Highly Efficient Recovery of Ammonium Nitrogen from Coking

In a seven-step process, the TAN-removal efficiency could be maintained at >80% with .... 0–1000 W MW power output) operating at 2.45 GHz and 800 W ...
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Research Article pubs.acs.org/journal/ascecg

Highly Efficient Recovery of Ammonium Nitrogen from Coking Wastewater by Coupling Struvite Precipitation and Microwave Radiation Technology Haiming Huang,* Jiahui Liu, Jing Xiao, Peng Zhang, and Faming Gao Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, 438 Hebei Avenue, Qinhuangdao 066004, People’s Republic of China ABSTRACT: In this study, to reduce the removal cost of total ammonia nitrogen (TAN) from coking wastewater via struvite crystallization, a struvite recycling process combined with microwave radiation technology was investigated. The results indicated that the nitrogenous heterocyclic and phenolic compounds present in coking wastewater obviously influenced struvite crystallization. If the formed struvite was decomposed by microwave (MW) radiation at OH−:NH4+ of 1.15:1 for 300 s, about >97% of ammonium could be released from the struvite. In the decomposition process of MW radiation, it was found that the solid−liquid concentration of struvite in the solution system had a negligible effect on ammonium release. When the resulting decomposition product was recycled, to remove TAN from the coking wastewater at pH 8.5 and P:N molar ratio of 1:1 for 50 min, approximately 88% of TAN removal was achieved. In a seven-step process, the TAN-removal efficiency could be maintained at >80% with a little bittern supplementation. Cost analysis revealed that the proposed recycling process could reduce 54.4% of the struvite precipitation cost as compared to the nonrecycling process. KEYWORDS: Microwave radiation, Struvite, Recycling, Coking wastewater

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al.6 has reported that 81% of TAN in nylon wastewater can be removed by using brucite and phosphoric acid as a precursor of struvite precipitation at an Mg:N:P molar ratio of 3:1:1. The effective removal of TAN by struvite precipitation is mainly based on the principle that struvite can naturally crystallize when the combined ion concentration of NH4+, Mg2+, and PO43− exceeds its solubility (23 mg per 100 mL water).7 According to the basic principle, it can be confirmed that there is a need for supplementation of large amounts of magnesium and phosphate salts to wastewaters if high removal of TAN is desired. The use of pure chemicals commonly incurs a high cost of wastewater treatment, blocking the practical application of struvite precipitation. Although various low-cost magnesium sources are used in struvite precipitation,8−11 their contribution to the reduction of struvite precipitation costs is extremely limited. For instance, Gunay et al.10 have reduced 18% of the struvite precipitation cost by using magnesite as the magnesium source. Lahav et al.9 have used a nanofiltration concentrate as the magnesium source and saved 25% of the struvite precipitation cost. Struvite recycling is an effective method to decrease precipitation cost. However, there are still many

INTRODUCTION Nitrogen is an indispensable nutrimental element for all living organisms. However, excess nitrogen, especially total ammonia nitrogen (TAN, NH4+, and NH3), in receiving waters is one of the main causes of eutrophication. It deteriorates the quality of water and increases dissolved oxygen depletion and fish toxicity. Coking wastewater produced during activities such as hightemperature carbonation, coal gas purification, and chemical product refining, in a coke plant, generally contains a large amount of complicated and toxic compounds, such as phenolic compounds, thiocyanate (SCN−), and TAN.1,2 As the growth of nitrifying bacteria is significantly inhibited by these refractory and toxic compounds,1 it is difficult to facilitate removal of high-concentration TAN from coking wastewater by conventional biological processes. Therefore, it is extremely essential to establish effective methods to remove TAN from coking wastewater for the prevention of water eutrophication. In recent times, the use of struvite precipitation as a pretreatment process has been developed and proposed for removal of TAN from various forms of wastewater, such as landfill leachate,3 swine wastewater,4 fertilizer wastewater,5 and nylon wastewater.6 From a technical perspective, it has been proved that struvite precipitation is extremely effective for TAN removal. Iaconi et al.3 has used magnesium oxide as the magnesium source of struvite precipitation and achieved a TAN-removal efficiency of 95% from landfill leachate. Huang et © XXXX American Chemical Society

