Solvent Extraction of Chromium and Cadmium from Contaminated Soil
Kapil Kumar Rana (T.E Chemical,
Ruphar Dhawan (T.E Chemical, Bharati Vidyapeeth College of Engineering, Pune)
Abstract:
The contaminants were selectively extracted by contacting the soil with the carrier solvent in which the appropriate chelating agents had been dissolved. The main thing was the use of an environmentally acceptable water soluble carrier solvent such as ethanol, showing that the carrier solvents themselves do not have to be good solvents of the contaminants .In fact, their main purpose is to carry with them appropriate chelating agents that, being soluble in the solvent mixture, can first penetrate the pores in the soil and then form complexes with contaminants that thereby can be washed out of the soil. Soils contaminated with chromium and cadmium can be remediated first by wetting them with water and then washing them with a 99% water and 1% ethanol solution which contains an appropriate chelators, namely diphenylcarbazide for chromium and 1,5- diphenylthiocarbazone for cadmium with a 5-to-1 molar ratio between chelates and metal.
1.1Introduction
Excessive accumulation of heavy metals in soils is a side effect of the development of Contamination of the environment with heavy metals has become one of the major ecological issues in the past few years. Chromium is one of those heavy metals the concentration of which is steadily increasing due to industrial growth, especially the development of metal, chemical and tanning industries [1]. Other sources of chromium permeating the environment are air and water erosion of rocks, power plants on liquid fuels, brown and hard coal, and industrial and
municipal waste [2]. Although there is no risk of chromium contamination on a global scale, local permeation of the metal to soil, water or the atmosphere might result in excessive amounts of this pollutant in biogeochemical circulation [3, 4]. Chromium in soil and water is usually present as trivalent or hexavalent ions. The occurrence of chromium as tri- or hexavalent ions depends mainly on soil pH, granulo metric composition, redox potential and content of humus [5]. Trivalent chromium is weakly soluble in highly acid and alkaline soils, whereas hexavalent Cr dissolves well in acid and alkaline soils .Cr (VI) in soil is reduced to Cr (III), which is not well available for plants. Chromium (VI) has a harmful effect on soil microorganisms by depressing their biological activity. Like other heavy metals, chromium may influence the enzymatic activity of soils by affecting soil microorganisms as well as by modifying the environment in which they live, and which is rich in many enzymes.Environment protection agency [EPA] presents approximately 1000 sites in U.S that pose significant environment health risks. About 40 percent of these sites have been reported to have metals problems .Figure -1 summarizes the occurance and distribution of metals at these sites.
Fig.1 Metals most commonly present in all matrices at Superfund sites.
Cadmium contamination of soil is a major concern in the biosphere. Beyond the suite of available physico Chemical treatment methods, green and more efficient technologies are desired to reduce cadmium and other heavy metal contaminants to acceptable levels. Heavy metals are of great concern to environmental safety because of their toxicity and intrinsically non-degradable nature. They tend to accumulate in soil through adsorption and pose a serious health threat to humans and animals. The cleanup of many contaminated sites remains a challenging task but an essential one for site restoration. Different decontamination methods including physical, chemical, thermal, and/or biological processes have been reported for soil treatment. Particularly, soil washing is a technology that extracts heavy metals into a wash solution either by desorption or solubilization . Several classes of chemical reagents have been investigated including acids, bases, chelating agents, oxidizing agents, and surfactants. However, the use of chemical reagents alters not only the soil characteristics in an adverse manner but is also non-biodegradable (e.g. EDTA) and may result in additional pollution.
1.2 Methods to Extract Chromium and Cadmium
1.2.1 Extraction of chromium
Chromium was extracted from a volcanic soil typical of the region near
Cr(III) chelate ion presents absorption spectra with a 550 nm peak, a spectrophotometer was used to be calibrated and chromium concentration was determined.
At first a solvent solution containing 50% water and 50% acetone was used in which an approximate amount of diphenylcarbazide was dissolved, so that the number of moles of extractant was, in every test, about five times larger than that of chromium ions. More benign solvents like ethanol can be used in very small amounts (i.e. less than 1%) without experiencing any reduction of extraction yield, thereby showing that the role of a solvent mixture is to simply transport the chelating mixture in the soil.
