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
CHAPTER ONE
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:
- investigate the biosorption of some heavy metals by Adansonia digitata roots, stem powder and activated carbon made from its stem.
- identify the optimum conditions for the removal of the heavy metals by Adansonia digitata plant parts.
- compare the ability of Adansonia digitata activated carbon as adsorbent to activated carbon from some plants.
- 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.
- develop adsorption kinetic models for the studied processes and
- carry out desorption studies on the heavy metals loaded activated carbon (ADSAC).
CHAPTER TWO
LITERATURE REVIEW
Pollution
The term pollution is a derivation of the word ‘pollute’, which means, to make something dirty or no longer pure, especially by adding harmful or unpleasant substances to it. Environmental pollution is used to mean an undesirable change in the environment through harmful substances, waste materials and resources, caused by man’s activity or natural disaster which also results to the degradation of the environment with its attendant consequences on biodiversity. Water pollution is a major environmental problem faced by modern society that leads to ecological disequilibrium and health hazards. Heavy metal ions such as lead, copper, nickel, cadmium, cobalt and chromium are often found in industrial wastewater. Their presence results in acute toxicity to aquatic and terrestrial life, including humans. The discharge of effluents into the environment is a major concern. Many conventional methods, such as chemical precipitation, ion exchange, membrane processes etc. have been used to remove heavy metal ions from various aqueous solutions. The application of such processes is often restricted because of technical or economic constraints16.
Heavy Metal Pollution
The increase of industrial activities has attracted the public attention to various environmental pollution problems that has caused the deterioration of ecosystems with the accumulation of toxic metals. For instance, heavy metals are discharged from different types of industries such as ceramic and glass manufacturing, storage batteries, textile and metallurgical processes. Basically, heavy metals are metallic elements which have higher atomic weight and greater density than water. Heavy metals are not biodegradable and persistent in the environment.
Heavy metals enter into the environment mainly via three routes, which are through deposition of atmospheric particulates, disposal of metal enriched sewage sludge as well as sewage effluents and by-products from metal mining processes. Heavy metals can be released into the environment by both natural and anthropogenic sources. The major causes of introduction of these metals are the anthropogenic sources, specifically mining operations17. The released metals are likely to persist in the environment even longer after mining activities have ceased. Through mining activities, water bodies are most polluted, as these metals may leach to sloppy areas and they are carried by acidic water downstream.
Heavy metal pollution represents a severe ecological and health problem due to the toxic effect on living organisms. For example, heavy metal pollution of surface and underground water sources will lead to considerable soil pollution. When agricultural soils and water are polluted, these metals are taken up by plants and animals and are consequently accumulated in their tissues18. As a result, humans are also exposed to heavy metals by consuming contaminated plants and animals, which result in a range of biochemical disorders. This proves that all living organisms within the same ecosystem are contaminated along their cycles of food chain.
Human exposure to heavy metals has risen dramatically in the last 50 years as a result of an exponential increase in the use of heavy metals in industrial processes and products19. Nowadays, many occupations involve daily heavy metal exposure; it is observed that there are over 50 professions entail exposure to lead and cadmium. For example, lead has been used in plumbing, and lead arsenate has been used to control insects in apple orchards20. Inhalation of heavy metal particles, even at levels well below those considered non-toxic, can have serious health effects. Virtually all aspects of human and animal immune system functions are compromised by the inhalation of heavy metal particulates. Toxic metals can enhance allergic reactions and even cause genetic mutation. Heavy metals can also increase the acidity of the blood. The body draws calcium from the bones to help restore the proper blood pH.
The removal and recovery of heavy metals is very important with respect to the environmental and economical considerations21.
Industrial and Agricultural Pollution
Industrial pollution is still a major source of environmental pollution. Industrial pollution poses a serious problem to the entire populace, especially in nations which are rapidly industrializing. According to the Environmental Protection Agency (epa), it has been estimated that industrial pollution is responsible for almost 50 percent of the pollution in the United States22. Industrial pollution poses a serious problem to the entire planet, especially in nations which are rapidly industrializing. Pollutant given off by various industries and factories are often considered to be one of the prime factors contributing to air, water and soil pollution. Toxic chemicals contaminate salt and fresh water bodies. The kind of pollutants that is released to the environment depends on the type of industry and product produced. Hydrogen sulphide emanates, from chemical plants, paper mills, textile mills and tanneries. They are products of anaerobic decay of organic matter containing sulphur. Hydrogen sulphide is evolved as a gaseous pollutant from geothermal waters. Sulphite is found in some industrial wastewaters. Sodium sulphite is commonly added to boiler feed water as an oxygen scavenger to consumes most of the dissolved oxygen in water23. In the case of oil spill, it is a type of industrial accident in which oil spills or leaks from a source into the environment. An oil tanker running aground and leaking oil into the ocean is an example. Some industries may dump oil illegally to cut costs.
