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1 © 2019 The Authors Journal of Water Reuse and Desalination | in press | 2019
Adsorption of Se(IV) in aqueous solution by zeolites synthesized from fly ashes with different compositions Xiao Zhang, Xinyuan Li, Fan Zhang, Shaohao Peng, Sadam Hussain Tumrani and Xiaodong Ji
ABSTRACT
Low-calcium fly ash (LC-F) and high-calcium fly ash (HC-F) were used to synthesize corresponding Xiao Zhang Xinyuan Li zeolites (LC-Z and HC-Z), then for adsorption of Se(IV) in water. The results showed that c zeolites can Fan Zhang ¼ Shaohao Peng effectively adsorb Se(IV). The optimal adsorption conditions were set at contact time 360 min; Sadam Hussain Tumrani pH ¼ 2.0; the amount of adsorbent ¼ 5.0 g·L 1; temperature ¼ 25 C; initial Se(IV) concentration ¼ Xiaodong Ji (corresponding author) School of Soil and Water Conservation, 10 mg·L 1. The removal efficiency of HC-Z was higher than the LC-Z after it had fully reacted because Beijing Forestry University, Beijing 100083, the specific surface area (SSA) of HC-Z was higher than LC-Z. The adsorption kinetics model of Se(IV) China E-mail: [email protected] 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 2 4.16 mg/g for the HC-Z and 3.93 mg/g for the LC-Z. For the coexisting anions, SO4 barely affected 3 fi Se(IV) removal, while PO4 signi cant affected it. Regenerated zeolites still had high capacity for Se(IV) removal. In conclusion, zeolites synthesized from fly 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-deficient areas. Key words | high-calcium fly ash, low-calcium fly ash, selenium(IV) removal, synthesized zeolite, wastewater treatment
INTRODUCTION
Selenium is an essential trace element with multiple biologi- Selenium is also a common and intractable metalloid cal functions in many organisms, including humans. The contamination with high toxicity in water, and the WHO recommended nutrient intake by the World Health recommends a maximum permissible selenium level in Organization is 40–200 μg/day, depending on gender and drinking water of 10 μg/L (Hu et al. ; Sharp et al. ; life-stage group (Kurniawati et al. ). Both an excess and Tan et al. ). Hudak () mapped and statistically deficiency of selenium can cause serious health problems in analyzed selenium contamination from 112 water wells in humans (Sun et al. ). Low Se intakes are associated south Texas groundwater and found that 21% of selenium with considerable health disorders while a toxic dose of Se observations exceeded the 20 μg/L advisory level for irriga- results in selenosis (Zhu et al. ; Kurniawati et al. ). tion water, and 5% surpassed the 50 μg/L standard for drinking water. The Lower Arkansas River Valley (LARV) This is an Open Access article distributed under the terms of the Creative in Colorado is designated as an impaired watershed by the Commons Attribution Licence (CC BY 4.0), which permits copying, Environmental Protection Agency due to high levels of adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). selenium and uranium, which endangers aquatic life and
doi: 10.2166/wrd.2019.036
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2 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
impairs domestic water sources (Sharp et al. ). The ammonium, and phosphorus (Zhang et al. b; Ji et al. Dorno-Gobi Aimag in Mongolia represents the highest , ). However, there are few studies on the adsorption drinking water Se levels ranging from 1.2 to 18.6 μg/L of Se(IV) from aqueous solution by synthetic zeolites with (Golubkina et al. ). Aquatic selenium pollution is a different calcium content. universal and chronic global problem (Bailey ; He In this study, we chose synthetic zeolite from fly et al. ). Selenium exists predominantly as two species, ash with different calcium content as an adsorbent to 2 2 selenite (SeO3 , Se(IV)) and selenite (SeO4 , Se(VI)), in remove Se(IV) from aqueous solutions. The zeolites the water and both oxyanions could result in health were synthesized by an alkali fusion-assisted hydrothermal problems (Sun et al. ). treatment. The impacts of contact time, pH value, adsorbent Numerous methods for the removal of selenium from dosage, initial concentration of Se(IV), and coexisting aquatic environments have been studied, including anions on the adsorption of Se(IV) were investigated coagulation, flocculation, ion exchange, precipitation, to determine the optimal adsorption conditions. The adsorp- bioremediation, membrane filtration, ozone oxidation, tion kinetic and equilibrium were also investigated. Further, electrochemistry treatment, and adsorption (Geoffroy & adsorbent regeneration and feasibility of slow release of Demopoulos ; Sen ; Hamed et al. ; He et al. selenium by the selenium-loaded zeolite were studied in ; Jacobson & Fan ). Of these, adsorption is one of order to develop recycling applications. the most effective and simple