Received: February 3, 2016 Revised: May 13, 2016

A

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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placed on a magnetic stirrer, followed by a separate addition of phenol/quinoline with different concentrations of organic compounds (0−600 mg/L). Second, bittern and disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) were fed to the synthetic wastewater in a magnesium:nitrogen:phosphorus (Mg:N:P) stoichiometric ratio. Third, the mixed solution was stirred for 20 min and the solution pH was kept at pH 9.5, by the addition of 0.1 M NaOH during the stirring, and then the formed struvite was allowed to precipitate for 30 min. Finally, 10 mL of the supernatant was removed and filtered through a 0.45 μm filter membrane for component analysis. In addition, after determination of the effects of phenol and quinoline, the struvite used for the decomposition was formed in the actual coking wastewater, at different Mg:N molar ratios (0.95:1−1.45:1, P:N molar ratio, 1:1). The operational procedures of the experiments were similar to those mentioned above. The struvite precipitates collected during the experiments were washed thrice with deionized water and then dried in an oven at 40 °C for 12 h. The dried struvite precipitates was dissolved in 0.5% nitric acid solution to determine the content of struvite in the precipitates (i.e., purity). Struvite Decomposition by Microwave Radiation. To determine the action of MW radiation on the decomposition of struvite, a series of batch experiments were performed in a modified MW radiation reaction system (MG34EP-B1; 0−1000 W MW power output) operating at 2.45 GHz and 800 W power. The schematic diagram of the experimental operation is shown in Figure 1. A 250 mL glass container was placed in the

problems that need to be overcome in the conventional pyrogenation of struvite, which occur in the solid system, resulting in excessive time consumption (>3 h) and the generation of magnesium pyrophosphate (Mg2P2O7) with no capacity to remove TAN.12,13 To resolve these problems, in this study, a novel decomposition process by MW radiation is proposed, to release ammonium from struvite. To the best of our knowledge, to date, there exist no reports dealing with the decomposition of struvite by MW radiation. In recent years, MW radiation has attracted much attention and has been widely applied in organic and inorganic synthesis, polymerization processes, biological aspects and drying, pasteurization, and sterilization in the food industry.14,15 MW irradiation has several advantages, including, higher heating rates and uniformity, greater heating control, and reduced equipment size, as compared to the conventional heating techniques.16,17 Consequently, decomposing struvite by MW radiation in the solution system can offer the following advantages as compared to the conventional pyrogenation processes in the solid system: (1) No Mg2P2O7 formation; (2) greatly shortened decomposition time; and (3) easy recovery of the ammonia released. The main objective of this study is to investigate the feasibility of recycling the decomposition product of struvite by MW radiation for removal of TAN from the coking wastewater. Owing to the complexity of coking wastewater, it is necessary to evaluate the effects of some organic matters on the crystallization of struvite with synthetic wastewater, before removal of TAN from real coking wastewater. Therefore, the effects of organic matters on the crystallization of struvite have to be first investigated. Next, the investigations need to be focused on the optimum conditions for struvite formation in coking wastewater. Third, the formed struvite is decomposed by MW radiation, and the resulting product is recycled to remove TAN from the coking wastewater. Finally, a cost analysis is conducted to determine the economical feasibility of the proposed struvite recycling process. Materials and Methods. Materials. The raw coking wastewater used in the experiments was collected from a coking wastewater treatment plant in Tangshan, China. Prior to use, the wastewater was pretreated to remove any suspended substances. The characteristics of the pretreated wastewater were as follows: pH 8.25 ± 0.07, TN 472 ± 25 mg/L, TAN 450 ± 18 mg/L, chemical oxygen demand (COD) 6092 ± 382 mg/ L, and total volatile phenols 758 ± 83 mg/L. In this study, the bittern used as the magnesium source of struvite precipitation was collected from a solar salt field in Tianjin, China. The Mg content in the bittern was 44 ± 2.1 g/L and its other main components are listed in our previous article.18 Phenol and quinoline served as the phenolic compounds and nitrogenous heterocyclic compounds, respectively. Both the chemicals were of analytical grade and purchased from the Tianjin Fengchuang Chemical Reagent Plant, China. Experiments for the Formation of Struvite in Coking Wastewater. As coking wastewater contains large amounts of nitrogenous heterocyclic and phenolic compounds, batch experiments were first performed, to determine the effects of these compounds on struvite precipitation. In the experiments, synthetic coking wastewater with the same TAN concentration as that in actual coking wastewater was prepared by dissolving Ammonium Chloride (NH4Cl) into deionized water. The experimental procedure was as follows: First, 100 mL of synthetic coking wastewater was added to a 200 mL breaker

Figure 1. Schematic diagram of struvite decomposition by MW radiation.

