Environ. Sci. Technol. 2002, 36, 1851-1855
In Situ Remediation of Groundwater Contaminated by Heavy- and Transition-Metal Ions by Selective Ion-Exchange Methods M A R K Y . V I L E N S K Y , †,‡ B R I A N B E R K O W I T Z , * ,‡ A N D A B R A H A M W A R S H A W S K Y †,§ Department of Organic Chemistry, and Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel
Laboratory studies were conducted to investigate the feasibility of using ion-exchange resins in permeable reactive barriers (PRBs) for the remediation of groundwater contaminated by heavy and transition metals. Ion-exchange resins represent an essentially neglected class of materials which may, in addition to iron, activated carbon, and zeolites, prove effective for use in PRBs. Four resins were considered: two commercially available resins, Duolite GT73 (Rohm and Haas) and Amberlite IRC-748 (Rohm and Haas), and two solvent-impregnated resins (SIRs). The SIRs were prepared from Amberlite IRA-96 (Rohm and Haas) and two different thiophosphoric extractants. All four resins are able to reduce cadmium, lead, and copper concentrations from 1000 µg/L (typical for contaminated groundwaters) to below 5 µg/L. Significantly, all of the resins are effective for the capture of cadmium, copper, and lead, even in the presence of CaCl2 and clay. Because of their high hydraulic conductivity, the use of these resins in clusters of wells, as an alternative to continuous walls, is considered in the design of effective PRBs. Numerical solution of the groundwater flow equations shows that, depending on the well configuration, most (or all) of the contaminated groundwater can pass through the resins. These results demonstrate the possibility of using selective ion-exchange resins as an effective, active material in PRBs for in situ groundwater remediation.
Introduction An important class of chemical wastes found in groundwater includes heavy- and transition-metal ions, such as nickel, cobalt, chromium, copper, lead, mercury, and cadmium. The treatment of groundwater polluted by these ions is very problematic, because their concentrations are very low (e.g., 100-500 µg/L), and the water itself is found at depths of several to hundreds of meters below the ground surface. Another aspect that complicates remediation is the coexistence of alkali and alkali-earth metals in much higher concentrations (30-300 mg/L). And yet, treatment of these contaminants is essential because of their high toxicity to humans and other organisms (1, 2). * Corresponding author phone: 972-8-934-2098; fax: 972-8-9344124; e-mail:
[email protected]. † Department of Organic Chemistry. ‡ Department of Environmental Sciences and Energy Research. § Deceased. 10.1021/es010313+ CCC: $22.00 Published on Web 03/16/2002
2002 American Chemical Society
Standard above-ground water treatment methods do not generally provide a suitable answer to this problem. Methods of converting soluble metal ion salts to the corresponding hydroxides are not useful, because the sparingly soluble hydroxides are still soluble at those concentrations. Calculated (3) equilibrium concentrations of cadmium, lead, and mercury, over the entire pH range, highly exceed permitted values (4). For example, as shown in Figure 1, dissolved cadmium, lead, and mercury complexes (1 mg/L initial concentration of each metal) remain high in the presence of 0.02 M Cl- and 0.01 M Ca2+. Other methods, such as electrochemical reduction, are very expensive (5). Current approaches under investigation for the treatment of transition-metal ions focus on emplacement of permeable reactive barriers (PRBs), usually consisting of zerovalent iron particles, below the ground surface. Iron reduces mobile and soluble ions such as CrO4- to the insoluble, and therefore immobile, form Cr3+. However, PRBs constructed with zerovalent iron are nonspecific and react with a wide spectrum of dissolved compounds, leading to a period of life for these kinds of barriers which is shorter than would be expected from stoichiometric considerations (6-8). In addition to iron and its compounds, zeolites are being developed for metal contaminant removal. Inorganic ion exchangers are usually natural or synthetically prepared zeolites. Zeolites are inexpensive and, because of their stability, they are used to treat water containing radioactive contaminants (9, 10). Synthetic inorganic ion exchangers display high selectivity (11) but slow kinetics, low capacity (10-3 mol/kg), and low hydraulic conductivity (12). Materials from biological sources (e.g., sea weed, algae, and bacterial biomass) have been tested for their ability to treat contaminated water. These materials have, in general, poor selectivity (13, 14) and low breakthrough capacity (14, 15). Another class of materials tested for ion-exchange ability includes humic acids and peat. Humic acids have good selectivity toward heavy metals; however, the capacity of this material is only about 10-2 mol/kg (16). Also, peat has been found to be highly effective, with