WO2023218124A1

WO2023218124A1 - A method for preparing porous geopolymer granules, porous geopolymer granules and use thereof

Info

Publication number: WO2023218124A1
Application number: PCT/FI2023/050247
Authority: WO
Inventors: Tero Luukkonen; Yangmei YU
Original assignee: Oulu University of
Current assignee: Oulu University of
Priority date: 2022-05-09
Filing date: 2023-05-08
Publication date: 2023-11-16
Anticipated expiration: 2024-11-09
Also published as: EP4522578A1

Abstract

The present application provides a method for preparing porous geopolymer granules, the method comprising providing geopolymer precursor material, providing solid alkali activator, combining the geopolymer precursor material and the solid alkali activator to obtain a mixture, optionally milling the mixture to obtain a milled mixture, granulating the mixture or optionally the milled mixture by adding blowing agent solution, such as hydrogen peroxide solution, during granulating to obtain granules, and curing the granules at ambient or elevated temperature to obtain porous geopolymer granules. The present application also provides porous geopolymer granules, material comprising the porous geopolymer granules and use of the porous geopolymer granules.

Description

A method for preparing porous geopolymer granules, porous geopolymer granules and use thereof

Field of the application

The present disclosure relates to method for preparing porous geopolymer granules, and to granules obtained with the methods. The present application also provides use of the granules in a variety of end applications.

Background

According to some definitions geopolymers are a class of inorganic, aluminosilicate based ceramics or ceramic-lime material that are charge-balanced by group I cations. They form long-range, covalently bonded, non-crystalline (amorphous) networks. They are rigid gels, which are cured under ambient or near-ambient conditions of temperature below 100°C and ambient pressure, and which can subsequently be converted to crystalline zeolitic materials. Geopolymers are framework structures produced by polycondensation of tetrahedral aluminate or silicate units. Conventionally, geopolymers are synthesized from a so-called two-part mix, consisting of an alkaline solution (often soluble alkali silicate) and solid aluminosilicate precursors. Dissolution of solid aluminosilicate raw materials in alkaline solutions leads to the formation of dissolved aluminate and silicate species, which form precipitation nuclei, polycondensate, and grow into a new aluminosilicate gel phase to form a solid binder. One important property of geopolymers is their cation-exchange capacity, which is utilized in several of the applications.

In several of their applications, it is desired to obtain porous granules from geopolymers. The existing literature on the topic has utilized the granulationgeopolymerization process without any particular method to improve or control the porosity. In that case, the resulting granules have low porosity. Another method presented in the literature is to mix aluminosilicate precursor, alkali activator solution, and blowing agent component to make paste and that paste is added dropwise to hot oil bath to generate granules. Such a method may be called as a suspension-solidification method. The oil used in this method needs to be washed or burned away, and still remains of the oil may be left in the granules. Therefore, the method is laborious, it is not possible to generate large number of granules quickly, and it is not easily up-scalable. Yet another method is to prepare a block of highly porous geopolymer and crush it into milli- or centimeter-sized particles and use those, but it is also time consuming and not convenient to up-scale.

Still yet another method is an alkali-activating sintering, which uses very high temperatures of 800-1500°C, which requires specific equipment, consumes energy and is hazardous.

There is a need to find improved methods for preparing porous geopolymer granules, which are simple, economical, safe, controllable, up-scalable and do not require complex or expensive equipment and/or extreme conditions. There is also a need to obtain porous geopolymer granules with controlled size, shape and structure. It is desired to avoid the drawbacks of the prior art methods.

Summary

In the present invention it was found out how to prepare geopolymer granules with an adjustable level of porosity in a combined granulation-direct foaming-alkali- activation process. Drawbacks of the prior art could be avoided. The invention may involve a high-shear granulator, aluminosilicate precursor(s), alkali activator component(s), and blowing agent component(s). Compared to prior art methods the present method is much simpler, faster, safer and can be carried out with simple and inexpensive equipment and up-scaled easily. It is possible to prepare granules in industrial scale processes. High temperatures or other extreme conditions can be avoided.

Geopolymer granules have many applications as artificial aggregates for construction, thermal insulation, acoustic panels; ion-exchangers for wastewater treatment; catalyst supports and so on. Many of the applications would require that the granules are porous.

The present disclosure provides a method for preparing porous geopolymer granules, the method comprising

-providing geopolymer precursor material,

-providing solid alkali activator,

-combining the geopolymer precursor material and the solid alkali activator to obtain a mixture,

-optionally milling the mixture to obtain a milled mixture, -granulating the mixture or optionally the milled mixture by adding blowing agent solution during granulating to obtain granules, and

-curing the granules at ambient or elevated temperature to obtain porous geopolymer granules.

The present disclosure also provides porous geopolymer granules, which may be obtained with the method disclosed herein.

The present disclosure also provides materials comprising the porous geopolymer granules.

The present disclosure also provides use of the porous geopolymer granules for a variety of applications and end uses.

