Technical field

This specification relates to a material for collecting metals from an aqueous solution, which material comprises porous silicon. Some aspects of the disclosure relate to a system for collecting metals from an aqueous solution, which system comprises said material. Some aspects of the disclosure relate to a method for collecting metals from an aqueous solution by a material or a system disclosed.

Background

In natural waters as well as waters for industrial and household use, the occurrence of metals harmful to nature and health, such as copper, zinc, chromium, uranium, manganese and arsenic, is relatively common. Industrial metal emissions to surrounding waters are measured frequently, and analyzing well waters every third year is recommended by the authorities in Finland. Conventionally, a water sample is taken on-site and sent to a laboratory for analysis. In the laboratory, highly educated personnel analyze the sample typically by inductively coupled plasma mass or atomic emission spectrometry (ICP-MS or ICP-AES). These instruments are expensive to purchase, use and maintain. Moreover, they are not portable, and do not allow rapid analysis of samples.

Pre-concentration is a technique, wherein analytes of a sample are collected and concentrated into a smaller volume prior qualifying and/or quantifying the analytes. The technique can be used to enhance the sensitivity of certain analytical methods. However, for use on the field conditions, the pre concentration methods are often too laborious, time consuming and difficult to perform. Furthermore, natural waters typically have complex compositions that may cause the pre-concentration to be unreliable. Summary

The above disclosed deficiencies may be addressed by providing a material, a system and a method which enable collecting metals from an aqueous solution by a material comprising porous silicon structure and a metal binding ligand covalently bonded to it. An advantage is, that metals contained by the aqueous solutions may be collected, concentrated and analyzed in a fast, efficient and simple manner. A further advantage is that the material, the system and the method may be used for versatility of aqueous samples, as the material and the system provided are stable in highly versatile pH. Moreover, the collection and analysis of the metals, as well as the regeneration of the material or the system may be executed on-site. When regenerated, the material and the system may be reusable. Thus, a cost-efficient and environmentally friendly solution for collecting metals from aqueous solutions is provided. Furthermore, rapid and portable analytical methods for analyzing low concentrations of metals on-site are enabled.

According to an aspect of the disclosure, a material for collecting metals from an aqueous solution is provided. The material comprises porous silicon structure including a surface layer. When manufactured, the surface of the porous silicon structure is passivated in order to prevent undesired chemical reactivity of the surface and/or in order to enable functionalization of the surface to exhibit metal binding properties. The surface layer of the porous silicon structure thus comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The surface layer of the porous silicon structure does not comprise hydrocarbon(s). The surface of the porous silicon structure is functionalized by attaching a metal binding ligand by a covalent bond to the surface layer. The covalent bond to the surface layer provides stability and allows the material to withstand well both acidic and basic conditions. The metal binding ligand may be selected from versatility of functionalities capable of metal binding. The material may be in a form of a metal collecting porous film or metal collecting porous particles. The material disclosed enables the metals collected by the material to be released and the material to be regenerated by contacting the material with a solution having a pH of 3 or lower or with a solution having a pH of 1 1 or higher. The stability of the material enables the cycle of metal collecting and regeneration to be repeated for several tens of times without losing the material’s ability to collect metals from aqueous solutions.

Therefore, there is provided a material for collecting metals from an aqueous solution. The material comprises

- porous silicon structure including a surface layer, wherein the surface layer comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide; and

- a metal binding ligand covalently bonded to said surface layer, the metal binding ligand being selected from functionalities containing at least one of the following bond types: P-O, P=0, S- O, S=0, C-O, C=0, P-N, S-H.

According to another aspect of the disclosure, a system for collecting metals from an aqueous solution is provided. The system comprises material as disclosed above. The material of the system may be a metal collecting porous film or metal collecting porous particles. The material may be arranged on a support or a substrate. The support or the substrate may comprise for example silicon wafer, filter paper or glass plate. Arranging the material on a permeable support or substrate allows the system to be used for example in flow-through type of set-ups for collecting metals.

According to yet another aspect of the disclosure, a method for collecting metals from an aqueous solution is provided. In the method, the material or the system according to the disclosure is contacted with an aqueous solution containing metals. The metal binding ligands of the material or the system then interact with at least part of the metals contained in the aqueous solution, thus causing at least part of the metals to be collected by the material or the system. The metals are concentrated from a larger volume into a smaller volume when collected by the material or the system. The metals collected and concentrated by the material or the system may be analyzed using an analytical process or instrument. The metals collected by the material or the system may be qualified and/or quantified directly from the material or the system. The quantification of the metals collected by the material or the system may be used for quantification of the metal concentration of the aqueous solution. The metals collected by the material or the system may be released from the material or the system with a solution having a pH of 3 or lower or with a solution having a pH of 1 1 or higher. As a result, an eluent comprising the metals released from the material or the system may be formed, and the material or the system may be regenerated.