heavy metals being reduced to concentrations of about 3 µg/L, albeit in the absence of calcium and magnesium (17). Activated carbon is not an ion exchanger, but it adsorbs contaminants; therefore, it is important to mention it here. Regular activated carbon has poor selectivity, but it can be modified by oxidation to introduce carbonyl, carboxyl, and nitro groups (18). Modified activated carbon has a higher selectivity but still a low sorption capacity (about 100 times lower than resins) (19, 20). Significantly, in an extensive review, Scherer et al. (21) discuss and contrast a wide variety of materials used for PRBs but make no reference to ion-exchange resins. Ionexchange resins consist of solid, insoluble matrixes carrying functional groups that can exchange cations or anions with an electrolyte solution. The reaction of the dissolved ions with functional groups results in the creation of coordinating or electrostatic bonding. On the other hand, in hydrometallurgy and some industrial wastewater treatment schemes, ion-exchange resins have been used successfully to selectively separate metal ions in relatively high concentrations (1-10 mg/L). Many ion-exchange applications have been designed with an emphasis on recovering metals of interest, such as noble or rare earth metals or strategic metals such as uranium, emanating either from metallurgical sources or wastewater from different plants and water bodies (22). In all of these cases, metal concentrations are in the range of tens of mg/L, while the wastewater pH lies in the range of 2-6 (23). VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Dependence of total molar concentration of dissolved cadmium, lead, and mercury complexes on pH in the presence of 0.02 M Cl-, 0.01 M Ca2+, and 1 mg/L initial concentration of heavy metals (8.896 × 10-6 M, 4.831 × 10-6 M, and 5.00 × 10-6 M, respectively). The lowest possible concentrations of cadmium, lead, and mercury that can be achieved by precipitation are 2.1 × 10-7 M, 9.37 × 10-10 M, and 5.0 × 10-6 M, respectively, which equals 24 µg/L cadmium, 0.19 µg/L lead, and 1000 µg/L mercury. The concentrations of cadmium and mercury are higher than that allowed by drinking water standards set by the U.S. Environmental Protection Agency (4). The data were calculated using the HYDRAQL program (3).
TABLE 1. Properties of the Resins Useda
a
name
polymer type
functionality
ion-exchange capacity, eq/L
Duolite GT-73 Amberlite IRC-748 Amberlite IRA-96
polystyrene-divinylbenzene polystyrene-divinylbenzene polystyrene-divinylbenzene
thiol in H+ form iminodiacetic in Na+ form tertiary amine as free base
1.35 1.0 1.25
Reference 35 provides background on general resin properties. All resins have particle sizes in the range of 600-800 µm.
An important class of ion-exchange resins includes solvent-impregnated resins (SIRs). These materials combine the advantages of liquid-liquid extraction and ion exchange (which involves a separate solid phase). SIRs are prepared from an extractant loaded onto a polymeric support. There are two types of SIRs. In type I SIRs, extractant molecules are not bound chemically to a nonfunctional polymeric matrix (e.g., polystyrene-divinylbenzene) but remain inside the network because of hydrophobic interactions. In type II SIRs, extractant molecules are bound to a functional matrix because of acid-base interactions (24, 25). The use of SIRs allows for the synthesis of quite complicated extractants needed for selective separation. Moreover, these extractant molecules have higher mobility than the corresponding bound ligands and create an exact cavity needed for ion complexation (26). The idea to use ion-exchange resins for tap water remediation was suggested recently (27). This usage is characterized by the low concentration limit (below 1 mg/L) of contaminants, the high concentration of water microelements, such as calcium, magnesium, sodium, potassium, and chloride (essential for all living organisms), a neutral pH, and the presence of a variety of other anions which form ionic complexes in a solution phase, in competition for available sites on the ion-exchange resins. These conditions are more extreme in natural groundwater systems, because of even higher concentrations of microelements as well as the presence of small organic ligands (such as humic acids) which can form very stable complexes. 1852
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The objective of this study was to examine the feasibility of commercial ion-exchange resins and synthetically prepared type II SIRs for groundwater remediation as a material for use in PRBs. Particular emphasis was placed on analyzing the ability of these resins to remove low concentrations of toxic metals under natural groundwater conditions (i.e., in the presence of natural microelements such as calcium and chloride) and in sand-clay environments. In addition, the possibility of emplacing the materials using well configurations rather than a conventional, continuous wall was considered.
