Ammonium nitrogen removal from coking wastewater by chemical precipitation recycle technology

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Tao Zhang a, Lili Ding a, Hongqiang Ren a,*, Xiang Xiong b a State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, Jiangsu, PR China b National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, Jiangsu, PR China a r t i c l e i n f o Article history: Received 26 April 2009 Received in revised form 20 August 2009 Accepted 20 August 2009 Available online 1 October 2009 Keywords: Ammonium nitrogen Coking wastewater Chemical precipitation recycle technology (CPRT) Magnesium ammonium phosphate hexahydrate (MAP) Physico-chemical treatment a b s t r a c t Ammonium nitrogen removal from wastewater has been of considerable concern for several decades. In the present research, we examined chemical precipitation recycle technology (CPRT) for ammonium nitrogen removal from coking wastewater. The pyrolysate resulting from magnesium ammonium phosphate (MAP) pyrogenation in sodium hydroxide (NaOH) solution was recycled for ammonium nitrogen removal from coking wastewater. The objective of this study was to investigate the conditions for MAP pyrogenation and to characterize of MAP pyrolysate for its feasibility in recycling. Furthermore, MAP pyrolysate was characterized by scanning electron microscope (FESEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) as well as X-ray diffraction (XRD). The MAP pyrolysate could be produced at the optimal condition of a hydroxyl (OH) to ammonium molar ratio of 2:1, a heating temperature of 110 C, and a heating time of 3 h. Surface characterization analysis indicated that the main component of the pyrolysate was amorphous magnesium sodium phosphate (MgNaPO4). The pyrolysate could be recycled as a magnesium and phosphate source at an optimum pH of 9.5. When the recycle times were increased, the ammonium nitrogen removal ratio gradually decreased if the pyrolysate was used without supplementation. When the recycle times were increased, the ammonium nitrogen removal efficiency was not decreased if the added pyrolysate was supplemented with MgCl2$6H2O plus Na2HPO4$12H2O during treatment. A high ammonium nitrogen removal ratio was obtained by using pre-formed MAP as seeding material. ª 2009 Elsevier Ltd. All rights reserved. 1. Introduction One of the best understood consequences of human pollution associated with environment is water eutrophication (Holloway et al., 1998; Cooperband and Good, 2002). Ammonium nitrogen is one nutrient essential that can lead to water eutrophication when present in excess (Liikanen and Martikainen, 2003). As a consequence, people have suffered a great deal from pollution by wastewater containing ammonium nitrogen. Coking wastewater, a typical industrial wastewater, usually contains high and toxic concentrations of ammonium nitrogen (Zhang et al., 1998; Vazquez et al., 2006). Biological (Lay-Son and Drakides, 2008) and physicochemical treatment methods are the conventionally used strategies for ammonium nitrogen removal. The biological process is economical for wastewater treatment, but it is often * Corresponding author. Tel.: þ86 25 83596781; fax: þ86 25 83707304. E-mail address: hqren@nju.edu.cn (H. Ren). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.08.054 w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 5 2 0 9 – 5 2 1 5 not effective for high concentration of ammonium nitrogen removal due to a shortage of carbon sources for denitrification. Various methods of physico-chemical processes can be applied to treatment high concentrations of ammonium nitrogen, such as air stripping (Marttinen et al., 2002), ion exchange (Jorgensen and Weatherley, 2003), membrane separation (Palma et al. 2002), and chemical precipitation (Hao et al., 2008; Renou et al., 2008; Zhang et al., 2009). Chemical precipitation of ammonium nitrogen removal by adding magnesium salt and phosphate to form magnesium ammonium