THE POTENTIALS OF ADANSONIA DIGITATA ROOT AND STEM POWDERS AND STEM ACTIVATED CARBON AS LOW-COST ADSORBENTS FOR THE REMOVAL OF HEAVY METALS FROM AQUEOUS SOLUTIONS

ABSTRACT

The potentials of Adansonia digitata root (ADRP) and stem powders (ADSP) and the stem activated carbon (ADSAC) for adsorption of Pb2+, Cd2+, Cu2+ and Co2+ from aqueous solutions were investigated. The activated carbon was prepared via chemical activation using ZnCl2 as the activating agent. Physico-chemical analysis of the adsorbents (ADRP, ADSP and  ADSAC) showed the moisture contents (< 20 %), volatile matter (9.2 – 11.2 %), ash content showed < 5.0

%, fixed carbon (68 – 81.0 %), pore volume (3.0 – 4.0×10-3 m3/g), bulk density (0.19 – 0.38 gml-1)

and conductivity (221 – 301 µS/cm). ADSAC showed the highest values for all the physico- chemical parameters determined except for moisture and ash content (5.2 % and 2.6 %). The FTIR analysis of the adsorbents (ADSAC and ADRP) before and after adsorption revealed that hydroxyl, carbonyl and amino groups were predominant on the surface of the adsorbents. The Scanning Electron Microscope (SEM) image revealed high porosity and irregular pores in the adsorbents indicating good adsorbents while the Energy Dispersive X-ray Spectrum showed carbon as the major element with 57.2 %, oxygen (36.4 %), and phosphorus (6.5 %) for ADSAC while ADSP showed 75 % oxygen, 13.4 % carbon, 0.9 % calcium and 9.8 % nitrogen. For ADRP, 53.0 % Nitrogen, 23.8 % carbon, 9.1 % calcium, 7.5 % potassium and 6.6 % magnesium were present. Batch adsorption was carried out to evaluate the optimal operational conditions such as initial concentration, pH, contact time, adsorbent dose, particle size and carbonization

temperature on the removal of Pb2+, Cd2+, Cu2+ and Co2+. Removal of the metals decreased with

increase in initial concentrations of the metal ions (0.5-40 mg/L) and in particle size of the adsorbents (32-250 µm) while removal of Pb2+, Cd2+, Cu2+  and Co2+     ions increased with increase in pH (2-5), contact time (30-90 min), adsorbent dose (0.1 – 0.4 g) and carbonization temperature (250 – 350 oC) for ADRP, ADSP and ADSAC respectively. The optimal conditions for the adsorptions are initial concentration of the metal ions = 0.5 mg/L, pH = 5, contact time =

90 min, adsorbent dose = 0.4 g, particle size = 32 µm and carbonization temperature = 350 oC. The competitive adsorption of Pb2+, Cd2+, Cu2+  and Co2+  mixed metal solutions showed that

ADSAC was more efficient with % removal (75.0 – 99.9 %), ADSP (59.8 – 86.7 %) and ADRP

(66.1 – 78.3 %). Desorption studies indicate that regeneration and recovery of the adsorbates are possible with acidic reagent with desorption above 90 % at 180 min while below 60 % at 180 min  was  recorded  with  basic  reagent.  Among  the  adsorption  isotherms  tested,  Freundlich isotherm showed good fit to the adsorption of Pb2+, Cd2+, Cu2+ and Co2+ onto ADRP, ADSP and

ADSAC while Langmuir isotherm indicate a favourable adsorption process between the adsorbents and metal ions in solution. Dubinin-Radushkevich isotherm revealed that the adsorption processes were physisorption having mean free energy value below 8 KJmol-1 (1.581

– 3.536 KJmol-1). The kinetics studies showed that Pseudo-second order provides the best fit to the experimental data for ADSAC while the presence of the boundary layer effect in intraparticle

diffusion showed the existence of the surface sorption indicating that intraparticle diffusion was

not the only rate-limiting step. Hence, the studied parameters showed that ADRP, ADSP and ADSAC can be effectively used as a low cost adsorbent.