Experimental Analysis Resins. Duolite GT-73 and Amberlite IRC-748, produced by Rohm and Haas, were considered in this analysis. In addition, Amberlite IRA-96, produced by Rohm and Haas, was used as a support for SIR preparation. Properties of all resins are summarized in Table 1. The resins were washed in water, methanol, and methylene chloride to remove any impurities in the preparation process. The hydraulic conductivity, K, of Duolite GT-73 was determined to be 1.31 × 10-3 m/s. This measurement was made by using a falling head apparatus and on the basis of Darcy’s law, according to ln (h/h0) ) -(AK/aL)(t - t0), where h is the hydraulic head, h0 is the hydraulic head at t0, t is the time, A is the cross-sectional area of the sample, a is a crosssection of the inlet pipe, and L is the sample length (28). Because all of the resins have similar mechanical and
TABLE 2. Initial Concentrations of Metals in Feed Solutions in Column Experiments
Duolite GT-73 Amberlite IRC-748 SIR 1 SIR 2
[Cd], mg/L
[Cu], mg/L
[Pb], mg/L
[Ca], mg/L
pH
0.50 1.00 1.00 1.00
0.50 1.00 1.00 1.00
0.50 1.00 1.00 1.00
270 270 270 270
7.13 6.50 7.01 7.00
TABLE 3. Summary of Properties of the Different Sand Types mesh size
geometrical properties, a value of 10-3 m/s was used for the numerical calculations. SIR Preparation. Two SIRs were prepared, one using di2-ethylhexyldithiophosphoric acid (D2EHDTPA) and the second using di-2,4,4-trimethylpentylmonothiophosphinic acid (Cyanex 302 by Cytec). D2EHDTPA was synthesized as described in the literature (29) and verified by NMR spectroscopy. SIRs were prepared by contacting resin and extractant for 12 h, by shaking of 1 g of air-dried resin with 10 mL of acetonitrile solution of extractant at 220 rpm. Extractant mass was calculated by using the known ion-exchange capacity of the resin (Table 1) with a resultant loading (as determined from a mass balance between the initial and supernatant solutions) of SIR 1 (Amberlite IRA-96 impregnated with D2EHDTPA, 0.61 g of extractant/g of resin) and SIR 2 (Amberlite IRA-96 impregnated with Cyanex 302, 0.69 g of extractant/g of resin). The procedure for SIR preparation, including the resin and extractant types and the solvent, was developed earlier (30). In this previous study, a macroporous polystyrene divinylbenzene copolymer with a tertiary amine functionality was found to be a suitable and convenient support material. Before use, SIRs were washed on sintered glass filters with 50 mL of water, 50 mL of methanol/water (1:1), and 50 mL of water. Concentration and pH Measurements. Metal concentrations were measured by atomic absorption spectroscopy (AAS) on a Perkin-Elmer 5100PC instrument according to corresponding Aldrich AAS standards and then by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Spectro Flame Modula E instrument) according to the Merck 23-element ICP standard in 1% HNO3. The ICP-AES measurements were made only on samples that were below the detection limits of AAS. The phosphorus concentration also was measured on ICP-AES. Acidity was adjusted using an Oakton pH 2500 series meter. Column Experiments. The columns containing resins were fed with solutions of known metal concentrations and pH values. Solutions were prepared from cadmium, copper, lead, and calcium in the form of cadmium sulfate, copper sulfate, lead nitrate, and calcium chloride, as described in Table 2. The acidity of the feed solution was adjusted to a neutral pH value by the addition of 0.1 N NaON and 1.0 N HCl volumetric standards. One liter (equivalent to 500 bed volumes (BV)) of metal solution was passed through a plastic column which contained 2 mL of washed resin; the effluent was collected into 50 mL fractions. In all experiments, the volumetric flow rate was 0.25 BV/min, which corresponds to a linear flow velocity of 0.016 cm/s. The flow rate was set to be as low as possible (yet maintaining reasonable experiment length of 33 h), because typical flow velocities in groundwater aquifers are of the order of 10-5 cm/s. After collection, the effluent fractions were examined for metal concentration. Samples from the columns filled with SIRs were also tested for phosphorus content, to examine extractant leakage. To test whether small clay particles influence the efficiency of ion exchange, an additional experiment using metal concentrations and pH values, described in Table 2, was
coarse sand medium sand fine sand
12/20 30/40 50/70
grain diameter, hydraulic conductivity, mm cm/s 1.105 0.532 0.231
0.50 0.15 0.014
TABLE 4. Outlet Concentrations of Metals in Column Experimentsa
Duolite GT-73 Amberlite IRC-748 SIR 1 SIR 2
[Cd], mg/L
[Cu], mg/L
[Pb], mg/L
0.005 0.003 0.001 0.005
0.000* 0.000* 0.003 0.003
0.000* 0.000* 0.001 0.001
a Entries marked by an asterisk indicate metal concentrations below the detection limit of the ICP-AES.
