Synthesis, characterization, and DFT analysis of a brushite/PEG6000 composite for Pb(II) adsorption from aqueous solutions

The pollution of industrial effluents by hazardous heavy metals, particularly lead (Pb 2+ ), constitutes a significant environmental dilemma owing to their inherent toxicity, persistence in the environment, and propensity for bioaccumulation. In this regard, a novel composite of brushite and polyethylene glycol 6000 (PEG6000) was synthesized utilizing a dissolution–precipitation methodology to augment the stability and adsorption efficacy of unmodified brushite. FTIR confirmed the coexistence of phosphate and PEG6000 functional groups, with slight shifts indicating interfacial interactions. SEM–EDS analyses revealed a homogeneous morphology and appropriate elemental composition (Ca, P, O, and C). TGA/DTA demonstrated brushite stability up to ∼200 °C and the successful incorporation of PEG6000. X-ray diffraction (XRD) analysis, combined with Rietveld refinement, confirmed a phase-pure orthorhombic structure with good reliability factors. Notably, the refinement enabled accurate determination of the electron density distribution within the unit cell using the GFourier method, providing detailed insight into bonding charge density, chemical bonding, and atomic interactions within the composite. Batch adsorption assays revealed that the elimination of Pb 2+ is influenced by variables such as thermal conditions, initial heavy metal concentration, pH levels, and duration of contact. The findings from the experiments were consistent with the Langmuir isotherm model, suggesting a maximum adsorption capacity of 34.53 mg/g for the composite, which exceeds that of unmodified brushite. Kinetic investigations conformed to a pseudo-second-order (PSO) model. Thermodynamic assessments suggested that the process is characterized as both spontaneous and endothermic in nature. Theoretical analyses grounded in RDG and NCI methodologies substantiated that the adsorption of Pb 2+ is primarily driven by coordination interactions, hydrogen bonding, and van der Waals forces, thereby underscoring the considerable potential of the brushite/PEG6000 composite for applications in wastewater treatment.