Cadmium and lead adsorption capacity of selected nsukka urban soils

Thesis Overview

<p> </p><ul><li><strong>INTRODUCTION</strong></li><li><strong>Background of the Study</strong></li></ul><p>Soil is one of the most important components of all terrestrial ecosystems and is of essential importance and it plays a pivotal role in the sustenance of all life forms on the planet. Some of the roles played by soil range from the simplest of functions like providing anchorage and nutrients for the growth of crops, trees and grassland, and regulating water supplies, to more complex functions such as helping in maintaining a clean environment via degradation and transfer of biomass and being a source and sink for atmospheric gases1. The importance of soils to all life forms cannot be overemphasize.</p><p>Soil is a very complex heterogeneous mixture, which consists of solid phases (the soil matrix) containing minerals and organic matter and fluid phase (the soil water and the soil air), which react with each other and ions entering the soil system2.</p><p>Due to growing industrialization and urbanization, heavy metals are increasingly introduced into the environment mainly soil, via a variety of sources. These sources include industrial processes, application of sewage sludges, fertilizers, pesticides, and fungicidal sprays applied to plants, atmospheric deposition, municipal effluent, and disposal of electronic waste.3 Unfortunately, some of these heavy metals can be taken up by crops, thereby entering the food chain. Hence, soils provide a potential pathway with which heavy metals may become bioavailable to humans.</p><p>Soils in urban environment have direct influence on public health, this is because they receive higher than normal loads of contaminants from anthropogenic activities, mostly in industrial areas. Most of the heavy metals are bound to particles in sediments, but only a small quantity becomes dissolved in the water and it can spread widely in the food chain4.</p><p>The term heavy metal refers to any metallic element that has a relatively high density, toxic or poisonous at low concentrations, that are stable and cannot be degraded or eliminated5. Heavy metals have been classified into essential and non-essential metals. The essential metals are needed by living organisms in trace quantities for optimum performance of life processes. They include Ni, Fe, Zn, Co, Mo, e.t.c.6. Insufficient supply of these essential metals in an organism, leads to problems associated with growth and ability to complete its life cycle, while sufficient supply results in optimum conditions &nbsp; and excess supply results in toxic effects and possibly death7.</p><p>The non-essential elements include As, Ag, Cd, Hg, and Pb and the ability of various organisms to accommodate these non-essential metals are limited8. They may be tolerated at very low concentrations, some are toxic even if their concentration is very low, and their toxicity increases with accumulation in water and soils9.</p><p>Heavy metal ions are the most toxic inorganic pollutants which are present in soils and can be of natural or anthropogenic origin10. Heavy metals may be found in soils11, ground water12, sediments, plants13 and even in dust14. They cause many health problems, some of which include cancer, renal damage, Wilson’s disease(neurological or psychiatric symptoms of liver disease, compounded with heavy metal deposits), lung damage, dermatitis, nausea, chronic asthma, headache, dizziness, rapid respiration, e.t.c.12, 15.</p><ul><li><strong>Behaviour of Metals in Soils</strong></li></ul><p>The soil is the primary recipient either by design or accident of diverse waste products and chemicals used in industry16. Natural source of trace metal elements in soils is parent materials of the soil. Anthropogenic sources, including industrial emissions and effluents, bio-solids, fertilizers, and pesticides can also contribute to the increase in the amount of heavy metals in soils17. Heavy metals are spread to the ground in several ways, such as spillage or corrosion of products that contain heavy metals. The heavy metals that are spread to the ground by airborne particles remain mainly in the ground where they are deposited, because heavy metals have strong affinity for the soil matrix17. In the ground, heavy metals are distributed as either soluble in the soil solution, sorbed to the soil matrix or precipitated as solids17. While in the soil solution, heavy metal can exist as a free ion, as well as soluble complexes with different organic and inorganic ligands17. Common inorganic ligands in the soil are SO42-, Cl¯, OH¯, PO43-, NO3¯ and CO32-, and common organic ligands are low molecular weight aliphatic, aromatics, amino acids and soluble constituents of fulvic acids such as phenol17.</p><p>The persistence of heavy metals in the soil and its reduction in mobility involves the phenomena of sorption, desorption, precipitation, complexation, oxidation/reduction and dissolution. Although these phenomenons can occur simultaneously, adsorption mechanisms are known to be determinant in the control of metal availability and solubility in the soil18. Adsorption phenomenon in soils controls the concentration of metal ions and complexes in the soil solution and thus can exert a major influence on translocation of the metal ions in plant19.