Modification of biochar derived from sawdust and its application in removal of tetracycline and copper from aqueous solution: Adsorption mechanism and modelling
Graphical abstract
Introduction
Antibiotics are emerging pollutants of particular concern because of their widespread use in animal and human medicine (Norvill et al., 2017). Tetracycline (TET), a common antibiotic, has been widely used in the agriculture and livestock industry, and 60–90% of the parent compound is discharged into aquatic environments in original and metabolized forms because of the incomplete metabolism of TET by animals (Zhu et al., 2014). Selvam et al. analyzed the influence of livestock activities on the concentrations of TET in Yuen Long, Kam Tin, and Shing Mun rivers of Hong Kong, concluding that TET were at the concentration range of 30–497 ng/L (Selvam et al. (2017)). Copper (Cu) is also widely used in livestock and poultry industry as feed additives due to its growth-promoting effect (Ostermann et al., 2015). Ji and co-workers reported that the concentrations of Cu(II) and TET in animal manures (i.e., dry swine, poultry and cattle manures) in Shanghai, China were at the range of 32.3–730.1 mg/kg and 4.54–24.66 mg/kg, respectively (Ji et al., 2012). Obviously, Cu(II) and TET could easily form complexes, and the waterbody and soil environment nearby livestock farms are contaminated by both of them (Wang et al., 2016b).
In addition, antibiotics like TET could induce the generation of antibiotic-resistant genes (ARGs) in microorganisms (Wang et al., 2017b), which can proliferate and widely disseminate in ecosystems, thereby posing a great danger to human health (Huang et al., 2016b). Furthermore, the presence of heavy metals, especially, Cu(II) has been shown to increased occurrence of ARGs even at relatively low levels, thus elevating the health risk (Poole, 2017). The co-existence of Cu(II) and TET occur as complex solute mixtures in contaminated water and soil, and pose more serious toxicological problems to the environment because of their relative mobility and combined toxicity. Therefore, it is necessary to develop integrated techniques realizing the removal of Cu(II) and TET simultaneously.
There are several methods (e.g., adsorption, advanced oxidation, electrochemical methods, biological treatment) that have been extensively applied to remove antibiotics and/or heavy metals (Ahmed et al., 2017, Chen et al., 2017, Liu et al., 2017). Among these methods, the adsorption technology was a priority choice because of a synthetic consideration of removal efficiency, simplicity, safety and economic feasibility in the treatment process (Yu et al., 2017). Furthermore, various adsorbents such as functionalized carbon nanotubes (Li et al., 2014), graphene-based materials (Huang et al., 2016a), polystyrene-divinylbenzene resin (Ling et al., 2016), biosorbents (Wang and Chen, 2014) and etc. have been applied to remove the coexisting pollutants of antibiotics and/or heavy metals with a positive effect in aqueous solution previously.
Moreover, searching for new efficient, simple and inexpensive method to control heavy metals and antibiotics is also of considerable interest. Nowadays, biochar-based materials have been extensively studied and widely applied in the environmental remediation because of its inherent characteristics: (i) biochar is cheap, non-toxic and easy to obtain, (ii) biochar possess intrinsic high specific surface areas, large pore volumes, enabling the possibility of physisorption and hydrophobic interaction and electrostatic adsorption with pollutants efficiently, (iii) biochar can be modified briefly and possess an increasing number of oxygen containing functional groups on biochar’s surface, which enabling the possibility of specific binding (e.g., hydrogen bonding, π-π electron-donor–acceptor interactions as well as covalent binding) for contaminants efficiently (Mohan et al., 2014, Wang et al., 2017a). Hence, to enhance the adsorption effect of pristine biochar, the modification of biochar was essential to improve its surface properties (e.g., surface area, pore volume, hydrophobic/hydrophilic property, or surface charge). And the most popular reagents which are used for functionalization of biochar surface include nanoscale zerovalent iron (Dong et al., 2017), bimetallic layered double hydroxide (e.g., Mg/Al, Ni/Mn, Mg/Fe) (Tan et al., 2016b, Wan et al., 2017), amino groups (Ma et al., 2014a), oxygen-containing functional groups (Huang et al., 2015). Results suggest that functionalization of raw biochar could toward dramatically improve its adsorption ability. For examples, Wang et al. and Liang et al. used manganese oxide-modified biochar composites for the sorption of As(V), Pb(II) and Cd(II) with highly sorption capacity because of the strong As(V), Pb(II) and Cd(II) affinity of its birnessite particles (Liang et al., 2017, Wang et al., 2015). Tan and co-workers reported Na2S modified biochar enhanced the sorption capacity for Hg(II) because of carboxyl, hydroxyl groups and sulfur impregnation on the sorbents contributed to Hg(II) sorption (Tan et al., 2016a).
