Alkali-activation (or geopolymer) technology has gained a great deal of interest for its potential applications in water and wastewater treatment during the last decade. Alkali-activated materials can be prepared via a relatively simple and low-energy process, most commonly by treating aluminosilicate precursors with concentrated alkali hydroxide and/or silicate solutions at (near) ambient conditions. The resulting materials are, in general, amorphous, have good physical and chemical stability, ion-exchange properties, and a porous structure. Several of the precursors are industrial by-products or other readily available low-cost materials, which further enhances the environmental and economic feasibility. The application areas of alkali-activated materials in water and wastewater treatment are adsorbents/ion-exchangers, photocatalysts, high-pressure membranes, filter media, anti-microbial materials, pH buffers, carrier media in bioreactors, and solidification/stabilization of water treatment residues. The purpose of this review is to present a comprehensive evaluation of the rapidly growing prospects of alkali-activation technology in water and wastewater treatment.

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Catalytic wet air oxidation of an aqueous solution, bisphenol A (BPA), was carried out at 160°C and at 20 bar of air in a batch reactor. Silver catalysts (2.5 wt%) prepared by wet impregnation and complexation on commercial CeO 2 in addition to Ce 0.85 Zr 0.15 O 2 and Ce 0.2 Zr 0.8 O 2 sol-gel mixed oxides were synthesized and used as catalysts in the reaction. Characterizations of the catalysts were performed by using FESEM, XRD, BET, XPS and ICP-OES techniques. Residual BPA concentration was analyzed by using an UV-Vis technique and organic compound content was measured via the total organic carbon method. Commercial CeO 2 showed a smaller specific surface area and a larger crystallite size than laboratory prepared Ce-Zr mixed oxides. The highest BPA removal (76 %) was achieved after 3 h with CeO 2 , Ce 0.85 Zr 0.15 O 2 and Ag/Ce 0.85 Zr 0.15 O 2 catalysts revealing that the addition of silver had no effect on the catalytic activity of the pure supports. However, the loading of active metal to the supports by complexation decreased the adsorption of the BPA during the heating period and hence the Ag/Ce 0.85 Zr 0.15 O 2 prepared via complexation was the most active catalyst with only 1 % adsorption of BPA. Moreover, the activity of the catalysts was not related to the surface area of the samples. According to the ICP-OES analysis of the terminal water samples, leaching of the silver was occurred during oxidation experiments explaining the behavior of Ag catalysts in the reaction.

In this study, novel and cost-effective alkali-activated materials (AAMs) for catalytic applications were developed by using an industrial side stream, i.e., blast furnace slag (BFS). AAMs can be prepared from aluminosilicate precursors under mild conditions (room temperature using non-hazardous chemicals). AAMs were synthesized by mixing BFS and a 50 wt % sodium hydroxide (NaOH) solution at different BFS/NaOH ratios. The pastes were poured into molds, followed by consolidation at 20 or 60 °C. As the active metal, Fe was impregnated into the prepared AAMs by ion exchange. The prepared materials were examined as catalysts for the catalytic wet peroxide oxidation (CWPO) of a bisphenol A (BPA) aqueous solution. As-prepared AAMs exhibited a moderate surface area and mesoporous structure, and they exhibited moderate activity for the CWPO of BPA, while the iron ion-exchanged, BFS-based catalyst (Fe/BFS30-60) exhibited the maximum removal of BPA (50%) during 3 h of oxidation at pH 3.5 at 70 °C. Therefore, these new, inexpensive, AAM-based catalysts could be interesting alternatives for catalytic wastewater treatment applications.

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The wet granulation process was used to prepare new, efficient, and cost-effective granular biomass-based composite catalysts for catalytic wet peroxide oxidation (CWPO) of bisphenol A (BPA). The most stable composite granules was prepared by mixing biomass-based carbon residue (CR) with metakaolin (MK) combined with calcium oxide (CaO) or cement and a solvent (NaOH or KOH). For all the prepared composite granules, the optimized binding agents to carbon ratio was 0.3, the solvent to carbon ratio 1.2, and the agitation rate 1200 rpm. The specific surface area of the prepared catalysts was 152-205 m 2 /g. The composite granular catalyst (CR+MK+CaO+NaOH) had the most durable and stable structure (compressive strength of 27 N) and the most basic surface (15 mmol/g) measured with temperature programmed desorption. This catalyst was the most active in CWPO of BPA and total organic carbon removal of 50% and 48%, respectively.

Cerium-zirconia-supported ruthenium and activated carbon catalysts were used in catalytic wet air (CWAO) and wet peroxide (CWPO) oxidation of industrial wastewater. Both catalysts were active in the removal of TOC and COD from the wastewater. The degree of biodegradation of organic matter increased during CWAO. Therefore, the CWAO process could be considered as a potential pre-treatment process integrated with subsequent biological treatment to achieve the required level of purification of wastewater.