Therefore, there is provided a method for collecting metals from an aqueous solution. The method comprises

- contacting a material or a system according to the disclosure with an aqueous solution that contains metals,

- allowing the metal binding ligands of the material or the system to interact with at least part of the metals contained in the aqueous solution, in order to at least part of the metals to be collected by the material or the system.

Objects according to this disclosure are further described in the appended claims.

Detailed description

Within context of this specification,“and/or” stands for three different kind of situations, that all are equally possible. For example,“A and/or B” may refer to merely A, merely B, or both A and B.

In this specification, a material for collecting metals from an aqueous solution is provided. The material comprises porous silicon structure and a metal binding ligand covalently bonded to a surface of said porous silicon structure.

Porous silicon, abbreviated as PSi, is a form of chemical element silicon. Porous silicon has nanometer or micrometer scale sized pores in its microstructure, rendering a large surface to volume ratio. The surface of porous silicon may be modified to exhibit different properties. Porosity is defined as the fraction of void within the PSi material. The porosity of porous silicon may range from 4 vol-% to 95 vol-%. The surface area of porous silicon may range from 10 m ² /g to 1000 m ² /g. The porosity and the surface area of porous silicon may be varied depending on its application areas. Based on the pore size, that is, pore diameter, the porous silicon may be divided into three categories. Microporous silicon has an intermediate pore width of less than 2 nm. In mesoporous silicon, the intermediate pore width may vary between 2 and 50 nm. For macroporous silicon the intermediate pore width is larger than 50 nm.

For producing porous silicon structure, silicon may be provided for example in form of silicon wafer, silica or silicon salts.

Porous silicon structure may be produced by electrochemical etching of silicon wafer for example in hydrofluoric acid (HF) solution. The wafer acts as an anode. Cathode made of for example platinum acts as a counter electrode in the system. In the process, pores are formed on top of the silicon wafer. The properties of thus formed porous layer may be controlled by etching parameters, such as current density, electrolyte composition, etching time, and both wafer type and conductivity. After etching, high current pulse may be utilized to remove a formed layer of PSi from the substrate wafer. The layer formed may be used as such. Alternatively, it may be fractured to particles of desired size.

PSi may also be formed by etching without an external source of electric voltage. These methods include for example stain etching and metal assisted etching.

Alternatively, porous silicon structure may be produced by magnesiothermic reaction from silicon dioxide i.e. silica. Silica may have been synthesized with various processes or it may be of biological or geological origin. Some plants and other organisms have the ability to accumulate silica in them. Examples of these are rice, barley, bamboo and diatoms. In the magnesiothermic reaction silica is reduced to silicon and magnesium is oxidized to magnesium oxide. Magnesium oxide nanocrystals may be dissolved with acid leaching revealing a PSi structure. Material may be washed for example with HF to remove silicon oxides and other impurities. Porous silicon structure produced by magnesiothermic manner may be disordered and more uncontrollable compared to above described electrochemical etching. Flowever, source materials such as rice husks are common wastes of agriculture. By using this approach a reuse for the waste materials is provided. Yet another alternative for the production of porous silicon structure is using silicon salts as precursors. As an example, a reaction between NaSi and NhUBr may be mentioned. In an exemplary reaction, the salts are mixed and dispersed with NaBr. Then the mixture is placed under nitrogen atmosphere and heated to over 200 °C. The reaction between NaSi and NhUBr creates small silicon nanocrystals. The silicon nanocrystals aggregate with each other, forming pores therebetween. Impurities may be dissolved for example with HCI and HF.

Porosity of the porous silicon structure disclosed may be at least 10 vol-%. The porous silicon structure may have the porosity of at least 50 vol-%. Preferably, the porosity is between 50 - 95 vol-%. The porous silicon structure may have a surface area of at least 10 m ² /g. The surface area may be between 10 - 1000 m ² /g. Average pore diameter of the porous silicon structure disclosed may be smaller than 500 nm. Preferably, the average pore diameter is between 2 - 500 nm.

Surface of PSi manufactured by any method described above, having Si-H species, is highly reactive after etching or HF washing and may be easily oxidized. Thus, it may be beneficial to passivate the surface of the porous silicon structure in order to improve stability. Within context of this specification, passivation refers to a process by which undesired chemical reactions of the surface are prevented and/or functionalization of the surface in order to exhibit metal binding properties is enabled. Passivation may also be called stabilization or protection. Moreover, the surface may be modified in order to exhibit various properties.

The surface of the porous silicon structure may be passivated for example by hydrocarbonization, carbonization, hydrosilylation, nitridation or oxidation. As a result of hydrocarbonization the surface of the porous silicon structure may comprise silicon carbide and/or carbon with hydrocarbons. As a result of carbonization, the surface of the porous silicon structure may comprise silicon carbide, silicon oxycarbide and/or carbon without hydrocarbons. After hydrosilylation, the surface of the porous silicon structure may comprise hydrocarbon molecules. As a result of nitridation, the surface of the porous silicon structure may comprise Si-N entities. After oxidation, the surface of the porous silicon structure may comprise Si-O-Si entities and hydroxyl groups.

