ABSTRACT
This paper presents the adsorption of humic acid from aqueous solution onto crosslinked chitosan derivative (carboxymethylchitosan), formed by additionless irradiation technique. The surface charge and swelling properties of crosslinked samples were investigated. The adsorption of humic acid onto crosslinked carboxymethylchitosan was carried out by the batch method at room temperature, and it was found to be strongly pH-dependent. Maximum amount of humic acid was adsorbed under acidic conditions at the optimum pH value of 3.5. Adsorption kinetic studies indicated the adsorption process was transport-limited at the same pH. The adsorption isotherm analysis data under various initial humic acid concentrations confirms that experimental data fitted well into the Langmuir equation. X-ray photoelectron spectroscopy (XPS) revealed that the amino groups of carboxymethylated chitosan were protonated, suggesting the formation of organic complex between the protonated amino groups and humic acid. From these preliminary evaluations, it was concluded that crosslinked carboxymethylated chitosan derivatives have a great potential in water treatment for the removal of humic acid and other polarized or electrically charged species.
Subject(s)
Adsorption , Biotechnology/methods , Chitosan/analogs & derivatives , Chitosan/chemistry , Cross-Linking Reagents/chemistry , Humic Substances , Water Purification , Dose-Response Relationship, Radiation , Hydrogen-Ion Concentration , Kinetics , Models, Statistical , Protons , Spectrometry, X-Ray Emission , Water/chemistryABSTRACT
All theories related to the evolution of Deinococcus radiodurans have a common denominator: the strong positive correlation between ionizing-radiation resistance and desiccation tolerance. Currently, the widespread hypothesis is that D. radiodurans' ionizing-radiation resistance is a consequence of this organism's adaptation to desiccation (desiccation adaptation hypothesis). Here, we draw attention to major discrepancy that has emerged between the "desiccation adaptation hypothesis" and recent findings in computational biology, experimental research, and terrestrial subsurface surveys. We explain why the alternative hypothesis, suggesting that D. radiodurans' desiccation tolerance could be a consequence of this organism's adaptation to ionizing radiation (radiation adaptation hypothesis), should be considered on equal basis with the "desiccation adaptation hypothesis".
Subject(s)
Biological Evolution , Deinococcus/genetics , Deinococcus/physiology , Deinococcus/radiation effects , Radiation Tolerance/physiologyABSTRACT
A DNA fragment carrying the gene encoding poly(3-hydroxybutyrate) (P(3HB)) depolymerase was cloned from the genomic DNA of Marinobacter sp. DNA sequencing analysis revealed that the Marinobacter sp. P(3HB) depolymerase gene is composed of 1734bp and encodes 578 amino acids with a molecular mass of 61,757Da. A sequence homology search showed that the deduced protein contains the signal peptide, catalytic domain (CD), cadherin-type linker domain (LD), and two substrate-binding domain (SBD). The fusion proteins of glutathione S-transferase (GST) with the CD showed the hydrolytic activity for denatured P(3HB) (dP(3HB)), P(3HB) emulsion (eP(3HB)) and p-nitrophenylbutyrate. On the other hand, the fusion proteins lacking the SBD showed much lower hydrolytic activity for dP(3HB) compared to the proteins containing both CD and SBD. In addition, binding tests revealed that the SBDs are specifically bound not to eP(3HB) but dP(3HB). These suggest that the SBDs play a crucial role in the enzymatic hydrolysis of dP(3HB) that is a solid substrate.