methods for point-of-use appli- cations (Awual et al. ; Jacobson & Fan ). Many materials for adsorption of selenium have been studied, MATERIALS AND METHODS such as metal nanoparticles and metal oxides (Verbinnen et al. ; Mafu et al. ; Cui et al. ; Ma et al. ). Zeolite synthesis In recent years, wastewater treatment using zeolite as low- cost adsorbent has been examined by many researchers. The low-calcium and high-calcium fly ashes (LC-F and It is found that the zeolite, either natural or synthetic, HC-F) used in this study were obtained from a power can improve water quality and wastewater treatment plant located in Hebei Province in China. An alkaline effectiveness by removing substance such as heavy metals, fusion method followed by a hydrothermal treatment was ammonium, phosphorus, chemical oxygen demand (COD), adopted for the synthesis of zeolites (Wang et al. ; dissolved organic matter, cations, and radioactive elements Zhang et al. a). In brief, 10 g of fly ashes was mixed (Chen & Lu ; Kussainova et al. ; Kotoulas et al. with 12 g of NaOH powder (analytical reagent grade) to ). Yusof et al. () performed column studies to obtain a homogeneous mixture. The mass ratio of the fly evaluate the performance of modified zeolite-Y with ion ash to NaOH powder was 1:1.2 (w/w). The homogeneous Fe in removing As(III), As(V), Se(IV) and Se(VI) from mixture was then heated in a nickel crucible in 600 C groundwater and proved that the modified zeolite Y could air for 180 min. The fusion products were ground and be an effective adsorbent material. poured into a flask, to which distilled water was then Fly ash is an industrial solid waste material generated by added to form a mixture. The mass ratio of the fusion pro- coal-fired thermal power plants. Currently, over 750 million ducts to water was 0.1725 (w/w). The mixture was stirred tonnes of fly ash are produced annually throughout the intensely at 80 C for 2 h to form aluminosilicate gel and world, with global average utilization estimated to be was subsequently poured into a stainless alloy autoclave only approximately 25% (Blissett & Rowson ; Cardoso and kept in an oven at 100 C for 9 h. After hydrothermal et al. ; Wang et al. ). As a resource utilization treatment, solid samples were extracted and then washed method, the synthesis of zeolites from fly ash has been thoroughly with distilled water until their pH was less widely studied (Zhang et al. a; Jin et al. ; Koshy & than 10. The resultant solid products were dried at 100 C Singh ). Synthetic zeolite from fly ash with different for 12 h and ground to pass through a 100-mesh sieve for calcium content had been used to remove heavy metals, further use.
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3 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
Characterization method. Nitrogen adsorption experiments were conducted at 77 K using QuadraSorb SI (Cantata Instruments The chemical compositions of fly ash and synthesized Co., USA). zeolites were determined by X-ray fluorescence (XRF) (Philips PW2404, Philips Co., Holland). The percentage Batch experiments and oxide contents of each element were determined by the chemical analysis method of silicate rock in GB/ Stock solutions of Se(IV) (1,000 mg/L) were prepared from fl T14506.28-93. The mineralogical compositions of the y anhydrous Na2SeO3 (analytical grade), which was diluted to ashes and synthesized zeolites were determined using a the desired experimental concentrations by adding distilled powder X-ray diffraction (XRD) instrument (Rikaku water. All adsorption experiments were conducted in D/max-RB, Rikaku, Japan), as shown in Figure 1. Scattering 150-mL stoppered conical flasks on a temperature-controlled patterns were collected from 5 to 70 with a scan time of shaker (HZQ-F160) with continuous stirring at 150 rpm. 1 min per 2 steps. Photomicrographs of the two aforemen- The effect of contact time on Se(IV) adsorption by the tioned materials were obtained using a scanning electron two synthesized zeolites was investigated under different microscope (SEM) (S-3000N, Hitachi, Japan). The results contact times ranging from 0 to 600 min at 25 C and the are shown in Figure 2. Before the experiment, about 3 g adsorbent dosage was 0.5 g. The initial concentration of of fly ash and 0.1 g of synthesized zeolite were degassed Se(IV) was 10 mg/L, and the initial pH of the solutions in a vacuum at 200 C for 2 h, then the specificsurface was adjusted to 2.0 ± 0.05. area (SSA) of the materials was calculated by using the The effect of pH was investigated in the pH range from 1 BET equation. The SSA of the fly ashes and synthesized to 10 at 25 C; 0.5 g of the synthesized zeolite was put into zeolites was determined using the nitrogen adsorption 150-mL conical flasks containing 100 mL of a 10 mg/L
Figure 1 | XRD patterns of fly ashes (a) LC-F and (b) HC-F and zeolites (c) LC-Z and (d) HC-Z.
