Removing tetracycline and Hg(II) with ball-milled magnetic nanobiochar and its potential on polluted irrigation water reclamation

https://doi.org/10.1016/j.jhazmat.2019.121095Get rights and content

Highlights

  • Ball-milling production of magnetic nanobiochar was investigated.

  • More than 268 mg TC and 127 mg Hg(II) were removed by per gram BMBC700.

  • TC and Hg(II) contaminated irrigation water was reclaimed by BMBC700.

  • Hybrid nanobiochar composite was easily recovered with a magnet after use.

Abstract

The feasibility of ball-milled magnetic nanobiochars (BMBCs) derived from wheat straw for adsorptive removal of tetracycline (TC) and Hg(II) from aqueous solution was assessed against that of pristine magnetic biochars (PMBCs). Ball milling conversion of PMBCs into BMBCs greatly improved TC and Hg(II) removal, and ≥ 99% TC and Hg(II) were adsorbed by BMBC prepared at 700 °C (BMBC700) within 12 h. The maximum adsorptive removal capacities of BMBC700 for TC and Hg(II) were 268.3 and 127.4 mg/g, respectively. The amounts of TC and Hg(II) removed by BMBC700 decreased gradually as the ionic strength of the solution increased, but increased as the solution temperature increased from 25 to 45 °C. The further FTIR and XPS analysis confirmed removal of TC was predominately regulated by the combination of electrostatic interactions, hydrogen bonds, and Cπ–Cπ interaction, while, the adsorption of Hg(II) was mainly governed by several mechanisms, including electrostatic attractions, Hg–Cπ bond formation, and surface complexation. Overall, BMBC700 presented great potential for TC and Hg(II) removal from polluted irrigation water and exhibited acceptable recyclability performance as well as magnetic separation advantage in use.

Introduction

China is currently struggling with cultivable land pollution, which is mostly attributed to potentially toxic metals (PTMs) and organic contaminants (The Ministry of Environmental Protection (MEP), 2014). Proper management of aqueous waste streams is essential to meet the strict environmental regulations in China, because irrigation water is one of the primary sources of contaminants (Meng et al., 2016; Pan and Chu, 2018). One option is the creation of alternative technological innovations, such as adsorption (Li et al., 2018a; Wei et al., 2018). It is believed that adsorption is an energy-saving and cost-effective method for the purification of effluents (Li et al., 2018a), particularly for agricultural wastewater that contains moderate or low concentrations of PTMs and organic pollutants (Wei et al., 2018). Numerous materials can be used as adsorbents, but the selection of adsorbents primarily takes into account their efficiency and cost of practice (Huang et al., 2018). For example, as one of the most well-known and effective adsorbents, granular activated carbon can effectively remove PTMs and organic pollutants from waters; however, its potential as adsorbent for water treatment appears limited by its high cost ($340–22,000 USD/t) (Alhashimi and Aktas, 2017), due to the high temperature and additional activation processes requirement during its production in commercial (Fu et al., 2019). In view of sustainable development, many scientists focused on searching for cost-effective adsorbents, with characteristics including affordability, natural abundance, environmental sustainability, minimum processing requirement, good mechanical and chemical resistance, and acceptable adsorption capacity for the target contaminants (Fu et al., 2019; Shen, 2015; Patil et al., 2019).

Lately, biochar, the carbon-rich porous solid from the pyrolysis of waste biomass in the absence of oxygen, has attracted considerable attention in fields of organic solid waste management (Shen, 2015), carbon sequestration (Lehmann et al., 2006), soil amendment (Cao and Harris, 2010), greenhouse gases emission reduction (Kettunen and Saarnio, 2013), fertilizer production (Tu et al., 2019), energy storage (Jia et al., 2018), and water purification (Xu et al., 2016). The beneficial properties of the emerging biochar materials, such as abundant feedstock, convenient preparation, special surface chemical properties, porous structure, and high environmental stability (Huang et al., 2019; Yang et al., 2019), make biochars as promising adsorbents for contaminants removal from waters (Li et al., 2018b; Park et al., 2019a, b; Zeng et al., 2019). The maximum adsorption capacities of biochars obtained from wheat straw and anaerobic digestion sludge, which are cost-effective adsorbents, were determined to be 46.6–62.5 and 51.20 mg/g for methylene blue and Pb(II), respectively (Li et al., 2016; Ho et al., 2017). The hybridization of sugarcane leafy trash biochar with MgO nanoparticles increased the adsorption capacities for methylene blue and Pb(II) to 297 and 103 mg/g, respectively (Li et al., 2018c). Moreover, the adsorption capacities of alkaline nanominerals-enriched engineered celery waste biochars could even be improved to 288–304 mg/g for Pb(II) (Zhang et al., 2017). These acceptable PTMs and organic contaminants removal capacities confer engineered biochars great potential as adsorbents. However, these engineered biochars present limitations, including the consumption of chemicals or nanomaterials and complicated manufacturing procedures at large-scale production (Li et al., 2019). Moreover, contaminant adsorption onto most engineered biochars is a time-consuming process (Li et al., 2016). The adsorption equilibrium times for the adsorption of methylene blue and Pb(II) onto sugarcane waste biochar were reported to be >48 h (Li et al., 2018c), those for the adsorption of phenol onto coconut shell biochar were determined to be >144 h (Hao et al., 2018) and those for the adsorption of tetracycline (TC) onto rice husk biochar even >320 h (Jing et al., 2014). The mentioned drawbacks of biochars make their utilization for full-scale practical adsorption operations difficult, particularly for continuous-flow systems where the rapid and efficient removal of contaminant is required (Gwenzi et al., 2017).

