PROJECT TOPIC ON REMOVAL OF SOME HEAVY METALS FROM CONTAMINATED SOILS USING TWO COMPLEXING AGENTS
A study on the removal of heavy metals from polluted site where metal-works activities are carried out was studied. The soil samples were treated separately with aqueous solution of two complexing agents – namely, tiron and glutamic acid to determine the removal efficiency of heavy metals. Before treatment with the complexing agents, the soils showed remarkably high levels of heavy metals above background concentrations. The distribution pattern were in the following order: Pb> Zn> Cr> Cd> Ni. Across all the sampling locations, Pb showed the highest (199.97 mg Kg-1) mean concentration and Cd (1.35 mg Kg-1) the least mean concentration respectively. Key factors that determine the absorption of heavy metal by complexing agents such as pH, concentration, amount of soil, and contact time were considered and the process optimized by the application of 24 factorial design of experiment. Results indicated optimum removal of Cr 98.81%, Ni 86.00%, Pb 76.18%, Zn 62.63% and Cd 83.95% using tiron while the optimum absorption for glutamic acid was 37.68% Cd, 56.41% Cr, 37.68% Ni, 1.81% Pb, and 63.23% Zn respectively. Optimum removal Cr 98.81% was obtained at pH 3, 25 mg dm-3, 10g amount of soil and 60 minutes contact time. The optimum removal of Ni 86.00% obtained at pH 5, 25 mg dm-3, 10g amount of soil and 60 minutes contact time. Optimum removal of Pb 76.16% was obtained at pH 5, 10 mg dm-3, 5 g amount of soil and 60 minutes contact time. Zn 62.63% was obtained at pH 3, 5g amount of soil and 60 minutes contact time. Cd 83.95% was obtained at pH 5, 25 mg dm-3, 10 g amount of soil and 30 minutes contact time using tiron. This indicates a good yield when compared to other used chelating agents like EDTA, EDDS, and NTA. Optimum removal of glutamic acid was Cr 99.98%, Ni 53.26%, Pb 99.99%, Zn 99.98% and Cd 99.98%.Cd 99.98% obtained at pH 5, concentration of 10 mg dm-3, 10g amount of soil, and 60 minutes contact time. Cr 99.98% obtained at pH 5, concentration of 25 mg dm-3, 5 g amount of soil, and 60 minutes contact time. Ni 53.26% obtained at 25 mg dm-3 concentration, 10 g of soil, at pH 3, and 60 minutes contact time. The optimum removal of Pb was 99.99% at pH 3, 10 mg dm-3, 10 g amount of soil, and 60 minutes contact time. Optimum removal of Zn was 99.98% achieved at 25 mg dm-3 concentration of glutamic acid, 10 g of soil, 30 minutes contact time, and pH 5. The results of cube plots to show the interaction of the absorption process by factorial design of experiment indicate 83.22% Cr, 9.16% Cd, 61.03% Ni, 69.86% Pb and 59.32% Zn removal by tiron. In conclusion, the results from this study suggested that tiron and glutamic acid have high removal efficiency. However, glutamic acid is more promising extracting agent than tiron.
Industrial development and urbanization have led to increase in the amount of solid wastes frequently discharged into the natural environment. This has increased the amount of many chemical substances in the environment. The chemical substances which are of great environmental interest are the heavy metals. Heavy metals are of special interest because of the major risks they contribute to the environment. They are toxic to humans as well as other organisms; they are released by metal-bearing soil constituents and migrate through the soil solution downward to the water table (Van Oort et al., 2006; Shina and Alok, 2010). The contamination by these metals is a threat to the quality of groundwater, if these metals are not properly treated. Unlike organic compounds that can be biodegraded with time or can be incinerated, metals are robust and remain a potential threat to the environment and human health for a long time (Hong et al., 2002). Soil and sediments are rich in these heavy metals since they are not subject to degradation phenomena. They can easily be suspended or dissolved by surface water, hence become available to plankton, nekton and deposit feeders. The consequence of this is that, they can enter the food chain and become concentrated in fish and other edible organisms (Di Palma and Mecozzi, 2007).
The environmental impact of soil contamination depends not only on the total amount of metals in the soil but mainly on their mobility and availability. This is influenced by leaching and interactions with other components of the ecosystem such as air and water. Soil washing remediation technology is used to remove undesirable contaminants in soil and sediments by dissolving or suspending them in a washing solution (Freeman and Harris, 1995; Moutsatsou et al., 2006; Weihua et al., 2010), and also by concentrating contaminants in small volume of soil through particle size separation (Detzner et al., 1998; McCready et al., 2003). This is based on findings that contaminants tend to bind either physically or chemically to clay, silt or organic soil particles.
