Download 43.56 Kb.
|Development of a pollutant trapping, deep soil delivery technology for in-situ subsurface decontamination|
Jamil Rima, Lizette Aouezova, Marwan Saad, Qing X. Li
It is well recognized that zerovalent metal particles are powerful reducing agents for chlorinated hydrocarbons [Rima et al. 1999, 2000, 2001a&b, Keum and Li 2004 and 2005] and for Cr(VI) [Boronina et al. 1995, Powell et al. 1995, Blowes et al. 1997] and Pb(II) ions [Blowes et al. 1997]. They also have high oxidation powders at appropriate conditions [Rima, Aouezova, Li, patent application reference UOH.012A/UOH.012VPC]. Therefore, we will employ this technology in an application of soil decontamination.
We propose to develop a delivery vehicle for iron powder or iron nanoparticles using a mixture of cyclodextrin and detergents. This solution will facilitate the mass transfer of pollutant molecules from the soil to the cyclodextrin component. We expect to develop a technology by which an oxidation reaction will be able to treat soil contaminated by organic chemicals. A decontaminating solution will be delivered into subsurface soil via semi-permeable membranes in contact with the soil. Finally, air will be circulated into the polluted area by air pumps. The eluates will be collected for further treatment using our oxidative iron technology to completely eliminate the organic pollutants.
The objective of this project is to develop a delivery technology for metallic iron into deep soil using cyclodextrin (cornstarch-based sugar) as an iron carrier and pollutant trapper. Using control and experimental soil columns, we have demonstrated the feasibility of our technology to destroy atrazine.
We have conducted the following soil column experiments:
(1) deliver cyclodextrin and detergents into deep soil and collect the eluates,
(2) deliver cyclodextrin-trapped iron and detergents into deep soil and collect the eluates, and
(3), in conjunction with (1) and (2), mineralize the organic pollutants present on the eluates using our proprietary oxidative iron powder technology.
Atrazine (2-chloro-4-ethylamino-6-isopropyl amino-1,3,5-triazine) is a widely used herbicide since the late 1980’s for the control of weeds. Use of atrazine was greatly restricted in 1993 due to health risks. Long-term exposure to amounts greater than 300 ppb of atrazine can result in cardiovascular damage, retinal damage, muscle degeneration, and cancer.
-cyclodextrin and iron powder were purchased from Sigma Aldrich, and used as received. Atrazine was purchased from Rodel-dehein. Distilled water was used for preparing aqueous solution of -cyclodextrin and atrazine stock solutions.
Atrazine was analyzed by a UV–visible spectrophotometer (UV-1601, Shimadzu). Agilent 1100 serials HPLC system was also used. The pH values are measured in open bottles with a WTW pH/mv, and hand-held meter 330/set.
Soil was collected from an agricultural field, and was transported in coolers to the laboratory where was air-dried and ground to pass through a 2 mm sieve followed by fortification with atrazine at 10 µg/g, 2,4-DNT at 50 µg/g and heptachlor epoxide at 1 µg/g. The soil was clayey.
Sorption and desorption behavior of atrazine
Atrazine was used as the model contaminant in this study. Atrazine is an herbicide with a molecular weight of 215.7 g/mol and the water solubility about 28 mg/L at 20 oC. Atrazine is frequently detected in soil and groundwater. In this experiment, Atrazine was dissolved in methanol to form a 100 ppm stock solution. All the glass vials containing soils and atrazine solutions were shaken until the measurement time was reached. For each batch experiment, blank samples were prepared and monitored (i.e., atrazine solutions without soil). The blank samples did not indicate any significant atrazine degradation or sorptive losses on the glassware during the course of the experiment. Maximum atrazine losses in these blank samples were 2.6% within the experiment period, so the lost atrazine from the solution phase could be considered safely sorbed onto the soil solid phase. Aqueous phase atrazine concentrations were determined with an Agilent 1100 HPLC system equipped with a vacuum degasser, quaternary pump, autosampler, column compartment, diode array and multiple wavelength detectors (DAD), and a hypersil reversed-phase ODS-C-18 column (Agilent). The conditions for atrazine analysis were: mobile phase consisting of 1% acetic acid (10%) and methanol (90%) at a flow rate of 1.00 mL/min, signal wavelength of 222 nm with 20 nm bandwidth, and a reference wavelength of 300 nm with 50 nm bandwidth. The atrazine concentration was quantified with an external standard.
