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ISSN : 1225-7672(Print)
ISSN : 2287-822X(Online)
Journal of the Korean Society of Water and Wastewater Vol.27 No.5 pp.631-639
DOI : https://doi.org/10.11001/jksww.2013.27.5.631

Fe-Mn-Si 산화물을 이용한 인제거 흡착연구

맹민수, 이해균, 독고석*

Phosphate removal using novel combined Fe-Mn-Si oxide adsorbent

Seok Dockko*, Minsoo Maeng, Haegyun Lee
Dept. of Civil and Environ. Eng., Dankook Univ.
Received 26 September 2013, revised 14 October 2013, accepted 15 October 2013.

Abstract

The removal of phosphate from surface water is becoming increasingly vital to prevent problems such as eutrophication, particularly near urban areas. Recent requirements to reduce high concentrations of phosphate rely on physicochemical methods and adsorbents that must be effective even under strict conditions. The phosphate removal efficiencies of two adsorbents, Fe-Mn-Si oxide and Fe-Mn oxide, were investigated and the data used to compare kinetics and isotherm models. The maximum adsorption capacities of the two adsorbents were 47.8 and 35.5 mg-PO43-/g, respectively. Adsorptions in both cases were highly pH dependent; i.e., when the pH increased from 3 to 9, the average adsorption capacities of the two adsorbents decreased approximately 32.7 % and 20.3 %, respectively. The Freundlich isotherm model fitted the adsorption of Fe-Mn-Si oxide more closely than did the Langmuir model. Additionally, anionic solutions decreased adsorption because of competition with the anions in the adsorbing phosphate. Although affected by the presence of competing anions or a humic substance, Fe-Mn-Si oxide has better adsorption capacity than Fe-Mn oxide.

0006-01-0027-0005-11.pdf1.13MB

1. Introduction

 Waste containing phosphate is produced in large quantities in the industrial production of materials such as fertilizers, detergents, food and drinks and the smelting and refining of various metals. These phosphate byproducts are discharged into municipal and industrial water effluent streams, affecting surface-water quality via the well-known process of eutrophication (Galarneau and Gehr, 1997; Karageorgiou et al., 2007). To handle excess phosphate discharge, the removal of phosphate employing various technologies has been investigated using physical (Clark et al., 1997; Omoike and Vanloon, 1999), chemical (Ruixia et al., 2002) and biological (Stensel, 1991) methods. However, chemical precipitation and biological removal have distinct disadvantages. Adsorption methods, on the other hand, have long been preferred owing to their low cost, effective treatment of dilute solutions of phosphates and excellent adsorption capacity (Zhang et al., 2009). Beginning in 2012, the maximum discharge limit of total phosphorous in Korea was tightened from 2.0 to 0.2 mg/L. Inefficient conventional treatments cannot achieve this new requirement. However, the adoption of physicochemical processes using adsorbents will be advocated by environmental engineers and water facility operators because of their low cost and high efficiency. Different types of low-cost, high-capacity materials have long been used to remove phosphate, such as granulated coal ash (Asaoka and Yamamoto, 2010), red mud (Liu et al., 2007; Zhao et al., 2009; Yue et al., 2010) and lanthanum-doped vesuvianite (Li et al., 2009). Researchers have investigated ferric manganese oxide as an adsorbent for phosphate and arsenic removal (Zhang et al., 2007, 2009). Current interest in phosphate adsorbents stems from phosphorus and arsenic both belonging to periodic group V and thus possessing similar chemical properties (Gao and Mucci, 2003). As a result, a recent publication studying silica-containing iron oxide as an adsorbent for arsenic removal suggests that the addition of silica to iron (Fe) oxide could enhance its phosphate adsorption capacity (Zeng, 2003).

 This research investigates the phosphate adsorption capacities of Fe-Mn-Si and Fe-Mn oxides and compares their performance via data analysis. The purpose of this study is to report the improvement in adsorption performance when inorganic silicon is added to ferric manganese oxide adsorbent.

2. Methods

2.1 Materials

 Adsorbents were synthesized using a precipitation method and the chemicals potassium permanganate (KMnO4), ferrous sulfate heptahydrate (FeSO4·7H2O) and ferric chloride (FeCl3) with and without added silicon dioxide anhydrous (SiO2). Phosphate solutions were prepared by dissolving potassium dihydrogen orthophosphate (KH2PO4), manufactured by J. T. Baker (USA); the pH was adjusted using solutions of NaOH and/or HNO3 (Duksan Chemical).

