Clay-biochar composites for sorptive removal of tetracycline antibiotic in aqueous media

Highlights

• Successful Clay-biochar composites were tested for antibiotic removal in water.

• Montmorillonite-BC composite showed high tetracycline removal capacity.

• Adsorption occurred via both physisorption and chemisorption mechanisms.

Abstract

The focus of this research was to synthesize novel clay-biochar composites by incorporating montmorillonite (MMT) and red earth (RE) clay materials in a municipal solid waste (MSW) biochar for the adsorptive removal of tetracycline (TC) from aqueous media. X-ray Fluorescence Analysis (XRF), Fourier Transform Infrared Spectroscopy (FTIR), Powder X-ray Diffraction (PXRD) and Scanning Electron Microscopy (SEM) were used for the characterization of the synthesized raw biochar (MSW-BC) and clay-biochar composites (MSW-MMT and MSW-RE). Results showed that minute clay particles were dispersed on biochar surfaces. The FTIR bands due to Si-O functional group vibrations in the spectra of the clay-biochar composites provided further evidence for successful composite formation. The kinetic TC adsorption data of MSW-MMT were well fitted to the Elovich model expressing high surface activity of biochar and involvement of multiple mechanisms in the adsorption. The kinetic TC adsorption data of MSW-BC and MSW-RE were fitted to the pseudo second order model indicating dominant contribution of chemisorption mechanism during the adsorption. The adsorption differentiation obtained in the kinetic studies was mainly due to the structure of the combined clay material. The adsorption isotherm data of all the adsorbents were well fitted to the Freundlich model suggesting that the adsorption of TC onto the materials occurred via both physisorption and chemisorption mechanisms. In comparison to the raw biochar and MSW-RE, MSW-MMT exhibited higher TC adsorption capacity. Therefore, MSW-MMT clay-biochar composite could be applied in the remediation of TC antibiotic residues in contaminated aqueous media.

Introduction

Aquatic ecosystem pollution via antibiotics is becoming a serious environmental issue. The tetracycline (TC) antibiotics were discovered in 1940s, and started to be used as a therapeutic agent in preserving human heath in 1950s (Eliopoulos and Roberts, 2003). The TC molecule consists of three ionizable functional groups which can protonate and deprotonate, and form different conformations depending on the solution pH (Parolo et al., 2008). Usually at pH < 3.3, cationic form of TC is predominant, at pH 3.3 < pH < 7.7 zwitter ions become dominant, and at pH > 7.7 anionic form becomes dominant (Chang et al., 2014).

Most of the gram negative and gram positive bacteria are vulnerable to antibacterial activity of TC (Abdulghani et al., 2013). Compared to other antibiotics, TC is considered as a cheap antibiotic as a result of which it is popular in developing countries (Roberts et al., 2012). Although the human use of TC has been restricted in many countries due to the development of resistant bacterial strains (López-Peñalver et al., 2010), it is still extensively used in veterinary medicines as well as a growth promoter in livestock maintenance, poultry farming and aquaculture (Sarmah et al., 2006). When TC is administrated to animals, about 25–75% and 70–90% of the antibiotic are excreted into the environment in active forms via urine and feces, respectively (Halling-Sørensen, 2000). Therefore, antibiotics residues eventually contaminate the soil and surface water via leaching and run off. These residues develop antibiotic resistance to microorganisms, inhibit growth of some aquatic species, and directly influence the steroidogenic pathway which leads to endocrine disruption in humans (Halling-Sørensen, 2000; Daghrir and Drogui, 2013; Ji et al., 2010). A study in Sri Lanka showed that effluents released from poultry and livestock farms contained approximately 45, 35 and 20% of TC, oxytetracycline (OTC) and amoxicillin (AMX), respectively (Liyanage and Manage, 2017). The average concentration ranges of TC, OTC and AMX in effluents of selected poultry farms are 0.001–0.005, 0.001–0.004 and 0.001–0.005 mgL⁻¹ (Liyanage and Manage, 2017).

Therefore, it is necessary to eliminate TC residues from effluents prior to their release in the environment. Compared to current water treatment techniques available, such as membrane processes, photochemical processes, electrochemical processes, photocatalytic and photoelectrocatalytic processes, ozonation and advanced oxidation (Košutić et al., 2007; Koyuncu et al., 2008; Chen et al., 2010; Lee et al., 2011), adsorption is considered as a simple, cost effective, and less harmful technique for the removal of TC from waste water. Different types of adsorbents have been used for the adsorptive removal of antibiotics of all categories, but not TC (Chang et al., 2016; Rajapaksha et al., 2015). Most of the adsorbents used in adsorption studies are abundant, naturally originated and relatively cheap, such as biochar and clays (Ahmad et al., 2014; Uddin, 2008).