Equilibrium sorption experiments
All equilibrium sorption experiments were conducted in triplicate in 35-mL glass vials. The soil samples were mixed with variable atrazine concentrations and the vials sealed and shaken for 24 h in the dark at 20 °C. Different solid-to-water ratios varied due to the soil samples having different sorption capacities (i.e., to achieve sufficient sorption so that it could be easily quantified while keeping the aqueous phase concentration above the detection limit). The solid-to-water ratios were 0.5:30, 2:30, 5:30, 10:30, and 20:30 (w:w) at different test sites. Before analysis, the samples were centrifuged at 3000 rpm for 20 min at 20 °C. The atrazine concentration in the supernatant was determined by HPLC.
Batch sorption and desorption experiments
In this study, an individual experiment consisted of repeated cycles of sorption and desorption. A cycle of sorption and desorption consisted of sorption followed by several successive desorption steps. The procedure for the sorption cycles was the same as in the equilibrium experiments. All sorption experiments were conducted in triplicate in 35-mL glass vials. The solid-to-water ratio was different for different soil samples. The vials were stored at 20 °C in the dark. After an incubation period of 24 h, the soil was separated from solution by centrifugation at 3000 rpm for 20 min. The supernatant was analyzed for solution-phase atrazine concentration by HPLC. After the sorption experiments, desorption was induced by the successive placement of 90% supernatant with an atrazine-free electrolyte solution (actual amounts determined by weight), and the vials were sealed and shaken for 24 h at 3000 rpm at 20 °C in the dark. The desorption period of 24 h was chosen after a preliminary desorption kinetic experiment was conducted for 72 h, in which the data revealed a concentration plateau after 18 h. For operation convenience, the desorption period was set at 24 h, the same as the sorption period.
At the end of the desorption period (24 h), the vials were centrifuged at 3000 rpm for 20 min, and the supernatant analyzed for the concentration of atrazine in solution-phase by HPLC. The desorption procedure was repeated until the atrazine concentration in the supernatant was below the detection limit. After the successive desorption steps, the solution was decanted and the wet soil was left in the vials (actual amounts were determined by weighing). Thirty mL of atrazine solution were added to the vials to conduct another cycle of sorption/desorption experiments. Desorption processes were repeated until the amount of atrazine desorbed from the soil was below detection limit.
Atrazine extraction by -cyclodextrin
Adsorption batch experiments were performed in triplicate in 50 ml glass centrifuge tubes by mixing 10 g of soil with 50 ml of atrazine background solution. The samples were shaken at 22 ± 2 °C for 3 days to allow adsorption equilibrium. The pH was measured during the experiments. Desorption or extraction experiments were initiated immediately after ascertaining sorption by replacing the supernatant by the same mass of de-ionized water or CD solution. The tubes were shaken again, allowing desorption equilibration for an additional 2 d period. The concentrations of atrazine in the supernatants were determined by HPLC. The amount of atrazine adsorbed or desorbed was calculated from the difference between its concentration before and after adsorption or desorption experiments. Extraction experiments were performed in triplicates. The reproducibility of these measurements was around 5%.
Atrazine dilutions were prepared in deionized water. The concentration of atrazine was 6 ppm. Each sample consisted of 5 g of soil mixed with 25 mL of the atrazine solution in 50 ml glass centrifuge tube sealed with screw caps. The tubes were agitated on a rotary shaker for 24 h at 20 ± 1ºC to achieve equilibrium. Blanks without soil prepared in the same way showed no adsorption of atrazine on the tube wall. The amount of atrazine adsorbed by the soil was calculated from the difference between the initial and equilibrium atrazine concentrations in solution. The adsorption equilibration process was made in three replicates. Desorption of atrazine was determined on triplicate soil samples equilibrated with 6 ppm of atrazine after equilibration the soil saturated by atrazine was shaken with β-cyclodextrin solution (0.01 M) and the concentration of atrazine removed from the soil was monitored at different intervals of times within 24 h. After 3 h of shaking we observed that the concentration of atrazine trapping from the soil did not change. To eliminate all the atrazine from the soil we changed the CD solution.
