An alginate gel adsorbent to remove heavy metal ions according to the present invention is prepared by adding dropwisely 0.1-5 wt % alginate solution to a polyvalent cationic solution thereby cross-linking alginic acid with polyvalent cations. An alginate gel adsorbent containing activated carbon capable of simultaneously removing heavy metal ions and organotoxic materials, that is, activated carbon/alginate gel adsorbent, is prepared by adding dropwisely a mixed solution of 0.17-10 wt % of alginate and 0.1-10 wt % of activated carbon powder to a polyvalent cationic solution thereby cross-linking alginic acid with polyvalent cations in order to immobilize polyvalent cation to alginic acid containing activated carbon. The polyvalent cationic solution is selected from the group consisting of calcium chloride (CaCl₂), strontium chloride (SrCl₂), barium chloride (BaCl₂) and aluminium chloride (AlCl₃). A flat board or a thin membrane coated alginate gel adsorbent can be prepared by coating alginate gels onto a supporter such as paper, wood plate and textile fabrics. The alginate gel adsorbent for water purification can be applied to a water purifier as well as a process of wastewater treatment. The alginate gel coated onto a supporter is prepared by immersing a supporter in 0.05-5 wt % alginate solution so that the supporter may uniformly adsorb the alginate solution, immersing the supporter in a polyvalent cationic solution thereby forming alginate gel on the surface of the supporter, and drying the supporter formed with alginate gel thereon.