Synthesis and characterization of zeolite-based composites functionalized with nanoscale zero-valent iron for removing arsenic in the presence of selenium from water
Graphical abstract
Introduction
The application of adsorbent nanomaterials for the removal of arsenic (As) present in aqueous matrices has been of great interest in recent years, in response to the high levels of this pollutant in the environment [[1], [2], [3]]. The main interest and use of nanomaterials is due to their beneficial physicochemical properties, size, and reactivity. A nanomaterial widely used in the removal of organic and inorganic pollutants [[4], [5], [6], [7]] has been the nanoscale zero-valent iron (nZVI), due to its easy synthesis procedure and high contaminant removal efficiencies [8,9].
The morphology of nZVI in open aqueous systems is characterized by an Fe° nucleus and an outer surface composed of Fe oxides and/or oxyhydroxides [10], which predominantly depends on several environmental factors, such as pH, dissolved O2 content, dissolved organic carbon content, ionic strength, temperature, and the presence of co-existing contaminant species other than a target contaminant. The nZVI promote pollutant removal through various processes, such as reduction (Cr, Cu, U, and Hg), oxidation (U, As, Se and Pb), adsorption (Cr, U, Pb, and Se), co-precipitation (Se, Cr, and Ni), and precipitation (Zn, Co, Pb, Cd and Cu) [[11], [12], [13]]. A detailed study by Yan et al. [13] used high resolution x-ray photoelectron spectroscopy (HR-XPS) to show As(III) removal by nZVI through multiple processes, including oxidation-reduction, adsorption, and precipitation, that occur in different zones of these nanoparticles. However, the contaminant removal efficiency of nZVI is strongly affected by aqueous matrices in which magnetic forces and van der Waals type interactions encourage the formation of agglomerates and decrease the effective surface area of the adsorbent [14]. Additionally, the high reduction potential of nZVI (-0.44 V (Fe2+/Fe°)) facilitates rapid oxidation of nZVI in the aqueous systems that causes a considerable decrease in the contaminant sorption capacity of the material [15,16]. A solution that stands out for addressing the above problems is to support nZVI on synthetic and natural substrates, including natural geomaterials such as montmorillonite, kaolinite, palygorskite, and zeolite, to immobilize the nanoparticles [12,14,17]. The geomaterials provide greater chemical stability, mainly through decreasing the agglomeration and oxidation processes [[18], [19], [20], [21]]. In particular, zeolites are alkaline aluminosilicates that have exchangeable cations, mainly Na+ and Ca2+, which are coordinated with water molecules in their structure. The main functional groups of zeolite are aluminols (AlOH) and silanols (SiOH), which are ionized as a function of pH. However, due to the isomorphic substitution of Si4+ for Al3+, this aluminosilicate has a permanent negative charge which is balanced by monovalent or divalent cations dissolved in the aqueous medium [22]. The hydrated cations (Na+, Ca2+) thus attracted to the aluminasilicates impart high cation exchange capacity (CEC) to the geomaterials. The geomaterials like clays and zeolites are commonly used to remove heavy metals (Pb2+, Cu2+, and Cd2+) from contaminated waters because their permanent negative charge and CEC favour the adsorption of cations, in addition to having high porosity and a large internal and external surface area [[22], [23], [24], [25], [26]]. In this context, the use of zeolite as the support material of nZVI (zeolite-n-ZVI composites) has been reported, considering their extensive applications in pollutant removal from aqueous matrices [10,14,15,27,28]. However, the presence of a large diversity of ions such as NO3−, PO43−, SO42–, MnO4–, Cd2+, Cu2+ and Ni2+ in contaminated surface waters can be a challenging issue in achieving the best pollutant removal performance of nZVI and zeolite-nZVI composites [[27], [28], [29]]. The co-existing ions pose competition with the target contaminant ions for the different types of adsorption sites of the adsorbing material [[29], [30], [31]]. For example, Boparai et al. [32] reported that the removal of Cd2+ using nZVI as the adsorbent material was strongly decreased by the presence of cations such as Zn2+, Co2+, Mg2+, Mn2+, Cu2+, Ca2+, Na+, and K+ in the aqueous medium.