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4 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
Figure 2 | SEM images of (a) low-calcium fly ash, (b) high-calcium fly ash, (c) low-calcium zeolite and (d) high-calcium zeolite.
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Se(IV) solution. Each solution’s pH was adjusted via is the testing solution volume (L), and W is the mass of
the addition of the appropriate volume of 0.1 M HNO3 the adsorbent (g). or NaOH, and the pH was measured using a pH meter (PHS-3C). Adsorbent regeneration The effect of the synthesized zeolite dosage on the Se(IV) removal was studied in the range from 1 to 20 g/L To investigate the practical reusability of the synthesized at 25 C. The initial concentration of Se(IV) was 10 mg/L. zeolites as a potential adsorbent, a regeneration experiment The initial pH of the solutions was adjusted to 2.0 ± 0.05. was carried out using 0.1 M NaOH (Bleiman & Mishael The effect of the initial concentration of Se(IV) was ). In this experiment 50 mL of the NaOH solution was studied in the range of 5–30 mg/L at 25 C. The adsorbent blended with 5 g of Se(IV)-saturated zeolite, and the mixture dosage was 5.0 g/L. The pH of the solutions was adjusted was stirred at 150 rpm and 25 C for 90 min in a tempera- to 2.0 ± 0.05. ture-controlled shaker (HZQ-F160). The aforementioned To investigate the effects of coexisting anions on Se(IV) regeneration process was repeated three times. The solid removal by synthesized zeolites, two types of oxyanion precipitant was then washed with distilled water about 2 3 (SO4 and PO4 ) of similar molecular structures were 10 times until neutral. The obtained solid samples were tested individually. Stock solutions of two oxyanions were dried at 100 C for 12 h and sieved through a 100 mesh
prepared by dissolving anhydrous Na2SO4 and Na2PO4 for further study. After regeneration, the adsorbent was (analytical grade) into distilled water. Again, 0.5 g synthesized reused for Se(IV) removal at doses ranging from 1 to 20 g/L zeolite was added into each Se(IV) solution (10 mg/L) at via batch experiments with the same reaction conditions as 25 C, with initial pH set to 2.0 ± 0.05. The two oxyanions those above. The results were compared with those from were fixed at three concentrations (0.1, 1, and 10 mM). the original zeolites. The adsorption isotherm experiments were conducted at different anion concentrations ranging from 5 to 30 mg/L. A total of 0.5 g of adsorbent was added to the different Se(IV) RESULTS AND DISCUSSION solutions at 25 C. The initial pH of the solutions was adjusted to 2.0 ± 0.05. Characterization of fly ashes and synthesized zeolites All of the aforementioned batch adsorption experiments were performed in duplicate. After the experiment, the sus- The composition (XRF), XRD patterns and SEM images pensions of the batch experiments were centrifuged and of fly ashes and synthesized zeolites are shown in Table 1, then filtered with a 0.45-μm cellulose acetate membrane. Figures 1 and 2, respectively. The detailed description of The concentrations of Se(IV) in equilibrium supernatants the above indicators was consistent with the research done were analyzed by inductively coupled plasma optical emis- by Ji et al. () and is not discussed in detail here. sion spectroscopy (ICP-OES, Optima 2000, PerkinElmer Co., USA). The amount of Se(IV) adsorption by the syn- Effect of contact time thesized zeolites (q(mg/g)) and the removal efficiency was calculated using Equations (1) and (2), respectively: Figure 3 shows the effect of contact time on the Se(IV) removal efficiencies by HC-Z and LC-Z. The results indicate C C Adsorption capacity ¼ 0 e × V (1) that adsorption of Se(IV) by the two synthesized zeolites W was a very fast process. HC-Z had better removal efficiency C C than LC-Z at the end. At 10 min, the removal efficiency of Removal efficiency(%) ¼ 0 e × 100(%) (2) C0 LC-Z was 80.67% while that of HC-Z was 41.62%. After 60 minutes, the removal efficiency of Se(IV) by synthetic
