Simultaneous removal of ammonium and phosphate by zeolite synthesized from coal fly ash as influenced by acid treatment

Journal of Environmental Sciences 19(2007) 540–545

Simultaneous removal of ammonium and phosphate by zeolite synthesized from coal fly ash as influenced by acid treatment ZHANG Bao-hua, WU De-yi ∗, WANG Chong, HE Sheng-bing, ZHANG Zhen-jia, KONG Hai-nan School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail: [email protected] Received 22 May 2006; revised 4 August 2006; accepted 21 August 2006

Abstract Zeolite synthesized from fly ash (ZFA) without modification is not efficient for the purification of NH4 + and phosphate at low concentrations that occur in real effluents, despite the high potential removal capacity. To develop an effective technique to enhance the removal efficiency of ammonium and phosphate at low concentrations, ZFA was modified with acid treatment and the simultaneous removal of ammonium and phosphate in a wide range of concentration was investigated. It was seen that when compared with untreated ZFA, only the treatment by 0.01 mol/L of H2 SO4 significantly improved the removal efficiency of ammonium at low initial concentrations. The behavior was well explained by the pH effect. Treatment by more concentrated H2 SO4 led to the deterioration of the ZFA structure and a decrease in the cation exchange capacity. Treatment by 0.01 mol/L H2 SO4 improved the removal efficiency of phosphate by ZFA at all initial P concentrations, while the treatment by concentrated H2 SO4 (> 0.9 mol/L) resulted in a limited maximum phosphate immobilization capacity (PIC). It was concluded that through a previous mild acid treatment (e.g. 0.01 mol/L of H2 SO4 ), ZFA can be used in the simultaneous removal of NH4 + and P at low concentrations in simulating real effluent. Key words: zeolite; fly ash; acid treatment; ammonium; phosphate; removal

Introduction The accumulation of nitrogen and phosphorus in relatively stagnant water is usually the leading cause of eutrophication. Therefore, removal of nitrogen and phosphorus from domestic and agro-industrial wastewater prior to discharge, as well as from eutrophicated natural water is obligatory. The application of efficient solid materials, involving natural materials, synthesized materials, and solid wastes, in ammonium and phosphate removal from aqueous solutions has been widely investigated in recent years. Natural clays, especially zeolites, were proved to be efficient for ammonium removal (Booker et al., 1996; Komarowski and Yu, 1997; Ro´zi´c et al., 2000; Sarioglu, 2005), while a variety of materials such as blast furnace slag, activated alumina, fly ash, aluminum oxide hydroxide, synthetic iron oxide-gypsum compound, and other materials were investigated for their ability to treat phosphate-laden wastes (Neufeld and Thodos, 1969; Baker et al., 1998; Sakadevan and Bavor, 1998; Bastin et al., 1999; Drizo et al., 1999; Cheung and Venkitachalam, 2000; Johansson and Gustafsson, 2000; Tanada et al., 2003). Although both ammonium and phosphate have to be scavenged from wastewater, the simultaneous removal by Project supported by the Chinese Ministry of Science and Technology Funding (No. 2002AA601013). *Corresponding author. E-mail: [email protected]

one material has been rarely reported hitherto. The merit of using only one material to simultaneously eliminate ammonium and phosphate from wastewater is obvious. Generally, two kinds of reagents must be applied for the removal of NH4 + and PO4 3− . For example, the combination of a cation exchanger and an anion exchanger, or a cation exchanger and an inorganic cohesion precipitant are considered. In any case, the cost is high, and the counter anion such as Cl− and SO4 2− remains in the treated water. In the previous studies (Wu et al., 2005, 2006a, 2006b; Chen et al., 2006, 2007; Zhao et al., 2006), it was reported that both the CEC (cation exchange capacity) value and the phosphate immobilization capacity (PIC) value of zeolite synthesized from fly ash (ZFA) were improved greatly when compared with the corresponding raw fly ash. It was noted that ZFA may be a promising material for the simultaneous removal of ammonium and phosphate from wastewater. Unfortunately, the preliminary study found that the removal efficiency of ZFA for both ammonium and phosphate at low concentrations simulating real effluents was quite limited, despite their high maximum potential of removability. It is deemed, however, that an excellent material must have both a high maximum removal capacity and a high removal efficiency at low nutrients concentration, which occurs in most real wastes. In an attempt to improve the treatability especially at low nutrient concentrations, the simultaneous removal of

