8) Irreversible or reversible sorption process

Question:

8a) A sorbent is dispersed in a given volume of water V_W with a given concentration C_aq of a pollutant. What will happen if the sorption process of the pollutant to the sorbent is (i) an irreversible process or (ii) a reversible process (i.e., true sorption equlibrium)?

8b) A given volume of water with a given concentration of a pollutant is filtered through the same sorbent. What will happen if the sorption process is a) an irreversible process, b) a reversible process (i.e., true sorption equilibrium)?

Answer:

8a) (i) The sorbent will either sorb all pollutants from the aqueous phase (i.e., create completely clean water) if the so-sorbed concentration remains smaller than the sorption capacity of the sorbent or it will sorb a fraction of the pollutants that defines the sorption capacity (i.e., the sorbent has no free "binding sites" for the pollutant left). In this case, a fraction of the pollutant will remain in the water.

8a) (ii) The sorbent will always sorb a certain fraction of the pollutant so that the water will be partly but never completely cleaned. This fraction can be calculated from Equation 9 in the script if the partition coefficient is known. The sorbed amount (i.e. the "sorption capacity") will increase with increasing aqueous concentration of the pollutant and water volumes.

8b) (i) All pollutant molecules will be irreversebly removed from the filterd water until the sorption capacity of the filter is reached (i.e., the sorbed pollutant concentration corresponds to the maximum amount that can be sorbed). Once the sorption capacity is reached, the sorbent cannot take up any more pollutant molecules. Hence, additional water that passes through the filter will stay unaffected: if clean water passes through the filter it will stay clean (note: no pollutant desorbs as the sorption proecess was aid to be irreversible); pollutants from contaminated water will not be taken up (i.e., the water stays polluted) if the filter has already reached its sorption capcity.

8b) (ii) A certain volume of water (depending on the sorption coefficient and the mass of filter) will be cleaned by the filter (i.e., the outflow concentration is smaller than, for example, 99% of the inflow concentration). Beyond this volume of water, the pollutant concentration in the outflow will steadily increase until it reaches 100% of the inflow concentration. This outcome may seem similar to the outcome of the irreversible filter. However, in this case, if clean water is flushed through the filter that was previously exposed to and had sorbed pollutants, then the clean water will take up some of these contaminants (i.e., partition equilibrium is re-established). The formerly clean water will then contain contaminants after having passed through the filter.

Once the outflow concentration has reached 100% of the inflow concentration, the filter and the water are in equilibrium. If the concentration of pollutants in the inflow is then increased, the additional contaminant concentration will not immediately break through (as it would in case of a saturated irreversible filter) but will be retained again ... of course only until equilibrium between the filter and the new pollutant concentration in the water is reestablished.

It is a common misbelief among laypersons that a sorbent sorbs the first X gram of pollutant that arrive at the filter and then the sorbent is saturated and cannot sorb anymore of the pollutant. If this was true (ie., if sorption was an irreversible process) then one could indeed characterize the capacity of a filter by such a numer of X gram per Y gram of sorbent. But there are very few cases where sorption occurs as an irreversible process. The typical situation that we have instead is a dynamic sorption equilibrium that is characterized by a sorbtion constant. In this case, any information about the sorption capacity can only be made for a specific concentration of the pollutant in the water.