Adsorption Isotherm, Kinetic and Thermodynamic Studies for The Removal of Rhodamine B and Heavy Metals (Co2+, Cu2+ and Pb2+) from Wastewater using Rice Husks, Coconut Shells and Clay

Abstract

The contamination of aquatic environments by heavy metals and synthetic dyes requires cost-effective and sustainable remediation strategies. This study evaluates the efficiency of powdered adsorbents derived from rice husks (ARC), coconut shells (ACC) and clay soil (ACS) for the removal of Co^2+, Cu²⁺, Pb²⁺, and Rhodamine B (RB) dye. The adsorbents were characterized using XRD, XRF, FTIR, and BET analysis to determine their mineral phases, elemental composition, functional groups, and surface area. Metal concentrations were analyzed using Atomic Absorption Spectroscopy (AAS). Batch adsorption experiments were conducted to examine the effects of temperature, contact time, solution pH, and initial concentration dosage. For Co²⁺, Cu²⁺, Pb²⁺ ions, and RB, equilibrium and kinetic studies were conducted at 25 °C with particle sizes of 100 μm at optimal pH values of 4, 7, and 12. Thermodynamic assessments were also conducted, including variations in free energy, enthalpy, and entropy. The adsorption kinetics followed pseudo-second order kinetics, and the adsorption data fitted well the Langmuir isotherm model, with a correlation coefficient (R²) > 0.99 for all adsorbates for the three adsorbents. Thermodynamic parameters revealed endothermic and non-spontaneous metal ion adsorption at low temperatures, however, with spontaneous adsorption at higher temperatures. High negative free energy change (ΔG^o) implies spontaneity, reflecting a more energetically favourable process driven by entropy. This study demonstrates carbonized biomass (rice husks, and coconut shells) and modified clay to be promising low-cost adsorbents for the removal of metal ions and dyes from aqueous solutions.