CHAPTER ONE
INTRODUCTION
1.1 Background of the Study Lead is a common contaminant of soil and considered to be a risk to human health. Lead may contaminate soil through vehicle exhausts, sewage-sludge biosolids, mining, and smelting. Toxicity from Pb-contaminated soils primarily occurs from direct ingestion. Symptoms of lead poisoning in human beings include irritability, poor muscle coordination, nerve damage, increased blood pressure, developmental delays etc. Heavy metal concentration in soil solution is of great importance for all eco logical consideration because the plants are likely to take up the available metals from soil solution (Doumett et al. 2014). The transport of metals within the soil or even to groundwater depends on the metal concentration of the solution phase. It was suggested that the fate and transport of toxic metal ions in the environment are generally controlled by adsorption reactions, complexation etc. Studies on Pb (II) adsorption were performed on soils, clay minerals and oxides. These studies showed that soil type, ionic strength, ion type, contact time affected the adsorption of heavy metals onto soils and clay minerals. Other factors such as liquid: solid ratio, solution pH, ionic strength and temperature also affect sorption process. It is well known, tropical and subtropical regions are distributed with large areas of variable charge soils. These soils usually carry both positive and negative charges on their surfaces, therefore can adsorbed both anions and cations (Ishii et al. 2013). It has been observed that increasing the contact time favor the adsorption of metal ions because the sorbed phase of metals changed from loosely bounded phase to strongly bounded phases with increasing contact time. However, there are few studies and limited information on the effect of these operating variables on the adsorption and affinity of Pb onto variable charge soils. The process of adsorption, though widely applied, is still only partially understood. This paper therefore investigated impact of reaction variables like contact time and temperature and on adsorption and affinity of Pb2+ onto three variable charge soils. The aim is to provide scientific information that will help in the management of lead contaminated soils (Manouchehri et al. 2016). Kaolin is abundantly found in nature and has shown the potential to be used as a low-cost adsorbent for toxic pollutants in aquatic environments. It has high chemical stability, cation exchange capacity, modifiable layered structure, small negative charge, and high specific surface area (Rhodes, 2012). Its layered mineral structure consists of alumina octahedral and silica tetrahedral sheets with shared oxygen atoms. In addition, the surface of alumina octahedral layers is covered with hydroxyl groups that enable bridging between the layers through hydrogen bonds (Ademoroti, 2016). The enhancement of surface properties and adsorption ability of kaolin can be done through activation and calcination of materials. The calcination of kaolin has shown to induce its reactivity, with the temperature used usually ranging from 550 to 950°C, which changes the material to metakaolin (MK) (Ehsan et al. 2016). The heating process also removes the impurities and improves the surface area of kaolin. However, calcination could affect the structural integrity of kaolin, and eliminate the hydroxyl group. Therefore, acid and alkali treatments were performed by many researchers to increase the surface area and surface functional groups. Further improvement of kaolin performance for the removal of cationic dyes has been performed using various types of nanoparticles, such as Fe3O4, TiO2 and CuFe2O4, as well as carbonaceous material such as carbon nanotube and graphene oxide (GO) (Jean et al. 2016). Effluents discharge from industries is a major source of environmental pollution globally. Heavy metals are one of the dominant contaminants in industrial effluents and discharging heavy-metal laden into soil at elevated concentrations is considered lethal to living organisms. Excessive accumulation of metal ions in living tissue impedes the biological functions of vital organs resulting into health problems (Manouchehri et al. 2016). Several treatment methods have been devised for removal of heavy metals from soil. Adsorption technique is considered one of the effective low-cost methods for removal of heavy metals. Adsorption of heavy metals from aqueous solutions has been studied using different adsorbents including clay. The use of kaolinite clay for adsorption is worthwhile because of its abundance in deposit especially in Nigeria (Naidu et al. 2011). Kaolinite clay is a phyllosilicate clay mineral of interlayer structure type 1:1 with ideal structural formula of Al Si O (OH). Several researchers have investigated the adsorption of heavy metal ions in aqueous solution on kaolinite 2+ clay. The data obtained from adsorption of Pb, 2+ 3+ 2+ 2+Cu, Fe , Mn and Zn ions on kaolinite showed that there was favourable adsorption of the metal ions (Sanborn et al. 2015). Investigation of 2+ 2+ 2+ 2+ adsorption of Pb , Cu , Cd , and Zn ions on 2+ different clays have shown that Pb ion, has greater affinity for the clays than the other metal ions (Zachara et al. 2016). Nigerian kaolinite clay have been used to simultaneous adsorb Ni and 2+ Mn ions from wastewater.