Sorption and Leaching Characteristics of Heavy Metals through Clay
Sorption and Leaching Characteristics of Heavy Metals through Clay
S.K Singh
Asst Professor, Civil Engineering Department, PEC University of Technology, Chandigarh–160012, India.
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ABSTRACT: Human activities may result in the introduction of heavy metals into the subsoil and/or subsurface aquifer systems, either due to negligence or by accident. The presence of heavy metals constitutes a potential threat to human health and ecosystems. The potential sources of heavy metals as a contaminant are from industrial effluents and leachates from municipal and mining solid waste disposal sites. Understanding the fundamental interaction is a pre-requisite to solve the soil contamination and heavy metals mobility problems and for any proposed clean- up or remediation measures. The leaching column test is a very useful and versatile tool that can be used to study the interactions between soils and various contaminants. The migration of heavy metals through compacted soil columns are monitored in leaching column tests to study the soil interactions with heavy metals. This paper presents the results of batch tests and leaching column tests conducted to examine the interactions of heavy metals (Cd, Cu, Pb) with cohesive soil with low permeability as it can be used as liner in the engineered landfill site or any containment system. The results are compared and it is found that cadmium has lowest retardation or largest mobility while lead has lowest mobility due to highest adsorption degree among all the metals considered.
Key Words: Heavy metals, Contaminants, Sorption, Leachate, Batch and Column Tests.
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Sorption and Leaching Characteristics of Heavy Metals through Clay
1. INTRODUCTION
Now-a day’s contamination of soil by human activities has emerged as a widespread problem. Among the most commonly encountered soil contaminants are heavy metals, petroleum hydrocarbon and halogenated organics. Many of these contaminants are highly toxic and potential carcinogens. Mining, smelting, and industrial activity have caused extensive heavy-metal contamination of the environment by directly introducing heavy metals into the surrounding atmosphere, waterways, and soil (Ernst, 1995). Another environmental concern is the potential for contamination from heavy metals leached from containment facilities such as landfills and surface impoundments. Engineered clay barriers are commonly used in both landfills and surface impoundments to prevent the migration of heavy metals. They are practical and economical, especially when clay sources are locally available. For areas afflicted with heavy-metal–contaminated soil or ground water, vertical clay barriers may be used to prevent heavy metals from reaching drinking water sources. For landfills and surface impoundments, clay barriers may act as the low-permeability (<10–9 m/s) component in liner systems. Migration of contaminants present in the leachate through clay barriers are retarded due to its low permeability and sorption of contaminants in clay barrier. It is desired that liner system should be able to restrict discharge of leachate to admissible concentration level of contaminants in sub-soil and ground water. The mobility studies with respect to those contaminants are required to be studied. The metals most frequently found in the identified hazardous sites under Superfund programs are: As, Cr, Pb, Zn, Ni, Cd, Cu, and Hg (USEPA 2004a; Williford Bricka 2000). Elevated concentrations of heavy metals in soils are of potential long-term environmental and health concerns because of their persistence and cumulative tendency in the environment, and their associated toxicity to biological organisms (Nriagu 1979, 1988; Nriagu Pacyna 1988). This paper presents the study of adsorption and migration behaviour of three heavy metals (Cd, Cu, Pb) in cohesive soil with low permeability (bentonite enriched local soil) through batch tests and column tests. Consideration of contaminant retention capacity of the soil in the performance-oriented design of liners may promote the utilization of local soil (Rowe 1887; Sackelford & Rowe 1998).
2. MATERIALS
2.1 Soil
The soil for the study was collected locally from open pits at Mohali in the outskirts of Chandigarh. The locally available soil was mechanically sieved to remove coarse fractions grater than 4.75 mm sieve. Physico-chemical properties of soil was determined (Table 1). The soil was mixed with 10% of commercially available Ca-bentonite (CaOAl2O35SiO2 2H2O) by weight in order to make its less permeable that can be used as clay barrier. Index properties and compaction characteristics of the soil-bentonite mix are given in Table 2.