Liberated metal ions are variously trapped by a wide range of reactive soil constituents that is organic matter, iron, manganese oxides, hydroxides, silicates, phosphates, carbonates. A fraction of these metals can thereafter be remobilized, either in dissolved or colloidal form before migrating downward. Mobilization is defined here, as the potential capacity of metals to be transferred from the solid phase into the soil solution. It depends on the various links between metals and reactive sites of solid phase surfaces. (McGrath, 1996; Gleyzes et al., 2002; Krishnamurti et al., 2002; Chaignon et al., 2003; Feng et al., 2005;). Metals readily extracted, were called „„labile‟‟ by (Fangueiro et al., 2005), and more slowly extracted metals, called „„less labile‟‟, which might be reasonably attributed to potentially „„mobile‟‟ and/ or „„bioavailable‟‟ metal fractions (Bermond et al., 2005). Degryse et al., (2006) showed that the proportion of labile metals was well-correlated with metal uptake in plants.
Chelating agents are most effective extractants, which can be introduced in the soil washing to enhance heavy metal extraction from contaminated soils. The advantages of chelating agent include high efficiency of metal extraction, high thermodynamic stabilities of the metal complexes, and low absorption of the chelating agents to a catalyst (Jerome et al., 2007). In addition, the chelating agents cause only minor impact on the physical and chemical properties of the solid matrix as compared to acids (Lee and Marshall, 2002).
PROJECT TOPIC ON REMOVAL OF SOME HEAVY METALS FROM CONTAMINATED SOILS USING TWO COMPLEXING AGENTS
1.1 Statement of Research Problem
Metal contaminated soils are a serious environmental problem with implications for human health. The presence of metals in the soil has two main origins: the alteration of the bedrock and human activities, the latter being the major cause of high level of metals in the soil. The risks are related to the mobility and bioavailability of the metals and consequently to their speciation in soil. Chelating agents have been widely investigated as efficient extracting agent to enhance the performance of soil washing (Peters, 1999).
Naidu and Harter (1998) stated that metals extracted by a mixture of organic acids are well-correlated with the mobile metal fraction in the soil solution. Low molecular weight organic acids, naturally exuded by plant roots or produced by microbial activity (Fox and Comerford, 1990), have been hypothesized to influence nutrient mobilization (Van Hees et al., 2002) or translocation of metals in soil profiles (Van Hees and Lundstrom, 2000; Li et al., 2006). Ethylene diaminetetraacetic acid (EDTA) is a well-known strong chelating agent and has been widely used in agronomy for estimating the total extractable metal pool (Alvarez et al., 2006; Manouchehri et al., 2006).
1.2 Justification of Research
Soil washing process, amongst other remediation technology gives high removal efficiency for remediating sites contaminated with heavy metals and organics using suitable chelating agents, surfactant, acids alkalis and complexing agents because it can be applied to large contaminated areas due to its operational easiness and economic efficiency (US-EPA, 2001). Several types of extractants can be used to extract heavy metals and metalloids from contaminated soil for soil washing technology. The extractants can be acids, chelating agents, electrolytes, oxidizing agents and surfactants (Reddy and Chinthamreddy, 2000; and Sun et al., 2001), out of which acids and chelating agents are the most used extractive reagents for heavy metal decontamination. Acid washing leads to decreased soil productivity and adverse changes in the chemical and physical structures of soil due to mineral dissolution (Reed et al., 1996). Chelating agents are regarded as more attractive alternatives to acids because they can form strong metal-ligand complexes and are thus highly effective in remediating heavy metal contaminated soil (Kim and Ong, 1998 and Wei et al., 2011). EDTA continues to be explored extensively for soil remediation because of its ability to mobilize metal cations efficiently coupled with only a minor impact on the physical and chemical properties of the soil matrix (Lee and Marshall, 2003). However, of the use of dominant complexing agents (tiron and glutamic acid) could be employed as alternatives to EDTA in soil washing and for chelant-enhanced phytoremediation. The use of nontoxic and biodegradable biopolymer for wastewater and soil treatment is becoming increasingly recognized (No and Meyers, 2000).
1.3 Aim and Objectives
This study was aimed at determining the pollution level of the soils of some metal-work sites and estimating the percentage removal of different heavy metal (Cd, Cr, Ni, Pb and Zn) by washing the most polluted soil with complexing agents (tiron and glutamic acid).The specific objectives include:
- Determination of some physicochemical parameters of the contaminated soil, such as pH, carbonate content, particle size and cation exchange capacity.
- Determination of the concentrations of cadmium, chromium, nickel, lead and zinc in parent soil.
- Assessing the metal removal efficiency of the used chemical extractant through batch extraction using solutions of the two complexing agents at different contact times, pH, masses and concentrations.
- Applying factorial design of experiment to determine the optimum operational condition for the process.