The present disclosure also provides a method for removing substances from wastewater, such as municipal wastewater, the method comprising -providing the porous geopolymer granules, -providing wastewater,

-contacting the wastewater with the porous geopolymer granules to remove the substances from the wastewater

The main embodiments are characterized in the independent claims. Various embodiments are disclosed in the dependent claims. The embodiments and examples recited in the claims and the specification are mutually freely combinable unless otherwise explicitly stated.

Brief description of the figures

Figure 1 shows a photograph of non-porous granules obtain with a prior art method

Figure 2 shows a photograph of porous granules obtained with the present method

Figure 3 shows a graph illustrating the effect of 25 cycles of adsorption/desorption and regeneration to the absorption capacity of the granules

Detailed description In this specification, percentage values, unless specifically indicated otherwise, are based on weight (w/w). If any numerical ranges are provided, the ranges include also the upper and lower values. The open term “comprise” also includes a closed term “consisting of’ as one option. The diameters disclosed herein, unless specifically indicated otherwise, may refer to the smallest diameter, and may be presented as average or number-average diameter. The diameter may be also presented as equivalent spherical diameter. The diameter may be determined microscopically or by other optical methods, which may comprise using a camera and a dedicated software to analyse the results, and/or by sieve analysis.

The present disclosure provides a method for preparing porous geopolymer granules, the method comprising -providing geopolymer precursor material, -providing solid alkali activator, and

-combining the geopolymer precursor material and the solid alkali activator to obtain a mixture.

In one embodiment the geopolymer precursor material comprises aluminosilicate, such as metakaolin. Metakaolin is the dehydroxylated calcined form of the clay mineral kaolinite.

In one embodiment the solid alkali activator comprises solid sodium silicate, such as solid sodium metasilicate, or solid sodium hydroxide. The solid sodium silicate may have a SiO2:Na2O ratio in the range of 0.8-2.5, such as in the range of 0.8- 2.0, for example about 1 .

As the ingredients are provided in dry and solid form, the obtained mixture is dry and solid. The ingredients may be mixed with a mixer, and/or mixing may be obtained in milling, grinding or other disintegration step.

The method may comprise disintegrating, such as milling or grinding, the mixture to obtain a disintegrated or milled mixture. The grain size may be adjusted in this step, for example by selecting a suitable method, device, processing time, processing conditions, processing energy and/or the like features. Examples of suitable disintegrating devices include a ring grinder and a disc mill. The milling, grinding or other disintegrating may be carried out for 30 seconds - 10 minutes, such as 1-5 minutes. The obtained grain size, which may be an average particle diameter or number-average particle diameter, may be 50 pm or less, such as in the range of 0.1-50 pm, such as 1-50 pm.

In a next step the mixture in a milled, grinded or disintegrated form is formed into granules of desired form, such as into a form having a desired size, porosity, density, weight and/or hardness, and/or other property disclosed herein. A suitable granulator may be used, such as a high-shear granulator.

The method may comprise providing and/or adding solution comprising gasforming agent, i.e. a blowing agent, such as an agent forming gas at alkaline or high pH, such as at a pH of 10 or more, preferably 11 or more. The blowing agent may comprise or be for example peroxide, such as inorganic and/or organic peroxide, for example hydrogen peroxide and/or peracetic acid, sodium hypochlorite and/or ammonium. The gas-forming agent is preferably provided as solubilized, i.e. it is not solid, and it is preferably not provided as a dispersion.

The method comprises granulating the disintegrated or milled mixture to obtain granules, preferably by adding peroxide solution or other blowing agent solution during granulating. This may be carried out in the granulator. The blowing agent, such as the peroxide, is added in a form of an aqueous solution, and it acts as blowing agent in a later heating step. The peroxide may be hydrogen peroxide, but other peroxides may be used, such as peracetic acid. The peroxide solution also provides the required water to dissolve the alkali-activator component and initiate the geopolymerization.

One embodiment provides a method for preparing porous geopolymer granules, the method comprising -providing a granulator,

-providing geopolymer precursor material,

-providing solid alkali activator,

-providing blowing agent solution, such as peroxide solution,

-optionally milling the mixture to obtain a milled mixture,

-adding the mixture or optionally the milled mixture and the blowing agent solution to the granulator, such as adding the blowing agent solution during granulating, and granulating to obtain granules, and -curing the granules at ambient or elevated temperature to obtain porous geopolymer granules.

The method may be a method for preparing spherical and/or substantially spherical porous geopolymer granules. The method may also be a method for preparing the porous geopolymer granules by granulating.

Providing the blowing agent in a form of a solution was found advantageous as it provides very homogeneous distribution of the blowing agent through the forming granules. Further, no residues of blowing agent are left in the final product, especially when hydrogen peroxide is used. These are major advantages compared to methods wherein the blowing agent is added in a solid form, wherein for example silicon or aluminium residues from the blowing agent usually remain in the final products. Using a solution also facilitates controlling and speeding up the process. For example the solution can spread evenly and fast into the material, and the water of the solution dissolves the solids, such as metasilicate or sodium hydroxide, which subsequently react with aluminosilicate precursor to bind the material together to form the granules. It is possible to bind the separate granules together, in this step or later, to form products comprising a plurality of interconnected granules, such as sheets, blocks and the like structures.