where C0 and Ce are the initial and equilibrium Se(IV) zeolites exceeded 80%. Both synthetic zeolites displayed concentrations of the test solution (mg/L), respectively. V a rapid initial remove rate (<60 min), and afterward the
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Table 1 | Chemical compositions of fly ashes and synthesized zeolites (wt.%)
Content (%)
Component LC-F LC-Z HC-F HC-Z
SiO2 53.30 43.55 37.95 33.65
Al2O3 16.98 16.27 14.52 12.62
Fe2O3 8.57 10.07 16.73 11.70 CaO 3.14 3.69 17.86 15.00
TiO2 0.60 2.96 1.09 0.79
K2O 0.97 0.71 1.44 0.46 MgO 0.91 0.99 1.09 0.70 MnO 0.10 0.12 0.44 0.31
Na2O 0.08 21.10 1.24 23.16
Figure 4 | Effect of pH on Se(IV) removal by synthesized zeolites and Zeta potential of
synthesized zeolites (C0 ¼ 10 mg/L; adsorbent dosage ¼ 5 g/L; contact time ¼ removal efficiency of LC-Z slightly increased but basically 360 minutes; temperature ¼ 25 C). remained steady, whereas the removal efficiency of HC-Z slowly rose. Such a quick initial removal rate could be due removal efficiency. The highest removal efficiency of HC-Z to a considerable amount of surface sites and a large area and LC-Z was 97.4% and 88.49%, respectively. of the zeolites that retains the ions compared with the Se(IV) concentration (Awual et al. ; Hamed et al. Effect of initial pH ). According to the SEM and SSA results of synthetic zeolites in Ji et al. () and this study, LF-Z was smoother Figure 4 shows the effect of initial pH on the adsorption of than HC-Z, whereas HC-Z had a larger SSA. These two Se(IV) at a pH of 1 to 10 at 25 C. pH played a significant pieces of evidence explain why the removal efficiency and role in Se(IV) removal. When the initial pH of the solution removal rate of LC-Z was higher than HC-Z in the first increased from 1.0 to 2.0, the removal efficiency by HC-Z 60 min but lower after 240 min. Adequate contact time and LC-Z increased dramatically and reached the maximum allows the zeolite with a larger SSA to have a higher value at a pH of 2. The biggest removal efficiency of the synthetic zeolites was 96.54% and 87.53%, respectively. However, as pH increased above 3.0, the removal efficiency by HC-Z and LC-Z decreased, and gradually leveled off above a pH of 10.0. These behaviors could be explained by the change in the density of hydrogen and NO3 . The dominant ionic species of Se(IV) and the surface charge of the synthetic zeolite was a function of solvent pH. The initial solution pH was
adjusted by HNO3 or NaOH. As shown in Figure 4, the
point of zero charge (pHPZC) of the HC-Z and LC-Z was at a pH ∼ 2.4 and 2.1, respectively, as characterized by zeta potential measurements. At this pH, synthetic zeolites are electrically neutral. Below those pH values, the adsor- bent surface sites carried a positive charge, while above those pH values negatively charged sites dominated Figure 3 | Effect of contact time on Se(IV) removal by synthesized zeolites (C0 ¼ 10 mg/L; adsorbent dosage ¼ 5 g/L; pH ¼ 2.0 ± 0.05; temperature ¼ 25 C). (Szlachta et al. ). In an aqueous solution, Se(IV) species
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generally contained selenious acid (H2SeO3), biselenite 2 (HSeO3 ), and selenite (SeO3 ) according to the relationship between Se(IV) dissociation and pH (Sun et al. ). The