In hydrocarbonization, the surface of porous silicon structure may be treated with an organic molecule, such as a polymer or gaseous acetylene (C2H2), at a temperature above 400 °C. Hydrocarbonization with acetylene may be performed by removing oxygen with a nitrogen flush and then introducing C2H2 into the system and elevating the temperature. This provides the surface of the porous silicon structure with silicon carbide and/or carbon with hydrocarbons on the surface. The hydrogen atoms may be removed from the surface leaving only silicon carbide or carbon surface with free radicals. This may be done by adsorbing C2H2 gas on the surface and elevating temperature to over 600 °C. This process is referred to as carbonization. When exposed to air the free radicals may react with oxygen of air and moisture, forming silicon oxycarbide (SixCyOz) entities on the surface of the porous silicon structure. The surface of the porous silicon structure comprising silicon carbide, silicon oxycarbide and/or carbon is stable and non-reactive in both acidic and basic conditions.

By carbonization, the porous silicon structure is provided with a surface layer. Within context of this specification, the terms “carbonized surface” and “carbonized PSi” refer to the surface layer of the porous silicon structure. The surface layer of the porous silicon structure comprises at least two atomic layers of solid material different in atomic composition compared to the porous silicon structure. The surface layer forms an interface between the porous silicon structure and the surroundings. The surface layer may be on the external surface of the porous silicon structure or inside the porous silicon structure, namely in the pores. The surface layer of the porous silicon structure comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The surface layer of the porous silicon structure does not comprise hydrocarbon(s). The surface layer of the porous silicon structure is stable and non-reactive in both acidic and basic conditions. The surface layer of the porous silicon structure is not hydrophobic. To the surface layer functional molecules may be attached. Carbonized surface of the porous silicon structure may be functionalized in order to enable it to exhibit desirable properties. Within context of this specification, a material with metal binding capability is searched for. Thus, the surface layer of the porous silicon structure is functionalized in order to arrange a metal binding entity on it. The surface layer of the porous silicon structure is functionalized in order to form a material for collecting metals from an aqueous solution. This is performed by attaching a functional molecule bearing a metal binding ligand onto the surface layer of the porous silicon structure. The attachment is of a covalent type. The attachment may also be called conjugation. The functional molecules may comprise for example terminal double carbon-carbon bond, triple carbon-carbon bond, alkoxysilane or silanol group for bonding with the carbonized PSi surface, as well as the metal binding ligand.

The chain length between the metal binding ligand and the group capable for bonding with the carbonized PSi surface may vary. Whereupon a shorter chain length, for example C2-C20, is applied, it may allow the functionalization density of the surface to be higher, as smaller molecules more easily fit to the surface. With a longer chain length, it may be challenging to obtain a high functionalization density of the surface, as the functional molecules require more space. Moreover, the molecules having longer chain length may be disoriented more easily as the intermolecular forces between the neighboring alkyl chains may cause the chains to be bundled or clustered together. This may be disadvantageous to the metal binding capacity of the material.

The metal binding ligand may be selected from functionalities containing at least one the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P-N, S- H. The metal binding ligand may be selected from the following: phosphonate, sulfonate, thiol, carboxyl, carbonyl, hydroxyl. Bond types P-0 and P=0 may belong to a bisphosphonate structure. The functional molecule may comprise one or more metal binding functional groups.

Bisphosphonates comprise a P-C-P bridge, wherein two phosphonate (PO(OH)2, in its protonated form) groups are covalently linked to a central carbon atom. The central (bridging) carbon atom also has one or two side chains covalently linked to it. The nature of the side chains determines the chemical properties of the bisphosphonates. Bisphosphonates have a variety of uses, the most prominent ones perhaps relating to their use for treatment of bone diseases and disorders of calcium metabolism, such as osteoporosis. Within context of this disclosure, the bridging carbon atom typically has a side chain covalently linked to it, which side chain may be of variable length and is utilized for bonding the bisphosphonate molecule to the surface of the porous silicon structure. Typically, the bridging carbon atom also has a functional group, such as a hydroxyl group. The functional group may adjust the metal binding properties of the P-C-P moiety.