Soil contamination. The soil was spiked by adding 50 ml of a water solution of potassium dichromate (K2Cr2O7) containing 20 ppm of chromium to each gram of soil and mixing for 3 hr. At the end of that time, the chromium concentration in water was measured, reveling that 23%of chromium was absorbed, corresponding to a 46 ppm chromium contamination. This measurement was repeated five times, obtaining similar results within a 10% error.
Contaminant extraction. After filtering the soil, we let it dry at ambient conditions, and performed a series of experiments using 50% water-50%acetone( in volume) mixture, containing a quantity of diphenylcarbazide so as to have approximately 5 moles of chelant per each mole of chromium. The results obtained by mixing 1gm of contaminated soil for 2 hr with 20 ml and 40 ml of extracting solution, containing, respectively 500 and 1000 ppm of diphenylcarbazide are reported in the table.
| Table.1 |
| |
| | Cr in soil Initially | Cr in soil after process | Cr Extracted | Cr Extracted | Partition coefficient |
| 20 ml | 0.046 mg | 0.0308 mg | 0.0125 mg | 33% | 0.025 |
| 40 ml | 0.046 mg | 0.0234 mg | 0.0226 mg | 49% | 0.025 |
The results are expressed in terms of partition coefficient ,P , defined as ratio between chromium concentration in the extracting solution and in the soil particles at equilibrium, that is, P= Cs\Cp. Here Cs is the ratio of the mass of extracted chromium and that of the extracting solution, while Cp is the ratio between the mass of chromium remaining in the soil and the mass of soil itself. The above given table clearly indicates that the value of partition coefficient is practically constant and is equal to about 0.025 independent of the amount of extracting solution used in the process.
Blank extraction. The experiments were repeated, with contaminated soil washed with deionized water. With 20 ml of water added to each gram of contaminated soil, it was found that after mixing for 1 hr, only 2% of the chromium was extracted.
Dry v/s wet soil. Another set of measurements was performed with wet soil, obtained by soaking each gram of dry contaminated soil wit 1.3 ml of water for 1hr. It was found that the extraction yield improved dramatically, with the partition coefficient increasing from 0.025 to 0.036.
Use of benign solvent mixtures.:At this point we investigate how the composition of the solvent mixtures influence the extraction yield, using first a 50% water-50% ethanol solvent and then a 99% water-1% ethanol solvent solution (pure water is not a feasible solvent, since it does not dissolve diphenylcarbazide), with again a 5-to-1 ratio of chelate and chromium, in both cases we obtain the same results when using acronitrile(table 1),
showing that the role of the solvent solution is solely to carry the complexing agent inside the pores of the soil particles. This is an important result, since it shows that the amount of solvent required to decontaminate the soil is very limited and such solvent does not have to be a good solvent of the metal ions at all, and therefore a non-toxic one can be chosen. In a separate set of experiments it was found that the yield of the process starts to decrease only when the molar ratio between the chelate and metal becomes less than 2.
| Table.2 |
| Cr in soil initial | Cr in soil after | Cr extracted | Cr extracted | Partition coeff. |
| | | | | |
| .046 | .0266 | .0194 | 42% | .036 |
| .046 | .0264 | .0196 | 42% | .036 |
| .046 | .026 | .020 | 40% | .035 |
Multiple stage extraction process. These experiments were repeated in an attempt to mimic a multiple-stage process. For this the extraction process was repeated using the soil that had been extracted a day before and then let dry overnight at ambient conditions. It was found that partition coefficient was about the same as for first extraction.
Kinetics of the process.:The percent of chromium extracted was measured as the function of time, using both dry and wet soil samples, with the extracting solution composed of 99% water-1% ethanol and with a 5-to-1 molar ratio of diphenylcarbazide to chromium.[7].