H2SeO3 is the dominant species below a pH of 3.5, the HSeO3 is the dominant species at a pH of 3.5 to 9, and 2 the SeO3 species is dominant above a pH of 9 (Awual et al. ; Dorraji et al. ). Therefore, the fact that the adsorption of Se(IV) reached the maximum point at a pH of 2 can be attributed to the decrease of NO3 ions in solution, leading to a decrease of the competition of the H2SeO3 and NO3 ions for adsorption sites onto zeolite particles. At a studied pH from 3.5 to 10, biselenite and selenite were the dominant ionization state Figure 5 | Effect of adsorbent dosage on Se(IV) removal by synthesized zeolites (C0 ¼ of Se(IV) which had a similar negative charge, so the surface 10 mg/L; pH ¼ 2.0 ± 0.05; contact time ¼ 360 minutes; temperature ¼ 25 C). fi charge of adsorbent is the determinative factor in the Note: The Y-axis on the left corresponds to the curve of removal ef ciency; the Y-axis on the right corresponds to the curve of adsorption capacity (q). removal efficiency. So we choose 2.0 as the optimized pH value. Similar trends in selenium anion adsorption were reported in the other literature (Hu et al. ; Sun et al. by the LC-Z increased from 80.40% to 88.74% with an ). For some biological materials, such as sulfuric acid- increase in the adsorbent dosage from 1.0 to 5.0 g/L; treatment peanut shell and modified rice husk, optimum however, the adsorption capacity of the LC-Z decreased adsorption was obtained at pH 1.5 (El-Shafey ; Soda from 8.04 to 1.77 mg/g. Removal efficiency peaked at an et al. ). LC-Z dosage of 5.0 g/L. With higher dosage, more active At initial pH range from 1 to 2, the removal efficiency of sites were available for rapid Se(IV) adsorption, resulting LC-Z was smaller than HC-Z, but from pH ¼ 3 and above, in greater removal efficiency. The decrease in adsorption the removal efficiency of LC-Z was higher than HC-Z. capacity in conjunction with increased zeolite dosage may This can be attributed to the impact of SSA. The SSA of result from a decrease in the Se(IV) concentration gradient HC-Z was larger than LC-Z. When the pH ranges from 1 per mass unit of adsorbent (Zhang et al. a). to 2, HC-Z could provide more contact sites than LC-Z, The removal efficiency of Se(IV) decreased when the but when the pH was equal to or higher than 3, contact adsorbent dosage started from 8 g/L of LC-Z and 10 g/L sites carried negative charges. The larger the SSA of zeolite, of HC-Z. At an adsorbent dosage of 20 g/L, the removal the more negative charges it carries, leading to stronger efficiency of Se(IV) decreased to 72.50% and 31.30% for competition with anions and lower removal efficiency. the HC-Z and LC-Z, respectively. The adsorption capacity also decreased to 0.36 and 0.16 mg/g, respectively. Accord- Effect of adsorbent dosage ing to Figure 6, these changes related to the change of the solution pH. The pH of the solution rose rapidly and The effect of synthesized zeolite quantity on the Se(IV) exceeded 3 as the reaction progressed and reached a plateau removal efficiency and adsorption capacity (q) was investi- at 180 min. The higher the adsorbent dosage, the higher gated to determine the optimal adsorbent dosage, as the solution pH. As discussed in the previous section, pH shown in Figure 5. With the increase of the dosage of the is an important parameter in the removal of Se(IV) by HC-Z from 1.0 to 8.0 g/L, the removal efficiency of Se(IV) synthesized zeolites, contributing to reduced adsorption increased from 80.83% to 99.9% whereas the adsorption efficiency. When the pH of the solution was higher than 3, capacity of the HC-Z decreased from 8.08 to 1.25 mg/g. the removal efficiency of zeolites was lower. Therefore, The maximum removal efficiency was achieved with an the adsorbent dosage of 5 g/L was the most appropriate HC-Z dosage of 8.0 g/L. The removal efficiency of Se(IV) amount.
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4.16 mg/g for the HC-Z and 3.93 mg/g for the LC-Z. The decrease in removal efficiency can be explained by limited sorption sites on the adsorbents (Perego et al. ; Ivanova et al. ). The increase in q was probably due to the greater driving force because of the increasing concentration gradi- ent with greater initial Se(IV) concentrations.