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to be associated with the phosphate removal, are listed in Table 1. The zeolitic phase(s) in the products were identified by the powder X-ray diffraction method on a D8 ADVANCE X-ray diffractometer using Ni filtered Cu-kα radiation (40 kV, 40 mA).

ammonium and phosphate by ZFA that received acid pretreatments was investigated. A wide range of ammonium and phosphate concentration, with the nutrients co-existing in aqueous solution, was employed so as to evaluate the effectiveness of the acid pretreatment on the potential removal capacity and the treatability at low nutrients concentration. It was concluded that through a previous mild acid treatment, ZFA can be used in the simultaneous removal of NH4 + and P at low concentrations simulating real effluent.

1.2 Batch immobilization studies An original stock solution (A) containing 5000 mgN/L and 1000 mgP/L, and a stock solution (B) containing 10000 mgN/L and 5000 mgP/L were prepared from anhydrous (NH4 )2 HPO4 and anhydrous NH4 Cl. Batch equilibration experiments were conducted using aqueous solutions containing both ammonium and phosphate with the combinations of initial ammonium and phosphate concentration as shown in Table 2. The initial ammonium and phosphate concentration ranged from 2.5 to 1200 mgN/L and from 0.5 to 1000 mgP/L, respectively. Since the concentrations of ammonium and phosphate present in domestic wastewater in China typically have the N/P ratio of 5, the combinations with P concentration 6 12 mg/L were prepared from the original stock solution (A) to simulate real sewage, while the combinations with P concentration > 25 mg/L were prepared from the original stock solution (B). For the batch equilibration experiments, 40 ml of the aqueous solutions were added to centrifuge tubes containing 0.4 g of the sample. The tubes were then shaken for 24 h at room temperature (24 h was found to be sufficient for ammonium and phosphate to achieve equilibrium in the pre-experiments). After 24 h, the suspensions were centrifuged and the supernatants were determined for phosphate by the molybdenum-blue ascorbic acid method (APHA, 1995) and for ammonium by the Nessler method (APHA, 1995) using a Unico spectrophotometer (model

1 Materials and methods 1.1 Materials A fly ash sample was supplied by the second electric power station of Wujing, Shanghai, China. Fifteen grams of fly ash were placed in a flask, mixed with 150 ml of 2.0 mol/L NaOH solution, and boiled with reflux for 48 h. At the end of the synthesis process, the solid phase was separated by centrifugation, washed with re-distilled water three times and with ethanol twice, and dried in an oven at 45°C. For acid treatments, ZFA was mixed with 0.01, 0.1, 0.9, and 1.8 mol/L H2 SO4 using a liquid/solid ratio of 6, and boiled with reflux for 6 h. The suspensions were separated by centrifugation, washed with re-distilled water and ethanol, and dried in a similar manner. The obtained products were finally ground to pass an 80-mesh sieve prior to use. The methods for chemical analysis, CEC (cation exchange capacity), PIC, and fractionation of immobilized phosphorus were the same as in the previous articles (Chen et al., 2006; Wu et al., 2006b). The CEC value and the total amount of Si, Al, Fe, Ca, and Mg, which were believed

Table 1 Some chemical composition of ZFA untreated and treated by H2 SO4 Sample

SiO2 (%)

Al2 O3 (%)

CaO (%)

MgO (%)

Fe2 O3 (%)

CEC (cmol/kg)

Untreated H2 SO4 (0.01 mol/L) H2 SO4 (0.1 mol/L) H2 SO4 (0.9 mol/L) H2 SO4 (1.8 mol/L

34.4 35.3 39.3 34.2 50.7

18.9 18.4 18.6 14.8 10.1

8.1 8.0 8.6 6.4 7.5

1.0 1.0 1.1 0.3 0.1

9.0 8.2 6.6 6.1 1.2

213 175 169 72 13

Table 2 pH values in equilibrium solutions containing ammonium, phosphate, and ZFA untreated and treated by H2 SO4 Initial concentration

pH in equilibrium solution

N (mg/L)

P (mg/L)

No ZFAa

Untreated

H2 SO4 (0.01 mol/L)

H2 SO4 (0.1 mol/L)

H2 SO4 (0.9 mol/L)

H2 SO4 (1.8 mol/L)