performed. One liter of solution was passed with a volumetric flow rate of 0.25 BV/min. Duolite GT-73 was placed downflow to the mixture of different types of sand (coarse, medium, and fine; Table 3) and clay (bentonite powder and kaolin fines). Bentonite powder has a particle size from 2.5 to 4 µm, while kaolin fines have particle sizes ranging from 0.78 to 3.8 µm. The amount of clay suspended in solution was determined by gravimetric analysis to be 0.219 g/L. As a control, the adsorption of metal on clay (without the presence of resin) was measured.
Results and Discussion Cadmium was selected to simulate contaminated groundwater because of its high toxicity and because it can be easily measured by atomic absorption spectroscopy even at the level of 30 µg/L. Copper is much less toxic, but it usually appears in concentrations higher than cadmium. Moreover, most commercial ion exchangers are more selective toward copper, so the introduction of copper tests the ability of the resins to remove competitor metals. Lead was selected as representative of heavy metals, also because of its toxicity and relative simplicity of detection. The particular cation exchangers used here were chosen because they are inexpensive, widely used, and have selectivity toward heavy and transition metals over alkali and alkali-earth metals. The particular anion exchangers for preparation of SIRs were selected because their utility has been demonstrated in previous studies (30). All of the resins are effective in the range of pH values between 5 and 8, and they can therefore be considered for groundwater treatment. The column experiments show that all four of the resins can reduce lead, cadmium, and copper concentrations from values of 1.00 to as low as 0.001 mg/L (i.e., below the detection limit of ICP-AES), as shown in Table 4. In comparison, for example, the maximal value permitted by the U.S. Environmental Protection Agency for drinking water is 5 µg/L for cadmium (4). Most ion-exchange resins are designed for the treatment of fast flowing water. In the current experiments, effective treatment (remediation) over residence times as short as 4 min (i.e., a volumetric flow rate of 0.25 BV/min for all experiments) was achieved. This flow rate is, of course, much faster than typical rates relevant to in situ groundwater remediation; at slower flow rates, even higher levels of metal uptake can be expected. Thus, significantly, these materials are suitable for the treatment of all flow velocities occurring in groundwater systems. The presence of calcium and chloride in groundwater is a key factor, as natural calcium concentrations may be VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Illustration of pathlines in an aquifer containing a PRB, here designed as an array of two staggered rows of wells. Shown here is a section of an isotropic sandy aquifer with a hydraulic conductivity of 10-5 m/s. Wells which completely penetrate the aquifer contain resins having a hydraulic conductivity of 10-3 m/s. The steady-state groundwater flow equations are solved using the software package Visual MODFLOW, version 2.8.2.48 (Waterloo Hydrogeologic, Inc.). One pathline is drawn for each element along the inlet boundary (i.e., the left side of the figure). Observe that all of the pathlines pass through the reactive material emplaced in the wells. sufficiently high to interfere with metal uptake by these resins, and chloride forms stable complexes with different transition metals. Preliminary analyses indicated that competition with regard to HCO3-, SO42-, Cl-, NO3-, Na+, Mg2+, and K+ was not significant when their concentrations in the feed solutions were as much as 5 times lower than the concentration of Ca2+. Such a result was also reported elsewhere (27). The calcium breakthrough point occurs at less than 10 BV in all four resins, after which all of the calcium is released and escapes from the column. As stated previously, the ion exchangers used here are more selective to transition metals, such as cadmium and copper, than to calcium. Competition exists between transition metals, which is governed by stability constants of corresponding complexes. As such, high concentrations of copper can remove previously captured cadmium from the resin. As a consequence, field application of these resins requires, as for any remediation system, on-site monitoring of contaminant concentrations. In a simple batch experiment which involved loading the metal followed by contact with water, metal release from full resin to pure water was found to be no higher than 10%. Effluent samples obtained from columns filled with SIRs were checked for leakage of each of the extractants. Each extractant molecule contains one phosphorus atom; therefore, analysis of phosphorus levels is a convenient method to determine extractant concentrations in the effluent. The extractants themselves are toxic organic molecules, and as such, their introduction into groundwater systems must be avoided. The experiments here found that extractant leakage stops already after 180 BV, while the experiments were carried 1854
