Novel green technology for wastewater treatment: Geo-material/geopolymer applications for heavy metal removal from aquatic media

The aim of this paper is to show the concise chemico-physical adsorbent performance of water purification systems utilizing geo- (e.g., allophane, clinoptilolite, and smectite) and bio-polymer materials (e.g., chitosan or cellulose nanocomposite materials) and to propose an optimal ground-water remediation technique. The performance of geo-materials is evaluated based on the individual sorption and immobilization capacities for various priority substances and pollutants (e.g., lead, zinc, cadmium, copper, arsenic, and others), their availability, and cost-efficient use. A systematic assessment of the sorption potential of geo-materials in comparison to other available sorbents used for the removal of harmful aqueous metal ions is made through a literature review. This paper introduces novel sustainable technologies based on natural and tailored silicate-polymerized substances (geo-materials and geopolymers), and highlights their applicability in the treatment of water and solid matter contaminated by heavy metal ions. The advantages of geo-materials and geopolymers over other commercially available sorbents used for heavy metal ion removal from solution are presented through a literature review. Benefits and current challenges of geo-materials and geopolymers applications in water processing technologies and in environmental remediation are discussed, with recognition of their performance, individual sorption and immobilization capacities, availability, and cost-efficient use. The applications described here comprise: (i) the removal of heavy metal ions from contaminated water using in-situ remediation strategies; (ii) heavy metal ion immobilization through co-precipitation with silicate binders in underground stabilization and waste solidification scenarios; and (iii) a proposal for a new geo-material/geopolymer-based solidification and stabilization technology for efficient, sustainable, and simultaneous treatment of soil/sediment and groundwater at environmental hotspots. Clay-substituted geopolymers, smectite, and zeolites are distinguished by their superb sorption and immobilization capacities for heavy metal ions, while biosorbents can play an important role in the removal of metals, metalloids (e.g., arsenic), and other contaminants. More research on individual removal mechanisms of heavy metals will provide new clues on the development of remediation strategies in advanced scientific and field applications, and on atomic-to-micron scale processes occurring at the solid–liquid interface.