COMPARISON BETWEEN EXTRACTION AND ACID DIGESTION TECHNIQUES TO REMOVE HEAVY METAL FROM SOIL

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1 COMPARISON BETWEEN EXTRACTION AND ACID DIGESTION TECHNIQUES TO REMOVE HEAVY METAL FROM SOIL Abstract: Haifa El-Sadi 1, Maria Elektorowicz 2, Ammar Badawieh 3 1 Wentworth Institute of technology, Mechanical Engineering and technology, Boston, MA, USA, ,3 Concordia University, Building, Civil and Environmental Engineering, Montreal, QC, Canada Correspondent author elsadih@wit.edu A series of laboratory tests were conducted in order to compare between two techniques supercritical extraction and acid digestion and the effect of shaking time on the extraction efficiency of heavy metals from clayey soil. Supercritical fluid extraction and acid digestion were used for the analysis of heavy metals concentrations after the completion of Electro-kinetic experimentation. Supercritical fluid (carbon dioxide) extraction is a new technique used to extract the heavy metal (lead, Nickel, Calcium and Potassium) from clayey soil. The comparison between supercritical extraction and acid digestion of different metals was carried out. Supercritical fluid extraction, using Ethylenediaminetetraacetic acid (EDTA) as a modifier, proved to be efficient and safer technique than acid digestion technique in extracting metals from clayey soil. Mixing time of soil with EDTA before extracting heavy metals from clayey soil was investigated. The optimum and most practical shaking time for the extraction of lead, Nickel, calcium and Potassium was two hours. Keywords: clay soil, heavy metals, supercritical fluid extraction, acid digestion 1. Introduction From an engineering standpoint, natural soil presents a medium that is complex in structure, physic-chemical properties and general behavior. Knowledge of the soil horizon forms the initial foundation for the study of soil composition. The efficiency of any remediation effort is predicated on a knowledge of soil composition, since this presents an idea related to the fate (transport, transformation and remediation). Soil consists of a myriad of components that can be subdivided based on their physical structure and their chemical behavior related to contaminant fate. The soil has been used in this study is illites. Illites consist of repeated layers of alumina sheets between two silica sheets with oxygen atoms shared. Each layer is bonded by potassium ions which fit exactly into the hexagonal gaps provided by the silica sheets. As a result of the presence of these potassium ions, a positive charge results, which is dissipated by the substitution of aluminum for silicon. The structure and physical properties of illite are favorable to the retardation of chemicals, particularly heavy metals. Illite is characterized by a small particle diameter, less substitution of aluminum for silicon, and more sites for exchangeable cations than kaolinite and chlorite [3]. Metals play an important role in environment. Heavy metal contamination develops from accidental spills, fume emission, electroplating operations etc. metals are universal and cause the toxicity of plants and humans. Also, the clay involves of tight structure and very small pores which affect difficulty on clay remediation. The fate of metals in the subsurface, which includes their physical transport and chemical transformation is crucial to understand which type of remediation technique can be used. Since the subsurface is heterogeneous and anisotropic, the metal content is subject to spatial variability [1]. Heavy metals contain different kind of metals such as Lead, Nickel, Potassium and Calcium. The retention of nickel is primarily due to the presence of organic matter, precipitation and due to the tendency of nickel for hydrolysis. Harter [2] established that nickel adsorption in soil primarily follows a Langmuir isotherm and is not as strongly nor as quickly adsorbed by a given soil as is lead. In addition, retention of lead by insoluble humic and soluble fulvic acid has been reported by Yong, et al. and Harter [2,3]. Fergusson, [4] 7

