WO2018038683A1

WO2018038683A1 - Co2-enabled regeneration and reuse of responsive adsorbents

Info

Publication number: WO2018038683A1
Application number: PCT/SG2017/050418
Authority: WO
Inventors: Xiao Hu; Yu Bai; Yen Nan LIANG
Original assignee: Nanyang Technological University
Priority date: 2016-08-26
Filing date: 2017-08-25
Publication date: 2018-03-01
Also published as: JP7129706B2; JP2019526440A; CN109715567A

Abstract

This invention relates to a switchable adsorption and desorption process triggered by carbon dioxide (CO₂) to remove contaminants such as heavy metal ions using CO₂-responsive adsorbent materials for water purification. The method comprising the steps of: (a) contacting a wastewater that comprises at least one contaminant with a polymeric adsorbent material, which comprises CO₂-responsive functional groups that are in a form capable of complexing the at least one contaminant; (b) separating the adsorbent-contaminant complexes from the wastewater; (c) bubbling CO₂ into the concentrated aqueous solutions of such complexes to regenerate the polymeric adsorbent material by releasing the contaminant from the polymeric adsorbent material, wherein the CO₂-responsive functional groups are in a CO₂-replete form; (d) separating the released contaminants from the polymeric adsorbent material. The pKb of the polymeric adsorbent material is from 7.5 to 9.0. The CO₂-responsive polymeric adsorbent materials disclosed herein can be complexed, released and be regenerated without the need for heat or acid to be introduced into the decomplexation or regeneration steps.

Description

C0₂-enabled regeneration and reuse of responsive adsorbents Field of Invention This invention relates to the switchable adsorption and desorption process triggered by C0₂ to remove contaminants such as heavy metal ions using C0₂-responsive adsorbent materials for water purification, particularly the recovery or regeneration of the adsorbents using C0₂. Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

In recent decades, the booming battery, electronics, mining and many other industries have resulted in widespread heavy metal pollution, including Cu, Pb and Cd, in agricultural soil, lakes, rivers and even groundwater. These toxic metallic elements are able to accumulate in organisms and cause cell malfunction, genetic disease and enzyme deficiencies even at a very low-level of exposure. Therefore, convenient and efficient techniques to remove and recycle the heavy metals are highly desirable for both environmental protection and economic interest.

Conventional methods to remove the heavy metal ions from wastewater include chemical precipitation, ion-exchange, adsorption, electrochemical treatment and membrane filtration. Although these methods have been used widely for decades, it is a big challenge to regenerate the various adsorbents (including ion-exchange resins) and to recover the metal effectively and efficiently. Regeneration of adsorbents and recovery of the heavy metals have begun to receive more attention in recent years. However, acid treatment is still the prevailing method, which often involves the use of concentrated acids, such as hydrochloric or nitric acid. The use of such concentrated acids may lead to secondary pollution and it is not cost effective. Therefore, it is highly desirable to find a new process to remove heavy metal ions that is both more eco-friendly and sustainable. C0₂ is economic, non-hazardous and easy to remove and can be used to switch various properties, including polarity, hydrophilicity, phase transition, gelation and crosslinking, of different types of materials such as solvents, particles, surfactants, polymers and hydrogels. The C0₂-switchable process is widely used in forward osmosis, polymerization and gelation control, C0₂ capture and detection applications. Recently, C0₂ has been used in microextraction to separate chelated cadmium and uranium complexes in a polarity switchable solvent.

Polyethyleneimine (PEI) has been used as an absorbent to extract a specific tocopherol from a mixture of tocopherols. However, the de-complexation of the tocopherol proved to be difficult and required either the use of a mineral acid or the addition of heat in addition to the bubbling of C0₂. This makes PEI impractical for large-scale industrial usage.

Summary of Invention

Disclosed herein is a C0₂-switchable adsorption and desorption process to remove contaminants, such as heavy metal ions, by C0₂-responsive materials. Ideally, the C0₂- responsive materials disclosed herein can be complexed, released and be regenerated without the need for heat or acid to be introduced into the decomplexation or regeneration steps.

In a first aspect of the invention, there is provided a method of removing contaminants from a wastewater, comprising the steps of:

(a) contacting a wastewater that comprises at least one contaminant with a polymeric adsorbent material, which comprises C0₂-responsive functional groups that are in a form capable of complexing the at least one contaminant, to form a first mixture;

(b) separating the first mixture to provide a first portion that is substantially free of the at least one contaminant and the polymeric adsorbent material and a second portion comprising the at least one contaminant complexed to the C0₂-responsive functional groups of the adsorbent material;

(c) bubbling C0₂ into the second portion of step (b) to release the at least one contaminant to form a second mixture comprising the at least one contaminant in an uncomplexed state and the polymeric adsorbent material where the C0₂-responsive functional groups are in a C0₂-replete form; and

(d) separating the second mixture to provide a preconcentrate solution portion comprising the at least one contaminant in water and a polymeric adsorbent material portion comprising the polymeric adsorbent material where the C0₂-responsive functional groups are in the C0₂-replete form, wherein

the pKb of the polymeric adsorbent material is from 7.5 to 9.0. In embodiments of the invention, the process may further comprise contacting the polymeric adsorbent material portion of step (d) with an inert gas or air to regenerate the C0₂- responsive functional groups of the polymeric adsorbent material from the C0₂-replete form to the form where the C0₂-responsive functional groups are capable of complexing the at least one contaminant, where:

(i) this step may be conducted as a separate step and the regenerated polymeric adsorbent material comprising C0₂-responsive functional groups that are in a form capable of complexing the at least one contaminant is recycled into step (a); or

(ii) this step may be conducted at the same time as step (a), such that the polymeric adsorbent material in step (a) is initially provided in the form where the C0₂- responsive functional groups are in the C0₂-replete state and the C0₂-responsive functional groups are converted into the form capable of complexing the at least one contaminant during the contacting step. In yet further embodiments of the invention:

(i) each step of the method may be conducted within a temperature range of from 0°C to 45°C;

