The effect of pH on coagulation (particle-particle, particle-fabric) was studied so as to find some substitute for a phosphate builder, using ferric oxide (Fe
2O
3) as a model of particulate soil. The ζ-potentials of Fe
2O
3 particles and nylon fabrics were mesured in a solution of various pH with HCl or NaOH adjusted to 5×10
-3 ionic strength with NaCl. Fe
2O
3 deposited on the nylon fabrics was also measured in the same solution. The particle size distribution of Fe
2O
3 in the solution was measured and interpreted in terms of the potential energy of the interaction between particles. Fe
2O
3 deposited on nylon fabrics is discussed on the basis of the heterocoagulation theory for a sphere and an infinite flat plate.
The increase in the ζ-potential of Fe
2O
3 in a highly acidic or alkaline solution promoted the dispersion stability of particles, and corresponded to the potential energy curves. It was difficult to express Fe
2O
3-deposition on nylon fabrics in terms of potential energy. However, deposition was found to depend on dispersion stability in aqueous solution. Consequently, the redeposition of particulate soil may possibly be prevented by enhancing the ζ-potential so as to promote dispersion stability.
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