Efficient dye removal using well-crystallized layered double hydroxides and their oxides: insights into kinetics, thermodynamics, isotherm and the memory effect

Dyes are toxic organic compounds that can adversely affect living organisms, and their removal from wastewater is a critical environmental concern. This study primarily focuses on the synthesis and adsorption behavior towards both cationic and anionic dyes of well-crystallized Mg-Al layered double hydroxides (MAH), made using highly concentrated metal salts, and their thermally treated derivatives, Mg-Al layered double oxides (MAO). Whilst both MAH and MAO exhibit high adsorption capacities for the methyl orange (MO) and acid blue 113 (AB) dyes, MAO, however, demonstrates slightly greater adsorption efficiency due to its increased surface area resulting from calcination. The adsorption behavior of these nanocomposites was studied by varying parameters such as contact time, temperature, pH, ionic strength, initial dye concentration and adsorbent dosage. The adsorption kinetics were best described by a pseudo second order model. Thermodynamic parameters such as the enthalpy, entropy and Gibb's free energy were calculated and the negative value for the free energy confirms the spontaneity and feasibility of the process, while the negative value of Δ<i>H</i> indicates that the adsorption process is physisorption and exothermic in nature. Two of the most widely used adsorption isotherms, the Freundlich and Langmuir models, were applied to fit the data. Both materials exhibited a good fit with the Langmuir model. The memory effect of MAO was studied by allowing the material to rehydrate to its LDH structure by moisture absorption from the environment over different durations (in days) followed by examining its adsorption behavior towards the MO dye. The reusability of the MAO material for MO dye adsorption was tested over multiple cycles, with washing between each cycle using solvents, acids, and bases. Additionally, the MO dye desorbed from the MAO material remained chemically intact and was recovered after washing the adsorbent with ethanol. This study demonstrates that both the adsorbent and the adsorbate materials are reusable.