Effective and simultaneous removal of organic/inorganic arsenic using polymer-based hydrated iron oxide adsorbent: Capacity evaluation and mechanism
Graphical abstract
Introduction
Organoarsenicals are derivatives of phenylarsonic acid compounds (PACs), which are commonly used in the livestock and poultry industry to promote animal growth, control diseases, and enhance the feeding efficiency (Christen, 2001; Silbergeld and Nachman, 2008). As a common PAC, p-Arsanilic acid (p-ASA) is one of the most widely used feed additives in China and other developing countries (Chapman and Johnson, 2002; Makris et al., 2008). Few of these compounds remain in animals, whereas most are excreted unchanged in the urine and feces (Zhu et al., 2014). Moreover, these compounds easily migrate into the water through rainfall and irrigation due to the extremely high water solubility (Chen et al., 2020). The concentration of PACs in water environments ranges within 0.5–5000 μg/L (Tian et al., 2018). PACs are not stable and can be further transformed into stable but toxic inorganic arsenic species (e.g., arsenite and arsenate (As(V))) through biogeochemical degradations and/or abiotic transformation in water environments (Lv et al., 2020). Inorganic arsenic primarily exists as As(V) under aerobic and higher pH conditions (Jain and Ali, 2000; Jones, 2007). Prolonged exposure to inorganic arsenic increases the risk of cancer for humans, which poses a serious threat to the ecological environment and residential health (Erickson et al., 2019). Therefore, the water pollution and safety problems caused by PACs and inorganic arsenic should be addressed urgently.
Adsorption can effectively separate pollutants from aqueous phase without generating toxic secondary products, which is a simple and low-cost process. This process is one of the most extensively used and effective methods for removing inorganic or organic pollutants from water environment. Researchers have been developing various adsorbents, such as carbonaceous (e.g., activated carbon (AC)) (Dabrowski et al., 2005; Dias et al., 2007), metal oxide (e.g., Al2O3) (Singh and Pant, 2004; You et al., 2019), inorganic non-metallic materials(e.g., functionalized Silica Gel)(Fan et al., 2012b; Fan et al., 2011), and polymer adsorbents (Pereao et al., 2017; Qin et al., 2016). AC is commonly used to remove various pollutants because of its large specific surface area and abundant functional groups. However, some disadvantages of this material, including high cost and difficulty in separation, recovery, and regeneration, should be addressed in practical applications (Gupta et al., 2016). Metal oxide adsorbents have high adsorption capacity and desirable regeneration ability, which attract widespread attention from researchers. However, such materials were difficult to apply directly to turbulent operations because of low mechanical strength and easy clogging. Polymer adsorbents possess several outstanding features, including the nanoporous structure, adjustable surface chemistry and effective regeneration ability. Besides, this material does not show obvious capacity loss during long-term use and is considered as an alternative to AC for removing many pollutants (Kiani et al., 2017). In addition, there have been some previous studies on the adsorption of inorganic arsenic by functionalized inorganic non-metallic materials (Fan et al., 2012a; Fan et al., 2011). It has been found that functionalized silica gel showed desirable removal capacity for inorganic arsenic. Nevertheless, the ability of functionalized inorganic non-metallic materials to remove organic arsenic is not yet known. Therefore, it is believed logically that the abovementioned adsorbents cannot satisfactorily remove inorganic and organic pollutants simultaneously in one system because the removal pathway of pollutants is limited to a single physical or chemical adsorption. Adsorbate-limited sorbents are generally unsuitable for the treatment of wastewater containing multiple pollutants. Therefore, a new type of functional adsorbent should be developed to achieve the simultaneous removal of different arsenic pollutants. The adsorption process of two types of pollutants is achieved through different pathways. Inorganic water pollutants exist primarily in the form of ions. Consequently, electrostatic interaction and/or coordination are the dominant pathways of adsorption (Zhang et al., 2008a; Zhang et al., 2008b). For organic pollutants, Van Der Waals force is the common interaction shared by various adsorption materials. Micropore filling is another important adsorption pathway for materials with microporous structure (Yang et al., 2015). The common interactions that occur between the organic pollutants and functional groups on the surface of adsorbents include acid-base interaction, hydrogen bonding, and π–π–electron donor–acceptor interaction (π–π–EDA) (Joshi et al., 2017c; Sarker et al., 2017). The key to the simultaneous removal of organic toxicants and inorganic pollutants in water is to effectively combine the different adsorption sites into one unit. Apparently, it is difficult to develop adsorbent materials that can simultaneously remove PACs and inorganic arsenic.
Anion-exchange is currently one of the most appropriate removal methods because inorganic arsenic exists mostly in the form of As(V). However, the abundance of anions in water environment (e.g., Clˉ, NO3ˉ, and SO42ˉ) can reduce the selectivity and treatment ability of the anion-exchange resins for arsenic removal. Many studies reported that a strong complexation exists between hydrated iron oxide (HFO) and As(V), and the adsorption performance of HFO is stable regardless of pH value (Zhang et al., 2008a; Zowada and Foudazi, 2019). As(V) is selectively immobilized on the surface, thereby forming a dual nuclear-bridged inner surface ligand with HFO. However, HFO nanoparticles are difficult to apply directly to fixed-bed or other continuous-flow operating systems (Wang et al., 2011; Zhang et al., 2008b). Immobilizing HFO on the surface of porous carrier is an effective method to solve the aforementioned issues (Bui et al., 2019; Li et al., 2016a). Among the carrier materials, ion exchange resin has several advantages, such as high mechanical strength and excellent regeneration performance. In conclusion, resin can be an ideal carrier for HFO nanoparticles (Cumbal and SenGupta, 2005; Zhang et al., 2005). HFO immobilization and enhanced removal of As(V) can be achieved by utilizing the structural characteristics of the anion-exchange resin. Donnan membrane effect can enhance the penetration in the pores, as well as the electrostatic attraction due to the positively charged groups. Constructed resin-based nanocomposite adsorbent possesses the advantages of nanoparticles and ion exchange resin, which is given excellent macroscopic properties and renders both materials suitable for the selective adsorption of As(V). The rich microporous structure of organic host exhibits attractive selectivity and potential for PACs removal in theory (Yang et al., 2015). Therefore, resin-based HFO nanocomposite is expected to become a promising adsorbent for the one-step removal of PACs and As(V) in water environments.
