Time dependent changes in zeta potential of freshly precipitated calcium carbonate

We were interested in monitoring zeta potential of CaCO3 freshly precipitated from Na2CO3 and CaCl2 solutions at 30 degreesC and in pH changes of the slurry. This seemed interesting to us in aspect of hard scale formation. The influence of impurity ions and magnetic field (MF) on the parameters were also investigated. Before calcium carbonate precipitation the solutions were exposed to a permanent N-S (B = 0.4 T) MF applied orthogonally to silicone tubes in which the solutions circulated for 30 min. Then, after CaCO3 precipitation time dependent changes of zeta potential and pH were measured. The same parameters were also determined for the reference systems in which the solutions were not MF-treated. Besides the zeta potentials of CaCO3 precipitated from equimolar (8 x 10(-3) M) Ca2+/CO32-, also these precipitated from excess 1.2 x 10(-2) M of Ca2+ and 8 x 10(-3) M CO32-, or reverse, were measured up to 60 min (in some systems up to 240 min). Simultaneously pH of the suspensions was recorded. It was found that MF effect on the zeta potential depended on the Ca2+/CO32- ratio and the "memory effect" lasted for a longer time if calcium carbonate was precipitated at the excess of Ca2+ or CO32- ions. The experiments of CaCO3 precipitation in the presence of impurity ions: Mg2+, Fe2+, or SO42-, which are very common in natural deposit of the scale showed that both the ions and MF affect all the investigated parameters. In these systems precipitation and sedimentation rates were determined by measuring the light absorbance at lambda = 543.3 nm. The effects depended on the kind of the impurity ion present. (C) 2003 Elsevier B.V. All rights reserved.