phosphate hexahydrate (MAP) is a useful method (Stratful et al., 2001). MAP is a white crystal substance consisting of equal molar concentrations of magnesium, ammonium and phosphorus. The chemical reaction is expressed in Eq. (1) (Siegrist, 1996; Tunay et al. 1997; Doyle and Parsons, 2002): Mg2þ þ NHþ 4 þ PO3 4 þ 6H2O5MgNH4PO4$6H2OY: (1) pKs ¼ 12:6ð25 CÞ: (2) The method of chemical precipitation has been studied widely for the treatment of high strength ammonium nitrogen wastewater. Li and Zhao (2001) reported that under an equal molar ratio of Mg2þ:NH4 þ:PO4 3, ammonium nitrogen concentration could quickly be reduced from 5618 mg/L to 112 mg/L using chemical precipitation as the pretreatment. Altinbas et al. (2002) treated anaerobically pre-treated domestic wastewater and landfill leachate mixture by MAP precipitation. An ammonium removal ratio of 68% was achieved at pH of 9.2 at the stoichiometric ratio, which could be increased to 72% when more than the stoichiometric ratio was added at the same pH. Ryu et al. (2008) studied the MAP precipitation process on semiconductor wastewater in field-scale and was able to accomplish over 89% ammonium nitrogen removal. MAP precipitation has been considered an effective technology for ammonium nitrogen removal. However, the high cost of magnesium salts and phosphate consumption hamper the wide application of this type of chemical precipitation. Our research objective in this study was to investigate the chemical precipitation recycle technology (CPRT) for ammonium nitrogen removal, with the aim of reducing the running costs incurred by addition of magnesium and phosphate. The conditions of MAP pyrogenation and the characteristics of recycled MAP pyrolysate were investigated in laboratory scale experiments. MAP pyrolysate was also characterized by scanning electron microscope (FESEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). 2. Materials and methods 2.1. Raw wastewater The raw wastewater used in the experiments was collected from the outlet of the activated slugged process of a coke wastewater plant near Nanjing. Table 1 shows some parameters of the wastewater sample. 2.2. Formation of MAP Experiments for MAP formation were performed as follows. Firstly, magnesium chloride (MgCl2$6H2O) and disodium hydrogen phosphate (Na2HPO4$12H2O) were added to the coking wastewater (1000 mL) at Mg2þ:NH4 þ:PO4 3 molar ratios of 1:1:1. Secondly, the reaction solution was agitated by magnetic stirrers for 30 min at pH 9.5. Thirdly, the formed MAP was allowed to settle in the reaction solution for 30 min. Lastly, the reaction solution was filtered through a 0.45 mm membrane filter and the MAP precipitate was collected. 2.3. Experimental procedures After the formation of MAP, experiments for ammonium nitrogen removal of CPRT process were performed, as follows. (1) Releasing ammonium nitrogen from MAP precipitate: the formed MAP precipitate was washed with deionized water 3 times to wash off the co-precipitation organic compounds in the coking wastewater, then mixed with sodium hydroxide (NaOH) solution to reduce the MAP pyrogenation temperature, and heated at the given experimental temperature (80–130 C) and time (1–5 h) to release ammonium. (2) Preparing the pyrolysate: the pyrolysate of MAP precipitate was washed with deionized water 3 times, and then dried in an oven at 40 C for 48 h. (3) Recycling the pyrolysate as magnesium and phosphate sources: all of the pyrolysate was added to the coking wastewater (1000 mL) at the experimental conditions. The reaction solution was agitated by stirrers for 90 min at given experimental pH (8.5–10.5). The reaction solution was then allowed to settle for 60 min. The collected MAP precipitate was recycled (i.e., steps (1), (2), (3) were repeated). The effluent was filtered with a 0.45 mm membrane filter and then hydrochloric acid (HCl) was added to adjust the pH to 6.5–7.5 to determine the concentration of the remaining ammonium nitrogen. 