2 demonstrated that lead compounds are enriched in mosses, podzolic soils, ombrotropic and in other environmental targets in southernmost Norway. there are different techniques can be utilized to remediate the soil from heavy metal such as soil washing which is expensive from a standpoint of capital costs. However, Electrokinetics (EK) soil remediation has confirmed to be an effective on a pilot scale for remediation of lead-spiked Georgia Kaolinite [4]. On the other hand, the transport of ionic species and pore water between cathode and anode, via electroosmosis and electrolytic migration is more effective than standard hydraulic gradients [6]. Two techniques, acid digestion and supercritical fluid extraction have been used after the completion of Electrokinetic experimentation to analyze the heavy metal concentrations. Soil constituents and the properties have a significant influence on the fate of contaminants. A contaminant present in the subsurface is transported to other locations in the soil by pore water potential, advection, dispersion and diffusion. In clay soils, molecular diffusion is the typical transport mechanism due to the low permeability of clay. While the contaminant is transported through the media, retardation occurs which aids in removing these contaminants from the liquid phase and onto the solid phase, thereby ceasing their mobility. Retardation of cationic substance, particularly metals, is of particular importance to remediation engineering. In any remediation process, accessibility of metals to transport (i.e. electro kinetics) is paramount. Heavy metals have the ability to accumulate in living organisms and can therefore cause toxicity within the food chain. It is generally understood that the degree of mobility and activity of metals is influenced by temperature, cation exchange capacity of the solid phase, complex metal and soil composition and the concentration of metal in the soil solution. In natural clay soil, the use of EDTA has the potential to keep more heavy metals in the solution phase, thereby allowing Electrokinetic method to transport an increasingly higher amount of metals. Ethylenediaminetetraacetic (EDTA) has been used to extract heavy metals from contaminated soil, through solubility and mobility [11]. It is very soluble in water and dissociates into a wide variety of species such: H 3 Y -, H 2 T -2, HY -3 and Y -4. The pka value are shown in table 1. Table 1. Ethylenediaminetetraacetic acid (EDTA) species and pka values (Yeung and Menon, 1996) Ionic species pka H 3 Y - 2 H 2 T HY Y Each EDTA ion can coordinate bond to a metal at six different sites, namely, each of the four acetate sites and the two nitrogen sites, which have free electron pairs available to coordinate bond formation. The structure and configuration of the metal-edta complex is shown in Figure 1. The numerous coordination sites inherent to EDTA ions creates complexes that are highly stable and favorable to chelation technology. Figure 1. Structure and location of metal-edta complication sites [11] Supercritical fluid extraction (SFE) is a technique used for the analysis of heavy metal concentrations after the completion of electrokinetic experimentation. SFE requires a supercritical fluid such as carbon dioxide and a 8

3 modifier if needed [7, 8] extracted metals from liquid and solid materials using lithium bis-triflouroethyldithiocarbamate (FDDC) as a modifier. This modifier revealed high solubility in supercritical fluid carbon dioxide. Wang and Marshall [12] performed a series of SFE tests in order to improve the solubility of the metals in supercritical fluid CO 2 by using tetrabutylammonium dibutyldithiocarbamate as a modifier. The objectives of this research are to determine the extraction efficiency of SFE technique versus acid digestion and to study the most optimum and practical shaking time for extraction of metals. 2. Materials and Methods Tests were conducted on laboratory-prepared contaminated samples to assess SFE extraction efficiency, acid digestion and Atomic absorption spectrometry (AAS) of heavy metals from clayey soil. 2.1 Materials Four metals were analyzed versus shaking time for samples employing EDTA and supercritical extraction as shown in table 1. Ethylenediaminetetraacetic acid (EDTA) is a highly branched and high-molecular weight acidic compound. EDTA has been extensively used to extract heavy metals from contaminated soil, through solubility and mobility enhancement [9]. Table 2 shows the soil characterization which has been used for the experiments. Table 2. Soil characterization Parameter value Mineral composition Cation exchange capacity 21meq/1g ph 7.6 Organic matter content 1.3% Carbonate content 4.5% Specific gravity Total Kjeldahl nitrogen.42% Sulfate test -2.6 ppm as SO 4 Metal content Fe 273mg/kgdry soil K 176mg/kg dry soil Ca 25mg/kgdry soil Ni 31 mg/kg dry soil Pb 9.3 mg/kg dry soil 1.2 Supercritical Fluid extraction equipment and procedure All extraction experiments were performed by the SFX 22 extraction System, which consists of an SFX 22 Extractor and SFX 2 Controller, and 1 ml syringe pumps (ISCO Model 1DX). Both the pumps and the extractor and connected to a SFX 2 Controller, which controls all pumping and extraction operations by changing the parameters such as temperature and pressure (Fig. 2). The SFX 22 Extractor is a bench top, dual chamber (cartridge filter with a 5/8-inch diameter filter element), which fits on the top of an SFX 2 Controller. The extractor incorporates six motor-actuated valves, which are controlled by the SFX 2 controller. The fluid source for the extractor is supplied by a D-series pump and the other pump for the modifier. Fused-silica tubing with an inner diameter of 5 µm, 3 cm long, was used as an outlet restrictor, allowing analysts to be conveniently collected in the test tubes. A vent valve allows rapid depressurization of the chamber after the extraction is completed. Extracted analyte was collected out-side of the oven at room temperature by placing the outlet end of the restrictor into a 3-ml vial containing 7-1 ml of solvent. 9

