Hybrid Geopolymeric Foams for the Removal of Metallic Ions from Aqueous Waste Solutions

Abstract

For the first time, hybrid organic–inorganic geopolymeric foams were successfully used as monolithic adsorbents for the removal of metallic ions pollutants from wastewaters. The foams were realized by the in situ foaming of a hybrid geopolymer obtained by a reaction of metakaolin and polysiloxane oligomers under strong alkaline conditions and then cured at room temperature. In this way, porous materials with densities ranging from 0.4 to 0.7 g/cm3 and showing good mechanical properties were produced. With the aim of producing self-standing monolithic adsorbents for the removal of metallic ions pollutants from wastewaters, these porous hybrid geopolymers were subjected to a washing pretreatment with ultrapure water, dried, and then used for absorption tests by dipping them into an aqueous solution with an initial concentration of 20 ppm of Pb2+, Cd2+, Cu2+, and Zn2+ ions. Preliminary results indicated that all the tested materials are effective in the adsorption of the tested metal ions and do not release the removed metal ions upon sinking in ultrapure water, even for a very long time. Interestingly, compressive strength tests performed before and after the washing treatments show that the foamed samples remain intact and maintain their physical–mechanical characteristics, suggesting that these kinds of materials are promising candidates for the production of self-standing, monolithic adsorbent substrates that can be easily collected when exhausted, which is a major advantage in comparison with the use of powdered adsorbents. Moreover, since these materials can be obtained by a simple and versatile experimental procedure, they could be easily shaped or directly foamed into precast molds to be used in packed beds as membranes.

Geopolymer Foams—Will They Ever Become a Viable Alternative to Popular Insulation Materials?—A Critical Opinion

“…However, as can be seen from numerous scientific studies, this is a challenging task due to the fact that foamed geopolymers are highly unstable even before curing. Beyond the standard possibilities of using foamed geopolymers as thermal insulation in construction [9,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41], they can be applied as hybrid organic-inorganic foams for the removal of metal ion contaminants from wastewater [24], making foam filters with the addition of lignocellulose from sawdust, and for the filtration of dyes and wastewater [42].…”

Red Mud-Blast Furnace Slag-Based Alkali-Activated Materials

“…Based on the wide authors' experience on the preparation of alkali-activated mixes [26,27,29,32,33], a procedure was chosen which consisted in adding small quantities of RM powder to the alkaline activated solution under constant mixing. After the addition of RM, BFS powder was slowly added under 500 rpm frequency stirring.…”

Sustainable Materials and their Contribution to the Sustainable Development Goals (SDGs): A Critical Review Based on an Italian Example

“…In this context, alginate-based smart sponges play a key role and open exciting opportunities for future applications [ 47 ]. Additionally, porous materials were synthesized to obtain membranes and foams [ 48 ], with good results as an adsorbent to remove heavy metals. These materials are flexible and can be easily shaped or foamed into precast molds.…”