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Colloidal particles are of particular interest in water treatment because they correlate with the presence of pathogenic organisms, interfere with disinfection, and negatively impact drinking water quality. Colloidal particles (0.001-1.0 μm) are difficult to remove by gravity sedimentation because of their low settling velocities.
Naturally occurring colloids typically have a negative surface charge and electrostatic repulsion acts to hinder particle aggregation. Use of a coagulant such as alum (Al2Al 2(SO4SO 4 )3*14(H20H 2 0)) is commonly employed in water treatment to neutralize the negative colloid surface charge. Alum dissolves and forms positively charged species such as Al 3+3, Al(OH) 2+2 and Al(OH) 2+ that could adsorb to the surface of colloids. Precipitation of Al(OH) 3(s) also occurs on colloid surfaces. The solid surface charge of Al(OH) 3(s) is positively charged at circumneutral pH .
Aluminum can also form polymer species in water. In solution, Al2 Al 2 to Al6 Al 6 polymers can form fairly rapidly, however longer polymers that aid bridging between floc particles can take days to form utilizing alum. Different forms of polyaluminum chloride (PACl) can assist the formation of longer chain polymers facilitating bridging between particles in much shorter time.
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