Silicate adsorbent material and use of silicate adsorbent material in water treatment

Abstract

Silicate adsorbent material in a form of gel, which is obtained by a method comprising a precipitation of an aqueous solution of sodium silicate by a water-soluble inorganic salt(s) in presence of cationic compound comprising amine functionalities, and / or powder activated carbon (PAC) Silicate adsorbent material is used in water treatment for removal of micropollutant(s), such as stable per- and polyfluoroalkyl substances (PFAS).

Description

[0001] SILICATE ADSORBENT MATERIAL AND USE OF SILICATE ADSORBENT MATERIAL IN WATER TREATMENT

[0002] Field of the invention

[0003] The present invention relates to a silicate adsorbent material and use of a silicate adsorbent material in water treatment, especially for micropollutant(s) removal. of the invention

[0004] Micropollutants have become a global concern in contamination of water bodies. One of the most challenging group of micropollutants is chemically and thermally stable per- and polyfluoroalkyl substances (PFAS) which are used in wide range of applications and are extremely difficult to degrade in typical water treatment processes.

[0005] Currently, the leading removal method of PFAS is use of activated carbon (AC) adsorbents. The porous AC adsorbents show good PFAS removal performance. However, the production of AC adsorbents have generally significant environmental impacts due to fossil-based raw materials and high-temperature treatment needed for activation. Furthermore, the price of AC adsorbent products is rather high. Hence, there is a need for affordable and more sustainable products for removal of micropollutants, such as PFAS from water bodies and generally in water treatment.

[0006] Summary of the invention

[0007] It is an object of the present invention to reduce or even eliminate the above-mentioned problems appearing in prior art.

[0008] The object of the invention is to provide a novel affordable silicate adsorbent material that can be used in water treatment, such as for micropollutant(s) removal. Further, an object of the present invention is to provide a novel silicate adsorbent material that can be produced in a simple method. In order to achieve among others the objects presented above, the invention is characterized by what is presented in the characterizing parts of the enclosed independent claims.

[0009] The embodiments and advantages mentioned in this text relate, where applicable, both to the silicate adsorbent material, the method for producing it as well as to the uses of the silicate adsorbent material according to the invention, even though it is not always specifically mentioned.

[0010] Typical silicate adsorbent material according to the present invention is in a form of gel and comprises

[0011] - polymerised silicate precipitated from an aqueous solution of sodium silicate by water-soluble inorganic salt(s), and

[0012] - a cationic compound comprising amine functionalities, and / or powder activated carbon (PAC), which are dispersed in the gel.

[0013] Typical silicate adsorbent material according to the present invention in a form of gel is obtained by the method comprising a precipitation of an aqueous solution of sodium silicate by a water-soluble inorganic salt(s) in presence of cationic compound comprising amine functionalities, and / or powder activated carbon (PAC).

[0014] Silicate adsorbent material according to the present invention is a moist gel product, which is not dried prior to use of it. Hence, silicate adsorbent material according to the present invention is a non-dried adsorbent product.

[0015] Silicate adsorbent material according to the present invention can be used in water treatment, such as removal of micropollutant(s). According to the present invention, water treatment may comprise a treatment of water, water body and / or wastewater. Water treatment may comprise drinking water treatment or treatment of wastewaters, such as municipal wastewaters, industrial wastewaters or wastewater sludges. Silicate adsorbent material according to the present invention is found to be an efficient adsorbent for removal of stable per- and polyfluoroalkyl substances (PFAS), but the adsorbent material of the present invention can also be used for removal of other micropollutants, particularly hydrophobic and anionic compounds.

[0016] The present invention provides a modified silicate adsorbent material produced from low-cost materials and with simple preparation method. The silicate adsorbent material according to the present invention is prepared from affordable aqueous solution of sodium silicate, also called a water glass solution or reagent. Water-soluble inorganic salt or combination of two or more water-soluble inorganic salts are used for precipitation or polymerization of water glass to produce gel-type, water permeable and high surface area adsorbent material. Further, cationic compound(s) can be used for modification of the silicate adsorbent material. Favourably, amphiphilic compounds with amine functionalities are used as cationic compound(s) for modification of the adsorbent material. Cationicity and hydrophobicity of the cationic compound improves adsorbent material’s micropollutants removal capacity. In the adsorbent material according to the present invention, the cationic compound(s) are well -dispersed in the gel-type silicate adsorbent material, as the polymerization is done in-situ in a solution containing cationic compound(s). In an embodiment of the present invention, powder activated carbon (PAC) can be added to the solution before the polymerization of water glass to create PAC-silicate type gel adsorbent material.