2.4. Experimental procedures for seeding technique Seeding technique studies of MAP precipitation were performed with 1000 mL coking wastewater at 25 C. Firstly, the pre-formed MAP added to coking wastewater was used as a seeding material. The reaction was then started after adding the pyrolysate. The reaction solution was agitated by stirrers for 90 min at given experimental pH (8.5–10.5). The reaction solution was then allowed to settle for 60 min. The effluent was filtered with a 0.45 mm membrane filter and then HCl was added to adjust the pH to 6.5–7.5 for analysis. Table 1 – Characteristics of raw wastewater. Parameter Unit Concentrations COD mg L1 200  10 NH4 þ–N mg L1 520  20 pH – 7.2  0.1 BOD5 mg L1 60  10 TOC mg L1 100  10 SS mg L1 80  10 5210 w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 5 2 0 9 – 5 2 1 5 2.5. Analytical methods The concentration of ammonium nitrogen was measured according to Standard Methods (APHE, 1998). The collected pyrolysate was washed with deionized water 3 times, dried in an oven at 40 C for 48 h, and then analyzed by a scanning electron microscope (FESEM, LEO 1530VP, Germany), a transmission electron microscope (TEM, JEM-2100, Japan), Fourier transform infrared spectroscopy (FTIR, NEXUS870, USA), X-ray diffraction (XRD, X/TRA, ARL, Switzerland). 3. Results and discussion 3.1. MAP pyrogenation experiments Factors affecting ammonium nitrogen release from MAP during pyrogenation were studied, including heating temperature, heating time, and the molar ratio of hydroxyl (OH) and ammonium. MAP was mixed with a NaOH solution, and the OH to ammonium molar ratio was controlled in the range of 1:1–3:1, the heating temperature from 80 to 130 C, the heating time over 1–5 h. The results (Fig. 1a and b) showed that when the OH to ammonium molar ratio increased from 1:1 to 2:1, the ammonium nitrogen release ratio also increased, but further increases in the ratio of OH caused no further increase in the release ratio of ammonium nitrogen. When the heating temperature was 110 C and the heating time was 3 h, the ammonium nitrogen release ratio was more than 90% at an OH to ammonium molar ratio of 2:1. However, any increased heating temperature or prolonged heating time was not economical for additional ammonium nitrogen release. Ammonium nitrogen release behavior could be qualitatively explained by hard and soft acids and bases (HASB) (Pearson, 1963). As MAP mixed with NaOH solution, a NH4 þ– OH–Mg2þ–Naþ–PO4 3 solution system was formed. As the Lewis acids listed, NH4 þ interacted preferentially with OH in the NH4 þ–OH–Mg2þ–Naþ–PO4 3 solution system. Based on the Eq. NHþ 4 þ OH5NH3$H2O, an increase in the concentration of OH could accelerate ammonium transformation to NH3$H2O, which could lead to NH3(g) release from the solution under the given experimental temperature. Therefore, the thermal treatment of MAP for ammonium nitrogen release was accomplished. In the literature, there are some papers dealing with MAP pyrogenation. Abdelrazig and Sharp (1988) found that MAP could be pyrolyzed at the temperature of 300 C. Janus and van der Roest (1997) reported that the NH3 in the treated MAPsludge was recycled to the MAP reactor, which could lead to a decrease in ammonium removal efficiency, although the thermal process of the MAP-sludge treatment could produce reusable chemicals. In order to make the produced MAP pyrolysate performance well for ammonium nitrogen removal, we performed the steps of dewatering and drying of the MAP pyrolysate as mentioned elsewhere (Wilsenach et al., 2007), and the MAP pyrogenation technology parameters were optimized in our research. Turker and Celen (2007) reported that MAP with addition of NaOH was decomposed by heating and that the residues could be recycled for ammonium removal in good performance. 