Adsorption And Reduction Of Cr(?) On Surface Of Modified Corn Stalk In Water

Chromium has been widespreadly used in various industries,e.g.electroplating, leather tanning,textile dying and metal finishing,generating large volumes of Cr-containing wastewater.In aqueous solutions,Cr mainly exists in two oxidation states,Cr?III?and Cr?VI?.Compared with Cr?III?,Cr?VI?is a powerful carcinogenic and mutagenic agent.For the removal of Cr?VI?,various methods have been developed,among which biosorption using waste biomaterials is economically favorable and technically feasible to separate Cr?VI?from aqueous solutions.It was found that Cr?VI?could be easily reduced to Cr?III?during the adsorption on biomass materials.However,the reduction characteristics and mechanism of Cr?VI?during the adsorption process have not been entirely clear.The main contents and results of this paper are as follows.In this study,an anion biosorbent was prepared from corn stalk by quaternary modification method.SEM,FTIR,XPS and ¹³C-NMR characterization showed that the main functional groups of modified corn stalk were quaternary amino and hydroxyl groups.Results of batch adsorption experiments indicated that best adsorption of Cr?VI?by modified corn stalk was obtained at pH=4.The adsorption isotherm was found to agree very well with the Langmuir model and the theoretical adsorption capacity was 35.9 mg/g.Cr?VI?was mainly removed by ion exchange with chlorideTo quantitatively measure the adsorption and the reduction of Cr???a desorption method of Cr from biosorbent was developed,enabling to determine Cr???and Cr?III?concentration both in liquid and solid phase.95%of Cr was recovered by adding 20 mL 2 mol/L H₃PO₄ solution at 25?with a 10 min contact time.Based on this analysis method,the effect to p H and initial Cr?VI?concentration on reduction of Cr?VI?on modified corn stalk were studied.The results showed that low pH and high initial concentration favored reduction of Cr?VI?.Kinetic studies showed that the initial stage Cr?VI?reduction was in accordance with zero-order reaction model and the activation energy?Ea?of the reduction reaction was calculated to be 36.8 kJ/mol.Furthermore,the mechanism of Cr?VI?reduction was discussed by solid-phase characterization and analysis of some liquid phase products the possible Cr?VI?reduction mechanism was that Cr?VI?were adsorbed on the surface of modified corn stalk by ion exchange with chloride,then part of Cr?VI??<30%?been reducted by the electron donor,such as quaternary amino and hydroxyl groups near the adsorption sites.Meanwhile quaternary amino was oxidized to HCOOH and NO₃^-.Most of the Cr?III??>55%?exists on the surface of adsorbent,the residual Cr?III?were released into the solution.The adsorption and reduction properties of modified corn stalk on Cr?VI?column study were investigated by using both chromate solution and actual electroplating wastewater.The results showed that larger height diameter ratio had a longer breakthrough time and a larger the adsorption capacity.However,with the increase of the flow and initial Cr?VI?concentration,the breakthrough time was significantly reduced.In the process of column adsorption,the reduction of Cr?VI?would also been observed.however,the amount of Cr?III?was less than that in batch experiments.