Effect of coexisting anions
The effect of coexisting ions on the adsorption of Se(IV) by synthesized zeolites is shown in Figure 8. The two oxyanions had a negative impact on the adsorption of Se(IV) that was Figure 6 | Solution pH over time with different dosages of (a) HC-Z and (b) LC-Z (C ¼ 0 generally more pronounced with the HC-Z than with 10 mg/L; pH ¼ 2.0 ± 0.05; contact time ¼ 360 minutes; temperature ¼ 25 C). 2 the LC-Z. With the HC-Z, the presence of SO4 did not sig- fi 3 Effect of the initial concentration of Se(IV) ni cantly affect adsorption of Se(IV). PO4 had the most significant effect on Se(IV) removal, even at low concen- trations, and the result is similar to previous findings with The adsorption of Se(IV) by synthesized zeolites was studied AlCl (AC) and FeCl (FC) adsorbents (Hu et al. ). under different initial concentrations of Se(IV) within the 3 3 With the LC-Z, SO2 did not significantly affect Se(IV) range from 5 to 30 mg/L. As shown in Figure 7, the initial 4 removal even at concentrations of 10 mM, but PO3 did Se(IV) concentration was a significant parameter in the 4 lead to a large decrease in Se(IV) removal at this concen- adsorption of Se(IV). With an increase in initial Se(IV) tration. These results can be explained by the triple-layer concentration from 5 to 30 mg/L, the removal efficiency model, which is widely applied to solid–liquid interface decreased from 100% to 65.55% for the LC-Z and from adsorption (Stumm & Morgan ). PO3 can bind 100% to 69.36% for the HC-Z. However, the adsorption 4 strongly to surface sites of metal hydroxides, forming capacity (q) of the two adsorbents increased with an inner-sphere (o-plane) surface complexes that are barely increase in initial Se(IV) concentration, peaking at affected by ionic strength. However, anions such as sulfate
Figure 7 | Effect of initial Se(IV) concentration on Se(IV) removal by synthesized zeolites (contact time ¼ 360 minutes; adsorbent dosage ¼ 5 g/L; pH ¼ 2.0 ± 0.05; temperature ¼ 25 C). Note: The Y-axis on the left corresponds to the curve of Figure 8 | Effect of coexisting anions on Se(IV) removal by (a) HC-Z and (b) LC-Z removal efficiency; the Y-axis on the right corresponds to the curve of (C0 ¼ 10 mg/L; adsorbent dosage ¼ 5 g/L; pH ¼ 2.0 ± 0.05; temperature ¼ 25 C; adsorption capacity (q). contact time ¼ 360 minutes).
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bind relatively weakly to surface sites. Complexes often form Table 2 | Rate constants and correlation coefficients for the studied kinetic models in the outer sphere (β-plane), which is significantly affected Pseudo-first-order model Pseudo-second-order model by ionic strength. As mentioned above, the negative effect Q k R2 Q k R2 of coexisting anions on Se(IV) removal by the HC-Z was Adsorbent e (mg/g) 1 e (mg/g) 2 3 greater than that by LC-Z. Since PO4 is usually present in HC-Z 1.80 0.0376 0.8729 1.95 0.0276 0.9351 wastewater, its effect should not be ignored. LC-Z 1.72 0.2668 0.3291 1.75 0.5657 0.7876
Note: C0 ¼ 10 mg/L; adsorbent dosage ¼ 5 g/L; pH ¼ 2.0 ± 0.05; temperature ¼ 25 C.
