Removal and recovery of heavy metals by amorphous TiO₂nanoparticles and Ca-alginate immobilized TiO₂beads

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Removal and recovery of heavy metals by amorphous TiO₂nanoparticles and Ca-alginate immobilized TiO₂beads

 

Author: Leung, Pui-sze
Title: Removal and recovery of heavy metals by amorphous TiO₂nanoparticles and Ca-alginate immobilized TiO₂beads
Degree: M.Phil.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations
Water -- Purification -- Technological innovations
Heavy metals -- Toxicology
Titanium dioxide -- Environmental aspects.
Nanoparticles -- Environmental aspects.
Department: Dept. of Applied Biology and Chemical Technology
Pages: xxxiii, 258 p. : ill. ; 31 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2342985
URI: http://theses.lib.polyu.edu.hk/handle/200/5715
Abstract: Heavy metal contamination in aquatic environment has become an ecotoxicological hazard of prime interest. Conventional treatment technologies, such as chemical precipitation, coagulation and ion-exchange, are often restricted because of some technical or economic constraints. In view of this, the search for a more innovative and effective adsorbent for the removal of toxic metal ions has intensified in recent years. In this study, the technical feasibility of self-synthesized titanium dioxide for the removal of various heavy metals was investigated in batch studies using synthetic wastewater. A series of experiments were carried out to optimize different parameters of the TiO₂production. The results showed that the amorphous TiO₂sample synthesized by the precursor of titanium butoxide (TBO) and the solvent of ethanol with distilled-deionized (DDI) water in a volume ratio of 5:30:1 exhibited the highest adsorption capacities for both lead and cadmium. The performance of this adsorbent was also found to be much more superior to many other commercial TiO₂products and chemically modified activated carbons. Novel adsorbents developed by immobilizing TiO₂in different polymeric matrices, including calcium alginate/polyacrylamide, sol-gel/polyvinylalcohol, polysulfone and carboxylmethylcellulose, were evaluated. Of these immobilized adsorbents, the titanium dioxide-alginate (Alg-TiO₂) gel beads exhibited the best uptake capacities for both Pb(II) and Cd(II) ions. Additionally, these beads were rigid, spherical and uniform-sized. They not only could enhance the solid-liquid separation for the adsorption process but also increase the possibility of regeneration or reuse.
Further studies were carried out to examine the effects of pH, temperature, adsorbent dose and initial metal concentration on Pb(II) and Cd(II) adsorption by TiO₂ in both suspended and immobilized forms. It was found that the adsorption of metal ions was highly dependent on the solution pH and the maximum adsorption capacities of Pb(II) and Cd(II) were reached at pH 5.5 and pH 8.0, respectively. The adsorbent doses of the freely-suspended and immobilized TiO₂ selected for the metal adsorption processes were 0.5 g-TiO₂/L and 1.0 g-beads/L, respectively. The effect of temperature in the range of 10 to 40°C on suspended TiO₂ was comparatively more critical than that on Alg-TiO₂. Changes in temperature not only affected the removal efficiency of the metal adsorption process, but also the kinetics of the reaction. Various thermodynamic parameters, including the Arrhenius activation energy (Ea), the changes of standard Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), for the adsorption reaction of Pb(II) and Cd(II) by TiO₂ and Alg-TiO₂ were also evaluated. The obtained values indicated that the adsorption processes were spontaneous and endothermic in nature. Isotherms for the adsorption of lead and cadmium on suspended and immobilized TiO₂ were developed. Similar results were obtained from both adsorbents. While the equilibrium data of Pb(II) fitted well to the Langmuir isotherm model only, the Cd(II) data were well described by both the Langmuir and Freundlich models. The kinetic data of both metal adsorption by suspended TiO₂ were best simulated by the linearized pseudo-second order kinetic model. As for the kinetics of Alg-TiO₂, the experimental data of both metal adsorption were well described by both the linearized and non-linearized pseudo-second order models. In addition, desorption studies were conducted to determine the metal recovery efficiency of the metal-laden adsorbents using various desorbing agents with different concentrations and solid/liquid ratios. The reusability of the suspended TiO₂ was compared to that of Alg-TiO₂ by performing repeated adsorption/desorption cycles. In contrast to the suspended adsorbent, the immobilized TiO₂ can solve the problem of adsorbent material loss in multiple cycle operations. It was found that the Alg-TiO₂ beads could still achieve a very outstanding performance after ten adsorption/desorption cycles when 4 M and 2 M CaCl₂ were used as the eluting agents for Pb(II) and Cd(II) ions, respectively. These results indicated that titanium dioxide encapsulated in calcium alginate/polyacrylamide matrix could be a very efficient and economical adsorbing material for the removal of heavy metals in actual processes.

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