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Title: Removal of dyestuffs and heavy metal from effluent via encapsulation technology
Authors: Luk, Chi Him Jim
Degree: Ph.D.
Issue Date: 2017
Abstract: Pollution problems instigated by human activities and economic development have been threatening the environment for decades. Water pollution, one of the most serious problems, is attributed to contamination from biochemical substances and chemicals. Synthetic dyes and heavy metal ions are common pollutants from the effluents of dyeing industries, electrochemical plants, tannery industries, etc. Both synthetic dyes and heavy metal ions, even in a part-per-million scale, give rise to serious environmental damage. Although conventional methods such as precipitation, ion exchange, filtration, etc. are being used to treat polluted water, these methods have their own shortcomings or implementation is costly for satisfactory results. As an alternative, adsorption has been introduced into water treatment and this method is still being extensively explored by researchers. Adsorption is well known for its excellent removal ability of a number of pollutants, and not limited to the major targets in this study. Biosorption is a kind of adsorption making use of biological adsorbents to physically or chemically remove the target adsorbates. Biosorbents have been explored in-depth in terms of utilizing their removal ability with physical and chemical treatments. Besides biosorption, biological functions from live cultures have also been one of the promising way to treat synthetic dyes and heavy metals. In this study, encapsulation was used to prepare chitosan-based and alginate-based biosorbents. The encapsulation of microorganisms and subsequent evaluation of the scavenger of the pollutants is the main objective of the present study. Although biosorption that incorporates cultures are effective, the practical separation issues in treatment plants are a concern. The rationale of the encapsulation is to minimize the separation problems as well as keep the original biological and chemical characteristics of the combined biosorbents, which includes possible biodegradation of synthetic dyes and biological reactions with heavy metal ions. After the encapsulation of microorganisms and evaluation of the biosorption performance, a further mechanistic study was also performed to understand the biosorption. The research methods of this study are as follows: (1) synthesis of biosorbents and preparation of microorganisms, (2) characterisation by using various instrumentations, (3) removal of synthetic dyes and heavy metal ions, as well as (4) kinetics and isotherm model fitting. A number of bacteria and yeast were screened to observe for possible biodegradation of synthetic dyes. Afterward, suitable encapsulating materials were chosen for the synthesis of biosorbents. The biosorbents were characterized by using Fourier transform infrared (FTIR) spectroscopy, zeta potential measurement and scanning electron microscopy (SEM). The viability of the encapsulated culture was also examined to determine whether live culture encapsulation is successfully carried out. Biosorption and mechanistic studies were subsequently conducted to evaluate the biosorption performance of the synthesized biosorbents.
Lactobacillus casei (L. casei) was encapsulated into chitosan hydrogel beads synthesized through a coacervation process with alkali neutralization and ionotropic crosslinking. Further chemical crosslinking of the chitosan beads was also conducted to strengthen their chemical stability. Direct Red 80, Reactive Yellow 25 and Acid Blue 25 are used to evaluate the biosorption performance. The effects of pH, temperature, crosslinking and encapsulating L. casei were examined to evaluate the biosorption of the combined biosorbents. No biodegradation was observed as the encapsulated L. casei were not live culture. Nevertheless, L. casei had positive effects on the biosorption of Direct Red 80 with satisfactory enhancement. Further kinetic and isotherm modelling revealed that the biosorption of all three dyes followed a pseudo-second-order kinetic and also the Freundlich isotherm. Another synthetic dye, Reactive Blue 19, was also biosorbed by L. casei-encapsulated chitosan beads in which its viability was maintained. Freeze-drying can improve the physical and chemical stabilities of the beads. It is found that plain beads could remove Reactive blue 19 with a high removal efficiency in both acidic and alkaline environments. A kinetic study revealed that encapsulating L. casei in a chitosan matrix could bring about minor enhancement to biosorption. A yeast strain, Candida krusei (C. krusei), was encapsulated into alginate-based hydrogel beads and the viability of the culture was also maintained. The biosorption of heavy metal ions was carried out by using free and encapsulated C. krusei with calcium alginate. However, the biosorption of chromium(VI) and nickel(II) is not as expected while copper(II) and lead(II) were well biosorbed. Furthermore, biosorption of copper(II) was conducted and the pH and temperature effects were also evaluated. A kinetic study revealed that all three biosorbents follow pseudo-second-order kinetics. The isotherm modelling shows good fit to Temkin isotherm with C. krusei. Calcium alginate and encapsulated C. krusei-calcium alginate both followed Langmuir isotherm.
Subjects: Hong Kong Polytechnic University -- Dissertations
Dyes and dyeing -- Waste disposal
Sewage -- Purification -- Biological treatment
Pages: xvii, 187 pages : color illustrations
Appears in Collections:Thesis

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