Effects of synthetic processes on the swelling capacity and modification mechanism of CMC-modified bentonite composites

Abstract

Polymer-modified bentonite is widely utilized in wastewater treatments and barrier projects for contaminants due to its excellent performance. Despite numerous studies being conducted on the properties of different types of polymer-modified bentonite, the impact of synthetic processes has not been systematically explored. Hence, two common processes, the wet process and the semi-dry process, were investigated and compared. Sodium carboxymethyl cellulose (CMC)-modified bentonite was selected as an example here. Response surface methodology (RSM) was employed to develop relationship models of various factors versus the swell index. Specifically, the influence of modification temperature and stirring duration in the wet process, as well as pressure, moisture content, and ageing time in the semi-dry process were investigated. The feasibility and accuracy of the models were verified by the analysis of variance (ANOVA) and hypothesis testing. Results indicate that the semi-dry process leads to greater expansibility than the wet process. CMC dosage is a critical factor affecting performance. The following recommendations were made for the practical production: for the wet process, long-duration stirring at high temperature should be avoided at low CMC dosage, while the opposite is true at high CMC dosage (> 10%); for the semi-dry process, a longer ageing period should be selected at higher polymer contents, which can obviously improve the performance. Microscopic experiments were conducted to compare the differences in the modification mechanism of the two processes. Polymer/bentonite composites mainly form co-existing exfoliated and intercalated structures in wet and semi-dry processes, and the latter has a higher degree of exfoliation. The intensity of cationic bridging, hydrogen bonding, and group interaction between the polymer and bentonite is stronger when adopting the semi-dry process, resulting in better modification effects.