Diethylenetriaminepentaacetic acid–thiourea-modified magnetic chitosan for adsorption of hexavalent chromium from aqueous solutions
Graphical abstract
Introduction
Heavy metal contamination of water bodies is a serious problem given that these heavy metals tend to persist and accumulate in the environment, and among the heavy metals present in wastewater, Cr is one of the most toxic. According to the drinking water guidelines set by the World Health Organization, the maximum allowable limit for total chromium is 0.05 mg L−1 (Cui et al., 2017). When present in higher concentrations, chromium can cause major health problems, and particularly, Cr(VI) is 500 times more toxic, mutagenic, and carcinogenic than Cr(III) (Sun et al., 2016). In this regard, several treatment technologies, such as chemical precipitation, ion exchange methods, reduction techniques, electrochemical precipitation, and solvent extraction, have been developed for the removal of Cr(VI) from aqueous media (Song et al., 2014). Among these methods, adsorption, which is most widely used, has also been identified as the most effective method by which Cr(VI) removal from wastewater can be realized (Shi et al., 2017). However, its application is limited due to the associated cost. Hence, low-cost adsorbents are increasingly attracting attention.
In recent years, researchers have studied a number of adsorbents, such as chitosan (CS) (Pakade & Chimuka, 2013), cyclodextrin (Ngah & Fatinathan, 2008), citric acid (Abbas et al., 2015), graphene (Abbas et al., 2017), and carbon nanotubes (Sheha & El-Zahhar, 2008). Among them, CS, as one of the most abundant native polymers, is hydrophilic, biodegradable, and harmless to living things. In addition, it undergoes chemical derivatization readily and contains many functional groups on its macromolecular chains that can chelate heavy metals (S. Li et al., 2012). However, its low mechanical strength when raw as well as its tendency to dissolve readily in acidic solutions and leach organics such as carbohydrates are significant drawbacks that have limited its applicability in the adsorption of Cr(VI) from wastewater, given that many industrial effluents tend to be acidic (Pandey & Mishra, 2011). Therefore, chemical modification to introduce functional groups that are necessary for Cr(VI) adsorption in acidic environments is essential. The presence of sulfur-containing groups, such as xanthates (Deng & Ting, 2005), dithiocarbamates (Bhaumik et al., 2011), mercapto groups (Sherlala et al., 2018), acetyl groups (J. Xu et al., 2018), and thiourea (Burakov et al., 2018), can enhance its adsorption capacity. In particular, thiourea-modified CS composites show very high adsorption efficiency. Reportedly, aminothiourea-modified chitosan has shown removal efficiencies of 66.4–99.9% for As(V), Ni(II), Cu(II), and Cd(II) (Li et al., 2014). Additionally, Chauhan et al. (2012) improved the Cr(VI) removal efficiency of CS by preparing a thiocarbamoyl CS composite.
Diethylenetriaminepentaacetic acid (DTPA), which contains a large number of carboxyl groups (-COOH), is a typical chelating agent and is capable of forming highly stable chelates with Cr(VI) (Zendehdel et al., 2019). Recently, different supporting materials, functionalized with DTPA, have been explored. For instance, Huang et al. (2018) modified CS microgels using DTPA and obtained Cu2+ adsorbents with high adsorption capacities of up to 106.0 ± 2.1 mg g−1. Additionally, it has also been observed that DTPA-functionalized CS exhibits high adsorption performance toward Cd(II), Pb(II), Co(II), and Ni(II) with maximum adsorption capacities of 175, 182, 66, and 70 mg g−1, respectively (Zhao et al., 2015). DTPA-modified CS/polyethylene oxide nanofibers have also shown maximal adsorption capacities of 177, 142, and 56 mg g−1 for Cu(II), Pb(II) and Ni(II), respectively (Surgutskaiaa et al., 2020). However, DTPA cannot be directly used for metal adsorption applications owing to its high solubility in water (Upadhyay et al., 2021).
In previous studies, the modification of chitosan involved the grafting of only a single sulfhydryl or carboxyl group. However, it is worth exploring whether sulfhydryl and carboxyl groups can be simultaneously grafted into chitosan to obtain more adsorption active sites. Therefore, we hypothesized that co-inarching DPTA and thiourea into the structure of CS derivatives would yield a more superior structure and enhance the metal ion removal performances of the resulting composite. However, the heavy-metal-ion adsorption performance and highly cross-linked structure of DPTA-thiourea-modified magnetic CS are still unknown, and its potential for Cr(VI) ion adsorption has not been reported.
Therefore, the goal of this study was to develop a CS-based adsorbent material that exhibits a high adsorption capacity and allows for the easy removal of Cr(VI) from water. In this regard, diethylenetriaminepentaacetic acid–co-thiourea-modified magnetic CS (DTCS-Fe3O4) was synthesized for Cr(VI) removal. The effects of treatment conditions on its adsorption performance as well as the associated kinetics were analyzed. Moreover, based on observations of the structure and morphology of the composite, the possible Cr(VI) adsorption mechanism was proposed.
Section snippets
Materials
CS (deacetylation degree >90%), thiourea, DTPA (99%), sodium hydroxide, and acetic acid were provided by Tianjin Guangfu Fine Chemical Research Institute. Glutaraldehyde and FeCl3·6H2O were purchased in Tianjin Kemiou Chemical Reagent Company. Cr(VI) stock solution was prepared using K2Cr2O7 (BDH Chemicals).
Adsorbent preparation
First, 2.0 g of CS was added to 150 mL of an aqueous solution of acetic acid. Subsequently, the solution was spiked with 1 g of thiourea followed by stirring. At the same time, 1 g of DTPA
Characterization of adsorbent
The chitosan presented a weight average molecular weight (Mw) of 10,600 Da and a number average molecular weight (Mn) of 3257 Da. Further, its molecular weight distribution (polydispersity) was 3.3. The % transmittance FTIR spectra of CS, DTCS, and DTCS-Fe3O4 are shown in Fig. 1(a). Specifically, the FTIR spectrum of CS showed characteristic peak absorbances corresponding to the stretching vibrations of OH and NH bonds at 3440 cm−1 (Aliabadi et al., 2013). Conversely, the spectra of DTCS and
Conclusions
In this study, we synthesized the magnetic adsorbent, DTCS-Fe3O4, for application in Cr(VI) removal from aqueous solutions. The optimum conditions for the adsorption of Cr(VI), which were determined to be a pH of 3 and temperature of 313 K, resulted in a maximum adsorption capacity of 321.3 ± 6.0 mg g−1 which was beyond those of most previous reported adsorbents. Further, the adsorption process could be modeled using both the Langmuir isotherm model, which indicated the involvement of monolayer
CRediT authorship contribution statement
Shejiang Liu: Supervision, Conceptualization, Funding acquisition, Writing – review & editing. Jing Gao: Data curation, Writing – review & editing. Li Zhang: Methodology, Data curation, Writing – original draft. Yongkui Yang: Supervision, Writing – review & editing. Xiuli Liu: Supervision.
Acknowledgments
This study was financially supported by the Key Research and Development Plan of Tianjin, China (No. 19YFZCSF01090).
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