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Drinking Water Treatment > Activated Carbon > Flashcards

Activated Carbon Flashcards

1
Q

Give a definition of Adsorption?

A

Accumulation of a substance in solution onto the surface

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2
Q

Draw a diagram and give a short definition of each item:

  1. Adsorbate
  2. Adsorbent
  3. Desorption
  4. Adsorption
  5. Solute
A
  1. Adsorbate = substance that is adsorbing
  2. Adsorbent = substance that accepts solutes to adsorb onto its surface
  3. Desorption = release from the surface
  4. Adsorption = sticking to the surface
  5. Solute = molecules dissolved in water
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3
Q

What are 5 typical applications for adsorption on activated carbon?

A

To remove:

  1. Taste and Oder
  2. Colour and DOC (Natural organic matter)
  3. Cyano-toxins
  4. Organic micropollutants
  5. Metal Oxides (Arsenic)
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4
Q

Draw a micro-structure of activated carbon and label the three types of pores present and their size in nm when adsorbing a pesticide?

A

Macropores - the largest ones (>50nm)

Mesopores - middle ones (2-50 nm)

Micropores - the smallest - (<2nm)

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5
Q

How does the pore structure and size of activated carbon influence adsorption capacity and kinetics?

A

Pore structure depends on pore size and can adsorb different contaminates:

Very large pores can adsorb bigger contaminants but smaller pores have a larger surface per volume and therefore have more adsorption sites but only for smaller contaminates

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6
Q

What is competitive adsorption?

A

Water usually contains more than one adsorbate (mixture of different micro-pollutants and background organics) these all compete with each other for adsorption sites

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7
Q

What is pore-blocking in relation to activated carbon?

A

Larger background organic substances such as humics particles can block the pore spaces which will reduce the available adsorption surface area reducing the overall adsorption capacity

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8
Q

When does pre-loading occur in relation to activated carbon?

A

When there are fewer available adsorption sites for the target pollutant which reduces the adsorption capacity

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9
Q

What is equilibrium?

A

Adsorption = Desorption and Ce remains constant

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10
Q

What is ment by the carbon loading in equilibrium? Use a formula to explain your answer.

A

The Mass of adsorbed pollutant per mass of Activated Carbon in equilibrium conditions:

qe = Co - Ce / Dose of AC

where Co is the initial concentration of the pollutant and Ce is the concentration of the pollutant after adsorption

Co - Ce is the removal of the pollutant

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11
Q

What does the carbon loading (qe) depend on mention 6 possible reasons?

A
  1. Gradient (Co)
  2. Temperature - better adsorption at lower water temperatures
  3. The type of AC
  4. The properties of the pollutant (size etc)
  5. Water matrix
  6. Competition effects
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12
Q

When should a Langmuir Model be used to describe the adsorption of AC?

Sketch the model and describe the formula used in standard and linearised form

A

When investigating a monolayer of particles adsorbing.

x axis: C (ug/L)
y axis: q (ug/mg)

the max loading capacity (qmax) will be reached at a certain C as all adsorption sites will be full.

qe = abCe/(1+a*Ce)

Where a = number of adsorption sites

b = adsorption energy

Linerised:

1/q = 1/b + 1/abCe

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13
Q

When should a Freundlich Model be used to describe the adsorption of AC?

Sketch the linearised model and describe the formula used in the empirical and linearised form

A

Empirical:

qe = Kf * Ce^n

Where:

qe = equilibrium loading capacity (g/kg)
Ce = equilibrium concentration (g/m3)
x = adsorbed mass of substance (g)
m = mass of AC (kg)
Kf = Freundlich constant (g/kg)*(m3/g)^n
n = Freundlich Constant (-)

Linearisation:

Log(qe) = Log(Kf) + n * Log(Ce)

Kf = y intercept

n = gradient (x2-x1/y2-y1)

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14
Q

Looking at a Freundlich Model: if n is constant how will K differ between pollutants?

A

The K value determines the loading capacity of a pollutant, if n is constant then each pollutant will have a different loading capacity - the higher the k value the better will will adsorb

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15
Q

Looking at a Freundlich Model: if Kf is kept constant how will the model look if n = 1, n > 1 and n < 1 explain?

