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Title: Modification of fungal biomass by immobilization and hydrothermal carbonization for the removal of Cr(VI)
Authors: Ho, Kwok Pan
Degree: Ph.D.
Issue Date: 2020
Abstract: Fungal biomass, Mucor rouxii (MR), was immobilized by a novel silica-doped calcium alginate method and the immobilized MR was applied as an adsorbent for the removal of Cr(VI) from aqueous solution. The Cr(VI) removal ability of the silica-doped calcium-alginate-immobilized Mucor rouxii (SCAIM) was examined in batch and continuous modes. The batch Cr(VI) removal studies were investigated as a function of contact time, solution pH, initial Cr(VI) concentration, adsorbent dosage, temperature and agitation speed. The experimental results demonstrated that the obtained SCAIM displayed higher experimental maximum Cr(VI) removal capacities (QCr(VI) = 131.8 ± 1.4 mg/g) than MR (79.54 ± 1.19 mg/g). The maximum Cr(VI) removal capacity of MR was greatly enhanced after immobilization. Continuous removal of Cr(VI) was studied in a fixed-bed column by varying the flow rate and the initial concentration of Cr(VI) influent. Moreover, the surface morphology and elemental composition of pristine and Cr-loaded adsorbents were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Based on the results of adsorption studies and characterizations, a four-step Cr(VI) adsorption-reduction mechanism was proposed: (1) adsorption of the anionic Cr(VI) on the positively-charged adsorption sites (e.g.,amine groups) through the electrostatic interaction; (2) reduction of the adsorbed Cr(VI) to Cr(III) by the adjacent reductive surface groups (e.g., hydroxyl groups); (3) adsorption of the Cr(III) by the carboxylic acid groups; and (4) desorption of the Cr(III) due to the electrostatic repulsion. The obtained results suggest that the SCAIM is a promising adsorbent for the removal and detoxification of Cr(VI) from aqueous solutions. Silica-doped zirconium-alginate-immobilized M. rouxii (SZAIM) beads were successfully prepared by the subsequent soaking of the SCAIM beads in the 4% zirconyl chloride solution. The XPS results demonstrated the complete replacement of the Ca(II) ions in calcium alginate by the Zr(IV) ions through the ion-exchange mechanism. The obtained SZAIM beads exhibited improved experimental maximum Cr(VI) removal capacity (QCr(VI) = 169.6 ± 2.6mg/g) than that of the SCAIM beads since the Zr(IV) ions in SZAIM beads served as extra adsorption sites for Cr(VI). Further, Mucor rouxii was transformed to hydrochars by a one-pot hydrothermal carbonization process, and the hydrothermal carbonized MR (HTC-MR) was applied as an adsorbent for the removal of Cr(VI) from aqueous solutions. Moreover, the doping of nitrogen groups onto HTC-MR was done by the co-carbonization of MR with the amine-containing modifying agents, such as ammonium hydroxide, acrylamide (AA), diethanolamine (DEA), polyethyleneimine (PEI), triethylenetetramine (TETA) and urea. The Cr(VI) removal capacity (QCr(VI)) of HTC-MR was significantly enhanced by the functionalization of nitrogen groups. The hydrothermal carbonization process was then optimized by varying different reaction parameters, including reaction temperature, reaction time and modifying agent dosage. Besides, the HTC-MR was synthesized in the presence of eutectic salt (CaCl2-FeCl3)in order to improve the surface area and porosity of HTC-MR.
Batch adsorption experiments were conducted to evaluate the Cr(VI) removal capacities (QCr(VI)) of HTC-MR, 0.5DEA-HTC-MR and 0.5PEI-HTC-MR. It was clearly observed that the three HTC materials showed excellent removal capacity to Cr(VI). The Cr(VI) removal capacities of HTC-MR,0.5DEA-HTC-MR and 0.5PEI-HTC-MR were optimized with several factors, including contact time, solution pH, initial Cr(VI) concentration, adsorbent dosage and temperature. The kinetic profile of QCr(VI) demonstrated that the Cr(VI) was rapidly removed by HTC-MR, 0.5DEA-HTC-MR and 0.5PEI-HTC-MR from the solution phase in the first one hour, and the Cr(VI) removal rate slowed down afterward. The HTC-MR showed faster Cr(VI) removal rate and higher equilibrium Cr(VI) removal capacity than the raw MR. Moreover, the Cr(VI) removal capacities of HTC-MR, 0.5DEA-HTC-MR and 0.5PEI-HTC-MR at 3-day contact time were 158.4 ± 0.7, 209.5 ± 2.5 and 239.3 ± 2.7, respectively. The adsorption results indicated that DEA and PEI were effective for further increasing the Cr(VI) removal capacity of HTC-MR. The optimum pH for Cr(VI) removal by HTC-MR was pH 2.0, while a high Cr(VI) removal capacity by 0.5DEA-HTC-MR and 0.5PEI-HTC-MR was obtained at pH 2.0 to 4.0. The anionic Cr(VI) ions were adsorbed onto the positively-charged adsorbent surface through the electrostatic interaction. The fractional power model was the best-fit model to simulate the Cr(VI) removal kinetics of HTC-MR and 0.5PEI-HTC-MR with the lowest RMSE value and the highest correlation coefficient. Meanwhile, the Cr(VI) removal kinetics of 0.5DEA-HTC-MR was better described by the Elovich kinetic model. The Langmuir isotherm was the best-fit model to fit the experimental isotherm data of HTC-MR, 0.5DEA-HTC-MR and 0.5PEI-HTC-MR, and the simulated maximum total Cr adsorption capacity was 115.7 ± 10.8, 269.8 ± 5.9 and 414.6 ± 11.5 mg/g, respectively. Four different desorption agents (e.g., 0.5 M HNO3, 2.0 M HNO3, 0.5 M NaOH and 2.0 M NaOH) were tested for their Cr(VI) desorption performance on the Cr-laden HTC materials. Desorption results showed that both HNO3 and NaOH solutions were effective desorption agents but with different Cr desorption mechanisms. Physical and chemical characteristics of HTC materials were examined by elemental analysis (EA), N2 adsorption/desorption isotherm, Fourier-transformed Infrared spectroscopy (FTIR), 13C nuclear magnetic resonance spectroscopy (13C NMR), X-ray photoelectron spectroscopy (XPS) and isothermal titration calorimetry (ITC). In summary, the hydrothermal carbonized M. rouxii (HTC-MR) was a promising adsorbent for the removal of Cr(VI) in the aqueous phase. Diethanolamine (DEA) and polyethyleneimine (PEI) effectively improved the Cr(VI) removal capacity of HTC-MR by surface functionalization. The Cr(VI) was removed from the aqueous solution by HTC materials through (1) direct adsorption mechanism and (2) adsorption-coupled reduction mechanism. Finally, the magnetic Fe3O4-0.5PEI-HTC-MR was fabricated by incorporating Fe3O4 nanoparticles into 0.5PEI-HTC-MR, so the Fe3O4-0.5PEI-HTC-MR could be recovered by external magnetic field after Cr(VI) adsorption process.
Subjects: Sewage -- Purification -- Chromium removal
Chromium-plating -- Waste disposal
Hong Kong Polytechnic University -- Dissertations
Pages: xxvii, 331 pages : color illustrations
Appears in Collections:Thesis

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