Optimization of methyl orange adsorption on MgFeAl-LTH through the manipulation of solution chemistry and synthesis conditions

Adsorption is an attractive process for wastewater treatment due to its simplicity and mild operating conditions. However, the efficacy of this process depends on the development of effective adsorbents. Layered double/triple hydroxides (LDHs/LTHs) are promising materials for the adsorptive removal of water pollutants. In this study, the one-factor-at-a-time approach was utilized in order to optimize the methyl orange (MO) adsorption on MgFeAl-LTH through the variations of the LTH synthesis conditions and the solution chemistry. Specifically, the supersaturation degree of the synthesis solution, the divalent/trivalent metal ratio, the Al/Fe ratio, the synthesis temperature, and the post-synthesis treatment temperature were varied and their effects on the subsequent MO adsorption were investigated. To the best of our knowledge, this is the first study utilizing such an approach (i.e., optimizing the adsorptive removal of any water pollutant through the variations of the adsorbent synthesis chemistry and conditions). The obtained results revealed that MgFeAl-LTH prepared from a low supersaturated solution outperformed the one prepared from a high supersaturated solution in the adsorptive removal of MO from synthetic wastewater samples. Similarly, M²⁺/M³⁺ ratio of 2, Al/Fe ratio of 4, and room temperature synthesis produced MgFeAl-LTH materials with higher MO uptake capacities relative to other M²⁺/M³⁺ and Al/Fe ratios, and synthesis temperatures. Although the calcination of MgFeAl-LTH at 450 ^∘C increased its MO adsorption performance, calcination at a higher temperature (i.e., 600 ^∘C) did not boost the MO adsorption further. The XRD and BET characterization analyses proved the inexistence of any consistent relationship between the MO uptake capacity by MgFeAl-LTH and its crystallinity, the unit cell parameters, and textural properties (i.e., BET surface area, pore volume, and pore size).