Project Abstract

Project Overview

<p> NTRODUCTION<br>1.1 Background of the study<br>The amount of heavy metals released to the environment has been increasing<br>significantly resulting from industrial activities and technology development1. Contaminations<br>by heavy metals exist in aqueous waste streams of many industries such as metal purification,<br>metal finishing, chemical manufacturing, mining operations, smelting, battery manufacturing,<br>and electroplating. As a result of industrial activities and technological development, the amount<br>of heavy metals discharged into streams and rivers by industrial and municipal wastewater have<br>been increasing incessantly2.<br>Heavy metals are member of a loosely-defined subset of elements that exhibit metallic<br>properties, which mainly includes the transition metals, some metalloids, lanthanides, and<br>actinides. Certain heavy metals such as iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn)<br>are required by humans for normal biological functioning. However, heavy metals such as<br>mercury, lead, cadmium, cobalt are toxic to organisms. Most of the health disorders are linked<br>with specific tendency of heavy metals to bioaccumulate in living tissues and their disruptive<br>integration into normal biochemical processes3.<br>Increased use of metals and chemicals in the process industries has resulted in generation<br>of large quantities of effluent that contains high level of toxic heavy metals and their presence<br>poses environmental disposal problems due to their non-degradable and persistence nature.<br>Soil particles tend to have a variety of charged sites on their surfaces, some are positive while<br>some are negative. The negative charges of these soil particles tend to attract and bind the<br>positively charged metal cations, preventing them from becoming soluble in water. The soluble<br>form of metals is more dangerous because it is easily transported, hence more readily available to<br>plants and animals.<br>Metal behaviour in the aquatic environment is similar to that outside a water body.<br>Sediments at the bed of streams, lakes and rivers exhibit the same binding characteristics as soil<br>particles mentioned earlier. Hence, many heavy metals tend to be sequestered at the bottom of<br>water bodies. Yet, some of these heavy metals will dissolve. The aquatic environment is more<br>susceptible to the harmful effects of heavy metal pollution. Metal ions in the environment<br>bioaccumulate and are biomagnified along the food chain. The effect of heavy metals is more<br>2<br>pronounced in animals at higher trophic levels4. Some metals may be either beneficial or toxic,<br>depending on concentration.<br>Lead (Pb) is the most significant toxic of the heavy metals and its effects are of a<br>toxicological and neurotoxic nature including irreversible brain damage in humans. Inorganic<br>forms of lead typically affect the central nervous system, peripheral nervous system, and<br>hematopoietic, gastrointestinal, cardiovascular, and reproductive systems. Organic lead toxicity<br>predominantly tends to affect the central nervous system. Other hazardous effects of lead are<br>visual disturbances, convulsions, loss of cognitive ability, antisocial behavior, constipation,<br>anemia, tenderness, nausea, vomiting, severe abdominal pain, and gradual paralysis in the<br>muscles5. However, human activity has resulted in atmospheric Lead, mainly as PbSO4 and<br>PbCO3. Industries such as coating, paint, lead smelting and mining generate large quantities of<br>wastewater containing various concentrations of lead.<br>Another element of interest is copper; copper (Cu) is a chemical element, a soft reddishbrown<br>metal used for making electric wires, pipes, etc. it is also beneficial to organisms. The<br>American Medical Association has recommended 1.2 – 1.3 mg/day as the dietary requirements<br>for Copper. On the average, drinking water accounts for less than 5 % of our daily copper intake.<br>The U.S. Environmental Protection Agency (U.S. EPA) has determined that copper level in<br>drinking water should not exceed 1300 ug/L. In 1974, congress passed the safe drinking water<br>Act. This law requires Environmental Protection Agency (EPA) to determine safe levels of<br>chemicals in drinking water which may cause health problems. The Maximum Contamination<br>Level Goals (MCLG) for copper has been set at 1.3 parts per million (ppm) because,<br>Environmental Protection Agency believes that this level of protection would not cause any of<br>the potential health problems resulting from excess level of Cu6. Short periods of exposure can<br>cause gastrointestinal disturbance, including nausea and vomiting while Long-term exposure to<br>copper can cause irritation of the nose, mouth and eyes and it causes headaches, stomachaches,<br>dizziness and diarrhea. Use of water that exceeds the maximum Level of copper over many years<br>could cause liver or kidney damage.