Response surface methodology based modelling and optimisation of anionic dyes adsorption onto nitrate intercalated Zn2Al layer double hydroxide adsorbent

In the present work, nitrate intercalated zinc aluminium layer double hydroxide (ZA-LDH) nanoparticles in the hydrodynamic particle size range between 210 and 530 nm were synthesised using co-precipitation method under reflux condition and nitrogen atmosphere. The pristine ZA-LDH nanoadsorbent was characterised using X-ray diffraction (XRD), Fourier transformation spectroscopy (FTIR), and field emission scanning electron microscopy (FESEM). The results revealed the formation of single-phase ZA-LDH nanopowder with sheet-like morphology. The optimisation of adsorption kinetic parameters with the fixed dose of 10 mg for 50 ml of 0.03 mg/ml MO and AY dye solution was performed using the Box Behnken design model (BBD). The effects of three independent variables such as incubation time, temperature and pH of the suspended ZA-LDH nanoadsorbents in the MO and AY dye solution on the adsorption kinetics were studied. Optimum values for the maximum adsorption capacity were evaluated that included the incubation time of 52.4 min, the temperature of 35.45 +/- 0.55degree celsius, and the pH of 5.75 +/- 0.75. According to the BBD model, ZA-LDH nanoadsorbent exhibited the maximum adsorption percentage of 99.84% and 99.99% of corresponding MO and AY dye, respectively. The ZA-LDH adsorbent retained up to 72.3% for the MO and 60.43% for the AY dye of regeneration capacity after 5 cycles of regeneration. The adsorption kinetic and isotherm of the MO and AY dyes onto the ZA-LDH nanoadsorbent were well fitted with pseudo second order kinetic model and Langmuir model, respectively. According to Langmuir isotherm, the maximum adsorption capacities of 601.62 mg/g for the MO dye and 462.48 mg/g for the AY dye onto the ZA-LDH were found and those confirmed the monolayer chemisorption mechanism in this case.