3.2. Surface characterization analysis SEM analysis was performed to identify the surface characterization of the pyrolysate (Fig. 2), which showed the unshaped spherical crystal was coarse, and its size was irregular (radius 15–35 nm). TEM image (Fig. 3) of the pyrolysate further indicated that the fringe of crystals was coarse and the size of most crystals was less than 30 nm. The FTIR pattern (Fig. 4) implied that the ammonium nitrogen releases from MAP because the ammonium characteristic bands completely disappeared (Stefov et al., 2004). As suggested by XRD studies (Fig. 5), the pyrolysate was amorphous solid and could be amorphous magnesium sodium phosphate Fig. 1 – Ammonium nitrogen release ratio of MAP pyrogenation at different conditions (a) Effect of hydroxyl: ammonium and temperature on ammonium nitrogen release ratio (heating time [ 3 h) (b) Effect of hydroxyl: ammonium and time on ammonium nitrogen release ratio (heating temperature [ 110 8C). w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 5 2 0 9 – 5 2 1 5 5211 (MgNaPO4) as elucidated elsewhere (He et al., 2007), which is isomorphous analogues with MAP (Mathew et al., 1982). A MAP crystal is an orthorhombic structure that consists of PO4 3 tetrahedral, Mg(6H2O)2þ octahedral and NH4 þ, held together by hydrogen bonds (Whitaker and Jeffery, 1970). A MAP isomorphous analogue can incorporate a wide range of univalent ions differing in size, such as MgKPO4$6H2O (Mathew and Schroeder, 1979; Stefov et al., 2004), MgRbPO4$6H2O, MgCsPO4$6H2O and so on (Banks et al., 1975). The MAP isomorphous analogues are less stable with decreasing size of the univalent ion (Banks et al., 1975). MgNaPO4$7H2O and MAP are isostructural and instead of NH4 þ by the smaller Na(H2O)þ. Due to the extensive face sharing of PO4 3 tetrahedral and Mg(6H2O)2þ octahedral in MgNaPO4$7H2O, considerable strain is to be expected. As a result, it might be less stable than those involving larger univalent ions such as NH4 þ (Mathew et al., 1982). Therefore, the substitution of NH4 þ in MgNaPO4 could form a more stable MAP. 3.3. Optimum pH experiments pH is an important factor for MAP precipitation. Taking into consideration the optimum pH range required to obtain minimum MAP solubility in the wastewater treatment, more practicable pieces of data have been published. The optimum pH of 8.9–9.25 (Nelson et al., 2003), 9.0–9.4 (Booker et al., 1999), 9.2 (Altinbas et al., 2002; Ryu et al., 2008), 9.4 (Wilsenach et al., 2007), 10.0 (Stratful et al., 2001), 10.3 (Ohlinger et al., 1998), 10.7 (Snoeyink and Jenkins, 1980; Stumm and Morgan, 1970) for MAP precipitation was reported. This experiment was to study the optimum pH of CPRT for ammonium nitrogen removal from the coking wastewater. This was tested by adding the dried pyrolysate into the samples. The pH range was 8.5–10.5. The results (Fig. 6) showed that the maximum ammonium nitrogen removal ratio for coking wastewater was 9.5. When the pH was higher than the optimum point, Mg3(PO4)2 was formed instead of MAP with pH increased, which lead to the decrease of the ammonium nitrogen removal ratio. Hþ in reaction solution would inhibit MAP precipitation when the Fig. 2 – Scanning electron microscopy analysis of MAP pyrolysate. Fig. 3 – Transmission electron microscope analysis of MAP pyrolysate. Fig. 4 – Fourier transform infrared spectroscopy analysis of MAP pyrolysate. Fig. 5 – X-ray diffraction analysis of MAP pyrolysate. 5212 w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 5 2 0 9 – 5 2 1 5 pH was lower than the optimum point. Therefore, the ammonium nitrogen removal efficiency was lower. The optimum pH for ammonium nitrogen removal of coking wastewater was 9.5. He et al. (2007) recycled used decomposed MAP residues at the pH of 9.0 for ammonium removal in landfill leachate. 