Exam questions

Question 1

Explain the structure of anisotropic membranes using a scheme

Answer 1

• Layered structure with varying porosity across layers.
• Dense, selective top layer filters specific particles.
• Porous, supportive bottom layer provides structural integrity.

Loeb-Sourirajan Anisotropic Membrane:
* Gradual transition from a dense, selective top layer to a more porous sublayer.
* High selectivity with support from the porous layer.
* Design maintains structural integrity.

Thin-Film Composite Anisotropic Membrane:
* Distinct, thin, selective coating on top of a porous support layer.
* Optimizes separation while allowing for higher flux.

Question 2

Explain (with the help of a scheme) possible scenarios on how support affect the performance of a microporous anisotropic membranes.

Answer 2

Ideal Situation (1): No penetration into the support layer, with high pore density, resulting in optimal filtration performance and high selectivity.

Low Pore Density (2): Reduced pore density, which lowers the flux, meaning the membrane has a reduced flow rate.

Pore Penetration (3): The selective coating penetrates into the support layer, causing a decrease in flux due to increased resistance in the membrane structure.

Incomplete Coating with Pinholes (4): This leads to low selectivity (as contaminants may bypass the selective layer) and high flux, but it compromises overall filtration quality.

Support with Thin, Dense Skin Layer (5): The dense skin layer affects selectivity and flow rate, potentially creating anomalous selectivity issues, as the support plays a direct role in filtration properties.

Question 3

Ultrafiltration and microfiltration membranes have similarities as well as differences. Give a similarity and a difference between the two types of membranes

Answer 3

Physical Filtration Mechanism: Both use a physical barrier to separate particles based on size, with no chemical reaction involved in the separation process.

Microfiltration membranes act as screen filters, blocking contaminants on the surface due to their larger pores, like a sieve.
Ultrafiltration membranes function more as depth filters, trapping smaller particles within the membrane structure,

Question 4

Describe in words and with a figure the differences between two cases: One type of
molecule that adsorbs weakly to the surface of a porous material; another type that adsorbs more strongly to the surface of the same type of porous material.

Answer 4

Zeolites have cations that interact differently with gases like nitrogen (N₂) and oxygen (O₂):

Weak Adsorption (O₂): Oxygen adsorbs weakly on zeolite due to a lower electric field gradient (EFG) interaction with the cations. This weak interaction allows O₂ to desorb easily.

Strong Adsorption (N₂): Nitrogen adsorbs strongly on zeolite, particularly in Li-zeolite-X, due to a stronger EFG–quadrupole interaction with the cations. This stable adsorption makes N₂ less likely to desorb.

Summary: N₂ has stronger adsorption on zeolite than O₂ due to the quadrupole interaction with cations, while O₂ shows weaker, reversible adsorption.

Question 5

What do we call the molecules that adsorb? What do we call the material possessing the surface at which the molecules adsorb?

Answer 5

Adsorbing Molecules: Adsorbate
Surface Material: Adsorbent

Question 6

How large are micropores? How large are mesopores?

Answer 6

Micropores: <2 nanometers
Mesopores: 2–50 nanometers

Question 7

Describe a workflow of chemical activation processes of activated carbon

Answer 7

1. Precursor Selection: Choose a carbon-rich raw material, such as biomass, coal, or polymers.
2. Pretreatment (Optional): Heat the precursor in nitrogen (N₂) or water to remove volatile components.
3. Mixing: Impregnate the precursor with a strong acid (H₃PO₄) or base (KOH) to promote porosity.
4. Activation: Heat the mixture in an inert nitrogen atmosphere to develop the pore structure.
5. Washing: Rinse with water to remove residual chemicals from the activating agent.
6. After Treatment (Optional): Perform additional acid or base washing and high-temperature treatment if needed for specific properties.

Question 8

Describe the pros and cons for using powder and granulated forms of activated carbon in water treatment.

Answer 8

• Powdered activated carbon (PAC) offers rapid contaminant removal due to high surface area and fast adsorption, ideal for sudden contamination spikes.
• PAC is harder to remove and may increase operating costs.
• Granulated activated carbon (GAC) is easier to handle, reusable through regeneration, and cost-effective long-term.
• GAC’s larger particles slow adsorption but suit continuous, stable treatment systems.
• Choosing PAC or GAC depends on flexibility, cost, and contaminant type.

Question 9

Polysaccharides are used in water treatment. Name three polysaccharides used in water treatment and the related mechanisms involved in the removal of pollutants from water.

Answer 9

Chitosan
* Biopolymer from crustacean shells, cationic in nature.
* Binds with negatively charged contaminants (e.g., heavy metals, dyes) via electrostatic interactions.
* Acts as a coagulant, clumping small particles for removal.

Alginate
* Derived from brown algae, contains carboxyl (-COOH) groups.
* Binds heavy metals and cationic pollutants, forming stable complexes for removal.
* Forms gels to trap contaminants, aiding in adsorption-based filtration.

TEMPO-Oxidized Cellulose Nanofibers (TOCNF)
* Oxidized cellulose with carboxyl groups, creating a high-charge, nanoscale fiber network.
* Effective for organic pollutants and metal ions, using ion exchange and electrostatic attraction.
* High surface area and nano-structure allow efficient adsorption of dissolved organics and fine particles.

Question 10

What are mixed matrix membranes?

Answer 10

A type of hybrid membrane that incorporates inorganic fillers (such as zeolites, metal-organic frameworks (MOFs), or silica particles) into a polymer matrix.