MW radiation reactor and was connected to two absorption bottles by a glass condensing system. The absorption bottles were filled with 150 mL of H2SO4 solution (10%) to absorb the escaped ammonia. The experimental procedures are as follows. A given amount of dried struvite (10−40 g) and 100 mL of pure water were added to the glass container (i.e., the solid− liquid [SL] concentration; 100−400 g/L), followed by the addition of NaOH at different OH−:NH4+ molar ratios (0.7− 1.45). Next, the solid−liquid system was radiated for 60−420 s, and the remaining solution was dissolved in 0.5% nitric acid solution, from which 5 mL of the solution was taken for component analysis. Recycling of Struvite Decomposition Residues Produced from Microwave Radiation. To investigate the performance of TAN removal by reusing the struvite B

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering decomposition residues produced from MW radiation, a series of tests were carried out, and the specific experimental arrangements are shown in Table 1. The conditioning

were measured using an atomic adsorption photometer (AA6800; Shimadzu, Japan). The solution pH was monitored using a pH meter (pHS-3C; China). The residues after the decomposition of struvite by MW radiation were filtered through a filter paper, and the remaining solids were washed with pure water, thrice, followed by drying at 40 °C for 12 h. The morphology of the dried solids, resulting from struvite decomposition, was characterized by using a scanning electron microscopy-energy dispersive spectrometry instrument (SEMEDS; SUPRA 55 SAPPHIRE; Germany), and its composition was determined using a X-ray diffraction analyzer (XRD; DMAX-RB; Rigaku, Japan), X-ray photoelectron spectrometer (XPS; Phi-5000 Versa Probe; ULVCA-PHI, USA), and Fouriertransformed infrared spectroscopy (FTIR; NEXUS870, USA). In this study, all the tests were performed in triplicate, and their average value was reported.

Table 1. Arrangement of the Tests of Recycling the Decomposition Residues experiment condition experiment multiple recycling

testing pH

reaction time (min)

number of recycling

8, 8.5, 9, 9.5

65

1

8.5

50

7

experiments for the recycling of the decomposition residues were carried out according to the following procedure. The decomposition residues (i.e., solid plus solution) were added to 100 mL of coking wastewater at a molar ratio of P in the decomposition residues to the TAN in the coking wastewater, of 1:1, and then stirred at different pH values (8−9.5) for 65 min. During the experiments, 0.1 M NaOH solution was used to adjust the pH, and 1 mL of each sample was removed at different time intervals and filtered through 0.22 μm filter membranes for composition analysis. Following the conditioning experiments, multiple recycling experiments of the decomposition residues were conducted. The experimental procedures were similar to those mentioned in the section of Struvite Decomposition by MW Radiation and the conditioning experiments of the decomposition residues. To be precise, the decomposition of struvite and the recycling of the decomposition residues were repeatedly performed. Analytical Methods. Various parameters of coking wastewater were analyzed according to the American Public Health Association (APHA) standard methods.19 TAN and PO4−P concentrations were determined colorimetrically by using the 752N spectrophotometer (China). The concentrations of Mg2+

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RESULTS AND DISCUSSION

Formation of Struvite from Coking Wastewater. The experimental results of the effects of phenol and quinoline on struvite crystallization are shown in Figure 2a,b. It can be observed that the presence of phenol and quinoline has an obvious influence on struvite crystallization. With increasing concentrations of phenol and quinoline, the TAN-removal efficiencies decreased progressively (Figure 2a), whereas the remaining PO4−P concentrations increased gradually (Figure 2b). In addition, it can be seen that the effect of quinoline on struvite crystallization was slightly higher than that of phenol. When the concentrations of phenol and quinoline increased from 0 to 600 mg/L, the TAN-removal efficiencies decreased progressively from the initial 90.1% to 82.6 and 80.3%, respectively, whereas, the remaining PO4−P concentrations increased from the initial 43 to 74.6 and 80 mg/L, respectively. Quinoline is a typical N-heterocyclic aromatic compound and is

Figure 2. (a) TAN-removal efficiencies and (b) remaining PO4−P concentrations at different phenol and quinoline concentrations (pH = 9.5, Mg:N:P = 1:1:1), and (c) TAN-removal efficiencies and struvite purity at different Mg:N molar ratios (pH = 9.5, P:N = 1:1). C