0 2.5 5.0 10.0 20.0 40.0 50.0 60.0 100.0 200.0 400.0 800.0 1200.0 –

0 0.5 1.0 2.0 4.0 8.0 12.0 25.0 50.0 100.0 200.0 400.0 600.0 1000.0b

6.02 5.25 5.28 5.31 5.25 5.74 5.52 5.15 5.62 5.43 4.72 4.58 4.46 4.98

11.45 11.24 11.44 11.41 11.40 11.09 10.92 10.47 10.09 9.62 9.20 8.40 7.63 7.28

10.45 10.56 10.06 10.08 10.11 10.05 9.84 10.02 9.76 9.33 8.90 8.26 7.47 7.13

9.19 9.17 9.15 8.92 8.82 8.69 8.55 9.86 8.41 8.21 7.72 7.15 7.20 6.79

3.99 3.98 3.95 3.94 3.90 3.87 3.83 4.08 3.93 3.89 3.92 4.01 4.10 4.36

3.05 3.07 3.22 3.07 3.06 3.06 3.07 3.27 3.20 3.19 3.23 3.28 3.35 3.60

a

pH values of solutions containing ammonium and phosphate without the addition of ZFA; b KH2 PO4 solution.

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UV-2102PCS), respectively. Ammonium and phosphate immobilized from the solution were calculated from the difference between the initial and the final concentrations. 1.3 Effect of pH on ammonium removal One gram of untreated ZFA was placed in flasks containing 100 ml re-distilled water. The suspensions were adjusted to the desired pH levels by adding 2 mol/L HCl or NaOH. Appropriate volume of re-distilled water was supplemented for some flasks so that all suspensions with different pH values contained the same volume of water. After being shaken for 24 h, the pH was monitored using a Hach 51910 pH meter. The immobilization studies were carried out using an initial ammonium concentration of about 25 mgN/L by adding 0.5 ml of the original stock solution (A).

2 Results and discussion 2.1 Characterization of ZFA with and without acid treatment The XRD patterns of ZFA with and without acid treatment are illustrated in Fig.1. Following the alkaline activation process, monomineral of NaP1 zeolite (Na6 Al6 Si10 O32 ·12H2 O, Si/Al=1.73) was produced. Small amounts of quartz, mullite, and calcite remained in ZFA. NaP1 is a low-silica zeolite, which is known to deteriorate under acid conditions. With the treatment of 0.01 mol/L H2 SO4 , the crystallinity of NaP1 dropped slightly. The production of a new crystal phase (gypsum) with d values of 7.59 and 3.07 initiated at the concentration of 0.1 mol/L, while the crystallinity of NaP1 was destroyed entirely, and gypsum and anhydrite were formed upon treatment with 0.9 mol/L H2 SO4 . Though the CEC value decreased obviously following the treatment by 0.01 and 0.1 mol/L H2 SO4 (Table 1), it was still considerably high. A drastic decrease in the CEC value occurred when the H2 SO4 concentration exceeded

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0.1 mol/L. 2.2 Removal efficiency of ammonium The removal efficiency of ammonium as a function of the initial concentration is shown in Fig.2. A logarithmic scale was used for the horizontal axis so that the results at low concentrations can be clearly presented. The results in Fig.2 indicated that ZFA treated with 0.01 mol/L H2 SO4 had the highest removal efficiency at all initial concentrations. Although ZFA without acid treatment had removal efficiency compatible with the 0.01 mol/L H2 SO4 treated one at high initial concentrations (> 200 mg/L), the removal efficiency at low initial NH4 + concentrations (6 40 mg/L) was very low (< 20%). When compared with untreated ZFA, treatments with 0.1 and 0.9 mol/L H2 SO4 slightly improved the removal efficiency at low initial concentrations, but lowered the removal efficiency at high initial concentrations. On the other hand, the removal efficiency by ZFA treated with 1.8 mol/L H2 SO4 was very low at all initial concentrations. Given that both the maximum removal capacity and the removal efficiency at low ammonium concentration that exist in real effluent are important in practical usage, the obtained results suggest that the treatment of ZFA by dilute H2 SO4 (0.01 mol/L) is the most advantageous.

Fig. 2 Ammonium removal efficiency by ZFA untreated and treated by H2 SO4 with different concentrations as a function of the initial ammonium concentration.

2.3 Effect of pH on ammonium removal

Fig. 1 XRD patterns of ZFA untreated and treated by H2 SO4 with different concentrations. (P) NaP1 zeolite; (Q) quartz; (M) mullite; (C) calcite; (A) anhydrite; (G) gypsum.