The blowing agent solution, such as the peroxide solution, may be added dropwise, such as during the granulation or substantially during the granulation, and part of it may be added before and/or after granulation. The solution of peroxide, such as a solution of hydrogen peroxide, may have a peroxide concentration of for example 1-50% by weight, such as 5-50% by weight or 5- 40% by weight. In one embodiment the concentration of the peroxide in the solution is in the range of 10-30% by weight, such as 10-20% by weight, 15-30% by weight or 20-30% by weight. The peroxide solution is an aqueous solution. The granulation and/or adding the peroxide solution may be carried out for a time period suitable to obtain desired porosity, granulation and/or distribution of the peroxide in the mixture, such as for 10-60 minutes, for example 10-30 minutes, such as about 20 minutes. The examples relating to peroxide solution disclosed herein can be applied to other blowing agent solutions. For example similar amounts and reaction times can be used for other blowing agents as well, as the rection mechanism is similar. Finally the granules are cured. This may be carried out at ambient temperature or in a heat treatment, for example in an oven or the like heat-controlled device or at similar conditions, which enable providing and controlling elevated temperature. During curing, it is also important to protect granules from excessive evaporation of water. The curing may be carried out in a closed container or device, and/or at conditions enabling controlling and/or preventing the evaporation of water.

The method comprises curing the granules, preferably at elevated temperature, to obtain porous geopolymer granules. The granules will also dry during curing. The elevated temperature may be 30-100°C, 40-80°C, or 55-70°C, such as 55-65°C. In most cases about 60°C is preferred. During the drying, curing and/or heating the blowing agent, such as the peroxide, which has entered into the mixture, decomposes because of high pH and the elevated temperature thus releasing oxygen gas bubbles inside the granules causing expansion of granules due to the gas bubble formation and forming the pore structure. The granules also harden during the heat treatment. This process may be called a direct foaming process or approach.

The temperatures used in the present method are ambient or moderate, and temperatures above 100°C are not required, which enables carrying out the method with a variety of equipment, saves energy and increases the safety of the method. It is thus possible to avoid very high temperatures, such as temperatures used in processes involving burning oil or sintering, which may be hundreds of degrees Centigrade and which would require specific heating means, such as a kiln. The present temperatures may be achieved with common ovens and the like unexpensive equipment.

The curing is carried out for a suitable time to obtain a plurality of granules with desired dry matter content and/or other properties, such as desired hardness and/or porosity. The curing may be carried out for example for 15 or more, such as 60 minutes or more, 120 minutes or more, or 240 minutes of more. In one embodiment the curing is carried out for 15-1440 minutes, such as 15-240 minutes, 60-240 minutes or 120-240 minutes.

After curing the granules, or a product formed from granules, are/is recovered. The granules or the product may be used, stored, packed and/or further processed immediately, or they may be stored for a period of time before such steps, such as 1-48 hours, or at least for 12-48 hours or more, for example at least for about 24 hours, and/or at room or ambient temperature, which may be approximately 22°C, to obtain granules with final properties. The granules or product comprising granules may be stored in a sealed package or container or at otherwise isolated and/or air-proof and/or water-proof conditions, for example in plastic bags or other containers, to obtain prolonged shelf-life. The granules or the product, especially when packed in sealed packages, can be stored, handled, transported and/or provided for desired end uses at a desired location, and the granules and products tolerate well such conditions and handling.

Before using the granules for certain applications, their pore solution may be neutralized with mild acid solution. After that, the granules do not increase pH if they are submerged in water. The acid treatment can be carried out with weak or strong acid, such as acetic acid, nitric acid, hydrochloric acid, or sulfuric acid. Acetic acid is preferred. The concentration of acid, such as the acetic acid, may be in the range 0.01-5 M, such as 0.05-0.5 M or about 0.1 M. In one example the acid is acetic acid having a concentration of about 0.1 M. The treatment may be conducted by submerging the granules in the acid solution or by placing the granules in a column to form a granule bed in the column, and pumping acid solution through the granule bed, or using any other suitable method for contacting the granules with the acid.

In one embodiment the method comprises neutralizing the obtained granules, i.e. cured granules, with an acid solution, such as acid selected from acetic acid, nitric acid, hydrochloric acid and sulfuric acid, preferable with acid concentration in the range of 0.01-5 M.

The granules may be fractionated, such as sieved, to obtain one or more fractions with a desired average or range of particle (granule) size(s), such as an average diameter, which may be number-average diameter, and/or to obtain one or more fractions with a desired granule weight, density, porosity and/or other property, which may have an impact to suitability of the granules to a specific end use. The average particle diameter of the granules may be determined as a volume median particle size or diameter. Particle sizes or diameters of non-spherical granules can be determined or presented as equivalent spherical diameter (ESD). The equivalent spherical diameter of an irregularly shaped object is the diameter of a sphere of equivalent volume. The average diameter may be for example in the range of 1-5 mm, such as 1-4 mm, 1-3 mm, 2-4 mm, or 3-5 mm. Preferably majority of the granules have a diameter in the disclosed range. The majority may refer to at least 50%, to at least 60%, to at least 70%, to at least 80%, to at least 90%, or to at least 95%, which may be determined by volume or by number. Regarding the shape, majority, or preferably all or substantially all of the granules have the spherical shape, for example at least 80%, at least 90% or at least 95% have the spherical shape.

