Kinetic, equilibrium, thermodynamic, and desorption studies for sequestration of acid dye using waste biomass as sustainable adsorbents

In the present study, tea waste (TW) and cashew nut shell (CNS), waste biomasses are employed in removal of Acid Green 25 dye from aqueous media. Adsorbents are characterized using BET, SEM, zeta potential, and FTIR analyses. BET surface area was obtained as 25.70 and 68.82 m2 g−1 for CNS and TW, respectively. The efficacy of dye removal in the batch study was determined by varying the operating parameters like pH, adsorbent loading, contact time, influent concentration, and temperature. Maximum adsorption was obtained at a pH of 1 using TW and CNS. Adsorbent loading was established at 0.15 g of TW and 0.2 g of CNS. Adsorption was favored at the higher temperatures. Maximum uptake capacities were obtained as 123.46 and 76.34 mg g−1 for TW and CNS, respectively. Better results of dye removal were established for TW in comparison with CNS. Equilibrium data have been well exemplified by Langmuir model, whereas kinetic data suited well to the pseudo-second-order equation for dye removal using TW and CNS. Evaluation of thermodynamic parameters confirmed the spontaneous and physical nature of the studied adsorption. Weber-Morris model established the involvement of different stages in the rate-controlling mechanism. Reusability study performed in five cycles confirmed the potential of TW in removal of Acid Green 25 dye from aqueous media as a slight decrease in uptake capacity from 32.71 mg g−1 for the 1st cycle to 30.52 mg g−1 for the 5th cycle was reported. The overall adsorption study confirmed the efficacy of TW and CNS in removing Acid Green 25 from wastewater.