Effects of coagulants on the surface forces of colloidal alumina under water treatment conditions

The effect of sulfate and selenate anions on the rate of coagulation of alumina colloids by Al(III) salts has been investigated under conditions typical of potable water treatment. Methods used included conventional light scattering and jar test coagulation experiments, determination of zeta potential and measurements of surface forces using an atomic force microscope. The presence of sulfate or selenate increases the initial rate of Brownian coagulation of the colloids, compared to an Al(NO3)3 coagulant by approximately two orders of magnitude. In jar testing, Al(NO3)3 coagulant produced fluffy, low density flocs with the greatest floc formation occurring at the highest pH (pH 8), whereas with Al2(SO4)3, floc density was greater and optimum floc formation occurred at an intermediate pH (pH 6). AFM measurements reveal that sulfate (as Na2SO4) exhibits surface forces that cannot be modeled by simple DLVO theory, with a long-range (>10 nm) attractive force dominating, with hysteresis on the compression and decompression curves indicative of a bridging mechanism. The same behaviour was observed with the analogue Na2SeO4. With Al(NO3)3 electrolyte, simple forces that can be modeled using DLVO theory were observed, but the uniformly repulsive forces were independent of pH. The force curves in the presence of Al2(SO4)3 were more complex than those with Na2SO4, showing van der Waals attraction dominating at small separation at the PZC and largely repulsive forces at other pH values. The overall results underline the unusual surface effects induced by sulfate (and selenate) with aluminium oxyhydroxide surfaces.