Surface properties and aggregation of basic aluminium sulphate hydrolysis products

Abstract

The action of pre-hydrolysed coagulants such as basic aluminium chloride (BAC) and sulphate (BAS) in water treatment is of considerable practical and fundamental interest. Variables such as working solution concentration, dosage, pH and ionic strength are known to be important. We have investigated the influence of these variables on the ζ-potential and degree of hydration of hydrolysis products of basic aluminium sulphate samples with different basicities. The products were characterised by a timed ferron complexation procedure, giving information on the degree of polymerisation of the BAS samples. Electrokinetic measurements showed that the ζ-potentials of hydrolysis product particles were consistently higher (more positive) for the higher basicity samples. The ζ-potential decreased with increasing concentration of the working solution and with increased dosage. The degree of hydration of the hydrolysis products was investigated using the spin-lattice relaxation time of protons of water molecules bound to their surfaces. This was found to decrease with increasing basicity of BAS, indicating a greater degree of hydration. Since this effect is expected to decrease ζ potential by a displacement of the electrokinetic shear plane, the observed increase in ζ is probably due to the adsorption of highly charged positive hydrolysis products of BAS.