Adsorption of Se(IV) in aqueous solution by zeolites synthesized from ﬂ y ashes with different compositions

Low-calcium ﬂ y ash (LC-F) and high-calcium ﬂ y ash (HC-F) were used to synthesize corresponding zeolites (LC-Z and HC-Z), then for adsorption of Se(IV) in water. The results showed that c zeolites can effectively adsorb Se(IV). The optimal adsorption conditions were set at contact time ¼ 360 min; pH ¼ 2.0; the amount of adsorbent ¼ 5.0 g·L (cid:2) 1 ; temperature ¼ 25 (cid:3) C; initial Se(IV) concentration ¼ 10 mg·L (cid:2) 1 . The removal ef ﬁ ciency of HC-Z was higher than the LC-Z after it had fully reacted because the speci ﬁ c surface area (SSA) of HC-Z was higher than LC-Z. The adsorption kinetics model of Se(IV) uptake by HC-Z followed the pseudo-second-order model. The Freundlich isotherm model agreed better with the equilibrium data for HC-Z and LC-Z. The maximum Se(IV) adsorption capacity was 4.16 mg/g for the HC-Z and 3.93 mg/g for the LC-Z. For the coexisting anions, SO 2 (cid:2) 4 barely affected Se(IV) removal, while PO 3 (cid:2) 4 signi ﬁ cant affected it. Regenerated zeolites still had high capacity for Se(IV) removal. In conclusion, zeolites synthesized from ﬂ y ashes are a promising material for adsorbing Se(IV) from wastewater, and selenium-loaded zeolite has the potential to be used as a Se fertilizer to release selenium in Se-de ﬁ cient areas. Low-calcium fly ash (LC-F) and high-calcium fly ash (HC-F) were used to synthesize


INTRODUCTION
Selenium is an essential trace element with multiple biological functions in many organisms, including humans. The In recent years, wastewater treatment using zeolite as lowcost adsorbent has been examined by many researchers.
It is found that the zeolite, either natural or synthetic, can improve water quality and wastewater treatment effectiveness by removing substance such as heavy metals,

Zeolite synthesis
The low-calcium and high-calcium fly ashes (LC-F and HC-F) used in this study were obtained from a power plant located in Hebei Province in China. An alkaline fusion method followed by a hydrothermal treatment was adopted for the synthesis of zeolites (Wang et al. ; Zhang et al. a). In brief, 10 g of fly ashes was mixed with 12 g of NaOH powder (analytical reagent grade) to obtain a homogeneous mixture. The mass ratio of the fly ash to NaOH powder was 1:1.2 (w/w). The homogeneous mixture was then heated in a nickel crucible in 600 C air for 180 min. The fusion products were ground and poured into a flask, to which distilled water was then added to form a mixture. The mass ratio of the fusion products to water was 0.1725 (w/w). The mixture was stirred intensely at 80 C for 2 h to form aluminosilicate gel and was subsequently poured into a stainless alloy autoclave and kept in an oven at 100 C for 9 h. After hydrothermal treatment, solid samples were extracted and then washed thoroughly with distilled water until their pH was less than 10. The resultant solid products were dried at 100 C for 12 h and ground to pass through a 100-mesh sieve for further use.  Co., USA). The amount of Se(IV) adsorption by the synthesized zeolites (q(mg/g)) and the removal efficiency was calculated using Equations (1) and (2), respectively: Removal efficiency(%) where C 0 and C e are the initial and equilibrium Se(IV) concentrations of the test solution (mg/L), respectively. V is the testing solution volume (L), and W is the mass of the adsorbent (g).

Adsorbent regeneration
To investigate the practical reusability of the synthesized zeolites as a potential adsorbent, a regeneration experiment was carried out using 0.1 M NaOH (Bleiman & Mishael ). In this experiment 50 mL of the NaOH solution was

RESULTS AND DISCUSSION
Characterization of fly ashes and synthesized zeolites The composition (XRF), XRD patterns and SEM images of fly ashes and synthesized zeolites are shown in Table 1,    At initial pH range from 1 to 2, the removal efficiency of LC-Z was smaller than HC-Z, but from pH ¼ 3 and above, the removal efficiency of LC-Z was higher than HC-Z.
This can be attributed to the impact of SSA. The SSA of HC-Z was larger than LC-Z. When the pH ranges from 1 to 2, HC-Z could provide more contact sites than LC-Z, but when the pH was equal to or higher than 3, contact sites carried negative charges. The larger the SSA of zeolite, the more negative charges it carries, leading to stronger competition with anions and lower removal efficiency.