(ii) the polymeric adsorbent material may comprise C0₂-responsive functional groups selected from one or more of the group consisting of amino, carboxylic acid, amidinyl, guanidinyl, pyridinyl, hydroxyl and ether groups, provided that when ether groups are present at least one other functional group selected from amino, carboxylic acid, amidinyl, guanidinyl, pyridinyl, and hydroxyl is also present, optionally wherein the functional groups comprise amino and may further comprise one or more functional groups selected from carboxylic acid, amidinyl, guanidinyl, pyridinyl, hydroxyl and ether groups;

(iii) the polymeric material may be selected from one or more of the group consisting of poly(dimethylaminoethyl methacrylate) (P DMA EM A), polyacrylic acid, polymethacrylic acid, homologues thereof, and copolymers thereof, for example, the polymeric material may be poly(dimethylaminoethyl methacrylate);

(iv) the polymeric material may be provided in a high chain-density form, such as in the form of a branched polymeric structure, a star-shaped polymeric structure, a dendritic polymeric structure (e.g. when the polymeric material is provided in the form of a star- shaped PDMAEMA structure having a molecular weight of from 50,000 to 200,000 Daltons, optionally wherein the molecular weight is from 90,000 to 150,000 Daltons, such as about 124,000 Daltons; or in the form of a linear PDMAEMA structure having a molecular weight of from 5,000 to 20,000 Daltons, optionally wherein the molecular weight is from 8,000 to 15,000 Daltons, such as about 11 ,000 Daltons); (v) the polymeric material may be provided in the form of a plurality of linear or branched polymeric chains attached to a nano-substrate material, optionally wherein the nano-substrate material is a molecular cage or is a nanoparticle, optionally wherein the nano-substrate material is a silsesquioxane and the plurality of linear or branched polymeric chains attached to the silsesquioxane are polymer chains selected from one or more of the group consisting of poly(dimethylaminoethyl methacrylate), polyacrylic acid, and polymethacrylic acid, optionally wherein the silsesquioxane is a polyhedral oligomeric silsesquioxane and the plurality of linear or branched polymeric chains attached to the silsesquioxane are poly(dimethylaminoethyl methacrylate) polymer chains;

(vi) the polymeric material may be attached to a micro- or macro-substrate material, optionally wherein the micro- or macro-substrate material is selected from one or more of the group consisting of porous or non-porous structures, e.g., gels, particles, membranes, mesh and fibres;

(vii) the pKb of the polymeric adsorbent material may be from 8.0 to 8.9, such as above 8.75;

(viii) separating steps (b) and/or (d) may involve one or more of a filtration process, a sedimentation process, a magnetic separation process or a centrifugation process, optionally wherein separating steps (b) and (d) involve a filtration process (e.g. an ultrafiltration process);

(ix) each step of the method may be conducted within a temperature range of from 10°C to 40°C such as from 20°C to 35°C;

(x) the at least one contaminant may be selected from one or more of the group consisting of heavy metal ions, organic molecules, dyes, solvent, and pesticides;

(xi) the pH of step (a) may be from 5.0 to 6.0, such as a pH of 5.5.

Drawings

Figure 1 depicts a general scheme of contaminant adsorption and subsequent regeneration of the adsorbent material.

Figure 2 shows the general scheme of ultrafiltration under N₂ or C0₂ purging: (a) the adsorption process under N₂ purging where the concentration of metal ions are [M]₀ and [M]N2 before and after filtration; and (b) The desorption process under C0₂ purging where the concentration of metal ions are [M]₀ and [M] co2 before and after filtration.

Figure 3 (a) shows the adsorption capacity Q_e of Cu²⁺, Cd²⁺, Zn²⁺, Pb²⁺, Cr³⁺ and Ni²⁺ on star-shaped PDMAEMA at pH 5.5 with no ionic background. The inset shows the Q_e value in mmol/g scale; and (b) shows the metal recovery percentage of Cu²⁺, Cd²⁺, Zn²⁺, Pb²⁺, Cr³⁺ and Ni²⁺ under C0₂ purging.

Figure 4 shows the scheme of the preparation of a) star-shaped and b) linear PDMAEMA via the ARGET-ATRP method.

Figure 5 shows the scheme of the competition between the polymer-metal chelation and the polymer-C0₂ protonation. Figure 6 (a) shows a typical adsorption capacity titration at pH 5.5 (square and triangle) and the calculated maximum adsorption capacity Q_e (circle). During the titration, the 10 mM Cu(N0₃)₂ solution was continuously injected into 10ml_ of 0.268 mg/mL star-shaped PDMAEMA solution under N₂ bubbling and monitoring of Cu-ISE and pH electrodes; (b) shows the maximum adsorption capacity Q_e of star-shaped and linear PDMAEMA at different pH values.

Figure 7 shows the pH-dependent adsorption isotherm of Cu²⁺ by a) linear PDMAEMA and b) star-shaped PDMAEMA at 25 °C. Figure 8 shows the calculated protonation percentage of PDMAEMA at different pH values.

Figure 9 shows the fast and reversible C0₂ triggered Cu²⁺ adsorption and desorption process. Figure 10 shows: (a) a representative scheme to adsorb, separate and preconcentrate the contaminants from wastewater by C0₂-switchable adsorption and desorption process; and (b) the exemplified scheme using water-soluble C0₂-responsive polymer for heavy metal removal by a C0₂-assisted LRP method. Figure 1 1 shows the different retention profiles under N₂ and C0₂ purging of a solution containing 100 ml_ of 0.09 mM Cu(N0₃)₂ and 3 mg/mL star-shaped PDMAEMA.

