Molecular insights through computational modeling of methylene blue adsorption onto low-cost adsorbents derived from natural materials: A multi-model's approach

Highligths

• Thermal treatment enables the generation of new micropores for activated carbon.

• Activated carbon prepared from biomass exhibits excellent methylene blue removal.

• Statistical physics approach showed that MB lies parallel on the adsorbents.

• Isosteric heat of adsorption allows to investigate the surface heterogeneity.

• MEP map and Fukui index highlight the adsorption reactive sites.

Abstract

The fundamental phenomena involved in methylene blue adsorption onto three different activated carbons (a raw adsorbent and two samples derived from either chemical or thermal treatment of the raw sample) are elucidated by coupling different multi-physics modeling approaches. Statistical physics approach leads to understand that methylene blue adsorption is mainly affected by the porosity of sorbents rather than their functional groups. Electrostatic interactions, Van der Waals forces or hydrogen bonding might occur between dye cations and carboxylate anions on adsorbent surface. The quantum chemical calculations suggest that dispersive interactions and pore characteristics of the activated carbon derived from thermal treatment predominantly contribute. The investigated reactive sites show that the same preferable sites for both electrophilic and nucleophilic attacks are detected for the sample derived from thermal treatment, allowing explaining the best performances of this adsorbent. Finally, the most stable energy configuration of methylene blue adsorption on activated carbon is obtained by Monte Carlo simulations.

Introduction

The large quantity of dye effluents deriving from industrial processes (including e.g. paint, textiles and plastics) is one of the major environmental problems that must be faced to assure sustainable productions (Mouni et al., 2018). Most of the dyes contained in discharged effluents are toxic and harmful for both human health and environment (Bhatti et al., 2012; Albayati et al., 2016; Kono et al., 2016; Hiew et al., 2018). The most commonly used dye is undoubtedly methylene blue (MB). The efficient removal of dyes from wastewater can be achieved through different physico-chemical methods such as adsorption, coagulation, ozonation, electrolysis, photodegradation, membrane filtration etc. (Rafatullah et al., 2010; Shi et al., 2018; Mashkoo and Nasar, 2020; Santoso et al., 2020). Adsorption is a well-known technique widely adopted for wastewater treatment thanks to its simple design and easy operation. A broad range of different adsorbents can be proficiently used to remove the dissolved pollutants. However, most of these porous materials suffer of various drawbacks such as low availability and high cost of preparation (Guo et al., 2012; Magdy and Altaher, 2018;). Due to the large amount of dye effluents annually produced, the availability of cheap and efficient alternative adsorbents has become strictly necessary. To this aim, natural and waste materials could be interesting adsorbents for the adsorption of various pollutants, for a sustainable approach to the problem (Santoso et al., 2020; Gomes et al., 2015; Bentahar et al., 2018). In particular, biomasses can be used as adsorbent materials for the adsorption of dyes from polluted effluents thanks to their easy availability and favorable adsorptive properties. Simultaneously, a waste biomass can be a suitable precursor for the production of activated carbons (ACs), which has been the subject of past and recent researches (Islam and Rouf, 2012; Maneerung et al., 2016). ACs are highly porous materials, typically with a high surface area and high adsorption capacity for various organic compounds (Maneerung et al., 2016). Moreover, AC precursor materials can be categorized as renewable, low-cost and environmentally benign products, hence can be proficiently proposed as a sustainable solutions to dye water pollution.