Table 1: Physico-Chemical Properties of Local Soil
Properties / Values / Chemical content / ValuesSand (%) / 10 / SiO2 (%) / 62.50
Silt (%) / 67 / Al2O3 (%) / 26.00
Clay (%) / 23 / Fe2O3 (%) / 2.50
LL (%) / 32.7 / CaO (%) / 1.60
PL (%) / 19.8 / MgO (%) / 3.20
SL (%) / 16.4 / Na2O (%) / 0.07
CEC / 12.7 meq/100gm / K2O (%) / 0.04
pH / 8.4 / Organic Matter (%) / 2.87
Table 2: Properties of Soil-Bentonite Mix
LL (%) / PL (%) / Sp.Gr(G) / OMC (%) / Max γd
(kN/m3)
80 / 32.8 / 2.71 / 19.5 / 15.6
2.2 Heavy Metal Solutions
Heavy-metal solutions (Pb, Cu and Cd) were permeated through locally available soil mixed with Ca-bentonite in batch and column studies. All the solutions were prepared by dissolving calculated weight of each solid crystals of Pb(NO3)2, Cu(NO3)2 and Cd(NO3)2 into distilled water separately for known concentration of the metals in the solutions. Diluted nitric acid was used to adjust the pH of all the solutions as 5 for both batch and column experiments.
3. METHODS
3.1 Batch Adsorption Test
Batch adsorption tests involve mixing a batch of solutions of Pb(NO3)2 , Cu(NO3)2 and Cd(NO3)2, each with the same volume with known initial solute concentration, with a fixed mass of adsorbent in each reaction vessel. The resulting change in solute concentrations after contact with the adsorbent provides the basis for the construction of isotherms (EPA 1992).The procedure of these tests was based on ASTM D4319 guideline, with soil-solution ratio 1:4 (25 g of dry soil to 100 mL of solution). Equilibration time, defined as the minimum time needed to establish a rate of change of the solute concentration in solution that is equal to or below 5% for a 24 h interval (EPA 1992), was determined as 24 hrs. The suspensions were placed inside 250 mL inert flasks at an orbital shaker for 24 h. Liquid and solid phases were separated by filtration, and the concentration in the liquid phases was analyzed by atomic Adsorption spectrometer (Make ECI, EC AAS 4103). Lead, copper and cadmium were tested separately. Duplicate were conducted for each test.
3.2 Leaching Column Test
The experimental set up (Figure 1) consisted of a cylindrical column made of PVC and measuring 7.73 cm inner diameter and 10 cm in height. End diffusers made of perforated PVC plates of 1.5 cm were placed on the top and bottom of compacted soil specimen in the column for uniform discharge through soil column. The height of the soil column in the PVC cell was 7 cm. The other major components of the experimental setup were Influent Reservoir, 10 L capacity made of polypropylene, pressure system and effluent collector.
Fig. 1: Schematic Diagram of Experimental Set-up
The oven dried soils of required quantity was mixed with necessary amount of water separately as to prepare sample of required density. The soils were mixed thoroughly and kept in polythene bag in humid desiccators overnight to achieve uniform moisture content. The soil was then compacted in the PVC cell in three equal layers with dynamic compaction to achieve 0.85 times of Proctor's maximum dry density i.e. 13.26 kN/m3 at water content 2% wet side of optimum water content. The soil in the column is saturated by passing distilled water through influent reservoir consisting of with two opening, one at the top for transferring the source solution of interest with known concentration (100 ppm) into it and the other at the bottom to allow it to migrate through the soil specimen. After saturation with water for 24 hours, the solution of interest is placed in this influent reservoir and is stirred at frequent intervals so as to maintain constant initial concentration. The solution is then passed through the soil compacted in the column at constant hydraulic gradient of 25 to reduce the testing duration to reasonable period. Pressure gauge is connected to the influent reservoir and to the column assembly. A uniform pressure of 17.5 kPa is maintained throughout the experimental period by controlling the flow rate from the influent reservoir. The effluent is collected in the effluent collector consisting of a measuring jar covered at the top so as to avoid evaporation of collected leachate. The volume of the effluent that comes out of the column with time was monitored at regular intervals (usually two days) and the concentrations of metals were measured. The test duration was 15–20 days. Knowing the initial and concentration of metals at after different intervals, relative concentration (C/Co) is calculated and the breakthrough curves are plotted.