Question 11

Write two methods for mixed matrix membranes

Answer 11

Solution Casting and Phase Inversion

Question 12

Explain Solution Casting

Answer 12

Description: In the solution casting method, the polymer and inorganic filler are mixed together in a solvent to form a homogeneous solution or suspension. This mixture is then cast onto a flat surface or into a mold to form a membrane.
Process: After casting, the solvent is evaporated, leaving behind a solidified membrane with evenly dispersed inorganic fillers. This method ensures that the fillers are well-integrated within the polymer matrix, resulting in a uniform structure.
Advantages: Solution casting is relatively simple and allows precise control over the composition and thickness of the membrane. It’s also effective for producing dense, defect-free MMMs.

Question 13

Explain phase inversion

Answer 13

Description: Phase inversion is a technique used to create porous membranes by causing phase separation in a polymer-filler solution, resulting in a controlled, porous structure.

Process: The polymer-filler solution is first cast as a thin film, then submerged in a non-solvent bath (such as water). The non-solvent begins to enter the film, while the solvent diffuses out. This rapid exchange between the solvent and non-solvent causes the polymer to precipitate, or solidify, and form a stable structure. The areas where the non-solvent enters create voids, leading to the formation of pores throughout the membrane as phase separation occurs.

Advantages: Phase inversion allows precise control over pore size and distribution, which is valuable for filtration applications. The resulting porous structure enhances the membrane’s flux (flow rate) and selective permeability, making it well-suited for processes requiring high filtration efficiency.

Question 14

Describe briefly the three different types of treatments that are used for domestic wastewater treatment today

Answer 14

Physical or mechanical separation of large particles:
* Grates remove large debris (e.g., paper, pads) from influent to prevent clogging.
* A sand filter captures larger particles like gravel and sand sediment, reducing suspended solids for later treatment stages.

Biological processes to degrade organic matter:
* Microorganisms are introduced to break down organic matter in the wastewater. During this process, organic compounds degrade into simpler substances like carbon dioxide (CO₂), minerals, and ammonia.

Chemical treatment: Chemical treatment can target nutrients, heavy metals, or other contaminants that are difficult to remove through physical or biological processes. This step helps prevent issues like eutrophication in receiving water bodies by reducing nutrient loads.

Question 15

Describe with reactions the photolysis of ozone with UV light, into hydrogen peroxide in the first reaction and then into a specific reactive oxygenated species (ROS) in the second reaction. Which ROS is this? Give the formula and some of its properties that are useful in water treatment processes

Answer 15

O₃ + H₂O (with hν) → O₂ + H₂O₂

H₂O₂ (with hν) → 2 OH*

The ROS is the OH radical.

Properties of Hydroxyl Radical (OH*) Useful for Water Treatment:

High Reactivity: Hydroxyl radicals can quickly attack and break down organic pollutants.
Non-Selective Oxidation: They can react with various organic and inorganic compounds, making them effective against a range of contaminants.
Pathogen Inactivation: Hydroxyl radicals can kill bacteria, viruses, and other pathogens, aiding in water disinfection.

Question 16

Describe text and pictures of how a semiconductor can be used to promote reductive and oxidative reactions that can be suitable for water treatment. Include words such as bandgap, holes, electrons, UV-or-visible light, and acceptors and donors in your description

Answer 16

1. Introduction to Semiconductors in Water Treatment
   Semiconductors, like titanium dioxide (TiO₂), are used as photocatalysts in water treatment. They can drive both oxidative and reductive reactions that help break down contaminants.
2. Bandgap and Light Activation
   Semiconductors have a bandgap, which is the energy difference between their valence band (where electrons are initially located) and their conduction band (where electrons can move freely).
   When the semiconductor is exposed to UV or visible light with energy equal to or greater than its bandgap, it can absorb that energy. This excitation moves an electron from the valence band to the conduction band, creating an electron-hole pair.
3. Formation of Electrons and Holes
   Electron (e⁻): The excited electron moves into the conduction band, leaving a vacancy in the valence band.
   Hole (h⁺): The vacancy left behind in the valence band acts as a positively charged “hole.” Holes have oxidative properties, while electrons in the conduction band have reductive properties.
4. Role of Acceptors and Donors
   Acceptors: These are molecules in the water that accept electrons from the conduction band. For example, oxygen (O₂) in water can act as an electron acceptor and get reduced to form superoxide radicals (O₂⁻), which can further react to degrade contaminants.
   Donors: These are molecules that donate electrons to the holes in the valence band. For example, water (H₂O) or hydroxide ions (OH⁻) can donate electrons to the holes, generating hydroxyl radicals (OH*), which are powerful oxidizing agents that break down organic pollutants and kill pathogens.
5. Why This Process is Effective in Water Treatment
   The semiconductor-based photocatalytic process continuously generates reactive species (like OH* and O₂⁻) under light exposure. These species have strong oxidizing and reducing powers, making them effective at decomposing pollutants and disinfecting water without leaving harmful residues.

Question 17

Describe the principles for PSA and TSA

Answer 17

PSA (Pressure Swing Adsorption): PSA relies on pressure changes to separate gases. Under high pressure, specific gas molecules adhere to an adsorbent (like zeolite), allowing selective adsorption. When the pressure is reduced, these gases are released/desorbed.

TSA (Temperature Swing Adsorption): TSA uses temperature changes to control adsorption. At lower temperatures, the adsorbent captures target gases effectively. To release (desorb) the gases, the temperature is raised, which weakens the adsorbent’s hold.

Question 18

Describe how O2 can be purified from air with PSA, and give an example of sorbent for O2 production

Answer 18

In PSA, O₂ can be purified from air by exploiting the fact that N₂ adsorbs more strongly than O₂ onto certain adsorbents. A common sorbent used for this purpose is zeolite, which selectively adsorbs N₂ from the air mixture due to its stronger interaction with N₂. During the high-pressure phase, N₂ molecules adhere to the zeolite, while O₂, which is less strongly adsorbed, remains in the gas phase and can be collected as a purified stream. In the low-pressure phase, N₂ is desorbed from the zeolite, regenerating the adsorbent for the next cycle. This process effectively separates O₂ from N₂ in the air, producing an O₂-enriched product.