2.2 Adsorbent preparation

 In the laboratory, Si was added to ferric manganese oxide employing a coprecipitation process. Ferric chloride (FeCl3, 7.31 g) was dissolved in 200 mL of distilled water. Ferrous sulfate heptahydrate (FeSO4·7H2O, 12.51 g) and anhydrous silicon dioxide (SiO2, 18 g) were then added to this solution. Potassium permanganate (KMnO4, 2.37 g) solution was prepared by dissolving KMnO4 in 200 mL of distilled water. The iron and silicon were added to the potassium permanganate solution during vigorous stirring and the pH was adjusted to approximately 4-5 with the addition of 1 M NaOH solution. The solution was then stirred continuously for 1 hour. The precipitate formed at room temperature over the next 4 hours; finally, the precipitate was separated from the liquid portion. The sedimentation-containing liquor was then dried at 90 ℃ for 12 hours. The dried adsorbent, having a dark-brown color, was powdered and stored in a desiccator for later use. Fe-Mn-Si oxide was thus obtained. When anhydrous silicon dioxide was not added in the process, Fe-Mn binary oxide was produced.

2.3 Solution preparation

 Synthetic water was prepared by adding Na2HPO4·7H2O to distilled water. To investigate the effect of anions such as Cl- and SO42- on the process of adsorption, 1 mM sodium chloride (NaCl) solution and 10 mM sodium sulfate (Na2SO4) solution were prepared.

2.4 Apparatus

 Ion chromatography (Metrohm, 881 Compact IC) was employed to determine the phosphate concentration throughout the experiment. A zeta potential analyzer (Zeta-Meter, USA) was used to measure the electrokinetic characteristics of the adsorbents. The surface morphology and adsorption capacity were measured employing scanning electron microscopy (SEM) and a batch experiment.

2.5 Experimental procedure

2.5.1 Zeta potential and SEM analysis

 The zeta potentials of Fe-Mn-Si and Fe-Mn adsorbents were analyzed with a zeta potential analyzer under different pH conditions. The background electrolyte of the solutions was 100 mg/L NaCl to maintain constant ionic strength. SEM was employed to analyze the Fe-Mn-Si oxide before and after reaction with phosphate solution having a concentration of 35 mg PO43-/L for a contact time of 10 min.

2.5.2 Adsorption kinetics and isotherms

 Adsorption kinetics experiments were carried out using phosphate solutions with concentrations of 35 mg PO43-/L at room temperature (25 ℃±1 ℃). Quantities of 0.2 g of each adsorbent were added to a 1 L beaker and a 10 mL sample was taken at 5, 10, 20 and 30 min and 1, 2, 4, 8 and 24 hrs. All solutions were mixed with a magnetic stirrer at an agitation speed of 140 rpm throughout the reaction. The pH was adjusted to 4.5±1. The samples were then passed through a 0.45 μm filter before analyzing the phosphate residual. In the isotherm experiments, 5, 10, 20, 30, 40, 50, 60, 70, 80 and 90 mg quantities of each adsorbent were placed with phosphate solutions of 35 mg-PO43-/L in 50 mL vials. The vials were shaken for 24 hrs at a speed of 140 rpm.

2.5.3 Effect of pH, coexisting anions and desorption

The effect of coexisting anions, namely Cl- and SO42-, was investigated at different pH values by adding 1 mM NaCl and 10 mM Na2SO4, respectively. The contact time was 5 min for each 2 g/L dose of adsorbent. In the study of desorption, the absorbents were saturated with 35 mg-PO43-/L solution for 24 hrs. The amount of adsorbed phosphate was calculated by measuring the supernatant of the solution. The saturated adsorbents were filled with 0.001, 0.01 and 0.1 M NaOH, 10 mM NaCl, 10 mM Na2SO4 and 4 mg/L humic acid solutions. The solutions were stirred for 30 min at a speed of 140 rpm. Finally, the desorbed amount of phosphate was calculated by subtracting the residual phosphate from the initially adsorbed amount of phosphate.

2.6 Analytical methods

2.6.1 Langmuir and Freundlich isotherm models

 By applying the experimental data to Langmuir and Freundlich models, it is possible to determine the adsorption capacity of Fe-Mn-Si and Fe-Mn oxides in different concentrations of phosphate solution (Gao and Mucci, 2003; Zeng, 2003 ; Elzinga and Sparks, 2007). Model equations are given below.