Description It has been surprisingly found that polymeric C0₂-responsive materials can be used to separate, recover and preconcentrate heavy metal ions, and it is possible to regenerate these C0₂-responsive materials using simple gas purges (e.g. C0₂ and air purges) instead of strong acids. This C0₂-thggered adsorption and desorption process may be particularly useful in the treatment and purification of wastewaters that contain heavy metal ions. This method is highly attractive due to its simplicity and eco-friendliness and is illustrated in Figure 1 , where 100 is the material substrate, 110 is a C0₂-responsive group, 120 is a contaminant and 130 is a proton or a carbamate. When used herein the term "heavy metal" may refer to the presence of heavy metal ions in a wastewater and a discussion on the removal of heavy metals from wastewater may be interpreted accordingly.

The current invention relates to a method of removing contaminants from a wastewater, comprising the steps of:

the pKb of the polymeric adsorbent material is from 7.5 to 9.0.

It will be appreciated that the polymeric adsorbent material used herein may be regenerated itself, leading to a reduction in side products and waste from the decontamination process. As such, the process may further comprise contacting the polymeric adsorbent material portion of step (d) with an inert gas or air to regenerate the C0₂-responsive functional groups of the polymeric adsorbent material from the C0₂-replete form to the form where the C0₂-responsive functional groups are capable of complexing the at least one contaminant, where:

(i) this step is conducted as a separate step and the regenerated polymeric adsorbent material comprising C0₂-responsive functional groups that are in a form capable of complexing the at least one contaminant is recycled into step (a); or (ii) this step is conducted at the same time as step (a), such that the polymeric adsorbent material in step (a) is initially provided in the form where the C0₂- responsive functional groups are in the C0₂-replete state and the C0₂-responsive functional groups are converted into the form capable of complexing the at least one contaminant during the contacting step.

The methods disclosed above may be particularly suitable for use at industrially useful temperature ranges, such as a temperature range of from 0°C to 45°C (e.g. 10°C to 40°C, such as from 20°C to 35°C).

As noted above, the C0₂-responsive materials used in this method are (or at least comprise) a polymeric component in the form of a polymeric adsorbent material that comprises C0₂- responsive functional groups. Suitable C0₂-responsive functional groups that may be mentioned herein include, but are not limited to amino, carboxylic, amidine, pyridine, guanidine, hydroxyl, ether, or other nitrogen- and oxygen-rich groups. It will be appreciated that certain of these functional groups may act alone (e.g. amino), while others may need to be used in conjunction with one or more of the other functional groups mentioned herein (e.g. ether functional groups - where an oxygen atom is bonded to two alkyl groups, such as Ci_₃ alkyl groups). In certain embodiments of the invention, amino functional groups may be used in conjunction with one or more of the other functional groups mentioned herein. Without wishing to be bound by theory, it is believed that the presence of amino functional groups may enhance the C0₂-responsive nature of the other functional groups.

Any suitable contaminant that can be complexed to the C0₂-responsive materials may be subject to the process described herein. Contaminants that may be mentioned herein include, but are not limited to heavy metal ions, organic molecules, dyes, solvent, pesticides and combinations thereof.

Suitable polymeric adsorbent materials comprising C0₂-responsive functional groups that may be mentioned herein include, but are not limited to poly(dimethylaminoethyl methacrylate) (PDMAEMA), polyacrylic acid, polymethacrylic acid, homologues thereof, and copolymers thereof. A particular polymeric adsorbent material that comprises C0₂- responsive functional groups that may be mentioned herein is poly(dimethylaminoethyl methacrylate) (PDMAEMA). A suitable form of PDMAEMA may be a linear PDMAEMA structure having a molecular weight of from 5,000 to 20,000 Daltons (e.g. from 8,000 to 15,000 Daltons, such as about 1 1 ,000 Daltons). As discussed hereinbelow in the examples, it may be advantageous in certain embodiments to provide the polymeric adsorbent material in high chain-density form. Suitable high chain- density forms include but are not limited to a branched polymeric structure, a star-shaped polymeric structure, and a dendritic polymeric structure. For example, when the polymeric adsorbent material is PDMAEMA, the PDMAEMA may be provided in the form of a star- shaped structure having a molecular weight of from 50,000 to 200,000 Daltons (e.g. from 90,000 to 150,000 Daltons, such as about 124,000 Daltons).

It will be appreciated that the polymeric adsorbent materials may themselves be attached to a suitable substrate material that may help facilitate separation (e.g. onto a porous substrate, such as a ceramic or zeolite material that is easy to remove and replace), or they may comprise additive materials (e.g. magnetic materials) within the polymeric matrix that may assist in facilitating separation.

Additionally or alternatively, the polymeric material may be provided in the form of a plurality of linear or branched polymeric chains attached to a nano-substrate material. For example, the nano-substrate material may be a molecular cage or is a nanoparticle. A particular nano-substrate that may be mentioned herein is a silsesquioxane, which is used to anchor a plurality of linear or branched polymeric chains. In certain embodiments that may be mentioned herein, the plurality of linear or branched polymeric chains attached to the silsesquioxane may be polymer chains including, but not limited to, poly(dimethylaminoethyl methacrylate), polyacrylic acid, and polymethacrylic acid, copolymers thereof and combinations thereof. In particular embodiments of the invention that may be mentioned herein, the silsesquioxane may be a polyhedral oligomeric silsesquioxane and the plurality of linear or branched polymeric chains attached to the silsesquioxane may be poly(dimethylaminoethyl methacrylate) polymer chains, which may be referred to herein as a PDMAEMA having a star-shaped structure in certain embodiments and examples. Thus, in certain embodiments of the invention, there may be provided a polyhedral oligomeric silsesquioxane with a linear or branched PDMAEMA units, where the resulting polymeric material has a molecular weight of from 50,000 to 200,000 Daltons (e.g. from 90,000 to 150,000 Daltons, such as about 124,000 Daltons).

In yet further alternatives, the polymeric material may be attached to a micro- or macro- substrate material. Suitable micro- or macro-substrate materials may be selected from one or more of the group including, but not limited to, porous or non-porous structures, e.g., gels, particles, membranes, mesh and fibres. When used herein, the term "nano-substrate material" relates to a particulate material that has a particle size of less 100 nm (e.g. from 1 nm to 99.99 nm). When used herein, the term "micro-substrate material" relates to a particulate material that has a particle size of from 100 nm to less than 2,500 nm (e.g. from 100 nm to 2,499.99 nm). When used herein, the term "macro-substrate material" may relate to a particulate material that has a particle size of from 2,500 nm to 10,000 nm or to a substrate material in the form of a membrane, a mesh or fibres. It will be appreciated that the nano- and micro-substrate materials may be incorporated into larger substrate materials, such as membranes, meshes, gels and fibres.