CHAPTER THREE
MATERIALS AND METHODS
Experimental works were conducted to determine the efficiency of the Adansonia digitata root, stem and activated carbon made from the stem in removing heavy metals from aqueous solution. The effects of various variables, such as pH, initial metal ion concentration, contact time, adsorbent dosage, particle size and temperature were investigated. Desorption studies were carried out on the activated carbon. The following apparatus were used in this study.
Apparatus and Chemicals used:
The following apparatus were used in this study.
- Thermostatically controled oven (model: OV-160)
- Analytical balance (model: FEJ-600)
- Grinding mill (multi-purpose) (model: CF-158)
- Micron rotary sieve (venner 1152-60)
- pH Meter (EUTECH pH510)
- Vecster muffle furnace
- Multifix Rotary Shaker (M-80)
- Flame atomic absorption spectrophotometer (FAAS) (model: thermo-elemental, serial no: se: no 710694).
- Furrier transform Infrared spectrophotometer (FTIR); (FTIR-8400S)
- Scanning Electron Microscopy (SEM) (model: Phenom World ProX).
- Conductivity meter (model: LF-90)
Chemicals used:
All chemicals used were of analytical grade and were used without any further purifications. The chemicals were procured from commercial source BDH laboratories, Germany.
CHAPTER FOUR
RESULTS AND DISCUSSION
This chapter presents the results obtained from the batch studies of adsorption of the metal ions by ADRP, ADSP and ADSAC. Desorption results were also presented in this section. The studied metals include Pb2+, Cd2+, Cu2+ and Co2+.
The results, Table 4.1 showed a low amount of % moisture, ash, and volatile mater, indicating that the particle density is relatively small and that the biomaterial should be an excellent raw material for adsorbents to be used in adsorption experiment. Ash content can also affect activated carbon. It also reduces the efficiency of reaction ration, the lower the ash value therefore the better the material for use as adsorbent.
The fixed carbon yield of the adsorbents (ADRP, ADSP and ADSAC) showed a range of 68-71 %. The highest fixed carbon amongst the adsorbent was observed for ADSAC (71 %) while ADSP (69 %) and ADRP (68 %) was the lowest.
CHAPTER FIVE
CONCLUSION
The present investigation shows that adansonia digitata plant parts (ADRP, ADSP and ADSAC) can be used as an effective adsorbent for the treatment of waste waters containing metal ions like Pb(II), Cd(II), Cu(II) and Co(II). Adansonia digitata plant parts were successfully utilized as a low-cost adsorbent.
The adsorption process was found to be dependent on many factors such as the solution pH, initial concentration of metal ions, contact time, adsorbent does, adsorbent particle size and also effect of carbonization temperature and activation. The adsorption of Pb(II) and Cd(II) ions by ADRP, ADSP and ADSAC were found to be higher than that obtained for Cu(II) and Co(II) ions.
The percentage removal of all the four metal ions decreased with increased in initial metal ion concentration for all the adsorbent used. The uptake of metals increased with increase in the agitation time till the equilibrium was reached. The effect of adsorbent dosage on the adsorption of metals showed that the percentage of metal ions removed increased with increase in adsorbent dosage due to increased adsorption surface area. For all the adsorbents studied, adsorbent dosage of 0.1 g-0.5 g was sufficient for adsorption of 96.0 % for Pb(II), 99.1 % for Cd(II), 81.0 % for Cu(II) and 75.0 % for Co(II) unto ADRP. The adsorbent dosage of 0.1 g-0.4 g was sufficient for the adsorption of 97.8 % for Pb(II), 99.7 % for Cd(II), 74.8 % for Cu(II) and 84.9 % for Co(II) onto ADSP respectively. While 99.9 % for Pb(II), 95.7 % for Cd(II), 77.2 % for Cu(II) and 79.9 % Co(II) was adsorbed unto ADSAC within the adsorbent range of 0.1 g-0.4 g.
Irrespective of the type of the adsorbent, the optimum pH for the removal of the metal ions, Pb(II), Cd(II), Cu(II) and Co(II) was maximally adsorbed at pH 5.0. The amount of the metal removed at optimum pH increased with increase in initial metal concentration but the percentage adsorbed decreased with increase in initial metal concentration.
The optimum contact time of the adsorption of the metal ions unto ADRP, ADSP and ADSAC was achieved at 90 min (66.2 % – 99.9 %) for all the studied metal ions.
