset-Z, although it adheres more to actual soils than surrogate soils, may not adhere significantly to natural soils, thus improving its potential for Pb remediation. Pb Chelation from Soil. After determining that Metaset-Z rapidly chelates soluble Pb and does not have a high affinity for quartz, soil chelation experiments were performed. Both surrogate and actual contaminated soils were investigated and compared. Figure 4 shows the batch extraction of the surrogate waste, lead contaminated quartz. To test for the presence of polymer mass transfer effects, three different initial polymer concentrations were used: 0.315, 0.909, and 1.52 mg of chelator/mL, which were approximately 3, 9, and 15× excess polymer for the total amount of lead present. Parallel control experiments without polymer showed no removal of lead from the quartz. The results showed that chelation rate was independent of polymer concentration over the range tested. The amount of lead retained in the sample was determined after polymer extraction by soil acid extraction. The Pb mass balances closed. Because a similar amount of lead was retained by the quartz, even at high concentration gradients with the polymer, the model used to fit extraction data was one with an equilibrium surface concentration. Also, since the surface reaction for Pb adsorption is known (10) and since Pb does not adsorb tightly to quartz (16), a first-order, one-site semiempirical model was used:
dCs ) -k(Cs - Cse) dt
(1)
where Cs (mg of lead/g of quartz) is the surface lead concentration on quartz, Cse (mg of lead/g of quartz) is the equilibrium quartz lead concentration or the amount still retained on the surface of the quartz, and k (h-1) is the fitted rate constant. Integration of eq 1, using the boundary condition of Cs ) Cs0 at t ) 0, leads to the expression
(Cs - Cse) ) (Cs - Cse)0e-kt
(2)
TableCurve, a curve fitting software package, was used to find the best fit curve for the extraction, yielding an r2 ) 0.94. The final form of the equation containing the fitted parameters Cse and k is
(Cs - 0.05) ) 0.26 e-8.0t
(3)
where t is in hours. This one-site, first-order equilibrium model describes the data well (Figure 4). The removal efficiency of the lead was 90%. The surrogate waste soil was successfully cleaned to below EPA standards in a single batch extraction with chelator. However, some lead remained on the quartz surface. Because lead does not bind tightly in artificially contaminated soil (16), it is hypothesized that the lead was no longer adsorbed directly to the quartz surface, but instead was chelated to adsorbed Metaset-Z. The following simple calculation supports this hypothesis: in the experiments, 10 g of contaminated quartz were used; which, at 0.08 mg of polymer adsorbed/g of quartz, would mean approximately 0.8 mg of polymer adsorbed at the surface. Using the loading rate of 4 mg of lead/10 mg of polymer, a maximum of 0.32 mg of VOL. 32, NO. 7, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Batch extraction of Pb from surrogate quartz waste; (]) 3× excess chelator; (0) 9X excess chelator; (4) 15X excess chelator; (s) one site, first-order equilibrium model.
FIGURE 5. Batch extraction of Pb contaminated Cal-West soil; 0.5% chelator. ([) Experimental data; (s) best fit one-site model and prediction based on surrogate soil, one-site model; (‚‚‚) best fit based on two-site model. lead could be chelated to adsorbed polymer. The average amount of total lead not extracted into solution by the chelator was also 0.32 mg. After successfully removing lead from a surrogate soil, it was desirable to extract lead from an actual contaminated soil obtained from the Cal-West Superfund Site. The rates were expected to be different since the soil was weathered, thus, the lead was more tightly bound to the surface. The results of the extraction are presented in Figure 5. Lead was removed to below EPA required levels based on TCLP analysis after 10 h. Control experiments done without chelator showed no lead removal. Originally, eq 2 was fit to these 992
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data; k ) 0.44 h-1; Cse ) 0.06 mgof Pb/g of soil; Cs0 ) 0.92 mg of Pb/g of soil. However, there was not good agreement between data and the first-order equilibrium equation. In particular, the agreement was unacceptable at long times (>10 h). Next, the data were fit with two parallel, first-order, irreversible reaction rates. Multiple site, first-order reaction models have been widely used by soil chemists to describe the extraction of metals from soil (17-21). Two removal rates corresponds to the presence of two discrete binding sites for the lead, one from which lead is easily removed, and one from which removal is more difficult (22). The fit equation is
(Cs - Cse) ) a1(1 - e-1.52t) + a2(1 - e-0.177t)
(4)
where a1 is the amount of lead removed by the fast reaction and a2 is the amount of lead removed by the second reaction and t is in hours; Cse ) 0.05 mg of Pb/g of soil. The curve fit yielded an r2 ) 0.998. Comparing the relative amounts, 48% of the lead was removed by the fast reaction, and 52% was removed by the slower reaction. The overall removal efficiency of the lead was approximately 85%. A two-site reaction model has also been used to describe chelation of Cu, Zn, Fe, and Mn by EDTA (23). Good agreement was obtained for the Fe and Mn data; however, an empirical model better described the Cu and Zn extraction data. Empirical models were not used here because, as Yu and Klarup (23) point out, these models are not based on a physical mechanism. Also shown in Figure 5 is the predicted rate of lead extraction based on quartz, surrogate soil experiments (eq 3). The surrogate results predict that 95% of the lead would be removed after only 1 h, when actually 95% of the lead was removed after 12 h. This is a large time difference which is important when designing both batch and continuous flow extraction procedures and reinforces the necessity for actual site trials. Overall, Metaset-Z was shown to effectively remediate lead contaminated soils in a batch extraction process. The polymer does not adhere greatly to actual soils whether or not it is bound with lead, and removes lead to below the TCLP limits. The rate constants indicate that lead removal occurs on the time scale of hours and thus is a feasible method for site remediation. The chelation process appears to be relatively insensitive to ionic strength over the typical range found in ground or tap water. In addition, the process is not affected by the presence of calcium. Some preliminary comparisons can be made to other chelators such as EDTA and NTA. First, this chelator is more selective for calcium than EDTA and has comparable selectivity for lead over calcium as NTA. Also, the removal efficiency for lead from surrogate soils in batch experiments using Metaset-Z is greater than that observed by other researchers who used EDTA to extract lead from surrogate soils [90% compared to 60-80% (6, 22)]. Although the work presented here is very encouraging, the total remediation potential of Metaset-Z will not be known until continuous flow experiments with actual soils are completed and modeled; these experiments are the focus of our current work.
Acknowledgments We would like to acknowledge Nancy Sauer and Debra Ehler (LANL) for providing the Metaset-Z. This work was funded by the Waste Management Symposium, Tucson, AZ.
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Received for review July 15, 1997. Revised manuscript received January 5, 1998. Accepted January 20, 1998. ES9706256
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