Turning to Figure 1 , the substrate material 100, or the polymeric backbone of the polymeric adsorbent material, comprises C0₂-responsive functional groups 110 that can complex to a contaminant 120 (e.g. a heavy metal ion). The resulting complexes may serve as media for contaminant removal and can be easily separated from the purified water in concentrated forms, leading to the purification of the bulk of the wastewater. When C0₂ is purged into concentrated aqueous solutions of such complexes the release of metals (i.e. metal ions) and/or other contaminants will occur, as the C0₂-responsive functional groups in the adsorbent material are protonated or otherwise functionalised by C0₂ 130 (e.g. the formation of a carbamate group). It is then possible to separate the released metal ions and/or other contaminants from the adsorbent material, resulting in the recovery of the metal ions and/or other contaminants in concentrated form. As will be appreciated, the separated polymeric adsorbent material may then be recycled.

As the separated polymeric adsorbent material is provided in a form where the C0₂- responsive functional groups have been exposed to C0₂, they are in a C0₂-replete form. That is, the C0₂-responsive functional groups are protonated or otherwise functionalised by C0₂ in some way (e.g. formation of a carbamate). In order to reverse this functionalisation and return the C0₂-responsive functional groups into a form that is capable of complexing with a contaminant material, the polymeric adsorbent material may simply be treated by purging it with an inert gas (e.g. N₂), or even air (which contains more than 78% of inert gases), to drive out C0₂ and regenerate the C0₂-responsive functional groups for reuse. It will be appreciated that this regeneration process may be accomplished most easily while the polymeric adsorbent material is in some liquid medium, such as water. Depending on the separation method used to separate the polymeric adsorbent material, it may be provided in a concentrated aqueous solution or in a dry or semi-dry form. In the latter case, a liquid medium (e.g. water) may be added to help facilitate the regeneration process. This regeneration process may be accelerated by low-grade heating including waste heat. The adsorption, C0₂-driven desorption, and inert gas driven regeneration processes may be affected by both the intrinsic conditions and the extrinsic parameters. The intrinsic factors include the types of contaminants, types of functional groups and their local chemical environments (e.g., polymer chain structure or surface grafted states). The extrinsic parameters include temperature, gas pressure and flow rates, contaminant concentration and background ionic strength.

One important factor for the proper functioning of the process is the ability of the polymeric material to complex and release both the contaminant and C0₂. In this respect, it is believed that polymeric materials that have a pKb in the range of from 7.5 to 9.0 are particularly suitable. Without wishing to be bound by theory, it is believed that polymeric materials having this property (and suitable C0₂-responsive functional groups) are able to complex and decomplex contaminants and C0₂ within an industrially useful temperature range, such as from 0°C to 45°C (e.g. 10°C to 40°C, such as from 20°C to 35°C). As will be understood, the ability to operate the process at ambient temperatures enables the process to be used industrially at low cost without the need to heat water to elevated temperatures (e.g. over 45°C) in order to effect complexation or decomplexation of contaminants and/or C0₂. In addition, the process does not require the introduction of further contaminant materials (e.g. mineral acids) to effect decomplexation, it simply requires the use of a gas that comprises a substantial proportion of an inert gas, such as air itself. Particular pKb values that polymeric materials of the invention may have include pKb values of from 8.0 to 8.9, such as a pKb value of about 8.75.

In certain embodiments of the invention, it may be useful to control the pH of the first step of the process. In other words, it may be useful to control or monitor the pH of step (a) as discussed above to keep it within the range of from pH 5.0 to pH 6.0, such as pH 5.5. Without wishing to be bound by theory, it is believed that maintaining the pH within this range may assist in the complexation of the contaminant(s) with the C0₂-responsive functional groups of the adsorbent polymeric material. A general method 1000 to utilize the C0₂-switchable adsorption and desorption by a C0₂- responsive material for water purification applications is depicted in Figure 10a. Wastewater 1010 is contacted with a polymeric adsorbent material in a contacting step 1020, which material can be in the form of a water-soluble polymer, hydrogel beads or surface functionalized particles, fibres, membranes or other substrates which contains the C0₂- responsive groups and structures. It will be appreciated that any suitable form of the polymeric adsorbent material described hereinbefore may be used. If the polymeric adsorbent material has been previously used, the contacting step may also involve the introduction of a gas that comprises a substantial amount of an inert gas (e.g. nitrogen argon or air) in a purge 1030 to regenerate the C0₂-responsive groups of the polymeric adsorbent material into a form that can complex with the contaminant. The contaminant can be heavy metal ions, organic dyes, pesticides, proteins and the like. Once the purge 1030 and contacting steps 1020 have been conducted for a sufficient amount of time, the treated water 1050 may be separated from the adsorbent polymeric material. The separation method 1040 of the adsorbed contaminant from purified water can be filtration, centrifugation, sedimentation, magnetic collection or other processes according to the form, morphology and properties of the adsorbent polymeric material. The adsorbent polymeric material can then be subjected to a decomplexation step 1060 by the introduction of C0₂ to release the preconcentrated contaminants. The resulting mixture may then be separated 1070 to provide preconcentrated contaminants 1080 and a polymeric adsorbent material with C0₂- replete functional groups that can be recycled for use in step 1020 and which step must be accompanied by inert gas purge step 1030 to enable the C0₂-responsive functional groups to return to a state where they can complex with a contaminant.

