Sustainable Synthesis of Ni-Based Metal–Organic Framework from Waste Plastic as a Potential Electrochemical Sensor for Detecting Heavy Metals

The widespread contamination of soil and surface water by waste plastics and heavy metals has become a critical environmental challenge. This research highlights sustainable waste-to-resource management by converting discarded polyethylene terephthalate (PET) bottles into terephthalic acid (TPA) for the synthesis of metal–organic frameworks (Ni-MOFs), enabling highly sensitive electrochemical detection of Cd2+ ions. This research also focuses on the Sustainable Development Goals (SDGs) through reusing waste PET bottles and converting them into terephthalic acid, which was used to synthesize a value-added product, Ni-MOF. The synthesized terephthalic acid and Ni-MOFs were characterized using several sophisticated instruments, including TEM, FTIR, XPS, XRD, FESEM, Raman spectroscopy, VSM, TGA, and DSC. The X-ray diffractogram revealed the formation of high-purity terephthalic acid (TPA) and Ni-MOFs. The presence of –COOH and Ni–O in terephthalic acid and Ni-MOFs, respectively, was confirmed by the FTIR spectrum, while SEM demonstrated a bimodal particle morphology. The VSM data of Ni-MOFs revealed distinctive magnetic properties (Ms: 0.041 emu/g, Hc: 1460 Oe), whereas TG analysis disclosed three stages of mass change. Potassium ferrocyanide was used to measure the sensitivity of the fabricated electrode. The Ni-MOF-fabricated glassy carbon working electrode was able to detect heavy metal (Cd) through electrochemical techniques (LSASVlinear sweep anodic stripping voltammetry and DPASVdifferential pulse anodic stripping voltammetry) because the modified electrode provides a high surface area with accessible Ni2+ coordination sites and oxygen-containing functional groups, which enhance the binding and electron transfer of Cd2+. LSASV achieved exceptional sensing performance with a linear range of 0.001–10 ppm, while DPASV demonstrated a superior sensitivity (linear range of 0.00001–7 ppm) of 2.95 × 10−5 A/ ppmrepresenting a 43.05% improvement over LSASVwith reproducibility showing relative standard deviations below 11%.