Author: Lo, Lai Shan
Title: The performances and mechanisms of Cd(II) and Cr(VI) removal by a low-cost filamentous fungal biomass
Advisors: Lo, Wai-hung (ABCT)
Degree: Ph.D.
Year: 2015
Subject: Sewage -- Purification -- Heavy metals removal.
Metal ions -- Absorption and adsorption.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Biology and Chemical Technology
Pages: xxxvi, 367 pages : illustrations
Language: English
Abstract: Wastewater laden with toxic heavy metals must first be treated to minimize the amounts of metal ions in water before being discharged into the environment. Conventional treatment technologies, such as ion-exchange,membrane separation and chemical precipitation, have their inherent limitations and constraints. A promising, cost-effective and environmentally friendly alternative is to utilize microbial biomass as a biosorbent to remove and recover toxic metals from wastewater. In this study, the performances and mechanisms of Cd(II) and Cr(VI) removal by a low-cost filamentous fungal biomass were examined. The cultivation temperature and duration of the fungal biomass were found to be important parameters governing not only the growth yield but also its metal removal abilities.Results showed that the optimum incubation temperature and incubation duration were 30℃ and 3 days,respectively.The fungal biomasses cultivated at 25,30 and 37℃ demonstrated similar biosorption capacities towards Cr(VI) while a significant decrease was observed in the Cd(II) uptake ability of the biomass incubated at 37oC. The biomasses harvested after 2 and 7 days exhibited lower biosorption capacities towards Cr(VI) and Cd(II),respectively. The surface properties of the biomass were then characterized by zeta potential measurement,high resolution potentiometric titration and spectroscopic techniques.The point of zero charge (PZC) of the fungal biomass was 3.5.The integrated results of potentiometric titration,energy dispersive analysis of X-rays (EDAX),fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) indicated the presence of phosphodiester (pKa = 3.22±0.52), carboxyl (pKa = 4.92±0.43), amino (pKa = 6.79±0.21) and hydroxyl (pKa = 8.77±0.31) groups on the fungal biomass surface with the corresponding site concentrations of 0.55±0.09,0.30 ±0.09,0.37±0.05 and 1.44±0.20 mmol/g-biomass, respectively. Batch biosorption studies were conducted to evaluate the effects of pH, biomass concentration, initial metal concentration,contact time, agitation speed and temperature on Cd(II) and Cr(VI) removal by the fungal biomass. Results showed that the optimum pH values for Cd(II) and Cr(VI) removal were 7.5 and 2.0,respectively. The equilibrium data were simulated by the Langmuir, Freundlich, Sips, Redlich-Peterson, Temkin and Dubinin-Radushkevich isotherm models. The kinetic data were analyzed by the pseudo-first order, pseudo second order,Elovich,fractional power and intraparticle diffusion equations. Results showed that the Sips model provided the best correlation of the Cd(II) equilibrium data,while the non-linearized Langmuir, Sips and Redlich-Peterson models presented the best simulation of the isotherm data of total Cr and Cr(VI) at 6 and 24 h contact times. The pseudo-second order equation and intraparticle diffusion equation well simulated the kinetic data of removal of Cd(II) and Cr(VI),respectively. The intraparticle diffusion may not be the only rate-determining step in the Cd(II) and Cr(VI) biosorption processes, while the external mass transfer may control the processes to some extent. Three thermodynamic parameters (ΔH., ΔS. and ΔG.) were determined from the slope and intercept of ln Kd versus 1/T. Results suggested that the Cd(II) biosorption process was endothermic, thermodynamically feasible and spontaneous under the examined conditions.
The surface characteristics of the fungal biomass before and after Cd(II) and Cr(VI) removal were examined with scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDAX),FTIR and XPS. In EDAX analysis, no alkali or alkaline earth metal ions were observed on the pristine biomass surface, indicating that Cd(II) adsorption process might not be due to ion exchange between Cd(II) ions and the alkali or alkaline earth metal ions. Results from the FTIR and XPS studies demonstrated that phosphodiester, carboxyl and amino groups were involved in the Cd(II) binding process. On the other hand,the Cr(VI) removal from aqueous solution by the fungal biomass was through adsorption-coupled reduction. XPS was employed to determine the oxidation state of the Cr bound onto the biomass surface; it revealed that most of the chromium bound on it was in trivalent form,showing that most of the Cr(VI) ions were reduced to Cr(III) by the fungal biomass. The integrated results of FTIR and XPS demonstrated that amine, carboxyl and phosphodiester groups might be involved in the Cr(VI) removal process. These results show that the anionic Cr(VI) ions bound onto the positively-charged biomass surface by electrostatic attraction and the electron donating groups on the biomass surface were capable of reducing Cr(VI) to Cr(III).The bio-reduced Cr(III) ions appeared in the aqueous solution or were partly bound onto the biomass surface.This research has demonstrated that the filamentous fungal biomass shows great potential to be applied in developing low-cost and effective biosorbents for removing Cd(II) and Cr(VI) ions from contaminated water and wastewater.
Rights: All rights reserved
Access: open access

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