Question 19

Give an example of packing sorbent powders in shapes for PSA

Answer 19

Granulation and extrudationare common
The microporous powder is mixed with a binder or set of binders; typically clays or polymers
Then the powder is either granulated or extruded

For microporous solids, it is common to use an inclined table and a moist mixture of e.g. a zeolite and binder which is then stirred or agitated to form small granules.
The granules form in analogy to that of a snowball

Question 20

Describe CCS of the post-combustion capture type and speculate why the Benfield process was selected over the standard amine scrubbers for the planned installation here in Stockholm.

Answer 20

Post-combustion carbon capture involves capturing CO₂ from the flue gas emitted after combustion. In a power plant, fuel combustion generates flue gas containing CO₂, which can be captured by chemical or physical processes.
The captured CO₂ is then compressed and transported to a storage site, where it’s injected into deep geological formations, such as depleted oil and gas fields or saline aquifers, for long-term storage. This prevents the captured CO₂ from entering the atmosphere, helping to mitigate greenhouse gas emissions.

They chose it in sthlm because of Lower Operating Costs, No amine emission.

Question 21

Describe the Benfield process chemistry and the amine scrubbing chemistry in some detail

Answer 21

Benfield Process Chemistry: The Benfield process uses potassium carbonate (K₂CO₃) as the primary absorbent to capture CO₂. The flue gas is contacted with K₂CO₃ solution, which reacts with CO₂ to form bicarbonate (KHCO₃):
CO₂ + K₂CO₃ + H₂O → 2 KHCO₃
To regenerate the solution, the bicarbonate is heated, releasing CO₂ and returning K₂CO₃ to the solution, ready for reuse.

Amine Scrubbing Chemistry: Amine scrubbing typically uses monoethanolamine (MEA) as the absorbent. CO₂ reacts with MEA to form a carbamate intermediate:
CO₂ + 2 MEA → MEA–CO₂ + MEA (carbamate)
The carbamate can then be heated to release CO₂ and regenerate the MEA for reuse.

Question 22

The thermodynamics of direct air capture (DAC) is not as appealing as for regular CCS. Describe the pros and cons of DAC vs. CCS.

Answer 22

Direct Air Capture (DAC):
Pros:
Flexibility in Location: DAC can be implemented anywhere, as it captures CO₂ directly from the air, regardless of its source. This makes DAC versatile, particularly for removing emissions from dispersed sources like transportation and agriculture.
Negative Emissions Potential: DAC can help achieve net-negative emissions by removing CO₂ that’s already in the atmosphere, an essential feature for addressing past emissions and reaching climate targets.
Cons:
High Energy Demand: Due to the low concentration of CO₂ in the air, DAC requires significant energy to capture CO₂, making it thermodynamically less efficient and costly. This high energy requirement limits its scalability unless low-cost, renewable energy sources are used.
Costly Infrastructure: DAC technologies are expensive to build and operate, making them economically challenging compared to CCS. The cost per ton of CO₂ captured is typically much higher with DAC.

Carbon Capture and Storage (CCS):
Pros:
Efficiency for Point Sources: CCS is well-suited for capturing CO₂ from large, concentrated sources, such as power plants and industrial facilities, where CO₂ is emitted in high concentrations. This concentration improves thermodynamic efficiency, reducing energy requirements and costs.
Established Technology: CCS is already in use at various industrial sites and has a more established infrastructure. It is widely seen as a near-term solution to reduce emissions from industrial processes and fossil fuel-based energy production.
Cons:
Limited to Point Sources: CCS is only practical for concentrated sources of CO₂ emissions. It cannot capture CO₂ from dispersed sources like transportation, which limits its potential for achieving global CO₂ reductions.
Storage Risks: Storing large volumes of CO₂ underground can pose environmental risks, including the possibility of leaks, which could release CO₂ back into the atmosphere and potentially harm local ecosystems and human health.

Question 23

What is your guess for why people have so diverging views on DAC and CCS?

Answer 23

Supporters of DAC believe it’s important for capturing emissions from dispersed sources like cars and agriculture, and it offers a way to remove CO₂ directly from the atmosphere. However, critics point out that DAC is very energy-intensive and costly compared to CCS.

CCS is viewed by its supporters as a practical, immediate solution for capturing emissions from large sources like power plants. But others worry that it might keep us dependent on fossil fuels and has potential environmental risks, like CO₂ leaks from storage sites.

Overall, people have mixed views because they prioritize different factors like cost, scalability, and impact on the environment.

Question 24

Explain three membrane processing techniques commonly used to prepare anisotropic
membranes.

Answer 24

Phase Inversion:
In phase inversion, a polymer solution is cast onto a surface and then immersed in a non-solvent bath (e.g., water). The rapid exchange between the solvent and non-solvent causes the polymer to precipitate, creating a dense selective layer on the top and a porous substructure beneath. This asymmetric or anisotropic structure allows for high permeability with selective separation, making it suitable for ultrafiltration, gas separation, and other filtration applications.

Solution-Coated Composite Membranes:
This technique applies a thin polymer solution layer onto a porous support, which is then dried or cured to form a selective membrane layer. The porous support provides mechanical strength, while the thin coating layer offers selectivity. By combining these properties, solution-coated composite membranes are particularly useful for applications requiring durability and precise filtration, such as microfiltration and reverse osmosis.