Soil column experiment
Experiments were conducted in bench-top soil columns as shown in Figure 1. The external dimensions of the columns were 30 cm x 30 cm x 150 cm. The internal dimensions were 10 cm x10 cm x 150 cm. The soil was filled between the two compartments. The column was filled with approximately 52 kg of the clayey soil. One soil column experiment was to deliver cyclodextrin into deep soil and collect the eluates. Based on our previous work of cyclodextrin inclusion of atrazine (see Related Work, below), we propose that cyclodextrin can be delivered into deep soil, and the cyclodextrin can trap the target pollutant molecules. Detergent may facilitate dissociation of the target pollutant molecules from soil particles and their transfer into cyclodextrin. Sodium dodecyl sulfate (SDS) was used as a primary detergent tested. β-cyclodextrin (β-CD) was used at a concentration of approximately 20 g/L. Eluates were collected and analyzed for the concentration of the target pollutants and their metabolites, particularly their reduction products. The remaining eluate was treated with our oxidative iron powder technology for complete mineralization of the pollutants.
Figure 1. Prototype soil column treatment using cyclodextrin, detergents and zero valent iron. Column external dimensions are 30 cm x 30 cm x 150 cm; and internal dimensions are 10 cm x 10 cm x 150 cm. This compartment is perforated from the bottom to the top. Sampling heights from the top of the column for analysis are: 5 cm, 55 cm, and 110 cm.
Results and discussion
Evidence of atrazine trapped -cyclodextrin
UV/VIS spectra were obtained for atrazine and -CD-atrazine complex in water at pH 6.5. Figure 3 shows the absorption spectra of a constant amount of atrazine (9.26x10-6 M) dissolved in solutions varying concentrations (1x10-3 – 9x10-3 M) of -CD. The absorption intensity of atrazine increased as the concentrations of -CD increased. The remarkable changes suggest an interaction between -CD with atrazine, in which atrazine molecules preferentially reside in the non-polar CD cavity.
Figure 2. UV/VIS absorption spectra of atrazine at 9.26x10-6 M in varying concentrations of β-CD solutions (1x10-3 - 9x10-3 M). The higher concentrations of β-CD, the higher absorbance.
Stoichiometry of the inclusion complex
The stoichiometry of -CD-atrazine complex was analyzed by the Scatchard and Benesi-Hildebrand plots [Connors 1987 , Benesi and Hildebrand 1949 ]. According to Scatchard’s method (equation 1), we assume that -CD forms an inclusion complex with atrazine by 1:1 ratio. If the -CD-atrazine complex is formed in a 1:1 ratio, a plot of (A-A0)/ [-CD] versus (A-A0) should give a straight line. Figure 3a shows a linear relationship of (A-A0)/ [-CD] versus (A-A0).
–––––– = (A∞-A0)K1 - (A-A0)K1 (1)
Where A0 denotes the absorbance intensity of atrazine in the absence of -CD, A∞ denotes the absorbance intensity when all of the guest molecules are essentially complexed with -CD, A is the observed absorbance at each -CD concentration tested, K1 is the association constant and [-CD]0 is -CD concentration tested.
When a Benesi-Hildebrand plot of 1/(A-A0) versus 1/[-CD] is constructed according equition-2, a straight line is obtained. Figure 3b shows straight lines. When the plot of 1/(A-A0) versus 1/[-CD]2 is considered, a downward concave curvature is obtained, confirming that the stoichiometry of the [-CD-atrazine] complex is not 1:2.