The use of PDMAEMA to remove Cu²⁺ with membrane filtration as the separation method 1100 is shown in Figure 10b as an example of the application and viability of this process. As before, wastewater 1110 containing Cu²⁺ is contacted with PDMAEMA in step 1 120, which may also be accompanied by a purge 1130 with nitrogen or air if the PDMAEMA has been previously been used. Following a suitable contacting time, the separation step 1 140 is conducted using an ultrafiltration membrane. This ultrafiltration process was used because the polymer can be retained by the ultrafiltration membrane 1145, which was selected to have an appropriate molecular weight cut-off value, while allowing the treated water filtrate 1150 to be collected separately. The resulting retentate was then subjected to treatment with C0₂ in purge step 1160 to provide a mixture of the polymer in a C0₂-replete state and the free metal ions. This mixture was again subjected to an ultrafiltration step 1170, such that the polymer was retained as retentate, while the free metal ions can pass through the ultrafiltration membrane 1175 (again selected to have an appropriate molecular weight cut-off value) as a preconcentrated mixture 1180, which is known as the liquid-phase polymer-based retention (LPR). Subsequently, the polymer may be recycled and used in the process once again where the inclusion of purging step 1130 is now mandatory.

In the process above, the metal ions can be retained by the ultrafiltration membrane (1145) when they are coordinated with the polymer, but desorbed and filtered when C0₂ is purged into the solution. It will be appreciated that the above processes may take place using the same apparatus and so ultrafiltration membranes 1145 and 1 175 may be the same membrane. However, the process may be conducted in a continuous/batch flow apparatus, in which case, membranes 1145 and 1 175 may be different. Compared to conventional LPR, the adsorption and desorption of the metal ions can be conveniently controlled by C0₂ and inert gas purging.

In embodiments herein, the word "comprising" may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word "comprising" may also relate to the situation where only the components/features listed are intended to be present (e.g. the word "comprising" may be replaced by the phrases "consists of" or "consists essentially of"). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word "comprising" and synonyms thereof may be replaced by the phrase "consisting of or the phrase "consists essentially of" or synonyms thereof and vice versa.

Examples

Preparation of linear and star-shaped poly(2-dimethylaminoethyl methacrylate) Star-shaped and linear PDMAEMA polymers can be prepared via several routes and, in this example, they were prepared using an activator regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP). The preparation of the 8-arm star polymer based on polyhedral oligomeric silsesquioxane (POSS) was carried out according to a previous report (Bai, Y., et al., Colloid and Polymer Science, 201 1. 290(6): p. 507-515). When used in the examples below, reference to a "star-shaped PDMAEMA polymer" means the 8-arm star polymer based on polyhedral oligomeric silsesquioxane (POSS).

As a typical polymerization procedure, the linear initiator ethyl 2-bromoisobutyrate or the 8- arm POSS initiator, DMAEMA, CuBr₂, 1 , 1 ,4,7, 10, 10-Hexamethyl-triethylenetetramine (HMTETA) and tin(ll) 2-ethylhexanoate (Sn 2-Eh) are dissolved in appropriate amount of 2- propanol in a sealed flask with molar ratio of [Br groups in initiator]:[DMAEMA]:[CuBr₂]:[HMTETA]:[Sn 2-Eh] = [1]:[100]:[0.05]:[0.25]:[5] and stirred for 6 to 8 hours at 50 °C. The solvent was removed after polymerization and the mixture was re- dissolved in acetone and passed through a basic Al₂0₃ column, followed by dialysis in a Spectra/Por™ 8kDa tubing against deionised (Dl) water. Centrifugation was used to remove precipitants and the solution was freeze-dried to obtain the polymer. The polymer was dissolved in 0.1 M NaN0₃ solution and adjusted to the desired pH for other experiments. The star-shaped (POSS-PDMAEMA) and linear PDMAEMA used herein to demonstrate the cupric ion adsorption and desorption are prepared by ARGET-ATRP polymerization (Figure 4) because the controlled living polymerization can provide excellent molecular weight and architecture control. Compared to conventional ATRP, the ARGET-ATRP uses much less organic ligand and metal catalyst which cause less polymer purification problems and less interference on the cupric ion concentration measurements in later experiments. The number average molecular weight M_n of the polymer was ca. 124,000 Daltons for the star-shaped PDMAEMA and 11 ,000 Daltons for the linear PDMAEMA, as determined by the ¹ H NMR integration method.

Measurement of Cu²⁺ concentration and pH

The concentration of Cu²⁺ was determined using a Thermo Scientific Orion cupric ion- selective electrode 9629BNWP (Cu-ISE) with a Metrohm Titrando 905 titration system at 25 °C. The Cu-ISE electrode was calibrated with freshly prepared standard Cu(N0₃)₂ solutions with the concentration 10^"2, 10^"3, 10^"4, 10^"5 and 10^"6 M and 0.1 M NaN0₃ as the ionic backgrounds which stabilize the ionic strength and result in constant and accurate acquisition.

The pH value was measured by a Metrohm 6.0262.100 electrode with Metrohm Titrando 905 titration system. The electrode was calibrated before measurement.

Example 1

As one of the most common heavy metal pollution species widely found in wastewater sources from electronics, paint, wiring and printing industries, copper was chosen to demonstrate the C0₂ assisted adsorption and desorption. Based on the polymer-metal complexation mechanism, many other multivalent chelatable heavy metals, such as Cu²⁺, Cd²⁺, Pb²⁺, Pb⁴⁺, Cr³⁺, Co²⁺, Ni²⁺, Zn²⁺ etc. are also applicable for this process (as shown below in Example 3).