Interfacial Polymerization:
Interfacial polymerization occurs when two monomers dissolved in different phases (one aqueous, one organic) react at the interface between the phases, forming a thin, dense polymer layer on a support membrane. The result is a highly selective and ultrathin layer, typically less than 1 micron thick. This method is frequently used for creating reverse osmosis and nanofiltration membranes due to its high selectivity and efficiency in separating small molecules from solutions.

Question 25

Describe the general difference between the Langmuir and BET isotherms.

Answer 25

Langmuir Isotherm:
This model assumes monolayer adsorption on a surface with a finite number of identical, non-interacting adsorption sites. Each site holds only one adsorbate molecule, leading to a saturation point when all sites are occupied. It applies best to surfaces with homogeneous characteristics, making it suitable for describing adsorption on non-porous materials or surfaces with uniform energy sites.

BET Isotherm:
The BET isotherm extends the Langmuir model to account for multilayer adsorption, where multiple layers of adsorbate molecules form on the surface. This model is widely used for porous materials with large surface areas, as it considers interactions between adsorbed molecules and allows for continuous adsorption beyond the first monolayer, making it suitable for higher-pressure conditions.

Question 26

If one type of porous silica has a wall thickness of 3 nm and a surface area of 500 m2/g, what
would the surface area be for another porous silica with a wall thickness of 1.5 nm

Answer 26

If the wall thickness of the porous silica is halved from 3 nm to 1.5 nm, the surface area roughly doubles. This is because, in porous materials, reducing wall thickness increases the accessible surface area by exposing more internal surfaces within the same mass.

Question 27

Explain the synthesis of mesoporous silica through hydrothermal process?

Answer 27

The surfactant templates are dissolved in water and mixed with inorganic precursors (either hydrolyzed or non-hydrolyzed) along with an acid or base catalyst to speed up the reaction. This mixture forms a “sol,” creating a hybrid material of silica and surfactants, which gradually transforms into a particle gel. The gel is then subjected to hydrothermal treatment, promoting the condensation and solidification of the silica framework. Finally, the surfactant templates are removed, typically by calcination (heating in air), leaving a porous silica structure. Acidic or basic pH conditions help accelerate these reactions.

Question 28

Metal organic frameworks (MOFS) are hybrid porous materials used in water treatment.
What are the possible interactions of MOFs with water pollutants?

Answer 28

Electrostatic Interaction: MOFs can attract oppositely charged adsorbate molecules, enhancing pollutant capture through electrostatic forces.

Influence of Framework Metal: The metal ions in MOFs can be tailored to selectively interact with specific molecules (e.g., selecting pollutant A over B), improving target-specific adsorption.

Hydrogen Bonding: Functional groups in MOFs can form hydrogen bonds with pollutants, increasing adsorption of specific molecules based on their polarity.

Hydrophobic Interaction: Hydrophobic MOFs attract hydrophobic pollutants, helping in the separation of non-polar contaminants from water.

Acid-Base Interaction: MOFs can participate in acid-base interactions, where acidic or basic groups on pollutants interact with complementary sites in MOFs.

π-π Interaction: MOFs with aromatic structures can attract pollutants with π-electrons (e.g., aromatic hydrocarbons), enhancing adsorption through π-π stacking.

Question 29

What are the advantagesand disadvantages of MOFs in environmental remediation?

Answer 29

Advantages of MOFs in Environmental Remediation:

Hydrophilic and Hydrophobic Coexistence: Allows MOFs to adsorb a diverse range of contaminants.
Variety of Metal Cations: Enables customization for targeted pollutant interactions.
Functionalized Organic Linkers: MOFs can be modified with functional groups, enhancing selectivity and versatility.
High Surface Area and Uptake Capacities: MOFs offer large adsorption capacities, making them effective even in low concentrations of pollutants.

Disadvantages of MOFs in Environmental Remediation:

Thermal inStability: Many MOFs have limited thermal stability, restricting their use in high-temperature conditions.
Sensitivity to Water and pH: Some MOFs degrade or lose performance in certain aqueous or extreme pH environments, limiting their robustness.

Question 30

Describe one advantage and one disadvantage of using OH* in water purification processes.

Answer 30

Advantage: High Reactivity – OH* (hydroxyl radicals) are extremely reactive and can rapidly degrade a wide range of organic pollutants, including those that are otherwise resistant to conventional treatment methods. This makes them highly effective for breaking down complex contaminants in water purification.

Disadvantage: Non-Selectivity – Due to their high reactivity, OH* radicals are non-selective, meaning they can react with almost any organic molecule present. This lack of specificity can lead to the formation of unwanted by-products and may reduce efficiency if other non-target organic compounds are also present in the water.

Question 31

Describe the regular Fenton process for water treatment and compare it with an UV mediated
version, giving advantages for the latter.

Answer 31

The regular Fenton process for water treatment uses hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺) to generate hydroxyl radicals (OH)
Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH
Fe³⁺ + H₂O₂ → Fe²⁺ + HOO* + H⁺

In this process, Fe²⁺ needs to be continuously added to maintain the reaction since it is converted to Fe³⁺.

UV-Mediated Fenton Process:
In the UV-Fenton or photo-Fenton process, UV light is applied to regenerate Fe²⁺ from Fe³⁺, allowing continuous production of OH* radicals. The UV light also directly breaks down H₂O₂ into additional OH* radicals. Key reactions include:

Fe(OH)²⁺ + UV (hv) → OH* + Fe²⁺
H₂O₂ + UV (hv) → 2 OH*
Advantages of the UV-Mediated Fenton Process:

Higher Efficiency: Continuous regeneration of Fe²⁺ under UV light allows sustained OH* production, leading to faster and more effective pollutant degradation.
Reduced Chemical Requirement: The regeneration of Fe²⁺ minimizes the need for extra iron, making the process more cost-effective.
Enhanced Degradation of Resistant Pollutants: UV light enhances the breakdown of persistent contaminants that are harder to degrade with the regular Fenton process alone.

