Benzimidazole based zeolitic imidazolate framework as an efficient adsorbent for palladium recovery from wastewater

Abstract

This study presents the cost-effective synthesis of a cobalt-based zeolitic imidazolate framework (Co-ZIF) using benzimidazole as a linker for the adsorption of palladium (Pd(II)) ions from wastewater. The synthesised Co-ZIF exhibited high crystallinity, confirmed by XRD with a crystallite size of 20.06 nm, and FTIR spectra showing a distinct Co–N stretching band at 420 cm⁻¹, indicating successful coordination between cobalt ions and the nitrogen atoms of benzimidazole. SEM–EDS analysis revealed a uniform elemental distribution of cobalt (26%), carbon (32%), nitrogen (24%), and oxygen (18%), confirming compositional homogeneity. Batch adsorption experiments showed that 10 mg of Co-ZIF achieved 94.38% removal of Pd(II) under acidic conditions (pH 3), with an experimental maximum adsorption capacity (qₘ) of 471.9 ± 1.0 mg/g. The adsorption followed the Langmuir isotherm model (with theoretical qₘ = 354.3 mg/g), suggesting monolayer adsorption on a homogeneous surface. Kinetic analysis followed the pseudo-second-order model, indicating chemisorption as the dominant mechanism, while thermodynamic parameters confirmed a spontaneous and endothermic process. Density Functional Theory (DFT) calculations supported strong Pd–N interactions and high binding affinity at the cobalt sites. The Co-ZIF demonstrated excellent selectivity for Pd(II), maintaining over 80% removal in the presence of competing ions. However, regeneration studies revealed complete structural degradation after a single desorption cycle, limiting its reusability despite its high initial performance. • Synthesis of a cobalt-based zeolitic imidazolate framework. • The synthesised Co-ZIF exhibited high crystallinity with a distinct Co–N vibration. • Batch adsorption experiments showed that Co-ZIF achieved 94.38% removal of Pd(II) under acidic conditions. • The adsorption followed the Langmuir isotherm model. • Density Functional Theory (DFT) calculations supported strong Pd–N interactions.