Adsorption of selected organic emerging chemical pollutants from aqueous media using graphene wool composites

Abstract

In this project, a comprehensive review of existing and emerging technologies for the mitigation of PAH pollution in water was conducted. Furthermore, the status of antiretroviral drugs in African surface water, toxicological impacts, and potential remediation strategies were assessed. Through these reviews, the current status of carbon-based/graphene-based materials as an efficient alternative adsorbent for the removal of antiretroviral drugs and PAHs from water was evaluated. It was discovered that there is scanty/no report on the development of adsorbents for the removal of ARVDs, especially those prevalent in Africa- namely efavirenz (EFV) and nevirapine (NVP). The underlying mechanisms of interaction between ARVDs and carbon-based materials has not been reported. Other knowledge gaps include the fact that regeneration and reusability is a major challenge for those adsorbents which have been tested for adsorption of PAHs, due to loss of active sites, difficulty in clean-up and retrieval of many nano-scale materials. Graphene wool is a novel material which was applied for the adsorption of chemical pollutants in water in this study for the first time. Graphene is two-dimensional (2D) with sp2 hybridized carbon atoms arranged hexagonally in a closely packed crystal lattice structure containing σ- and π-bonds. The large specific surface area, thermal stability, thermal conductivity, high tensile strength, chemical robustness, charge mobility, flexibility, and thin film thickness provide the basis for the vast applications of graphene and its composites in many fields of science. In this study, a graphene wool composite was utilized for the adsorption of selected PAHs and ARVDs under variable conditions such as pH, concentration, ionic strength, and temperature. Sorption isotherms and sorption kinetic models were used to fit experimental data to elucidate the mechanism of interactions and predict conditions for optimum adsorption efficiency of the composites. The role of natural organic matter (NOM), which is a ubiquitous component of aquatic systems, was also studied and its impact on the mechanism of interaction and efficiency of the material was evaluated. Generally, adsorption of PAHs by GW is best described by the Sips (Freundlich-Langmuir) model and the Freundlich multilayer adsorption mechanism for single-solute and competitive batch adsorption studies, and is mainly controlled by hydrophobic and π-π interactions. Thermodynamic studies of PAH adsorption revealed that the process is spontaneous and endothermic, with a negative value of Gibb’s free energy (ΔG) and a positive value of adsorption enthalpy (ΔH). On the contrary, the adsorption of antiretroviral drugs, specifically EFV and NVP, is slightly more complex and the nature of interaction may vary for different types of ARVD due to the variations in chemical structure thereof. Similar to PAHs, ARVD adsorption onto GW was best described by Sips (EFV) and Freundlich (NVP) models. However, while GW-EFV interaction was endothermic, GW-NVP was exothermic. Several mechanisms such as hydrophobic, π-π, covalent, van der Waal’s, and hydrogen bonding interactions are possible, depending on the molecular properties and conformations of the ARVD as revealed by supporting computational studies. Furthermore, the study carried out on the influence of NOM on the adsorption of pyrene revealed that the mineral-rich fraction of NOM significantly diminished the removal efficiency of graphene wool, as both adsorption capacity (Kd) and efficiency reduced from 16.9 L g-1 and 95.4% to 0.3 L g-1 and 18.5%, respectively. Fractions with higher % organic carbon (natural sediment, black carbon and mineral deficient fractions) had higher maximum adsorption capacities for several PAHs. Aromaticity and hydrophobic moieties of the different PAHs and NOM significantly influenced the π-π and hydrophobic–organophilic interactions between sorbates and sorbents that may have occurred, which led to some degree of irreversible sorption as shown by hysteresis indices. Adsorption isotherm parameters suggested that GW adsorbed NVP slightly better with stronger binding strength than EFV, with removal efficiencies of 84% (NVP) and 80% (EFV) under optimum conditions. The overall GW removal efficiencies of the target compounds (PAHs and ARVDs) in this project ranged from 81 – 100% under optimum conditions. It was reported that the removal efficiency is dependent on the choice of operational parameters, such as concentration of adsorbate and adsorbent dosage, etc. Graphene wool doped with stabilized silver nanoparticles (GW-αAgNP) was synthesized and its adsorption capacity was evaluated using benzo(a)pyrene contaminated water. GW-αAgNP were found to have an adsorption capacity (Kd) and Sips maximum adsorption capacity (qm) of 2.75 L g-1 and 97.62 µg g-1 respectively, much higher than GW with 0.93 L g-1 and 59.76 µg g-1 respectively. Furthermore, GW-αAgNP and GW were tested against Gram-negative and Gram-positive bacteria (Pseudomonas aeruginosa and Bacillus subtilis). While GW showed no significant inhibition at the concentrations tested, 1000 mg L-1 dosage of GW-αAgNP significantly inhibited the growth of both bacteria. This hybrid material thus has the potential to serve as a smart solution to chemical and microbiological water pollution. This project demonstrated how advancement in material sciences could be harnessed for the development of novel solutions to environmental challenges and pollution remediation.