Pectin-based adsorbents for heavy metal ions: A review

Background: Over the past decades, the growing industrialization caused continuously increased release of heavy metal into environment. Through the food chain, heavy metals can be enriched in the body, causing serious harm. The removal and recycling of heavy metals are of great significance to environment protection, health concern as well as resource reuse. Pectic materials are available from a variety of natural sources and can be used as versatile adsorbents for heavy metal. Scope and approach: This paper reviews the current state of research on the adsorption capacity, internal (i.e. molecular structure) and external factors (i.e. pH, ionic strength, equilibrium time, temperature, and initial concentration) that influence adsorption properties, as well as adsorption mechanism of pectin and its modified forms (hydrogels and chemical modified pectin) for the removal of heavy metals. Besides, the corresponding key problems that need further studies are emphasized. Some novel trends in this field have also been discussed. Key findings and conclusions: Pectin possesses good capacities for adsorption of heavy metals, but drawbacks such as low mechanical strength and difficulty in separation limit it's application. Therefore, development of pectin derivatives such as hydrogels and chemical modified pectins were encouraged. The adsorption properties were affected by kinds of external factors and were related to molecular structure of pectin. The adsorption mechanisms of pectin-based adsorbents were mostly described to "egg-box" model, ionic exchange, complexation, and/or physical restraint.