Question 32

Describe how N2 can be purified from air with an adsorption process and highlight a suitable
process?

Answer 32

Nitrogen (N₂) can be purified from air using an adsorption-based process that selectively adsorbs other gases, particularly oxygen (O₂), leaving behind enriched N₂. A commonly used method for this is Pressure Swing Adsorption (PSA).

Carbon Molecular Sieves are commonly used in PSA for nitrogen purification because they selectively adsorb oxygen over nitrogen, aided by the difference in diffusion rates. This selective adsorption enables effective nitrogen purification from air.

Question 33

What are the mechanisms involved in separation of pollutants in air filtration membranes? How
does pollutant particle size correlate with the mechanisms.

Answer 33

Size Exclusion (Sieving):
This mechanism physically blocks particles that are larger than the membrane’s pore size, trapping them on the surface or within the pores. It is most effective for larger particles (e.g., dust, pollen) and relies on pore size matching to the pollutant size.

Electrostatic Attraction:
Many filtration membranes are charged or polar, allowing them to attract and capture oppositely charged particles or polar molecules. This mechanism is effective for capturing smaller particles, such as fine particulate matter (PM2.5 and PM0.1) and certain pollutants like smoke particles, which can carry a slight charge.

Chemical Adsorption:
In membranes coated with or made of adsorptive materials (e.g., activated carbon), pollutants like volatile organic compounds (VOCs) and certain gases are adsorbed onto the membrane surface. This mechanism is effective for gaseous pollutants or very small particles that interact chemically with the membrane material.

Diffusion:
Small particles (especially ultrafine particles) move randomly through Brownian motion and may collide with and adhere to the membrane surface, even if they are smaller than the pore size. This mechanism becomes more effective as particle size decreases, especially for particles smaller than 0.1 micrometers (PM0.1).

Question 34

Explain with two examples how carbon based materials are used in air purification

Answer 34

Activated carbon is extensively used in air filters to remove volatile organic compounds (VOCs), odors, and gases from the air. Its high surface area and porous structure allow it to adsorb pollutants effectively. This material is commonly found in household air purifiers, industrial ventilation systems, and gas masks, where it traps harmful gases and improves air quality.

Question 35

Describe the general approach of CCS and CCU and argue for advantages for each of the
approaches

Answer 35

Carbon Capture and Storage (CCS) involves capturing CO₂ from industrial sources, transporting it, and injecting it deep underground into geological formations for permanent storage.

Advantages:
Long-Term CO₂ Reduction: Permanently removes CO₂ from the atmosphere, directly mitigating climate change.
Suitable for Hard-to-Decarbonize Industries: Essential for sectors like cement and steel, where renewable energy alone can’t eliminate emissions.
Supports Climate Goals: Enables significant emission cuts, aiding in meeting global climate targets.
Carbon Capture and Utilization (CCU) captures CO₂ and repurposes it to make products like fuels, chemicals, and building materials.

Advantages:
Economic Value Creation: Turns CO₂ into valuable products, incentivizing carbon capture.
Circular Carbon Economy: Recycles CO₂, reducing reliance on fossil resources and minimizing waste.
Indirect Emission Reduction: CO₂-derived products can replace more carbon-intensive alternatives, indirectly lowering emissions.
In summary, CCS offers long-term climate benefits by storing CO₂ permanently, while CCU supports economic and environmental sustainability by repurposing CO₂, creating a circular carbon economy.

Question 36

What is an isotropic membrane?

Answer 36

Uniform structure and properties throughout, used in applications where even filtration is required.

Question 37

Explain in detail 2 methods used for characterisation of pore structure of microporous membranes.

Answer 37

Scanning Electron Microscopy (SEM):

SEM involves focusing a beam of electrons onto the membrane surface. These electrons interact with the sample to create high-resolution images.
It provides detailed visual data on surface structure, pore size, and pore distribution, which is crucial for understanding how pore structure impacts membrane performance.
SEM is effective for examining surface pores and identifying structural defects that might affect filtration efficacy.

Gas Adsorption Method:
This method measures the amount of gas adsorbed onto the membrane surface under controlled pressure, allowing calculation of pore volume, surface area, and distribution.
The BET theory, a common approach in gas adsorption, extends Langmuir’s model to multilayer adsorption, making it suitable for microporous structures.
By plotting adsorption-desorption isotherms, gas adsorption reveals pore characteristics like micropore and mesopore presence, essential for applications requiring specific pore size ranges for separation or adsorption.

Question 38

Give one example of a chemical reaction with CO2 where the carbon oxidation state is not
changed and one in which it is changed.

Answer 38

No Change in Carbon Oxidation State:
In this reaction, the carbon in CO₂ retains its oxidation state (+4):

CO₂ + CaO → CaCO₃

Here, CO₂ reacts with calcium oxide to form calcium carbonate (CaCO₃), with the carbon remaining at a +4 oxidation state.

Change in Carbon Oxidation State:
In this reaction, CO₂ is reduced, and the oxidation state of carbon decreases from +4 to +2:

CO₂ + H₂ → CO + H₂O

Here, CO₂ reacts with hydrogen to form carbon monoxide (CO) and water, with the carbon in CO having an oxidation state of +2.

Question 39

If a silica material (SiO2, amorphous) has a smooth wall with a thickness of 2 nm, what is the specific surface area? Assume that the material has a lamellar structure and a local silica density of 2000 kg/m3.

