Synthesis, characterization, and adsorption performance of novel acidic calixarene-modified LDH/polyaniline nanohybrid for efficient Cr(VI) removal

Herein, aniline was oxidatively polymerized in the presence of modified Mg-Al/LDH to prepare an acidic calix[4]arene modified Mg-Al/LDH-polyaniline based nanohybrid (MLDH-NH₂@PANI). The prepared nanohybrid was studied using a variety of characterization techniques, including FT-IR, PXRD, TGA, FE-SEM, EDX-MAPP, and Zeta potential. To remove Cr(VI) from the aqueous solution, different parameters were examined, including pH, sorbent dosage, contact time, initial concentration of Cr(VI), and removal ability in the presence of interfering ions. The results showed that MLDH-NH₂ co-existence improved the active sites in MLDH-NH₂@PANI nanohybrid and reduced the agglomeration of polymeric chains. The pseudo-second-order model provided a better fitting for the Cr(VI) adsorption onto MLDH-NH₂@PANI, according to kinetic experiments. The highest adsorption capacity of Cr(VI) on MLDH-NH₂@PANI was 420.0 mg /g at pH = 3.0, which was more than that of typical adsorbents. The isotherms were accurately reproduced using the Langmuir model. At pH = 3, It is proposed that the oxyanion forms of Cr(VI) (e.g., HCrO₄^-) can interact electrostatically with protonated amine groups and form hydrogen bonds with calix[4]arene moieties as a potential mode of contact with the adsorbent. Moreover, a portion of the adsorbed Cr(VI) undergoes reduction to Cr(III) through an adsorption-coupled reduction process facilitated by the PANI chains, and the resulting Cr(III) is subsequently chelated onto the nanohybrid. The reusability studies showed that the adsorbents could be reused up to 4 cycles with over 80% adsorption efficiency. These findings suggest that the MLDH-NH₂@PANI nanohybrid proved to be a good candidate for Cr(VI) removal from contaminated water.