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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 (Al2(SO4)3*14(H20)) is commonly employed in water treatment to neutralize the negative colloid surface charge. Alum dissolves and forms positively charged species such as Al+3, Al(OH) +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 .
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