Carbonized surface of the porous silicon structure may be functionalized using a functional molecule comprising a terminal double or triple carbon-carbon bond and one or more metal binding functional groups. Functionalization of the carbonized surface of porous silicon may be performed at an elevated temperature, such as at 120 °C. The functional molecule may react with Si and/or C atoms on the surface. Density of the reacted functional molecules on carbonized PSi may be improved by keeping the carbonized PSi under an inert atmosphere (for example N, He, Ar, Ne) after the carbonization. The improved density is due to free radicals formed on the surface during carbonization which react with the double or triple bond. The covalent bond formed between the carbonized surface of the porous silicon and the functional molecule is stable, withstanding both acidic and basic conditions well . The covalent bond exhibits good stability also in challenging flow-through systems wherein a liquid flows through/past the material. As an example, undecylenic acid with carboxyl group as a metal binding ligand may be conjugated with this method to the surface of the carbonized PSi. Similarly, bisphosphonates with terminal double or triple carbon-carbon bond may be conjugated.

Another example of functionalization of carbonized PSi surface is using functional molecules comprising silanol or alkoxysilane groups, which may be conjugated to the surface by silanization. For this carbonized surface is oxidized with nitric acid, hydrogen peroxide or other oxidizing agent in order to form hydroxyl groups on the surface. Metal binding ligand with a terminal silanol or alkoxysilane group may be dissolved to organic solvent. The PSi particles are allowed to contact with the solution. The silanization reaction occurs between the silanol or alkoxysilane groups and the hydroxyl groups of the surface. Density of the reacted functional molecules on carbonized PSi may be increased by using elevated reaction temperature such as 65°C and by removing oxygen, which may prevent the silanization reaction. As an example, (3-mercaptopropyl)trimethoxysilane with a primary thiol as a metal binding ligand may be conjugated with this method to the surface of the carbonized PSi.

As disclosed above, a variety of functional molecules may be conjugated to the surface of the carbonized PSi. Both examples of thiol and carboxyl functionalized porous silicon structures may be used as such for collection of metals from aqueous solution or they can be further functionalized with different functional molecules comprising metal binding ligands. Similarly as thiol silanes above, amino silanes may be conjugated to carbonized PSi surface in order to further functionalize the surface with different functional molecules comprising metal binding ligands. Ability to modify for example the amine, thiol and carboxyl groups further extends the selection of suitable metal binding ligands even more.

A material for collecting metals from an aqueous solution may be produced as described above. The material comprises porous silicon structure including a surface layer. The surface layer of the porous silicon structure has a different atomic composition compared to the porous silicon structure. The surface of the porous silicon structure is passivated by carbonization. The surface layer of the porous silicon structure thus comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The surface layer of the porous silicon structure does not comprise hydrocarbon(s). The porous silicon structure also comprises a metal binding ligand covalently bonded to the surface layer.

As presented above, the metal binding ligand may be selected from functionalities containing at least one of the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P-N, S-H. The metal binding ligand may be selected from the following: phosphonate, sulfonate, thiol, carboxyl, carbonyl, hydroxyl. Within context of this specification“ligand” refers to an atom, ion, or functional group that may donate one or more of its electron pairs through at least one coordinate bond to one or more atoms or ions. Ligand may also be called complexing agent. A coordination complex consists of a central atom or ion, which may be metallic and may be called a coordination centre, and a surrounding array of bound molecules or ions, known as ligands or complexing agents. The coordination complex whose centre is a metal atom is called a metal complex. Chelation is one example of coordination complex forming processes. In chelation, two or more coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom/ion may be formed. Thus, a chelation complex is formed.

Typically, ligand’s ability to donate electron pair(s) depends on the structure of the ligand and the chemical nature of coordinating atoms. In principle, less electronegative atoms have better ability to donate one or more electron pairs. The electronegativity series can be presented as following: H<C=P<S<Br<N=CI<0<F, wherein H and C do not have free electron pair(s) to be donated. The other way around, electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons or electron density towards itself. Functional groups that contain oxygen are effective in binding metals, since oxygen polarizes the group effectively and attracts the electron pair(s) from the other atoms. Therefore, higher electronegativity makes oxygen effective in metal binding, when bonded to less electronegative atom(s). Oxygen atom with double bond is more polarized than with a single bond, thus being more effective in metal binding. Therefore, ligands containing P=0, S=0, C=0, P-O, S-0 and/or C-0 bond(s) are potential for metal binding. Ligands containing P and S can have typically more above mentioned bonds than ligands containing C, because of their higher number of valence electrons. Therefore, ligands containing P and/or S, such as sulfonates and phosphonates, are especially potential for metal binding.

The material for collecting metals from an aqueous solution may be in form of a metal collecting porous film or metal collecting porous particles. The metal collecting porous film comprises porous silicon structure including a surface layer. The surface layer of the porous silicon structure comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The metal collecting porous film also comprises a metal binding ligand covalently bonded to the surface layer of the porous silicon structure. The metal binding ligand is selected from functionalities containing at least one of the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P-N, S-H.

Alternatively, the material may be metal collecting porous particles. The metal collecting porous particles comprise porous silicon structure including a surface layer. The surface layer of the porous silicon structure comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The metal collecting porous particles also comprise a metal binding ligand covalently bonded to the surface layer of the porous silicon structure. The metal binding ligand is selected from functionalities containing at least one of the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P- N, S-H. The metal collecting porous particles may have a diameter of 0.01 -500 pm.

