Recent developments in alginate-based adsorbents for removing phosphate ions from wastewater: a review

The huge development of the industrial sector has resulted in the release of large quantities of phosphate anions which adversely affect the environment, human health, and aquatic ecosystems. Naturally occurring biopolymers have attracted considerable attention as efficient adsorbents for phosphate anions due to their biocompatibility, biodegradability, environmentally-friendly nature, low-cost production, availability in nature, and ease of modification. Amongst them, alginate-based adsorbents are considered one of the most effective adsorbents for removing various types of pollutants from industrial wastewater. The presence of active COOH and OH- groups along the alginate backbone facilitate its physical and chemical modifications and participate in various possible adsorption mechanisms of phosphate anions. Herein, we focus our attention on presenting a comprehensive overview of recent advances in phosphate removal by alginate-based adsorbents. Modification of alginate by various materials, including clays, magnetic materials, layered double hydroxides, carbon materials, and multivalent metals, is addressed. The adsorption potentials of these modified forms for removing phosphate anions, in addition to their adsorption mechanisms are clearly discussed. It is concluded that ion exchange, complexation, precipitation, Lewis acid-base interaction and electrostatic interaction are the most common adsorption mechanisms of phosphate removal by alginate-based adsorbents. Pseudo-2(nd) order and Freundlich isotherms were figured out to be the major kinetic and isotherm models for the removal process of phosphate. The research findings revealed that some issues, including the high cost of production, leaching, and low efficiency of recyclability of alginate-based adsorbents still need to be resolved. Future trends that could inspire further studies to find the best solutions for removing phosphate anions from aquatic systems are also elaborated, such as the synthesis of magnetic-based alginate and various-shaped alginate nanocomposites that are capable of preventing the leaching of the active materials.