Adsorption kinetics model (R2 ¼ 0.8729 and 0.3291, respectively). Hence, the The following pseudo-first-order (Equation (3)) and pseudo- pseudo-second-order kinetic model better explained the kin- second-order (Equation (4)) kinetic models were applied to etic process of HC-Z, suggesting an effect of chemisorption. fi simulate the experimental data (Lagergren ; Ho & Nevertheless, neither the pseudo- rst-order kinetic model McKay ): nor the pseudo-second-order kinetic model explain the adsorption kinetics of LC-Z well. The differences in the ¼ adsorption kinetics of the HC-Z and LC-Z may relate to ln(Qe Qt) lnQe k1t (3) their composition and structure. The predicted values of t 1 t ¼ þ (4) Qe using the pseudo-second-order kinetic model were Q k Q2 Q t 2 e e 1.95 mg/g by the HC-Z and 1.75 mg/g by LC-Z, closely mod- eling the experimental data (1.95 mg/g by the HC-Z and where Qe and Qt are the amounts of Se(IV) adsorbed on the 1.77 mg/g by the LC-Z). synthesized zeolites (mg/g) at equilibrium and at time t, 1 1 1 respectively. The values of k1 (min ) and k2 (g·mg ·min ) are the sorption rate constants of the pseudo-first- and Adsorption isotherm pseudo-second-order kinetic models, respectively. The figures are shown in Figure 9. The constants are summarized The mechanism of interaction between adsorbent and in Table 2. adsorbate can be described by adsorption isotherms. Of In the sorption of Se(IV) by both the HC-Z and LC-Z, the many models that describe this process, the commonly the coefficients of determination for the pseudo-second- used Langmuir and Freundlich isotherms were applied in order kinetic model (R2 ¼ 0.9351 and 0.7876, respectively) this study to fit the adsorption isotherm data (Herbert were higher than that for the pseudo-first-order kinetic ; Langmuir ).
Figure 9 | Adsorption kinetics of Se(IV) by (a) HC-Z and (b) LC-Z.
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10 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
Langmuir isotherm 0.61228 for the LC-Z, which confirms that the Langmuir iso- therm equation cannot be supposed an accurate model for The Langmuir isotherm assumes that monolayer adsorption adsorption of Se(IV) by the synthesized zeolite adsorbent. occurs on an adsorbent surface, and there is no interaction between the adsorbate molecules. The Langmuir equation Freundlich isotherm is as follows: The Freundlich isotherm is an empirical equation, as follows: C 1 C e ¼ þ e (5) Qe KLQm Qm lg Qe ¼ lgKF þ nF lgCe (6)
where KL is the Langmuir coefficient (L/mg). Qm is the where KF and nF are the Freundlich parameter and adsorp- maximum monolayer adsorption capacity, and Qe is the tion intensity, respectively. The nF should be in the range of amount of adsorbed Se(IV) on a mass unit of the adsorbent 0.1–1 for beneficial adsorption. The results are shown and at equilibrium (mg/g). C is the equilibrium Se(IV) concen- e listed in Figure 10 and Table 3. For the HC-Z, the obtained tration (mg/L). KF was 2.190 mg/g, and nF was 0.276, indicating favorable The Langmuir isotherm-fitting results are summarized adsorption. For the LC-Z, the constant KF was 1.749 mg/g, in Table 3. For the HC-Z and LC-Z, Qm was 3.912 and and the nF was 0.236, also in the range for beneficial adsorp- 2.451 mg/g, respectively, both closer to the respective tion. The coefficient of determination (R2 ¼ 0.9654 for HC-Z experimental results of 4.161 and 2.831 mg/g. The value of and R2 ¼ 0.9578 for the LC-Z) was higher than those of the K was 1.664 L/mg for the HC-Z and 3.939 L/mg for the L Langmuir isotherm model, illustrating that the Freundlich LC-Z. The obtained R2 value was 0.74214 for HC-Z and isotherm model is a better fit for the experimental data and that the adsorption of selenium by synthetic zeolite is a spontaneous process. Table 3 | Rate constants and correlation coefficients for the studied kinetic models A comparison of the maximum Se(IV) sorption capacities of different adsorbents is shown in Table 4. The Langmuir Freundlich synthesized zeolites in this study showed higher sorption Q R2 K n R2 Adsorbent m (mg/g) b (L/mg) F (mg/g) F capacity in comparison with various other adsorbents. HC-Z 3.912 1.664 0.7421 2.190 0.276 0.9654 Within this study, the maximum sorption capacity of the LC-Z 2.451 3.939 0.6122 1.749 0.236 0.9578 HC-Z was slightly higher than that of LC-Z; this was
Note: C0 ¼ 10 mg/L; adsorbent dosage ¼ 5 g/L; pH ¼ 2.0 ± 0.05; temperature ¼ 25 C. because the HC-Z has larger SSA, which can provide
Figure 10 | Isotherm nonlinear plots on Se(IV) adsorption by (a) HC-Z and (b) LC-Z.