Porosity of the material disclosed may be at least 10 vol-%. The material may have the porosity of at least 50 vol-%. Preferably, the porosity is between 50 - 95 vol-%. The material may have a surface area of at least 10 m ² /g. The surface area may be between 10 - 1000 m ² /g. Average pore diameter of the material disclosed may be smaller than 500 nm. Preferably, the average pore diameter is between 2 - 500 nm.

The system for collecting metals from an aqueous solution comprises above disclosed material. The material comprises porous silicon structure including a surface layer. The surface layer of the porous silicon structure comprises at least two atomic layers of at least one of the following: carbon, silicon carbide, silicon oxycarbide. The surface of the porous silicon also comprises a metal binding ligand covalently bonded to the surface layer. The metal binding ligand is selected from functionalities containing at least one of the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P-N, S-H. The material may be a metal collecting porous film or metal collecting porous particles. The material may be arranged on a support or a substrate. The support or substrate may comprise for example silicon wafer, filter paper or glass plate. The support or substrate may be permeable.

For example, the system may be a metal collecting fixed particle bed. For producing the metal collecting fixed particle bed, a slurry comprising metal collecting porous particles as described above, water and at least one crosslinking binder polymer may be mixed and spread on a filter paper. The slurry with the filter paper may be heated in order to remove water and crosslink the polymer. As a result, a porous and fixed particle layer on top of the filter paper is obtained.

The material or the system as described above may be used for collecting metals from an aqueous solution. The method comprises contacting the material or the system disclosed with an aqueous solution that contains metals, and allowing the metal binding ligands of the material or the system to interact with at least part of the metals contained in the aqueous solution, in order to at least part of the metals to be collected by the material or the system.

Interaction between the metal binding ligands of the material or the system and the metals of the aqueous solution may be for example of a type of coordination, complexation or chelation. As a result of the interaction, at least part of the metals contained in the aqueous solution are collected by the material or the system. At least part of the metals are thus removed from the aqueous solution. The material or the system may collect the metals by adsorption. Adsorption refers to adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. When adsorbed, the metals may be concentrated to form a layer on a surface of the material or the system. At least part of the metals contained in the aqueous solution may thus be concentrated into the material or the system. When necessary, the aqueous solution, from which the metals are to be collected, may be pretreated before applying the method disclosed. The pretreatment may comprise for example filtration, acidification and/or alkalization.

Metals that may be collected by the material or the system include for example Cu, Zn, Fe, Sc, Pb, Eu, Cr, Mn, As and U. Metals that may be collected by the material or the system also include Ni, Hg and Sb. Metals may be in ionic or elemental form, or they may form part of a molecule. When in ionic form, the metals may be anionic or cationic. Oxidation states of the metals collected by the material or the system may vary. The concentration of the metals collected may vary from sub-ppm concentrations to concentrations of hundreds of ppm. Properties of the material and/or the system as well the conditions applied when using the method may be tailored in such a way, that it is possible to collect one metal species at a time, or several metals at once, as desired.

The material and the system are stable in a highly versatile pH range. The material and the system are stable in a highly versatile pH range also in a flow through system. The material and the system withstand both acidic and basic conditions well. The material and the system may be utilized for collecting metals even at pH of 1 .

Contacting the material or the system with aqueous solution may be performed in a batch process, in which the material or the system is placed in a container containing the aqueous solution. Alternatively, contacting may be performed in a flow-through type of setup, in which the aqueous solution containing metals is allowed to flow through the material or the system. Allowing the solution to flow through the material or the system may be called filtering. Filtering through the material or the system may be effectuated by gravitational force or by pressure. The filtering may be performed for example by utilizing a syringe pump.

The metals collected by the material or the system may be released from the material or the system by contacting the material or the system with a solution having a pH of 3 or lower. When in anionic form, the metals collected by the material or the system may require alkaline conditions to be released from the material or the system. In that case, the metals collected by the material or the system may be released from the material or the system by contacting the material or the system with a solution having a pH of 1 1 or higher. Contacting the material or the system with the solution may be performed in a batch process, in which the material or the system is placed in a container containing the solution having a pH of 3 or lower, or the solution having a pH of 1 1 or higher. Alternatively, contacting may be performed in a flow-through type of setup, in which the solution having a pH of 3 or lower or the solution having a pH of 1 1 or higher is allowed to flow through the material or the system. The solution having a pH of 3 or lower may be for example an aqueous solution of HCI or H ₂ SO ₄ . The solution having a pH of 1 1 or higher may be for example an aqueous solution of ammonium hydroxide or sodium hydroxide. The pH required for the release is metal dependent. As understood by a person skilled in the art, pH may vary in a range of from 0 to 14.

