Removal of Some Heavy Metals From Contaminated Soil Using Two Complexing Agents

Removal of Some Heavy Metals From Contaminated Soil Using Two Complexing Agents

CHAPTER ONE

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
Determination of the concentrations of cadmium, chromium, nickel, lead and zinc in parent soil.

CHAPTER TWO

LITERATURE REVIEW

Soil and Its Properties

Nicholas et al., (2004) stated that the soil is a key component of terrestrial ecosystem, being essential for the growth of plants and the degradation and recycling of dead biomass. It is a complex heterogeneous medium comprising mineral, organic solids, aqueous and gaseous components. Brady and Weil, (2002) stated that the solid components are usually clustered together in the form of aggregates, thus creating a system of interconnected bids (pores) of various sizes filled with either water or air. The solid components have the ability to absorb ions, but this differs between materials and is strongly influenced by the prevailing pH, redox conditions and the relative concentrations of the ions present in the aqueous soil solution. The structured heterogeneous mixture of organic and mineral components is the habitat for many organisms as well as the medium in which plant roots grow, extracting water, oxygen and ions. Roots also release carbon dioxide and exude organic compounds which are responsible for the intense microbial activity in the interracial zone between the root and the soil called the

„rhizosphere‟. Plant roots modify the chemical and physical properties of the soil around them and thus influence the bioavailability of some chemical elements. The soil is a dynamic system subject to short-term fluctuation such as variation in moisture status, pH and redox conditions and also undergoing gradual alterations in response to changes in management and environmental factors (Ann and Clain, 2005).

These changes in soil properties could affect the form and bioavailability of metals and need to be considered in decision on the management of polluted soil or the use of soil for disposal of waste materials (Gardiner and Miller, 2004). Soil can show marked spatial variability in physical and chemical properties at the macro and micro scales, thus emphasizing the need for the thorough sampling to include the range of variance in parameters at any site of investigation.

Soil texture

According to Basu, (2011) soil texture is the proportion of three mineral particles, sand, silt and clay in the soil. These particles are distinguished by size and make up the fine mineral fraction, particles over 2mm in diameter (the coarse mineral fraction) are not considered in texture. The relative amount of various particle sizes in a soil defines its texture, i.e. whether it is clay, loam, sandy-loam or other textural category. Gurevitch et al., (2002) stated that texture is the result of weathering, the physical and chemical breakdown of rocks and minerals, weathering is a relatively slow process, hence texture remains fairly constant and is not altered by management practice.

CHAPTER THREE

MATERIALS AND METHODS

Description of Study Area

Soil samples were collected in Sabon-gari, Samaru, and Tudun-wada areas, Zaria, Kaduna state, Nigeria (figure 3.1) where blacksmith activities (metal-works) were taking place, in March 2015.

CHAPTER FOUR

RESULTS

Physicochemical Parameters of Soil

The result of the physiocochemical properties of the soil samples are presented in Table 4.1.Preliminary visual inspection of the soils from polluted metal work sites from Samaru, Sabon-gari and Tudun-wada showed that the soils were dark grey in colour indicating a low amount of humus. Colour is one of the characteristics of soil, which tells much about the origin of the soil and its composition. The value of pH (water) and pH (CaCl₂) obtained for soil from Tudun-wada are 7.50 and 9.67 respectively; while the values of pH (water) and pH (CaCl₂) obtained for Sabon-gari was 8.43, and 9.03 respectively, indicating the soils are alkaline. The value of pH (water) and pH (CaCl₂) obtained for soil from Samaru are 6.47 and 6.60, indicating the soil is moderately acidic.

The CEC values (in Cmo1^-1kg) was 11.00 at Samaru, 16.33 at Sabon-gari, and 19.67 at Tudun- Wada. Particle size distribution for Samaru was 20.00.% clay, 19.67% silt and 67.33% sand. The particle size distributed for Sabon-gari was 17.00% clay, 17.00% silt and 69.67% silt while the particle size distribution of the soil from Tudun-wada was 12.67% clay, 12.67% silt and 80.00% sand.