4 Figure 2: Dual syringe pump system for modifier SFE The different extraction sequences can be described as follows: (1) 1 g of soil and 9 ml of.1 M EDTA are placed into a 2.5 ml cartridge body. Valves closed, the cartridge is loaded into extraction chamber. (2) Filters are placed at the top and bottom of the cartridge. (3) The supply valve is opened and the supercritical fluid is supplied by two 1DX syringe pumps. The fluid flows into the extraction chamber, filling the cartridge upwards. The samples were placed into the supercritical fluid extractor for 15 min dynamic extraction at 9C and 5 psi. The restrictor temperatures were maintained at 9 C. The samples were shaken at 3 minutes, 1 hour, 2 hours, 4 hours and 8 hours (shaking time) before were placed into the cartridge of extraction chamber. The recoveries of the metals from the collection were determined by using Atomic absorption spectrometry (AAS). 2.3 Acid Digestion experiment and procedure Acid digestion is applying a very strong acid to the matrix in order to dissolve certain elements (metals) that could become environmentally available, and prepare the collected samples for analysis by atomic absorption spectrometry. American Environmental Protection Agency EPA, generated a standard procedure for acid digestion [1]. The samples were prepared by transferring a 1-2 g sample (wet weight) and 1 g sample (dry weight) to a digestion vessel. The sample is used as long as digestion is completed. Then 1 ml of 1:1 HNO 3 was added to the sample, mix the slurry, and cover with a watch glass or vapor recovery device. The sample was heated to 95 o C ± 5 o C and reflux for 1 to 15 minutes without boiling. Then the sample was allowed to cool, then 5 ml of concentrated HNO 3 was added, the cover and reflux were replaced for 3 minutes. If brown fumes are generated indicating oxidation of the sample by HNO 3, repeat this step (addition of 5 ml of conc. HNO 3 ) over and over until no brown fumes are given off by the sample indicating the complete reaction with HNO 3. A ribbed watch glass or vapor recovery system were allowed either allows the solution to evaporate to approximately 5 ml without boiling or heat at 95 o C ± 5 o C without boiling for two hours. Maintain a covering of solution over the bottom of the vessel at all times. After the sample has been cooled, 2 ml of water was added and 3 ml of 3% H 2 O 2 was added with warming until the effervescence is minimal or until the general sample appearance is unchanged. Heat was continued until the volume has been reduced to approximately 5 ml without boiling for two hours. Maintain a covering of solution over the bottom of the vessel at all times. 1 ml concentrated HCl was added to the sample digest and cover with a watch glass or vapor recovery device and Placed in the heating source for 15 minutes. After adjusting to the required volume, sample is ready for atomic absorption spectrometry for metal detection. 2.4 Atomic absorption spectrometry (AAS) The recoveries of the metals from the collection were determined by using Atomic absorption spectrometry (AAS) during all the experiments. AAS is an analytical technique that measures the concentrations of elements. Atomic absorption is so sensitive that it can measure down to parts per billion of a gram (µg dm 3 ) in a sample. The technique makes use of the wavelengths of light specifically absorbed by an element. They correspond to the energies needed to promote electrons from one energy level to another, higher, energy level. AAS is used to analyze a sample to see if it contains a particular element means using light from that element, since atoms of different 1

