Biodegradable and Reusable Sponge Material Prepared from Pea Protein for the Effective Removal of Heavy Metals

There is a growing demand for biodegradable and sustainable materials, particularly those sourced from agricultural or industrial byproducts, for use in wastewater treatment. This study introduces an approach for developing a sponge-like adsorbent derived exclusively from pea protein (Pea_AS-PEI) using liquid foam templating for the removal of heavy metals. The pore size, surface area, and mechanical strength of the sponge were modulated by the extent of protein hydrophobic aggregation induced by ammonium sulfate (AS) immersion based on the salting-out effect. Raising AS concentration from 10 to 20% led to an increase in surface area from 30.69 to 127.41 m²/g and compressive stress from 151.02 ± 8.73 kPa to 316.10 ± 13.87 kPa at 90% strain. Polyethylenimine (PEI) grafting introduced additional amine groups for heavy metal adsorption, and the porosity of the sponge increased from 86.9 to 90.3% upon surface modification. As a result, the PEI-modified sponge showed favorable adsorption performance of Cu(II), Zn(II), and Ni(II) ions at pH 5 with maximum sorption capacities of 67.07, 115.61, and 55.86 mg/g, respectively. The adsorption kinetics and isotherm study suggested that the adsorption process was primarily chemisorption and occurred with monolayer interactions. The negative Gibbs free energy change (ΔG < 0) confirmed that the adsorption was thermodynamically spontaneous. Reusability tests for the Pea_AS-PEI sponge revealed that its adsorption capacity could be well maintained over five successive adsorption-desorption cycles, with the removal efficiency of Cu(II) over 95%. The pea protein sponge also exhibited excellent biodegradability in soil within 28 days, with weight losses of 86.1% and 67.5% before and after PEI grafting, respectively. Together, these results indicate the great potential of affordable and sustainable PEI-modified pea protein sponges for the remediation of water polluted with copper, zinc, and nickel ions.