Answer 39

Specific Surface Area = [1 / (t * ρ)]*2

Thickness (t) = 2 nm = 2 × 10⁻⁹ m
Density (ρ) = 2000 kg/m³

The equation is multiplied by 2 because of the lamellar structure which means it is 2 surfaces. one on each side of the lamells.

Question 40

Describe the workflow of synthesis of mesoporous silica

Answer 40

Preparation of the Precursor Solution:
A silica source is mixed with water, a surfactant, and an acid or base catalyst.
The surfactant molecules form micelles, which act as templates around which the silica will condense.

Hydrolysis and Condensation:
The silica source undergoes hydrolysis and condensation reactions, forming a silica network around the surfactant micelles.
This results in a gel-like material where the surfactant-filled pores are embedded in the silica matrix.

Hydrothermal Treatment:

The mixture is then heated (often in an autoclave) at elevated temperatures (around 100–150 °C) for several hours.
This step strengthens the silica framework and stabilizes the mesoporous structure.

Template Removal:
After cooling, the solid material is collected and dried.
The surfactant template is removed, usually by calcination (heating in air around 500 °C) or by solvent extraction, leaving behind the mesoporous silica with uniform pore sizes.

Question 41

Describe the advantages for using activated carbon in water treatment

Answer 41

High Adsorptive Capacity:
Activated carbon has a large surface area and porous structure, allowing it to adsorb a wide range of contaminants, including organic pollutants, pesticides, and certain heavy metals.

Versatile in Removing Various Contaminants:
It can effectively remove dissolved organics, odors, and chlorinated compounds, making it suitable for both industrial and drinking water purification.

Non-Toxic and Safe:
Activated carbon is chemically stable, non-toxic, and safe for use in water treatment without introducing harmful by-products.
Regenerable and Reusable:

It can be regenerated and reused multiple times, reducing operational costs and waste.

Question 42

Describe the limitations for using activated carbon in water treatment

Answer 42

Limited Removal of Inorganic Contaminants:

Activated carbon is less effective for removing inorganic ions, such as nitrates, ammonia, and certain heavy metals without additional treatment.

Potential for Saturation:
Over time, activated carbon becomes saturated with contaminants, reducing its effectiveness and requiring regeneration or replacement.

Cost of Regeneration and Replacement:
While regenerable, the regeneration process can be energy-intensive and costly, particularly for high-load industrial applications.

By-Product Release:
In some cases, activated carbon may release adsorbed contaminants back into the water if not maintained or replaced properly.

Question 43

Nanocellulose have great potential in water treatment. Explain two mechanisms by which nanocellulose based materials can purify water

Answer 43

Adsorption of Contaminants:

Nanocellulose has a large surface area and can be chemically modified with various functional groups, such as carboxyl or hydroxyl groups. These groups enable strong interactions with contaminants, allowing nanocellulose to adsorb heavy metals, dyes, and organic pollutants from water.
The adsorption process is highly effective because the nanocellulose fibers provide ample surface area for contaminants to attach, while the functional groups improve binding affinity for specific pollutants.
Filtration and Size Exclusion:

Nanocellulose can form dense, interconnected networks with nanoscale pores, acting as a physical barrier to trap particulate contaminants and microorganisms based on size exclusion.
This filtration mechanism is effective for removing bacteria, suspended solids, and even some viruses, as the small pore size physically blocks contaminants from passing through the nanocellulose membrane.
In summary, nanocellulose purifies water through adsorption of chemical contaminants and size-based filtration, making it an efficient, versatile material for various water treatment applications.

Question 44

Polymeric membranes can be effectively used in gas separation. How does the glass transition temperature of the polymer impact the separation performance. Explain using N2 gas separation as an example.

Answer 44

Polymer Chain Mobility:

Below the glass transition temperature (Tg), the polymer is in a rigid, glassy state. In this state, the polymer chains have limited mobility, creating smaller, more fixed free volume (or pore spaces) between chains.
Above Tg, the polymer becomes rubbery, with increased chain mobility and larger free volume, which allows gas molecules to pass through more easily.
Selectivity vs. Permeability:

Glassy Polymers (below Tg): These tend to have higher selectivity because the smaller, fixed free volume restricts the movement of larger or less permeable gases, like N₂, more than smaller, more permeable gases (such as O₂). This selectivity is advantageous for N₂ separation, as it allows for a more controlled and selective separation process.
Rubbery Polymers (above Tg): These have higher permeability due to the increased free volume. However, this often results in reduced selectivity, as larger gases like N₂ can pass through more easily, reducing the ability to distinguish between gases based on size or diffusivity.
Example with N₂ Separation:

In N₂/O₂ separation, a glassy polymer with a Tg above the operating temperature can selectively allow O₂ (smaller and more diffusive) to pass through while retaining more N₂ (larger and less diffusive). This makes glassy polymers better suited for N₂ separation.
A rubbery polymer with a lower Tg may allow both gases to pass through more freely, leading to higher permeability but lower selectivity.

Question 45

Discuss common water treatment processes (describe their three steps) and why research and development is made to enhance these processes.

Answer 45

Coagulation and Flocculation
Coagulation: Chemicals, like alum or ferric chloride, are added to the water to neutralize the charges on particles, allowing them to come together.
Flocculation: The neutralized particles clump together into larger aggregates, or “flocs,” that are easier to remove.
Purpose: This step is essential for removing suspended solids, colloids, and some dissolved substances, which might otherwise pass through the filtration stages.
2. Sedimentation and Filtration
Sedimentation: In this stage, gravity allows the heavy flocs formed during flocculation to settle at the bottom of the tank.
Filtration: The water is then passed through sand, gravel, or activated carbon filters to trap remaining particles, microorganisms, and some chemical contaminants.
Purpose: This step further purifies the water, removing both particles that didn’t settle and microorganisms.
3. Disinfection
After filtration, water undergoes disinfection to kill any remaining pathogens. Common disinfectants include chlorine, ozone, or UV light.
Purpose: This final step is crucial for eliminating disease-causing organisms, ensuring the treated water is safe for human consumption.