5 elements absorb characteristic wavelengths of light. Sample preparation for AAS is often simple, and the chemical form of the element is usually unimportant. This is because atomization converts the sample into free atoms irrespective of its initial state. The sample is weighed and made into a solution by suitable dilution. Elements in biological fluids such as urine and blood are often measured simply after a dilution of the original sample. When making reference solutions of the element under analysis, for calibration, the chemical environment of the sample should be matched as closely as possible where the analyte should be in the same compound and the same solvent. Teflon containers may be used when analyzing very dilute solutions because elements such as lead are sometimes leached out of glass vessels and can affect the results. 3. RESULTS AND DISCUSSION 3.1 SFE versus acid digestion Extraction efficiency and feasibility of SFE technique versus acid digestion was studied. The concentration of various metals measured in the extract of SFE samples and acid digestion samples was compared. A summary of results for various metals is shown in table 3. Table 3: Comparison of SFE technique versus acid digestion Metal type Concentration of Metals in Extract: SFE Technique Lead 9.3 below detectable limits Nickel 31.1 below detectable limits Calcium Potassium Iron 2 15 Concentration of Metals in Extract: Acid Digestion Table 3 shows that the utilization of SFE techniques with EDTA was significantly higher in the extraction of lead, nickel and calcium. The extraction from acid digestion produced lead and nickel concentrations that were below detectable limit during atomic absorption spectrophotometry. However, lead and nickel concentrations were detected in the extract from SFE. The calcium concentration in the SFE extract was 346% higher than that of the acid digestion extract. Therefore, supercritical fluid extraction with EDTA represents a viable and more efficient method for analyzing metals in clay soils, as is the case in the analysis of soil after the termination of electrokinetic treatment. 3.2 SFE tests: obtaining optimum shaking time SFE extraction with EDTA was more efficient in the extraction of heavy metals, particularly lead and nickel, establishment of the optimum shaking time of contaminated soil of heavy metal with EDTA acid was crucial. The samples were shaken at 6 rpm for 3 minutes, 1 hour, 2 hours, 4 hours and 8 hours using an AROS orbital shaker. It should be noted that four samples for each shaking time were used. The metals (lead, nickel, calcium and potassium) were analyzed versus shaking time, for samples employing EDTA and SFE extraction and EDTA without SFE extraction. All metals show a significant improvement in extract concentration when SFE is applied as shown in table 4. Table 4: Average percent increase in extracted Metals (SFE versus No SFE) 11

6 Metal extracted Metal Extracted American Journal of Engineering and Technology Research Vol. 15, No.2, 215 Metal Lead Nickel Calcium Potassium Iron % increase in Metal Extracted when utilizing SFE technique % Metal extracted with no SFE Calcium Iron potassium 5 Mixing Time (hours) 1 (a) Lead Nickel Mixing time (hours) (b) Figure 3: Metal extracted versus shaking time without SFE for (a) Calcium, iron and potassium and (b) Lead and nickel. 12

7 Metal extracted Metal Extracted American Journal of Engineering and Technology Research Vol. 15, No.2, Metal extracted with SFE Calcium Iron 5 1 Mixing Time (hours) (a) 1 8 Lead Nickel potassium Mixing time (hours) 1 (b) Figure 4. Metal extracted versus shaking time with SFE for (a) calcium and iron, Lead, (b) Nickel and potassium Figures 3 (a and b), 4 (a and b) show a general increasing trend in metal concentration versus shaking time, for all metals. The highest percent of metal in the extract occur from.5 to 2 hours of shaking. After 2 hours, the concentration of metals in the extract increase slightly. Table 5 shows the percent increase in metals extracted within each time interval. 13