 

 Here, qe is the amount of phosphate adsorbed per gram of adsorbent (mg/g); Ce is the total phosphate concentration of the solution (mg-PO43-/L); the Freundlich constant KF reflects the adsorption capacity of the adsorbent; and the experimental constant n represents the adsorption capacity of the adsorbent. n is a heterogeneity factor that has a lower value for a more heterogeneous surface.

 

 Here, the factors qe and Ce have been previously noted; KL is the equilibrium adsorption constant describing the affinity of binding sites (L/mg); and qm is the maximum amount of the phosphate per unit weight of adsorbent.

2.6.2 Pseudo second-order model

 The adsorption rate can be calculated from the duration of the adsorption process and analyzed using the pseudo second-order models of adsorption kinetics (Ho, 2006; Lindegren and Persson, 2010).

 

 Here, qt is the amount of adsorbate sorbed on the sorbent surface at time t (mg/g); k1 is the first-order rate constant of sorption (g/mg/min); k2 is the second-order rate constant of sorption (g/mg/min); qe is the amount of adsorbate removed from aqueous solution at equilibrium (mg/g); and t is the reaction time (min).

3. Results and discussion

3.1 Zeta potential

 The zeta potentials of Fe-Mn-Si and Fe-Mn oxides under different pH conditions decreased with increasing pH. As shown in Fig. 1, the zero point of charges (pHzpc) of Fe-Mn-Si and Fe-Mn were about 3.3±0.2 and 6.0±0.2, respectively. The negative charge of Fe-Mn-Si adsorbent is due to the Si-OH structure (Manning and Goldberg, 1997). Specific adsorption of phosphate as an anion makes the surface of oxides more negatively charged, which shifts the isoelectric point of the adsorbent to a lower pH value (Hsia et al., 1994).

Fig. 1. Zeta potentials of Fe-Mn-Si and Fe-Mn oxides.

3.2 Adsorption kinetics

 The kinetics of adsorption using equations (1), (2), (3) were determined from the contact time of the adsorption process for different initial phosphate concentrations. Results of the pseudo second-order model fit are shown in Fig. 2 together with the experiment kinetics results.

Fig. 2. Pseudo second-order model fits of experiment kinetics data for Fe-Mn-Si and Fe-Mn oxide adsorbents.

 The initial phosphate concentrations were 35 mg/L and the initial pH value of the solution was 4.5. In the first 2 hrs, 70 % of the equilibrium adsorption capacity was achieved in the case of Fe-Mn-Si oxide. The adsorption capacity of Fe-Mn-Si oxide was higher than that of Fe-Mn oxide. To calculate the rate of adsorption analytically, pseudo second-order models were employed using experimental data of phosphate adsorption (Ho, 2006). The pseudo second-order model fit the experimental data well with a high correlation coefficient of 0.935-0.998. All parameter values used in developing the pseudo second-order models are listed in Table 1.

Table 1. Parameters of pseudo second-order, Freundlich and Langmuir models for the adsorption of phosphate

3.3 Adsorption isotherm

 The adsorption isotherms considering the effects of the equilibrium concentration on the adsorption capacity are shown in Fig. 3. Data were analyzed using Freundlich and Langmuir isotherm models. The results and parameters of these models are given in Table 1. The Freundlich model fits the experimental data of the Fe-Mn-Si and Fe-Mn oxides well with relatively high correlation coefficients (R2) of 0.949 and 0.977, respectively. The maximum capacity of this adsorbent was 47.8 mg-PO43-/L, which was derived from the Langmuir model. The Freundlich model can be well applied to solids with heterogeneous surface properties. On the other hand, the fit of Fe-Mn adsorption had a poor correlation value (R2 = 0.825). Previous research has shown that the maximum adsorption capacity of Fe-Mn oxide is 33.2 mg-PO43-/L at a pH of 5.6±1 (Zhang et al., 2009). Consequently, Fe-Mn-Si has greater adsorption capacity than Fe-Mn oxide.

Fig. 3. Isotherms of phosphate adsorption by Fe-Mn-Si and Fe-Mn oxides.

