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Soil particle aggregation is closely related to soil quality and pollutant migration in the environment. Soil is a naturally complex system with multi-components, including different organic and inorganic particles. However, current studies with respect to particle aggregation by dynamic light scattering (DLS) have centred on single-component particles. Therefore, the application of DLS to mixture components is an important issue in soil science research. In this study, montmorillonite (Mont), kaolinite (Kaol) and humic acid (HA) particles were mixed in different mass ratios to form two-and three-mixture components, and then the aggregation kinetics of the mixture components in KNO 3 solutions were studied by DLS. The results showed that (a) for any given mixing ratio of two-and three-mixture component particles, there was only one critical coagulation concentration (CCC) value observed for the mixture components; (b) the CCCs of the two-mixture components aggregation were always between the CCC values of each single component in the mixture, and the CCC values of the three-mixture components aggregation were always between those of the two-components aggregation. Based on those experimental results we concluded that, for mixture components aggregation, all the components would participate in the aggregation with equal probability, and the aggregation should be driven by the average DLVO forces of the mixture components. Therefore, for the mixture components of natural soil, the soil particle aggregation would be a co-aggregation process, and the DLS technique would thus be a useful tool to study the real soil aggregation. Meanwhile, HA would decrease soil particle aggregation by increasing the DLVO/XDLVO repulsive forces, rather than promoting soil particle aggregation. Highlights• The aggregation of mixture colloids analysed by DLS is an important issue in soil science research.• There was only one CCC for mixture colloids with different colloidal components.• The aggregation of mixture colloids (soil) is a co-aggregation process.• HA will decrease soil particle aggregation by increasing the DLVO/XDLVO repulsive forces.

Background The interactions between colloidal particles in the binary systems or mixture colloids containing clay minerals and bacteria have important influences on formations and stabilities of soil aggregates, transportations of soil water, as well as biological activities of microorganisms. How the interfacial reaction of metal ions affects their interaction therefore becomes an important scientific issue. Methods Dynamic light scattering studies on the aggregation kinetics of mixture colloids containing kaolinite and Pseudomonas putida (P. putida) were conducted in this study. Results Aggregation could be observed between kaolinite and kaolinite, between kaolinite and P. putida when P. putida content was less than 33.3%. Additionally, aggregation rates decreased with increasing P. putida content. The critical coagulation concentrations and activation energies indicated that there were strong specific ion effects on the aggregation of mixture colloids. Most importantly, the activation energy increased sharply with increasing P. putida content, which might result from the lower Hamaker constant of P. putida compared with that of kaolinite. Contributions (1) Strong specific ion effects on mixture colloids aggregation of kaolinite-P. putida were observed; (2) the aggregation behavior of mixture colloids was determined by the average effects of mixture colloids, rather than the specific component. This finding provides an important methodological guide for further studies on the colloidal aggregation behavior of mixture systems with organic and inorganic materials.