CHAPTER ONE INTRODUCTION

1.1      Background of the study

The  amount  of  heavy  metals  released  to  the  environment  has  been  increasing significantly resulting from industrial activities and technology development1. Contaminations by heavy metals exist in aqueous waste streams of many industries such as metal purification, metal finishing, chemical manufacturing, mining operations, smelting, battery manufacturing, and electroplating. As a result of industrial activities and technological development, the amount of heavy metals discharged into streams and rivers by industrial and municipal wastewater have been increasing incessantly2.

Heavy metals are member of a loosely-defined subset of elements that exhibit metallic properties, which mainly includes the transition metals, some metalloids, lanthanides, and actinides. Certain heavy metals such as iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) are required by humans for normal biological functioning. However, heavy metals such as mercury, lead, cadmium, cobalt are toxic to organisms. Most of the health disorders are linked with specific tendency of heavy metals to bioaccumulate in living tissues and their disruptive

integration into normal biochemical processes3.

Increased use of metals and chemicals in the process industries has resulted in generation of large quantities of effluent that contains high level of toxic heavy metals and their presence poses environmental disposal problems due to their non-degradable and persistence nature.

Soil particles tend to have a variety of charged sites on their surfaces, some are positive while some are negative. The negative charges of these soil particles tend to attract and bind the positively charged metal cations, preventing them from becoming soluble in water. The soluble form of metals is more dangerous because it is easily transported, hence more readily available to plants and animals.

Metal behaviour in the aquatic environment is similar to that outside a water body. Sediments at the bed of streams, lakes and rivers exhibit the same binding characteristics as soil particles mentioned earlier. Hence, many heavy metals tend to be sequestered at the bottom of water bodies. Yet, some of these heavy metals will dissolve. The aquatic environment is more susceptible to the harmful effects of heavy metal pollution. Metal ions in the environment bioaccumulate and are biomagnified along the food chain. The effect of heavy metals is more

pronounced in animals at higher trophic levels4. Some metals may be either beneficial or toxic, depending on concentration.

Lead (Pb) is the most significant toxic of the heavy metals and its effects are of a toxicological and neurotoxic nature including irreversible brain damage in humans. Inorganic forms of lead typically affect the central nervous system, peripheral nervous system, and hematopoietic, gastrointestinal, cardiovascular, and reproductive systems. Organic lead toxicity predominantly tends to affect the central nervous system. Other hazardous effects of lead are visual disturbances, convulsions,  loss of cognitive ability,  antisocial  behavior, constipation, anemia,  tenderness,  nausea,  vomiting,  severe  abdominal pain,  and  gradual paralysis  in  the

muscles5. However, human activity has resulted in atmospheric Lead, mainly as PbSO4  and

PbCO3. Industries such as coating, paint, lead smelting and mining generate large quantities of wastewater containing various concentrations of lead.

Another element of interest is copper; copper (Cu) is a chemical element, a soft reddish- brown metal used for making electric wires, pipes, etc. it is also beneficial to organisms. The American Medical Association has recommended 1.2 – 1.3 mg/day as the dietary requirements for Copper. On the average, drinking water accounts for less than 5 % of our daily copper intake. The U.S. Environmental Protection Agency (U.S. EPA) has determined that copper level in drinking water should not exceed 1300 ug/L. In 1974, congress passed the safe drinking water Act. This law requires Environmental Protection Agency (EPA) to determine safe levels of chemicals in drinking water which may cause health problems. The Maximum Contamination Level Goals (MCLG) for copper has been set at 1.3 parts per million (ppm) because, Environmental Protection Agency believes that this level of protection would not cause any of

the potential health problems resulting from excess level of Cu6. Short periods of exposure can

cause gastrointestinal disturbance, including nausea and vomiting while Long-term exposure to copper can cause irritation of the nose, mouth and eyes and it causes headaches, stomachaches, dizziness and diarrhea. Use of water that exceeds the maximum Level of copper over many years could cause liver or kidney damage.

