Comparative evaluation of calcined paper mill sludge, montmorillonite, and their composite adsorbents for mature landfill leachate treatment.

Mature landfill leachate is difficult to treat because refractory dissolved organic matter produces high chemical oxygen demand (COD), intense color, and poor biodegradability. This study compared calcined paper mill sludge (PMS-A), montmorillonite (Mt), and a novel calcined PMS-Mt composite for simultaneous COD and color removal from mature landfill leachate. Adsorbents were characterized by SEM, EDAX, FTIR, nitrogen sorption, and point-of-zero-charge analysis, and evaluated in batch experiments. At the optimized conditions (pH ≈ 7, 80 g L⁻¹ adsorbent, 90 min, and 250 rpm), PMS-A achieved the highest color and COD removals (98% and 92%), followed by PMS-Mt composite (96% and 76%) and Mt (50% and 60%). Although Mt had the greatest specific surface area, its lower performance showed that surface chemistry and site accessibility were more influential than surface area alone. The pseudo-second-order and Elovich models provided the best fit to the kinetic data. COD adsorption was generally better described by the Freundlich isotherm, whereas color isotherm behavior differed among the adsorbents. These empirical fits were not interpreted as proof of a specific adsorption mechanism. The required dosage of 80 g L⁻¹ and energy-intensive calcination remain important scale-up constraints. Overall, PMS-A showed the strongest technical performance, supporting further work on dosage reduction, regeneration, continuous-flow operation, and techno-economic assessment.