Synthesis of low pH stable, highly photocatalytic alginate hydrogels for methylene blue removal from wastewater

Chemical compounds in wastewater, including dye molecules, have been identified as significant concerns for environmental integrity and human health. In this study, adsorbent materials were synthesized from biodegradable polymers to remove methylene blue (MB) from wastewater. Calcium alginate hydrogels were produced and coated with chitosan to enhance their pH stability. Furthermore, the calcium alginate gels were doped with silver nanoparticles to improve their photocatalytic properties, promoting MB degradation and enabling gel reuse. The adsorption behavior of the gels was comprehensively examined under dark conditions, considering factors such as time, solid-to-liquid ratio, temperature, and pollutant concentration, all of which significantly impact the adsorption process. The wastewater, containing 100 ppm of MB, was subjected to a cleaning process that resulted in a reduction of the initial concentration by over 95%. Adsorption kinetics, thermodynamic constants, and adsorption isotherms were studied to understand the underlying mechanisms better. It was observed that the adsorption kinetics were compatible with pseudo-second-order with R² values above 0.99 and the adsorption kinetics were compatible with Freundlich with 0.9996. Modeling and simulation studies were used to establish the correlation between adsorption behavior and the concentration of dye material. Subsequently, UV light exposure was applied, enhancing further efficiency and enabling gel regeneration. Even after five cycles, the gels maintained an efficiency of 88.42%. Structural characterizations, pH stability assessments, and cytotoxicity analyses were conducted to evaluate the suitability of the gels for wastewater treatment with minimal environmental or health impact.