The kinetics of growth and α-amylase production of a novel Candida wangnamkhiaoensis yeast strain were studied in single-stage steady-state continuous cultures. This was performed in a split-cylinder internal-loop airlift bioreactor, using a variety of carbon sources as fermentation substrates. Results showed that the steady-state yields of cell mass from carbohydrates were practically constant for the range of dilution rates assayed, equaling 0.535 ± 0.030, 0.456 ± 0.033, and 0.491 ± 0.035 g biomass/g carbohydrate, when glucose, maltose, and starch, respectively were used as carbon sources. No α-amylase activity was detected when glucose was used as the carbon source in the influent medium, indicating that α-amylase synthesis of C. wangnamkhiaoensis is catabolically repressed by glucose. Contrastingly, maltose and starch induce synthesis of α-amylase in C. wangnamkhiaoensis, with starch being the best α-amylase inducer. The highest α-amylase volumetric and specific activities (58400 ± 800 U/L and 16900 ± 200 U/g biomass, respectively), and productivities (14000 ± 200 U/L·h and 4050 ± 60 U/g biomass·h, respectively) were achieved at a dilution rate of 0.24 h-1 using starch as the carbon source. In conclusion, single-stage steady-state continuous culture in an airlift bioreactor represents a powerful tool, both for studying the regulatory mechanisms of α-amylase synthesis by C. wangnamkhiaoensis and for α-amylase production. Furthermore, results showed that C. wangnamkhiaoensis represents a potential yeast species for the biotechnological production of α-amylase, which can be used for the saccharification of starch. This offers an attractive renewable resource for the production of biofuels (particularly bioethanol), representing an alternative to fossil fuels with reduced cost of substrates.
The main purpose of the present work was to evaluate the potential of plum (P. domestica L.) tree bark (PDB) to remove Cr(VI) and total chromium from aqueous solutions in batch systems. Experimental data showed that the Cr(VI) and total chromium removal capacity was dependent on operating variables such as PDB particle size, PDB pretreatment, solution pH, initial Cr(VI) concentration, and contact time. The mechanism of Cr(VI) removal by PDB implies two simultaneous processes: 1) the reduction of Cr(VI) to Cr(III), and 2) the biosorption of chromium ions. Cr(VI) and total chromium removal rates were affected to a significant extent by PDB particle size. Hydrochloric acid pretreatment proved to be optimum to increase the total chromium biosorption capacity of PDB, whilst also reducing the time needed to reach equilibrium. The optimum pH value for removal of Cr(VI) and total chromium was 1.0-2.0 and 2.0, respectively. Significant enhancement of Cr(VI) and total chromium removal was observed by increasing initial Cr(VI) concentration. The biosorption kinetic data of total chromium were best described by the pseudo-second-order model. Freundlich´s model exhibited the best fit to experimental equilibrium biosorption data. FTIR studies indicated that the main functional groups responsible for total chromium biosorption consist of the amide and carboxyl groups, which may interact with Cr(VI) anionic species and Cr(III) cationic species, respectively. The removal characteristics of Cr(VI) and total chromium exhibited by PDB make it potentially useful for the detoxification of Cr(VI)-polluted water and wastewater.
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