Red Seaweed (Gracilaria verrucosa Greville) Based Polyurethane as Adsorptive Membrane for Ammonia Removal in Water

Polyurethane membranes are widely developed polymers by researchers because they can be made from synthetic materials or natural materials. Red seaweed (Gracilaria verrucosa Greville) is a natural material that can be developed as a raw material for polyurethane membranes. This study used red seaweed biomass (RSB) as a raw material to manufacture polyurethane as an adsorptive membrane for removing ammonia in water. The membrane composition was determined using the Box-Behnken design from Response Surface Methodology with three factors and three levels. In the ammonia adsorption process, the adsorption isotherm was determined by varying the concentration, while the adsorption kinetics was determined by varying the contact time. Red seaweed biomass-based polyurethane membrane (PUM-RSB) can adsorb ammonia in water with an adsorption capacity of 0.233 mg/g and an adsorption efficiency of 16.2%. The adsorption efficiency followed the quadratic model in the Box-Behnken design, which resulted in the optimal composition of RSB 0.15 g, TDI 3.0 g, and glycerin 0.4 g with predicted and actual adsorption capacities of 0.224 mg/g and 0.226 mg/g. The ammonia adsorption isotherm using PUM-RSB follows the Freundlich isotherm, with a high correlation coefficient (R²) of 0.977, while the Langmuir isotherm has a low R² value of 0.926. The Freundlich isotherm indicates that ammonia is adsorbed on the surface of the adsorbent as multilayer adsorption. In addition, based on the analysis of adsorption kinetics, the adsorption phenomenon follows pseudo-order II with a chemisorption mechanism, and it is assumed that the bond that occurs is between the anion -SO₄^2- with the NH₄⁺ cation to form ammonium sulfate (NH₄)₂SO₄ and between isocyanates (NCO) with NH₄⁺ cations to form substituted urea.