Removal of Alizarin red S by calcium-terephthalate MOF synthesized from recycled PET-waste using Box-Behnken and Taguchi designs optimization approaches

In this research, recovered terephthalic acid from polyethylene terephthalate (PET) waste was employed as a precursor for the high yield solvothermal synthesis of a calcium-terephthalate metal-organic framework ([Ca(BDC) (H2O)3]). The structure of this MOF was analyzed using FTIR, XRD, BET, EDX, and FESEM methods. For the first time, its adsorption ability was studied to remove Alizarin red S (ARS) dye from an aqueous solution. Experimental designs involving Box-Behnken (BBD) and Taguchi were used to optimize removal conditions. These two approaches were compared for optimization of the adsorption process. So, effective factors such as pH, adsorbent dosage (m), and initial ARS concentration (C) were examined. BBD with 15 runs led to a proper relation between the effective factors and either removal percent (R%) or adsorption capacity (q) response. The proposed BBD model yielded a maximize removal percent (R% ​= ​69.97) and adsorption capacity (q ​= ​788.3 ​mg˖g−1) of ARS for following optimum conditions: pH ​= ​6, m ​= ​2 ​mg, and C ​= ​379 ​mg˖L−1. The optimum conditions obtained by a Taguchi design with 9 runs are pH ​= ​5, m ​= ​2 ​mg, and C ​= ​300 ​mg˖L−1. The adsorbent-adsorbate behavior is described by the Langmuir-Freundlich isotherm model well because this isotherm has the largest value of nonlinear R2 (R2 ​= ​0.9966) form compared with others. The maximum adsorption capacity (qmax) of ARS is 979.0 ​mg∙g−1. The experimental data obey the pseudo-second-order (PSO) kinetic model (R2 ​= ​0.9702). According to the negative value of ΔH° (−13 ​kJ ​mol−1), ΔS° (−0.037 ​kJ∙mol−1), and ΔG° (−2.6 to −1.5 ​kJ∙mol−1), the ARS adsorption process is exothermic and spontaneous chemisorption in the thermal range of 293–323 ​K.