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11 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
Table 4 | Comparison of maximum sorption capacity of Se(IV) on different adsorbents
Maximum sorption Initial Se(IV) Adsorbent pH capacity (mg/g) concentration (mg/L) Dosage of adsorbent (g/L) References
Natural goethite (α-FeOOH) 4 7.74 0.05–0.25 0.01–0.5 Jacobson & Fan () Candida utilis ATCC 9950 yeast 5 5.47 60 Slightly lower amount Kieliszek et al. () Hematite 5.5 4.19 8.647 0.8 Jang et al. () Hydroxyapatite (FHAP) 5 5.51 0.01 4 Kongsri et al. ()
Microwave-assisted Fe3O4 4 2.40 0.1 2.5 Gonzalez et al. () Nano-Jacobsite 4 6.573 0.1 2.5 Gonzalez et al. () Iron-coated GAC 5 2.50 1–2 1.5–3 Zhang et al. () High-calcium zeolite (HC-Z) 2 ± 0.5 4.16 5–30 5 This study Low-calcium zeolite (LC-Z) 2 ± 0.5 3.93 5–30 5 This study
more adsorption sites, which would allow the increased zeolite reached the minimum (25.02% for HC-Z and 1.2% equilibrium maximum Se(IV) sorption capacity. 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 Adsorbent regeneration removal efficiency of regenerated zeolite relative to the initial zeolite may be attributable to the incomplete release The adsorption ability of regenerative zeolites was evaluated of Se(IV) adsorbed to the original zeolites during regener- to investigate their reusability. A comparison of Se(IV) ation, leading to a collapse of the structure of zeolites. adsorption between regenerated zeolites at different dosages These preliminary results indicate that regenerated zeolites and the original zeolites is shown in Figure 11. Under the still had a relatively high capacity for Se(IV) adsorption. same adsorbent dosage, the removal efficiency of the initial Hence, synthesized zeolites can be recycled and utilized zeolite was higher than the regenerated zeolite. When the for Se(IV) removal from wastewater. adsorbent dosage was 5 g/L, the removal efficiency of regen- erated zeolites reached the maximum and the differences in Application prospect removal efficiency between initial zeolite and regenerated
Selenium is an essential trace micronutrient for both human beings and animals. The concentration of selenium in soil influences its concentration in food (Mechora et al. ). Se-deficient areas of soils are relatively larger than those with a potentially harmful concentration of Se as China is located in a low Se area (Li et al. ). Soil applications of commercial fertilizers enriched with Se appear to be a safe method to increase the selenium uptake in Se-deficient areas (Temmerman et al. ). Bañuelos et al. () found that the Se-hyperaccumulator plants such as Stanleya pinnata can be used as organic Se fertilizer. Shang et al. () studied the adsorption of phosphate by the quaternary ammonium-based sugarcane bagasse (QA-SB) and its sustained release of laden phosphate in aqueous solution
Figure 11 | Comparison of Se(IV) removal by original and regenerated zeolites. and soil. The research found that the laden phosphate on
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12 X. Zhang et al. | Adsorption of Se (IV) in aqueous solution by zeolites of different compositions Journal of Water Reuse and Desalination | in press | 2019
the QA-SB could be desorbed efficiently and 27.8% of laden fly ash resources. Selenium-loaded zeolite has the potential phosphate released in soils after 4 days (Shang et al. ). to be used as a Se fertilizer to release selenium in Se- Meanwhile, the use of zeolites as feed supplements for ani- deficient areas. mals 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 ACKNOWLEDGEMENTS free nutrients (Ramesh & Reddy ). Compared with the above research, we could assume that selenium-loaded This work was supported by the Fundamental Research zeolite can be used as a Se fertilizer to release selenium in Funds for the Central Universities (No. 2016JX04), Se-deficient areas. Of course, selenium-loaded zeolites National Special Water Programs (No. 2017ZX07101002- may also release other chemical elements while releasing 002), and Fundamental Research Funds for the Central selenium, which needs to be clarified in the next study. In Universities (No. 2015ZCQ-SB-01). the subsequent studies, we will study the slow release of selenium by the selenium-loaded zeolite and the available REFERENCES potential and risk.
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First received 2 June 2019; accepted in revised form 12 July 2019. Available online 9 September 2019
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