A Novel Nanocomposite Bioadsorbent Derived From Natural CaCO3 for the Efficient Adsorption of Toxic Dyes: Synthesis, Characterization, and Application in Wastewater.

The textile dye industry releases dye-laden effluents that contaminate water, especially those containing Eriochrome Black T (EBT), a commonly employed anionic dye that poses health risks to humans. To address this problem, increasing attention is being paid to the use of abundant, low-cost, and nontoxic bioadsorbents as sustainable alternatives for wastewater treatment. This work assesses the potential of a new nanocomposite adsorbent (MnO:ES) obtained by combining natural calcium carbonate (CaCO3) derived from eggshells with manganese oxide (MnO) as an active agent to improve its adsorption capacity. A straightforward physical approach was employed to fabricate the MnO:ES through planetary milling conducted at room temperature for 2 h at a rotational speed of 350 rpm. The synthesized MnO:ES was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM/EDX). The MnO:ES nanocomposite was used to efficiently remove EBT from colored water, achieving a removal efficiency of 98% corresponding to a maximum adsorption capacity of 6.6 mg/g. The optimal conditions were achieved at natural pH, a temperature of 20°C, a contact time of 30 min, and an initial EBT concentration of 100 mg/L. The adsorption experiments were performed using 0.75 g of MnO:ES adsorbent composed of 10 wt% MnO and 90 wt% ES. MnO:ES was found to be dispersible, insoluble in water, and highly paramagnetic. The adsorption of the EBT dye followed pseudo-second-order kinetics and the Langmuir isotherm model, and thermodynamic analysis indicated spontaneous adsorption. Furthermore, microbiological analysis indicated a total absence of pathogenic germs, and a total aerobic bacterial count (TABC) of 190 CEU/mL was observed, which is relatively low for this treated water. The findings indicate that the incorporation of natural CaCO3 into a nanocomposite matrix significantly enhances its chemical stability and affinity adsorption efficiency toward target pollutants.