A

N = 1 : Qe = Kf*Ce liner relationship

N > 1 : exponential - unfavourable isotherm bc only starts to adsorb at very high adsorption pressure (high eq conc)

N < 1 : hyperbolic - favourable isotherm bc most of the loading happens at a low equilibrium concentration

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16
Q

What are the units for the Freundlich constant (Kf)?

A

(g/kg)*(m3/g)^n

17
Q

Explain the kinetics in 4 steps of a dissolved pollutant adsropbing to activated carbon?

A
  1. Transport of dissolved pollutant by advection and dispersion in the bulk solution towards the carbon grain - fast
  2. Transport in the stagnant film by diffusion - slow
  3. Transport in the pores by diffusion - slow
  4. Then when it reaches the carbon surface it adsorbs fast.
18
Q

How to improve the kinetics of adsoption processes?

A
  1. Improve flow conditions by increasing turbulence and mixing - reduces the thickness of the stagnant layer around the AC grain
  2. Pore structure influences the transport inside the pores
  3. Adsorption is driven by solute concentration gradient therefore, a high solute concentration and clean AC surface will improve kinetics
19
Q

10 mg/L of PAC and GAC are dosed during batch experiments. A change in the concentration of the target micropolutant is observed over time. sketch two curves to show which is PAC and GAC under these conditions and why?

A

PAC will reach equilibrium after 4 hours whereas GAC will take many days before it reaches equilibrium.

PAC adsorbs faster because it has a larger surface area per mass volume

GAC has large pore diameters and pollutants may take longer to diffuse to the carbons surface

20
Q

What are the main differnces between PAC and GAC?

A

GAC:

Used as a continuous process in granular filters where water will pass through it - have a thicker stagnate layer as less turbulence

  • Regneration/ Reactivation possible
  • Longer Contact Time

PAC:

-dosed in a slurry reactor or in the system intermittently and will mix and flow with the water until it is removed - smaller stagnant layer

  • Cannot be re-activated
  • Shorter Contact Time
21
Q

How can a breakthrough curve be used to determine when you should regenerate your GAC filter?

A

The breakthrough is determined when a GAC filter is already sufficiently loaded with the target pollutant but now only the mass transfer zone is left (no clean surfaces) then you will start to measure the pollutant concentration in the effluent. Depending on the regulations for your effluent then you will need to regenerate the AC if this concentration exceeds that value.

22
Q

What influences a GAC filter breakthrough curve? And state if each action will accelerate or slow down adsorption?

A

Postpone Breakthrough curve:

  • Longer contact time
  • Larger Gain Size: the column will be exhausted slower so your breakthrough will come later

Accelerate Breakthrough curve:

-Higher temperature: adsoption is less good (favours cooler temperatures - exothermic)

  • Compeition with NOM
  • Competition with other non mico-pollutnats
  • NOM pre-loading
  • Larger grain size: increases microporuse surface area behind a consitricted pore also increases resulting in a lower adsorption capacity per mass of adsorbent
23
Q

Explain the difference between a PAC process and a GAC process regarding the adsorption capacity of the AC?

A

PAC in reality has a too short contact time to reach absorption equilibrium therefore some of the PAC will be wasted.

GAC has a much longer contact time usually operated for hours to years until we achieve the desired breakthrough for the target pollutant in the effluent.

24
Q

Name 3 advantages and 3 disadvanatges of using PAC?

A

Advantages:

Minimal investment costs (if combined with coagulation/ sedimentation and filtration)

  1. No pre-loading with NOM (but there still is competition)
  2. Flexible operation (seasonal)

Disadvantages:

  1. Short contact time - equilibrium concentrations are not yet reached
  2. PAC capacity is really exhausted before disposal
  3. No reactivation possible
25
Q

What are 3 advantages and disadvantages of GAC?

A

Advantages GAC:

  1. Effluent concentration = zero until the breakthrough
  2. Reactivation is possible
  3. Robust against peak concentrations

Disadvantages:

  1. High investment costs
  2. Pore blocking
  3. Pre-loading with NOM

Drinking Water Treatment (6 decks)

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