Efficient removal of hexavalent chromium Cr (VI) using magnesium-iron layered double hydroxide supported on orange peel (Mg-Fe LDH@OPP): A synthetic experimental and mechanism studies

Herein, a series of meticulously designed batch and continuous mode sorption experiments were conducted to compare removal affinity of raw orange peel powder (OPP) and magnesium-iron layered double hydroxide impregnated OPP (Mg-Fe LDH@OPP) composite for the elimination of Cr (VI) ions from aqueous media. The relatively greater particle size, irregular pore distribution, effective surface area, and higher point of zero charge (pH_pzc) of Mg-Fe LDH@OPP as compared to OPP were principally responsible for its superior Cr (VI) sorption from aqueous media. It was observed that highest Cr (VI) removal (OPP: 81.84 %; Mg-Fe LDH@OPP: 100 %), was achieved at optimum conditions of pH (2), adsorbent dosage (2 g/L), contact time (180 min) and temperature (25 °C) for suspension containing 20 mg/L initial Cr (VI) concentration. The sorption data was adequately fitted with Freundlich and pseudo-second order model using both biosorbents. The FT-IR and XRD analysis further confirmed the reduction pathway followed by complexation reactions as major sorption mechanisms using Mg-Fe LDH@OPP. However, in the case of OPP, ligand exchange played a critical role in removing Cr (VI) ions from water. Furthermore, continuous mode column studies suggested applicability of Mg-Fe LDH@OPP compared to raw OPP. The results indicated breakthrough times of up to 366 min and 60 min, and exhaustion times of up to 800 min and 350 min, for Mg-Fe LDH@OPP and OPP, respectively. In general, Mg-Fe LDH@OPP was found to be a promising material with strong Cr (VI) sorption potential from polluted water.