C0₂-assisted Cu²⁺ removal and adsorbent regeneration process are described. Linear and star-shaped PDMAEMA were separately dissolved in 0.1 M NaN0₃ solution and the pH was adjusted to ca. 5.5. The Cu²⁺ solution, used here as a substitute for a wastewater, was prepared by dissolving Cu(N0₃)₂-6H₂0 in 0.1 M NaN0₃. The 0.1 M NaN0₃ is used as ionic background to stabilize the solution and provide an accurate measurement of the Cu²⁺ concentration in the experiment and is not necessary during actual application. The PDMAEMA and the Cu(N0₃)₂ solutions were mixed together and bubbled with N₂ during filtration through an ultrafiltration membrane assisted by vacuum suction to obtain the purified filtrate where the Cu²⁺ ions were removed based on the ratio of polymer to copper in the cell. The remaining retentate, which was about 20% of the initial volume, was bubbled with C0₂ and filtered again through the ultrafiltration membrane (NMWL = 5kDa) to obtain a preconcentrated Cu²⁺ ion solution as the filtrate, and a retentate PDMAEMA solution that can be reused in another cycle of mixing with Cu(N0₃)₂ solution in a repeat of the above processes. In this example, PDMAEMA is the adsorbent used to preconcentrate and remove Cu²⁺ and this process is assisted by ultrafiltration under control of C0₂ and N₂ which leads to regeneration and reuse of the polymer adsorbent. Therefore, this is a simple, fast, and eco- friendly approach associated with suitable separation technique for wastewater treatment and/or for heavy metal extraction. Example 2

As a typical procedure to determine the adsorption capacity, 1 mmol/L Cu(N0₃)₂ solutions were continuously injected into 10 mL 0.268 mg/mL PDMAEMA solution, which had been adjusted to the desired pH value prior to the injection. The Cu²⁺ concentration and pH were continuously measured by Cu-ISE and pH electrode. During the process, N₂ gas was continuously bubbled into the solution to stabilize the pH and to avoid the influence of atmospheric C0₂. The adsorption capacity (Q_e) is calculated according to the following equation:

_ (C₀ - C_V)V

m where C₀ is the total copper concentration, C_v is the measured free Cu²⁺ concentration, V s the solution volume and m is the mass of adsorbent polymer.

The adsorption isotherm was determined by adding designed amount of Cu(N0₃)₂ solution into a fixed amount of polymer solution under N₂ purging at 25 °C. Thus, different polymer/Cu ratios can be achieved and the corresponding Cu²⁺ concentrations were measured by Cu-ISE during the addition.

The adsorption capacity Q_e of Cu²⁺ by PDMAEMA is measured by addition of Cu(N0₃)₂ solution into PDMAEMA solution at an adjusted pH monitored by Cu-ISE. Because the Cu- ISE is only sensitive to free Cu²⁺ but not complexed, hydrolyzed or precipitated Cu/copper ions, the difference between measured concentration and calculated total concentration of Cu²⁺ can be considered as the adsorbed amount. The maximum adsorption capacity was found when the difference value is constant at continuous addition of Cu(N0₃)₂ solution as shown in Figure 6a. Using the same method, the adsorption capacity of star and linear PDMAEMA at different pH are characterized and shown in Figure 6b. The adsorption capacity at pH 5.5 is greater than pH 5.0 and pH 6.0 because too low a pH value will increase the protonation of the polymer's amino pendant groups, thereby inhibiting complexation, and a high pH value will promote the hydrolysis of copper. The star-shaped PDMAEMA used in this example exhibited an adsorption capacity of 145.1 mg/g at pH 5.5.

The star-shaped polymer exhibits a much higher adsorption capacity than linear polymers with a similar arm length, because the Cu²⁺ ions often form planar or tetrahedral 4- coordination complexes with weak chelators, while the polymer arms in a star-shaped polymer are spaced close enough together that two or more arms may chelate to the same Cu²⁺ ion, enabling the formation of multiple intra-molecular coordination bonds. It will be appreciated that the copper ions used in this example are provided as a model system and that similar chelation would be expected with respect to other heavy metal ions. For linear polymers, the inter- and intra-chain coordination are hindered by chain rigidity and electrostatic repulsion. This capacity difference between star-shaped and linear polymers indicates the importance of the molecular architecture on chain density and inter-chain interaction, which can thus affect the chelating adsorption capability. Therefore, it can be expected that a densely grafted surface or a hyper-branched molecule which possess high chain density will also exhibit high adsorption capacity.

Example 3

In the following example, several other metal ions, in addition to Cu²⁺, were tested. The metal ions were Cd²⁺, Zn²⁺, Pb²⁺, Cr³⁺ and Ni²⁺, and were evaluated with the ultrafiltration process with no NaN0₃ ionic background to simulate the practical wastewater treatment condition.

In the process, all of the metal salts are nitrates to avoid the formation of anion coordination bonds. As a typical procedure, the metal nitrate salt and star-shaped PDMAEMA (POSS- PDMAEMA) were dissolved in Dl water with the concentration of 0.4 mmol/L and 27.3 mg/L, respectively to form two identical solutions. The pH of each solution was adjusted to 5.5. The excess amount of metal over polymer, where the molar ratio of [metal]:[DMAEMA units] was c.a. 2.5: 1 , ensures that the maximum adsorption capacity is achieved at equilibrium. One of the identical solutions was purged by N₂ and the other by C0₂ before filtration to reach the adsorption or desorption equilibrium, respectively. The filtration was also conducted under gas purging and with the assistance of vacuum suction.

The scheme of the ultrafiltration is illustrated in Figure 2 with the indication of the metal concentrations before and after filtration. The metal concentrations are measured by microwave plasma atomic emission spectrometry (MP-AES) and used to calculate the adsorption capacity and metal recovery percentage by the following equations:

„ . ([M]p - [M]_N2) x V

Adsorption capacity =

^MPDMAEMA

[^M]c02

Metal recovery percentage = — ; where the [M]₀, [M]N2, [M]CO2 are the metal concentrations before filtration, after filtration under N₂ and after filtration under C0₂, respectively. V and ΠΊ_ΡΟΜΑΕΜΑ are the solution volume and mass of the star-shaped PDMAEMA in the solution. The calculated adsorption capacities Q_e are shown in Figure 3b and the inset normalizes the unit to mmol/g scale.