8 Metal Table 5. Metal Extraction: percent increase versus shaking time SHAKING INTERVAL.5-1. hrs. 1-2 hrs. 2-4 hrs. 4-8 hrs. Lead 6.7% 23.1% 2.5% 11.1% Nickel 4.3% 17.6% 2.2% 4.9% Calcium 1.3% 5.2% 1.7% -.1% Potassium.7% 6.2% 1.7% 2.3% Iron 3.5% 1% -1% 2.2% 3.3 Correlation between Extraction and shaking time In spite of the fact that the effectiveness of extraction for different metals is difficult to predict due to the soil complex and metal structure, an attempt was conducted in order to correlate the extraction efficiency of different metal with different shaking time. Nonlinear regression was considered to model extraction process and shaking time: c (t, a, a 1 ) = a Ln(t) + a 1 Where c is the extracted metal concentration, a and a 1 are constants and depend on metal structure and t is the shaking time. The recoveries of the metals from the collection were determined by using Atomic absorption spectrometry (AAS) during all the experiments to analyze the concentrations of extracted metals in collection vials of SFE. Extraction efficiency (E) was estimated as follows: VtC E V C metal i X1% Where V t is trapped volume, V metal is volume of metal, C is extracted concentration, Ci is initial concentration of metals in soil. Figure 6 shows that the calcium has the highest extraction efficiency, However, potassium has the lowest recovery because potassium has high bivalent ions. 1 recovery % 5 Pb Ni Ca k metal Figure 6: Recovery versus different metals (lead, Nickel, calcium and Potassium) 4. Conclusion A series of laboratory tests were conducted in order to compare between two techniques Supercritical fluid extraction (SFE) and acid digestion techniques and to study the effect of shaking time on the extraction efficiency of heavy metals from clayey soil. SFE and acid digestion have been used for the analysis of heavy metals after the completion of electrokinetic experimentation. The results show that SFE is safer than acid digestion. SFE requires supercritical CO 2 which don t pose a serious health threat. Acid digestion requires the use of high concentrations of nitric acid, hydraulic acid, sulfuric acid and 3% of hydrogen peroxide. These chemicals are highly reactive. SFE is 14

9 faster, each SFE sample can be obtained in 15-3 minutes, while acid digestion samples are typically obtained in 3-6 hours. Also, the results show that SFE technique with the use of EDTA has higher extraction efficiency than acid digestion for all metals tested. The optimum and most practical shaking time for the extraction of lead, Nickel, calcium and Potassium was two hours. Increasing the shaking time to four hours or eight hours has slight effect on the extraction efficiency. Also the results show that SFE technique is more safe and efficient than acid digestion and it can be applied to any porous medium. Atomic absorption spectrometry was used to analyze the concentration of the collect extract in 1 ml of distilled water. 5. References [1] Merian, P.D., Metals and their Compounds in the Environment: Occurrence, Analysis and Biological Relevance, VCH Publishing, New York, 1438 pages, [2] Harter, D.R., Effect of soil ph on Adsorption of Lead, Copper, Zinc, and Nickel, Soil Science Society American Journal, Vol. 47, pp , 1983 [3] Yong, R. N., A. M. O. Mohamed, B. P. Warkentin, 1992, Principles of Contaminant Transport in Soils, Elsevier Publishing, New York, 327 pages, [4] Acar, Y. B., A.N. Alshawabkeh, Electrokinetic Remediation I: Pilot-Scale Tests with Lead-Spiked Kaolinite, Journal of Geotechnical Engineering, Vol. 122, No. 3, pp. 173, [5] Ferguson, J. E., The heavy Elements: chemistry, Environmental Impact, and Health Effects, Pergamon Press, New York, 614 pages, 199. [6] Hamed, J., Y. B. Acar, R. J. Gale, Pb(II) Removal from Kaolinite by Electrokinetics, Journal of Geotechnical Engineering, Vol. 117, No. 2, pp , [7] Laintz, K. E., C. M. Wai, Extraction of Metal Ions from Liquid and Solid Materials by Supercritical Carbon Dioxide. Analytical Chemistry, Vol. 64, No. 22, pp , 1992 [8] Lin, Y, R. D. Brauer, K. E. Laintz, C. M. Wai, supercritical Fluid extraction of Lanthanides and Actinides from Solid Materials with A Fluorinated ß-Diketone, Analytical Chemistry, Vol. 65, No. 18, pp , 1993 [9] Albert T. Yeung, Cheng-non Hsu, Rajendra M. Menon, Physicochemical soil-contaminant interactions during electrokinetic extraction, Journal of Hazardous Materials, Vol. 55, pages , [1] EPA, Acid Digestion of Sediments, Sludge, and Soil, Method 35B, American Environmental Protection Agency, [11] Yeung, A. T., R. M. Menon, 1996, EDTA-Enhanced Electrokinetic Extraction of Lead, Journal of Geotechnical Engineering, Vol. 122, No. 8, pp [12] Wang, J., W. D. Marshall, 1994, Metal Speciation by Supercritical Fluid Extraction with On-Line Detection by Atomic Absorption Spectrometry, Analytical Chemistry, Vol. 66, No. 22, pp