Cr(VI) removal from aqueous solution using biochar modified with Mg/Al-layered double hydroxide intercalated with ethylenediaminetetraacetic acid
Introduction
Nowadays, with the improvement of people's environmental awareness, heavy metal pollution has drawn more and more extensive attention in society (Gong et al., 2018, Huang et al., 2017a, Huang et al., 2016b). Excess heavy metals have adverse effects on ecological environment and human health (He et al., 2018, Li et al., 2018). Heavy metals can not be biodegraded and would cause harm to the human body through the bioaccumulation of the food chain (Gong et al., 2017, Xue et al., 2017). Cr(VI) has been identified as one of the human carcinogens by international cancer research institutions (Farrell et al., 1989). The pollution of chromium (Cr) is mainly caused by industry such as mining industry and smelting industry (Knott, 1996). Cr is usually existed in the natural environment in the forms of Cr(III) and Cr(VI). Cr(III), as one of the essential trace elements of a mammal, can regulate the levels of insulin and blood glucose (Xu and Zhao, 2007). Cr(III) mainly exists in the form of Cr(OH)3 or Cr2O3, and it is easy to form a complex and stably exists in the sediment. The Cr(VI) toxicity is 100 times higher than Cr(III). Reportedly, long-term exposure of human skin to Cr(VI) wastewater is the origin of dermatitis and eczema. And the inhalation of Cr(VI) can cause sneezing, running nose, nosebleeds, ulcers, even lead to the kidneys and liver damage. In critical situations, Cr(VI) may could do harm to the human circulatory system or even threaten human life (Wang et al., 2000). Cr(VI) was highly mobile in water and soil (Fendorf et al., 2000). Cr(VI) in water is mainly in the form of oxygen-containing anions (CrO42−, Cr2O72−), and it can hardly be adsorbed by colloidal substances in the water. Phytoremediation is an effective method in the treatment of heavy metals in soil. Heavy metals can be directly utilized by plants, thus effectively removing heavy metals from soil (Zhou et al., 2018). Adsorption is one of the most effective technologies for the removing heavy metal ions from water (Wang et al., 2018a). Therefore, many materials have been designed and implemented as adsorbents to remove heavy metal from wastewater in academic research and industrial applications (Huang et al., 2015).
Biochar is the product of pyrolysis of biomass at high temperature under anoxic conditions (Huang et al., 2017b, Zhang et al., 2016). There is growing global interest in the role that biochar (BC) play in environmental management (Tan et al., 2015). Biochar could origin from many raw materials, including forest and agricultural waste, industrial by-products and waste, and municipal solid waste (Huang et al., 2017c). Biochar is an attractive carbon-rich material, which has attracted much more attention as a pharmaceutical preparation for water treatment and a soil improvement material (Ding et al., 2016, Tan et al., 2015). The use of biochar can reduce carbon emissions and slow down greenhouse effect to a certain extent (Wu et al., 2017). Owning to the excellent performance like its high porosity, large surface area and variable surface composition, biochar has displayed excellent adsorption capacity for pollutants (Wang et al., 2018a). In recent years, many papers have reported the adsorption of pollutants by biochar (Huang et al., 2016a). The ability of biochar to adsorb ionizable antibiotic sulfamethazine was enhanced by addition of carboxyl functionalized short multi-walled carbon nanotubes (Zhang et al., 2016). Biochar can not only be used in water pollution treatment but also in soil and sediment. Composite materials of biochar could immobilize Pb in contaminated sediment (Huang et al., 2018). Due to the limited function of biochar, the application of biochar as adsorbent is limited. Therefore, the functionalization and modification of biochar as an ultra-fine composite material is an important topic (Wang et al., 2016).
The chemical composition of layered double hydroxides (LDH) can be represented by the general formula [M2+1−xM3+x(OH)2]x+[An−]x/n·mH2O, where M2+(Mg2+, Mn2+, Fe2+, Zn2+, Cu2+, etc.) and M3+(Al3+, Fe3+, Co3+, etc.) represent divalent cation and trivalent cation, respectively. An-(CO32–, Cl−, NO3–, citrate, etc.) is an interlayer anion with negative charge, and m represents the number of interlayer water molecules, and × is M2+/ (M2+ + M3+) (Cavani et al., 1991). For high-purity LDH, the value of × is generally from 0.20 to 0.33 (Brindley, 1980). LDH have a certain degree of acidity which depends on the divalent and trivalent metal hydroxides and interlayer anions. At the same time, LDH are alkaline. The alkalinity is related to the nature of divalent and trivalent metal cations (M) and M−O bond on the plates. The anions in the LDH can exchange with other external anions (inorganic anions, organic anions, complex anions, etc.). LDH particles or shaped pieces are limited in the ability to handle contaminants (Cavani et al., 1991). Furthermore, biochar has recently been used as a support framework for nanomaterials, which has the effect of reducing the aggregation of nanomaterials (Huang et al., 2018). LDH have been used to load biochar to improve the ability to remove crystal violet and tetracycline in wastewater (Tan et al., 2016a, Tan et al., 2016b). Cr(VI) mainly exists in the form of anion in water. The application of pure biochar in adsorption of Cr(VI) is scarce.
The objectives of this study were as follows: (1) to synthesize the ethylenediaminetetraacetic acid (EDTA) intercalated Mg/Al-LDH supported biochar (BC@EDTA-LDH) using co-precipitation technique; (2) to analyze the properties of BC@EDTA-LDH using various characterization methods; (3) to study the adsorption performance of BC@EDTA-LDH for Cr(VI); (4) and to explore the adsorption mechanism of BC@EDTA-LDH.
Section snippets
Materials
The drugs used in the experiment were analytically pure magnesium chloride hexahydrate (MgCl2·6H2O), aluminum chloride hexahydrate (AlCl3·6H2O), sodium hydroxide (NaOH) and disodium edetate (EDTA-2Na), and used. The solution used in the experiment was ultrapure water (resistivity of 18.25 MΩ·cm−1), which was also used to rinse and clean the samples. The Cr(VI) stock solution was prepared by dissolving K2Cr2O7 in ultrapure water. The desired concentrations of Cr(VI) solution were achieved by the
Characterizations
BC had BET surface area of 48.894 m2/g, and BC@EDTA-LDH had BET surface area of 8.831 m2/g, which indicated that the voids on the surface of the biochar were filled with the loaded nano-material LDH. After loading the layered double hydroxides, the pores on the biochar were filled, and the BET surface area of the material became smaller. This was further verified by the SEM images (Supplementary material), which exhibited a smooth surface of the original biochar, as opposed to the remarkable
Conclusions
BC@EDTA-LDH could be produced by liquid phase coprecipitation of LDH on biochar substrates, providing an efficient adsorbent to remove Cr(VI) in aqueous solutions. The experimental results demonstrated that interlayer anion exchange and surface adsorption dominated Cr(VI) sorption process. The pseudo second-order and Langmuir-Freundlich models were well-fitted the sorption process, indicating chemisorption and monolayer adsorption were the main mechanisms of the adsorption. Since biochar was
Acknowledgments
This study was financially supported by the Program for the National Natural Science Foundation of China (51879101, 51579098, 51779090, 51709101, 51521006, 51809090, 51278176, 51378190), the National Program for Support of Top–Notch Young Professionals of China (2014), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), and Hunan Provincial Science and Technology Plan Project (2018SK20410, 2017SK2243, 2016RS3026), and the Fundamental Research Funds for
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