Electrokinetics couples with the adsorption of activated carbon-supported hydroxycarbonate green rust that enhances the removal of Sr cations from the stock solution in batch and column
Graphical abstract
Introduction
The dependence of economic development on energy consumption and the shortcomings highlighted in traditional energy sources have accelerated the utilization of nuclear energy in many countries including both developed and developing countries [1]. The discharge of wastewater containing radioactive ions and the disposal of nuclear wastes have become major technical problems limiting the sustainable development of the nuclear industry [2], [3]. Moreover, the accidental release of some radionuclides not only threatens food supplies for human also sustains a negative impact on natural ecosystems. In March 2011, an earthquake measuring 9.0 on the Richter scale caused the failure of two nuclear power plant reactors in Fukushima Prefecture, in which an abnormality occurred in a reactor in the first nuclear power plant, causing nuclear steam leakage [4], [5]. The accident caused a large area of contamination with the radionuclides of 137Cs, 134Cs, 131I, and 90Sr [6], [7], [8], [9]. Among these radionuclides, 90Sr isotope having a half-life of approximately 29 years, as a beta emitter, has attracted wide attention from researchers and environmentalists [10], [11]. strontium ions (i.e., Sr2+) have strong mobility in the water environment and can be easily incorporated into terrestrial and aquatic organisms. Therefore, it is critical to explore new materials and invent new techniques for the selective uptake or efficient removal of Sr2+ ions from the solution [11], [12].
There are several methods used to remove Sr ions from wastewater, such as evaporation, filtration (e.g., microfiltration and ultrafiltration), chemical precipitation or coprecipitation, chromatography, solvent extraction, reverse osmoses, adsorption, and ion exchange [13], [14], [15], [16]. Among these techniques, the adsorption method is more desired to remediate the wastewater with high volumes to guarantee the removal performance and to cause minimal disposal wastes. Besides, the adsorption of Sr2+ presents some unique advantages compared to the others including high efficiency, excellent selectivity, simple processing, and convenient operation [2], [15], [17], [18]. Commonly, for a specific pollutant, the adsorption performance depends largely on the physicochemical properties of the adsorbent and the changes in the competitive environments. Some traditional adsorbents such as zeolite, bentonite montmorillonite, biomass-carbonized materials, and chitosan, etc., and some synthesized adsorbents silicotitanate, WO3, niobate nanofibers, and metal hexacyanoferrates, etc., have developed and applied for the removal of Sr2+ ions [19], [20], [21], [22], [23], [24], [25], [26]. However, some existing issues which include the stability drawbacks of the organic-based materials, the complexity of the synthetic process, the high sensitivity to solution conditions, and the inapplicability of adsorbents in the column mode have restricted the industrial applications of these materials. It is essential to test and investigate more efficient and cheaper materials [2], [15], [27].
Over recent decades, the wide application of the mixed-valent iron minerals in processing environmental pollutions has drawn a significant amount of attention due to their important roles in influencing the transformation, toxicity, and mobility of inorganic pollutants in the engineered systems. The mixed-valent iron oxides mainly include magnetite and green rust (GR) [28], [29]. GRs are layered double hydroxides, containing both Fe(II) and Fe(III) cations in the brucite-like structures [30]. Following the type of intercalated anions (e.g., , , and ), there are some common types of GRs such as hydroxycarbonate GR (i.e., ), hydroxysulfate GR (i.e., ), and hydroxychloride GR (i.e., ) [31], [32]. GR can theoretically adsorb both inorganic anions and cationic metals based on its amphoteric surface hydroxyl groups. GRs have been used to transform several inorganic contaminants through redox processes [33], [34], [35], [36], [37], [38]. The cycling of 35 elements highlighting the involvement of GR has been reported in a comprehensive investigation [31], [32]. Generally, the interaction between cations and GR mainly experiences three major pathways: 1) the adsorption of cations onto the external surfaces of GR; 2) the incorporation of cations (e.g., divalent and trivalent metals) into the octahedral sheets of the GR interlayer; 3) redox transformations or chemical immobilization through anion exchange, complexation, and chemical precipitation [28], [29], [31], [32], [39], [40], [41]. The redox processes caused by GRs for the transformation of inorganic contaminants have been widely investigated [32]. However, the adsorption capabilities of GRs for cations in the solution environment have been rarely studied, which increases the uncertainty of the application of GR on the uptake of Sr2+ ions. To enhance the adsorption performance of GR on Sr2+, some other techniques are needed to improve the whole removal process.