Selenium (Se) is released into in environment by anthropogenic activities, such as agriculture, mining, combustion, glass manufacturing, coal and insecticide production [33]. In the nature, Se has four oxidation states, (-II), (0), (+IV) and (+VI) [34]. In aqueous matrices, the chemical forms of Se that dominate are selenite (SeO32−) and selenate (SeO42−) [35], where the predominance of these oxyanions in an aqueous medium depends on the pH and redox potential [36]. Particularly, SeO42− might be a potential competitor of As(V) for sorption sites on nZVI-based adsorbents because they have a tetrahedral structure.
The removal of Se using nZVI has been studied by various authors [7,[37], [38], [39], [40], [41]], who have found two adsorption mechanisms. In the first mechanism, the redox process of nZVI facilitates the reduction of Se(VI) to Se(IV), of which the latter is adsorbed on the oxyhydroxides groups. Another mechanism of removal of Se by nZVI considers the direct adsorption and subsequent reduction of Se(VI) to Se(IV) [41]. Yoon et al. [39] used x-ray absorption spectroscopy (XAS) to determine that the primary factor affecting Se(VI) adsorption and nZVI corrosion is the relative concentration of Se(VI) and nZVI. They concluded that when the Se(VI) concentrations exceeded 50 mg L−1, the formation of lepidocrocite (γ-FeOOH) was favored, and Se(VI) was adsorbed directly on the surface of nZVI, which did not have the capacity to reduce Se(0) or Se(-II).
No previous study have investigated the competitive adsorption of Se and As on nZVI-zeolite composite materials. The overarching aim of this study is to apply nZVI and nZVI-functionalized zeolite (Z-nZVI) for the removal of arsenic (As(V)) in the presence of selenate (Se(VI)) as a competitive ion. The adsorbent materials were synthesized and characterized at the morphological and physico-chemical levels pre and post sorption of As(V), Se(VI) and As(V)-Se(VI). Additionally, the variation of contaminants removal performances was evaluated considering changes in the rate and removal capacities of the adsorbents in mono-and multi-component aqueous systems.
Section snippets
Materials and methods
All chemical reagents used in this study were of analytical grade. FeCl3·6H2O, As2O5 in water (1000 mg L−1), Titrisol, NaBH4, NaCl, HCl, and NaOH (Merck), and analytical grade Na2SeO4 (Sigma) were procured and used. The zeolite was obtained from a geological deposit located at Linares city (36° 16′ S, 71° 40′ W). The zeolite was ground and passed through a sieve with 2 mm mesh [42]. The zeolite contains quartz traces, 2.0 mass% of Fe, a Si:Al ratio of 5:1, and corresponds to rehydrated
Pre- and post-sorption characterization of nZVI, zeolite and Z-nZVI
The chemical composition of Z-nZVI showed that the Fe content did not exceed 50%, while the Fe present in the natural zeolite was close to 2.0%. The morphology of nZVI supported on zeolite before and after sorption in single- and multi-component systems is shown in Fig. 1. Chain-type structures of nZVI (the thermodynamically favored state) were seen both pre- and post-sorption of the metalloids. These structures occur mainly due to hydrogen bonds in conjunction with magnetic forces and van der
Limitations and outlook of using nanoparticles and composites in complex aqueous matrices
The functionalization of zeolite with nZVI and the ability of the composites to remove pollutants in single-component systems were described previously by various researchers [14,15,56], with findings that the immobilization process had a synergic effect on removal capacities compared to the pristine materials prior to functionalization. However, the presence of two or more analytes might potentially generate a competition between analytes to occupy the adsorbent’s surface sites. As a result,
Conclusions
The characterization of the Z-nZVI composite showed that immobilization of nZVI on zeolite caused a decrease of aggregation of nZVI in addition to an increase of the external surface area as a result of destabilization of the nanoparticles. After sorption of As(V), nZVI and Z-nZVI lost a degree of Fe° crystallinity, which was associated with the in situ corrosion process of nZVI. Furthermore, it was determined that pH did not affect the sorption of As(V) on nZVI, with approximately 100% removal
Acknowledgements
This work was supported by FONDEF under Project ID17I10302 and by CEDENNA (FB0807). J.S.H acknowledges CONICYT Ph.D. scholarships Nº 21171685 and Convenio Marco Plurianual FRO1656. P.S.O acknowledges CONICYT-PFCHA/Doctorado Nacional/2017-21170040 and Dirección de Postgrado de la Vicerrectoría Académica de la Universidad de Santiago de Chile.
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