The present granules may be specified as having a very high sphericity and roundness, preferably close to 1 each, such as 0.80 or more, for example 0.90 or more. Sphericity is a measure of how closely the shape of an object resembles that of a perfect sphere. Roundness is the measure of how closely the shape of an object approaches that of a mathematically perfect circle. The very high sphericity and roundness indicate the preparation method, i.e. they are a result of granulation process, as it is not possible to obtain spherical and round granules by crushing, milling or casting. Crushing may comprise methods including breaking the structure of material to obtain smaller particles or blocks, such as milling and the like procedures. Such methods yield very small particles, even powder, which may have an average diameter of less than 1000 micrometers, less than 500 micrometers or less than 100 micrometers, and which may have an uneven shape.

It was found out that when the aluminosilicate precursor, especially metakaolin, was milled with the alkali activator, the reactivity of both could be improved when water/peroxide is added. In one specific example aluminosilicate precursor, such as metakaolin, is first milled with alkali activator (solid sodium silicate with SiO2/Na2O of approximately 1.0 or solid sodium hydroxide) using a ring grinder or similar mill for a few minutes. The mixture is placed inside a high shear granulator, which is turned on. Hydrogen peroxide solution with concentration of 30%, 20%, or 10% by weight, for example, is added dropwise when the granulator is running. Water in the hydrogen peroxide solution dissolves the metasilicate or sodium hydroxide, which subsequently reacts with aluminosilicate precursor to bind the granules together. The formed granules are placed in oven at 60°C, where hydrogen peroxide present inside the granules starts to decompose due to the high pH and elevated temperature. Hydrogen peroxide decomposes into oxygen gas bubbles inside the granules, which form the pore structure. Hardening of the granules occurs within 15 min - 4 h. After that, they can be removed from the oven and stored at room temperature. They are ready to use after approximately 1 day. The present disclosure provides porous geopolymer granules and products comprising thereof obtained with the method disclosed herein. Such porous geopolymer granules are free or substantially free of residues originating from blowing agent, and/or substances such as oil and/or residues originating from oil. For example hydrogen peroxide decomposes into oxygen and water, which are released in the heat treatment. The obtained porous geopolymer granules may exhibit different porosity and size of the granules, as these features may be adjusted in the process, such as during granulation and heat treatment. The composition of the granules may be affected by the choice of raw materials and amounts and/or ratios thereof as well as controlling the preparation steps. Further, the granules have an even shape, i.e. they are substantially spherical which distinguishes them e.g. from granules obtained by crushing or casting. The present granules, which are obtained by granulating, are therefore uncrushed and/or uncasted.

The porous geopolymer granules may be characterized with one or more further properties disclosed herein, or other properties. The properties may be determined by using standard methods, such as by microscopic methods, for example electron microscopy methods such as SEM, EDX, TGA, XRD, XRF, and the like commonly used for characterizing inorganic granules.

The porous geopolymer granules may have an average diameter in the range of 1-10 mm, such as 1-4 mm or 2-4 mm. The average diameter may be also determined with sieves in a mesh scale, for example in the range of 18-7/16.

Porosity of the granules may be defined as the ratio of the volume of the voids or pore space divided by the total volume, and expressed as a percentage. The porous geopolymer granules may have a porosity in the range of 30-90%, such as in the range of 30-60%, 40-70%, 40-80%, 50-80%, or 50-90%. Pore size and pore volumes of the samples can be measured using a N2 gas adsorptiondesorption isotherms, pycnometer, microscope combined with image analysis, or X-ray microtomography, preferably by using a dedicated instrument such as a porosimeter.

The porous geopolymer granules may have a cumulative NH4⁺ adsorption capacity, as determined in dynamic flow-through column experiments, in the range of 10-100 mg/g, 10-50 mg/g, 10-40 mg/g, 10-30 mg/g or 10-20 mg/g, such as 10-16 mg/g, 11-16 mg/g or 10-15 mg/g. It was noted that the adsorption capacity of the porous granules when tested in this set-up was multifold compared to non- porous granules. The cumulative NH4⁺ adsorption can be determined by cationexchange capacity measurement using NH4⁺ as the cation exchanged to the structure. In one example the measurement was conducted by pumping (at flow rate 0.5 l/h) NH4⁺-containing solution (200 mg N /I, pH = 7) through the granule bed (mass of 20 g in a plastic column with inner height 99 mm, diameter 44 mm, and volume 0.15 I) and measuring NH4⁺ before and after the bed periodically. The experiment was continued for 6 h and the cumulative adsorption capacity was then calculated. The cumulative adsorption capacity is the integral of NH4⁺ concentration vs. duration of the experiment.

