analysis of the adsorbent capacity of a carbon

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granulometry of an powdered activated carbon ( PAC )

Dry the carbon for 4 hours at 120°C. Precisely weigh out approximately 10 g of carbon and place on the first screen (125 mm). Having wetted the screen, pass the screen under a moderate pressure output from a water tap, washing the carbon that remains on the screen. This task is carried out over a white enamelled pot and the wash is continued until no more carbon is seen to be passing through the screen. Then place the screen in an oven for 4 hours at 120°C to dry it out. Weigh what remains on the screen. Use the difference in relation to the initial figure recorded, to calculate the amount of carbon that has passed through the screen. Express this ratio as a percentage.

Repeat these steps with the smaller mesh screens (90‑63‑45 mm).

adsorption isotherm = Freudlich isotherm

The adsorption capacity of an activated carbon compared with a given pollutant can be estimated using an adsorption isotherm.

The Freudlich mode (see adsorption) establishes the relationship between the mass of pollutant held back per unit of mass of carbon

Formula: adsorbent capacity of a carbon  - mass of pollutant held back per unit of mass of carbon

and the pollutant’s concentration in an aqueous phase in a state of equilibrium Ce with the carbon.

Formula: adsorbent capacity of a carbon - Freudlich  isotherm

K and n, two coefficients established through experimentation

These isotherm curves, see figure 25 for instance, can be used to establish:

  • a carbon’s maximum adsorption capacity that can be used to estimate the maximum mass of a pollutant held back in a state of equilibrium per unit of mass of carbon, for a concentration of pollutant in an aqueous phase that is equal to the original concentration of the pollution introduced:
  • a pollutant’s adsorption index (Q10) defined as the mass of pollution held back per unit of mass of adsorbent for a pollutant concentration equal to one tenth of the pollutant’s original concentration. This index is used to estimate the adsorption capacity of carbon under mean conditions that are closer to the normal operation of carbon in a fixed bed system.

If these capacities, indices, are to be used as the basis for comparing different suppliers and/or reception methods applicable to activated carbon, agreement must first be reached with the various suppliers on a very precise method of operation.

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Figure 25. Adsorption isotherm for an activated carbon

plotting an isotherm

  • take six 1.2 litre glass flasks ;
  • add 1 litre of water containing the pollutant whose removal is under examination. (comment: if we are dealing with a volatile pollutant, the flasks must be filled up to the brim in order to avoid losing any mass through volatilisation);
  • when studying a naturally polluted water, add the water as provided;
  • when studying a made-up water, as a rule, 1 mg of the pollutant under test must be injected into each flask; this applies to a study designed to appraise the potabilisation of water (comment: the same applies to IWW, adding 10 – 100 – 1 000 mg per flask depending on the case simulated);
  • reduce the carbon under examination to a powder by crushing it in a mortar and dry-sieving it through a 40 µm screen. Collect the particles that have passed through the screen;
  • dry the carbon for 4 hours at 120°C in a powder box;
  • add precisely weighed, increasingly high amounts of carbon into the flasks containing the polluted water under study:
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Table 9. Plotting an isotherm

After at least 5 days’ moderate agitation (30 to 40 rpm) at a constant temperature, filter each sample through a 0.45 µm cellulose acetate membrane. Remove the first 100 millilitres and measure the pollutant remaining in the rest of the filtrate. Thus, for each amount of carbon, we obtain the balanced pollutant concentration in water after contact.

Plot the isotherm curve using log-log coordinates, entering the concentration at equilibrium expressed in mg · L-1 on the X-axis and the mass of pollutant held back per gram of carbon expressed in milligrammes on the Y-axis (figure 26).

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Figure 26. Adsorption isotherm for an activated carbon

kinetics study

In the case of granular carbon, contact can also be achieved without crushing the carbon, by continuing agitation for a long time (several weeks). Measurement of concentration at equilibrium at different times during contact can be used to model the adsorption of the pollutant being studied.

carbon ash

Precisely weigh out approximately 1 g of dry carbon and place in a crucible; let P1 be the mass weighed.

Incinerate the carbon under study at 625 °C (± 25 °C). Thoroughly check for complete incineration. After cooling, weigh the ash; let P2 be this weight.

Ash content C is expressed as follows:

Formula: Ash content C

iodine index

Based on the iodine isotherm plotted for the carbon concerned, its iodine index represents the number of milligrams of iodine absorbed per gram of carbon for a 0.02N residual iodine concentration in the filtrate.

dechlorination capacity

The properties of the dechlorination capacity of a carbon are defined by the depth of the bed required to reduce the amount of existing chlorine by half at a percolation rate of 20 m · h–1.

Boil the carbon in distilled water to expel any air the carbon may contain. Place the damp carbon away from air in a 22 mm diameter tube until a precisely measured 10 cm high column is obtained.

Using a 7.5 pH sodium hypochlorite solution, prepare a chlorinated water solution containing 10 mg· L-1 of active chlorine.

Percolate this chlorinated water through the carbon column at a rate of 20 m · h–1. After 30 minutes operation, precisely titrate the chlorinated water at the column intake (a expressed in mg· L-1) and at the column outlet (b mg · L-1).

H being the layer depth in cm, calculate the half-dechlorination length G (in cm):

Formula: half-dechlorination
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