Assessment of the Competitive Adsorption of Some Heavy Metal Ions Onto Soil

Assessment of the Competitive Adsorption of Some Heavy Metal Ions Onto Soil

Chapter One

Aim and Objectives

The aim of this research was to study the competitive adsorption of Cd, Cu, Cr and Pb ions onto a standard reference soil. This aim was achieved through the following objectives:

optimization of the initial metal ion concentrations, adsorbent dosages and contact times for the adsorption of Cd²⁺, Cr⁶⁺, Cu²⁺and Pb²⁺ from aqueous solution onto soil
modeling the adsorption characteristics of Cd²⁺, Cr⁶⁺, Cu²⁺and Pb²⁺ using Langmuir and Freundlich adsorption
determination of the appropriate kinetic models that best describe the adsorption
investigation of the competitive adsorption of the selected metal ions in the binary, ternary and quaternary

CHAPTER TWO

LITERATURE REVIEW

Mechanism Adsorption of Heavy Metals onto Soil

Heavy metals adsorption is usually described in terms of two basic mechanisms which are; specific adsorption or surface complexation and non-specific adsorption or ion exchange (Mouni et al., 2009). The mechanism of a metal ion adsorption describes the process involved in the binding of the metal ion to an adsorbent and it serves as a basis for quantitative stoichiometric considerations. There are two major processes postulated to be active in adsorption (Mouni et al., 2009). They include; (i) Chemisorption (ii) Physical Adsorption

Chemisorption is an irreversible process. It is limited to monolayer coverage of the adsorbent. A covalent bond is usually formed between the adsorbate and the adsorbent in chemisorption. On desorption, the adsorbent undergoes a chemical change (Bhatia, 2006). The binding force involved in this sorption is very strong and the heat liberated during the process is very large. The enthalpy of chemisorption is within the region of 200 kJ/mol (Atkins, 2014). The energy required for a chemisorbed molecule to react with the molecular species may be considerably less than the energy required when the two species react directly in the gas phase (Atkins, 2014).

Physical adsorption also known as the non-specific adsorption is as a result of long range weak van der waal’s force between adsorbates and adsorbents. The energy released when a particle is physically adsorbed is of the same order of magnitude as the enthalpy of condensation. The enthalpy of physisorption can be measured by monitoring the rise in temperature of a sample of known heat capacity but mostly, typical values are usually in the region of 20 kJ/mol (Atkins, 2014). The adsorption process is exothermic and heat is liberated depending on the magnitude of the attractive force. Physisorption is usually directly proportional to the amount of solid surface available and the adsorption process is reversible (Dash, 2012).

Ion – exchange mechanism of heavy metals in soil

One of the mechanisms described for the adsorption of metal ions on natural soil material involves ion exchange. In most cases, the adsorption of metals on humic substances results in the release of H⁺ and possibly with other exchangeable species such as Ca²⁺, Mg²⁺, Na⁺ and K⁺ (Crist et al., 2004). Ion exchange involves electrostatic interactions between an ion on a charged particle surface and ions in a diffused cloud around the charged particle. It is usually rapid, diffusion-controlled, reversible, stoichiometric, and in most cases there is selectivity of one ion over another by the exchanging surface. Stoichiometry in terms of ion-exchange means that any ion that leaves the adsorbent surface is replaced by an equivalent amount of ion with respect to ionic charge (Sparks, 2003). This same principle applies to the displacement of an adsorbed heavy metal by a more preferred metal ion (Crist et al., 2004).

Factors Influencing Mobility and Adsorption of Heavy Metal ions in Soil

Heavy metal mobility in soil is affected by some factors which include: particle size distribution, pore structure, pH, temperature, organic matter content, competing ions, humic substances and ionic strength of the soil (Naidu et al., 2003).

Particle size distribution and resulting total surface area available for adsorption are important factors in adsorption processes and can affect metal mobility (Hines and Scholes, 2003). Particle size also affects heavy metal content in soils. Small particles with large surface area to mass ratios can adsorb metal ions more than large particles with small surface area to mass ratios (Ackay et al., 2003). Trace metals have potentials for adsorption onto clay minerals, hydrous oxides and organic matter due to their small particle sizes. The amount of extractable metal usually increases with decreasing particle size (Ackay et al., 2003).

CHAPTER THREE

MATERIALS ANDMETHOD

Apparatus and Equipment

Volumetric flasks (1000 cm³), measuring cylinders (50 cm³ and 10 cm³), conical flasks (250 cm³), polypropylene sample bottles (120 cm³), beakers (250 cm³), petri-dish, watch glass, refrigerator, wash bottles, funnels, glass rod, Whatman No1 filter papers, Analytical balance (A and D instrument GR-200EC model), mechanical shaker (Gallenkamp BKS-300-010F model), pH meter (Jenway), deionizer (Elgacan C115 model) and atomic adsorption spectrophotometer (AA240FS Varian).

