Assessment of sepiolite as a low-cost adsorbent for phenanthrene and pyrene removal: Kinetic and equilibrium studies
Introduction
Groundwater is a limited natural resource that may be considered as one of the most important components of our socio-economic framework. Groundwater provides enormous quantities of high-quality water for human activities such as agriculture, domestic uses and industry. Nevertheless, there are several sources of groundwater pollution as a result of anthropogenic activities, including septic tanks, agricultural areas where fertiliser or pesticides are used, underground storage tanks, and unauthorised dump sites. Thus, one of the consequences of groundwater pollution is the degradation of environmental quality, which poses a risk to public health (EPA, 1980).
In recent years, the amounts of emerging organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), found in the environment have increased (Pergent et al., 2011). PAHs primarily enter the environment as a result of anthropogenic activities, such as the combustion of fossil fuels, and their low solubility does not prevent their persistence in water bodies (Manoli and Samara, 1999). According to Mackay and Gschwend (2001), the hydrophobic organic compounds (HOCs) that are mobile in groundwater include both dissolved and colloid-associated species. Dissolved forms may include HOC molecules that are solvated by water or mixtures of water and non-aqueous solvents. The colloid-associated species include HOC molecules that are associated with macromolecular humic substances, biogenic exudates, micelles, microorganisms, and nanometre-to-micrometre-sized suspended mixed-phase solids. As result of the impacts of PAHs on the environment, these compounds are included in a list of pollutants provided by the United States Environmental Protection Agency and the EU Water Framework Directives (2000/60/EC) (EU, 2000, Haritash and Kaushik, 2009). In addition, current Spanish regulations allow only a small concentration of PAHs in waters, approximately 0.1–0.01 μM (Miñana et al., 2008).
The use of permeable reactive barriers (PRBs) for the treatment of a range of contaminants found in polluted groundwater is widespread throughout the world (Warner, 2011). The most common procedure for installing a PRB is to dig a trench in the polluted area and fill it with an appropriate reactive material (Cobas et al., 2013). As contaminated groundwater flows through the structure, the reactive material removes the pollutants by adsorption, ion exchange, precipitation or oxidation–reduction reactions (Thiruvenkatachari et al., 2008). This technology has been widely studied for the removal of inorganic compounds such as metals or nitrates (Fiore and Zanetti, 2009, Moraci et al., 2011). However, the use of this treatment to remove emerging organic pollutants such as PAHs is in the initial stages, and the number of reactive mediums available for this type of pollutant remediation is limited.
The suitability of a reactive material depends, in part, on its cost-effectiveness in comparison with other materials. Thus, the assessment of alternative low-cost materials as potential sorbents for the removal of pollutants has recently received a significant amount of attention within the scientific community (Ali et al., 2012, Doke and Khan, 2013).
Activated carbon is a highly preferred sorbent because it is one of the most effective sorbents, yields high-quality treated water, and can be used for a variety of contaminants, including metals, phenols, dyes, pesticides, PCBs, detergents and a variety of organic compounds, such as PAHs (Gupta et al., 2009, Yadav et al., 2013). However, activated carbon presents some disadvantages associated with the high cost and the difficulties related to its regeneration and/or reuse (Marchal et al., 2013). Thus, low-cost sorbents such as agricultural wastes, clay materials, biomass, and seafood-processing wastes have been studied (Irem et al., 2013, Kocaoba, 2009).
For the selection of an appropriate reactive material, the general requirements of a porous structure, environmental suitability, mechanical stability, hydraulic conductivity, and transport retardation should be satisfied. Therefore, the material considered in this work as a potentially suitable reactive material for use in PRB technology was the natural clay sepiolite, which has already been studied in terms of its adsorption of inorganic pollutants and dyes (Gupta et al., 2009, Yuan et al., 2011). This material has a high sorption capacity and selectivity because of its high porosity and molecular-sieving properties (Liu et al., 2013). Furthermore, this clay is available in large quantities and at a low-cost (Ongen et al., 2012). To date, there have been a number of studies that have used modified sepiolite as a sorbent (Dimos et al., 2012, Malamis and Katsou, 2013), but the present study employs a natural sepiolite because the modification process would increase the price of a sepiolite PRB.
The objective of this work is to identify and evaluate the natural clay sepiolite as a reactive material for use in PRBs installed for the treatment of PAHs. To evaluate the potential of this low-cost material, studies of the kinetics, adsorption mechanisms and isotherms were conducted.
Section snippets
Materials
In this study, phenanthrene (PHE) and pyrene (PYR) were used as the model PAHs. Both were obtained from Sigma Aldrich; the PHE and PYR had purities of 98% and 99%, respectively. The natural sepiolite clay was supplied by TOLSA, S.A (Spain). This sepiolite had a specific surface area of 263 m2 g−1. The mean pore diameter was 20.4 nm, and the pore size distribution was 0.04 cm3 g−1 (micropores), 0.32 cm3 g−1 (macropores) and 0.98 cm3 g−1 (mesopores) (Belzunce et al., 1998).
Material characterisation
The elemental analysis of
Sepiolite characterisation
Sepiolite is a fibrous silicate clay mineral that is rich in magnesium with a structure of layers and chains. The surface of the natural sepiolite used in this study was characterised by SEM (Fig. 1a and b). The sepiolite particle size ranged between 100 and 200 μm (Fig. 1a). The observed textural features confirmed that the studied samples had a fibrous morphology (Fig. 1b). The fibres had lengths in the order of micrometres and were grouped into parallel bundles of fibres, which creates an
Conclusions
Kinetic and equilibrium studies were conducted to examine the adsorption of PAHs from aqueous solutions onto sepiolite at 298 K. The suitability of the pseudo-first-order, pseudo-second-order and Elovich kinetic models were assessed. The pseudo-second-order kinetic model is in agreement with the dynamic behaviour for the adsorption of PAHs, both individually and as a mixture, onto sepiolite. Therefore, it was determined that there is no difference in the interaction between adsorbate and sorbent
Acknowledgements
This research was funded by the Spanish Ministry of Economy and Competitiveness and ERDF Funds (Project CTM 2011-25389). The authors thank the University of Vigo for the financial support of Marta Cobas under a predoctoral grant and to the Ramón y Cajal Program for financial support for Marta Pazos.
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