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

Abstract 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%.