Subsequently, an eluent comprising the metals released from the material or the system is recovered. The release of the metals may be called desorption. Contacting the material or the system with the solution having a pH of 3 or lower or the solution having a pH of 1 1 or higher causes the material or the system to be regenerated. When regenerated, the material or the system may be reused for collecting metals. The cycle of metal collecting and regeneration may be repeated for several tens of times without losing the material’s or system’s ability to collect metals from aqueous solutions.

The metals collected by the material or the system may be qualified and/or quantified using an analytical process or instrument. The analytical process or instrument may be for example an atomic absorption spectroscopy (AAS) process or instrument, an X-ray fluorescence (XRF) process or instrument or laser-induced breakdown spectroscopy (LIBS) process or instrument. The metals collected by the material or the system may be qualified and/or quantified directly from the material or the system. After release of the metals collected by the material or the system the metals may be qualified and/or quantified from the recovered eluent.

The material, system and/or method according to this specification have several applications in a variety of fields, wherein the collection of metals from aqueous solutions is needed. Those include, for example, purification of waste water, enrichment or recycling of rare metals or metals of high value, pre concentration of dilute solutions for analysis and collection of toxic metals.

Pre-concentration refers to a technique, in which analytes of sample, such as metals, are collected and concentrated into a smaller volume prior qualifying and/or quantifying the analytes. Pre-concentration serves to enhance the sensitivity of certain analytical methods. By pre-concentration, the analyte concentration may be increased and potentially interfering constituents in the sample removed. XRF provides a technique for direct multielemental analysis of solids. In the case of liquid samples, XRF has suffered from poor detection limits for the analytes. Thus, a solid phase material providing sufficient degree of pre concentration as well as compatible physical form to enable direct XRF analysis is needed.

The material or the system disclosed may serve as a system for pre concentration of metals. The metals collected by the material or the system may be qualified and/or quantified using an analytical process or instrument. The metals collected by the material or the system may be qualified and/or quantified directly from the material or the system. The analytical process or instrument may be an XRF process or instrument. The XRF instrument may be portable or handheld.

Quantification of metals collected by the material or the system may be utilized for evaluating the original metal concentration of the sample from which the metals have been collected. There is a correlation between the original metal concentration of the sample and for example XRF readings determined from the material or the system.

The material or the system may provide a pre-concentration system for portable XRF or other portable instrument in order to qualify and/or quantify metals in sub-ppm concentrations on field conditions.

Natural waters typically contain humic acids, which are major organic constituents of soil, peat and coal. The disclosed material, system and method are particularly useful for collecting metals from natural waters. When present in aqueous sample, the humic acids may interfere with the metal collection by binding the metals from the solution and preventing the binding to the material or the system. Flowever, acidic conditions may be utilized to release metals from humic acids. Thus, when using the material or system and the method disclosed, the metals may be collected from aqueous solutions having acidic pH, which prevents humic acids from binding the metals.

A material for collecting metals from an aqueous solution is provided. The material comprises porous silicon structure including a surface, wherein the surface comprises at least one of the following: carbon, silicon carbide, silicon oxycarbide; and a metal binding ligand covalently bonded to said surface. The metal binding ligand may be selected from functionalities containing at least one the following bond types: P-O, P=0, S-O, S=0, C-O, C=0, P-N, S-H. P-0 and/or P=0 may be part of a bisphosphonate structure. The material may be a metal collecting porous film or metal collecting porous particles.

A system for collecting metals from an aqueous solution is provided. The system comprises material as presented above. The material may be a metal collecting porous film or metal collecting porous particles, and the material may be arranged on a support or a substrate.

A method for collecting metals from an aqueous solution is provided. The method comprises contacting a material or a system as presented above with an aqueous solution that contains metals, allowing the metal binding ligands of the material or the system to interact with at least part of the metals contained in the aqueous solution, in order to at least part of the metals to be collected by the material or the system. The method may further comprise qualifying and/or quantifying the metals collected by the material or the system using an analytical process or instrument. The material or the system may be contacted with the aqueous solution that contains metals by allowing said aqueous solution to flow through the material or the system. The metals collected by the material or the system may be qualified and/or quantified directly from the material or the system. The metals collected by the material or the system may be released from the material or the system by contacting the material or the system with a solution having a pH of 3 or lower or with a solution having a pH of 1 1 or higher, for recovering an eluent comprising the metals released from the material or the system, whereby regenerating the material or the system. The metals collected by the material or the system may be qualified and/or quantified from the recovered eluent. The quantification of the metals collected by the material or the system may be used for quantification of the metal concentration of the aqueous solution. The analytical process or instrument may be an X-ray fluorescence process or instrument. The X-ray fluorescence instrument may be portable or handheld. Use of the material as presented above for collecting metals from an aqueous solution is provided.

Use of the system as presented above for collecting metals from an aqueous solution is provided.

