Overview of operational factors for selecting high-performance methylene blue adsorbents: a structured screening guide

Methylene blue (MB) is among the most extensively discharged cationic dyes in industrial effluents and remains a persistent environmental concern due to its high chemical stability, resistance to biodegradation, and adverse ecological impacts. Although hundreds of adsorbents have been developed for MB removal, the selection of practically applicable materials remains challenging because performance is commonly judged solely by adsorption capacity, while critical operational parameters are often overlooked. To address this limitation, this review proposes a comprehensive operational screening framework for identifying high-performance MB adsorbents suitable for real-world wastewater treatment applications. A total of 83 adsorbents reported in peer-reviewed studies published over the last five years were systematically evaluated across all major material classes. The proposed framework integrates five practical performance indicators: maximum adsorption capacity ( Q max > 100 mg/g), low adsorbent dosage (≤ 1 g/L), rapid equilibration time (≤ 300 min), exothermic adsorption behavior, and high swelling capacity for hydrogels, thereby enabling a multidimensional assessment beyond conventional capacity-based comparisons. Among the evaluated materials, 55 adsorbents satisfied the proposed selection criteria, including 43 activated carbons/biochars and 12 hydrogel-based adsorbents. Hydrogels emerged as the most promising class, exhibiting an average adsorption capacity of 1660 mg/g at an average dosage of only 0.4 g/L, combined with remarkable swelling capacities (90 g/g) and relatively short equilibration times (160 min). Sodium alginate–multi-walled carbon nanotube (SA-MWCNT) and poly(acrylic acid)/laponite hydrogels demonstrated the most balanced operational performance, achieving adsorption capacities of 1370–1600 mg/g, low dosages (0.33–0.75 g/L), rapid adsorption times (30–50 min), and swelling capacities ranging from 23 to 181 g/g. Carbon quantum dots derived from neem biomass were further identified as a promising emerging adsorbent, attaining 714 mg/g within only 30 min. This study introduces, for the first time, a practical decision-making framework that bridges the gap between laboratory-scale adsorption performance and engineering applicability. While the focus was on MB, the proposed performance indicators are adaptable for identifying adsorbents targeting other contaminants, including pesticides, anionic dyes and heavy metals.