[0017] Compared to the known adsorbent materials that comprise silica-based ceramic particles, the silicate adsorbent material according to the present invention is not a ceramic adsorbent, but a silicate-based material prepared without any heat treatment. A method according to the present invention enables a production of the adsorbent material with additives, such as with cationic compound and / or powder activated carbon, in such a manner that the additives are not covalently bound to the material.

[0018] Brief Figure 1 shows PFAS concentrations in untreated surface water (two water batches: the first used for adsorption tests of fresh adsorbents and the other used for adsorption tests of 2 months stored adsorbents). The total concentration of PFAS and the PFAS profile of the two water batches are similar.

[0019] Figure 2 shows PFAS removal efficiency of PAC slurry (reference) and silicate gel adsorbents according to the present invention from surface water sample. Average PFAS adsorption capacity: PAC slurry 44.7%, Silicate+C12 37%, Silicate + PAC 53%, Silicate + PAC + C12 63%.

[0020] Figure 3 shows PFAS (removal efficiency of fresh and 2 months stored silicate gel adsorbent produced according to Example 1 . The adsorbent product was stored in sealed bottle for 2 months at room temperature. Average PFAS adsorption capacity: fresh silicate+C12 37%, 2 months stored silicate+C12 21 %.

[0021] Figure 4 shows PFAS removal efficiency of fresh and 2 months stored silicate gel adsorbent produced according to Example 2. The adsorbent product was stored in sealed bottle for 2 months at room temperature. Average PFAS adsorption capacity: fresh silicate+PAC 53%, 2 months stored silicate+PAC 47%.

[0022] Figure 5 shows PFAS removal efficiency of fresh and 2 months stored silicate gel adsorbent produced according to Example 3. The adsorbent product was stored in sealed bottle for 2 months at room temperature. Average PFAS adsorption capacity: fresh silicate+PAC+C12 63%, 2 months stored silicate+PAC 84%.

[0023] Detailed description of the invention

[0024] A silicate adsorbent material according to the present invention is prepared in a simple manner by precipitating of an aqueous solution of sodium silicate by water-soluble inorganic salt(s) in presence of a cationic compound and / or powder activated carbon (PAC). The obtained silicate adsorbent material is in a form of a moist gel. The cationic compound and / or powder activated carbon (PAC) are dispersed in the gel as the polymerization is done in-situ in a solution containing cationic compound(s) and / or PAC.

[0025] A method according to the present invention is based on a polymerization of sodium silicate, also called a water glass, by adding water-soluble inorganic salt to an aqueous solution of sodium silicate, wherein the geltype silicate adsorbent material is obtained. According to one exemplary embodiment of the present invention, an aqueous solution of sodium silicate comprises 20 - 30 weight-% of silica (SiC ), particularly e.g. 25.5 - 28.5 weight-% of silica (SiC ).

[0026] According to an embodiment of the present invention a water-soluble inorganic salt comprises at least one salt selected from the group consisting of sodium, potassium, magnesium, calcium, iron and aluminium salts. In an embodiment of the present invention, an inorganic salt may comprise soluble alkali or alkali earth metal halides or sulphates. In an exemplary embodiment, an inorganic salt may be selected from a group consisting of sodium, potassium, calcium, magnesium , aluminium and ferric or ferrous chlorides or sulphates.

[0027] According to an embodiment of the present invention a water-soluble inorganic salt is used for a precipitation of an aqueous solution of sodium silicate in a such an amount that a stoichiometric molar ratio between the inorganic salt and silicon is between 1 and 20.

[0028] According to an embodiment of the present invention, a water-soluble inorganic salt or salts are dissolved in an aqueous solution prior to mixing with an aqueous solution of sodium silicate. Inorganic salt(s) may be dissolved e.g. to an aqueous solution of cationic compound(s) and / or powder activated carbon (PAC).

[0029] In a method for producing a silicate adsorbent material according to the present invention, a cationic compound comprising amine functionalities is present in the aqueous solution during precipitation or polymerisation of sodium silicate, wherein the cationic compound is well dispersed in the obtained gel-type adsorbent material. According to an embodiment of the present invention, a cationic compound comprises quaternary ammonium cation. In one preferred embodiment of the present invention cationic compound comprises a cationic amphiphilic compound comprising quaternary ammonium cation and hydrophobic moiety or moieties. In an exemplary embodiment according to the present invention, a cationic amphiphilic compound comprises a linear cationic surfactant with a hydrophobic alkyl chain with a chain length of C < 16. In an exemplary embodiment according to the present invention, a cationic amphiphilic compound may comprise octatrimethyl ammonium halide, decyltrimethylammonium halide, dodecyltrimethylammonium halide, tetradecylmethlyammonium halide, hexadecyltrimethylammonium halide (e.g. cetrimonium bromide (CTAB) or cetrimonium chloride (CTAC)), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT) or any combination thereof.

