۱- Introduction

۲- Chemistry of geopolymers

۳- Method and analysis section

۴- Synthesis of geopolymers and mechanisms involved

۵- Use of industrial waste as filler for the fabrication of crystalline metakaolin-based geopolymer

۶- Result and discussion on applications driven by the surface chemistry of geopolymers in water

۷- Barriers to utilization

۸- Future research and prospects

۹- Conclusion

References

Abstract

Geopolymer is a ceramic material, most often amorphous; finds applications in fire- and heat-resistant coatings and adhesives, medicines, refractory ceramics and binders, and manufacturing of radioactive waste container. Over the last decade, new cement based on geopolymers has been developed. Most relevant to this review is the fact that its porous nature and chemical similarity to zeolites is being employed for applications pertaining to wastewater treatment. Most of the work has been on the adsorptive treatment of water. However, using chemical reasoning and literature available we show that geopolymers have relevance for further activity on additional areas of relevance to waste water treatment such as photocatalysis, disinfection, and H2-energy production from waste water etc. These applications would depend strongly on the properties of geopolymers, which in turn would rely on the precursors employed and the synthetic methods used. The relevance of geopolymers for cleaner production is also highlighted. The use of fly ashes and metakaolin composites for the fabrication and surface tailoring of geopolymers (perhaps using relevant surfactants) is suggested as a plausible step in the right direction. Given the critical analysis of the state of the art, and the plausible directions identified, this article will benefit environmental scientists, engineers and chemists interested in deploying geopolymers for environmental remediation purposes. However moving forward, barriers are to be anticipated for the large scale implementation of geopolymers. Several barriers (e.g. legal, economical, technocrats and synthetic challenges) that are likely to hinder future research and translation are highlighted.

Introduction

Heavy metal removal and organic molecule degradation from wastewater have become a major topic of the public interest, due to their relevance to health and environment. Heavy metal and hazardous organic molecules are known to be toxic and a precursor to ill-health of the populace (Jaishankar et al., 2014). They are primarily released into the environment through human/industrial activities (Azimi et al., 2017). Hence research on multifunctional materials which relates to simultaneous environmental remediation and energy production are relevant to contemporary technologies connected with cleaner production. Geopolymers are a promising category of materials, for the removal of toxic substances from industrial and household effluents. It is usually prepared from a rather simple and eco-friendly reaction between an alkali such as NaOH/Na2SiO3 or KOH; and Al and Si source (s), making it a material amenable to clean production. Geopolymers obtained thus are often applied for the removal of metal ions such as Cs+ , Pb2+, Cd2+ etc. from wastewater. Geopolymers have come a long way, since their first discovery four decades ago (Davidovits, 1976). It has gained attention primarily because of the ease with which it can be synthesized with little or zero emission of green house gases (CO2. SO2, NOx etc.) (Noor ul et al., 2016). This coupled with its properties which include toughness, fire and heat resistance (i.e. its refractory nature), radiation hardness (making it relevant for radioactive waste containment) and pozzolanic action (Carsten, 2013), make geopolymers truly multifunctional.