Electrokinetic (EK) remediation is considered an outstanding method for the remediation of contaminated soil and solid wastes, which is effective in achieving the migration and accumulation of inorganic contaminants in a specific area by electrolysis (i.e., decomposition of water) and electromigration [42], [43], [44], [45]. The H+ and OH− ions generated in electrolysis can affect the adsorption and desorption of Sr2+ in the electrolyzer [46], [47]. The Sr2+ ions will be continuously migrated to the cathode area through electromigration during the EK process. In this study, the hydroxycarbonate GR loading on activated carbon (GR-AC) was synthesized to remove Sr2+ ions from the stock solution. The EK technique coupling with the adsorption was further designed to conduct the uptake of Sr2+ ions in the column-mode experiments. The parameters including the molar ratios of Fe(II) to Fe(III), the ratios of solid to liquid (g/mL), the molar ratios of OH− to Fe(II) + Fe(III), and the molar ratios of to Fe(II) + Fe(III) were comprehensively adjusted based on the adsorption results of Sr2+ in the static equilibrium-adsorption tests to choose an appropriate type of GR-AC for further experiments. Five kinetic models including the pseudo-first-order, pseudo-second-order, intraparticle diffusion, Elovich, and Chrastil’s models were employed to obtain reaction kinetic parameters in different pH ranges and elucidate mechanism pathways corresponding to the equilibrium adsorption. The factors in the column experiments including the voltage gradients (V/cm), the flow rates of feed solutions (mL/min), and the initial pH were changed within four levels in the form of orthogonal design to optimize the running performance of the EK system. Two models including Thomas and Bohart–Adams were further applied for the prediction of breakthrough curves. The mechanisms of Sr2+ removal from the stock solution in the column-mode experiments were comprehensively discussed and analyzed by combining the results of pH, minerals, and morphologies. The research provides an efficient method for removing radioactive Sr from water and also supplying a reference for removing other radioactive cations from wastewater.
Section snippets
Synthesis of GR-AC
Hydroxycarbonate GR loading on AC was synthesized under a coprecipitation method [32]. The AC powders were commercially purchased and were washed several times using ultrapure water, separated by centrifugation at 5000 rpm, dried at 100 °C, and immediately stored in a chamber (150 mbar) pumped by a vacuum machine (VT4.8, BEAKER, Germany) to remove some dissolvable impurities and oxygen gas (O2) before being employed for the synthesis process. The ultrapure water was boiled under atmospheric
Characterization of GR-AC
Characterizations of AC and GR-AC referring to SEM images, EDS, and FTIR spectra are shown in Fig. 2. The distribution of elements on the adsorbents that includes AC and GR-AC are correspondingly listed in Table S2 in the SI. As seen, for the morphology of AC (Fig. 2 a), the surface was rough and uneven with the random distributions of some small pores and cracks. Differently, for the morphology of GR-AC (Fig. 2 b), the brucite-like structures were observed, which demonstrated the layered
CRediT authorship contribution statement
Tao Huang: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Shu-wen Zhang: Validation, Resources, Supervision. Lulu Zhou: Data curation, Writing - review & editing. Long-fei Liu: Resources, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the China Postdoctoral Science Foundation (No. 2020M681774) and the Natural Science Foundation of the Jiangsu Higher Education institutions of China (No. 20KJB490001).
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2022, Journal of Environmental ManagementCitation Excerpt :Although CrVI is indispensable for the growth of some plants, excessive Cr in the soil would poison the plant root and thwart the adsorption and transportation of some nutrients to the botanical issues (Apollaro et al., 2019; Zhang et al., 2019). Some techniques including ion exchange, chemical precipitation, chemical reduction, electrokinetics, electroplating, adsorption, and microbial disposal have been used to remove heavy metals (HMs) from the wastewater (Bilal et al., 2020; Egbosiuba et al., 2020; Huang et al., 2021e; Qin et al., 2020; Zeng et al., 2020). The adsorption method among these choices has its advantages for the treatment of HM-contaminated wastewater such as operation simplicity, lower secondary pollution, and adsorbent recyclability (Egbosiuba et al., 2020; Jeon et al., 2020; Rasheed et al., 2020).