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In this study, porous hydroxyapatite-metakaolin geopolymer (HAP-MK-GP) granules were prepared with a novel method for adsorption of organic pollutants from aqueous solutions, using brilliant green dye as a model compound. The preparation involved combining finely powdered hydroxyapatite, metakaolin, and solid sodium metasilicate in a high-shear granulator. Then, hydrogen peroxide (H2O2) was added at different concentrations (0.0, 5.0, 7.5, or 10.0%) as a combined granulation fluid and blowing agent. In the resulting granules, metakaolin geopolymer bound the powdered hydroxyapatite as granules and contributed to the adsorption capacity. To determine their structural, morphological, and mechanical properties, the granules were characterized with infrared spectroscopy, X-ray fluorescence, scanning electron microscopy, specific surface area and porosity analysis, and compressive strength tests. The granules had a high porosity and promising compressive strength and Young's modulus of 0.8 and 10 MPa, respectively, potentially enabling their use in large-scale adsorption columns. The monolayer adsorption mechanism of brilliant green dye onto HAP-MK-GP granules was confirmed by fitting the obtained batch adsorption data to the Langmuir isotherm model which showed that the adsorption capacity was approximately 41 mg/g. The optimum conditions were: pH of 7, initial dye concentration of 100 mg/L, adsorbent dose of 3 g/L, agitation speed of 250 rpm, contact time of 30 min, and temperature of 298 K. The adsorption kinetic data indicated that the adsorption followed the pseudo-second-order kinetic model. The thermodynamic parameters, including Gibbs free energy, enthalpy, entropy, and activation energy, indicated that adsorption was spontaneous, endothermic, and was based on physisorption. The rate-limiting step was film diffusion. Based on the results, HAP-MK-GP granules could be a feasible adsorbent for the removal of dyes from industrial wastewater. Moreover, the facile granulation method presented here could be applied to other powdered adsorbents as well to prepare adsorbent granules.

High-value applications, such as adsorbents, have drawn attention to geopolymers. In several of those applications, having the geopolymer as porous spherical particles is beneficial. This study presents a novel process for fabricating porous metakaolin-based geopolymer granules using a combination of direct foaming, one-part alkali activation, and granulation. In short, the precursor (e.g., metakaolin) and solid activator (e.g., sodium silicate) are loaded in a granulator, in which an aqueous blowing agent (e.g., H2O2) is added while the granulator is running, and the obtained granules are cured at 60 °C. Characterization of the granules for physico-chemical and morphological properties indicated an increase in overall porosity, especially in the µm-scale pores. Also specific surface area (+50%) and nanoscale pore volume (+102%) increased when using more concentrated H2O2 (20 or 30%) compared to nonporous granules. The use of porous granules was also demonstrated in dynamic adsorption experiments for ammonium (NH4+) uptake, which showed up to ∼126% increase in cumulative adsorption amount compared to nonporous granules. The highest NH4+ uptake was obtained with 10% H2O2 solution as the granulation fluid. The results confirmed the feasibility of the method for introducing porosity to geopolymer granules, which enhances the adsorption properties of the granules.

Alkali-activated materials (AAMs) are actively studied as adsorbents in wastewater treatment. They are frequently classified as low-, medium-, or high-calcium content since the gel nanostructure of AAMs changes upon the introduction of network-modifying cations (e.g., Ca2+). Furthermore, AAMs undergo changes in the phase composition and pore structure upon aging. In the present study, these two aspects were assessed for the first time in the context of adsorption: three typical mix designs involving metakaolin, blast furnace slag, or their mixture were prepared and studied for ammonium (NH4+) uptake at 7-, 28-, or 90-day age. The adsorption capacity of the “zeolite-like” metakaolin-based geopolymer increased from 34 to 87 mg/g when material was cured up to 90 days at room temperature. However, the adsorption capacity increase appears to be also depending on the metakaolin structure as approx. 1-month and 8-year old samples prepared with another metakaolin grade indicated almost similar adsorption capacity. Adsorption was clearly hindered when blast furnace slag was introduced in the mix designs, which turns the gel into “tobermorite-like” chain structures. Thus, this study demonstrated that using the Ca-content of precursors and material age are significant parameters for the interpretation of the adsorption results and practical development of AAM-based adsorbents.

