Mechanism for the removal of Cr(VI) and Cr(III) by a microalgal isolate, chlorella miniata
一种小球藻 (Chlorella miniata) 去除六价铬和三价铬机制的研究
Student thesis: Doctoral Thesis
Contamination of hexavalent chromium Cr(VI) and trivalent chromium Cr(III) in surface water is of great environmental concern due to their toxic, mutagenic and carcinogenic effects on ecosystems and human beings. Recent studies have demonstrated that metal biosorption is an effective method to remove toxic metal ions from wastewater, but information on using microalgae to remove Cr ions is still limited. The present study aims to investigate the mechanisms for the removal of Cr(VI) and Cr(III) by a microalgal isolate, Chlorella miniata, and the effects of other anions on their removal. The study also explores the feasibility of producing biomass from culturing Chlorella miniata in domestic wastewater for Cr removal. In Cr(VI) removal, kinetic studies indicated that both biosorption and bioreduction processes were involved. Cr(VI) in wastewater was firstly adsorbed on the biomass; the adsorbed Cr(VI) was then reduced to Cr(III). Desorption studies revealed that most of the adsorption sites on the biomass were occupied by the reduced Cr(III). The equilibrium time for Cr(VI) removal was dependent on various factors, including initial pH, biomass quantity and Cr(VI) concentration. Equilibrium studies showed that the Cr(VI) removal capacity was negatively related to the initial pH, and the biosorption capacity of total Cr [Cr(III) and Cr(VI)] reached the maximum at an initial pH of 3.0. The spectrum of FTIR (Fourier Transform Infrared Spectrometer Analysis) further confirmed that the amino group on the algal biomass was the main adsorption site for Cr(VI) biosorption in acidic pH while the reduced Cr(III) was mainly sequestered by carboxyl group. The comparison between biosorption-bioreduction and direct bioreduction kinetic models proved that biosorption of Cr(VI) was the first step, followed by Cr(VI) bioreduction, and adsorption of the reduced Cr(III) on the algal biomass. In Cr(III) biosorption, the biosorption capacity increased with the increase of pH from 2.0 to 4.5, and no significant change was observed outside this pH range. Langmuir isotherm results indicated that the maximum Cr(III) sorption capacity of Chlorella miniata was 14.17, 28.72 and 41.12 mg g-1 dry weight biomass at pH 3.0, 4.0 and 4.5, respectively. Results from desorption studies, SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy) and EDX (Energy-dispersive X-ray Spectroscope) analyses confirmed that surface complexation was the main process involved in Cr(III) biosorption. Potentiometric titration revealed that carboxyl (pKa1=4.10), phosphonate (pKa2=6.36) and amine (pKa3=8.47) functional groups on the surface of Chlorella miniata were the possible sites for Cr(III) uptake, and their average amounts were 0.53, 0.39 and 0.36 mmol g-1 dry weight biomass, respectively. A surface complexation model further indicated that the carboxyl group played the main role in Cr(III) complexation, with a binding constant of K11=1.87×10-4 and K12 =6.11×10-4 for Cr3+ and Cr(OH)2+, respectively. This model also suggested that hydroxy species were easier to complex with the cell surface of Chlorella miniata than other forms of chromium. The effects of different anions on Cr(III) biosorption varied and the inhibitory order was SO42- > Cl- > NO3-. According to the software Mineql+ developed by Schecher and McAvoy in 1994, the strongest inhibition effect of the sulfate system was attributed to the formation of Cr(OH)SO4 aq. and the decrease of Cr(OH)2+ and Cr3+, while the inhibitory effect of the other two anions could be accounted by the formation of inner-sphere surface complex (ionic-strength independent) in the nitrate system and the outer-sphere surface complex (ionic-strength dependent) in the chloride system. The inhibitory order for Cr(VI) removal was NO3- > Cl- > SO42-, and the order for the removal of the reduced Cr(III) was SO42- > Cl- ≈ NO3-. The biosorption of Cr(VI) on the biomass was affected by the presence of other anions, due to their competition with HCrO4- for the adsorption sites. The biosorption-bioreduction model developed in the present study suggested that the reaction rate constant k was more sensitive in the initial anion concentrations ranging from 0 to 0.5 M and followed the order of SO42- > Cl- > NO3-, while the adsorption constant b was more sensitive in the range of 0-0.2 M. The feasibility of culturing the green microalgal isolate Chlorella miniata in domestic wastewater (DW) for Cr removal was investigated. Results showed that C. miniata cultured in DW had similar growth as that in commercial Bristol medium (BM) and the highest biomass was produced under continuous illumination (24-0 hours light-dark cycle). The carbohydrate concentrations of the microalgal biomass cultivated in different media and light-dark cycles were different, and declined in the order of DW(24-0) > BM(16-8) > DW(16-8). Conversely, the protein content showed an opposite trend of DW(16-8) > BM(16-8) > DW(24-0). The FTIR spectra revealed that the functional groups on the surface of these three kinds of biomass were comparable, except an additional peak at 1731 cm-1 was found in the biomass cultured in domestic wastewater, probably due to bacterial contamination. In spite of the differences in biochemical composition, comparable performances were found in the removal of both Cr(VI) and Cr(III) among three kinds of algal biomass with 75% Cr(III) and 100% Cr(VI) removal at equilibrium. It is therefore possible to culture Chlorella miniata in domestic wastewater and utilize the biomass for the removal of Cr(III) and Cr(VI).
- Environmental aspects, Purification, Sewage, Chromium, Microalgae, Chlorella, Chromium removal