To promote the adsorption efficiency and shorten the adsorption equilibrium time requirements in practice, adsorbents featuring developed porous structures and/or maximized surface areas have been often preferred and prepared (Sing, 2004). Typically, surface area and porosity increase as the adsorbent particle size decrease, which thereby, improves the interactions between adsorbent and contaminant, and in turn increases adsorption efficiency (Lonappan et al., 2016). Mechanical ball-milling is a green and energy-efficient method for increasing surface area (Peterson et al., 2012), reducing particle size (Naghdi et al., 2017), increasing the density of surface oxygen-containing functional groups (e.g., carboxyl, lactonic, and hydroxyl) (Lyu et al., 2018a), and improving the adsorption performance of adsorbents (Shan et al., 2016; Wang et al., 2018). For example, (Peterson et al., 2012) reported that ball-milling treatment effectively reduced the size of biochar particles from milli-/micro-meters to nanometers and increased the surface area of biochar up to 60 times compared with that of unmilled biochar (from 3 to 194 m2/g). (Naghdi et al., 2017) optimized the ball-milling parameters (ball-to-powder mass ratio, time, and rotational speed) for large-scale nanobiochar production and investigated the effect of ball-milling on the physicochemical properties of nanobiochar (e.g., water holding capacity, organic matter, oxidation-reduction potential, elemental composition, zeta potential, particle size, and specific surface area). Lyu et al. (2018b) reported that ball-milled nanobiochar presented high adsorption affinity for methylene blue, and its removal capacity was approximately 354 mg/g, which was much higher than that of pristine biochar (17.2 mg/g). All these results are encouraging for nanobiochar production using ball milling. However, knowledge gaps still exist in the utilization of nanobiochar for the adsorptive purification of wastewater. In addition, wastewater usually contains various dissolved PTMs and organic contaminants, and the potential use of ball-milled biochar for the simultaneous removal of these pollutants from wastewater has not been investigated. Moreover, nanobiochar particles are highly dispersed in aqueous systems, which hinder their separation after use (Li et al., 2019; Wang et al., 2018).

Magnetic biochar, produced by pyrolyzing the iron salt solution soaked biomass, can be easily retrieved with a magnet after use (Li et al., 2018a). And a series of Fe-biochar composites with different iron impregnation ratios had been successfully synthesized for removal of toxic Cu(II), Cr(VI), Zn(II), As(V), and hetero-chloride from wastewater (Sun et al., 2019). Therefore, a pristine magnetic biochar was prepared using the so-called iron-impregnation-pyrolysis integrated method, and it was further ball-milled into magnetic nanobiochar in this study. The magnetic nanobiochar was supposed to have acceptable applications and recyclable performances as well as magnetic separation advantages, which could make it promising for the large-scale removal of PTMs and antibiotic contaminants from aquatic environments. However, further research is required to better understand the feature of the as-prepared magnetic nanobiochar. Therefore, Tetracycline (TC) and Hg(II), two of the most frequently detected pollutants in wastewater, were chosen as the model contaminants in batch experiments. This work: (1) optimizes the pyrolysis temperature of biochar preparation; (2) characterizes the optimized nanobiochar composites; (3) discusses the adsorptive removal performances of the nanobiochar toward TC and Hg(II) from aqueous solutions and polluted irrigation water; and (4) investigates the possible mechanisms of TC and Hg(II) removal by the nanobiochar composites.

Section snippets

Materials

Feedstock, wheat straw, was collected from the 2nd Farm Test Station of the Northwest A&F University, Yangling. The wheat straw was sequentially washed with tap and deionized (DI) water (18.2 MΩ cm) to remove dust, followed by drying at 80 °C overnight in an oven and cutting into pieces smaller than 0.12 mm before use. Tetracycline (>98.5% purity) was obtained from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). The working Hg(II) stock solution was prepared by appropriately diluting a

Influences of pyrolysis temperature and ball milling on the physiochemical properties of resulting biochar composites

With the increase of pyrolysis temperature from 400 °C to 700 °C, the surface area of the BMBC biochars increased from 47.2 to 296.3 m2/g, which was greater than that of their precursors (corresponding PMBC samples). Moreover, after the ball-milling, the oxygen contents of the BMBC400, BMBC550 and BMBC700 were 25.07%, 25.47% and 27.89%, respectively, all higher than that of their corresponding PMBC biochars (Table S2, Supplementary Materials). The results prove that ball-milling can effectively

Conclusions

Ball-milling production of magnetic nanobiochar greatly improved TC and Hg(II) adsorption. More than 99% TC and Hg(II) were adsorbed onto BMBC700 within 12 h. The adsorption isotherms fit the Langmuir model, and the TC and Hg(II) adsorption capacities were 268.3 and 127.4 mg/g, respectively. The increase in the ionic strength of the solution slightly inhibited TC and Hg(II) adsorption. Several combined processes were involved in the adsorption of TC and Hg(II). The prepared BMBC700 could be a

Acknowledgments

This project was partly supported by the Science and Technology Program of Yangling Demonstration Zone (China) (2017NY−24) and the R&D Program of Shaanxi Province (2018ZDCXL−NY−02−02). The authors would like to thank Dr. Ran Zhao and Dr. Hu Li at Northwest A&F University for their help in BET, FTIR and XPS analysis. We also thank anonymous referees for their constructive comments.

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