Cadmium (Cd) is present in air in the form of particles in which cadmium oxide is probably an important constituent. Cigarette smoking increases cadmium concentrations inside houses. The average daily exposure from cigarette smoking (20 cigarettes a day) is 2 to 4 μg of cadmium. Cadmium concentrations in unpolluted natural waters are usually below 1 μg/dm3. Contamination of drinking water may occur as a result of the presence of cadmium as an

impurity in the zinc of galvanized pipes or cadmium-containing solders in fittings, water heaters, water coolers and taps. Food is the main source of cadmium intake for non-occupationally exposed people. Crops grown in polluted soil or irrigated with polluted water may contain increased concentration of Cd(II). Levels of Cd(II) concentrations in fruit, meat and vegetables are usually below 10 μg/kg, in liver 10–100 μg/kg and in kidney 100 – 1000 μg/kg. Cadmium concentrations in tissues increase with age. Both kidney and liver act as cadmium stores, 50–85

% is stored in kidney and liver, 30–60 % being stored in the kidney alone7.

Cobalt (Co) is used in many alloys (superalloys for parts in gas turbine aircraft engines, corrosion resistant  alloys,  high-speed  steels,  cemented  carbides),  in  magnets  and  magnetic recording media, as catalysts for the petroleum and chemical industries, as drying agents for paints and inks. Cobalt blue is an important part of artists’ palette and is used by craft workers in porcelain, pottery, stained glass, tiles and enamel jewelers. The radioactive isotopes, cobalt-60, is used in medical treatment and also to irradiate food, in order to preserve the food and protect the consumer. As cobalt is widely dispersed in the environment humans may be exposed to it by breathing air, drinking water and eating food that contains cobalt. Skin contact with soil or water that contains cobalt  may also enhance exposure. Cobalt is not often freely available in the environment, but when cobalt particles are not bound to soil or sediment particles the uptake by

plants and animals is higher and accumulation in plants and animals may occur8.

1.1.1   Hazards of Heavy Metal Contamination

The main threats to human health from heavy metals are associated with exposure to lead, cadmium, mercury, copper, cobalt and arsenic. These metals have been extensively studied and their effects on human health regularly reviewed by international bodies such as the World Health Organization (WHO). Heavy metals have been used by humans for thousands of years. Although several adverse health effects of heavy metals have been known for a long time, exposure to heavy metals continues, and is even increasing in some parts of the world, in particular in less developed countries, though its contaminant have declined in most developed countries over the last 100 years. For example, Cadmium compounds which are currently used in re-chargeable nickel–cadmium batteries have increased dramatically during the 20th century, one reason being that cadmium-containing products are rarely re-cycled, but often dumped together with household waste. Also, cigarette smoking is a major source of cadmium exposure, while in non-smokers; food is the most important source of cadmium exposure. Recent data indicate that

adverse health effects of heavy metals exposure may occur at lower exposure levels, primarily in the form of kidney damage but possibly also bone effects and fractures9. Many individuals already exceed these exposure levels and the margin is very narrow for large groups. Therefore, measures should be taken to reduce heavy metals exposure in the general population in order to minimize the risk of adverse health effects. The general population is primarily exposed to mercury via food, fish being a major source of methyl mercury exposure, and dental amalgam.

The general population does not face a significant health risk from methyl mercury, although certain groups with high fish consumption may attain blood levels associated with a low risk of neurological damage to adults. Since there is a risk to the fetus in particular, pregnant women should avoid a high intake of certain fish, such as shark, swordfish and tuna; fish (such as pike, walleye and bass) taken from polluted fresh waters should especially be avoided. There has been a debate on the safety of dental amalgams and claims  have been made that  mercury from amalgam may cause a variety of diseases. However, there are no studies so far that have been able to show any associations between amalgam fillings and ill health.

