Textural properties and adsorption behaviour of recyclable Ni-Al layered double hydroxides for Congo red and Alizarin red S dyes removal in single and binary systems

Purification of dye-containing wastewaters has been considered as serious challenge over the last decades due to the huge, discharged dye wastewaters in the environment and their chemical complexity and different behaviour. The characteristics of used adsorbent have a crucial role in dye sorption and selectivity. This work aims to design nickel-aluminium-based Layered Double Hydroxides (LDH) with different ratios (R = Ni2+/Al3+ = 1.5; 2.0 and 3.0), uncalcined (NiRAl-LDH), and calcined (NiRAl-LDO). As-prepared materials were tested comparatively for the adsorption of Congo red (CR) and Alizarin red S (AR) in single and binary systems. The structure and chemical characteristics were studied as a function to synthesis conditions and chemical composition. The effect of operating parameters on the removal of dyes in different systems was studied and discussed according to the properties of prepared adsorbents. The selectivity of LDH adsorbents toward CR and AR was estimated. AR exhibits a higher affinity compared to CR, where the maximum quantity adsorbed by LDH R: 1.5 was 126.2 and 307.7 mg g-1 for CR and AR, respectively, and 365.7 and 892.5 mg g-1 for CR and AR respectively, adsorbed by LDO R: 1.5. Furthermore, the modified Redlich-Peterson equation was applied to adsorption process, and it was found to accurately predict the binary data in this state with R2 values closer to unity and higher Wi values compared to the Extended Langmuir, Sheindorf-Rebuhn-Sheintuch, non-modified competitive Redlich-Peterson, and Extended Sips models. The mechanism behind the relationship between the Ni2+/Al3+ ratio in adsorbents and sorption behaviour of dyes was studied. The pseudo-second-order model was found to fit well with the adsorption process, while based on thermodynamic analysis, the sorption of dyes was spontaneous and endothermic. Overall, the study gives important discussion regarding the selective adsorption of dyes with respect to the synthesis conditions of Ni2+/Al3+ Layered Double Hydroxides.