Study of the scaling process on membranes

The state of water was studied by the method of differential scanning calorimetry in initial membranes and the membranes following their use in a water desalination process based on the baromembrane and electrodialysis methods. The analysis of data obtained makes it possible to determine the contribution of the CaSO4 scaling process deposited on the surface of reverse osmosis and ion exchange membranes in water desalination processes. We determined that an intensive accumulation of hardly soluble CaSO4 scale occurs on the surface of an MK-40 cation exchange membrane when the hardness water desalination process is held in an electrodialysis unit. This results in clogging of about 50% of mesopores having a radius of about 50Å. In the case of using an MA-40 anion exchange membrane one could observe only insignificant blocking of micro- and macropores in the polymer frame of the membrane. Any scale on the surface of anion exchange membrane MA-40 was not found. It was established that nucleating centers of CaSO4 formed in a solution were not located on the surface of membranes but penetrated deep into the pore space of a polymer frame in both cases of water desalination (based on electrodialysis and reverse osmosis). We calculated the amount of free and bound water in a membrane OPAM-KN under investigation following its use in processes of water desalination both with and without the addition of the PAA inhibitor. Variation of the amount of free and bound water makes it possible to assess the layout of CaSO4 nucleating centers in the polymer matrix of the membrane or on its surface. The using of polyacrylamide as an inhibitor in a baromembrane unit results in a partial blocking of big pores in a membrane. The use of the inhibitor in the water desalination process makes a polymer layer over the membrane more compact owing to binding of nucleating centers of CaSO4 by polyelectrolyte or by it blocking. Redistribution of pore space occurs in the membrane: a reduced radius of pores gives rise to the reduced amount of free water, while the amount of bound water increases.

It was determined that the water demineralization process provided by the electrodialysis method tends to about 50% mesopores having a radius of about 50Å stopping up in the MK-40 cation exchange membrane and leading to the intensive accumulation of hardly soluble CaSO4 scale occurring on the surface. At the anion exchange membrane MA-40 using observe only insignificant blocking of micro- and macropores in the polymer frame of the membrane. As a result any scale on the surface of anion exchange membrane MA-40 was not found. It was established that nucleating centers of CaSO4 formed in a solution were not located on the surface of membranes but penetrated deep into the pore space of a polymer frame in both cases of water desalination (based on electrodialysis and reverse osmosis). The amount of free and bound water in a membrane OPAM-KN under investigation was calculated after following its use in processes of water desalination both with and without the addition of the PAA inhibitor. Variation of the amount of free and bound water makes it possible to assess the layout of CaSO4 nucleating centers in the polymer matrix of the membrane (cCa2+ = 0.2 – 0.5 g/l) or on its surface (cCa2+ = 1.3 g/l). The use of the inhibitor in the water desalination process makes a polymer layer over the membrane more compact owing to binding of nucleating centers of CaSO4 by polyelectrolyte or by it blocking. Redistribution of pore space occurs in the membrane: a reduced radius of pores gives rise to the reduced amount of free water, while the amount of bound water increases. The inhibitor presences lead to the increasing of retention coefficient.