Compared to conventional adsorbents, the star-shaped PDMAEMA exhibits much higher or highly competitive Q_e. The metal recovery percentage of each metal is shown in Figure 3b, the values exceeding 100% are due to experimental error resulted from water evaporation during gas purging and vacuum suction. The six metal species exhibit almost the same concentration before and after filtration under C0₂ purging, indicating the capability of C0₂ to fully desorb all the tested metal species from PDMAEMA. In conclusion, the high Q_e and the full recovery of the different metal ions prove that the C0₂-enabled regeneration principle is promising to be universally applicable for many pollutant species.

Example 4

The effect of polymer to copper ratio on adsorption was evaluated at different pHs as shown in Figure 7. The different mass ratios of polymer to copper are achieved by the addition of Cu(N0₃)₂ solution into a fixed amount of polymer solution, and the adsorption percentage is considered as the difference between added copper and measured free Cu²⁺ amount. The same trend found in the adsorption capacity tests is followed here, as the star-shaped PDMAEMA (Fig. 7b) exhibits stronger affinity to Cu²⁺ than the linear polymer (Fig. 7a) and the pH also shows a 5.5 > 5.0 > 6.0 trend, indicating the best working pH for adsorption is around 5.5.

PDMAEMA has amino pendant groups that can be protonated by C0₂ in an aqueous solution. Assuming that all the DMAEMA units are in an equivalent chemical environment, the base dissociation constant of PDMAEMA can be calculated based on the following equilibrium:

P + H₂0 ≠P-H⁺ + OH^~

[P-H⁺] [OH^~] [OH-y

[P] [P]₀ - [OH-] where P is a repeating unit DMAEMA, [P]₀ is the total concentration of DMAEMA repeating units, [P-H⁺] is the concentration of protonated DMAEMA repeating units and [P] is the concentration of non-protonated DMAEMA repeating units. By assuming the [P-H⁺] equals to [OH^"] based on the equilibrium equation, the base dissociation constant of DMAEMA k_b can be calculated to be 1 .78* 10^"9 as the pH of a 4.94 mg/mL star-shape PDMAEMA solution was measured to be 8.87. Consequently, the relationship of pH and the percentage of protonation of PDMAEMA can be calculated and described in the following equation:

\Ρ-Η⁺λ k

Protonation% = _{Γ Ί} x 100% = r τ ^x 100%

[P]₀ k_b + [OH-]

Based on the equation, the protonation percentage of DMAEMA repeating units at different pH can be plotted in Figure 8. The DMAEMA repeating units are not protonated at pH higher than 7.0, and are almost fully protonated below pH 3.5. It must be noted that about 95% of the polymer are protonated at pH 4.0 which is around the pH value of a C0₂-purged aqueous solution at 1 atm. The protonation evaluation based on pH indicates that C0₂ is able to protonate PDMAEMA in an aqueous solution to trigger metal ion desorption. If the C0₂ pressure is increased to higher than 1 atm, e.g. , in an industrial plant, the pH can be made even lower to close to 3.0. In such a case, a higher degree of protonation, faster protonation and metal recovery speed can be anticipated. Example 5

To demonstrate the C0₂-switched adsorption and desorption of Cu by PDMAEMA, 10.0 ml_ of 3.52 mg/mL star-PDMAEMA solution, which is adjusted to pH 5.5, is mixed with 10 mL 1 mM Cu(N0₃)₂ solution in a beaker equipped with magnetic stirring. N₂ and C0₂ are alternatively bubbled into the solution through a needle at 25 °C to switch between the adsorption and desorption of the metal ions. The concentration of Cu²⁺ and pH are measured by Cu-ISE and pH electrode. The C0₂ triggered Cu²⁺ adsorption and desorption process was demonstrated by bubbling N₂ and C0₂ alternatively into a mixed solution containing the star-shaped PDMAEMA and Cu(N0₃)₂ under the monitoring of the Cu-ISE and pH electrode as shown in Figure 9. At first, N₂ is bubbled to reach adsorption equilibrium and remove the atmospheric C0₂ from the solution. When C0₂ is bubbled into the solution, the pH value is quickly reduced and the percentage of free Cu²⁺ increases close to 100% in several seconds, indicating a very fast process where the DMAEMA units are protonated and dissociate from the polymer-metal complex, and almost all of the chelated Cu²⁺ ions are released as free ions. The re- adsorption of Cu²⁺ is achieved when N₂ is purged into the solution again for a few minutes to remove the dissolved C0₂, and the percentage of free Cu²⁺ ions reduced to the same as that in the initial equilibrated solution. Although the processes of protonation by C0₂ and deprotonation by N₂ have previously been reported for PDMAEMA as draw solutes (e.g. see Cai, Y., et al., Chemical Communications, 2013. 49(75): p. 8377-8379), there is no report that this material can be used as a regenerable adsorbent for heavy metal or contaminant removal in water treatment. In favor of the single phase process, the adsorption can be achieved within ca. 2 minutes and the adsorption-desorption cycle can be completed within ca. 4 minutes which is much faster than other adsorbents. As shown in Figure 9, the adsorption-desorption cycles can be repeated many times and the slight drifting of the value of measured Cu²⁺ concentration is due to solvent evaporation which occurs due to gas bubbling through the solvent for a long time, leading to a more concentrated solution. In this demonstration, C0₂ exhibits its capability to switch the adsorption and desorption cycles within a few minutes, which is much faster, more convenient and eco-friendly than the conventional metal ion recovery from normal hydrogels or functionalized membranes and other bi-phase systems by acids or strong chelators which often takes tens of minutes or even several hours, or need additional bi-phase separation steps. Example 6

To demonstrate the C0₂ assisted Cu separation by liquid-phase polymer-based retention, 90 mL of 0.1 mM Cu(N0₃)₂ solution is mixed with 10ml_ 30.0 mg/mL star-shaped PDMAEMA and adjusted to pH 5.5. The mixed solution is filtered through a Millipore polyethersulfone ultrafiltration membrane with the nominal molecular weight limit (NMWL) of 5kDa under either N₂ or C0₂ bubbling through a needle. The volumes of the filtrate solution are measured to determine the filtration factor Z, which is the volume ratio of filtrate to cell solution, and the concentration of Cu²⁺ in the filtrate are measured by Cu-ISE after adjusting to ca. pH 1 to 2 by HN0₃ to make sure all the Cu²⁺ are not hydrolyzed or adsorbed. The volume and concentration of Cu²⁺ of the retentate are calculated from the volume and amount of Cu²⁺ in the filtrate.