In one embodiment the porous geopolymer granules have a cumulative NH4⁺ adsorption in the range of 10-100 mg/g, 10-50 mg/g, 10-40 mg/g, 10-30 mg/g or 10-20 mg/g, such as 10-16 mg/g, 11-16 mg/g or 10-15 mg/g, when determined using a dynamic column experiment after 6 h with an experimental set-up including: 20 g of granules with average diameter in the range of 1-4 mm in plastic column (inner height 99 mm, diameter 44 mm, and volume 0.15 I), 200 mg NH4⁺/I influent concentration, pH of 7, and flowrate of 0.5 l/h.

The porous geopolymer granules may have a specific surface area in the range of 10-300 m²/g, such as 10-200 m²/g, 10-100 m²/g, 15-300 m²/g 15-200 m²/g, 15- 100 m²/g, 20-300 m²/g, 20-200 m²/g, 20-100 m²/g or 15-50 m²/g. In some examples the porous geopolymer granules have a specific surface area in the range of 10-26 m²/g, 15-30 m²/g, 20-30 m²/g or 20-26 m²/g. The specific surface area can be further increased by flushing with water, low concentration acid solution, or by salt solution (such as KNO3). The specific surface area may be determined by liquid nitrogen adsorption method and BET isotherm.

The porous geopolymer granules may have a mechanical strength in the range of 0.8-5.0 MPa, such as 1.0-4.0 MPa. The mechanical strength may be determined by standard ISO/CD 11273-2: Soil quality determination of aggregate stability part 2: Method by shear test (British Society of Soil Science, 2000).

The porous geopolymer granules may have a zeta potential in the range of from -1 to -25 mV measured at pH 7, such as from -3 to -17 mV, for example from -10 to - 25 mV or from -12 to -21 mV. Zeta potential is a factor governing electrostatic interactions. Zeta potential may be determined by using a zeta potential analyzer, which may be based on dynamic light scattering.

The porous geopolymer granules may have an molar ratio of SiO2/Al2Os in the range of 2.50-4.50, such as 2.50-3.50, 2.50-3.00 or 2.55-2.65. This may be determined by XFR spectroscopy or EDX.

The present porous geopolymer granules may be used in a variety of methods including methods for preparing materials, such as materials disclosed herein for different uses, wherein the method may comprise providing the porous geopolymer granules and combining with other materials and/or forming the porous geopolymer granules into products, to obtain said materials or products comprising said materials.

The porous geopolymer granules may also be used in methods of treating gases and/or fluids such as waters, waste waters, effluents, solutions, dispersions, suspensions, colloids, and other fluids, the method comprising providing the porous geopolymer granules and contacting with the gas and/or fluid to treat the gas and/or fluid, for example to bind, remove, recover or otherwise treat substances in the gas and/or fluid. The substances may be for example contaminants, harmful substances or other substances which are to be removed or recovered, such as reaction products. The porous geopolymer granules may act for example as filtration material, absorption material and/or ion exchange material. One example of substances to be removed include ammonium, which may be present in wastewater. The porous geopolymer granules may be packed in a column of the like structure, which may allow a flowthrough of liquid, dispersion and/or gas to be treated.

The present application provides materials and/or products comprising the porous geopolymer granules, such as construction materials, filtration materials, absorption materials, ion exchange materials, support materials, disinfecting materials, nano and/or micro plastics collection materials, active capping materials, wate water treatment materials, pharmaceutical transporting or carrier materials, chromatography materials, oil collecting/recovery materials, soil stabilization materials, fertilizer or other substances carrier materials, hydrogen storage materials, permeable pavement materials, landfill materials, mine tailings or gangue cover or treatment materials, and the like materials, which may further comprise one or more materials, which may comprise a plurality of the granules, and/or which may be formulated into products, such as products having a form and/or a shape such as a sheet, a block, a tile, a plate, a tube, aggregate(s) of granules and the like.

The present application provides use of the porous geopolymer granules in one or more applications disclosed herein, as well as methods for carrying out such applications. The granules may be used as such in the methods or in materials or products used in the methods. The granules may be also modified for a specific use, for example by chemical, physical and/or mechanical modification. The granules may be used and/or they may act in two or more methods or by two or more mechanisms disclosed herein. For example water treatment may include two or more of filtration, ion exchange, pH adjustment, catalysis, capping and the like.

One embodiment provides use of the porous geopolymer granules as lightweight artificial aggregate in concrete.

One embodiment provides use of the porous geopolymer granules in construction materials or elements, such as in building blocks or sheets, for example breeze blocks and the like structures comprising aggregate of particles/granules. This may include similar uses as with general breeze block materials such as LECA (light-weight expanded clay aggregate) material or elements, and the like. The granules may be attached together to form the materials or elements, for example during the preparation or later, for example by adding a suitable adhesive or bonding agent.

One embodiment provides use of the porous geopolymer granules as or in sound absorbing material, such as in acoustic panels.

One embodiment provides use of the porous geopolymer granules as or in filtration material for liquids or gases.

One embodiment provides use of the porous geopolymer granules as or in adsorption material for gases and/or dissolved solids in liquids.

