Comparative performance study of ZnCl₂ and NaOH sludge based activated carbon for simultaneous aqueous uptake of phenolic compounds

In this study, different sludge-based adsorbents were synthesised using ZnCl₂ (Zn-SBACs) and NaOH (Na-SBACs) under varying experimental conditions and subsequently employed for simultaneous aqueous uptake of phenol, resorcinol, and catechol. Response surface modelling (RSM) technique was employed to evaluate and optimise the collective influences of the SBAC production operating conditions based on adsorptive performances of the produced SBACs. The adsorbent-adsorbate interactive mechanisms were explored through several structural and surface characterisation techniques such as Scanning Electronic Microscopy (SEM), Thermogravimetric Analysis (TGA), Brunauer, Emmett and Teller machine (BET), Fourier Transformed Infrared spectroscopy (FTIR), pKa pHpzc and zeta potentials. These techniques depicted the SBACs as finely granular, thermally stable and mesoporous adsorbents enriched with oxygen functionalities. There existed a higher affinity of the Zn-SBACs towards catechol, then followed by resorcinol with the Na-SBACs yielding lower performance. Respectively, the maximum adsorption capacities of (45.57 mg/g and 42.99 mg/g); (14.19 mg/g and 29.28 mg/g); (20.95 mg/g and 17.82 mg/g) were obtained with higher pronounced effects of chemical–sludge ratio and activation temperature. For the Zn-SBAC, the best operating conditions for combined optimisation of the phenolic compounds’ removal were obtained at 615°C, 1.06 ZnCl₂-sludge ratio and 86.74-min activation time that yielded 100%, 57% and 80% for catechol, phenol, and resorcinol, respectively. The adsorption mechanism insight taken into cognisance FTIR, pKa, isoelectric point, zeta potential, and the phenolics-SBAC-adsorbent surface charge interactions suggest the interplay of chemisorption involving strong surface electrostatic attraction. The high performance of the SBACs under three cycles of regeneration affirmed the economic potentials of the usability of the SBACs for the treatment of phenolic wastewater. This study establishes the aptness of creating optimal conditions for SBACs production through the evaluation of the performance of simultaneous removal of multi-solute pollutants from an aqueous phase.