Biochar is a porous carbonaceous material with large specific surface area, and it is produced via pyrolysis of biomass in sealed containers, under limited oxygen environment (Lehmann and Joseph, 2009). The initial use of biochar was limited to the enhancement of agricultural productivity via improvement of soil fertility, increasing soil nutrient levels and water retaining capacity, and decreasing greenhouse gas emission through carbon sequestration (Lehmann and Joseph, 2009; Mandal et al., 2016). However, in recent years, biochar has been used as a cost effective adsorbent for immobilizing both organic (antibiotics, pesticides, dyes) and inorganic contaminants (nutrients, heavy metals) (Vithanage et al., 2014, 2016; Yang et al., 2016; Yao, 2013; Han et al., 2013). Biochar can be produced using various feedstocks, mainly crop residues and waste biomasses, which are abundant and easily collectable, and considered as waste materials, hence indirectly supporting sustainable waste management (Ahmad et al., 2014).

Montmorillonite (MMT) is the most commonly used clay mineral as an adsorbent for the removal of variety of contaminants (Aristilde et al., 2016; Zhu et al., 2016; Krupskaya et al., 2017). It is an aluminosilicate clay mineral with 2:1 type structure in which aluminum and silicon are the main components of layers, and one aluminum octahedral sheet is stacked in-between two silicon tetrahedral sheets (Krupskaya et al., 2017). Negatively charged layers in the clay mineral are balanced by hydrated exchangeable cations (Na⁺, Ca²⁺, Mg²⁺) present in the interlayer space (Segad et al., 2010). Cations present in the interlayer space of MMT can exchange with positively charged contaminants via cation exchange mechanism (Yao, 2013). Such a cation exchange reaction is a physical mechanism which is referred to as intercalation when guest ions or molecules occupy the interlayer space by replacing the hydrated cations (Aristilde et al., 2016; Li et al., 2010; Perelomov et al., 2016).

Natural red earth clay (RE) is an iron coated quartz sand consisted of small amounts of ilmenite and magnetite, and abundantly found in the northwest coast of Sri Lanka covering the lime stone strata (Vithanage et al., 2007). According to available literature, RE mainly consists of silica (SiO₂) coated with considerable concentrations of Al₂O₃ and Fe₂O₃ (Vithanage et al., 2006, 2007). The Fe²⁺ level is comparatively high, and sometimes reaches up to 6%, and both aluminum and iron (>AlOH and >FeOH) act as active surface sites. As a result, the surface charge of RE can be varied with variation of pH in the surrounding environment. Although RE did not show porous or layered structure (Vithanage et al., 2007), it could have high adsorption capacity for heavy metals and metalloids like arsenic and nickel (Vithanage et al., 2006; Rajapaksha et al., 2011, 2012). However, previously RE clay was not used to determine its adsorption affinity for antibiotics likely due to the non-layered structure and low surface activity of the material.

Compared to activated carbon, biochar was found less promising in the context of adsorptive removal of contaminants from aqueous media because of the relatively low surface area of the latter material, and the influence of abiotic and/or biotic processes on its properties and adsorption capacity (Anderson et al., 2013). Hence, biochar composites prepared by impregnating biochar with specific materials such as clay minerals have been tested for the adsorption of contaminants like antibiotics and nutrients (Yao et al., 2014; Li et al., 2017; Chen et al., 2017). For instance, in biochar-clay composites, biochar could provide surfaces for the distribution of the clay particles (Yao et al., 2014), and thus improve contaminant adsorption capacity of the pristine material.

Nevertheless, the application of clay-biochar composites for antibiotic removal from aqueous media and associated mechanisms have seldom been investigated to the best of our knowledge. Therefore, current study was conducted to investigate the TC adsorption behavior of two different clay-biochar composites prepared by incorporating MMT and RE in municipal solid waste (MSW) derived biochar, and discuss the mechanisms involved in the adsorption of TC by these composites. The raw biochar and clay-biochar composites are henceforth referred as MSW-BC, MSW-MMT and MSW-RE, respectively.