###### Abstract  
Emulsified pickling waste liquid, derived from cleaning oily hardware, cause serious environmental and ecological issues. In this work, a series of grafted (3-aminopropyl)triethoxysilane (APTES) TiO2 pillared montmorillonite (Mt), Ti-Mt-APTES, are prepared and characterized for their assessment in demulsification of acidic oil-in-water emulsion. After titanium hydrate is introduced through ion exchange, montmorillonite is modified by hydrophobic groups coming from APTES. The Ti-Mt-APTES in acidic oil-in-water emulsion demulsification performance and mechanism are studied. Results show that the prepared Ti-Mt-APTES has favorable demulsification performance. The Ti-Mt-APTES demulsification efficiency (\(\mathrm{E_{D}}\)) increased to an upper limit value when the mass ratio of APTES to the prepared TiO2 pillared montmorillonite (Ti-Mt) (\(\mathrm{R_{A/M}}\)) was 0.10 g/g, and the 5 h is the optimal continuous stirring time for breaking the acidic oil-in-water emulsion by Ti-Mt-APTES. The \(\mathrm{E_{D}}\) increased to 94.8% when 2.5 g/L of Ti-Mt-APTES is added into the acidic oil-in-water emulsion after 5 h. An examination of the demulsification mechanism revealed that amphiphilicity and electrostatic interaction both played vital roles in oil-in-water separation. It is demonstrated that Ti-Mt-APTES is a promising, economical demulsifier for the efficient treatment of acidic oil-in-water emulsions.  
m o  
oils in the cleaning solution are an important factor leading to the scrapping of the pickling solution. Hence, separating emulsified oils from emulsified pickling solution can prolong the service life of the pickling solution, which leads to a reduction in the discharge of pickling waste liquid.  
Commonly, traditional methods of separating oil from oil-in-water or water-in-oil emulsions contain flotation [7], coagulation [8], ultracentrifugation, chemical and electrochemical demulsification [9], gravity separation [10], and membrane filtration [11]. Due to its high efficiency, easy operation, and low-energy consumption, chemical demulsification is a practical method for treating oil-in-water emulsions [12]. In addition, chemical demulsifiers can expedite the emulsion-breaking process, and their application conditions are neutral or weakly alkaline. So far, multitudinous chemical demulsifiers are suggested for demulsification, such as cationic polymers, non-ionic block polymers, and dendrimers [13,14]. As shown in previous papers, these demulsification performances are sensitive to pH, and especially poor under acidic conditions [15].  
Montmorillonite is a natural material with a layered structure, enormous surface area, high cation exchange capacity, and can absorb organic substances [16]. Nowadays, the preparation and application of montmorillonite and related products principally focus on environmental pollution improvement [17,18], nanocomposites [19], and catalyst carriers [20]. As for application of montmorillonite in environmental pollution, montmorillonite is primarily studied for the adsorption of heavy metal ions and organic pollutants.  
However, there have been limited studies on montmorillonite demulsification in emulsified pickling waste liquid [21]. Montmorillonite is relatively stable under acidic conditions [22] and organically modified to be introduced hydrophobic groups, making the clay amphiphilic. Therefore, organically modified montmorillonite can be used in an acidic environment. The demulsifiers that are being studied are almost never used in acidic conditions, especially in strong acid conditions. The idea that montmorillonite demulsifies in an acidic environment is novel. Organically modified montmorillonite achieves separation of the emulsified oil from the emulsion in an acidic environment, which can prolong the service life of pickling liquid and reduce the discharge of pickling waste liquid in the metal pickling process.