Recently, zinc-biochar (Gan et al., 2015), iron-biochar (Peng et al., 2014, Wu et al., 2016, Zhou et al., 2017c) and iron/zinc-biochar (Wang et al., 2016a) were synthesized in laboratory, and used for the effective removal of heavy metals and recalcitrant organic compounds from wastewater. Previous works indicated that the surface and physicochemical properties of biochar can be easily modified with functional groups or incorporated with zinc and/or iron, exhibiting improved physical or chemical properties for the removal of hazardous contaminants.
Hence, on the basis of previous works, the purposes of this research were to (1) study the adsorption ability of the prepared iron and zinc doped sawdust biochar (Fe/Zn-biochar) for tetracycline and Cu(II) simultaneously, (2) investigate the impacts of some key parameters, namely pH, and contact time on the adsorption capacity, (3) explore the modelling of adsorption isotherms and kinetics to help explain its adsorption mechanism of Cu(II) or tetracycline, (4) investigate the underlying mechanisms of tetracycline and Cu(II) adsorption onto Fe/Zn-biochar. This study provides new insights in the development of biochar-based materials and advances their applications in water treatment.
Section snippets
Materials and chemicals, and preparation of biochar based-materials
The used materials and chemicals are presented in supporting information. The preparation procedure of pristine biochar (P-biochar), Zn-biochar, Fe-biochar and Fe/Zn-biochar were prepared according to the strategy reported previously (Wang et al., 2016a). Typically, the mixture of sawdust (15 g) and the solution containing ZnCl2 (0.2354 g), FeCl3·6H2O (0.72321 g) and a certain volume of water was stirred for 24 h, then kept at 100 °C. The aqueous solution containing ZnCl2 and FeCl3·6H2O was
Effect of solution pH
The tetracycline is an amphoteric molecule with multiple ionizable functional groups and formed a series of species at different pH levels. In fact, TET is hydrophilic and the predominate TET species are TETH3+ at pH < 3.3, TETH20 at 3.3 < pH < 7.7, TETH− at 7.7 < pH < 9.7, and TET2− at pH > 9.7 (Huang et al., 2016a). The TET becomes more negatively charged as pH is increased, which may make its sorption behavior to biochar-based materials more complicated. The zeta potentials of the biochar-based
Conclusions
In this work, iron and zinc doped sawdust biochar was synthesized and used for the removal of TET and Cu(II) contaminated water. The adsorption mechanism of TET and/or Cu(II) onto Fe/Zn-biochar was investigated by different models. Fe/Zn-biochar contained hydrophobic site for Cu(II) removal, iron oxide for TET removal, and hydrophilic sites for TET and Cu(II) removal. The results of batch adsorption tests demonstrated that site recognition, bridge enhancement and site competition were the
Acknowledgements
The study was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 51709103), the National Program for Support of Top-Notch Young Professionals of China (2012), the Program for New Century Excellent Talents in University from the Ministry of Education of China (NCET-11-0129), Project of Science and Technology of Hunan Province (2015WK3016 and 2016WK2010).
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