[0030] According to an embodiment of the present invention, cationic compound can be added to an adsorbent material in an amount of 0.1 - 20 weight- %, preferably 1 - 10 weight-% or 1 - 5 weight-%, calculated from dry adsorbent weight.

[0031] Further, powder activated carbon (PAC) may be added to the adsorbent material. In a method for producing a silicate adsorbent material according to the present invention, powder activated carbon is also present in the aqueous solution during precipitation, wherein it is well dispersed in the obtained gel-type adsorbent material. Powder activated carbon is a finely ground form of activated carbon. Typically, powder activated carbon particles have a size of below 200 pm , preferably in the range of 0.1 - 100 pm.

[0032] An amount of powder activated carbon (PAC) used in the adsorbent material can vary. According to an embodiment of the present invention, PAC is used in the adsorbent material in an amount of 10 - 70 weight- %, preferably 10 - 50 weight-%, calculated from the total weight of the reagents, assuming no solvent evaporation. In some embodiments, PAC is used in an amount of at least 10 weight-%, such as 10 - 30 weight-% or 10 - 20 weight-%, calculated from the total weight of the reagents, assuming no solvent evaporation.

[0033] In an embodiment the present invention, both a cationic compound and powder activated carbon are added to the adsorbent material.

[0034] Typically, a silicate adsorbent material according to the present invention is prepared at a room temperature or at moderate temperature, typically at a temperature of 15 - 40 °C. The method for producing silicate adsorbent material of the present invention is free of any heat treatment after precipitation of gel-type adsorption material. The obtained adsorption material can be used as freshly, it is not dried or activated with any method prior to use.

[0035] According to an exemplary embodiment of the present invention, a method for producing silicate adsorbent gel material comprises

[0036] - obtaining an aqueous solution, which comprises a cationic compound comprising amine functionalities, such as quaternary amine cation, and / or powder activated carbon (PAC),

[0037] - dissolving at least one water-soluble inorganic salt to the aqueous solution comprising cationic compound and / or powder activated carbon to obtain a salt mixture,

[0038] - obtaining sodium silicate solution comprising 20 - 30 weight-% of silica (SiO2), and

[0039] - mixing the sodium silicate solution with the salt mixture to obtain a silicate adsorbent gel material.

[0040] Silicate adsorbent material in a form of moist gel is obtained by a method according to the present invention. According to an embodiment of the present invention a gel-type silicate adsorbent material is a moist gel having a theoretical moisture content of 20 - 95 w / w-%, more typically 40 - 90 w / w-%, and even more typically 70 - 90 w / w-%. Theoretical moisture content of the moist gels according to the present invention is calculated by solids contents of the reagents and water used in preparation. The theoretical moisture content (w / w-%) was determined by 100 w / w-% - calculated solids content (w / w-%).

[0041] Moisture content is the amount of water present in a moist gel. In addition to theoretical moisture content defined above, moisture content can be determined e.g. by gravimetric methods, where the moist gel is dried. The mass change during drying is interpreted as the moisture content of the gel. Moisture content can be calculated by weights obtained before and after the drying process. Drying of the moist gel can be performed in different ways. In an exemplary embodiment, a sample is placed in a drying oven until a steady mass is obtained or infrared drying or halogen drying, such as by Mettler Toledo Halogen moisture analyser. Regardless on the determination method, a moisture content of the moist gel according to the present invention is in the range of 20 - 95 w / w-%.

[0042] Typically, the performance of the gel weakens, if the gel is dried. Hence, the silicate adsorbent material according to the present invention is used without drying. A moist gel according to the present invention is not dried or subjected to any heat treatment. A gel according to the present invention is a moist product, which structure is essentially amorphous having non-crystalline structure or having a degree of crystallinity less than 10 % or less than 5 %. In some embodiments of the present invention a moist gel may comprise minor amounts of crystalline impurities.

[0043] A silicate adsorbent material according to the present invention is typically produced just prior to use. Silicate adsorbent material according to the present invention has also observed to be also kinetically relatively stable at moderate storing temperature, when stored for closed cap receptacles.