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out for 500 BV. Breakthrough points for metals are expected at 1032 BV, on the basis of known ion-exchange capacities and metal concentrations (31). Because extractant molecules used in the current study are hydrophobic weak acids, leakage of extractant may become permanent at pH values lower than 6. However contact with solution of even low ionic strength will decrease leakage. Thus, thorough washing of the SIRs permits their use in remediation of metalcontaminated water. Duolite GT-73 was placed downstream from a sand and clay mixture, to test whether migrating clay particles present in natural groundwater influence the metal uptake capabilities of the resin. Because all resins used in this study have the same macroreticular morphology (with macropores in the range of 200-1000 nm and micropores in the range of 2-5 nm), it was only necessary to test one resin. Analysis of the effluent concentrations demonstrated no loss of efficiency in the ion-exchange process, with the same output metal concentrations (as in Table 4) being achieved. Moreover, no adsorption of the metals on the sand and clay mixture was detected. The ion-exchange capacity of the resins (Table 1) considered here is at least 1 order of magnitude higher than that of alternative materials, such as zeolites (12) and activated carbon (19, 20). Moreover, the effective exchange capacity of these resins is even higher, taking into account their uptake of toxic metals only, due to their high selectivity. The capacity of the SIRs is equal to that of Amberlite IRA-96, as each amino group in the Amberlite support captures one extractant molecule. Because of high capacity, 1 kg of ion-exchange resin
can treat 1 m3 of water contaminated with 200 µg/L cadmium, which is a typical metal concentration in contaminated groundwater. A PRB is usually designed as a trench filled with reactive material, which forms a permeable reactive “wall” through which groundwater flows by natural gradients. A major limitation of PRBs is their relatively shallow (up to 15 m) depth of penetration. To permit access to deeper groundwater systems and to ease the installation and reduce the cost of PRBs, clusters of wells can be considered as an alternative. The high hydraulic conductivity of ion-exchange resins, relative to that of natural geological formations, allows for such designs. A major advantage of ion-exchange resins is that their hydraulic conductivity remains unaffected by metalion uptake. In sharp contrast, the hydraulic conductivity of PRBs consisting of iron, for example, decreases over time because of precipitation, which may even cause groundwater to ultimately bypass the barrier (6). To illustrate the possible design of a PRB consisting of discrete wells, the software package Visual MODFLOW, version 2.8.2.48 (Waterloo Hydrogeologic, Inc.) was used to solve the steady-state groundwater flow equations. Consider a section of an isotropic sandy aquifer with a hydraulic conductivity of 10-5 m/s (32) and a head gradient of 2%. This value, while higher than most natural gradients, was selected only for numerical accuracy in the simulations; because of the linearity of the flow equations, the actual streamlines are independent of the head gradient value. Wells that completely penetrate the aquifer are filled with resins having a hydraulic conductivity of 10-3 m/s. For a well field configuration consisting of two staggered rows of 1.4 m diameter wells, separated by intervals of 3.6 m, with a separation of 3.5 m between the two rows, and flow orthogonal to these rows, it is found that virtually 100% of the water will pass through the reactive material (see Figure 2). Even for an aquifer consisting of a very coarse sand with hydraulic conductivity of 10-4 m/s, 96% of the water will pass through the reactive material. Clearly, the most efficient configurations of such wells, and their diameter, can be determined from the analysis of the geology and flow properties of a particular aquifer and from technical/engineering considerations. Moreover, the possibility of pumping water directly from wells containing the resins can also be considered; the resulting modification of the flow field, as a function of pumping rate, must then be accounted for in design of the well configuration. Ion-exchange resins are capable of selectively removing heavy- and transition-metal ions from contaminated groundwater, despite high concentrations of natural components. Consequently, such resins can be considered and tested as a materials for use in PRBs. A major advantage of ionexchange resins, particularly in contrast to iron particles, zeolites, and other potential PRB materials, is that they can be effectively regenerated (27, 33). SIRs can reach regeneration efficiencies of 100% (30, 34). Use of such resins, particularly in configurations of wells, may offer a viable and cost-effective method for in situ groundwater remediation.
Acknowledgments The authors thank Mr. Michael Treitel and the Rohm and Haas company for supplying the resins. The financial support of the Angel Faivovich Foundation and the Brita Fund for Scholarships, Research and Education is gratefully acknowledged.
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Received for review December 3, 2001. Revised manuscript received February 11, 2002. Accepted February 14, 2002. ES010313+ VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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