4.3 Application of Pre-mixed Complexes as Flocculants
For a long time, the application of pre-mixed PECs as flocculants seemed to be a field
of academic work that had not found a practical application [20]. But, as found
earlier [82], such complexes made from oppositely charged polymer solutions have
interesting properties and are very effective for the flocculation of finely dispersed
inorganic particles. A reflocculation was not noticed in a wide range of
concentrations. Investigations on PECs as flocculants for inorganic particles like
silica are important for the understanding of flocculation mechanism. But, PECs will
never be applied as flocculant for such inorganic particles in practice. Nevertheless,
PECs play an important role as flocculants in certain applications for special
problems in wastewater treatment, such as low content of solids, soluble detrimental
substances, or small amount of dye that can color a big volume of water. Therefore
their removal is important.
As one of the first to study this, Somasundaran and coworkers [83] found that
combinations of polystyrene sulfonate and cationic polyacrylamide enhanced the
flocculation of (positively charged) alumina suspensions. The weight ratio of the
two polymers was kept at 1:1 for all experiments. But, pre-mixing of the two
polymers did not give as good results as those obtained when the polymers were
added step by step [82]. In our opinion, the charge ratio of both PECs is extremely
important and influences the flocculation mechanism. Effective flocculation can be
found with highly charged complexes as well as with complexes near the 1:1 ratio
of charges.
Onabe [84] analyzed the drainage behavior of pulp suspensions with a twocomponent system, with attention to polyion complex formation. With cationic and
anionic polyacrylamides (molecular weights 3,000,000 and 1,000,000 g/mol) added
individually, a polyion complex with an irregular three-dimensional structure was
precipitated on the fiber surface. This reduced the homogeneity of the paper sheets.
In contrast to this, a pre-mixed system of an anionic and cationic polyacrylamide [83]
resulted in an improvement in pulp retention and paper quality (paper strength).
Because of the very high molar mass and thus the big coil diameter, the formed
PECs are very big and are therefore good flocculants.
Wagberg and coworkers [85] described the preparation and characterization of
complexes for dry and wet strength improvement of paper. They investigated the
structure of complexes formed by two oppositely charged PEL commonly used in
the paper industry: polyamideamine epichlorohydrine condensate (PAE) and
carboxymethylcellulose (CMC). They found that complexes can have a good
resistance to PEC aggregation/dissolution at high salt concentrations and that it
was possible to adjust the net charge of the complex particle solutions, making the
PEC of great value in papermaking for covering the fibers with a high amount of
material in a one-step procedure.
CMC-rich cellulose sheets were prepared by Uematsu [86] with a cationic
retention aid. When 5% poly(NNN-trimethyl-N-(2-methacryloxyethyl) ammonium
Polyelectrolyte Complexes in Flocculation Applications
49
For a long time, the application of pre-mixed PECs as flocculants seemed to be a field
of academic work that had not found a practical application [20]. But, as found
earlier [82], such complexes made from oppositely charged polymer solutions have
interesting properties and are very effective for the flocculation of finely dispersed
inorganic particles. A reflocculation was not noticed in a wide range of
concentrations. Investigations on PECs as flocculants for inorganic particles like
silica are important for the understanding of flocculation mechanism. But, PECs will
never be applied as flocculant for such inorganic particles in practice. Nevertheless,
PECs play an important role as flocculants in certain applications for special
problems in wastewater treatment, such as low content of solids, soluble detrimental
substances, or small amount of dye that can color a big volume of water. Therefore
their removal is important.
As one of the first to study this, Somasundaran and coworkers [83] found that
combinations of polystyrene sulfonate and cationic polyacrylamide enhanced the
flocculation of (positively charged) alumina suspensions. The weight ratio of the
two polymers was kept at 1:1 for all experiments. But, pre-mixing of the two
polymers did not give as good results as those obtained when the polymers were
added step by step [82]. In our opinion, the charge ratio of both PECs is extremely
important and influences the flocculation mechanism. Effective flocculation can be
found with highly charged complexes as well as with complexes near the 1:1 ratio
of charges.
Onabe [84] analyzed the drainage behavior of pulp suspensions with a twocomponent system, with attention to polyion complex formation. With cationic and
anionic polyacrylamides (molecular weights 3,000,000 and 1,000,000 g/mol) added
individually, a polyion complex with an irregular three-dimensional structure was
precipitated on the fiber surface. This reduced the homogeneity of the paper sheets.
In contrast to this, a pre-mixed system of an anionic and cationic polyacrylamide [83]
resulted in an improvement in pulp retention and paper quality (paper strength).
Because of the very high molar mass and thus the big coil diameter, the formed
PECs are very big and are therefore good flocculants.
Wagberg and coworkers [85] described the preparation and characterization of
complexes for dry and wet strength improvement of paper. They investigated the
structure of complexes formed by two oppositely charged PEL commonly used in
the paper industry: polyamideamine epichlorohydrine condensate (PAE) and
carboxymethylcellulose (CMC). They found that complexes can have a good
resistance to PEC aggregation/dissolution at high salt concentrations and that it
was possible to adjust the net charge of the complex particle solutions, making the
PEC of great value in papermaking for covering the fibers with a high amount of
material in a one-step procedure.
CMC-rich cellulose sheets were prepared by Uematsu [86] with a cationic
retention aid. When 5% poly(NNN-trimethyl-N-(2-methacryloxyethyl) ammonium
Polyelectrolyte Complexes in Flocculation Applications
49