Hydroxypropyl sulfonated starch and Asperlligus oryzae biomass for cationic dye adsorption: characterization, mechanism, sorption modelling

In this work, hydroxypropyl starch sulfate (HPSS) and Aspergillus oryzae ( Asp. oryzae ) were successfully synthesized and investigated for aqueous methylene blue (MB) adsorption. The as-prepared adsorbents were also characterized extensively using FTIR, XRD, SEM, EDX, and BET surface area analyses to elucidate their functional, textural, and morphological properties. Also, the effects of initial dye concentration, contact time, and pH on the adsorption performance of both adsorbents were systematically investigated. Due to the significant surface area differences, the HPSS recorded a higher maximum adsorption capacity ( q max ) of 52.41 mg/g at 20 mg/L initial concentration, 60 min, and pH 8.0, while the Asp. oryzae recorded a q max of 37.26 mg/g at 20 mg/L initial concentration, 60 min, and pH 9.0. Specifically, the –SO 3 groups on the HPSS shared some electrostatic affinity with the MB dye cationic center (N + backbone), while a hydrogen bond is formed between the hydroxyl groups of the starch and N + backbone of the MB dye. Also, the nitrogen- and oxygen-containing groups on the Asp. oryzae provided active sites for the binding of MB species. Also, the XRD spectra of the loaded HPSS showed a decrease in the sharp crystalline peaks, while no structural changes were observed in the case of loaded Asp. oryzae . Therefore, the effectiveness of the HPSS and Asp. oryzae for adsorbing MB was established in the study.