[0044] A silicate adsorbent material according to the present invention can be used in a water treatment. In a typical embodiment of the present invention, a silicate adsorbent material of the invention is used in removal of micropollutant(s) in water treatment. Water to be treated may comprise natural water body or wastewater. According to an embodiment of the invention, a silicate adsorbent can be used in a drinking water treatment, or a treatment of wastewater, such as municipal or industrial wastewater or wastewater sludge. According to an embodiment of the present invention, a silicate adsorbent material is used in removal of stable per- and polyfluoroalkyl substances (PFAS) from water, water body and / or wastewater.

[0045] According to an embodiment of the present invention, a silicate adsorbent material can be added directly to water, water body and / or wastewater to be treated. In another embodiment of the present invention water, water body and / or wastewater to be treated is guided through a filter, a fluidised bed reactor and / or a column comprising a silicate adsorbent material.

[0046] A method according to an embodiment of the present invention for treating water, water body and / or wastewater for removing micropollutant(s) comprises

[0047] - adding silicate adsorbent material according to the present invention to water, water body and / or wastewater to be treated, and / or

[0048] - guiding water, water body and / or wastewater to be treated through a filter, a fluidised bed reactor and / or a column comprising silicate adsorbent material according to the present invention.

[0049] The following examples describe processes of the silicate adsorbent preparation according to some embodiments of the present invention, and use of the silicate adsorbent material according to the present invention in some applications.

[0050] EXPERIMENTAL

[0051] In the Examples, the following reagents are used. MgCl2 is used as soluble inorganic salt, and dodecyltrimethylammonium chloride (C12) as a cationic compound. Water glass solution (sodium silicate solution containing 25.5-28.5% SiC ) is used as the silicate source in the synthesis of the silicate adsorbent material. In Examples 2 and 3, Super SAE powder activated carbon (PAC) from Norit is used as the activated carbon source. 1 : Silicate adsorbent material with cationic

[0052] 0.1 g of C12 is dissolved in 15 g deionized (DI) water. The solution is mixed until clear. 3.42 g of MgCl2*6H2O is added to an aqueous solution of the surfactant and mixed until the solution is clear. 5 g of water glass solution is added dropwise to the salt-C12 mixture. Polymerization and gelling occurs rapidly after water glass addition.

[0053] 2: Silicate adsorbent material with activated carbon

[0054] 0.98 g of powder activated carbon (PAC) is mixed with 15 g deionized water. The PAC slurry is mixed overnight to ensure complete wetting of PAC. 3.42 g of MgCl2*6H2O is added to the PAC slurry and mixed for 10 minutes. 5 g of water glass solution is added dropwise to the PAC-salt slurry. Polymerization and gelling occurs rapidly after water glass addition. s 3: Silicate adsorbent material with cationic and activated carbon

[0055] 0.1 g of C12 is dissolved in 15 g deionized water. 0.98 g of PAC is added to an aqueous solution of the surfactant. The PAC-C12 slurry is mixed overnight to ensure complete wetting of PAC. 3.42 g of MgCl2*6H2O is added to the PAC-C12 slurry and mixed for 10 minutes. 5 g of water glass solution is added dropwise to the PAC-C12-salt slurry. Polymerization and gelling occurs rapidly after water glass addition.

[0056] PFAS tests

[0057] Perfluoroalkyl substance (PFAS) adsorption performance of the adsorbent gels produced in the Examples 1 -3 were tested in batch tests. The adsorbent gels of the Example 1 -3 or PAC slurry (Super SAE powder activated carbon) as reference were dosed to sealed sample bottles with 500 ml of surface water containing PFAS molecules. Reference PAC slurry contains 1 g PAC dissolved in 49 g deionized water. The samples were mixed with 250 rpm mixer plate for 22 hours. The samples and dosages are described in Table 1 . The concentrations of selected PFAS molecules were quantified from the surface water before and after the adsorption tests with LC-MS / MS (method CEN / TS 15968). Analyzed PFAS compounds are listed in Table 2. Detection limit, i.e. the lowest concentration of the analyte that can be reliably detected in the analysis, was 0.01 pg / L

[0058] Table 1 . Samples and dosages for PFAS adsorption tests

[0059] Table 2.

[0060] Stability tests

[0061] Stability of the adsorbents was studied by storing the moist adsorbent gel samples in sealed bottles at room temperature for 2 months, and by comparing the PFAS adsorption capacities of the freshly prepared and 2 months stored samples.

[0062] Results

[0063] Figure 1 shows PFAS concentrations in untreated surface water. The results of the adsorption tests are presented in Figures 2-5. Figure 2 shows removal-% of PFAS compounds found from the surface water sample in the adsorption tests with silicate adsorbent materials according to the present invention and PAC slurry (reference). The results of the stability tests are presented in Figures 3-5.

