Abstract
The threat of industrial pollutants such as dyes, to water quality is a pressing concern that requires urgent attention. Effective remediation strategies are essential to mitigate the harmful effects of these pollutants on both aquatic ecosystems and human health. Porous materials have emerged as promising solutions which offer a sustainable and cost-effective approach to water purification via pollutant adsorption, thereby preventing their entry into water bodies. Thus, utilizing these materials can greatly enhance water quality and protect the environment for future generations. In this study, a series of solvothermally synthesized microcrystalline coordination polymers is presented, employing benzene-1,4-dicarboxylic acid, benzene-1,4-dihydroxamic acid and 5-nitroisophthalic acid in conjunction with metal salts of Cu, Cr, Ce and La. Characterization of these polymers was conducted using a comprehensive suite of techniques, including 1H NMR, P-XRD, FTIR, TGA, SEM-EDX, ICP-OES and BET analysis. Subsequently, this study involved a systematic evaluation of how effectively the prepared coordination polymers could adsorb methylene blue, a widely used dye in various industries, by measuring parameters such as adsorption capacity, kinetics and factors influencing the adsorption process such as pH and initial dye concentration. The findings revealed that these polymers exhibited maximum adsorption capacity within ten minutes of dye exposure, following pseudo-second order kinetics (chemisorption) and that increasing pH levels augmented both the rate and extent of methylene blue absorption. Intra-particle diffusion studies showed that methylene blue uptake was controlled by both external surface adsorption and intraparticle diffusion mechanisms. Half-life studies based on pseudo-second order kinetics indicated that Cr-BHA was the fastest acting adsorbent toward methylene blue having the lowest methylene blue removal efficiency because of active site saturation whereas La-5NIP offered superior methylene blue removal efficiency despite being the slowest adsorbent toward methylene blue uptake among the coordination polymers investigated. Langmuir and Freundlich isotherm models suggested that methylene blue adsorption onto the CPs involved both monolayer and multilayer adsorption. The Dubinin-Radushkevich model suggests a pore-filling mechanism for MB uptake with possible physisorption-dominated interactions. The multimodal adsorption observed prompted further analysis using the three-parameter Redlich-Peterson (R-P) and Sips hybrid models. The β constant in the R-P model confirmed contributions from both Langmuir and Freundlich models whilst the Sips model heterogeneity factor (n) indicated methylene blue uptake via multimodal heterogeneous adsorption processes.