One of the most significant factor to be taken into account in the design of an adsorption process is the adsorbent performances, which can be retrieved from the determination of its adsorption isotherms. To get information about the nature of the interactions at the solid-liquid interface and to assess the efficiency of the adsorbent, several adsorption isotherm models have been adopted (Langmuir, 1916; Freundlich and Heller, 1939; Ahsaine et al., 2018). A thorough modeling of adsorption isotherm is a mandatory tool for a flexible design of operation and its simultaneous optimization. Some approaches for adsorption isotherm modeling based on statistical physics or molecular dynamics have been recently developed, aimed at describing and understanding the adsorption phenomena at a molecular level (Bergaoui et al., 2018a; Bergaoui et al., 2018b; Toumi et al., 2018a). Further approaches include some empirical modeling and simulation (Ben-Mansour et al., 2017; Kuang et al., 2020; Rahideh et al., 2016), molecular dynamics (Benabid et al., 2019; Mota et al., 2004), Quantum chemical calculations (Hussein et al., 2020; Regti et al., 2016) and the one derived from statistical physics formalism, which allows retrieving important parameters with physical meaning not possible when adopting classical empirical models (e.g. Langmuir) (Bergaoui et al., 2017; Khalfaoui et al., 2015; Khalfaoui et al., 2003; Nakhli et al., 2014). In this work, different modeling and simulation approaches were combined and compared with the statistical physics approach so to explore new ways to dissect and interpret the removal of dyes from wastewater. Indeed, in our previous works (Bergaoui et al., 2018b; Toumi et al., 2018b), for the first time we combined the molecular dynamics simulations with the statistical physics modeling for to consolidate the understanding of adsorption phenomena and to confirm the results obtained from each method. In order to further extend this methodological analysis, in the present work an extension of the comparative study by using different simulation methods was made, and the retrieved results were put in relation with the results obtained from the statistical physics approach. The statistical physics approach can be a challenging tool to understand the mechanism of dyes adsorption onto solid surfaces, as it allows to correlate the microscopic properties of the adsorbent (e.g. electronic structure) to the macroscopic ones (i.e. adsorption capacity) (Khalfaoui et al., 2003; Nakhli et a., 2018). On the other hand, computational studies such as Monte Carlo simulations and quantum chemical calculations have shown to be very effective in providing important information such as the most stable molecular conformation and the adsorption sites for a wide range of sorbent materials (Dohare et al., 2018). Quantum chemical methods have been widely used to elucidate various experimental observations in many areas. Moreover, they have proved to be a very powerful tool for studying adsorption mechanism of different systems, in order to understand at a molecular level the interactions between adsorbate and adsorbent (Zulfareen et al., 2017; Rezakazemi et al., 2018). Simultaneously, Density Functional Theory (DFT) (Pakdel et al., 2019) can be a very interesting instrument for understanding different aspects of chemical processes, being a powerful technique for performing calculations (Parr and Yang, 1984). Summing up, a significant help to experimental studies can be derived from software modeling and calculations, for a deep comprehension of the observed phenomena and their mathematical management in the framework of an advanced computer-aided intervention design.

In this work, we investigated the adsorption equilibrium of MB on three activated carbons (named JAC-1, JAC-2 and JAC-3) prepared from a biomass waste, i.e. the press-cake residue of Jatropha curcas L., as described by Kurniawan and Ismadji (2011). The chemical and microstructural characterization of JAC adsorbents surface was performed by SEM, XRD and nitrogen adsorption isotherms analysis. The goal of this paper is to extend the study of Kurniawan and Ismadji (2001) by analyzing the adsorption efficiency of the proposed adsorbent/adsorbate systems by calculating many fundamental theoretical parameters such as isosteric heat of adsorption (Q_st), number of adsorbed molecules per site (n), receptor sites density (N_m), characteristic concentration (w), energy of highest occupied molecular orbital (E_HOMO), energy of lowest unoccupied molecular orbital (E_LUMO), energy gap (E_gap), Molecular electrostatic potential (MEP), Fukui analysis and adsorption energy. These theoretical parameters were calculated from statistical physics approach, Density Functional Theory (DFT) and Monte Carlo simulations. Finally, a joint analysis of the aforementioned parameters was carried out in order to fully characterize the MB adsorption performances of the investigated adsorbents, for a thorough knowledge of their field and potentialities of application. The full methodology can be proposed as a standard set of analysis for the screening of effective adsorbents for a generic application and for the evaluation of their proficient applicability.