MnO2/g-C3N4/PPy Ternary Composite for Congo Red Adsorption from Water: Preparation and Comparative Performance

The presence of synthetic dyes in wastewater continues to create problems for water treatment and increases the need for adsorbent materials that can provide effective dye removal. MnO2, g-C3N4, and PPy have been widely studied for pollutant removal, but direct comparisons of their individual, binary, and ternary systems under common adsorption conditions remain limited. In this study, six adsorbents were prepared and compared for Congo Red (CR) removal in the dark. The adsorption experiments were conducted using approximately 40.0 mg of adsorbent in 40 mL of a 200 mg/L CR solution at ambient laboratory temperature. The prepared materials were characterized using XRD, SEM–EDS, XPS, FTIR, nitrogen adsorption–desorption analysis, UV–Vis spectroscopy, and PL spectroscopy to examine their structural, morphological, elemental, surface-chemical, textural, and optical properties. Among the tested samples, the ternary composite showed the highest final Congo Red removal, reaching 75.1% after 120 min, with an observed adsorption capacity q120 of 150.20 mg g−1. Three independent adsorption experiments also produced close final removal values, supporting the repeatability of the ternary composite. The removal ranking did not follow the BET surface-area ranking, suggesting that surface area alone could not explain the observed performance. Changes in the FTIR spectra after adsorption supported Congo Red removal but did not identify the specific interactions between the dye and the adsorbent surface.