In this work, the insolubilization of water-soluble sodium alginate, which is a representative biomass natural polymer, was investigated for various purposes, including coatings on paper cups. It was observed that sodium alginate could be insolubilized well in calcium chloride by formation of ionic crosslinks through replacement of Na⁺ ions with Ca²⁺ ions on a block of L-guluronic acid of alginate to form calcium alginate. Water-insolubilization of alginate is a critical prerequisite for the coatings on cups to retain water. However, the calcium alginate coating needs to undergo re-dissolution in water to enable recycling of the paper cups after use. It was observed that calcium alginate could be easily re-dissolved in a specific aqueous salt solution containing disodium phosphate (Na₂HPO₄). The re-dissolution of calcium alginate was confirmed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS). The process developed in this study is expected to be used for various eco-friendly applications without the need to use organic solvents.