Effect of adsorbent dosage
The effect of synthesized zeolite quantity on the Se(IV) removal efficiency and adsorption capacity (q) was investigated to determine the optimal adsorbent dosage, as shown in Figure 5.     bind relatively weakly to surface sites. Complexes often form in the outer sphere (β-plane), which is significantly affected by ionic strength. As mentioned above, the negative effect of coexisting anions on Se(IV) removal by the HC-Z was greater than that by LC-Z. Since PO 3À 4 is usually present in wastewater, its effect should not be ignored.

Adsorption kinetics
The following pseudo-first-order (Equation (3)) and pseudosecond-order (Equation (4)) kinetic models were applied to simulate the experimental data (Lagergren ; Ho & McKay ): where Q e and Q t are the amounts of Se(IV) adsorbed on the synthesized zeolites (mg/g) at equilibrium and at time t, respectively. The values of k 1 (min À1 ) and k 2 (g·mg À1 ·min À1 ) are the sorption rate constants of the pseudo-first-and pseudo-second-order kinetic models, respectively. The figures are shown in Figure 9. The constants are summarized in Table 2.
In the sorption of Se(IV) by both the HC-Z and LC-Z, the coefficients of determination for the pseudo-secondorder kinetic model (R 2 ¼ 0.9351 and 0.7876, respectively) were higher than that for the pseudo-first-order kinetic

Adsorption isotherm
The mechanism of interaction between adsorbent and adsorbate can be described by adsorption isotherms. Of the many models that describe this process, the commonly used Langmuir and Freundlich isotherms were applied in this study to fit the adsorption isotherm data (Herbert ; Langmuir ).

Langmuir isotherm
The Langmuir isotherm assumes that monolayer adsorption occurs on an adsorbent surface, and there is no interaction between the adsorbate molecules. The Langmuir equation is as follows: where K L is the Langmuir coefficient (L/mg). Q m is the maximum monolayer adsorption capacity, and Q e is the amount of adsorbed Se(IV) on a mass unit of the adsorbent at equilibrium (mg/g). C e is the equilibrium Se(IV) concentration (mg/L).
The Langmuir isotherm-fitting results are summarized in

Freundlich isotherm
The Freundlich isotherm is an empirical equation, as follows: where K F and n F are the Freundlich parameter and adsorption intensity, respectively. The n F should be in the range of 0.1-1 for beneficial adsorption. The results are shown and listed in Figure 10 and Within this study, the maximum sorption capacity of the HC-Z was slightly higher than that of LC-Z; this was because the HC-Z has larger SSA, which can provide

Adsorbent regeneration
The adsorption ability of regenerative zeolites was evaluated to investigate their reusability. A comparison of Se(IV) adsorption between regenerated zeolites at different dosages and the original zeolites is shown in Figure 11. Under the same adsorbent dosage, the removal efficiency of the initial zeolite was higher than the regenerated zeolite. When the adsorbent dosage was 5 g/L, the removal efficiency of regenerated zeolites reached the maximum and the differences in removal efficiency between initial zeolite and regenerated zeolite reached the minimum (25.02% for HC-Z and 1.2% for LC-Z). Subsequently increasing the regenerated zeolite dosage resulted in decreased removal efficiency, similar to the trend seen with the original zeolites. The reduction of removal efficiency of regenerated zeolite relative to the initial zeolite may be attributable to the incomplete release of Se(IV) adsorbed to the original zeolites during regeneration, leading to a collapse of the structure of zeolites.
These preliminary results indicate that regenerated zeolites still had a relatively high capacity for Se(IV) adsorption.
Hence, synthesized zeolites can be recycled and utilized for Se(IV) removal from wastewater.

Application prospect
Selenium is an essential trace micronutrient for both human   the QA-SB could be desorbed efficiently and 27.8% of laden phosphate released in soils after 4 days (Shang et al. ).
Meanwhile, the use of zeolites as feed supplements for animals and medical applications indicates that zeolites are not harmful to humans (Smedt et al. ). Zeolite had been used as both carriers of nutrients and medium for free nutrients (Ramesh & Reddy ). Compared with the above research, we could assume that selenium-loaded zeolite can be used as a Se fertilizer to release selenium in Se-deficient areas. Of course, selenium-loaded zeolites may also release other chemical elements while releasing selenium, which needs to be clarified in the next study. In the subsequent studies, we will study the slow release of selenium by the selenium-loaded zeolite and the available potential and risk.

CONCLUSIONS
This study confirms that the zeolites synthesized from differ- tion. The study shows that zeolites produced from fly ashes are inexpensive alternative adsorbents for the removal of Se(IV) from industrial wastewater. Furthermore, using fly ash to synthesize zeolite achieved the reutilization of the fly ash resources. Selenium-loaded zeolite has the potential to be used as a Se fertilizer to release selenium in Sedeficient areas.