This study investigated the effect of organic resin contained in glass wool on synthesis of alkali-activated binders. The study was performed on glass wool containing sugar or phenolic resin, comparing it with glass wool that did not contain resin, as a reference. The results showed that the organic resin could be qualitatively identified using Fourier-transform infrared spectroscopy (FTIR) and thermo gravimetry-mass spectrometer (TG-MS), with gradual decomposition occurring between 200 °C and 550 °C. The presence of organic resin reduced the milling efficiency of glass wool, modified the rheology by increasing the liquid demand, and slowed the strength development at room temperature. However, interestingly, the effect of the resin on the strength of the paste was less obvious at an age of 28 days. Curing for 24 h at 40 °C was beneficial for one-day strength development, in comparison to 20 °C and 60 °C, independent of the presence of the resin. All the cured paste samples, with and without resin, achieved a compressive strength of more than 40 MPa at 28 days, satisfying the requirement for many structural applications. Nevertheless, water immersion affected the materials’ strength, suggesting their suitability for dry environments or the need for suitable co-binders to increase their durability and water resistance.

Porous ceramics were generated by direct foaming the alkali-activated unprocessed kaolin (impure kaolin, iK) with the addition of hydrogen peroxide (H2O2) and calcining at a low temperature of 400 ​°C. Hydrogen peroxide, a blowing agent, decomposes producing oxygen gas bubbles forming the porous structure in the fresh alkali-activated iK paste. In the present study, three different molarity of alkali activator (5, 10, or 15 ​M NaOH) were employed. The dosage of blowing agent (H2O2) was varied between 0 and 2% with a constant dosage of 0.5% (wt. of binder) of surfactant. The gas bubbles in fresh-state geopolymer slurry are unstable with a tendency to coalesce. In such cases, surfactants are introduced to improve the stability of the gas-liquid interface. Two different surfactants: non-ionic and cationic ones were used to study the properties of alkali-activated porous material. Fresh properties such as viscosity, yield stress, foaming rate, and fresh density were compared between reference iK samples without surfactants and those with surfactants. The mechanical and microstructural properties of alkali-activated (AA) low-temperature porous iK ceramics were determined after calcination. Pore structures were characterized with electron microscope to understand the interaction of different parameters during fresh/hardened state and their relevance with the changes in the mechanical properties. Surfactants, irrespective of the type, highly influenced the fresh properties of the AA-iK samples, which in turn reflected on the compressive strength of the porous ceramics.

Geopolymers have been recently explored as sorbents for wastewater treatment, thanks to their mechanical and chemical stability and to their low-energy manufacturing process. One specific application could be the removal of ammonium (NH4+) through exchange with Na+ ions. Additive manufacturing (AM) represents an especially interesting option for fabrication, as it allows to tailor the size, distribution, shape, and interconnectivity of pores, and therefore the access to charge-bearing sites. The present study provides a proof of concept for NH4+ removal from wastewater using porous geopolymer components fabricated via direct ink writing (DIW) AM approach. A metakaolin-based ink was employed for the fabrication of a log-pile structure with 45° rotation between layers, producing continuous yet tortuous macropores which are responsible for the high permeability of the sorbents. The ink consolidates in an amorphous, mesoporous network, with the mesopores acting as preferential sites for ion exchange. The printed sorbents were characterized for their physicochemical and mechanical properties and the NH4+ removal capacity in continuous-flow column experiments by using a model effluent. The lattices present high permeability and high cation exchange capacity and maintained a high amount of active ions after four cycles, allowing to reuse them multiple times.

Worldwide, tons of lignin is produced annually in pulping plants and it is mainly considered as a waste material. Usually lignin is burned to produce energy for the pulping reactors. The production of value-added materials from renewable materials like lignin, has proved to be challenging. In this study, the effects of addition of three different types of lignin in the production of tannin/furanic foams is investigated. The foams were matured, first at 373 K and finally carbonized at 1073 K and the properties of them including mechanical strength, specific surface area and pore development are investigated before and after thermal treatment. According to the results, higher mechanical strength is obtained if samples are carbonized at 1073K compared to matured ones at 373K. Up to 10 times stronger materials are achieved this way, which makes them promising as insulating or constructive materials. With physical activation, it is possible to obtain specific surface areas and pore volumes close to 1200 m2/g and 0,55 cm3/g respectively. Mainly micropores are developed during the steam activation which makes these foams more suitable and selective to be used as catalyst support materials in the catalytic conversion of small molecules or in adsorption or gas storage application.