1 1 1
––––– = –––––––––––––– + –––––– (2)
(A-A0) (A∞-A0)K1[CD]0 (A∞-A0)
Association constant of the inclusion complex
The slope of the Scatchard plot is very close to the value that the intercept is divided by the slope of the Benesi-Hildebrand plot. A non-linear regression provides a more precise method than the double reciprocal plot for the determination of the complex formation constants [Benesi and Hildebrand 1949 ]. The determination is based on equation-3 [Connors 1987].
I = ––––––––––– (3)
(1 + K1H0)
here I = (A-A0) is the guest-induced absorbance intensity, and is equal to Imax = (A∞-A0) when every host exists as the inclusion complex. Imax is obtained from the double reciprocal plot equation 1. H0 is the initial concentration of the host. The formation constant (K1) is estimated by fitting Eq-3 to the data obtained. Figure 4 shows t
he curve fitting for atrazine. According to this curve the value of K1 is 285 ± 15 mol-1.
igure 3. Scatchard plot (a) and Benesi-Hildebrand plot (b) of β-CD-atrazine complex formation. The Scatchard plot is (A-A0)/[β-CD] vs. (A-A0); and the Benesi-Hildebrand plot 1/ (A-A0) vs. 1/[β-CD].
Figure 4. Fitting curve of the guest-induced absorbance intensity (I) vs. the initial concentration of the host β-CD (H0). Where I = (A-A0), and is equal to Imax = (A∞-A0) when every host exists as the inclusion complex.
Degradation of atrazine by metallic iron powder
The utility of fine-grained metallic iron to destroy atrazine in the soil extracts was investigated. Batch procedures under water treatment conditions (ambient temperature and pH of approximately 6) indicated that atrazine was degraded rapidly in the presence of iron powder (350 mesh, 4 g/l). The decline of atrazine concentrations, as shown in Figure 5, wa
Figure 5 Dissipation of atrazine destroyed by zero valent iron powder. The atrazine concentration was monitored with HPLC at 222 nm.
s monitored by HPLC. Experiments with un-buffered solutions showed a steady increase in pH values during the reactions. Therefore, experiments were run in buffered solutions. Different buffered solutions resulted in different degradation rates, indicating that the buffer plays an important role in enhancing the degradation process. Tests were also performed on an industrial effluent solution containing a variety of pesticides. Our HPLC results indicated the disappearance of all the parent pollutants. As expected, the oxygen content plays an important role on oxidation by which organic compounds are mineralized.
Soil column results
We polluted 52 kg of soil by 5 liters of 7.4 ppm atrazine solution. After filtration, desorption of atrazine was carried out with (a) 5 liters of deionized distilled water and (b) 5 litters of β-cyclodextrin solution 0.01 mol/L. Figure 6 shows desorption of atrazine using water and aqueous β-cyclodextrin solution.
Figure 6. Kinetics of atrazine desorption using water (a) and aqueous β-cyclodextrin solution 0.01 mol /L ( b)
e injected 4.5 liters of a mixture of β-cycoldextrin (0.01 M) and SDS (1 x 10-4 M) into a soil column which contained atrazine-polluted soil. After 30 hours of removing most atrazine into the cyclodextrin solution, we injected a mixture of 400 mL of phosphate buffer solution pH 4 with 4 g/L of iron powder (about 20 g). After that, we started to circulate air using an air compressor. We analyzed samples that were taken from different heights of the column (Figure 7).
Figure 7. UV/Vis spectra of the soil column eluates.
trazine in the soil eluates dissipated within about 30 min (Figure 8). The degradation kinetics was monitored by HPLC at 222 nm.
Figure 8. Degradation of atrazine in soil column eluates.
Based on the results of this work, the zero valent iron powder in the presence of oxygen is able to reduce and/or oxidize all the organic compounds in the soil into carbon dioxide, water and minerals. The significant advantage of our technique vs. current techniques is that it enables completely remediate not only soil but groundwater contaminated with toxic organic compounds within days instead of months to years, and leave behind no toxic end-products.
We performed a prototype soil column test at a laboratory scale. The prototype test showed degradation of atrazine. We plan to demonstrate our technology for other toxic organic compounds, particularly PCBs and PAHs. We will test the system in situ soils contaminated with one or more of these pollutants.