Tea factory waste as a cost-effective and sustainable Cadmium(II) adsorbent: Investigating adsorption isotherms, kinetics, and thermodynamics

Abstract

In the quest for sustainable wastewater treatment, the reuse of locally available agro-industrial waste materials presents a promising alternative. Tripura, one of India's major tea-producing states, generates abundant tea factory waste (TFW) that holds significant potential as an affordable adsorbent for heavy metal remediation. This study explores the adsorption efficiency of two chemically treated forms of TFW—Na₂CO₃-treated (NTW) and HCl-treated (HTW)—for the removal of Cadmium (II) from aqueous solutions. Comprehensive characterization using SEM, FTIR, and EDX revealed enhanced porosity and functional group availability in both materials, particularly NTW. Increasing adsorbent dose (1–3 g/L) improved Cd(II) removal, with NTW and HTW achieving 63.85 % and 61.7 %, respectively. Though percentage removal dropped at higher Cd(II) concentrations, adsorption capacity increased, reaching 85.3 %–94.6 % (NTW) and 82.6 %–92.6 % (HTW). Higher agitation rates further improved removal, peaking at 88.6 % (NTW) and 86.4 % (HTW). Results indicated that NTW achieving a maximum efficiency of 90.08 % under optimal conditions (pH 7, 2 g/L dose, 350 rpm, 318 K, 60 min). The adsorption followed Elovich kinetics and best fit the Langmuir isotherm, confirming monolayer chemisorption. Thermodynamic analysis showed the process to be spontaneous and endothermic, with an activation energy of 76.602 kJ/mol. Using NTW for Cd(II) adsorption, wastewater's chemical oxygen demand (COD) dropped significantly from 138 to 74 mg/L. While the study highlights the low cost (Rs. 7/kg for NTW) and potential circular reuse of TFW, it also recognizes the need for future research to include life cycle assessment (LCA), energy analysis, and regeneration performance.