Kinetic and isothermal sorption of antimony oxyanions onto iron hydroxide during water treatment by coagulation process
Graphical abstract
Introduction
Antimony (Sb) is the fourth element of group 5A in the periodic table with outer electron configuration (s2p3). Pentavalent species (Sb(V)) in aerobic surface waters and trivalent ions (Sb(III)) in underground anoxic conditions are the two most prevalent inorganic forms of Sb [1]. Under natural water pH conditions, the Sb(III) species is stable as Sb(OH)3, while Sb(V) exists as Sb(OH)6− at pH > 2.72 [2,3]. The trivalent form of Sb is ten-fold toxic than its pentavalent species [4]. Therefore, Sb compounds and their derivatives are placed on the priority list of the United States Environmental Protection Agency (USEPA) and the European Union (EU) [5]. In addition, Korea has presently added Sb to the water monitoring list [6]. The EU and USEPA have established regulatory levels of Sb to be 6 μg/L and 10 μg/L, respectively, for potable water [7]. In Pakistan and China, the permitted Sb level in drinking water is 5 μg/L or less [8,9]. To reduce potential associated risks, it is therefore important to eliminate Sb from drinking water supplies.
While Sb levels in unpolluted waters should be as low as 1.0 μg/L, higher Sb contamination has been recorded in water streams near mining and smelting areas [1,10]. Several methods i.e. nano-filtration [11], adsorption [2,7], and coagulation [3,6,[12], [13], [14], [15]] for Sb removal have been well documented. Owing to unique properties of iron oxide based sorbents, their usage is usually preferred by drinking water industry. Among them, hematite, magnetite, and maghemite are the most commonly used due to exceptional surface charge and redox behavior, polymorphism with temperature-induced phase transitions, and unique magnetism [16]. However, the drinking water industry still prefers a chemical coagulation method as it is an efficient and economical removal method for several heavy metals ions including Sb from aquatic environment [17]. Compared to aluminum sulphate coagulant, ferric chloride (FC) coagulant has been shown to be effective in remediating Sb from drinking water [5,18]. It has been demonstrated that the formation of iron hydroxide (FHO) can enhance Sb removal efficiencies during FC coagulation [3,6,18,19]. Our previous studies [3,6,12,15] have also suggested that the influence of coagulation conditions is the determining factor for Sb removal from various aquatic environments. For example, an alkaline pH condition can significantly reduce the formation of FHO in Sb(V) suspensions, thereby affecting the efficacy of a system [3,6,12,15]. Moreover, remarkable inhibitory effects of anions such as phosphate, sulphate, and organic matter on Sb species' sorption affinity for FHO have previously been reported [14,15,18,19]. In the presence of interfering species, a more prominent impact on Sb(V) species than Sb(III) has been observed during FC coagulation [19]. An earlier study has also reported a higher Sb adsorption potential of Sb species towards hydrous ferric oxide than other iron oxide minerals [20]. Although previous studies have provided a wide understanding on removal characteristics of Sb from water, studies on the interactive behavior of FHO and Sb in different aqueous environments are minimal. Thus, understanding the sorption behavior of Sb on FHO is critical for water treatment applications.
Previous studies have revealed that adsorption is the primary removal mechanism for Sb species during a chemical coagulation process [15,18,19]. Physical and chemical characteristics of water play an essential role in determining sorption characteristics of Sb species onto FHO surface. For instance, the presence of monovalent and divalent cations (K+, Mg2+) can affect the adsorption potential of Sb species onto FHO surface [15]. It has been demonstrated that cations can effectively compress the electrical double layer of FHO in solution, thereby destabilizing the formed FHO and exposing more surface sites to enhance Sb sorption in a solution [21]. Temperature is also considered as one of influential parameters that can affect the fate, mobility, and transport mechanisms of contaminants in water [22]. However, little is known about how the adsorption of Sb species differs at certain temperatures in traditional water treatment processes. Therefore, it is imperative to examine isothermal and thermodynamic features of Sb sorption onto FHO surface.
It has been demonstrated that Sb(III) and Sb(V) are mostly removed by hydrophobic and ionic bonding, respectively, during a conventional water treatment [5]. Previous studies [15,19] have shown that coprecipitation and adsorption mechanisms can explain the removal of Sb(III) species under various suboptimal conditions. It has been demonstrated that both surface and internal adsorption of Sb(V) onto FHO can adequately explain the elimination of pentavalent Sb with high sensitivity under the influence of competing ions [19]. Specifically, electrostatic interactions might be involved in the elimination process of Sb during a chemical coagulation [9]. However, there is a lack of research on how the adsorption activity of FHO would vary for different types of Sb contamination. Thus, it is essential to comprehend the potential adsorption mechanisms of Sb ions onto FHO under a natural aquatic environment.
Therefore, the objective of the current study was to investigate the sorption affinity of Sb species on FHO during an FC coagulation process. Effects of various operating conditions (i.e., pH, time, Sb concentrations, temperature, and ionic strength (IS)) were examined. FHO formation was also monitored under similar experimental conditions. The present study also addressed the possible elimination mechanism of Sb(III, V) ions at various matrices depending on reaction conditions. Finally, X-ray diffraction (XRD) spectrum of pristine compounds and Sb-loaded FHO were acquired to elucidate the mechanism involved in Sb removal during FC coagulation.
Section snippets
Reagents and stock solution preparation
Chemical reagents including potassium hexahydro-antimonate (KSb(OH)6) and antimony (III) oxide (Sb2O3) were procured from (Sigma Aldrich, USA). Other chemicals such as iron (III) chloride hexahydrate (FeCl3.6H2O), sodium nitrate (NaNO3), hydrochloric acid (HCl), nitric acid (HNO3), and sodium hydroxide were purchased from Samchun (Pyeongteak-si, Korea). Pure water used for sample preparation was obtained via millipore water purification system.
The coagulant stock solution (100 mM FC) was
Influence of pH on Sb sorption
The sorption of metals from fresh water stream is strongly influenced by the suspension pH because of variation in characteristics of metal ion. Accordingly, the effect of pH on FHO formation and Sb sorption was monitored for a wide pH range during FC coagulation. Results are shown in Fig. 1. It was observed that around 90 % sorption of both Sb ions was found between pH 6–8, showing good sorption affinity towards FHO under these conditions. The FHO formation in Sb(V) suspension was
Conclusions
In this study, FHO formation and Sb(III, V) removal from fresh water bodies were investigated and sorption kinetics, isotherms, and thermodynamics were discussed extensively. Rapid Sb sorption reaction by FHO depends on solution pH. An alkaline pH condition has inhibitory effect on FHO formation, thereby increasing Sb(V) mobility. An insignificant impact on FHO formation was observed under other experimental conditions (i.e., contact time, Sb loading, and temperature). The sorption of Sb by FHO
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare no conflict of interest.
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