Natural Fiber Welding (NFW) is a green engineering process that uses Ionic Liquids (IL) to prepare functional biocomposites for applications including water purification, energy storage, sensing, and thermal enhancement. The composition of the welding solution can significantly impact welding efficiency as well as the structure and composition of fiber-welded biocomposites. This study evaluates how NFW solvent composition influences metal leaching from lignocellulose fiber-supported Pd–In nanoparticles. Solutions were prepared from pure, dry 1-ethyl-3-methylimidazolium acetate, and controlled amounts of IL contaminants (water, acetic acid), and one molecular co-solvent (acetonitrile). Lignocellulose fibers with embedded Pd–In nanoparticles were welded in each solution and reconstituted in water; the recovered precipitates/filtrates were evaluated using ICP-MS to determine how much palladium and indium leached from each catalyst into the welding solvent. Our data show how excess acetic acid in the NFW solvent causes indium metal to dealloy from Pd–In nanoparticles. Because Pd–In nanoparticles are proven hydrogenation catalysts for water purification, nitrate reduction tests were performed with each fiber-welded product and compared to unwelded control samples. These data show how preferential leaching of indium directly impacted the catalysts nitrate reduction reactivity, and provide insight into how one might control this phenomenon to optimize development of fiber-welded catalyst systems.