Adsorption mechanism investigation of methylthioninium chloride dye onto some metal phosphates using Monte Carlo dynamic simulations and DFT calculations

In this study, Monte Carlo dynamics (MCD) simulations and DFT has been used to investigate the adsorption performance of methylthioninium chloride (MC), known as methylene blue, on Zn3(PO4)2 (11-4), Fe3(PO4)2 (111), Co3(PO4)2 (012) and Ca3(PO4)2 (130) surfaces. In order to understand the chemical properties of MC and its interaction mechanism with the adsorbent surfaces, quantum chemical (QC) descriptors were calculated based on lowest unoccupied molecular orbital energy (ELUMO) and highest occupied molecular orbital energy (EHOMO). On the other hand, the dynamic descriptors were calculated using the Adsorption Locator module. The small gap energy (Delta Egap) value indicated that the MC molecule is highly reactive. As a results, the obtained adsorption configurations show that MC adsorbed parallelly on all surfaces. The dynamic descriptors indicate exothermic and spontaneous adsorption due to negative adsorption energy (Eads) in various mediums. Also, the MC dye adsorption was high, especially in an acidic medium. Therefore, the MC/Co3(PO4)2 complex has a higher adsorption energy obtained as -3067.51, -878.87 and -403.95 kcal/mol, respectively, in acidic, neutral and basic mediums. While, both Fe3(PO4)2 and Co3(PO4)2 surfaces has a low value of dEads/dNiMC, that explained the more easily adsorbed MC molecule. The most stable adsorption configurations show that the interaction between the adsorbate and the surfaces is a chemisorption. This result is confirmed by the radial distribution function (RDF) analysis.