C0₂-assisted LPR was demonstrated by filtration of two identical 100 mL mixed solutions of Cu(N0₃)₂ and star-shaped PDMAEMA having the same concentration through a 5kDa ultrafiltration membrane under either N₂ or C0₂ bubbling to obtain the retention profiles as shown in Figure 11. When N₂ is purged into the solution, most metal ions are adsorbed by the polymer and retained by the membrane, while under C0₂ purging the Cu²⁺ ions are free and can be filtered through the membrane. The differential retention achieved by N₂/C0₂ gas bubbling proves the capability to remove and preconcentrate Cu²⁺ from wastewater by the C0₂-assisted LPR method. In this example, the C0₂ and N₂ are directly bubbled into the solution through a needle under atmosphere pressure, while a much lower pH and faster adsorption/desorption speed can be expected in an industrial plant which has higher pressure and faster purging rate.

Claims

1. A method of removing contaminants from a wastewater, comprising the steps of:

the pKb of the polymeric adsorbent material is from 7.5 to 9.0.

2. The method according to Claim 1 , wherein the process further comprises contacting the polymeric adsorbent material portion of step (d) with an inert gas or air to regenerate the C0₂-responsive functional groups of the polymeric adsorbent material from the C0₂-replete form to the form where the C0₂-responsive functional groups are capable of complexing the at least one contaminant, where:

(i) this step is conducted as a separate step and the regenerated polymeric adsorbent material comprising C0₂-responsive functional groups that are in a form capable of complexing the at least one contaminant is recycled into step (a); or

(ii) this step is conducted at the same time as step (a), such that the polymeric adsorbent material in step (a) is initially provided in the form where the C0₂- responsive functional groups are in the C0₂-replete state and the C0₂-responsive functional groups are converted into the form capable of complexing the at least one contaminant during the contacting step.

3. The method according to Claim 1 or Claim 2, wherein each step of the method is conducted within a temperature range of from 0°C to 45°C.

4. The method according to any one of the preceding claims, wherein the polymeric adsorbent material comprises C0₂-responsive functional groups selected from one or more of the group consisting of amino, carboxylic acid, amidinyl, guanidinyl, pyridinyl, hydroxyl and ether groups, provided that when ether groups are present at least one other functional group selected from amino, carboxylic acid, amidinyl, guanidinyl, pyridinyl, and hydroxyl is also present.

5. The method according to Claim 4, wherein the polymeric material is selected from one or more of the group consisting of poly(dimethylaminoethyl methacrylate) (PDMAEMA), polyacrylic acid, polymethacrylic acid, homologues thereof, and copolymers thereof.

6. The method according to Claim 5, wherein the polymeric material is poly(dimethylaminoethyl methacrylate).

7. The method according to any one of Claims 4 to 6, wherein the polymeric material is provided in a high chain-density form

8. The method according to Claim 7, wherein the polymeric material is provided in the form of a branched polymeric structure, a star-shaped polymeric structure, and a dendritic polymeric structure.

9. The method according to Claim 8, wherein the polymeric material is provided in the form of a star-shaped PDMAEMA structure having a molecular weight of from 50,000 to 200,000 Daltons, optionally wherein the molecular weight is from 90,000 to 150,000 Daltons, such as about 124,000 Daltons.

10. The method according to Claim 6, wherein the polymeric material is provided in the form of a linear PDMAEMA structure having a molecular weight of from 5,000 to 20,000 Daltons, optionally wherein the molecular weight is from 8,000 to 15,000 Daltons, such as about 11 ,000 Daltons.

11. The method according to Claim 8 or Claim 9, wherein the polymeric material is provided in the form of a plurality of linear or branched polymeric chains attached to a nano- substrate material, optionally wherein the nano-substrate material is a molecular cage or is a nanoparticle.

12. The method according to Claim 1 1 , wherein the nano-substrate material is a silsesquioxane and the plurality of linear or branched polymeric chains attached to the silsesquioxane are polymer chains selected from one or more of the group consisting of poly(dimethylaminoethyl methacrylate), polyacrylic acid, and polymethacrylic acid, optionally wherein the silsesquioxane is a polyhedral oligomeric silsesquioxane and the plurality of linear or branched polymeric chains attached to the silsesquioxane are poly(dimethylaminoethyl methacrylate) polymer chains.

13. The method according to any one of Claims 4 to 10, wherein the polymeric material is attached to a micro- or macro-substrate material, optionally wherein the micro- or macro- substrate material is selected from one or more of the group consisting of porous or non- porous structures, e.g., gels, particles, membranes, mesh and fibres.

14. The method according to any one of the preceding claims, wherein the pKb of the polymeric adsorbent material is from 8.0 to 8.9.

15. The method according to Claim 14, wherein the pKb of the polymeric adsorbent material is about 8.75.

16. The method according to any one of the preceding claims, wherein separating steps (b) and/or (d) involve one or more of a filtration process, a sedimentation process, a magnetic separation process or a centrifugation process.

17. The method according to Claim 16, wherein separating steps (b) and (d) involve a filtration process, optionally wherein the filtration process is an ultrafiltration process.

18. The method according to any one of the preceding claims, wherein each step of the method is conducted within a temperature range of from 10°C to 40°C.

19. The method according to Claim 18, wherein each step of the method is conducted within a temperature range of from 20°C to 35°C.

20. The method according to any one of the preceding claims, wherein the at least one contaminant is selected from one or more of the group consisting of heavy metal ions, organic molecules, dyes, solvent, and pesticides.

21. The method according to any one of the preceding claims, wherein the pH of step (a) in Claim 1 is from 5.0 to 6.0, such as a pH of 5.5.