The application of food/agro-waste and spent household products for the environmentally benign separation of thorium

Abstract

The cost-effective and environmentally benign separation of thorium from an aqueous acidic medium using spent food/agro-byproducts has been demonstrated utilizing used tea leaves, coffee powder, and coconut leaves. The presence of carbonyl functionalities, hydroxyl moieties, and amine moieties was found to be responsible for coordinating with Th⁴⁺ ions, while rough sorbent surfaces provide more surface area for Th⁴⁺ ions to interact with. The Langmuir isotherm model was found to be applicable, involving monolayer sorption, without any neighboring-group participation, and chemisorption. Pseudo-second-order rate kinetics were revealed, with rate constants for thorium sorption of 3.4 × 10⁻⁶ mg g⁻¹ min⁻¹, 2.7 × 10⁻⁵ mg g⁻¹ min⁻¹, and 1.9 × 10⁻⁵ mg g⁻¹ min⁻¹ for coconut leaves, tea leaves, and coffee powder, respectively. The sorption process was thermodynamically favourable, having ΔG⁰ values of −5.91 kJ mol⁻¹, −8.60 kJ mol⁻¹, and −7.22 kJ mol⁻¹ for coconut leaves, tea leaves, and coffee powder, respectively. The sorption processes were endothermic in nature (ΔHcoconu⁰_{t leaf} = 3.30 kJ mol⁻¹, ΔH⁰_{tea leaf} = 6.33 kJ mol⁻¹, and ΔH⁰_{coffee powder} = 3.70 kJ mol⁻¹). The overall enhancement in entropy (ΔS⁰_{coconut leaf} = 0.03 kJ mol⁻¹ K⁻¹, ΔS⁰_{tea leaf} = 0.04 kJ mol⁻¹ K⁻¹, and ΔS⁰_{coffee powder} = 0.04 kJ mol⁻¹ K⁻¹) showed the spontaneity of the process. Upon exposure to 100 kGy gamma radiation, the K_d values for thorium were reduced to ∼49.9% of the original values for coconut leaves and tea leaves, while K_d was reduced to 40.4% of the original value for coffee powder.