One embodiment provides use of the porous geopolymer granules as or in ionexchange material for dissolved solids in liquids and/or gases, for example to recover one or more substances or ions of interest. An applicable method may comprise providing ion exchange material comprising the porous geopolymer granules, for example packed in a column or other suitable container, and providing liquid and/or gas, such as aqueous liquid to the ion exchange material to bind one or more ions of interest from the liquid and/or gas. For example in methods including aqueous solutions, which may be for example waste waters or other effluents, substances such as ammonium may be removed and/or recovered with the ion exchange material. The ion exchange material may be regenerated multiple times with for example agents such as salt, acid and/or base, for example NaCI and NaOH solutions. Examples of regenerating agents include KNO3, K2SO4, NaCI, NaNO₃, Na₂SO₄, NaOH, 0.1 M NaOH + 0.2M NaCI, CH3COOH, which may be used as one or more solutions having a concentration of 0.3-0.9 M.

Such methods are simple, inexpensive and environmentally friendly as no addition of chemicals is required. The used granules are durable and can be handled without safety issues. The granules can be reused and regenerated for a plurality of times without significantly loosing the desired properties, such as adsorption and/or desorption capacity and regeneration efficiency. It was found out that the granules can be reused for at least 25 cycles of adsorption/desorption of ammonium. During these cycles, the regeneration efficiency, i.e. the proportion of ammonium, which can be desorbed from the adsorbent, and adsorption amount, i.e. adsorbed ammonium in mg per g of adsorbent, both remain relatively constant, such as at approximately 80% and 11 mg/g, respectively as shown in Example 3.

One example provides a method for removing ammonium from wastewater, such as municipal wastewater, the method comprising -providing the porous geopolymer granules, -providing wastewater,

-contacting the wastewater with the porous geopolymer granules, to remove ammonium from the wastewater. The method may further comprise regenerating the porous geopolymer granules and/or removing and/or recovering the ammonium from the porous geopolymer granules. In analogous way a method for removing nitrogen and/or other substances instead of or in addition to the ammonium may be provided. In one embodiment the substances are selected from one or more of ammonium, nitrogen, gases, dissolved solids and ions in liquids, and/or from any other substances disclosed herein.

One embodiment provides use of the porous geopolymer granules as or in biofilm support material for bioreactors. One embodiment provides use of the porous geopolymer granules as or in pH adjustment material in water, wastewater and/or in biogas generation. The granules provide buffering capacity, so the pH adjustment material may refer to buffering material.

One embodiment provides use of the porous geopolymer granules as or in catalyst support material, for example in environmental applications.

One embodiment provides use of the porous geopolymer granules as or in disinfecting material, preferably after modification. The modification may include for example modification with active agents or particles, such as nanoparticles, for example silver nanoparticles or other catalytic (nano)particles. The disinfecting material may be used for example for water disinfection.

One embodiment provides use of the porous geopolymer granules as or in nano/microplastics collector, preferably after modification.

One embodiment provides use of the porous geopolymer granules as or in active capping material in polluted sediment remediation.

One embodiment provides use of the porous geopolymer granules as or in reactive permeable barrier in passive wastewater treatment applications.

One embodiment provides use of the porous geopolymer granules as or in pharmaceutical compound carrier in medicine.

One embodiment provides use of the porous geopolymer granules as or in chromatography media, for example in analytical chemistry.

One embodiment provides use of the porous geopolymer granules as or in oil collector from water, preferably after modification.

One embodiment provides use of the porous geopolymer granules in soil stabilization or earth construction.

One embodiment provides use of the porous geopolymer granules as or in fertilizer carrier material. One embodiment provides use of the porous geopolymer granules as or in hydrogen storage material, preferably after modification.

One embodiment provides use of the porous geopolymer granules as or in permeable pavement material.

One embodiment provides use of the porous geopolymer granules as or in landfill foundation construction.

One embodiment provides use of the porous geopolymer granules as or in mine tailings pond construction.

The present disclosure also provides said materials comprising the porous geopolymer granules.

Examples

Example 1. Production of porous geopolymer granules

Porous geopolymer granules were produced through the granulation- geopolymerization-direct foaming approach disclosed herein. Metakaolin was first mixed with solid sodium metasilicate at Al/Na molar ratio of 1.0 and ground for 1 min using a vibratory disc mill (Retsch RS 200) with a speed of 1500 rpm. The mixture was placed in a high-shear granulator (Eirich EL1 ). Hydrogen peroxide solution (0 [i.e., only water], 10, 20, or 30 weight-%) was dosed dropwise to the mixture during about 20 min while mixing at a speed of 1200 rpm. The dropwise dosage was stopped when agglomeration of granules had occurred by a visual observation. After granulation, the granules were allowed to cure at 60°C in an oven for 4 hours to release gas bubbles from the decomposing hydrogen peroxide. The obtained granules were substantially spherical and very porous, as shown in Figure 2. The granules were then sieved into desired particle size between 2 and 4 mm in diameter, which is suitable for most uses, and placed in an airtight plastic bag at ambient temperature for one day.