3.4 Effects of pH and coexisting anions on adsorption

 The effects of pH and coexisting anions on the two adsorbents are shown in Fig. 4. It is clear that when the solution pH increases, the phosphate adsorption by both adsorbents decreases. Similarly, both Fe-Mn-Si and Fe-Mn oxides have greater adsorption capacity at low pH than at high pH. The explanation is that at high pH, the adsorbent surface becomes more negatively charged, increasing the repulsion between the phosphate cation and the adsorbent surface (Lindegren and Persson, 2010). Moreover, the ionic solution concentration also affects adsorption.

Fig. 4. Effects of pH and coexisting anions on the adsorption of Fe-Mn-Si and Fe-Mn oxides.

 Adsorption decreased slightly at a high concentration of coexisting anions, but it increased at low concentrations. The coexisting anions exerted less influence on the adsorption on Fe-Mn-Si oxide, allowing greater adsorption than was observed for Fe-Mn oxide. It is clear that a change in ionic strength from 1 to 10 mM played a role in decreasing the adsorption of phosphate. However, the valence of anions had little effect on the adsorption. This agrees with the finding that the phosphate ions strongly adsorbed at specific sites are rarely exchangeable even in a solution with a large excess of coexisting ions (Zhang et al., 2009). In inner-sphere complexes of adsorbent, the surface hydroxyl groups act as σ-donor ligands, which increase the electron density of the coordinated metal ion (Stumm, 1992). The adsorption of these anions was suppressed with weakly adsorbing anions such as Cl- and SO42-, as these electrolytes also formed outer-sphere complexes through electrostatic forces. These anions that were adsorbed by the inner-sphere association either showed little sensitivity to ionic strength or responded to higher ionic strength with greater adsorption. The binding of a metal ion by surface ligands is strongly pH-dependent as shown in Fig. 4.

3.5 Desorption

 Desorption was carried out using three different concentrations of alkaline solutions, two different anions and an organic substance. The percentage of desorbed phosphate is shown in Fig. 5. With the addition of NaOH, alkaline solution concentrations of 0.001, 0.01 and 0.1 M were created. The Fe-Mn-Si oxide and Fe-Mn oxide were then added to these three alkaline solutions. It is clear that the amount of phosphate desorption increased with an increase in alkalinity. The amount of phosphate desorbed by the Fe-Mn-Si oxide and Fe-Mn oxide was found to increase from 42 % to 59 % and from 2 % to 86 % as the solution concentrations increased from 0.001 to 0.1 M, respectively. The effect of the alkaline solutions on the desorption process was relatively weak for Fe-Mn-Si oxide but strong for Fe-Mn oxide. Furthermore, when adding the adsorbents to phosphates in the presence of the mono/divalent anions and organic substance, the desorption of phosphate approached that achieved in alkaline solutions. These results show that the phosphate-loaded Fe-Mn oxide can be more easily desorbed than Fe-Mn-Si oxide in NaOH solution. This means that Fe-Mn oxide has the potential to be used as a regenerated adsorbent for phosphate removal.

Fig. 5. Desorption performance after adsorption of phosphate by Fe-Mn-Si and Fe-Mn oxide adsorbents depending on the concentration of alkaline solution, anions and addition of an organic substance.

 The morphology of Fe-Mn-Si oxide obtained by SEM is shown in Fig. 6(a, b). The figure illustrates the difference in morphology between these two adsorbents before and after phosphate adsorption. The image of the Fe-Mn-Si adsorbent shows that the adsorbent consisted of many small particles having porous structures and rough surfaces. After phosphate adsorption, the morphology was not significantly different.

Fig. 6. SEM morphology of Fe-Mn-Si oxide (a) before and (b) after phosphate adsorption.

4. Conclusion

 In a comparative study of two new adsorbents, the addition of inorganic silicon to Fe-Mn oxide was shown to improve adsorption performance. The adsorption of both adsorbents was affected directly by pH: the adsorption capacity increased at low pH but decreased at high pH. Investigation revealed that a change in ionic strength from 1 to 10 mM decreased the adsorption of phosphate. However, the valence of anions had little effect on the adsorption. The isotherm of the Freundlich model predicted the adsorption of Fe-Mn-Si better than the Langmuir model. A pseudo second-order model is suitable for analyzing the adsorption kinetics. Anionic solutions and a humic substance decreased adsorption through competition between these anions and the phosphate anions in the adsorption of phosphate. Although affected by the presence of competing anions or a humic substance, Fe-Mn-Si oxide has better adsorption capacity than Fe-Mn oxide.

Acknowledgements

 This research was supported through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (013-2011-1-D00071) and partly funded by a grant from the Eco-Innovation (Gbest) Project.

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