Integrated QMMM and Monte Carlo methods for analysis of adsorptive interactions between goethite cluster, carbon nanotubes, and arsenate

Abstract

Iron-immobilized nanoporous carbon is a well-known adsorbent used in treating arseniccontaminated waters. In this contribution, we present findings on the adsorptive interactions and dynamics of arsenate-goethite cluster ([FeO(OH)] 6 ) with carbon nanotubes (CNTs) using hybridized quantum mechanics/molecular mechanics (QMMM) calculations. The CNTs adsorption mechanism is of interest since a better understanding of the fundamental interactions between arsenate, goethite, and carbon surfaces would translate to advances in CNT-based adsorbent production and utilization. Novel applications of general amber force field (GAFF) and isobaricisothermal Gibbs ensemble Monte Carlo (NpT-GEMC) methods are described. By the abovementioned methods, we postulate that the [FeO(OH)] 6 /CNT-2.3 (diameter 2.3 nm -mesoporous) system enhances the qualitative (i.e., improved chemisorption) rather than the quantitative adsorptive aspect (i.e., total HAsO 22 4 ions adsorbed) in comparison to the [FeO(OH)] 6 /CNT-1.6 (diameter 1.6 nm -microporous) system. K E Y W O R D S arsenate, carbon nanotubes, density functional theory, goethite, quantum mechanics/molecular mechanics 1 | I N T R O D U C T I O NArsenic groundwater contamination is a widespread global issue prominent in countries such as India, Bangladesh, and Vietnam. An effective approach to treating arsenic-contaminated waters is via adsorption using iron-impregnated (or iron-immobilized) nanoporous carbon (or activated carbon). [1,2] Such research has consequently inspired offshoot work based on the application of iron-based nanoparticles such as magnetite (Fe 3 O 4 ), [3] maghemite (g-Fe 2 O 3 ), [4] and goethite (a-FeOOH) nanoparticles [5] to remove arsenic-based contaminants. In an intuitive sense, by focusing on the nano-region, the higher surface area would enhance the affinity of arsenic-based contaminants to the bespoke iron-based adsorbent.Arsenate, one of the main forms of arsenic in natural waters (the other being arsenite) generally exists as oxyanions (H 2 AsO 2 4 or HAsO 22 4 ) within a pH range of 2-12. [6,7] Otte and co-researchers [8] investigated the adsorption mechanism of arsenate on the goethite using density functional theory (DFT) calculations and indicated that the FeAAs bond length was predominantly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. On a similar theme, Andjelkovic Int J Quantum Chem. 2018;e25653. https://doi.