**6.6. Membrane‐based separation method**

A membrane is a specific type of a barrier that enables the separation of species in a gas or liquid through various mechanisms such as diffusion, sieving or sorption. The selective sepa‐ ration occurs as a result of the semipermeable nature of the membranes. This is the ability of the membrane to allow the passage of certain substances through it while it prevents the pas‐ sage of others based on their sizes and/or molecular weights. Thus, in membrane‐based sepa‐ ration method of water purification, the water usually passes through the membrane while the suspended pollutant, usually with comparatively larger sizes and molecular weights, is unable to pass through the membrane. They get retained in the medium or on the membrane and later removed.

Membrane separation is a general term used to encompass different types of separation processes that are characteristically the same or similar since they all use membranes. The difference lies in the pore size of the membranes and the driving force involved in the separation process. The driving forces for separation may include high pressure applica‐ tion, the creation of concentration gradient and the use of electric potential [86]. These pro‐ cesses are categorised as microfiltration, ultrafiltration, nanofiltration and reverse osmosis [87]:


An ideal membrane system must have good fluxes and be highly selective. It must have excel‐ lent thermal, chemical and mechanical stability with low tendency of foul formation. Some advantages of membrane system of water purification include the following [87]:

**(1)** It has comparatively low energy requirements.

**(3)** Adsorption of the pollutants on the active surface sites.

432 Phenolic Compounds - Natural Sources, Importance and Applications

tion, solution pH and temperature.

and later removed.

[87]:

bacteria.

**6.6. Membrane‐based separation method**

**(4)** Migration of the adsorbed pollutants through diffusion onto the pores' surfaces.

Various researchers have studied phenol adsorption from polluted water with different types of adsorbents. Phenol adsorption efficiency of different adsorbents including bagasse ash, activated carbon and charcoal from wastewater was studied by [84]. The adsorption efficiency was assessed based on the influence of pH, concentration of EDTA, anions and adsorbent dose. Their result showed 98, 90 and 90% phenol removal efficiencies by activated carbon, wood charcoal and bagasse ash systems, respectively. Removal efficiency was observed to increase with a decrease in the pH of the system. Effects of EDTA and nitrate ion content of the solution were identified as the factors that influenced the adsorption process. Chloride ion, on the other hand, exerted a significant adverse effect on the efficiency of bagasse ash system. Film diffusion was noted to control the adsorption efficiencies of all the adsorbents used. Similarly, the use of sugarcane bagasse‐based activated carbons for effective phenol adsorption from aqueous medium was assessed by Akl et al. [85]. The result of the study pro‐ posed sugarcane bagasse‐based activated carbon (SCBAC) as a viable adsorbent for phenol elimination from water. The pollutant eradication process depended solely on its concentra‐

A membrane is a specific type of a barrier that enables the separation of species in a gas or liquid through various mechanisms such as diffusion, sieving or sorption. The selective sepa‐ ration occurs as a result of the semipermeable nature of the membranes. This is the ability of the membrane to allow the passage of certain substances through it while it prevents the pas‐ sage of others based on their sizes and/or molecular weights. Thus, in membrane‐based sepa‐ ration method of water purification, the water usually passes through the membrane while the suspended pollutant, usually with comparatively larger sizes and molecular weights, is unable to pass through the membrane. They get retained in the medium or on the membrane

Membrane separation is a general term used to encompass different types of separation processes that are characteristically the same or similar since they all use membranes. The difference lies in the pore size of the membranes and the driving force involved in the separation process. The driving forces for separation may include high pressure applica‐ tion, the creation of concentration gradient and the use of electric potential [86]. These pro‐ cesses are categorised as microfiltration, ultrafiltration, nanofiltration and reverse osmosis

• Microfiltration: The membrane's pore size of this technique ranges from 0.1 to 1.0 µm. It is normally used to filter suspended particles or colloidal solutions with large particles and

• Ultrafiltration: The pore diameter of this type of membrane ranges from 0.01 to 0.1 µm and can be used for filtration macromolecules such as polymers and proteins from solution.


The technology is also not without disadvantages. Some of these disadvantages include [88]:


Phenol has been separated from water with membrane‐based separation technique by using non‐modified, and ionically, and covalently cross‐linked ethylene methacrylic acid copolymer‐ based membranes [89]. They found out that the total flux increased with increasing phenol content in the feed while the enrichment factor decreased. They, however, observed lesser fluxes and higher enrichment factors when non‐modified membrane containing a higher amount of methacrylic acid monomer was used. Ionic cross‐linked membrane proved to be the most efficient membrane against the feed containing a high concentration of phenol.

Use of ionic liquids in the form of bulk liquid membranes for the elimination of phenol from water has also been studied by Ng et al. [90]. High hydrophobic ionic liquids includ‐ ing 1‐butyl‐3‐methylimidazolium hexafluorophosphate, 1‐butyl‐3‐methylimidazolium bis (trifluoromethylsulfonyl) imide and 1‐butyl‐3‐methylimidazolium tris(pentafluoroethyl) trifluorophosphate were used for the experiment. The stability, membrane loss and phenol elimination efficiency of these liquids were compared. Their results identified 1‐butyl‐3‐ methylimidazolium bis (trifluoromethylsulfonyl) imide as the best performing liquid in terms of phenol elimination and stripping efficiencies. This liquid exhibited phenol extrac‐ tion efficiency of 96.21% and stripping efficiency of 98.10%. These values were attained at optimum conditions of 225 and 135 rpm aqueous and membrane stirring speed, respectively. This solvent was identified to possess higher hydrogen bonding, basicity and low viscosity compared to the other two solvents used.