Advertisements

Reagents

All reagents used were of analytical grade and they include: Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O), potassium chromate(K2Cr2O4), copper nitrate trihydrate (Cu(NO3)2.3H2O), lead nitrate(Pb(NO3)2), tetraoxosulphate (VI) acid (H2SO4), sodium hydroxide (NaOH) and deionised water.

CHAPTER FOUR

RESULTS

Scanning Electron Microscope (SEM) Analysis

The SEM micrographs for the standard reference soil before and after analysis are shown in Figures 4.1 and 4.2 below. The Scanning Electron Micrograph shown in Figures 4.1 and 4.2 clearly reveals the surface texture of the soil before and after adsorption at 1000x magnifications. It was observed that the surface of the soil sample before adsorption was rough, fused together alongside with large particles, but after adsorption, it is evident from the Figure

displayed that the surface morphology of the soil had undergone remarkable physical disintegration due to adsorption of metal ions.

CHAPTER FIVE

DISCUSSION

Scanning Electron Microscope (SEM) Analysis

The Scanning Electron Micrograph shown in Figures 4.1 and 4.2 clearly reveals the surface texture of the soil before and after adsorption at 1000x magnifications. It was observed that the surface of the soil sample before adsorption was rough, fused together alongside with large particles, but after adsorption, it is evident from the Figure 4.2 displayed that the surface morphology of the soil had undergone remarkable physical disintegration. The roughness correlates with the specific surface area. Increasing roughness and porosity produces a greater specific surface area, which makes an adsorbent have a higher adsorption capacity (Noppadol and Pongsakorn, 2014). The disintegration of the soil after adsorption resulted in high surface interaction between metal ion and the binding sites on the surface of the soil. Similar SEM observations have been reported according to Noppadol and Pongsakorn, (2014) and Shakirullah et al., (2006).

Soil pH

The pH of the soil plays a major role in the adsorption of heavy metals and mostly the pH of soils usually ranges from 4 to 10 (Brady and Weil, 2002). Soil pH influences the concentration levels, mobility and the retention of heavy metals in the soil (McBride, 1994). The pH of the soil assessed in this study was found to be 5.5. This means that the soil is moderately acidic (Brady and Weil, 2002). Some heavy metals have been reported to be more stable in slightly or moderately acidic soils to neutral soil (pH 5.5 – 7.3) rather than highly acidic soils (Mouni et al., 2009). Also, according to Fonseca et al., (2011), in the study “Mobility of Cr, Pb, Cd, Cu and Zn in a loamy sand soil”, soils with pH 5.5 were found to have heavy metal ions concentrations below the national legislated limit for soils. It can therefore be inferred that the soil can serve as a good adsorbent for the adsorption of metal ions from aqueous solutions.

CHAPTER SIX

SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Summary

The aim of this research was to assess the competitive adsorption of Cd, Cu, Cr and Pb ions onto a standard reference soil. This research showed that soil is an efficient adsorbent for the sorption of the metal ions studied. The effects of varying parameters: concentration (10 to 40 mg/dm³), adsorbent dosage (0.5 to 2.0 g), contact time (1 – 3 hrs), at room temperature and pH

5.5 of the soil were investigated. The overall percentage adsorption for all the metal ions in the batch experiments except for Cr⁶⁺ (≤ 32.34%) ranged between 91.08 and 99.57 %. The SEM results showed a remarkable physical disintegration on the soil surface after adsorption of the metal ions. The percentage adsorption of Pb²⁺ increased as the initial concentration increased up to 40 mg/dm³ while the adsorption of Cd²⁺, Cr⁶⁺ and Cu²⁺ increased with initial concentration up to 20 mg/dm³, and then gradually decreased as the initial metal ion concentration increased.

Increments in adsorption were observed at adsorbent dosages of 0.5 g to 1.5 g for Cd²⁺ and 0.5 g to 1.0 g for Pb²⁺, Cu²⁺ and Cr⁶⁺ after which further increment of adsorbent resulted in a decline in adsorption capacity. The adsorption of all the metals (except cadmium at 3 hrs) reached equilibrium within 2 hrs. Freundlich isotherm gave a better fitting than Langmuir isotherm for all the metals in terms of R² values. Cd had the greatest sorption capacity as estimated by the maximum sorption parameter (qmax) of the Langmuir equation. The ranked affinity of the selected metals for the soil was Cu > Cd > Pb > Cr according to the Freundlich parameter Kf. The kinetic studies carried out showed that the R² values of the pseudo second order model were higher (≥0.951) than that of the pseudo- first order model (≤0.004) and the calculated adsorption capacities of all the metals by pseudo- second order model were closer to their respective experimentally determined adsorption capacities than those from the pseudo- first order model. Hence, adsorption of these metal ions on the soil can be said to have occurred as a result of electrostatic reactions.

