Effect of pH, temperature and adsorbent mass on Cr (VI) adsorption in residues from a coal washing plant.

Abstract

The coal extraction process often results in the generation of significant amounts of waste materials. Mineral coal waste from the coal region of Coahuila poses significant environmental challenges due to its accumulation in surrounding areas. However, repurposing this waste presents opportunities for various applications, including the removal of water contaminants. This study evaluated the potential of mineral coal waste as an adsorbent material for Cr (VI). The waste underwent neutral (LN) and acid washing (LA) to compare the effects of these treatments on its properties. Characterization methods included proximate analysis, thermogravimetric analysis, X-ray fluorescence, X-ray diffraction, FTIR, scanning electron microscopy, contact angle and zero-point charge determination. Adsorption conditions were optimized varying pH, temperature and adsorbent mass. The highest removal efficiency was achieved with acid-washed material at a pH of 3, an adsorbent-to-solution ratio of 5 g/L, and a temperature of 45 °C, with pH identified as the most significant variable. Experimental isotherm data aligned with the Langmuir model, revealing a maximum adsorption capacity (Qmax) of 1.41 mg/g for the LA material. Kinetic data were best described by the pseudo-second-order model, with a maximum contact time of 48 h yielding 67.54% removal efficiency for a 5 mg/L Cr (VI) solution. Acid washing was found to reduce the ash content of the material, enhancing its adsorption performance.