Propensity of a low-cost adsorbent derived from agricultural wastes to interact with cationic dyes in aqueous solutions

Ash collected from thrown-away by-products while preparing a popular traditional food additive, kolakhar of the Assamese community of North East, India, was used as an alternate cost-effective, porous bioadsorbent option from the conventional activated carbon for the purification of carcinogenic dyes laden water. The base material for kolakhar preparation was taken from the discarded banana stem waste to stimulate agricultural waste management. Methylene blue (MB) and basic fuchsin (BF) dyes were used as model cationic dyes. Characterization techniques like CHN, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscope (FE-SEM), energy dispersive X-ray (EDX), and Brunauer–Emmett–Teller (BET) analysis of the prepared banana stem ash (BSA) reveal the presence of high inorganic contents and functional groups in the irregular, porous bioadsorbent with surface area 55.534 m 2 g −1 . Various regulating parameters studied to optimize the adsorption capacity of BSA were bioadsorbent dose (0.1–3 g/L), temperature (298–318 K), contact time (0–150 min), pH (2–9), and initial dye concentrations (10–40 mg/L). Non-linear kinetic models suggested Elovich for both MB and BF adsorption, while the non-linear isotherm model suggested Langmuir and Temkin for MB and BF adsorption, respectively, as best-fitted curves. The monolayer adsorption capacity (qm) for MB and BF was 15.22 mg/g and 24.08 mg/g at 318 K, respectively, with more than 95% removal efficiency for both dyes. The thermodynamic parameters studied indicated that the adsorption is spontaneous. The ∆ H 0 values of MB and BF adsorptions were 2.303 kJ/mol (endothermic) and − 29.238 kJ/mol (exothermic), respectively.