The general population is also  exposed to  lead  from air  and  food in roughly equal proportions. During the  last  century,  lead exposure to  ambient air  has caused considerable pollution, mainly due to lead emissions from petrol. Children are particularly susceptible to lead exposure due to high gastrointestinal uptake and the permeable blood–brain barrier. Blood levels in  children  should  be  reduced  below  the  levels  so  far  considered  acceptable,  recent  data indicating that there may be neurotoxin effects of lead at lower levels of exposure. Although lead in petrol has dramatically decreased over the  last  decades, thereby reducing environmental exposure, phasing out any remaining uses of lead additives in motor fuels should be encouraged. The  use  of  lead-based  paints  should  be  abandoned,  and  lead  should  not  be  used  in  food containers. In particular, the public should be aware of glazed food containers, which may leach lead into food.

Exposure to arsenic is also mainly via intake of food and drinking water, food being the most important source in most populations. Long-term exposure to arsenic in drinking-water is mainly related to increased risks of skin cancer, but also some other cancers, as well as other skin lesions such as hyperkeratosis and pigmentation changes. Occupational exposure to arsenic, primarily by inhalation, is usually associated with lung cancer. Clear exposure–response relationships and high risks have been observed.

Biosorption  is  presented  as  an  alternative  to  traditional physicochemical  means  for removing toxic metals from ground-waters and wastewater. It is a relatively new process that has proven very promising in the removal of contaminants from aqueous solutions. It has been shown  to  be  an  economically  feasible  alternative  method  for  removing  heavy  metals. Mechanisms involved  in the  biosorption process  include chemisorptions, complexation,  ion exchange,   microprecipitation,   hydroxide   condensation   onto   the   biosurface   and   surface adsorption. The phenomenon of biosorption has been described in a wide range of non-living biomass like nile rose plant powder and ceramics10.

In this study, the adsorption of heavy metals onto biomaterials derived from Adansonia

digitata plant commonly known as baobab was investigated. Often referred to as grotesque by some authors, the main stem of larger baobab (Adansonia digitata) trees may reach enormous proportions of up to 28 metres. The massive squat cylindrical trunk gives rise to thick tapering branches resembling a root-system, which is why it has often been referred to as the upside-down tree. The stem is covered with a bark layer, which is 50-100 mm thick. The bark is greyish brown and normally smooth. The leaves are hand-sized and divided into 5-7 finger-like leaflets. Being deciduous, the leaves are dropped during the winter/harmattan and appear again in late

spring or early summer/rain11.

The usefulness of the biomass of many plant materials in removing metal ions from aqueous solution have been investigated by several researchers and all have shown that the plant base adsorbents have the potential of being used as cheap source of biosorbent for metal ions,12,13,14,15. However, no such work has been reported for Adansonia digitata to our knowledge.

1.2  Statement of problem

Heavy metals released by a number of industrial processes are major pollutants in marine, ground, industrial and even treated wastewaters. A high degree of industrialization and urbanization has substantially enhanced the degradation of our aquatic environment through the discharge of industrial wastewaters and domestic wastes. The presence of heavy metals in water, even at very low concentrations, is highly undesirable. The problem of heavy metal pollution in water needs continuous monitoring and surveillance as these elements do not degrade and tend to biomagnify in man through food chain.

This environmental problem has led to extensive research into developing effective alternative technologies  for  the  removal of these  potentially  damaging  substances  from effluents  and

industrial wastewaters. Moreover, recovery of heavy metals from industrial waste streams is becoming increasingly important to neutralize the hazard from the industrial waste harmful to plant and animal life. Hence there is a need to remove the heavy metals from the industrial wastewater before disposal.