<br>Cadmium (Cd) is present in air in the form of particles in which cadmium oxide is<br>probably an important constituent. Cigarette smoking increases cadmium concentrations inside<br>houses. The average daily exposure from cigarette smoking (20 cigarettes a day) is 2 to 4 μg of<br>cadmium. Cadmium concentrations in unpolluted natural waters are usually below 1 μg/dm3.<br>Contamination of drinking water may occur as a result of the presence of cadmium as an<br>3<br>impurity in the zinc of galvanized pipes or cadmium-containing solders in fittings, water heaters,<br>water coolers and taps. Food is the main source of cadmium intake for non-occupationally<br>exposed people. Crops grown in polluted soil or irrigated with polluted water may contain<br>increased concentration of Cd(II). Levels of Cd(II) concentrations in fruit, meat and vegetables<br>are usually below 10 μg/kg, in liver 10–100 μg/kg and in kidney 100 – 1000 μg/kg. Cadmium<br>concentrations in tissues increase with age. Both kidney and liver act as cadmium stores, 50–85<br>% is stored in kidney and liver, 30–60 % being stored in the kidney alone7.<br>Cobalt (Co) is used in many alloys (superalloys for parts in gas turbine aircraft engines,<br>corrosion resistant alloys, high-speed steels, cemented carbides), in magnets and magnetic<br>recording media, as catalysts for the petroleum and chemical industries, as drying agents for<br>paints and inks. Cobalt blue is an important part of artists’ palette and is used by craft workers in<br>porcelain, pottery, stained glass, tiles and enamel jewelers. The radioactive isotopes, cobalt-60, is<br>used in medical treatment and also to irradiate food, in order to preserve the food and protect the<br>consumer. As cobalt is widely dispersed in the environment humans may be exposed to it by<br>breathing air, drinking water and eating food that contains cobalt. Skin contact with soil or water<br>that contains cobalt may also enhance exposure. Cobalt is not often freely available in the<br>environment, but when cobalt particles are not bound to soil or sediment particles the uptake by<br>plants and animals is higher and accumulation in plants and animals may occur8.<br>1.1.1 Hazards of Heavy Metal Contamination<br>The main threats to human health from heavy metals are associated with exposure to<br>lead, cadmium, mercury, copper, cobalt and arsenic. These metals have been extensively studied<br>and their effects on human health regularly reviewed by international bodies such as the World<br>Health Organization (WHO). Heavy metals have been used by humans for thousands of years.<br>Although several adverse health effects of heavy metals have been known for a long time,<br>exposure to heavy metals continues, and is even increasing in some parts of the world, in<br>particular in less developed countries, though its contaminant have declined in most developed<br>countries over the last 100 years. For example, Cadmium compounds which are currently used in<br>re-chargeable nickel–cadmium batteries have increased dramatically during the 20th century, one<br>reason being that cadmium-containing products are rarely re-cycled, but often dumped together<br>with household waste. Also, cigarette smoking is a major source of cadmium exposure, while in<br>non-smokers; food is the most important source of cadmium exposure. Recent data indicate that<br>4<br>adverse health effects of heavy metals exposure may occur at lower exposure levels, primarily in<br>the form of kidney damage but possibly also bone effects and fractures9. Many individuals<br>already exceed these exposure levels and the margin is very narrow for large groups. Therefore,<br>measures should be taken to reduce heavy metals exposure in the general population in order to<br>minimize the risk of adverse health effects. The general population is primarily exposed to<br>mercury via food, fish being a major source of methyl mercury exposure, and dental amalgam.<br>The general population does not face a significant health risk from methyl mercury, although<br>certain groups with high fish consumption may attain blood levels associated with a low risk of<br>neurological damage to adults. Since there is a risk to the fetus in particular, pregnant women<br>should avoid a high intake of certain fish, such as shark, swordfish and tuna; fish (such as pike,<br>walleye and bass) taken from polluted fresh waters should especially be avoided. There has been<br>a debate on the safety of dental amalgams and claims have been made that mercury from<br>amalgam may cause a variety of diseases. However, there are no studies so far that have been<br>able to show any associations between amalgam fillings and ill health.