To probe into the reasons accounting for the influence of the acid treatment on ammonium sequestration, the removal of ammonium as a function of the pH value was investigated and the results are shown in Fig.3. It can be seen that favorable elimination of ammonium by ZFA produced from fly ash occurred within the pH range of 5.5–10.5. The decrease in ammonium elimination at the alkaline pH level out of the range is explained by the fact that ammonium is partially transformed into electrically neutral NH4 OH(NH3 ·H2 O), which would abate the sequestration of ammonium ion. Although it was expected that the volatilization of NH3 could contribute to the elimination of ammonium at alkaline pH values, the data obtained by repeated experiments indicated that only about 5% of the

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concentrations for untreated ZFA can be also explained by the change in the pH value (to be below 10.5). 2.4 PIC of ZFA and fractionation of immobilized phosphorus

Fig. 3 NH4 + immobilization at an initial concentration of 25 mgN/L as a function of pH in equilibrium suspension.

added ammonium was lost by volatilization for a solution (without ZFA) having an ammonium concentration of 25 mgN/L and a pH value of 11.4 in the experimental conditions. The negligible removal by volatilization process was probably attributed to the facts that the concentration of ammounium was low, and the removal experiment was done in capped centrifuge tubes. On the other hand, ammonium may compete with other cations including the hydrogen ion for the exchangeable site at pH<5.0, resulting in the reduction of ammonium removal. The pH values in equilibrium solutions are given in Table 2. The data in the third row of Table 2 indicates that the addition of untreated ZFA to distilled water results in an increase of the pH value, probably because of the hydrolysis and the dissolution of alkali and alkaline earth metal oxides in ZFA (Ro´zi´c et al., 2000). The acid treatment causes the decrease in the pH value: the higher the concentration of H2 SO4 , the lower the pH value. However, the addition of (NH4 )2 HPO4 and NH4 Cl into the aqueous suspension of ZFA leads to the decrease in the equilibrium pH values for the untreated, 0.01 and 0.1 mol/L H2 SO4 treated ZFA, while the pH values for 0.9 and 1.8 mol/L H2 SO4 -treated ZFA remain almost unchanged. It can be seen from Table 2, and Figs.2 and 3 that when compared with untreated ZFA, the treatment by 0.01 mol/L H2 SO4 lowered the pH value of the equilibrium solution at low initial NH4 + concentrations to be below 10.5, resulting in the increase of the ammonium removal efficiency. However, since the treatment by more concentrated H2 SO4 led to the deterioration of the zeolite structure, this probably decreased the ammonium removal efficiency. The increase in the ammonium removal efficiency at high initial NH4 +

The PIC values of ZFA with and without acid treatment are shown in Table 3. The treatment by dilute acid (0.01 and 0.1 mol/L H2 SO4 ) resulted in a significant increase in the PIC value, while the treatment by more concentrated H2 SO4 caused a marked decrease in the PIC value. The former behavior cannot be explained by the change in the total chemical composition (Table 1), and was presumably because of the change in the Ca ingredient from sparingly soluble CaCO3 (calcite, see Fig.1) to relatively soluble CaSO4 (gypsum and anhydrite, see Fig.1), which would favor the calcium phosphate precipitation. The sharp decrease in the removal capacity of phosphate by relatively concentrated H2 SO4 (0.9 or 1.8 mol/L) was clearly associated with the loss of Fe, Al, Ca, and Mg, which was believed to be involved in phosphate fixation (Table 1). To understand the mechanism of phosphate removal, fractionation of phosphorus immobilized from solution with an initial P concentration of 1000 mg/L was conducted and the results are shown in Table 3. The percentage of residual phosphorus was very low. For ZFA untreated and treated with dilute (0.01 or 0.1 mol/L) H2 SO4 , the main P fraction was either Ca+Mg-P (treated with 0.01 or 0.1 mol/L H2 SO4 ) or LB-P (untreated), though Fe+Al-P also accounted for a considerable part of the total immobilized P. It is speculated that LB-P is principally a part of Ca+MgP (probably di-calcium phosphate di-hydrate which can relatively easily re-dissolve into solution. Further, the low MgO content implies that immobilized phosphate was bound mainly to calcium components. It is considered that, for untreated and dilute (0.01 or 0.1 mol/L) H2 SO4 -treated ZFA, the formation of calcium phosphate precipitates may be the predominant mechanism for phosphate removal. ZFA treated with 0.9 or 1.8 mol/L H2 SO4 had an equilibrium pH of 4.36 and 3.60, respectively (Table 2). Since iron and aluminum based crystalline and amorphous phases will become positively charged and through the ligand exchange mechanism, their adsorption capabilities will increase at such acidic pH values (Parfitt, 1978; Geelhoed et al., 1997), Fe+Al-P predominated in immobilized P whereas Ca+Mg-P was negligible. It thus suggests that, for ZFA treated by 0.9 or 1.8 mol/L H2 SO4 , immobilized phosphate was bound mainly to Fe and Al related components.