4. RESULTS AND DISCUSSION
4.1 Batch Tests
The sorption capacity of the bentonite enriched local soil for heavy metals under consideration (Pb,Cd and Cu) in form of isotherm curves are shown in Figure 2. Freundlich isotherms, expressed as in Eq. (i), were fitted to the experimental data using nonlinear least-squares curve fittings.
S = Kf (Ce)b (i)
where S = adsorption degree (mg/g); Ce = equilibrium concentration (mg/L); Kf = Freundlich partition coefficient (L/g); b = empirical constant of the Freundlich isotherm.
The coefficient of regression fit ranges from 0.96-0.99 denoting that sorption of metals on the soil can be best described as Freundlich isotherms. The Freundlich equations for isotherms are presented in Table 3 to compare the relative sorption capacities of soil for meats under consideration.
Table 3: Equations of Freundlich Isotherms
Pb solutions / Cu solutions / Cd solutionsS = 0.13 (Ce)0.4 / S = 0.05 (Ce)0.488 / S = 0.05 (Ce)0.311
The sorption ranking deduced from the Table 3 is Cd < Cu < Pb. Therefore, mobility of cadmium and lead will be highest and lowest, respectively, among the considered heavy metals.
4.2 Leaching Column Test
Three solutions of Pb(NO3)2 , Cu(NO3)2 and Cd(NO3)2 into distilled water were permeated separately in three columns. Concentration of metal in each solution was 100 ppm to compare the results. Soil column studies are conveniently reported in terms of pore volumes where one pore volume represents the volume of water required for filling the voids of compacted soil column completely. One pore volume was calculated as 157.6 ml at the porosity of compacted soil column as 0.48. Breakthrough curves are presented as relative outflow concentration (C/C0) as a function of pore volume of flow in Figure 3. Breakthrough curves are useful in the understanding of retention or mobility characteristics of a soil with respect to particular contaminant.
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Sorption and Leaching Characteristics of Heavy Metals through Clay
Fig. 2: Sorption Isotherms
Fig. 3: Breakthrough Curves
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Sorption and Leaching Characteristics of Heavy Metals through Clay
From Figure 3 it can be observed that the curves are
S-shaped and relative concentrations for each heavy metal upto 6 pore volumes are almost zero. The mobility of heavy metals is low denoting strong affinity with solid phase and more difficulty/time-delay in leaching out the heavy metals through aqueous washing. The heavy metals may remain as persistent contaminants in the sub-soil under ordinary field conditions. However, physico-chemical alteration of the soil environment, soil acidification and heterogeneous preferential flow may affect the heavy metal mobility through the sub-soil system (Kabata-Pendias & Pendias 1991; Dube Galvez-Cloutier 1998).
Breakthrough point is considered at relative concentration of 0.5 i.e. half of the initial concentration. The pore volumes required to reach this breakthrough point for Pb, Cd and Cu are 16.5, 12 and 9.5, respectively. This implies that relative mobility of the metals are in the order of Cd > Cu > Pb. This is again in confirmation with the results obtained in the batch tests.
5. CONCLUSIONS
Sorption characteristics of the soil with heavy metals can be best described as Frundilic isotherm. Results of batch tests and column tests are in good agreement and hence convenient tools for determining the retention and migration of contaminants through the soil. Relative mobility of the metals are in the order of Cd > Cu > Pb. Consideration of contaminant retention capacity of the soil in the performance-oriented design of liners may promote the utilization of local soil.
ACKNOWLEDGEMENTS
The work presented in this paper is the part of research programme for attachment/detachment of contaminants in the soil system being conducted at Punjab Engineering College (Deemed University) Chandigarh (India). Funding for the research was provided by All India Council of Technical Institutions, New Delhi.
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