Without hydrogen peroxide (i.e., using only water) the obtained granules were practically non-porous and had uneven shape, as shown in Figure 1 . The obtained granules were characterized for different properties, and the results are shown in Table 1 .

Table 1

Example 2. Laboratory-scale column test

Aqueous NH4⁺ solution preparation

The stock solution with NH4⁺ concentration of 1000 mg/l was prepared from reagent-grade ammonium chloride (NH4CI analytical grade), which was diluted to the concentration of working solutions (200 mg/l) by dissolving in deionized water. 0.5 M HCI and 0.5 M NaOH solutions were used for pH adjustment.

Fix-bed column test

The column tests were conducted to investigate the NH4⁺ adsorption capacities of the 0.1 M acetic acid pre-treated granules. The NH4⁺ uptake experiments were carried out in a plastic column having an inner diameter of 4.4 cm, height of 9.9 cm, and a packed bed height of about 5 cm (/.e. 20 g of acetic acid pre-washed geopolymer granules with particle size 2-4 mm). The experiments were conducted at room temperature. A peristaltic pump (MINIPULS 3) was employed to feed the NH₄ ⁺ solution with initial concentration of 200 mg/l NH4⁺ through the column using flow rate of 0.5 l/h corresponding to about 9 minutes empty bed contact time (EBCT). During the continuous experiment, samples were collected from column effluent with 10-min interval for 6-hour duration. The NH4⁺ concentration in samples was measured by using an ammonium selective electrode (Model HQ4100). The adsorbed NH4⁺ at time t, (mg/g), was calculated using the following equation:

where Co (mg/l) is the initial concentration of ammonium in solution before the column; Ct (mgl) is the concentration of ammonium in the solution at time t; V (I) is the volume of the sample; and m (g) is the mass of the adsorbent in the column.

Example 3. Analysis of granule properties

Brunauer-Emmett-Teller (BET) isotherm/ Barrett-Joyner-Halenda (BJH) method

The specific surface area, nanometer-scale pore size and pore volumes of the samples were measured using a N2 gas adsorption-desorption isotherms at the temperature of liquid nitrogen (-196°C) by employing a Micrometrics ASAP 2020 instrument. The specific surface area was calculated based on the Brunauer- Emmett-Teller (BET) equation. Pore size distributions were calculated from the desorption data using the Barrett-Joyner-Halenda (BJH) method.

Mechanical strength

The compressive strength of granules was determined with Zwick Roell Z010 instrument with a 1 mm/min loading rate (10 kN load cell was used). The granules were selected so that their shape was spherical as closely as possible. Compressive strength (o, [MPa]) was calculated using Equations 2 and 3, respectively (F =peak force [N], A =surface area under compression [mm²], and d = diameter of granule [mm]). Equation 3 is based on the standard ISO/CD 11273- 2.

Zeta potential

The surface charge of granules was determined with the streaming potential (Fairbrother-Mastin) method using an Anton Paar SurPASS instrument. Measurement was performed in 0.1 M HCI solution, and the pH of sample solution was adjusted to 7 with 0.1 M NaOH and 0.1 M HCI prior to the analysis.

X-ray fluorescence (XRF)

The elemental composition of the granule samples was obtained using an X-ray fluorescence (XRF) spectrometer (PanAnalytical Minipal 4). The granules were ground to fine powder (< 100 pm) before the analysis was conducted. The loss on ignition (LOI) at 950°C was also determined.

Example 4. Reuse tests

Granules obtained with the method described in Example 1 were used for adsorption/desorption of ammonium for a plurality of cycles, and the granules were regenerated between the cycles.

Long-term adsorption/desorption performance of porous geopolymer granules was evaluated in laboratory-scale experiments in which 20 g of granules were placed in a plastic column and 500 mg/l NH4⁺ solution was pumped through the granules bed using 500 ml/h flow rate at pH of approximately 7. This ensured that the granules were effectively saturated with NH The regeneration of granules was tested with various salt, acid, or base solutions with 0.3 M concentration and their regeneration efficiency was found to be the following: KNO3 > K2SO4 > NaCI > NaNO₃ > Na₂SO₄ > 0.1 M NaOH + 0.2M NaCI > CH3COOH. In terms of concentration, 0.9 M was found as the optimum in the range of 0.3-0.9 M. The regeneration was studied over 25 cycles of adsorption/desorption and approximately 40% decrease in the adsorption amount of NH4⁺ was observed. Figure 3 shows results from first 25 cycles. However, the adsorption amount after 25 cycles (approximately 7 mg/g) was still comparable to natural zeolite granules.

Claims

1. A method for preparing porous geopolymer granules, the method comprising

-providing geopolymer precursor material,

-providing solid alkali activator,

-optionally milling the mixture to obtain a milled mixture,

-granulating the mixture or optionally the milled mixture by adding blowing agent solution during granulating to obtain granules, and

2. The method of claim 1 , wherein the method is a method for preparing spherical or substantially spherical porous geopolymer granules.