[0064] In Figure 2, average PFAS adsorption capacity (AVG removal-%): PAC slurry = 45%, Silicate+C12 = 37%, Silicate + PAC = 53%, Silicate + PAC + C12 = 63%. The results are calculated as the average removal of all detected PFAS according to Equations:

[0065] X PFAS removal — %

[0066] AV G removal — % = - n where PFAS removal-% is the removal percentage of single PFAS compound, PFASaftertreatment is the PFAS concentration after adsorption treatment, PFASinitiaiis the initial PFAS concentration of the surface water, AVG removal - % is the average removal percentage of all PFAS compounds detected in water, n is the number of PFAS compounds found from the water.

[0067] The silicate adsorbent materials with PAC (gel products prepared according to Examples 2 and 3) perform better for PFAS removal as compared to PAC alone or silicate adsorbent prepared according to Example 1 . However, silicate adsorbent material according to Example 1 removes efficiently sulfonated PFAS compounds, whereas the adsorbent material without C12 surfactant (gel product prepared according to Example 2) seems to be most efficient for removal of long-chain PFAS compounds. In stability test, it has been observer that silicate adsorbent materials according to the present invention have generally maintained their PFAS adsorption capacities.

Claims

Claims:

1. Silicate adsorbent material in a form of gel, wherein the silicate adsorbent material comprises- polymerised silicate precipitated from an aqueous solution of sodium silicate by water-soluble inorganic salt(s), and- a cationic compound comprising amine functionalities, and / or powder activated carbon (PAC), which are dispersed in the gel.

2. Silicate adsorbent material according to claim 1 , wherein the silicate adsorbent material is obtained by a method comprising a precipitation of an aqueous solution of sodium silicate by a water-soluble inorganic salt(s) in presence of the cationic compound comprising amine functionalities, and / or powder activated carbon (PAC).

3. Silicate adsorbent material according to claim 1 or 2, wherein a stoichiometric molar ratio between the water-soluble inorganic salt(s) and silicon is between 1 and 20.

4. Silicate adsorbent material according to any one of the preceding claims, wherein the water-soluble inorganic salt comprises at least one salt selected from the group consisting of sodium, potassium, magnesium, calcium, iron and aluminium salts.

5. Silicate adsorbent material according to any one of the preceding claims, wherein the cationic compound comprises quaternary ammonium cation.

6. Silicate adsorbent material according to any one of the preceding claims, wherein the cationic compound comprises a cationic amphiphilic compound comprising quaternary ammonium cation and hydrophobic moiety or moieties.

7. Silicate adsorbent material according to any one of the preceding claims, wherein an amount of the cationic compound is in the range of0.1 - 20 weight-%, preferably 1 - 10 weight-% or 1 - 5 weight-%, calculated from dry adsorbent weight.

8. Silicate adsorbent material according to any one of the preceding claims, wherein an amount of powder activated carbon (PAC) is in the range of 10 - 70 weight-%, preferably 10 - 50 weight-%, calculated from the total weight of the reagents, assuming no solvent evaporation.

9. Silicate adsorbent material according to any one of the preceding claims 2-8, wherein the method for producing silicate adsorbent material is free of any heat treatment.

10. Silicate adsorbent material according to any one of the preceding claims, wherein the gel of the silicate adsorbent material is a moist gel having a theoretical moisture content of 20 - 95 w / w-%.

11. Use of silicate adsorbent material according to any one of the preceding claims 1-10 in water treatment.

12. Use of silicate adsorbent according to claim 11 , wherein the silicate adsorbent material is used in removal of micropollutant(s).

13. Use of silicate adsorbent according to claim 11 or 12, wherein the silicate adsorbent material is used in removal of stable per- and polyfluoroalkyl substances (PFAS).

14. Use of silicate adsorbent according to any one of the preceding claims 11 - 13, wherein- the silicate adsorbent material is added directly to water, water body and / or wastewater to be treated, and / or- water, water body and / or wastewater is guided through a filter, a fluidised bed reactor and / or a column comprising a silicate adsorbent material.

15. Method for treating water, water body and / or wastewater for removing micropollutant(s), preferably stable per- and polyfluoroalkyl substances (PFAS), wherein the method comprises- adding silicate adsorbent material according to any one of the preceding claims 1 - 10 to water, water body and / or wastewater to be treated, and / or- guiding water, water body and / or wastewater through a filter, a fluidised bed reactor and / or a column comprising silicate adsorbent material according to any one of the preceding claims 1 - 10.

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