1.3      Objectives of the study

The effluent treatment in developing countries is expensive and high cost is associated with the dependence on imported technologies and chemicals. The indigenous development of treatment techniques and chemicals or use of locally available non-conventional materials to treat pollutants seems to be the solution to the increasing problem of treatment of effluents. In this regard, there has been a focus on the use of appropriate low cost technology for the treatment of wastewater in developing countries in recent years. Technically feasible and economically viable pretreatment procedures with suitable biomaterials based on better understanding of the metal biosorbent mechanism(s) are gaining importance. Activated carbon of agricultural waste products as low cost adsorbents has been reported till now. However, there is an additional cost involved in the conventional methods of waste water treatment, which is posing economic difficulties necessitating research on alternate adsorbents with equivalent potential of the conventional methods.

The objectives of this research were to:

(i)        investigate the biosorption of some heavy metals  by Adansonia digitata roots, stem powder and activated carbon made from its stem.

(ii)       identify the optimum conditions for the removal of the heavy metals by Adansonia digitata plant parts.

(iii)      compare the ability of Adansonia digitata activated carbon as adsorbent to activated carbon from some plants.

(iv)      investigate  simultaneous  removal  of  Pb(II),  Cd(II),  Cu(II),  and  Co(II)  from  mixed aqueous solution by Adansonia digitata root, stem powder and activated carbon made from the stem.

(v)       develop adsorption kinetic models for the studied processes and

(vi)      carry out desorption studies on the heavy metals loaded activated carbon (ADSAC).

1.4      Significance of the study

This study was to remove heavy metals [Pb(II), Cd(II), Co(II) and Cu(II)] which causes environmental problem from aqueous solutions. These heavy metals cannot be destroyed as they are not biodegradable. It  means that the pollution of heavy metals in the environment will continuously increase if there is no immediate action taken to remove these heavy metals. As people or living organisms are exposed to the dangers of these heavy metals, it can be dangerous because they tend to bioaccumulate and easily enter our bodies via food, drinking water, dermal contact  and air.  It  is  important to study the removal of these  heavy metals. This study, a biosorption  process,  is  a  biological  method  of  environmental control  as  an  alternative  to conventional methods that are ineffective or extremely expensive. Natural materials such as Adansonia digitata plant parts which are environmental friendly, easily available, and cheap is been used in this study. With this technology (biosorption) and the use of Adansonia digitata plant as an adsorbent, a huge success will be recorded in the treatment of waste waters.

1.5      Scope of the study

The scope is to study the removal of heavy metals, Pb(II), Cd(II), Co(II) and Cu(II) in aqueous solution using Adansonia digitata roots powder and activated carbon made from its stem through the biosorption process.

The following limit has been defined:

(i)        Determination of functional groups on the surface of the sample that contribute to the biosorption of heavy metals used in the study through infrared spectroscopy.

(ii)       Determination of the pore sizes in the adsorbent that enhance the adsorption capacity using scanning electron microscope (SEM).

(ii)        Determination of the agitation/equilibrium time, pH, dosage, carbonization temperature, particle size and effect of adsorbent at different initial metal concentrations.

(i)        Calculation of the adsorption capacity and intensity using Langmuir, Freundlich, Temkin and Dubinin- Radushkevich isotherm models.

(ii)       Biosorption kinetic studies on the adsorption are investigated

(iii)      Comparison between the Adansonia digitata activated carbon as adsorbents to activated carbon from some plants found in literature.

1.6      Research questions

Heavy metals are one of the important pollutants in wastewater and it has become a public health concern, because of its persistent nature. The toxicity of heavy metal is enhanced through accumulation in living tissues and consequent bio-magnification in food chain.  Nature has given many things which are far better than artificial products. Then came a thought “Why can’t we use Adansonia digitata plant parts as bioadsorbents for removing heavy metals from aqueous solution? With this view, experimental questions were aim at answering the questions below:

(i)        Do Adansonia digitata plant parts have the ability to adsorb heavy metals from aqueous solution?

(ii)       How effective is the use of Adansonia digitata plant powder and its activated carbon as adsorbent in the removal of heavy metals from aqueous solution?

(iii)      What are the optimum conditions in the removal of heavy metals from aqueous solution by Adansonia digitata plant? (iv)      What are the adsorption isotherm models for Adansonia digitata plant parts?

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