Calcium Carbonate/Expanded Graphite As an Efficient Catalyst for Catalytic Ozonation of Ethylenediaminetetraacetic Acid.

A novel calcium carbonate (CaCO₃)/expanded graphite (EG) catalyst was fabricated, characterized, and used in a catalytic ozonation process to increase degradation and mineralization of ethylenediaminetetraacetic acid (EDTA) aqueous solutions. Various techniques were used to analyze the crystalline structure, morphology, and elemental composition of the novel CaCO₃/EG catalyst, including X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. Mercury intrusion porosimetry was used to measure specific surface area, pore diameter, and pore volume of the CaCO₃/EG catalyst. Essential influencing factors such as CaCO₃/EG composite dosage and initial pH were also surveyed for different systems. Catalytic ozonation by CaCO₃/EG was notably increased by the removal of total organic carbon (TOC) of EDTA. After 60 min, 80% TOC removal efficiency from simulated wastewater having an initial TOC concentration of 600 mg/L and initial pH of 4.9 was achieved using 1.0 g/L CaCO₃/EG. This efficiency was higher than that seen for other systems, including O₃ alone (40%), EG/O₃ (39%), CaCO₃/O₃ (28%), and EG (4%). A small decrease in the CaCO₃/EG-catalyzed TOC removal rate was observed after three runs, but nonetheless the efficiency was >80% after 120 min. Assays with tert-butanol confirmed that CaCO₃/EG-mediated catalytic ozonation was dependent on a free radical pathway. The results support that CaCO₃/EG material could serve as an important reusable resource for catalytic oxidation reactions during treatment of wastewater containing EDTA. [ABSTRACT FROM AUTHOR]