Many variations of the material, system and method will suggest themselves to those skilled in the art in light of the description above. Such obvious variations are within the full intended scope of the appended claims.

Examples

The following examples are to be used for illustrative purposes only.

Example 1 : Preparation of porous silicon by electrochemical etching

P+ -type silicon wafer was electrochemically etched in electrolyte consisting 1 :1 mixture of hydrofluoric acid (HF, 38 - 40 %) and ethanol (99,6 %). The wafer acted as an anode and a platinum electrode was immersed in the electrolyte as a cathode. Current density of 30 mA/cm ² was applied for 40 min. Parallel pores on top of the wafer were formed. Afterwards, two 200 mA/cm ² current pulses lasting for 4 s were applied to remove the porous silicon (PSi) layer from the surface. PSi layers were collected and dried afterwards. Properties of the formed material were characterized by ish adsorption (Micromeritics Tristar III); porosity was 75 vol-%, surface area 230 m ² /g and pore diameter 9.7 nm. The PSi layers were ground in a planetary ball mill (Fritsch Pulverisette 7) and sieved to particle size (diameter) between 75 - 150 pm or under 25 pm.

Example 2: Passivation by carbonization

The PSi surface can be passivated with thermal carbonization in a tube oven. 0,5 g of PSi particles was placed to a quartz tube and oxygen was removed by nitrogen purging (1 l/min) for 30 min. Acetylene was introduced to the particles by purging the sample with a mixture of nitrogen/acetylene (1 l/min both) for 15 min. Subsequently, sample was heated at 500 °C under nitrogen/acetylene purge (1 l/min both) for 15 min. The sample was cooled to room temperature under nitrogen purge, after which acetylene was adsorbed on the surface using a mixture of nitrogen and acetylene gases both at flow rate of 1 l/min. The acetylene purge was then cut off and the sample was heated at 820 °C for 10 min, after which the sample was cooled down to room temperature under nitrogen atmosphere. This resulted in silicon carbide/carbon type of chemical structure with free radicals on the surface. The chemistry was confirmed with Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectrometry. The radicals may easily react with moisture and air forming silicon oxycarbides on the surface. The material was kept under inert atmosphere after the carbonization in order to prevent silicon oxycarbide formation, and in order to enable further functionalization.

Example 3: Carboxylic acid functionalization and stability

10 ml of neat undecylenic acid was transferred under nitrogen atmosphere onto the PSi particles carbonized as described in example 2. The sample was placed in the oven at 120 °C for overnight treatment. Afterwards, the sample was cooled down and washed with chloroform and subsequently with 1 :1 mixture of EtOFI and 1 M NaOH in order to remove excess undecylenic acid. The samples were further washed with 1 M HCI and water. The amount of undecylenic acid conjugated to the carbonized PSi surface was quantified to be about 2.3 % (w/w) with thermogravimetry (TG). The stability of the conjugation was studied by aging the sample in FhO, 1 M HCI and 1 M NaOH for 6 and 20 days and measuring the amount of conjugated undecylenic acid by TG afterwards. 79 % and 68 % of the conjugation was still present after 20 days of aging in H2O and 1 M HCI, respectively. Conjugation was stable for 6 days in 1 M NaOH. The stability was compared and found to be superior to undecylenic acid conjugation by hydrosilylation to freshly etched PSi surface without the carbonization step.

Example 4: Bisphosphonate functionalization

Bisphosphonates may be conjugated to carbonized PSi surface similarly to undecylenic acid in example 3. 250 mg of bisphosphonate (BP1 ) was dissolved in 10 ml of mesitylene and the solution was bubbled with nitrogen for 15 min in order to remove oxygen from the solution. Afterwards, under nitrogen atmosphere, the solution was transferred onto the PSi particles carbonized as described in example 2. The sample was placed in the oven at a temperature of 120 °C for overnight treatment. Afterwards, the sample was cooled down and washed with methanol and ethanol in order to remove excess BP1 molecules and to remove protective groups (SiMes) of the BP1 molecules. The BP1 amount conjugated to the carbonized PSi surface was quantified to be 2.5 % (w/w) with TG.

BP1

Example 5: Amine conjugation followed by bisphosphonate

functionalization (3-Aminopropyl)triethoxysilane (APTES) may be conjugated to the carbonized PSi. The PSi particle sample of example 2 was taken to out of the quartz tube to ambient air after the carbonization. The sample was then immersed in 1 :1 mixture of HF (38 - 40 %) and ethanol for 10 min. Subsequently, the particles were dried and immersed in 1 :1 :5 mixture of H ₂ q ₂ (20 %):HCI(37 %):H ₂ 0 with total volume of 10 ml for 15 min at 90 °C. The liquid phase treatments promoted formation of hydroxyl groups on the surface. The sample was washed with water and dried afterwards. After this, 2 ml of APTES was dissolved in 18 ml of isopropanol and bubbled with nitrogen for 20 min in order to remove oxygen from solution. The dried PSi sample was immersed in the solution for 4 h at 65 °C. The sample was washed with isopropanol and ethanol afterwards. This resulted in amine conjugated carbonized PSi particles verified by FTIR. The amount of APTES conjugated to the carbonized PSi surface was quantified to be 3.5 % (w/w) with TG.