In the competitive experiment, the results showed that the adsorption capacity of the metals decreased as the number of competing ion(s) increased such that the least adsorption capacity was recorded in the quaternary system. Apparently due to Pb’s chemical characteristics such as relatively high electronegativity, lower pKH, small hydrated radius and electronic structure, this metal was the strongest adsorbed than other studied metals. Additionally, statistical results showed that at 95 % confidence limit (α = 0.05), there were significant differences in the means of adsorption of Cd, Cr, Cu and Pb among the groups of systems examined. The lowest percentage of adsorbed Cd was obtained as 31.89 % in the presence of Cr and Pb (in the ternary system). Competition significantly decreased the adsorption of Cd, Cu and Pb onto the soil surface.

Conclusion

These studies showed that the standard reference soil material has a high capacity to adsorb metal ions especially those of Cd, Cr, Cu and Pb. Based on the results obtained from this research, the following conclusions can be deduced

The adsorption of Cd²⁺, Cr⁶⁺, Cu²⁺ and Pb²⁺ by the standard reference soil is dependent on the initial metal ions concentrations of the solution, the mass of soil used and the contact time of the adsorption process.

The pseudo- second order adsorption model is more suitable to describe the adsorption kinetics of Cd²⁺, Cr⁶⁺, Cu²⁺ and Pb²⁺ by the standard reference soil. This implies that the adsorption process is by chemical sorption.

The presence of competing ions led to a significant reduction in the adsorption of Cd²⁺, Cu²⁺ and Pb²⁺ onto the soil.

The selectivity of the metal ions (Cd²⁺, Cr⁶⁺, Cu²⁺ and Pb²⁺) onto the standard reference soil in the competitive system is highly influenced by the difference in their electronegativity and hydrolysis constant parameters.

Recommendation for Further Studies

The composition of the standard reference soil has been well-known, its physical characteristics are acceptably uniform and can be used in experiments without further preparation (washing or sizing). However, it is recommended that a single source be usedby participants in inter-laboratory studies to mitigate the effects of any differences in the experimental
Theapplication of the soil standard reference material as a model or reference standard for subsequent research on adsorption by other soil materials is also
Furtherwork on the removal of other heavy metals by this soil considering the effects of other parameters such as temperature, pH, ionic strength and agitation speed with appropriate modifications may be investigated. Similarly, the analysis of metal pairs using mole ratios instead of mass loading may be useful in understanding the stoichiometry involved in the adsorption
It is also advisable to try working at higher initial concentrations above 40 mg/dm³for selected metal systems, as adsorption behaviour may be different from that at low concentrations.

REFERENCES

Acharya, J., Sahu, J. N., Mohanty, C. R., and Meikap, B. C. (2009). Removal of Lead (II) from Wastewater by Activated Carbon Developed from Tamarind Wood by Zinc Chloride Activation. Chemical Engineering Journal, 149 (1-3): 249-262.
Ackay, H., Oguz, H. and Karapin, C. (2003). Study of Heavy Metals Pollution and Speciation in Buyak Menderes and Grediz River Sedimenst. Water Research, 37: 813 – 822.
Adelaja, O. A., Amoo I. A. and Aderibigbe, A. D. (2011). Biosorption of Lead (II) ions from Aqueous Solution using Moringa oleifera Pods. Archives of Applied Science Research, 3 (6): 50-60.
Ademiluyi F. T. and Ujile, A. A. (2013). Kinetics of Batch Adsorption of Iron II Ions from Aqueous Solution Using Activated Carbon from Nigerian Bamboo. International Journal of Engineering and Technology, 3(6): 623-631.
Ademoroti, C. M. A. (1996). Levels of Trace Heavy Metals on Bark and Fruit of Trees in Benin City, Nigeria. International Journal of Environmental pollution (Senci B), UK, 4; 241- 253.
Aelion, C. M., Harley, T. D., Suzanne, M. and Andrew, B.L. (2008). Metal Concentrations in Rural Topsoil in South Carolina: Potential for Human Health Impact. Science of the Total Environment. 402(2-3): 149-156.
Ahalya, N., Ramachandra, T.V. and Kanamadi, R.D. (2003). Biosorption of Heavy Metals.
Research Journal of Chemistry and Environment, 7(4):83-93.
Al-Degs, Y. S., El-Barghouthi, M. I., Issa, A. A., Khraisheh, M.A. and Walker, G. M. (2006). Sorption of Zn(II), Pb(II) and Co(II) Using Natural Sorbents: Equilibrum and Kinetics Studies. Water Resources, 40: 2645-2658.
Al-Omair, M. A. and El-Sharkawy, E. A. (2007). Removal of Heavy Metals via Adsorption on Activated Carbon Synthesized from Solid Wastes. Environmental technology, 28(4): 443- 451.

Other Topics