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering structurally related to benzene, with one C−H group replaced by a nitrogen atom.20 Quinoline can link with metal ions through the nitrogen atom. Therefore, it may play a role in the complexation and coordination of Mg2+,21 which results in the decrease in concentration of Mg2+ and the blocking of struvite crystallization. This explanation can be further supported by the change in the remaining Mg2+ concentration of the supernatant with the concentration of quinoline. When the concentration of quinoline increases from 120 to 600 mg/L, the remaining Mg2+ concentration decreases from 7.6 to 1.8 mg/L. Phenol is an organic compound that bonds directly with the hydroxyl group and is slightly acidic. On account of the presence of the phenolic hydroxyl group, phenol can also coordinate with Mg2+, resulting in a decrease in the concentration of Mg2+ in a solution. In addition, in the experiments performed, we have found that the concentration of phenol in the solution, after reaction, presents a slight decrease. This may be attributed to the fact that phenol can react with NaOH to form natrium phenolicum, which can be adsorbed on the surface of the critical crystal nucleus of struvite, owing to electrostatic interaction.22 This may increase the induction time of struvite nucleation and delay the growth of the struvite crystal, resulting in the decrease of TAN removal.23 To decrease the effects of nitrogenous heterocyclic and phenolic compounds in coking wastewater on struvite crystallization, a batch of experiments were conducted, to investigate the separate effect of excess Mg2+ on TAN removal and struvite purity. The results obtained are shown in Figure 2c, which indicate that, with increasing Mg:N molar ratio from 0.95:1 to 1.2:1, the TAN-removal efficiencies also increase from 78.7 to 86.9%. However, when the Mg:N molar ratio was further increased, no further increase in TAN removal was observed. The findings are consistent with those reported by Li et al.24 Moreover, it can be observed from Figure 2c that the purity of the formed struvite decreases slightly with an increase in the Mg:N molar ratio. This event can be attributed to the fact that an excess of Mg2+ results in an increase in the organic coordination compounds of magnesium and Mg3(PO4) 2 formed in struvite precipitation.8 In this study, it has been confirmed that the optimum Mg:N molar ratio for the formation of struvite is 1.2:1. Under this condition, the formed struvite has a purity of 86.3% and is used in the subsequent decomposition experiments. Furthermore, composition analysis after dissolving the formed struvite in 0.5% nitric acid solution indicates that the molar ratio of magnesium, nitrogen, and phosphorus in it is 1.32:1:1.13 (Mg:N:P). Struvite Decomposition by Microwave Radiation. Ammonium Release Ratio by Microwave Radiation. To determine the release ratio of ammonium in struvite by MW radiation, three factors including OH−:NH4+ molar ratio, radiation time, and SL concentration of struvite were investigated. Figure 3a demonstrates the changes in the ammonium release ratio with an OH−:NH4+ molar ratio and radiation time at an SL concentration of 100 g/L. It was found from Figure 3a that the OH−:NH4+ molar ratio and radiation time had an obvious influence on the release of ammonium in struvite. At a given OH−:NH4+ molar ratio, the ammonium release ratio rapidly increased during the initial radiation period of 180 s, followed by a slow increase between 180 and 300 s, to arrive finally at a plateau after approximately 300 s. Furthermore, at a given radiation time, the ammonium release ratio increased with an increase in the OH−:NH4+ molar ratio of 0.7:1−1.15:1, whereas no more increase was observed when

Figure 3. Release ratio of ammonium from struvite by MW radiation at different conditions: (a) at OH−:NH4+ 0.7:1−1.45:1 and SL 100 g/ L for 60−420 s and (b) at OH−:NH4+ 1.15:1 and SL 100−400 g/L for 60−420 s.

the OH−:NH4+ molar ratio was increased further. At the OH−:NH4+ molar ratio of 1.15:1 and a radiation time of 300 s, the ammonium release ratio exceeded 97%. Figure 3b illustrates the changes in the ammonium release ratio with the SL concentration and radiation time at the OH−:NH4+ molar ratio of 1.15:1. It shows that the SL concentration had a negligible effect on the release of ammonium, suggesting that the energy consumption for the decomposition of a unit mass of struvite would be greatly reduced with an increase in SL concentration. There are several articles in the literature dealing with struvite pyrogenation. Zhang et al.25 reported that an ammonium release ratio of >92% could be achieved when struvite was heated at a temperature of 110 °C and at an OH−:NH4+ molar ratio of 2:1 for 3 h. Tüker and Ç elen13 pyrolyzed struvite at an OH−:NH4+ ratio of 1:1 and temperature of 110 °C for 3 h, achieving an ammonium release ratio of 81%. He et al.7 found that struvite pyrogenation at an OH−:NH4+ ratio of 1:1 and temperature of 90 °C for 2 h led to the release of >96% of ammonium from struvite. Compared to these reported findings, we found that struvite decomposition by MW radiation not only achieved a comparable ammonium release ratio to that of struvite pyrogenation by conventional heating, but also greatly reduced the decomposition time. This meant that the volume of the reactor used for struvite decomposition could be markedly decreased with an evident reduction in the investment cost. In D

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) SEM picture, (b) XRD pattern, (c) FTIR spectroscopy, and (d) XPS pattern of the solid obtained from struvite decomposition by MW radiation.