Table 3 Fractionation of phosphorus immobilized by ZFA with and without acid treatment (values of P fractions in mgP/g dw) Sample Untreated H2 SO4 0.01 (mol/L) H2 SO4 0.1 (mol/L) H2 SO4 0.9 (mol/L) H2 SO4 1.8 (mol/L)

PIC 38.26 62.77 56.50 11.07 1.91

LB-P 18.80 17.62 16.07 2.14 0.56

Fe+Al-P 8.69 15.38 14.30 8.62 0.91

Ca+Mg-P 10.61 29.49 25.04 0.29 N.D.

Percentage in PIC (%)

Res.-P 0.16 0.28 1.07 N.D. 0.44

LB-P

Fe+Al-P

Ca+Mg-P

Res.-P

49.14 28.05 28.47 19.40 29.31

22.71 24.51 25.31 77.94 47.64

27.73 46.98 44.34 2.66 N.D.

0.42 0.46 1.88 N.D. 23.04

LB-P: loosely bound phosphorus; Ca+Mg-P: calcium and magnesium bound phosphorus; Fe+Al-P: iron and aluminum bound phosphorus and Res.-P: residual phosphorus.

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mild acid treatment caused no significant decrease in the potential immobilization capacity (CEC) for ammonium, but resulted in a significant increase in PIC. However, the treatment by more concentrated H2 SO4 caused a sharp decrease in the CEC and PIC values though it improved the removability of P and NH4 + at low concentrations. Therefore, mild acid treatment is the most advantageous to improve the simultaneous removal efficiency of NH4 + and P from wastewater at low nutrients concentration by ZFA.

References

Fig. 4 Phosphate removal efficiency by ZFA untreated and treated by H2 SO4 with different concentrations as a function of the initial P concentration.

2.5 Removal efficiency of phosphate The removal efficiency of phosphate as a function of the initial concentration is illustrated in Fig.4. A logarithmic scale was used for the horizontal axis so that the results at low concentrations can be clearly presented. At low initial concentrations, ZFA treated with 0.9 and 1.8 mol/L H2 SO4 showed the greatest affinity for phosphate. This was explained by the fact that the adsorption mechanism of phosphate was primarily associated with Fe and Al through ligand exchange at acidic pH levels, and it is known that the adsorption by this mechanism takes place even at dilute phosphate concentration (Parfitt, 1978; Geelhoed et al., 1997). However, the removal efficiency quickly declined as the initial P concentration increased because of the limited immobilization capacity. At all initial P concentrations, ZFA treated with 0.01 and 0.1 mol/L H2 SO4 showed notably higher removal efficiency than the untreated one. Treatment by H2 SO4 can cause the transformation of CaCO3 (Ksp =2.9×10−9 ) into more soluble calcium phase such as gypsum (Ksp =9.1×10−6 ) and anhydrite (Fig.1), thus favoring the dissolution of Ca2+ and precipitation of calcium phosphate. The removal efficiency initially increased linearly at low concentration in solution, and reached the maximum at intermediate concentration. This is because of the fact that the P concentration needs to increase above a certain level (the precipitation limit) to precipitate phosphate, depending on the concentration of calcium (Koutsoukos et al., 1980; Baker et al., 1998). However, the removal efficiency finally declined at high P concentration with the amount of immobilized P approaching the maximum immobilization capacity.

3 Conclusions ZFA was treated with H2 SO4 and the simultaneous removal of ammonium and phosphate in a wide range of concentration by modified ZFA was compared with the untreated one. The removal efficiency for both ammonium and phosphate at low concentrations were greatly enhanced by mild acid treatment (0.01 mol/L of H2 SO4 ). The

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