The Influence Of Ferric Iron In Calcined Nano-Mg/Al Hydrotalcite On Adsorption Of Cr(Ⅵ) From Aqueous Solution

The calcined hydrotalcites (CH-Mg/Al and CH-Mg/Al/Fe) were obtained by thermal decomposition of their corresponding precursors at 500℃. The precursors were synthesized by co-precipitation method using Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Fe(NO3)3·9H2O, NaOH and NaCO3. The influence of ferric iron in calcined nano-Mg/Al hydrotalcite on removal of Cr (VI) from aqueous solution was studied from aspects of structure characteristics, adsorption properties and mechanism discussions. The structure characteristics of the adsorbents were analyzed by means of XRD, TEM, TG-DTA, pHPZC and FTIR. The adsorption properties were studied as a function of pH, initial chromium concentration, sorbent dose, contact time, temperature and coexisting ions. Additionally, the removal mechanism was also discussed by comparing the analysis of FTIR before and after adsorption and the XRD patterns of adsorbents at different contact times during reconstruction process. The structure characteristics showed that replacing Al3+ with Fe3+ has almost not changed the structure of CH-Mg/Al but reducing the crystallinity and interlayer spacing. The removal of Cr (VI) gets the maximum at pH 3.0 for both CH-Mg/Al and CH-Mg/Al/Fe. The isotherms indicated that the adsorption both fitted better to Langmuir model, but the theoretical saturated adsorption capacity reduces by 10.2 mg/g by addition of Fe3+. Although thermodynamic parameter values revealed the adsorption natures on CH-Mg/Al and CH-Mg/Al/Fe are endothermic and spontaneous processes, the value changes indicated the addition of Fe3+ in CH-Mg/Al goes against the adsorption process. The removal mechanism involved not only intercalation but also adsorption on external surface of the layers and interlayer anion exchange for both CH-Mg/Al and CH-Mg/Al/Fe. Furthermore, it also indicated that intercalation accounts for a large proportion during the removal process whatever for CH-Mg/Al, or for CH-Mg/Al/Fe. Additionally, the replacement of Al3+ by Fe3+ in hydrotalcite led to the interlayer anion exchange more difficult.