200 mg of another bisphosphonate molecule (BP2) was dissolved in 10 ml of DMF. Amine conjugated PSi sample was immersed into the solution and the mixture was bubbled with nitrogen for 1 h, after which the temperature was elevated to 80 °C for 12 h. The sample was washed afterwards with DMF and 0.1 M HCI. The material was characterized with TG. Mass loss during TG measurement was 6.4 % for APTES conjugated bisphosphonate functionalized samples. Therefore BP amount conjugated to the APTES conjugated carbonized PSi surface was quantified to be 2.9 % (w/w).

BP2

Example 6: Removal of metals from aqueous solutions

50 mg of BP1 conjugated carbonized PSi particles of example 4 having a diameter between 75 - 150 pm were placed on a syringe filter to form a particle bed. 5 ml of standard solutions of various metals (1 ppm Cu, 1 ppm Zn) were filtered through the particle bed by gravity. Metal concentrations of the solutions were measured with atomic absorption spectrometer (AAS) before and after the filtration. The metals were adsorbed by the particles and thus removed from water effectively according to Table 1. The particles were regenerated with acid wash in a similar kind of gravitational filtration system. By using 0.1 M HOI the metals were shown to be desorbed effectively from the particles.

Table 1 : Removal efficiency of the metals from 5 ml sample run through 50 mg of BP1 conjugated PSi particles. Acid regeneration efficiency of the material, which indicates the desorption of the metals from the particles to the acid.

Example 7: Reusability

50 mg of BP1 conjugated carbonized PSi particles of example 4 having a diameter between 53 - 150 pm were placed on a syringe filter to form a particle bed. Following solutions were filtered through the particle bed by gravity: 10 ml of 10 ppm Cu, 5 ml of FteO, 5 ml of 0.1 M HCI and 5 ml of FteO. This filtering cycle was repeated 50 times. Metal concentrations of the solutions were measured with AAS before and after the filtration. Cu was adsorbed by the particles with efficiency of 80 % at the first cycles and was released by acid almost completely. The water samples between the adsorption and release had only negligible amounts of Cu. The adsorption efficiency remained between 80 - 60 % during the 50 cycles with only small decrease in the performance. The release of Cu to the acid solution behaved similarly showing reversibility of the adsorption. This shows that adsorption capacity of the particles withstands tens of repeated adsorption and release cycles in a flow through system even with acid washes, and that the conjugation is stable under these conditions.

Example 8: Pre-concentration

420 mg of BP1 conjugated carbonized PSi particles of example 4, having a diameter below 25 pm, 21 mg of sodium carboxymethyl cellulose (low viscosity) and 21 mg of polyacrylic acid (100 kDa) were mixed with 1.2 ml of water to form a slurry, which was spread on Whatmann Grade 3 filter paper with a doctor blade, which was set on 240 pm from the filter paper surface. The slurry with the filter paper was heated to 120 °C in vacuum for overnight to remove water and crosslink the polymers. This resulted in a smooth and fixed particle layer on top of the filter paper, forming a filter system. Circles having a diameter of 12 mm were cut from the filter system.

The circular filter systems were placed in 13 mm Swinnex syringe filter holders. 10 ml of tap water spiked with different Cu concentrations (1 , 0.5, 0.25 and

0.125 ppm) were run through the filter systems with a syringe pump. Flow rate was set to 0.1 ml / min. Afterwards, Cu concentrations were measured three times directly from the wet filter systems with portable XRF device (Olympus DP02000). The filter systems were dried and measured again with portable XRF. Results showed good linear correlation (R ² > 0.99) between the XRF measurement from both wet and dried filter and spiked Cu concentration of the tap water.

Example 9: Calibration and adsorption efficiency

Similar experiment (as in example 8) was repeated with lake water samples spiked separately with six different metals (Cu, Zn, Ni, Pb, U and Mn) with concentrations of approximately 50 ppb, 100 ppb, 200 ppb, 0.5 ppm, 1 ppm, 2 ppm 5 ppm and 10 ppm at flow rate of 1 ml/min. The metal concentration measured by XRF from filter system showed good linear correlation for each metal compared to the original metal concentration of the samples measured with ICP-MS technique. The results can be used as calibration equations for measuring metal concentrations of natural waters by pre-concentrating the metals to filter system following by XRF measurement.

ICP-MS was utilized to analyze the metal concentrations of water samples pumped through filter systems. The adsorption efficiency was then calculated for each metal and each concentration based on the ICP-MS results. The system removed typically 80 % or more of the metals from the solution.