Comparison on aggregation and sedimentation of titanium dioxide, titanate nanotubes and titanate nanotubes-TiO2: Influence of pH, ionic strength and natural organic matter

• Aggregation highly related to microstructure and composition of the nanomaterials.
• Micro-heterogeneity of TNs–TiO2 led to smaller size but higher sedimentation rate.
• Electrical reversal for charge of TNs and TNs–TiO2 occurred when increasing Ca2+.
• HA caused aggregation energy barrier while Ca2+ acted as ion bridge when co-existed.

Aggregation and sedimentation are of significant importance to the safe applications of nanomaterials. In this study, aggregation and sedimentation of three homologous nanomaterials, titanium dioxide (TiO2), titanate nanotubes (TNs), and titanate nanotubes–TiO2 (TNs–TiO2) were systematically studied under various aqueous conditions. The influences of pH, ionic strength (Na+ and Ca2+) and natural organic matter (humic acid, HA) on aggregate size, zeta potential and sedimentation rate were totally investigated. It was found that maximum aggregation invariably occurred at pH close to point of zero charge. However, the three nanomaterials underwent different aggregation processes, even though aggregation was always greatly enhanced with increasing ionic strength in either Na+ or Ca2+ solutions. In Ca2+ solutions, the aggregate size of TiO2 tended to increase solely with increasing ionic strength, while the size of TNs and TNs–TiO2 increased initially before decreasing, due to the surface charge changing from negative to positive at high ionic strengths. The aggregation energy barrier was enhanced considerably by HA, thus inhibiting aggregation. TNs and TNs–TiO2 exhibited a greater energy barrier transition than TiO2 due to their higher adsorption capacity for HA, and moreover, the co-existence of Ca2+ also exhibited ion bridge effects on aggregation. Different microstructure and chemical composition of the nanomaterials resulted in different aggregation and sedimentation behaviors, and the micro-heterogeneity of TNs–TiO2 led to smaller size but higher sedimentation rate than TNs.

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