Effect of Na⁺ and K⁺ on the Modification of Low-Rank Coal by an Ionic Surfactant and Its Adsorption Mechanism

Abstract

This study is dedicated to investigating the impacts of the inorganic salt concentration on low-rank coal modified by ionic surfactants [dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS)] and its adsorption mechanism using ζ potential determination, total organic carbon test, contact angle measurement, and Fourier transform infrared (FTIR) analysis, in addition to the interaction theory calculations and adsorption experiments. The results of interaction model calculation, adsorption capacity and contact angle showed that the salt solution (KCl and NaCl) could effectively increase the adsorption capacity of the surfactant on coal and improve the hydrophobicity of coal. Moreover, the relatively optimal salt concentrations for DTAC-and SDS-modified coal were different, which were 500 g/t coal of KCl and 5000 g/t coal of NaCl, respectively. The FTIR analysis results indicated that DTAC was adsorbed on the coal by strong electrostatic force and weak hydrogen bonding force, but the way of SDS molecule adsorption was mainly hydrogen bonds. The role of salt ions is to break the hydrogen bonds of coal−water and surfactant−water, in addition to enhance the electrostatic interaction, which makes the electrostatic adsorption of DTAC and hydrogen bond adsorption of SDS on the coal surface easier to occur.

Influence of Nanoparticle–Surfactant Compound Solution on Coal Wettability

“…The small size and high reactivity of nanoparticles promote interaction with water molecules via hydroxyl groups, forming hydrogen bonds that boost wettability . Conversely, when surfactants alone modify coal, they adsorb onto the coal surface through hydrogen bonding and electrostatic forces, altering the distribution of functional groups and thus enhancing wettability. , In a cooperative scenario, nanoparticles and surfactants fill vacant adsorption sites on the coal surface. Surfactants, attracted by van der Waals forces, adhere to the nanoparticles, and their hydrophilic groups draw water molecules, facilitating migration and diffusion toward the coal and improving surface wettability. , …”

Advances and Perspectives of Green and Sustainable Flotation of Low-Rank/Oxidized Coal: A Review

“…74 The addition of Na + and K + facilitated the ionization of water molecules, which can reduce the hydrogen bonding force between surfactant molecules (SDS and DTAC) and water molecules, and enhance the electrostatic interactions by breaking the hydrogen bonding on the surface of low-rank/ oxidized coal, thus increasing the adsorption sites of surfactants, reducing the adsorption resistance, and increasing the adsorption area Na + and K + . 75 Adding inorganic salt ions is a promising option to reduce the concentrate ash. Sun et al pretreated the coal samples using ultrasound, C 12 EO 15 , and anhydrous calcium chloride.…”

“…However, the effect of Fe 2+ may not be satisfactory for coal samples with a high hydrophilic mineral content due to the fact that O–Fe + may also form on the surface of hydrophilic mineral particles, which leads to a low concentrate yield . The addition of Na + and K + facilitated the ionization of water molecules, which can reduce the hydrogen bonding force between surfactant molecules (SDS and DTAC) and water molecules, and enhance the electrostatic interactions by breaking the hydrogen bonding on the surface of low-rank/oxidized coal, thus increasing the adsorption sites of surfactants, reducing the adsorption resistance, and increasing the adsorption area Na + and K + …”