ABSTRACT
Modification of coconut (Cocos nucicera L.) shell activated carbon with an Azo ligand: 1, 2– dihydro-1,5-dimethyl-2-phenyl-4-(E)–(2,3,4-trihydrophenyl)-3H-pyrazol-3-one (DDPTP) and its potentials for the removal of Cd2+, Pb2+ and Ni2+ from polluted water samples were studied. It was activated chemically using CaCO3 as the activating agent. Proximate analysis on the coconut shell showed 8.7 % moisture content, 10.4 % volatile matter, 3.2 % ash content and 77.7 % fixed carbon. The developed adsorbent has bulk density of 0.46 g/cm3, pore volume of 8.0 x 10-3 cm3 and the conductivity was 37.9 µS/cm. Fourier Transform Infrared (FTIR) analysis showed that hydroxyl, carbonyl, amino and azo groups are present on the surface of the adsorbent. Scanning Electron Microscope (SEM) showed the micro-pores in the Modified Coconut Shell Activated Carbon (MCSAC) while Energy Dispersive X-ray Spectrum exposed carbon as the major quantitative element with 57 %. Batch adsorption was carried out and the results obtained showed that, MCSAC adsorbed Pb2+ (98 %), Cd2+ (80 %) and Ni2+ (92.2 %) ions more than un-modified coconut shell activated carbon which adsorbed Pb2+ (79 %), Cd2+ (60.2 %) and Ni2+ (73.6 %) ions from aqueous solutions. The quantity of the metal ions adsorbed increased with increase in initial concentrations, contact time, temperature of carbonization, the degree of treatment of adsorbent and pH for each metal. The percentage removal decreased with increase in particle sizes of the adsorbent. It also increased initially with increase in ligand amount but later decreased. Competitive adsorption of Pb2+, Cd2+ and Ni2+ on MCSAC from their mixed solution showed that the percentage removal of Ni2+ was highest with 80.35 % followed by Pb2+, 71.05 % and Cd2+, 45.10 %. The analysis of adsorption isotherm showed that, adsorption of Ni2+ followed Langmuir isotherm than Cd2+ and Pb2+; Ni2+ and Cd2+ followed Freunlich isotherm than Pb2+; Ni2+ and Pb2+ followed Temkin isotherm than Cd2+. Kinetic studies showed that the sorption of the metal ions can also be described by pseudo-first-order (Pb2+ and Ni2+), pseudo-second-order (Cd2+ and Ni2+) and intra-particle diffusion models for the three metals.
CHAPTER ONE
1.0 Introduction
1.1 Background of the Study
The presence of trace heavy metals in natural water has aroused the interest of many Nigerian scientists as a result of their environmental effects on the health of both plants and animals. More so, concerns about environmental protection has increased due to the technology1 development which keeps on changing, producing industrial product, as well as waste. Manufacturing industries have played an important role for economic growth in major countries. This sector provides services and product for better way and quality of life. However, rapid change in industrialization produces vast amount of waste and will cause harm and deterioration of the environment and ecosystem if improperly managed. Pollutants from textiles industry was declared as one of the major sources of wastewater in Asian country1 as it is considered as possible carcinogenic or mutagen. Apart from that, heavy metals such as cadmium, chromium, lead, copper, manganese, zinc as well as mercury and nickel are widely discharged in the wastewater from industries and are very toxic and harmful to living organisms by lowering the reproductive success, preventing proper growth and even causing death2. Some of the heavy metals are important for our body requirement; however exceeding the tolerance limit may create harm to body functions.
The most toxic heavy metals are Cd, Pb and Hg ions due to their high attraction for sulphur which will disturb enzyme function by forming bon d with sulphur. The ions will hinder the transport process through the cell wall, thereby disturbing the cell function. Other pollutants from the industries are phenol; from refineries, petrochemical wastewater, pulp mills and coal mines. Presence of phenols in water bodies caused carbolic odor to receiving water bodies, thus causing toxic effects on aquatic flora and fauna3. Apart from that it is also toxic to humans and affects several biochemical functions4.
Unlike organic pollutants, heavy metals do not biodegrade and thus, pose a different kind of challenge for remediation. To alleviate the problem of water pollution by heavy metals, various methods have been used to remove them from waste water such as chemical precipitation, coagulation, floatation, adsorption, ion exchange, reverse osmosis and electrodialysis5-7. The production of the sludge in the precipitation methods poses challenges in handling treatment and hand filling of the solid sludge. Ion exchange usually requires a high – capital investment for the equipment as well as high operational cost. Electrolysis allows the removal of metal ions with the advantage that there is no need for additional chemicals and also there is no sludge generation. However, it is inefficient at a low metal concentration. Membrane processes such as reverse osmosis and electrodialysis tend to suffer from the in-stability of the membranes in salty or acidic conditions and fouling by inorganic and organic substances present in waste water8. Most of these techniques have some pretreatments and additional treatments. In addition, some of them are less effective and require high cost9.
It was only in the 1990s that a new scientific area, biosorption was developed that could help in the recovery of heavy metals. The first reports described how abundant biological materials could be used to remove, at very low cost, even small amounts of toxic heavy metals from industrial effluents9-11. Metal-sequestering properties of non-viable microbial biomass provide a basis for the removal of heavy metals when they occur at low concentrations9. Therefore, many researchers have applied regenerated wastes to treat heavy metals from aqueous solutions.
The main objective of the method is to treat the wastewater before discharging to water source, thus decreasing the threat and deterioration to the environment and promising better sustainability of the environment. There are many technologies that have been developed for purification and treatment of waste water including chemical precipitation, solvent extraction, oxidation, reduction, dialysis/electro dialysis, electrolytic extraction, reverse osmosis, ion-exchange, evaporation, cementation, dilution, adsorption, filtration, floatation, air stripping, steam stripping, flocculation, sedimentation and soil flushing/washing chelation12. The selection technologies must be analyzed accordingly based on several factors such as available space for construction of treatment facilities, ability of process equipment, limitation of waste disposal, desired final water quality and cost of operation. Mostly, all the technologies listed above are less likely to be selected because they required large financial input and their applications are limited due to the associated cost factors. Adsorption process is found to be the most suitable technique to remove pollutants from wastewater. It is mostly preferred due to its convenience, ease of operation and simplicity of design. Apart from removing many types of pollutants, it also has wide application in water pollution control. Activated carbon (AC) is widely used as absorbent due to its high surface area and pore volume as well as inert properties. However, conventional AC is expensive due to the depletion of coal-based source and especially for producing high quality AC13.
To counter the high cost of AC, low cost precursors have been of high interest for researchers to replace the conventional AC. The factors affecting substitution of raw material are high carbon content, low inorganic content, high density and sufficient volatile content, stability of supply in the countries, potential extent of activation and inexpensive material6. The AC is mainly comprised of carbon with large surface area, large pore volume and porosity where the adsorptions take place.
There are some reviews reporting the use of coconut and palm shell for the production of AC14; however such studies are restricted to either type of wastes, preparation procedures, or specific aqueous-phase applications. But, due to the abundant source of precursors, with high volatile, carbon contents, and hardness; coconut shells are an excellent raw material source to produce activated carbon suitable to replace conventional AC14. Moreover, this can be said to be, “substitution of waste to wealth”. The adsorption capacity of the adsorbent could be improved by its modification. This is because; the functional groups on the surface of the AC could be improved by modification with a ligand that has electron donating groups like hydroxyl group, amide group, etc.