<br>The general population is also exposed to lead from air and food in roughly equal<br>proportions. During the last century, lead exposure to ambient air has caused considerable<br>pollution, mainly due to lead emissions from petrol. Children are particularly susceptible to lead<br>exposure due to high gastrointestinal uptake and the permeable blood–brain barrier. Blood levels<br>in children should be reduced below the levels so far considered acceptable, recent data<br>indicating that there may be neurotoxin effects of lead at lower levels of exposure. Although lead<br>in petrol has dramatically decreased over the last decades, thereby reducing environmental<br>exposure, phasing out any remaining uses of lead additives in motor fuels should be encouraged.<br>The use of lead-based paints should be abandoned, and lead should not be used in food<br>containers. In particular, the public should be aware of glazed food containers, which may leach<br>lead into food.<br>Exposure to arsenic is also mainly via intake of food and drinking water, food being the<br>most important source in most populations. Long-term exposure to arsenic in drinking-water is<br>mainly related to increased risks of skin cancer, but also some other cancers, as well as other skin<br>lesions such as hyperkeratosis and pigmentation changes. Occupational exposure to arsenic,<br>primarily by inhalation, is usually associated with lung cancer. Clear exposure–response<br>relationships and high risks have been observed.<br>5<br>Biosorption is presented as an alternative to traditional physicochemical means for<br>removing toxic metals from ground-waters and wastewater. It is a relatively new process that has<br>proven very promising in the removal of contaminants from aqueous solutions. It has been<br>shown to be an economically feasible alternative method for removing heavy metals.<br>Mechanisms involved in the biosorption process include chemisorptions, complexation, ion<br>exchange, microprecipitation, hydroxide condensation onto the biosurface and surface<br>adsorption. The phenomenon of biosorption has been described in a wide range of non-living<br>biomass like nile rose plant powder and ceramics10.<br>In this study, the adsorption of heavy metals onto biomaterials derived from Adansonia<br>digitata plant commonly known as baobab was investigated. Often referred to as grotesque by<br>some authors, the main stem of larger baobab (Adansonia digitata) trees may reach enormous<br>proportions of up to 28 metres. The massive squat cylindrical trunk gives rise to thick tapering<br>branches resembling a root-system, which is why it has often been referred to as the upside-down<br>tree. The stem is covered with a bark layer, which is 50-100 mm thick. The bark is greyish<br>brown and normally smooth. The leaves are hand-sized and divided into 5-7 finger-like leaflets.<br>Being deciduous, the leaves are dropped during the winter/harmattan and appear again in late<br>spring or early summer/rain11.<br>The usefulness of the biomass of many plant materials in removing metal ions from aqueous<br>solution have been investigated by several researchers and all have shown that the plant base<br>adsorbents have the potential of being used as cheap source of biosorbent for metal ions,12,13,14,15.<br>However, no such work has been reported for Adansonia digitata to our knowledge.<br>1.2 Statement of problem<br>Heavy metals released by a number of industrial processes are major pollutants in marine,<br>ground, industrial and even treated wastewaters. A high degree of industrialization and<br>urbanization has substantially enhanced the degradation of our aquatic environment through the<br>discharge of industrial wastewaters and domestic wastes. The presence of heavy metals in water,<br>even at very low concentrations, is highly undesirable. The problem of heavy metal pollution in<br>water needs continuous monitoring and surveillance as these elements do not degrade and tend to<br>biomagnify in man through food chain.<br>This environmental problem has led to extensive research into developing effective alternative<br>technologies for the removal of these potentially damaging substances from effluents and<br>6<br>industrial wastewaters. Moreover, recovery of heavy metals from industrial waste streams is<br>becoming increasingly important to neutralize the hazard from the industrial waste harmful to<br>plant and animal life. Hence there is a need to remove the heavy metals from the industrial<br>wastewater before disposal.