NF membranes are called microporous, because their size (ranging from 0.5 to
2 nm) is similar to that of few water molecules. This process is based on a selective
separation between solutes on the basis of steric, electrostatic, and dielectric interactions between the solute and the charged (usually negatively) membrane. The
reason for the dielectric effect is the low relative permittivity (about 30) in pores,
which is due to exclusion effects on water molecules. Moreover, because of the high
surface area of NF membranes, adsorption of solutes may play a major role on their
removal at low concentrations. NF is used to separate weakly charged or uncharged
molecules from multivalent ions as long as the Donnan effect remains high (e.g., a
high charge density onto the membrane and a Debye length larger than the pore size,
i.e., ionic strength of solution not high). For example, in NF, transmission ranges
usually from 30 to 85% and 1 to 30% for monovalent and divalent ions, respectively
(Degrémont 2005). In remediation, NF is often used for the treatment of landfill
leachates as it removes efficiently both dissolved total organic carbon (TOC) and
MTEs (Chaudhari and Murthy 2010). For the removal of metal ions, NF is often
preferred to RO because it has higher permeate flow rates and lower driving
pressures (Liu et al. 2008). It has been used for the recovery of oxidants from
contaminated groundwater during the treatment of contaminated soils (Liang et al.
2007; Ahmed Mohamed 2014). NF has not been used for chelating agent recovery,
since the electric charge carried by free ligands is usually higher than those of their
metal complexes and ligands are retained instead of contaminants (Suarez et al.
2013).
MF membranes are called macroporous, since their pores, ranging from 0.1 to
10 μm are visible with an optic microscope and are the largest membrane materials.
It is used to remove suspended particles using a sieving effect at very low driving
pressures ( 0.03 MPa). In SW operations, it is mainly used to remove solid particles
to prevent clogging of NF, RO, or pervaporation membranes (Kujawski et al. 2009).
MF is often used to retain degrading bacteria in membrane bioreactors; however, it is
very difficult to combine such a treatment with ligand recovery (Degrémont 2005). It
has been assessed in order to remove contaminants from oil field-produced water;
however, in contrast to UF, it was not able to meet standards for rejection into the
environment (Bilstad and Espedal 1996). The main issue with MF in the separation
of emulsions from wastewater is that a substantial fraction of contaminants is usually
present in microemulsions and in micelles with sizes lower than those of the
membrane pores (Peng and Tremblay 2008).
UF membranes are called mesoporous, since their pore size ranges between those
of NF and MF. It removes colloids and polymers from water and it is used in many
industrial applications (Jönsson and Tragardh 1990). It is mainly used in the
disinfection of drinking water production or as antifouling treatment before RO,
since UF membranes are very effective to remove the suspended matter. The main
mechanisms involved in the separation are sieving and adsorption, since a large
surface area is contacting the feed solution. These phenomena are responsible for the
reduction in permeate flow rates. Highly hydrophilic membranes should be used to
prevent the adsorption of hydrophobic molecules. UF is the most interesting membrane technology for oily wastewater treatment, thanks to its high efficiency in oil
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
35
2 nm) is similar to that of few water molecules. This process is based on a selective
separation between solutes on the basis of steric, electrostatic, and dielectric interactions between the solute and the charged (usually negatively) membrane. The
reason for the dielectric effect is the low relative permittivity (about 30) in pores,
which is due to exclusion effects on water molecules. Moreover, because of the high
surface area of NF membranes, adsorption of solutes may play a major role on their
removal at low concentrations. NF is used to separate weakly charged or uncharged
molecules from multivalent ions as long as the Donnan effect remains high (e.g., a
high charge density onto the membrane and a Debye length larger than the pore size,
i.e., ionic strength of solution not high). For example, in NF, transmission ranges
usually from 30 to 85% and 1 to 30% for monovalent and divalent ions, respectively
(Degrémont 2005). In remediation, NF is often used for the treatment of landfill
leachates as it removes efficiently both dissolved total organic carbon (TOC) and
MTEs (Chaudhari and Murthy 2010). For the removal of metal ions, NF is often
preferred to RO because it has higher permeate flow rates and lower driving
pressures (Liu et al. 2008). It has been used for the recovery of oxidants from
contaminated groundwater during the treatment of contaminated soils (Liang et al.
2007; Ahmed Mohamed 2014). NF has not been used for chelating agent recovery,
since the electric charge carried by free ligands is usually higher than those of their
metal complexes and ligands are retained instead of contaminants (Suarez et al.
2013).
MF membranes are called macroporous, since their pores, ranging from 0.1 to
10 μm are visible with an optic microscope and are the largest membrane materials.
It is used to remove suspended particles using a sieving effect at very low driving
pressures ( 0.03 MPa). In SW operations, it is mainly used to remove solid particles
to prevent clogging of NF, RO, or pervaporation membranes (Kujawski et al. 2009).
MF is often used to retain degrading bacteria in membrane bioreactors; however, it is
very difficult to combine such a treatment with ligand recovery (Degrémont 2005). It
has been assessed in order to remove contaminants from oil field-produced water;
however, in contrast to UF, it was not able to meet standards for rejection into the
environment (Bilstad and Espedal 1996). The main issue with MF in the separation
of emulsions from wastewater is that a substantial fraction of contaminants is usually
present in microemulsions and in micelles with sizes lower than those of the
membrane pores (Peng and Tremblay 2008).
UF membranes are called mesoporous, since their pore size ranges between those
of NF and MF. It removes colloids and polymers from water and it is used in many
industrial applications (Jönsson and Tragardh 1990). It is mainly used in the
disinfection of drinking water production or as antifouling treatment before RO,
since UF membranes are very effective to remove the suspended matter. The main
mechanisms involved in the separation are sieving and adsorption, since a large
surface area is contacting the feed solution. These phenomena are responsible for the
reduction in permeate flow rates. Highly hydrophilic membranes should be used to
prevent the adsorption of hydrophobic molecules. UF is the most interesting membrane technology for oily wastewater treatment, thanks to its high efficiency in oil
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
35