Elsevier

Ecological Engineering

Volume 70, September 2014, Pages 287-294
Ecological Engineering

Assessment of sepiolite as a low-cost adsorbent for phenanthrene and pyrene removal: Kinetic and equilibrium studies

https://doi.org/10.1016/j.ecoleng.2014.06.014Get rights and content

Highlights

  • The use of sepiolite as a reactive material for construction of PRBs was evaluated.

  • High amount of phenanthrene and pyrene removal was obtained.

  • The kinetics, adsorption mechanisms and adsorption isotherms were studied.

  • The adsorption of PAHs to sepiolite follows pseudo-second order kinetics and the Freundlich isotherm model.

  • The exposure to successive batches indicates that sepiolite could be an effective reactive material for use in PRBs.

Abstract

The objective of this work is to identify and evaluate reactive materials that can serve as permeable reactive barriers to PAHs. The ability to of sepiolite to remove model PAHs, such as phenanthrene and pyrene, was tested in batch assays. The reactive material was assayed for individual PAHs and a mixture of PAHs at concentrations ranging from 50 to 400 μM. Initially, kinetics studies aimed at determining the behaviour of the system and the equilibrium concentration were conducted. The adsorption processes followed pseudo-second-order kinetics, with rate constants of 0.5027 and 0.2478 g μmol−1 h−1 for phenanthrene and pyrene, respectively. Moreover, it was determined that the mechanism that controls the sorption rate was the pore diffusion rate in this study system. Subsequently, equilibrium data were analysed by evaluating their Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms. In all cases, the Freundlich isotherm exhibited the best fit to the experimental sorption data, suggesting that stronger binding sites on a heterogeneous surface are occupied first and that the strength of binding decreases with an increasing degree of site occupation. Finally, several successive batches were assayed using the same sepiolite to evaluate the behaviour of the clay matrix as a permeable reactive barrier. The conclusion of this study is that sepiolite can adsorb phenanthrene and pyrene. This finding has interesting implications for environmental treatment applications, such as providing a source of low-cost reactive material for groundwater treatment using permeable reactive barriers.

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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