Application of tea leaves in the circular economy: A sustainable approach to remove synthetic dye from wastewater by alkali-activated spent tea leaves

Spent tea leaves (STL) were recycled and utilized to reduce water pollution caused by synthetic dyes. The current study used STL as a biosorbent to remove malachite green (MG) dye from aqueous solutions and fish aquarium wastewater. The dye removal efficiency was enhanced by alkali (NaOH) and thermal pretreatment and showed 98% removal efficiency. Biosorbent characteristics and dye removal mechanisms were established using BET-surface area, zeta-potential, SEM, and FTIR. An optimum adsorption capacity of 40.6 mgg-1 was obtained for MG. Equilibrium sorption data were analyzed by Langmuir and Freundlich isotherm. Sorption kinetics was determined by pseudo-first-order and pseudo-second-order kinetic models. Experimental data showed the best fit with Langmuir isotherm and the adsorption followed pseudo-second-order kinetics. The negative value of free energy (ΔG◦) and positive value of enthalpy (ΔH◦) changes obtained from the thermodynamic studies indicated the spontaneous and endothermic nature of the adsorption. In our previous study, the bioactive compound theaflavin was isolated from fresh black tea leaves to treat cancer. Also, the STL generated after the theaflavin extraction from the tea leaves or after the consumption of tea as a beverage can be used as a potential source of natural dye for the textile industries which could be a possible alternative to the harmful synthetic dyes. Taken together, our studies close the loop of circular economy in the application of tea leaves in therapeutics, textile industries, and waste management by reusing, recycling, and repurposing the product and its wastes through multiple generations.