Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char

Highlights

• High surface area activated carbons were produced from tyre pyrolysis char.

• KOH activation allowed obtaining highly mesoporous activated carbons.

• These activated carbons are as good as commercial ones used in wastewater treatment.

• Kinetics and thermodynamic studies of tetracycline adsorption were performed.

• Activated carbons would improve the economic balance of pyrolysis oil production.

Abstract

Tyre pyrolysis char (TPC), produced when manufacturing pyrolysis oil from waste tyre, was used as raw material to prepare activated carbons (ACs) by KOH activation. KOH to TPC weight ratios (W) between 0.5 and 6, and activation temperatures from 600 to 800 °C, were used. An increase in W resulted in a more efficient development of surface area, microporosity and mesoporosity. Thus, ACs derived from TPC (TPC-ACs) with specific surface areas up to 814 m² g⁻¹ were obtained. TPC, TPC-ACs and a commercial AC (CAC) were tested for removing Tetracycline (TC) in aqueous phase, and systematic adsorption studies, including equilibrium, kinetics and thermodynamic aspects, were performed. Kinetics was well described by the pseudo-first order model for TPC, and by a pseudo second-order kinetic model for ACs. TC adsorption equilibrium data were also fitted by different isotherm models: Langmuir, Freundlich, Sips, Dubinin–Radushkevich, Dubinin–Astokov, Temkin, Redlich–Peterson, Radke–Prausnitz and Toth. The thermodynamic study confirmed that TC adsorption onto TPC-ACs is a spontaneous process. TC adsorption data obtained in the present study were compared with those reported in the literature, and differences were explained in terms of textural properties and surface functionalities. TPC-ACs had similar performances to those of commercial ACs, and might significantly improve the economic balance of the production of pyrolysis oil from waste tyres.

Introduction

The final disposal of waste tyres is a worldwide environmental problem. The annual global production of tyres is indeed around 1.4 billion units, equivalent to 17 million tons of waste tyres per year (Sienkiewicz et al., 2012). Recycling waste tyres is definitely a challenge due to both their highly complex structure and their diverse composition (Sienkiewicz et al., 2012). Waste tyres can be employed as additives in road pavement or in other applications such as playground surfaces, rubber roofs, drainage systems and energy generation (Manchón-Vizuete et al., 2005). These applications are not environmentally satisfactory as they may produce additional pollution if carried out in an uncontrolled way. Pyrolysis might be considered as an effective and environment-friendly recycling method as far as pollutants removal from outlet gases is performed. Depending on pyrolysis conditions, char represents from 30 to 40% of the initial weight. Therefore, a smart utilization of tyre pyroliysis char (TPC) should be included in waste tyre recycling to get a more positive economic and environmental balance.

On the other hand, antibiotics have been used extensively to control infections, and their global market consumption increases steadily every year. Humans and animals often do not fully metabolize pharmaceuticals, and then drugs are excreted and accumulated in the environment (Lian et al., 2013). The gradual increase of antibiotics in the biosphere causes a serious threat to human health because bacteria develop drug-resistance and an increasing number of infections become challenging to treat with known antibiotics (Michael et al., 2013). Therefore, antibiotics removal from water is an issue of public concern and a real scientific challenge. Oxidation (Kümmerer, 2009), electrochemical treatments (Dirany et al., 2012), membrane filtration (Kovalova et al., 2012), and adsorption onto porous materials have been applied for the removal of antibiotics. Among the adsorbents used, clay minerals (Kang et al., 2011, Chang et al., 2009), mesoporous siliceous materials (Xu et al., 2011), polymers (Chao et al., 2014), and activated carbons (ACs) (Liu et al., 2011, Choi et al., 2008), can be cited as examples. ACs are versatile materials whose pore texture and surface functionalities can be easily tailored by selecting the preparation conditions. Moreover, any material containing carbon, whether of natural or synthetic origin, can virtually be a precursor of ACs. In particular, TPC can be used for preparing ACs (Fernández et al., 2012, Rofiqul Islam et al., 2008). ACs from waste tyres have indeed been already successfully applied to wastewater treatment, in the sorption of dyes (Mui et al., 2010a), organic pollutants (Alamo-Nole et al., 2011) and heavy metals (Manchón-Vizuete et al., 2005).

The present study is focused on the adsorption of tetracycline (TC) on ACs produced from TPC. TC, whose molecular structure is shown in Fig. S1, has been used for decades in human and animal medicines. Its removal from water has been studied using different biomass carbon-based materials (Liao et al., 2013, Sun et al., 2013, Zhu et al., 2014, Ji et al., 2011), sludge and commercial AC (Rivera-Utrilla et al., 2013), and TPC (Lian et al., 2013). In the latter study, the TC adsorption capacity was poor, mainly because the surface area of that TPC was low. However, the textural characteristics of sorbents produced from TPC can be considerably improved. The utilisation of KOH as activating agent produces much higher surface areas than H₃PO₄ (Basta et al., 2009, Basta et al., 2011), which is mainly used for biomass activation.

The objective of this study was therefore to find the best activation conditions to prepare ACs with well-developed textural properties from TPC. We showed that ACs prepared from those chars have similar performances as commercial ACs in terms of TC adsorption. Systematic studies including equilibrium, kinetics and thermodynamic aspects were carried out. Comparison with TC adsorption on ACs reported in the open literature was also provided and differences were explained considering their pore texture and surface chemistry.