Preparation Of Bacterial Cellulose And Its Adsorption Properties To Heavy Metal Ions

Heavy metal is a kind of contaminant that is greatly harmful to ecological environment. The treatment of wastewater containing heavy metal ions has been attracting attention all the time. At present, various technologies such as chemical precipitation, ion exchange, evaporation, and adsorption processes, all have been proposed and adopted for the removal of heavy metals ions, while the technology of adsorption for its rapid absorption, low cost, easy operation, no secondary pollution are very popular. However, adsorption is confined to use because adsorbents used to this purpose are too expensive. Therefore, developing of economic, effective and new adsorbents will become urgent demand for social and economic development. In this paper, bacterial cellulose (BC) was prepared by Gluconacetobacter xylinus Gyll isolated from brewing mass and dried corn steep liquor powder as nitrogen source. At the same time, Ammonium sulfamate-bacterial cellulose (ASBC) and bacterial cellulose-graft-allylamine-acroleic acid (al-AA-BC) were prepared respectively based on the mechanism of etherifying reaction and grafting copolymerization which were optimized by response surface methodology (RSM). Taken Cd2+, Pb2+, Cu2+and Cr (VI) as object of the research, this paper also analyzed the adsorption property and adsorption mechanism of BC, ASBC, al-AA-BC.The experiment results showed that strains separated from brewing mass were identified as Acetobacter, wood vinegar subspecies and Gluconacetobacter xylinus and given the name Gyll. The fermentation products was identified as the cellulose I crystals on the basis of Fourier transform-infrared spectrum (FT-IR), scanning electron microscopic (SEM) and X ray diffraction spectra (XRD). The cellulose have typical network structure with nanometer ribbons and high crystallinity. The optimum conditions of fermentation for BC were dried corn steep liquor powder50g/L, sucrose60g/L, the initial pH of medium6.0, inoculum120g/L, the age of second stage seed for culture enlargement20h and the static culture period 7d. At the optimal conditions, the highest yield of bacterial cellulose was12.36g/L. The dynamic character of BC fermentation by strain Gyll was analyzed. The kinetic models were proposed based on the Logistic equation, Luedeking-Piret equation and substrate consumption equation for cell growth, BC production and residual sugar consumption, respectively. The above models appeared to provide a reasonable description for the BC fermentation process.The sequence of BC adsorbed amount on heavy metal cations was as Cd2+>Pb2+>Cu2+. pH is a very important parameter to the adsorbed efficiency and optimal pH value was5.0. BC has some adsorption capacity to Cr (Ⅵ) and optimal pH value was1.5. ASBC has better adsorption and the sequence order of ASBC adsorbed amount was as Cd2+>Pb2+> Cu2+> Cr (VI).The optimal pH value to adsorb Pb2+, Cu2+, Cd2+and Cr (Ⅵ) were5.5and1.5-2.0respectively. With al-AA-BC as adsorbent, it has better adsorption capacity than BC and ASBC. The optimal pH value to adsorb Pb2+, Cu2+, Cd2+and Cr (Ⅵ) were5.0and1.5respectively. By comparison, after etherification and graft copolymerization, adsorption capacity of ASBC and al-AA-BC to Cd2+, Pb2+, Cu2+and Cr (VI) could improved from12.98mg/g,10.42mg/g,5.09mg/g,6.05mg/g to45.76mg/g,57.12mg/g,36.87mg/g,46.66mg/g.The adsorption isotherm of BC, ASBC to Pb2+, Cu2+, Cd2+and Cr (Ⅵ) can be described by Langmuir model, as shows the adsorption process belongs to typical single-molecule layer adsorption; while the adsorption isotherm of al-AA-BC can be better described by Langmuir-Freundlich model, as shows the adsorbent must has some heterogeneity after graft copolymerization of BC. The adsorption rate of BC, ASBC and al-AA-BC to heavy metal ions could be well fitted by pseudo-second rate model, as illustrates adsorbents adsorbed metal ions mainly via chemical adsorption. The constants B were obtained with a particles internal diffusion model. Since B is not zero, particles internal diffusion is not the only one controlling rate step.The mainly active groups adsorbing heavy metal ions on BC, ASBC and al-AA-BC were OH;-NH2,-OH;-NH2,-COOH and-OH, respectively. The adsorption mechanism of BC are surface adsorption, electrostatic adsorption and coordinate adsorption; while the mainly adsorption mechanism of ASBC and al-AA-BC are complexation, at the same time metal ions also can combine to the adsorbents by ion exchange, electrostatic attraction and surface adsorption.