1.2.2 Extracion of cadmium
The extraction of cadmium is performed using an identical procedure as for the extraction of chromium. 1,5-diphenylthiocarbazone is used as the chelating agent with a 5-to-1 molar ratio of chelant to metal. First, the samples of vesuvian soil were spike by adding 1 gm of soil to 50 ml of a 2 ppm cadmium solution( pure granulated cadmium is used). While chromium forms chromate anions, cadmium, like most metals form Cd2+ cations. After 3 hr of stirring 29% if cadmium was absorbed by the soil, corresponding to 18-ppm cadmium contamination. Then after a blank extraction, the extraction process was performed using a mixture composed of 99% distilled water and 1% ethanol that contained 2 ppm of 1,5-diphenylthiocarbazone, and found a partition coefficient of about 0.048 for wet soil. As before this result was unchanged when the percent of ethanol in the extracting mixture was increased to 50% and when ethanol was replaced with acetone[8].
1.3 OTHER’S TECHNIQUE’S
1.3.1 Isotope Exchange Kinetics
An isotope exchange kinetics (IEK) technique has been tested to describe the kinetic transfer of ions from the soil solution to the solid phase. Although the IEK technique has been successful in describing nutrient availability in soils, it has not been widely applied to study contaminant availability. In this study, experimental conditions to determine exchangeable Cd in soils using IEK were determined along with a measurement of Cd availability using the IEK technique for 20 topsoils [9].
Theory:
Essentially, when ions such as 65Zn or 63Ni are added carrier free to a soil solution system at steady state, the radioactivity in solution decreases with time (t, expressed in minutes)
where R is the total amount of radioactivity introduced into the system, r(1) and r(
) are the radioactivity remaining in the solution after 1 min and an infinite exchange time respectively, and n is a parameter describing the rate of disappearance of the radioactive tracer from the solution for time longer than 1 min of exchange. The parameter n is calculated as the slope of the linear regression between log r(t)/R and log (t) for exchange t 
60 min. The ratio r()/R is the maximum possible dilution of the isotope, and is approximated by the ratio of water soluble Cd to the total soil Cd concentration Eq. [1].
| r(∞)/R=10Ccd/CdT |
where CCd is total water-soluble Cd (mg Cd L–1) and CdT is total Cd digested in concentrated HNO3 and H2O2 expressed in mg kg–1 soil. The factor 10 arises from the soil/solution ratio of 1:10 so that 10 x CCd is equivalent to the water-soluble Cd quantity in the soil expressed in milligrams per kilogram (mg kg–1).Given that the soil system is at a steady state before the introduction of the isotope, the decrease in the radioisotope with time is assumed to be the result of ionic exchange between radioactive ions, for example, 109Cd added to the soil solution and stable Cd ions on exchange sites on the soil solid phase. Therefore the quantity E(t) (mg Cd kg–1) of isotopically exchangeable Cd at a time (t) can then be calculated using Eq. [2] .
| E(t)=10CCd[R/r(t)] |
Study:
The effect of filtering on the recovery of stable Cd ( given Table ) indicated that there was in general a total recovery of Cd added to either deionized water or a soil solution extract, indicating that there was neither significant sorption nor contamination of Cd from the 0.2-µm membrane filter.
Table .3
| | Recovery of cd aaded to deionized water sol. Or soil sol. Extract filtered through a .2µm membrane | |
| Cadmium added to the sol. | Deionized water Mean sem | Soil water extract Mean sem |
| 0 | 0 0 | .22 .02 |
| .5 | .59 .03 | .76 .01 |
| 1 | .93 .05 | 1.27 .01 |
It has been previously noted that there can be a significant adsorption of 109Cd onto filter membranes during filtration in a background of water. This is important if an accurate estimation of the total radioactivity introduced into the soil solution system, (R), is to be made. It was observed in the present study that the percentage recovery of 109Cd in deionzed water was only 3.5% of that added, compared with nearly total recovery in a background of 1 mM Ca(NO3)2 or in soil solution extract (Table 3). This was likely the result of the large amount of competing Ca ions in the dilute electrolyte or other cations in the soil solution, which prevented significant sorption of 109Cd. It was also noted that filtering through a 0.2-µm filter membrane produced better recoveries than the 0.1-µm filter (Table 3)The lower recovery of 109Cd for the 0.1-µm membranes is likely due to greater amounts of colloids in solution, while a lower recovery of 109Cd in the biosolids-amended soil maybe a function of greater amounts of organic colloidal particles. We therefore made all measurements in a background of soil solution and used 0.2-µm membrane filters.