The uptake of metal ions onto ADRP and ADSP is influenced by the particle size of the adsorbent. The percentage adsorption of the metal ions onto the adsorbent decreased with increased in particle size of adsorbents due to a decrease in the adsorption sites of the adsorbents.
Carbonization temperature and activation of the adsorbent ADSAC enhance the adsorption capacity of the metal ions110. There was a sharp increase (Figure 4.13) in the percentage removal of meal ions unto ADSAC at carbonization temperature range (250 – 350 oC) with a slight decrease at 400 oC due to damaged cells of the adsorption site which enhance the adsorption capacity.
In Figures 4.14, the competitive adsorption of the metal ions onto the adsorbents varies due to the fact that different cations have different affinities to cell binding sites. Pd(II) and Cd(II) records the highest percentage adsorption onto ADRP, ADSP and ADSAC at optimum conditions. The percentage removal of the metal ions from the mixed aqueous solution by ADRP were in the order 78.3 % for Pb(II), 75.0 % for Cd(II), 73.0 % for Co(II) and 66.1 % for Cu(II). 86.7 % for Pb(II), 70.2 % for Cd(II), 65.3 % for Cu(II) and 59.8 % for Co(II) onto ADSP. The competitive adsorption of metal ions followed the order, 99.9 % for Cd(II), 94.9 % for Pb(II), 78.2 % for Cu(II) and 75.0 % for Co(II) respectively.
The equilibrium data were tested using the Langmuir, Freundlich, Temkin and Dubimin-Radushkevich (D-R) isotherm models and the best fit was obtained with the Freundlich model for all the three adsorbents (ADRP, ADSP and ADSAC). The kinetic parameters were also analyzed using the Lagergren pseudo-first order, pseudo-second order and intraparticle diffusion rate equation. The pseudo-second order provides the best fit to the experimental data for ADSAC in Table 4.8. The result also indicated the presence of intraparticle diffusion on the sorption of the metal ions, though it was not the sole rate determining step.
The Langmuir isotherm, Tables 4.5 showed a favourable (0<RL<1) adsorption process between the adsorbent and metal ions in solution. Freundlich isotherm also indicates a favourable adsorption124 with the values of n between 1.248 and 2.381. Furthermore, the high adsorption capacity of 6.369 mg/g, 3.497 mg/g and 4.149 mg/g for Pb(II) onto ADRP, ADSP and ADSAC obtained from the langmuir isotherm, 3.745 mg/g, 3.610 mg/g and 4.098 mg/g for Cd(II), 2.817 mg/g, 2.132 mg/g, 3.717 mg/g for Cu(II) and 4.348 mg/g, 4.587 mg/g and 2.353 mg/g for Co(II) onto ADRP, ADSP and ADSAC obtained from langmuir isotherm respectively. These suggest that adansonia digitata plant parts powder and its activated carbon can be used as a low cost adsorbent for the removal of metal ions from wastewater. The comparison Table 4.9 of the adsorption capacity of the three adsorbents with that cited in literature reveals that ADRP, ADSP and ADSAC had a higher biosorption capacity than the adsorbents reported in literature. Dubinin-Radushkevich isotherm model reveal that the adsorption process is physisorption having the energy E value less than 8 KJ/mol.
Desorption and regeneration studies of the adsorbates showed that regeneration and recovery of the adsorbates is possible with acid reagents as with base regents have low percentage desorption. The desorption process yield the metals in a concentrated form, restore the biosorbent close to the original state for effective reuse with undiminished metal uptake and no physical changes or damages to the biomass.
The Infrared Spectra Analysis of the adsorbents (ADRP and ADSAC) showed that carbon bonded with hydrogen and oxygen atoms played a major role in the adsorption spectra revealed that –C-O, C-N, O-H, N-H and C O bonds were predominant in the surface of the adsorbents. The physical properties of the adsorbents revelaed the suitability of the adansonia digitate plant parts as effective adsorbents in biosorption studies.
The SEM image with high porosity and irregular pores in the adsorbent proved to be an efficient adsorbent in adsorption studies. The high concentration of carbon and oxygen in the adsorbents Figures 4.5b-4.7b play a key role in the adsorption of the metal ions from aqueous solution. Other elements might be present in the adsorbent which could not be detected by SEM due to low concentrations.
The ADRP, ADSP and ADSAC as plant based adsorbent have negligible cost and have also proved to be an efficient biosorbent for the removal of metal ions from aqueous solutions. Furthermore, these adsorbed metals can be easily desorbed. Its utility will be economical, easily assessable and eco-friendly.
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