| Bentonite is a local available and low cost natural absorbent. Tannin has been used as a water treatment agent due to its capasity of complexion with metal ions. In this paper, the tannin-loaded bentonites (TANB and TAAC) were obtained through hydrolysable tannin being adsorbed on Na-bentonite (NB) and activated clay (AC). TANB and TAAC were then used as adsorbents for the treatment of simulated chromium-containing wastewater and electroplating waste water. At same time, the removal mechanism was studied.The main objectives of this study were to investigat the influence factors of preparation and application of tannin-loaded bentonites in the removal of haxavalent chromium from wastewater; to investigat the treatment of simulated chromium-containing wastewate by the sodium alginate-inmmobilized beads of tannin-loaded bentonites (SAB) in a fluidized bed; to study mechanism of Cr adsorption by analyzing the adsorption kinetics, adsorption isotherms, scanning electronic microscope images, fourier transform infrared spectra, X-ray diffractometry and Zeta potential. The results of this work were summarized as follows:(1) Both of the NB and AC were effective for tannin adsorption. The tannin adsorption by NB is mainly a chemical process, and the tannin adsorption by AC is mainly a physical procrss.(2) The major influencing factors of Cr(Ⅵ) removal by TANB, TAAC were pH and temperature. Under the optimum conditions, the Cr(Ⅵ) removal efficiency by TANB, TAAC were up to almost100%for50mg·L-1Cr(Ⅵ).(3) The removal efficiences of Cr(Ⅵ) and total chromium from electroplating wastewater by SAB were100%and99.34%, respectively. The results of fluidized bed experiments indicated that the concentration of Cr(Ⅵ) could be reduced from50mg·L-1to0.5mg·L-1by fourthly fluidized bed reactors.(4) The removal of Cr(Ⅵ) by TANB, TAAC, SAB includes reduce of Cr(Ⅵ) and adsorption of Cr(Ⅵ) and Cr(Ⅲ). The pseudo-second-order was discrided well the Cr(Ⅵ) sorption onto TANB, TAAC and SAB. The Cr(Ⅵ) sorption on TANB was agreement with both Langmuir and Freundlich. The Cr(Ⅵ) sorption on TAAC, SAB were well fitted with Freundlich and Langmuir, respectively.(5) The adsorption mechanism of Cr(Ⅵ) by tannin-loaded bentontie is verified as follow three parts:1) the porousness of bentontie makes it absorb a bit of Cr(Ⅵ) ions;2) partly TCr in form of the anionic species were adsorbed through esterification reaction with phenolic groups of tannin;3) the adsorption of reduced Cr(Ⅲ) ions was occurred through the ion exchange reaction or complexation reaction with hydroxyl and/or carboxyl groups of tannin or carboxyl group, forming from oxidation of tannin molecules.This research could be very useful in obtaining valuable data on removal of hexavalent chromium from waste using tannin-loaded bentonites (TANB and TAAC). It provies a new high perfomance, low cast technology for heavy metal pollution and opens a new dimension of adsorbent reuse, especially in electroplating industries. |
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