Why Research and Development Is Important
Research and development in water treatment aim to enhance these processes to achieve greater efficiency, cost-effectiveness, and environmental sustainability. Key goals include:

Improving Contaminant Removal: Advanced research helps develop new materials (e.g., nanomaterials, membrane technologies) that can target specific pollutants, including emerging contaminants like pharmaceuticals and microplastics, which traditional methods struggle to remove.
Reducing Chemical Usage: Many treatment processes rely on chemicals, which can create by-products and environmental impacts. R&D focuses on alternative methods, like biological treatments or advanced oxidation, that reduce or eliminate the need for chemicals.
Enhancing Energy Efficiency: Energy consumption in water treatment is a major cost and environmental factor. Developing low-energy or energy-recovering processes helps make treatment more sustainable and affordable.

Question 46

Describe why zeolites can be used for gas separation

Answer 46

Uniform Microporous Structure:
Zeolites have a well-defined, crystalline framework with pores and channels of precise molecular dimensions, typically in the range of 0.3–1 nm. This allows them to separate gases based on molecular size (size exclusion), where smaller molecules can pass through the pores, while larger ones are excluded.

High Surface Area and Adsorption Capacity:
The internal surface area of zeolites is very high, which increases their capacity to adsorb gases. This makes them effective for adsorbing and separating gases based on their adsorption affinity.

Selective Adsorption Due to Polarity:
Zeolites contain charged sites (such as aluminum in the framework) that can create local electric fields. This makes them effective at selectively adsorbing polar or polarizable molecules (like CO₂ and H₂O) over nonpolar molecules (like N₂ and O₂).

Pressure and Temperature Swing Capabilities:
Zeolites are stable under varying pressures and temperatures, making them suitable for Pressure Swing Adsorption (PSA) or Temperature Swing Adsorption (TSA) processes, where changes in pressure or temperature are used to adsorb and desorb gases selectively.

Question 47

Compare direct air capture (DAC) with biochars or reforestration, give advantages and disadvantages. These three approaches are examples of one set of technologies, which?

Answer 47

Direct Air Capture (DAC), biochar production, and reforestation are all examples of Carbon Capture and Storage (CCS) technologies in the broader sense. They focus on capturing CO₂ and storing it either temporarily or permanently to reduce atmospheric CO₂ levels.

Direct Air Capture (DAC)
Advantages:
Scalable and can be deployed independently of natural ecosystems.
Efficient CO₂ Capture at high purity, allowing for controlled storage or utilization.
Disadvantages:
Energy-Intensive and Costly: High operational costs and significant energy demand.
Infrastructure Requirements: Needs suitable CO₂ storage facilities or utilization pathways.
2. Biochar
Advantages:
Solid Carbon Storage: Biochar is stable and can store carbon long-term in soils.
Improves Soil Health: Enhances water retention and nutrient availability in soils.
Disadvantages:
Limited Biomass Availability: Depends on sustainable biomass sources, which may compete with other land uses.
Lower Carbon Removal Scale compared to industrial CCS technologies.
3. Reforestation
Advantages:
Ecosystem Co-Benefits: Supports biodiversity, water regulation, and soil health.
Low Cost and Self-Sustaining: Uses natural photosynthesis for carbon capture.
Disadvantages:
Land and Resource Demands: Requires large areas that may compete with agriculture.
Vulnerability to Environmental Changes: Forests can release stored CO₂ during fires, pests, or deforestation.

Each of these CCS approaches—DAC, biochar, and reforestation—plays a role in capturing and storing CO₂ but varies in scalability, cost, and co-benefits. They contribute to the larger goal of carbon dioxide removal within the CCS framework to address climate change.

Question 48

Name two oxidic adsorbents and their use in environmental remediation

Answer 48

Activated Alumina (Al₂O₃):

Use: Activated alumina is used to remove fluoride, arsenic, and selenium from water sources. It effectively adsorbs these contaminants due to its high surface area and porosity, making it widely applied in drinking water purification and wastewater treatment.
Titanium Dioxide (TiO₂):

Use: TiO₂ is used for photocatalytic degradation of organic pollutants in air and water. Under UV light, it generates reactive species that can break down contaminants like volatile organic compounds (VOCs), pesticides, and dyes, making it useful in air purification and wastewater treatment systems.

Question 49

Explain in detail 3 methods used for characterization of pore structure in membranes

Answer 49

1. Scanning Electron Microscopy (SEM)
   Principle: SEM uses a focused beam of electrons to generate high-resolution images of a sample’s surface. By scanning cross-sections of the membrane, SEM reveals pore structure, size, and distribution.
   Application: SEM provides visual data on pore shape, roughness, and connectivity, making it useful for analyzing mesopores and macropores (pores larger than 10 nm).
   Limitations: SEM only provides surface information and requires conductive coating on non-conductive samples, and it may not effectively visualize very small pores (below a few nanometers).
2. Gas Adsorption Analysis (BET Method)
   Principle: In this method, nitrogen gas is adsorbed onto the sample surface at varying pressures. The amount of gas adsorbed is analyzed to determine specific surface area and pore size distribution.
   Application: The BET method is ideal for characterizing micropores (below 2 nm) and mesopores (2-50 nm), providing quantitative data on surface area, pore volume, and distribution.
   Limitations: BET analysis may not capture larger macropores effectively and requires precise control of gas adsorption conditions.
3. Mercury Intrusion Porosimetry
   Principle: Mercury is forced into the pores under pressure, with higher pressures required for smaller pores. The pressure-volume relationship provides data on pore size and distribution.
   Application: Useful for measuring mesoporous and macroporous structures, mercury intrusion gives quantitative information on pore size distribution and total porosity.
   Limitations: This method involves toxic mercury and high pressures, which can damage fragile structures and may not be suitable for very small pores (below 3 nm).