Struvite is a mineral with an orthorhombic structure consisting of a PO43− tetrahedral, a Mg(6H2O)2+ octahedral, and an NH4+ group, held together by hydrogen bonds.26 When struvite remained in a high alkali solution, the crystal structure became unstable, and the NH4+ group combined with OH− to form NH3·H2O. During MW radiation, the NH3 molecules could escape from the solution by the thermal and nonthermal effects of MW radiation.14 On the one hand, the solution temperature could be rapidly increased by the thermal effect generated by the intermolecular friction caused by alternation in the electromagnetic field, which induced the rotation of the dipoles of polar compounds such as H2O.27 During the course of the experiments, it was observed that struvite solution boiled after approximately 100 s of radiation. Therefore, high solution temperature played a vital role in the escape of ammonia from struvite, resulting in the transformation of the solution system from the initial NH4+−OH−−Mg2+−Na+−PO43− state to the final Mg2+−Na+−PO43− state. On the other hand, the N−H···O and O−H···N intermolecular hydrogen bond present in the solution could be fractured and weakened by inducing a nonthermal effect generated by the frequent pendulum vibration of molecular NH3 and H2O, polarized by MW radiation.14,28 Consequently, the nonthermal effect of MW radiation could be another reason for the rapid escape of ammonia from the solution. As MgNaPO4 is an isomorphous analog of struvite,29 the Mg2+−Na+−PO43− system was found to be unstable in the alkaline solution after the escape of ammonia. Under this condition, most of the Mg2+ favorably combined with PO43− to form Mg3(PO4)2·8H2O. In addition, some Mg2+ might react with OH− to form Mg(OH)2.8 As a result, the amount of remaining MgNaPO4 in the solid product

addition, compared to the conventional pyrogenation process, the decomposition process by MW radiation was found to be extremely beneficial for the recovery of the released ammonia. In the experiments, several discrete measures indicated that the released ammonia was completely absorbed by the H2SO4 solution in the adsorption bottles. Decomposition Mechanism of Struvite by Microwave Radiation. To determine the struvite decomposition mechanism, the struvite solution system with an SL concentration of 100 g/L was radiated at an OH−:NH4+ molar ratio of 1.15:1 for 300 s, after which the solid−liquid separation of the decomposition residues was conducted. The pH and the Mg2+ and PO4−P concentrations of the filtrate (about 40 mL) were measured to be 10.7 ± 0.03, 2.8 ± 0.5 mg/L, and 4179 ± 120 mg/L, respectively. The obtained solid product was characterized by SEM, XPS, XRD, and FTIR. SEM analysis (Figure 4a) revealed that the unique crystal structure of struvite after MW radiation disappeared, implying that struvite decomposition had occurred. The FTIR pattern (Figure 4b) revealed that the characteristic bands of the ammonium group (1435 cm−1) completely disappeared, suggesting the release of ammonium from struvite. The XRD spectra (Figure 4c) showed that the main composition of the remaining solid product was magnesium phosphate 8-hydrate (Mg3(PO4)2· 8H2O). Moreover, on the basis of the XPS spectrum (Figure 4d), it was found that the solid product simultaneously contained the elements of Mg, Na, P, and O, indicating that MgNaPO4 may be a derivative of struvite decomposition. As the pH in the remaining solution was >10.5, Mg(OH)2 may also be formed, thereby serving as the derivative of struvite decomposition. E

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. TAN-removal efficiencies (a) and remaining PO4−P concentrations (b) by recycling the struvite decomposition product at different pH values and reaction times.

might be small, which was inconsistent with the findings of struvite pyrogenation by the conventional process, the main product of which was MgNaPO4.7,13,25 This difference could be derived from the decomposition environment between the proposed process (solution environment) and the conventional process (solid environment). Recycling of the Struvite Decomposition Product. Effects of Reaction Time and Solution pH on Decomposition Product Recycling. Solution pH has been widely proved to be an important factor in struvite formation. Various optimum pH values such as 8.5−9,24 8.8−9.4,10 9.4,30 9.5,25 and 10.031 have been reported. In the recycling process, to determine the reuse performance of the decomposition product (the solid−liquid residues that form under the conditions of OH−:NH4+ 1.15:1, radiation time 300 s, and SL concentration 100 g/L), recycling has been conducted at the pH range of 8−9.5 for 65 min, the results of which are shown in Figure 5. It can be seen from Figure 5a that the reaction time has an obvious effect on the removal of TAN removal by recycling the decomposition product as the magnesium and phosphate sources of struvite precipitation. At pH 8.5, the TAN-removal efficiency increases from 49.2 to 88%, on increasing the reaction time from 5 to 50 min. The reaction time is commonly considered as a negligible factor for struvite precipitation when soluble salts are used as magnesium and phosphate sources. Stratful et al.31 has reported that the crystallization of struvite can be completed within 1 min. However, in our study, the dissolution−precipitation of large amounts of insoluble compounds in the struvite decomposition product must have been responsible for the phenomenon. At a given reaction time, the TAN-removal efficiency increases in the pH range of 8−8.5, reaching a peak value at pH 8.5, followed by a decrease in the pH range of 8.5− 9.5. In addition, it can be observed from Figure 5b that the remaining PO4−P concentration decreases with an increase in the solution pH and reaction time. When the solution pH is 8.5 and the reaction time is 50 min, the remaining PO4−P concentration is approximately 27 mg/L. According to the composition analysis of the struvite decomposition product in the section of Decomposition Mechanism of Struvite by MW Radiation, the recycled product mainly contained dissolved PO43− and solid Mg3(PO4)2·8H2O, MgNaPO4, and Mg(OH)2. Therefore, when the product was recycled for struvite formation, the following reactions occurred:

Mg 3(PO4 )2 ·8H 2O + 2H+ → 3Mg 2 + + 2HPO4 2 − + 8H 2O

(1)

Mg 2 + + NH4 + + HPO4 2 − + 6H 2O → MgNH4PO4 · 6H 2O + H+

(2)

Mg(OH)2 + NH4 + + HPO4 2 − + 5H 2O → MgNH4PO4 · 6H 2O + OH−

(3)

NH4 + + MgNaPO4 + 6H 2O → MgNH4PO4 · 6H 2O + Na +

(4)

During the initial reaction stage of 5 min, the rapid increase in TAN removal was attributed mainly to the presence of MgNaPO4, Mg(OH)2, and HPO42− in the solution, which could immediately react with NH4+ to form MgNH4PO4·6H2O. At the subsequent reaction stage, the removal of TAN from coking wastewater occurred through the dissolution of Mg3(PO4)2·8H2O to release Mg2+ and HPO42−, followed by the precipitation of struvite by the reaction among NH4+, Mg2+, and HPO42− in the solution.12 The dissolution−precipitation mechanism could be supported by the slow increase in TANremoval efficiency, between 25 and 65 min. Although the minimum solubility of struvite is in the range of 8.8−9.4,10 at a lower pH this is conducive to the dissolution of magnesium phosphate compounds. Consequently, recycling of the decomposition product at pH 8.5 was considered to be optimal for TAN removal. In addition, as compared to the findings illustrated in Figure 2, the TAN-removal efficiencies by recycling the decomposition product were slightly higher than those by using soluble magnesium and phosphate salts. This phenomenon could be attributed to struvite formation at the former stage of reaction, which could be used as the favorable seeding material in the subsequent stages, thereby remarkably improving the TAN-removal efficiency.32 Multiple-Recycling of the Struvite Decomposition Product. On the basis of the above-mentioned recycling results, three modes of multiple-recycling of the decomposition product for TAN removal were investigated at pH 8.5 for 50 min. In the first mode, the decomposition product was directly recycled without any chemical supplementation. In the second mode, F

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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92% to 77% in the fifth recycle cycle. He et al.7 reported that when struvite pyrolysate was recycled six times, the TANremoval efficiency was initially >90% and gradually decreased to 62% in the sixth cycle. Yu et al.33 reported that the TANremoval efficiency was 90.7% in the first cycle and 79.4% in the sixth cycle, which was achieved by the recycling of struvite pyrolysate with acidolysis. Hence, as compared to the reported recycling processes, the proposed recycling process demonstrated a comparable performance with regard to TAN removal. Cost Analysis. In this investigation, a cost analysis of recycling the struvite decomposition product to remove TAN from coking wastewater for seven cycles was performed. In addition, the recycling cost was compared to that of struvite precipitation without recycling (bittern + Na2HPO4·12H2O, Mg:N:P = 1:1:1) and other reported processes. In this assessment, based on the experimental results mentioned above, the average TAN removal ratios of struvite recycling and nonrecycling were set at 85 and 90%, respectively, in the cost calculation. As the manpower and maintenance costs were closely related to the scale of wastewater treatment and the level of management, to facilitate the evaluation, they were not taken into account in the calculation. At present, as struvite is not used as a fertilizer for agricultural production in China, it can only be sold as phosphate ore. On the basis of the international market price of phosphate ore and the phosphorus content in the recovered struvite, it was considered that 0.17 $/kg of the recovered struvite was reasonable. Furthermore, as the recovery of the TAN released from struvite as ammonium sulfate needed large amounts of acid sulfate and energy, the value of the recovered ammonium sulfate just counteracted the recovery cost. Therefore, the value of recovered TAN was not considered in this assessment. Prices of the consumed chemicals, and the energy and recovered struvite and the calculation results are shown in Table 2. It was

the decomposition product was fed to the coking wastewater with supplementation of bittern at an Mg:N molar ratio of 0.15:1 at each recycling cycle. In the third mode, the decomposition product was recycled with the supplementation of bittern and Na2HPO4·12H2O at the Mg:N:P molar ratio of 0.05:1:0.05 per recycle cycle. The investigation results are described in Figure 6. It was observed that in the first mode, the

Figure 6. Changes in TAN-removal efficiencies and remaining PO4−P concentrations at different operational modes and number of recycles (Mode 1, without the supplementation of chemicals; Mode 2, with supplementation of bittern; Mode 3, with supplementation of bittern and Na2HPO4·12H2O).

