Enhance Cr(Ⅵ) Removal By Modified Activated Carbons

In recent years, heavy metal chromium is widely used in electroplating, metallurgy, leather and textile industries, those industries discharged wastewater containing chromium could cause serious pollution, and cause great harm to human health directly or indirectly. Adsorption is widely used in wastewater treatment, because of simple equipment, wide range adaptation, superior treatment effect. However, Activated carbon prepared by conventional methods are not suitable for Cr(VI) removal from wastewater.The study selected corn straw as raw material, preparing modified activated carbon (AC) through two methods (ferric salts auxiliary activation and quaternary amine anchor), the modified ACs were used to remove Cr (VI) from wastewater, the removal mechanism was discussed by means of multiple analyses. The main contents and results are as follows:(1) Fe(NO3)3 of different concentrations were used as the assisted activation agent in activated carbon preparation by H3PO4 activation. Characterization results showed that adding iron salt can greatly improve the yield and specific surface area of AC, impeding the generation of acidic functional groups. Adsorption experiments showed that, the uptake of Cr(VI) increased with the decrease in the pH of the solution by all 5 kinds of ACs, the adsorption equilibrium time was 720 min; compared to other models,5 kinds of ACs adsorption behavior more in line with the pseudo-second-order kinetic model and Freundlich model; Langmuir equation simulation result shows that the maximum amount of 5 kinds of ACs adsorption of Cr(VI) were:AC3(30.412 mg/g)> AC2(27.473 mg/g)> AC4(26.596 mg/g)> AC1(14.096 mg/g)> AC5(13.573 mg/g).(2) Adsorbents quaternary amine-anchored were prepared from commercial activated carbon (AC) by epoxide-induced methods:AC-T and AC-E. The effect of pH, contact time, temperature and initial Cr(VI) concentration on adsorption was investigated to compare Cr(VI) removal capacity of those two modified ACs. The adsorption capacity for Cr(VI) of AC, AC-T and AC-E was 10.57,26.25 and 112.36 mg/g, respectively. The rate of Cr(VI) adsorption onto AC-E was more rapid than that of AC and AC-T. In addition, AC-E performed a higher adsorption capacity during a wider range of pH (2.0～10.0). The physicochemical properties of ACs were investigated by Zeta potential, elemental analysis, N2 adsorption/desorption, scanning electron microscope and Fourier transformed infrared. By means of multiple analyses, the authors surmised that (a) AC-T showed both pore-filling and electrostatic adsorption processes, while AC-E almost depended on the electrostatic attraction to remove Cr(VI); (b) the active quaternary ammonium sites within AC-E were more exposed and accessible for Cr(VI) removal than those within AC-T.(3) Desorption regeneration experiments of modified activated carbons. Experiment selected four kinds of desorbents, the regeneration efficiency in the order of:HCl> NaCl> NaOH> H2O; The desorption efficiencies of AC-E were 86.93% after five sorption-desorption cycles by HCl, which shows that AC-E had good regeneration potential.In this study, corn stalks were used to prepare modified activated carbon through two methods. All those modified ACs exhibited higher Cr(VI) adsorption capacity than conventional AC.