Question 50

Describe a workflow of hydrothermal synthesis of zeolite

Answer 50

Preparation of the Gel Mixture
Source Materials: Silica and alumina sources (like sodium silicate and sodium aluminate) are combined with water and a mineralizing agent, often a strong base like sodium hydroxide (NaOH).
Mixing: The mixture is stirred thoroughly to form a homogenous gel with the desired Si/Al ratio, which determines the specific type of zeolite formed.
2. Hydrothermal Treatment
Sealing in Autoclave: The gel is transferred into a sealed autoclave or pressure vessel that withstands high temperatures and pressures.
Heating: The autoclave is heated to temperatures typically between 100–200 °C. Under these conditions, crystallization of the gel occurs, forming the zeolite structure.
Time and Control: The reaction time can range from hours to several days, depending on the desired crystal size and phase. Temperature and duration are carefully controlled to optimize crystal growth.
3. Recovery and Post-Treatment
Cooling and Filtration: After the synthesis is complete, the autoclave is allowed to cool. The crystalline zeolite is then recovered through filtration.
Washing and Drying: The recovered zeolite crystals are washed to remove residual chemicals and then dried at moderate temperatures.
Calcination (if needed): Some zeolites require calcination (heating to around 500 °C) to remove organic templates used during synthesis, leaving the final porous zeolite structure.
Summary
The workflow involves mixing a precursor gel, hydrothermal crystallization in an autoclave, and recovery with washing, drying, and optional calcination. This process yields a crystalline, porous zeolite structure suitable for various applications in environmental and industrial processes.

Question 51

Describe how H2 can be purified from CO2 and H2O with PSA, and give an example of sorbent for H2 purification

Answer 51

Pressure Swing Adsorption (PSA) for H₂ Purification:

In PSA for H₂ purification, different adsorbent beds are used in a sequential setup to capture impurities selectively, allowing H₂ to pass through and achieve a high-purity product.

Workflow:
First Stage: Activated Carbon Columns:

Columns with activated carbon are placed first in the PSA system to remove strongly adsorbing CO₂ and H₂O. Activated carbon has a high affinity for these compounds, effectively capturing them from the gas mixture.
This initial step simplifies the process, as CO₂ and H₂O can be easily recovered from the carbon adsorbent during the desorption phase.
Second Stage: Zeolite 5A Columns:

After CO₂ and H₂O removal, the remaining gas mixture passes through Zeolite 5A columns. Zeolite 5A adsorbs gases such as CH₄, CO, and N₂, which are smaller in size and more challenging to separate.
This step leaves a purified stream of H₂, as the zeolite selectively adsorbs the remaining impurities.
Carbon-Before-Zeolite Approach:

Placing activated carbon before zeolite optimizes the process. Since CO₂ and H₂O are easily adsorbed and released from activated carbon, it simplifies desorption and prevents these gases from reaching the zeolite stage, where they would be harder to remove.
Example of Sorbent:
Activated Carbon: Used to adsorb CO₂ and H₂O efficiently.
Zeolite 5A: Used to capture CH₄, CO, and N₂, allowing high-purity H₂ to be collected at the output.

Question 52

What is advanced oxidation process (AOPs). Describe one advantage and one disadvantage of AOPs

Answer 52

Advanced Oxidation Processes (AOPs) are water treatment methods that produce highly reactive hydroxyl radicals (OH*) to degrade tough organic pollutants. Common AOPs include ozonation, Fenton reactions, and UV/H₂O₂ treatment, which break down contaminants that conventional treatments can’t handle.

Advantage: Effective on Persistent Pollutants – AOPs can degrade resistant organic contaminants, making them ideal for wastewater treatment.
Disadvantage: High Costs and Energy Use – AOPs require significant energy and chemicals, increasing operational costs and sometimes producing by-products that need careful management.

Question 53

Photocatalysis is used for chemical water treatment. What are the differences between homogeneous and heterogenous photocatalysis used in water treatment. Give example for each type of photocatalytic water treatment

Answer 53

Photocatalysis is a chemical process that uses light (usually UV) to activate a catalyst, generating reactive species (like hydroxyl radicals) that degrade contaminants in water. In water treatment, photocatalysis can be classified into two types: homogeneous and heterogeneous.

1. Homogeneous Photocatalysis
   Definition: In homogeneous photocatalysis, the catalyst is in the same phase as the contaminants, typically dissolved in the water. The catalyst molecules or ions react uniformly throughout the solution.
   Example: The photo-Fenton process is a common example of homogeneous photocatalysis, where Fe²⁺ ions in solution react with H₂O₂ under UV light to produce hydroxyl radicals that degrade contaminants.
2. Heterogeneous Photocatalysis
   Definition: In heterogeneous photocatalysis, the catalyst is in a different phase (solid) than the contaminants (liquid). The reaction occurs on the surface of the solid catalyst, which is activated by light.
   Example: Titanium dioxide (TiO₂) is commonly used in heterogeneous photocatalysis. Under UV light, TiO₂ generates reactive species on its surface, which break down organic pollutants in the water.
   Key Differences:
   Phase: Homogeneous catalysts are dissolved in the solution, while heterogeneous catalysts are solid particles.
   Catalyst Recovery: Heterogeneous catalysts can be easily separated and reused, while homogeneous catalysts are harder to recover from the solution.