Table 2. Market Price of Chemicals and Energy and the Costs for Recycling and Nonrecycling Processes chemical/energy

TAN-removal efficiency rapidly decreased from 88.3% in the first cycle to 68.4% in the seventh cycle, and the remaining PO4−P concentration of effluent was maintained at approximately 24 mg/L during the seven recycling cycles. The decrease in TAN removal can be attributed to loss of Mg2+ and PO43− in the effluent, per recycle cycle. When bittern is separately supplemented to the recycling in the second mode, the decrease extent of the TAN-removal efficiency is obviously suppressed; otherwise, more importantly, the remaining PO4− P concentration is decreased to approximately 5 mg/L. The decrease in the loss of PO43− may be responsible for the slow decrease in the TAN-removal efficiency. When the decomposition product was recycled according to the third mode, the TAN-removal efficiency was basically maintained at a stable value, with a remaining PO4−P concentration of approximately 30 mg/L. However, the operational mode was found to be uneconomical. Therefore, on the basis of the comprehensive consideration of TANremoval efficiency and the treatment cost, the second mode was confirmed to be optimum for multiple-recycling of the decomposition product, in which the TAN-removal efficiency was decreased only by 5% after seven recycling cycles. Türker and Ç elen13 recycled the struvite pyrolysate for five cycles and found a decrease in the TAN-removal efficiency from an initial

Na2HPO4·12H2O NaOH bittern energy recovered struvite total

market price ($/kg) 0.35 0.27 0.01 0.1 $/kWh 0.17

average cost for seven recycling cycles ($/kg TAN)

cost of nonrecycling ($/kg TAN)

1.27 1.15 0.1 1.82

10.15 1.05 0.44 0.05

−0.37 3.97

−2.98 8.71

calculated that the cost for removing 1 kg of TAN from coking wastewater by struvite precipitation without recycling was 8.71$, whereas, it was 3.97$/kg TAN when the formed struvite was recycled for seven cycles. The cost of removing TAN by the proposed recycling process was thus comparable to that (3−5.5$/kg TAN) of using a bioreactor without sludge retention for TAN removal34 and close to that (3$/kg TAN) of the ion-exchange and electrochemical regeneration process for TAN removal.35 Furthermore, as compared to struvite precipitation without recycling, the proposed recycling process could reduce approximately 54.4% of the TAN removal cost. In the literature on struvite recycling, He et al.7 reduced 44% of the cost for removing TAN from the landfill leachate by recycling the struvite pyrolysate obtained through the conventional pyrogenation process. In addition, Liu et al.36 reported G

DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

ammonium from fertilizer wastewater. Bioresour. Technol. 2012, 124, 292−298. (6) Huang, H.; Song, Q.; Wang, W.; Wu, S.; Dai, J. Treatment of anaerobic digester effluents of nylon wastewater through chemical precipitation and a sequencing batch reactor process. J. Environ. Manage. 2012, 101, 68−74. (7) He, S.; Zhang, Y.; Yang, M.; Du, W.; Harada, H. Repeated use of MAP decomposition residues for the removal of high ammonium concentration from landfill leachate. Chemosphere 2007, 66, 2233− 2238. (8) Lee, S. I.; Weon, S. Y.; Lee, C. W.; Koopman, B. Removal of nitrogen and phosphate from wastewater by addition of bittern. Chemosphere 2003, 51, 265−271. (9) Lahav, O.; Telzhensky, M.; Zewuhn, A.; Gendel, Y.; Gerth, J.; Calmano, W.; Birnhack, L. Struvite recovery from municipalwastewater sludge centrifuge supernatant using seawater NF concentrate as a cheap Mg(II) source. Sep. Purif. Technol. 2013, 108, 103−110. (10) Gunay, A.; Karadag, D.; Tosun, I.; Ozturk, M. Use of magnesit as a magnesium source for ammonium removal from leachate. J. Hazard. Mater. 2008, 156, 619−623. (11) Romero-Güiza, M. S.; Astals, S.; Chimenos, J. M.; Martínez, M.; Mata-Alvarez, J. Improving anaerobic digestion of pig manure by adding in the same reactor a stabilizing agent formulated with lowgrade magnesium oxide. Biomass Bioenergy 2014, 67, 243−251. (12) Sugiyama, S.; Yokoyama, M.; et al. Removal of aqueous ammonium with magnesium phosphates obtained from the ammonium-elimination of magnesium ammonium phosphate. J. Colloid Interface Sci. 2005, 292, 133−138. (13) Türker, M.; Ç Elen, I. Removal of ammonia as struvite from anaerobic digester effluents and recycling of magnesium and phosphate. Bioresour. Technol. 2007, 98, 1529−1534. (14) Lin, L.; Yuan, S.; Chen, J.; Xu, Z.; Lu, X. Removal of ammonia nitrogen in wastewater by microwave radiation. J. Hazard. Mater. 2009, 161, 1063−1068. (15) Menendez, J. A.; Inguanzo, M.; Pis, J. J. Microwave induced pyrolysis of sewage sludge. Water Res. 2002, 36, 3261−3264. (16) Liao, P. H.; Wong, W. T.; Lo, K. V. Release of phosphorus from sewage sludge using microwave technology. J. Environ. Eng. Sci. 2005, 4 (1), 77−81. (17) Xiao, D.; Huang, H.; Jiang, Y.; Ding, L. Recovery of phosphate from the supernatant of activated sludge pretreated by microwave irradiation through chemical precipitation. Environ. Sci. Pollut. Res. 2015, DOI: 10.1007/s11356-015-4504-9. (18) Huang, H.; Yang, J.; Li, D. Recovery and removal of ammonia nitrogen and phosphate from swine wastewater by internal recycling of struvite chlorination product. Bioresour. Technol. 2014, 172, 253−259. (19) APHA. Standard methods for the examination of water and wastewater, 19th ed.; American Public Health Association/American Water Works Association/Water Environment Federation: Washington, DC, 1998. (20) Tuo, B.; Yan, J.; Fan, B.; Yang, Z.; Liu, J. Biodegradation characteristics and bio-augmentation potential of a novel quinolinedegrading strain of Bacillus sp. isolated from petroleum-contaminated soil. Bioresour. Technol. 2012, 107, 55−60. (21) Vico, L. I.; Acebal, S. G. Some aspects about the adsorption of quinoline on fibrous silicates and Patagonian saponite. Appl. Clay Sci. 2006, 33, 142−148. (22) Wierzbicki, A.; Sallis, J. D.; Stevens, E. D.; Smith, M.; Sikes, C. S. Crystal growth and molecular modeling studies of inhibition of struvite by phosphocitrate. Calcif. Tissue Int. 1997, 61, 216−222. (23) Kofina, A. N.; Demadis, K. D.; Koutsoukos, P. G. The effect of citrate and phosphocitrate on struvite spontaneous precipitation. Cryst. Growth Des. 2007, 7, 2705−2712. (24) Li, X. Z.; Zhao, Q. L.; Hao, X. D. Ammonium removal from landfilll leachate by chemical precipitation. Waste Manage. 1999, 19, 409−415.

that approximately 60% of cost could be saved by recycling the electrochemical decomposition product of struvite. On the basis of the analysis and comparison of the cost, it can be confirmed that the proposed recycling process is economically sound.

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CONCLUSIONS This article has presented a study concerning the removal of TAN from coking wastewater by a struvite recycling process combined with MW radiation technology. The following conclusions can be drawn from this: The TAN removal efficiency by struvite precipitation decreased with an increase in the concentrations of phenol and quinoline in coking wastewater. Nevertheless, when the Mg dosage was increased, the influence of the nitrogenous heterocyclic and phenolic compounds on the TAN removal efficiency could be counteracted. The optimum parameters for struvite decomposition by MW radiation were found to be an OH−:NH4+ molar ratio of 1.15:1 and a radiation time of 300 s. Under the decomposition conditions, the main composition of the generated active product was dissolved PO43− and solid Mg3(PO4)2·8H2O, MgNaPO4, and Mg(OH)2. When the decomposition product was recycled for the removal of TAN from coking wastewater at pH 8.5 and a P:N ratio of 1:1, it was found that 88% of TAN removal efficiency could be achieved. In a seven-cycle process, >80% of the TAN-removal efficiency could be maintained well with an approximate supplementation of bittern. An economic analysis indicated that 54.4% of the struvite precipitation cost could be saved by the proposed recycling process, compared to struvite precipitation without recycling.

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AUTHOR INFORMATION

Corresponding Author

*H. Huang. E-mail: [email protected]. Tel.: +86335-8387-741. Fax: +86-335-8061-549. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51408529), the Natural Science Foundation of Hebei Province (Grant No. E2014203080), and China Postdoctoral Science Foundation Funded Project (Grant No. 2015M580215).

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REFERENCES

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DOI: 10.1021/acssuschemeng.6b00247 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX