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DEVELOPMENT OF ADSORBENTS AND PHOTOCATALYSTS USING NIGERIAN PINDIGA BENTONITIC CLAY

ABSTRACT

The potential of Pindiga bentonitic clay for phenol adsorption from aqueous solution and photocatalytic degradation was studied. Pindiga bentonic clay was treated with oxalic acid and calcined at a temperature of 1000oC and was successfully used as an adsorbent and for the degradation of phenol under visible light illumination. The processes were investigated by X-Ray Fluorescence (XRF), X-Ray diffraction (XRD), and surface area analysis. The clay treated with acid for 60 min (PB-60) and 5 min (PB-5) gave higher surface areas of 363.61 m2/g and 265.99 m2/g respectively compared with the raw (PB) and the raw calcined (PBC) with surface areas of 151.69 m2/g and 47.13 m2/g respectively. The adsorptions of phenol by the acid treated clays were studied using pseudo-first order, pseudo-second order kinetic models and intra-particle diffusion model. The adsorption data does not fit well with pseudo-second order kinetic model. The Freundlich and Langmuir adsorption models were used for the mathematical description of adsorption equilibrium and it was found that the experimental data fitted very well to the Freundlich model. The clay treated with acid for 60 minutes (PB-60) showed a better monolayer coverage capacity and greater affinity for phenol compared with the PB, PBC and PB-5. The increase in pH values from 5 – 11 was observed to hinder adsorption processes. Better adsorption was observed at lower pH value. Increase in catalyst dosage increases the adsorption rate. Langmuir-Hinshelwood kinetic model was employed for photocatalysis processes and the values of krand KLH were calculated as 6.8483 mgl-1min-1 and 0.0034lmg-1 for PB-60. PB-60 degraded phenol better than PB, PBC and PB-5 under visible light illumination. Catalyst dosages of 1.5, 2.0, 2.5 and 3.0g were used and the optimum catalyst dosage was found to be 2.5g/l for the photocatalytic degradation.

CHAPTER ONE

INTRODUCTION

1.1   Preamble

Bentonite is an absorbent aluminium phyllosilicate, impure clay consisting mostly of montmorillonite. The absorbent clay was given the name bentonite by Wilbur C. Knight in 1898, after the Cretaceous Benton Shale near Rock River, Wyoming (Hosterman and Patterson, 1992). There are different types of bentonite, each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminium (Al). Experts debate a number of nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as similar clay called tonstein, has been used to describe clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite.

Photocatalysis, one of the advanced physico-chemical technology applicable in photodegradation of organic pollutants, has attracted much attention in recent years. Photocatalysis can generally be described as a process in which light is used to activate a substance.The photocatalyst that aid this process is itself not involved in the chemical transformation.