3.4. Multi-recycle precipitation experiments Two modes of pyrolysate, recycled as magnesium and phosphate sources for ammonium nitrogen removal from coking wastewater at pH 9.5, were investigated. Firstly, the pyrolysate was added directly. Secondly, both the pyrolysate and the supplementation of MgCl2$6H2O plus Na2HPO4$12H2O at Mg2þ:NH4 þ:PO4 3 molar ratio of 0.05:1:0.05 were added per recycle time. When the pyrolysate was added without supplementation, the ammonium nitrogen removal ratio was gradually decreased with the increase in recycle times (Fig. 7). The decrease in ammonium nitrogen removal might be responsible for inactive Mg3(PO4)2 or Mg2P2O7 increases in recycled pyrolysate (Schulze-Rettmer et al., 2001) and the losses of Mg2þ and PO4 3 in the supernatant per recycle time. When adding the pyrolysate with supplementation of MgCl2$6H2O plus Na2HPO4$12H2O, the ammonium nitrogen removal ratio was not decreased with the increase in recycle times. 3.5. MAP seeding technique Seeding techniques have been tested for MAP crystallization onto seed materials such as pre-formed MAP crystals (Kim et al., 2007; Liu et al., 2008), sand (Battistoni et al., 2002), or stainless steel structures (Le Corre et al., 2007). This experiment was to study the seeding technique for MAP precipitation in coking wastewater by CPRT. Pre-formed MAP used as seeding material was added into the samples at the beginning Fig. 6 – NH4 D removal ratio of chemical precipitation recycle technology at different pH. Fig. 7 – NH4 D removal ratio of chemical precipitation recycle technology at different recycle times. Fig. 8 – NH4 D removal ratio of MAP seeding technique at different factors (a) NH4 D removal ratio of MAP seeding technique at different pH (pre-formed MAP [ 2.0 g/L) (b) NH4 D removal ratio of MAP seeding technique at different added amount of pre-formed MAP (pH [ 9.5). w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 5 2 0 9 – 5 2 1 5 5213 of the experiments. Recycled pyrolysate used as a magnesium and phosphate source was subsequently added. The pH range was 8.5–10.5 and the added pre-formed MAP amount was from 1.0 to 3.5 g/L. High efficiency of removal of ammonium nitrogen could be obtained using the seeding samples (Fig. 8a and b). The ammonium nitrogen removal ratio was more than 88% at a pH of 9.5 and an addition of pre-formed MAP of 2.0 g/L. However, any overdosing of pre-formed MAP did not bring further significant increases in ammonium nitrogen removal. At the lower amount of seeding material, both mechanisms of crystal nucleation and crystal growth affect the MAP precipitation (Kim et al., 2007). This could reduce the losses of Mg2þ and PO4 3 in the supernatant and increase the recycled efficiency of removal of ammonium nitrogen. In contrast, at the higher amount of seeding material, the mechanism of crystal nucleation has a weaker effect than does crystal growth. Therefore, no improvement in the ammonium nitrogen removal ratio would be expected. Wang et al. (2006) found that the seeding technique could increase MAP crystal size and improve MAP settleability. Liu et al. (2008) also reported that the seeding technique could enhance the MAP formation rate and improve MAP crystal growth. The XRD analysis (Fig. 9) showed that the characteristic peaks of both precipitates, with and without the seeding technique, were similar to that of the pattern for the MAP standard (JCPDS 15- 0762). 4. Conclusions As a treatment for ammonium nitrogen removal from coking wastewater, the chemical precipitation recycle technology was investigated and the following conclusions were drawn. The optimal MAP pyrolysate production condition was controlled at an OH to ammonium molar ratio of 2:1, a heating temperature of 110 C, and a heating time of 3 h. When the pyrolysate was recycled as magnesium and phosphate sources, the optimum pH for CPRT was 9.5. When the treatment of added pyrolysate without supplementation was employed, the ammonium nitrogen removal ratio was gradually decreased with the increasing number of recycle times. When supplemented with MgCl2$6H2O plus Na2HPO4$12H2O, the ammonium nitrogen removal efficiency was not decreased with the increasing number of recycle times. 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