Recovering urea from human urine by bio-sorption onto Microwave Activated Carbonized Coconut Shells: Equilibrium, kinetics, optimization and field studies

• Microwave activated carbonized coconut shell was used for recovery of urea.
• Maximum monolayer adsorption of 256.41 mg g−1 obtained with Langmuir isotherm.
• Optimization of adsorption variables performed using Response Surface Methodology.
• Practical study to show feasibility of biomaterials to recover urea from urine.
• Urea adsorbed MACCS was shown to enhance plant growth and biomass yield.

Microwave Activated Carbonized Coconut Shell (MACCS) was used to recover urea from human urine. Batch adsorption studies were conducted to evaluate the effect of initial adsorbate concentration (25%–100%), contact time, carbon loading (1–3 g) and shaking speed (150–200 rpm) on the removal of urea at 30 °C. Microwave activation was performed at 180 W (microwave output power) for 10 min. The sorption data were fitted to Langmuir, Freundlich, Tempkin, Flory–Huggins and Dubinin–Radushkevich isotherm models. Results showed that the maximum monolayer adsorption capacity of the MACCS powder was 256.41 mg g−1. The Flory–Huggins model was found to best describe the urea uptake process since it demonstrated the minimum deviations from the experimental data. The kinetic data was fitted to pseudo-first-order, pseudo-second-order and intra-particle diffusion models, and was found to follow closely the pseudo-first order kinetic model. Based on the Central Composite Rotary Design, a five factor interaction model and a quadratic model were respectively developed to correlate the adsorption variables to the adsorption capacity. Field studies were conducted to determine the percentage biomass increase and relative agronomic effectiveness for soil treated with the urea adsorbed MACCS powder. Microwave activated carbonized coconut shell was shown to be a promising adsorbent for recovery and removal of urea from human urine solutions.