Zeta potential and particle size measurement of humic acid-heavy metal complex

(1) Measurement conditions
Humic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in a 0.1M NaOH aqueous solution to prepare a 5 wt% aqueous solution. This solution was diluted with distilled water to make a 10 ppm solution, which was adjusted to pH 5 with 0.1M HCl. From each metal shown in Fig. 2, three types of metal ions with different adsorption abilities-Cu²⁺, Zn²⁺, Mn²⁺ (standard material for atomic absorption manufactured by Wako Pure Chemical Industries, Ltd., all nitrates) were adjusted to each concentration (1 to 100ppm). Each solution was mixed with a 10 ppm solution of humic acid, adjusted to pH 5, and then the zeta potential and particle size were measured. A laser zeta potential meter manufactured by Otsuka Electronics Co., Ltd. was used for zeta potential measurement, and a dynamic light scattering photometer manufactured by Otsuka Electronics Co., Ltd. was used for particle size measurement.

Figure 3. Zeta potential of complex of 10ppm humic acid and each metal ion

(2) Measurement results and discussion
Fig. 3 shows the results of the zeta potential measurement after mixing a solution with different concentrations of each metal ion and a 10 ppm solution of humic acid. It can be seen that the zeta potential shows a negative value in all metal concentration, but its absolute value decreases as the metal ion concentration increases, and the charge approaches zero because it forms a complex with metal cations. In addition, zeta potential of each metal ion is in the order of Mn²⁺> Zn²⁺ > Cu²⁺ in the absolute value up to the metal ion concentration of 10ppm and corresponds well in the order of low adsorption performance shown in Fig. 2. It is thought that the higher the adsorption performance of a metal to humic acid is, the closer the charge approaches to zero due to the action of the metal cations. Fig. 4 shows the measurement result of the average particle size measured in the same way, and Fig. 5 shows the particle size distribution of Cu²⁺. These results indicate that there was no significant difference of particle size between each metal ion up to a metal ion concentration of about 5 ppm, but when it was 10 ppm, the average particle size increased, and the particle size distribution changed from a sharp peak to a broad peak. And the particle size also increased due to the formation of the complex and the aggregation of the complexes.

Figure 4. Average particle size of the complex of 10 ppm humic acid and each metal ion 

Figure 5. Change in particle size distribution (10 ppm humic acid + Cu²⁺ (0-20 ppm))

These results indicate that the particle size of each metal ion increased significantly in the order of Cu²⁺> Zn²⁺ > Mn²⁺ when the metal ion concentration was from 10 ppm to 20 ppm, and the metal ion with higher adsorption performance to humic acid has a larger average particle size. As observed in the particle size distribution, if it is added excessively, it is expected that large aggregates are formed and precipitated. However, when the metal ion concentration is close to 50 ppm, the average particle size becomes smaller. It is possible that the particle size has become too large and floating particles were measured after precipitating. 

The zeta potential showed a difference between various metal ions even when the metal ion concentration was relatively low, 10 ppm or less, and the absolute value of the surface potential of humic acid gradually decreased due to the adsorption of metal ions of opposite charge. increase. On the contrary, in the region where the metal ion concentration is low, the particle size did not show a remarkable difference even though the complex was formed, and the complex increased rapidly from a certain concentration was observed, which indicates the possibility that the complexes aggregate with each other.