<br>1.3 Objectives of the study<br>The effluent treatment in developing countries is expensive and high cost is associated<br>with the dependence on imported technologies and chemicals. The indigenous development of<br>treatment techniques and chemicals or use of locally available non-conventional materials to<br>treat pollutants seems to be the solution to the increasing problem of treatment of effluents. In<br>this regard, there has been a focus on the use of appropriate low cost technology for the treatment<br>of wastewater in developing countries in recent years. Technically feasible and economically<br>viable pretreatment procedures with suitable biomaterials based on better understanding of the<br>metal biosorbent mechanism(s) are gaining importance. Activated carbon of agricultural waste<br>products as low cost adsorbents has been reported till now. However, there is an additional cost<br>involved in the conventional methods of waste water treatment, which is posing economic<br>difficulties necessitating research on alternate adsorbents with equivalent potential of the<br>conventional methods.<br>The objectives of this research were to:<br>(i) investigate the biosorption of some heavy metals by Adansonia digitata roots, stem<br>powder and activated carbon made from its stem.<br>(ii) identify the optimum conditions for the removal of the heavy metals by Adansonia<br>digitata plant parts.<br>(iii) compare the ability of Adansonia digitata activated carbon as adsorbent to activated<br>carbon from some plants.<br>(iv) investigate simultaneous removal of Pb(II), Cd(II), Cu(II), and Co(II) from mixed<br>aqueous solution by Adansonia digitata root, stem powder and activated carbon made<br>from the stem.<br>(v) develop adsorption kinetic models for the studied processes and<br>7<br>(vi) carry out desorption studies on the heavy metals loaded activated carbon (ADSAC).<br>1.4 Significance of the study<br>This study was to remove heavy metals [Pb(II), Cd(II), Co(II) and Cu(II)] which causes<br>environmental problem from aqueous solutions. These heavy metals cannot be destroyed as they<br>are not biodegradable. It means that the pollution of heavy metals in the environment will<br>continuously increase if there is no immediate action taken to remove these heavy metals. As<br>people or living organisms are exposed to the dangers of these heavy metals, it can be dangerous<br>because they tend to bioaccumulate and easily enter our bodies via food, drinking water, dermal<br>contact and air. It is important to study the removal of these heavy metals. This study, a<br>biosorption process, is a biological method of environmental control as an alternative to<br>conventional methods that are ineffective or extremely expensive. Natural materials such as<br>Adansonia digitata plant parts which are environmental friendly, easily available, and cheap is<br>been used in this study. With this technology (biosorption) and the use of Adansonia digitata<br>plant as an adsorbent, a huge success will be recorded in the treatment of waste waters.<br>1.5 Scope of the study<br>The scope is to study the removal of heavy metals, Pb(II), Cd(II), Co(II) and Cu(II) in<br>aqueous solution using Adansonia digitata roots powder and activated carbon made from its<br>stem through the biosorption process.<br>The following limit has been defined:<br>(i) Determination of functional groups on the surface of the sample that contribute to the<br>biosorption of heavy metals used in the study through infrared spectroscopy.<br>(ii) Determination of the pore sizes in the adsorbent that enhance the adsorption capacity<br>using scanning electron microscope (SEM).<br>(ii) Determination of the agitation/equilibrium time, pH, dosage, carbonization temperature,<br>particle size and effect of adsorbent at different initial metal concentrations.<br>(i) Calculation of the adsorption capacity and intensity using Langmuir, Freundlich, Temkin<br>and Dubinin- Radushkevich isotherm models.<br>(ii) Biosorption kinetic studies on the adsorption are investigated<br>(iii) Comparison between the Adansonia digitata activated carbon as adsorbents to activated<br>carbon from some plants found in literature.<br>8<br>1.6 Research questions<br>Heavy metals are one of the important pollutants in wastewater and it has become a<br>public health concern, because of its persistent nature. The toxicity of heavy metal is enhanced<br>through accumulation in living tissues and consequent bio-magnification in food chain. Nature<br>has given many things which are far better than artificial products. Then came a thought “Why<br>can’t we use Adansonia digitata plant parts as bioadsorbents for removing heavy metals from<br>aqueous solution? With this view, experimental questions were aim at answering the questions<br>below:<br>(i) Do Adansonia digitata plant parts have the ability to adsorb heavy metals from aqueous<br>solution?<br>(ii) How effective is the use of Adansonia digitata plant powder and its activated carbon as<br>adsorbent in the removal of heavy metals from aqueous solution?<br>(iii) What are the optimum conditions in the removal of heavy metals from aqueous solution<br>by Adansonia digitata plant?<br>(iv) What are the adsorption isotherm models for Adansonia digitata plant parts?<br>9 <br></p>