Note:
Cadmium availability in soils with relatively low Cd concentrations can be assessed in soils using the IEK technique. The IEK technique approach could simultaneously provide information on the intensity factor (CCd) the quantity factor [E(t)] and a parameter related to Cd sorption / desorption in soils [r(1)/R]. A compartment analysis revealed that there were differences in the distribution of Cd in exchange pools between soils contaminated with Cd from either long-term land application of treated biosolids or from annual applications of phosphate fertilizer. Extraction of soils with 1 M CaCl2 can provide a useful estimation of potentially isotopically exchangeable Cd in soils. Isotopic exchange kinetics has potential as a technique to provide useful information on Cd availability in soils although further work is still required to find a robust measure of the potentially exchangeable parameter that is, r()/R which is required in the IEK equation..
1.3.2 Electrokinetics
Electrokinetics is a process that separates and extracts heavy metals, radionuclides, and organic contaminants from saturated or unsaturated soils, sludges, and sediments. Electrokinetic treatment concentrates contaminants in the solution around the electrodes. Contaminants can be removed from this solution by electroplating or precipitation/coprecipitation at the electrodes, or by pumping the contaminant and processing fluid to the surface and treating with ion exchange resins other methods to recover the extracted metal and reuse the processing fluid in the electrokinetic (Figure.4) is a schematic of the process.
fig.2 Electrokinetic process.
Equipment required for the application of this technology is of a fairly specialized nature. Anodes and cathodes are placed in permeable or porous casings.
The above ground system requires a pump to remove contaminated water from the cathode to a processing system. Tanks and meters are needed for holding waste to be processed, and water solutions or chemical additives that are used. A low voltage power supply is required. Other specialized equipment such as controllers, valves, vacuum pumps, and gauges may be required[10].
Electrokinetic remediation is a developing innovative technology with a specialized nature. Its main focus has been the treatment of low permeability soils where other technologies would not be successful or their use may not be cost-effective. Because of its specialized nature.
Perfomance and cost
This process was developed to remove Cr from unsaturated soil. The field-scale demonstration results showed that the process removed approximately 200 g of Cr during operation, and had an overall removal efficiency of approximately 0.14 g of Cr per kilowatt hour (kWh). However, comparison of pre-and post-treatment soil sample results did not show much improvement in Cr levels or TCLP levels in the treatment zone. The post-treatment median TCLP concentration of 20.4 mg/L exceeded the TCLP regulatory limit of 5.0 mg/L for the demonstration site. The total treatment costs for the ISEE System to treat 16 yd3 of soil were estimated to be $1,830/m3 ($1,400/yd3) for removing 200 g of Cr(VI). This cost will vary depending on cleanup goals, soil type, treatment volume, and system design changes.
1.3.3 Phytoremediation
Phytoremediation is a method of remedying environment contaminated with hazardous substances utilizing the innate capabilities of plants as shown in Fig. 3[11].
Functions :
1.Absorbing and accumulating the hazardous substances.
2.Degrading and detoxifying it.
3.Stabilizing it around the root
4.Activating microbes around the roots to degrade and detoxifying it.
For treatment of various contaminants
The targets for which the methods is best suited are heavy metals that cannot be neutralized and degraded by bacteria and chemical procedure .some institute is conducting research on cadmium, the regulation of which has been tightened in recent years, due to an expected reduction in permitted intake of cadmium by the FAO/WHO of the heavy metals found in comparatively large quantities in the soil.
Conclusion
Soil contaminated with chromium and cadmium can be remediated first by wetting them water then washing them with water and ethanol solution. which contain appropriate chelator namely diphenylcarbazide for chromium and 1,5-diphenylthiocarbazone for cadmium with some ratio between chelant and metal. The most important information was that soil remediation can be accomplished using very poor rather benign (i.e ethanol)solvents in which biodegrable and selective complexing agent have been dissolved.
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9.C.W Gray R.G Mclaren .68,2004.
10..USEPA Notes”OCT 2000”.
11.Toshihiro yoshiharo”phytoremediation”