</p><p>The term ‘sorption’ is often broadly used to encompass all processes that involve either the retention or release of contaminants in soil. Sorption process is one of the most important chemical processes that occurs in soils, it governs the rate and quantity of nutrient uptake by plants, transport of heavy metals, radionuclides, pesticides, organic pollutants and other materials within soil systems. Sorption processes are complex and are influenced by soil properties such as texture, bulk density, pH, organic matter, cation exchange capacity, and the type and amount of the different clay minerals present in any particular soils20.</p><p>Adsorption of heavy metals in soil involves two mechanisms. One is selective adsorption, in which surface complexes are formed; while the other is non-selective adsorption, in which the metallic cations act as counter-ions in the diffuse layer. The importance of each of these mechanisms depends on the metal and the nature of soil21.</p><p>Contamination of the environment most especially soils, by heavy metals, through increasing anthropogenic activities, has been on the rise in recent years and this has been of considerable concern due to environmental hazards, and various health problems it poses such as cancer, gene mutation, neurological disorder, e.t.c. Moreover, since soils offer a possible sink for pollutants, there is a need to investigate how pollutants are sorbed by soil and the fate of the pollutants in the environment. Furthermore, the presence of heavy metals in Nsukka urban soil has been well documented by researchers22, 23.However, the adsorption capacity of the soil for heavy metals has not been reported. Also, reports on how Nigerian soils function as sorbent for heavy metals are limited in literature. In addition, IUPAC observed that the majority of investigations of fate of pollutants in the environment have been conducted in temperate soils, predominately in Europe and North America, with limited report on tropic soils most especially Africa24. However, approximately one-half of the earth’s population and roughly one-third of its land mass are found in the tropics24. Hence, this has necessitated and underscores the need for this study.</p><ul><li><strong>Aims and Objectives of the Study</strong></li></ul><p><strong>1.4.1 &nbsp; Aims of Study</strong></p><p>The main aim of the present work is to study the retention capacity of cadmium (Cd) and lead (Pb) by selected Nsukka urban soils as well as to investigate the effect of some parameters on the adsorption of Cd and Pb by the soils using the batch technique.</p><p><strong>&nbsp;</strong></p><p><strong>&nbsp;</strong></p><p><strong>&nbsp;</strong></p><p><strong>&nbsp;</strong></p><p><strong>&nbsp;</strong></p><p><strong>1.4.2 &nbsp; Objectives of Study</strong></p><p><strong>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </strong>The objectives of this research work are to:</p><ol><li>ascertain the physico-chemical properties of soil of the study area.</li><li>determine cadmium and lead adsorption capacity of Nsukka urban soils by investigating</li></ol><p>the effects of pH, concentration, temperature and contact time on the adsorption process.</p><p>iii. evaluate the adsorption isotherm of the adsorption process using the empirical models; &nbsp; Langmuir, Freundlich and Temkin isotherm.</p><ol><li>propose the mechanism of the adsorption process using kinetics models of pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion.</li><li>provide baseline data on cadmium and lead adsorption capacity of the soils of the study areas.</li></ol><p><strong>1.5 &nbsp; &nbsp; Significance of the Study</strong></p><p>This study is of a great significance to the researcher as the knowledge created in relation to the study will be of immense benefit towards the understanding of how cadmium and lead are sorbed by the soil and the bioavailability of these metal ions in soil of the study areas. Additionally, the knowledge created regarding the study can give a clue on cadmium and lead translocation in plants grown on the soils.</p><p>Also, the outcome of this study can be of help towards contributing to the design of effective remediation strategies and providing possible alternatives for a preventive environmental control that is needed by environmentalist via formulation of guidelines.</p><p>Furthermore, the outcome of the study can serve as additional database to the researchers in onward adsorption studies of heavy metals by soils, most especially tropical soils.</p><p><strong>&nbsp;</strong></p><p><strong>&nbsp;</strong></p><p><strong>1.6 &nbsp; &nbsp; &nbsp; Scope of Study</strong></p><p>The research focuses on and is limited to cadmium and lead adsorption capacity of Nsukka urban soils using batch equilibrium techniques. The investigation was done by varying operational variables such as pH, initial concentration, contact time, and temperature to determine the effects of the variables on the adsorption process. The adsorption capacity of the soils for the metal ions is described using empirical models; Langmuir, Freundlich and Temkins adsorption isotherm model. The mechanism of the reaction is described using the pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion kinetics model.</p> <br><p></p>