In fact, many researchers have been working on finding an economical and simple way to repair arsenic-contaminated water, mostly for As(V) by adsorption or PACs by adsorption or oxidation. However, there are few studies on controlling pollution caused by multiple arsenic-specific pollutants in water environment. It is necessary to pay attention to the complex pollution caused by specific arsenic compounds (e.g. PACs and As(V)). In this study, p-ASA and As(V) were selected as the target pollutants and the prepared resin-based HFO composite (HFOR) was used as the adsorbent to effectively and simultaneously remove PACs and As(V) from water environment. Many monofunctional adsorbents are insufficient for the treatment of wastewater containing multiple pollutants. Therefore, based on the molecular structure and physicochemical properties of different arsenic species, HFOR was prepared from the perspective of framework properties of the adsorbent and its affinity and selectivity for the adsorbate. HFOR possesses many characteristics, such as high affinity for arsenic species, unique structure, excellent hydraulic and mechanical properties, and application potential (as host). It is expected to maximize its functionality based on practical applications. In this work, the “surface adsorption” of the adsorption resin was extended to “group adsorption”, “non-specific adsorption” was expanded to “specific adsorption”, whereas the “unit adsorption” was extended to “multiple adsorption”. The proposed method improved the adsorption selectivity and functionality of the adsorbent, as well as enabled the simultaneous removal of different pollutants in a system. This study introduced a new way for the simultaneous removal of different arsenic species coexisting in the water environment under nanoconfinement on the basis of the adsorbent immobilization, in order to achieve the goal of one-step repair of complex pollution caused by organic arsenic and inorganic arsenic in water environment. The findings of this study will of great value to the application of new composite adsorption materials for the removal of specific coexisting pollutants.
The anion-exchange resin was used as the HFO carrier to synthesize HFOR, which was then used as a functional material to simultaneously remove p-ASA and As(V) from water. Firstly, the physical and chemical properties of HFOR were analyzed. The adsorption performance of HFOR on different arsenic species was comprehensively evaluated. Secondly, the isotherm equilibrium and adsorption kinetics of p-ASA and As(V) by HFOR were systematically studied, and the adsorption pathways of different arsenic species were revealed. Then, the effects of pH and coexisting substances on the adsorption performance of HFOR were investigated. Lastly, the stability and reusability of HFOR were evaluated, and cyclic adsorption-regeneration and fixed-bed column experiments were conducted to assess the feasibility of material in long-term applications.
Section snippets
Materials
The macroporous strongly basic anion-exchange resin (D201) was purchased from China Zhengguang Resin Co., Ltd. The multi-walled carbon nanotubes (MWCNTs) (95%), AC, Al2O3, and molecular sieve (MCM-41) were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China). The p-ASA and C6H8AsNO3 (98%) were purchased from Macleans Corporation (Shanghai, China), whereas the Na2HAsO4·7H2O (98%) and FeCl3·6H2O (98%) were obtained from Sigma-Aldrich Corporation. The EtOH was purchased from Nanjing
Characterization of adsorbents
The physical and chemical characteristics of the HFOR are presented in Table S1 and Fig. S1. HFOR beads were spherical and dark brown in color (Fig. S1(a)). The average pore diameter of adsorbent decreased from 3.49 nm to 2.98 nm after loading HFO, whereas the BET surface area increased from 14.55 m2·g−1 to 29.08 m2·g−1. Similarly, the pore volume increased from 0.0100 cm3·g−1 to 0.0198 cm3·g−1. This change in the pore size distribution can be ascribed to the aggregation of HFO nanoparticles,
Conclusions
A multifunctional adsorbent HFOR was prepared to simultaneously remove As(V) and p-ASA in the water environment. Compared with other commonly used adsorption materials, the ability of HFOR to remove the two different types of arsenic pollutants was more prominent. The adsorption capacity of As(V) and p-ASA reached 60 mg As/g and 22 mg/g, respectively. This advantage can be attributed to the unique structure of HFOR and the synergy of the multiple functional groups, especially the specific
CRediT authorship contribution statement
Biming Liu: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Writing - review & editing. Zhenxue Liu: Conceptualization, Methodology, Software, Visualization, Investigation. Haixia Wu: Visualization, Investigation. Shunlong Pan: Supervision. Xing Cheng: Software, Validation. Yongjun Sun: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Supervision, Software, Validation, Writing - review & editing. Yanhua Xu: Supervision,
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.
Acknowledgements
This research was supported by National Key Research and Development Program of China (2017YFB0602500), the University Science Research Project of Jiangsu Province (16KJA610002), National Natural Science Foundation of China (No. 21607074, No. 51707093), and 2018 Six Talent Peaks Project in Jiangsu Province (JNHB-038).
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These authors contributed equally to this work and should be considered as co-first authors.