3. The method of claim 1 or 2, wherein the method is a method for preparing porous geopolymer granules by granulating.

4. The method of any of the preceding claims, wherein the blowing agent comprises or is peroxide, such as hydrogen peroxide,

5. The method of any of the preceding claims, wherein the blowing agent comprises or is peracetic acid.

6. The method of any of the preceding claims, comprising

-providing a granulator,

-providing geopolymer precursor material,

-providing solid alkali activator,

-providing blowing agent solution,

-optionally milling the mixture to obtain a milled mixture,

7. The method of any of the preceding claims, wherein the geopolymer precursor material comprises aluminosilicate, such as metakaolin.

8. The method of any of the preceding claims, wherein the solid alkali activator comprises solid sodium silicate or solid sodium hydroxide, such as solid sodium silicate with SiO2:Na2O ratio in the range of 0.8-2.5, for example about 1 .

9. The method of any of the preceding claims, wherein the milling is carried out with a ring grinder.

10. The method of any of the preceding claims, wherein the milling is carried out for 30 seconds - 10 minutes, such as 1-5 minutes.

11 . The method of any of the preceding claims, wherein the concentration of the peroxide in the solution is in the range of 1-50% by weight, such as 10-30% by weight.

12. The method of any of the preceding claims, wherein the elevated temperature during curing is 55-70°C, such as about 60°C.

13. The method of any of the preceding claims, wherein the curing is carried out for 15-240 minutes.

14. The method of any of the preceding claims comprising neutralizing the obtained granules with an acid solution, such as acid selected from acetic acid, nitric acid, hydrochloric acid and sulfuric acid, preferable with acid concentration in the range of 0.01-5 M.

15. Porous geopolymer granules having an average diameter in the range of 1-10 mm, such as 1-4 mm, and/or determined with sieves in a mesh scale in the range of 18-7/16, and a specific surface area in the range of 10-300 m²/g.

16. The porous geopolymer granules of claim 15, wherein the granules are substantially spherical.

17. The porous geopolymer granules of claim 15 or 16 having porosity in the range of 30-90%, such as in the range of 30-60%, 40-70%, 40-80%, 50- 80%, or 50-90%.

18. The porous geopolymer granules of any of claims 15-17 having a cumulative NH4⁺ adsorption in the range of 10-100 mg/g when determined using a dynamic column experiment after 6 h in an experimental set-up including: 20 g of granules with average diameter in the range of 1-4 mm in plastic column (inner height 99 mm, diameter 44 mm, and volume 0.15 1), 200 mg NH4⁺/I influent concentration, pH of 7, and flowrate of 0.5 l/h.

19. The porous geopolymer granules of any of claims 15-18 having a mechanical strength in the range of 0.8-5.0 MPa, such as 1 .0-4.0 MPa.

20. The porous geopolymer granules of any of the claims 15-19 having a zeta potential in the range of from -1 to -25 mV measured at pH 7.

21 . The porous geopolymer granules of any of the claims 15-20 having a molar ratio of SiO2/Al2Os in the range of 2.50-4.50.

22. The porous geopolymer granules of any of the claims 15-21 obtained by granulating.

23. The porous geopolymer granules of any of the claims 15-22 obtained with the method of any of the claims 1-14.

24. Material comprising the porous geopolymer granules of any of the claims 15-23, selected from construction materials, filtration materials, absorption materials, ion exchange materials, support materials, disinfecting materials, nano and/or micro plastics collection materials, active capping materials, wate water treatment materials, pharmaceutical carrier materials, chromatography materials, oil collecting/recovery materials, soil stabilization materials, fertilizer or other substances carrier materials, hydrogen storage materials, permeable pavement materials, landfill materials, mining materials and mine tailings treatment materials.

25. Use of the porous geopolymer granules of any of the claims 15-23 as lightweight artificial aggregate in concrete, in construction materials or elements, such as in building blocks, as or in sound absorbing material, such as in acoustic panels, as or in filtration material for liquids or gases, as or in adsorption material for gases and/or dissolved solids in liquids, as or in ion-exchange material for dissolved solids in liquids and/or gases, as or in biofilm support material for bioreactors, as or in pH adjustment material in water, wastewater and/or in biogas generation, as or in catalyst support material, as or in disinfecting material, as or in nano/microplastics collector, as or in active capping material in polluted sediment remediation, as or in reactive permeable barrier in passive wastewater treatment applications, as or in pharmaceutical compound carrier in medicine, as or in chromatography media, for example in analytical chemistry, as or in oil collector from water, in soil stabilization or earth construction, as or in fertilizer carrier material, as or in hydrogen storage material, as or in permeable pavement material, as or in landfill foundation construction, and/or as or in mine tailings pond construction.

26. A method for removing substances from wastewater, such as municipal wastewater, the method comprising

-providing the porous geopolymer granules of any of the claims 15-23,

-providing wastewater,

-contacting the wastewater with the porous geopolymer granules to remove the substances from the wastewater.

27. The method of claim 26, wherein the substances are selected from one or more of ammonium, nitrogen, gases and dissolved solids and ions in liquids.