CHAPTER FIVE

DISCUSSION

Physicochemical Properties of Soil

Particle size analysis across all sampling locations showed the preponderance of sand fraction ranged from ( 67.33 – 80.00%), followed by silt and clay ranging from (12.67 – 19.67%), thus classifying the parent soils as loamy-sand (soil survey staff,1998). The CEC parameter particularly measures the ability of soils to allow for easy exchange of cations between its surface and solution. The relatively low levels of silt, clay and high CEC indicate the high permeability (Eshan et al., 2006; Atafar, 2010). Preliminary characterization also revealed very high metal concentration. Consequently, there was no need for the soil to be spiked with a mixture of the metal nitrates to achieve higher levels of Cd, Cr, Ni, Pb and Zn. The pH values recorded for the soils in Samaru and Tudun-wada is within the range of agricultural soils, while that recorded slightly higher than the range of agricultural soils. Soil pH play a major function in heavy metals as it directly influences ion-pair formation (Tokalioglu et al., 2006). Each soil had low carbonate and bicarbonate content and high CEC ranging between (11-20). The relatively low levels of carbonate and bicarbonate, and high CEC indicate high permeability hence leachability of heavy metals in the soil. (Eshan et al., 2006; Atafar et al., 2010). Particle size distributed across the sampling locations indicate very high percentage of sand, and low levels of clay and silt respectively, classifying the soils as sandy-loam.

CHAPTER SIX

SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Summary

This study has allowed the determination of the level of some important heavy metals at locations where blacksmith activities are carried out. The blacksmith (metal-work) site is getting polluted particularly with Pb, Zn, Cr and Ni. This is a reflection of anthropogenic contribution which might partly result from continuous smelting of iron and aluminum containers. Based on the concentration of heavy metals in the soils, the soils can generally be classified as moderately polluted. The entire blacksmith site soils are considered to be of pollution concern. There was an indication of uneven distribution of the metals in soils from various sampling locations.

The polluted soil samples was treated separately with aqueous solution of tiron and glutamic acid using full factorial design of experiment. The effect of pH, concentration of complexing agent, amount of soil and contact time on the absorption of: Cd, Cr, Ni, Pb and Zn on the absorption process was also investigated. Eighteen (18) experimental runs including two (2) central points were used for each complexing agent as suggested by the 2⁴ full factorial design generated from design expert 6.0 software.

The interactions between variables were evaluated by response surface methodology and cube plots. The study clearly shows that factorial design of experiment is one of the suitable methods for optimizing absorption process. Statistical analysis of results proved the significance of the models from optimization data. The removal efficiency of tiron was Cd 9.16%, Cr 83.22%, Ni 61.03%, Pb 69.86%, and Zn 59.32%.

Removal efficiency of glutamic acid was 61.44% for Cd, 56.41% for Cr, 37.68% for Ni, 64.79% for Pb and 63.23% for Zn, with large surface presented by high low amount of soil, low pH and high initial concentration interacting to enhance removal efficiency.

Conclusion

From the results of the various studies carried out in this investigation the following conclusions are hereby made:

Total metal concentration across all sampling locations in Samaru and Sabon-gari is moderate but high in Tudun-wada, indicating that metal work activities in this area impact the environment negatively due to metal
The relatively low levels of silt, clay, organic matter and cation exchange capacity indicates high permeability and hence leachability of heavy
There was variation in metal removal efficiency. The removal efficiency showed that complete solubilization of metals did not occur, as not all the chelating agent added to the soil, binded the target metal. This might be due to the presence of other ions such as Ca and Fe which have relatively high stable

Recommendations

Further work on the removal of heavy metals by complexing agents may include the use of other complexing agents like heme, porphine, malate,
Additionally the essential metals Ca, Fe, Mg and Mn may be studied with respect to negative side effects of
The application of tiron and glutamic acid on other polluted sites like dumpsites and heavily fertilized soils, are

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