Chemosphere 141 (2015) 94–99

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Effect of humic substances on phosphorus removal by struvite precipitation Zhen Zhou ⇑, Dalong Hu, Weichao Ren, Yuzeng Zhao, Lu-Man Jiang, Luochun Wang College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, China College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Changyang Rd #2588, Shanghai 200090, China

h i g h l i g h t s  Inhibitory effect of humic substances on struvite precipitation was studied.  Humic substances inhibited struvite precipitation at pH 8.0–9.0 and low Mg/P ratio.  Humic substances changed structure and morphology of precipitated struvite crystals.  Coprecipitation of humic substances compromised precipitated struvite purity.

a r t i c l e

i n f o

Article history: Received 1 August 2014 Received in revised form 26 May 2015 Accepted 29 June 2015

Keywords: Struvite Humic substances Phosphorus Precipitation Wastewater

a b s t r a c t Humic substances (HS) are a major fraction of dissolved organic matters in wastewater. The effect of HS on phosphorus removal by struvite precipitation was investigated using synthetic wastewater under different initial pH values, Mg/P molar ratios and HS concentrations. The composition, morphology and thermal properties of harvested precipitates were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and thermo-gravimetric analysis (TGA), respectively. It showed that inhibition effect of HS reached its maximum value of 48.9% at pH 8.0, and decreased to below 10% at pH > 9.0. The increase of Mg/P ratio enhanced phosphorus removal efficiency, and thus reduced the influence of HS on struvite precipitation. At pH 9.0, the inhibitory effect of initial HS concentration matched the modified Monod model with half maximum inhibition concentration of 356 mg L1, and 29% HS was removed in conjunction with struvite crystallisation. XRD analysis revealed that the crystal form of struvite precipitates was changed in the presence of HS. The morphology of harvested struvite was transformed from prismatic to pyramid owing to the coprecipitation of HS on crystal surface. TGA results revealed that the presence of HS could compromise struvite purity. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Phosphorus (P) is a finite resource, and it was estimated that global reserve life of phosphate rock can only be mined economically somewhere between 50 and 100 years owing to its worldwide application in both industry and agriculture (Jordaan et al., 2010; Muster et al., 2013). In addition to being diminishing resource, P in the discharged wastewater is a threat to surface water quality (Wang et al., 2010; Triger et al., 2012; Liu et al., 2013). Therefore, P recovery from wastewater has gained attention both as a way of pollutant removal and to make the recovered P available for beneficial use. One of the most efficient ways to recover P from wastewater is through simultaneous precipitation ⇑ Corresponding author. E-mail address: [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.chemosphere.2015.06.089 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

of soluble ortho-phosphate (PO3 4 -P) and ammonium nitrogen (NH+4-N) in the formation of struvite (MgNH4PO46H2O). The struvite method has been applied to recover N and P from livestock wastewater (Song et al., 2007; Jordaan et al., 2010; Huang et al., 2011; Foletto et al., 2013; Kim et al., 2014), reject water from sludge digestion and dewatering process (Yoshino et al., 2003; Iqbal et al., 2008; Xu et al., 2011b; Kruk et al., 2014), urine (Triger et al., 2012; Liu et al., 2013), landfill leachate (Kochany and Lipczynska-Kochany, 2009), fertilizer industry (Hutnik et al., 2013), etc. Struvite is a valuable slow-release fertilizer, with higher purity and lower heavy metal content than commercial phosphate fertilizers, and could be directly used for horticulture and agriculture without traditional sludge handling process (Jordaan et al., 2010; Wang et al., 2010). Owing to the high concentration of dissolved organic matters (DOM) in P-rich wastewater (livestock wastewater, anaerobic

Z. Zhou et al. / Chemosphere 141 (2015) 94–99

supernatant, urine, and/or landfill leachate), simultaneous removal of DOM was observed during struvite precipitation (Kochany and Lipczynska-Kochany, 2009; Foletto et al., 2013), and the decrease of DOM was found to be in favor of struvite crystallization (Kim et al., 2014). Humic substances (HS) are a major fraction of DOM in most of P-rich wastewater (Kochany and Lipczynska-Kochany, 2009; Foletto et al., 2013). For instance, HS dominate the organic fraction of mature landfill leachate by as much as 60%, and usually ranged from tens to hundreds mg L1 (Sir et al., 2012). HS contain carboxylic, phenolic, alcoholic, quinone, amino and amido groups, and the presence of these groups results in their abilities of colloid-like adsorption, ionic exchange, complex formation and oxidation/reduction (Lipczynska-Kochany and Kochany, 2008). For P recovery from wastewater, it is necessary to get information about the effect of HS on struvite precipitation. There are a limited number of studies on growth inhibition of struvite crystals by inorganic ions (e.g. Ca2+, K+, CO2 3 ) (Xu et al., 2011b; Muster et al., 2013; Kruk et al., 2014), acetate (Iqbal et al., 2008), acetohydroxamic acid (Downey et al., 1992), phosphorcitrate (Wierzbicki et al., 1997) and herbal extracts (Chauhan and Joshi, 2013). However, to date, interference effects of HS, one of the major fractions in wastewater, on struvite precipitation are scarcely reported in the literature, and it should be very useful for understanding the struvite precipitation in real wastewater rich in DOM. The objective of this study is to investigate the inhibitory effect of HS on the struvite precipitation under different initial pH values, Mg/P molar ratios and HS concentrations. After struvite precipitates were recovered, their composition, morphology and thermal properties were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and thermo-gravimetric analysis (TGA), respectively. The results obtained in this study are expected to provide an insight for P removal by struvite precipitation in the presence of HS. 2. Materials and methods 2.1. Preparation of wastewater Stock solutions were prepared to form artificial wastewater by dissolving an analytical reagent grade of Na2HPO46H2O and NH4Cl chemicals in distilled water to get an initial PO3 4 -P and NH+4-N concentration of 95 and 272 mg L1, respectively. The initial pH was then adjusted to 7.0–12.0 using 0.45 M NaOH or 1 M HCl solutions. 6 g L1 HS solution with fulvic acid P90% was prepared and mixed with artificial wastewater before experiment. Stock solution of 0.6 M MgCl26H2O was added to the solutions in the batch reactors immediately before the experiments were initiated.

95

ratio of 1.2. Six Mg/P ratios (0.2, 0.5, 0.8, 1.0, 1.5 and 2.0) were used to study the effect of Mg/P ratio on struvite precipitation. The initial pH was maintained at 9.0 in the presence (30 mg L1) and absence of HS. Ten initial HS concentrations (6, 12, 18, 24, 30, 36, 42, 48, 54 and 60 mg L1) were used to evaluate the effect of HS concentration on struvite precipitation. All the experiments were performed in duplicate. The initial pH and Mg/P ratio were maintained at 9.0 and 1.2 for the batch test, respectively. 2.3. Inhibition model The inhibition ratio (IR) of HS on P removal efficiency (PRE) is defined as

IR ¼

PRE0  PREi  100% PRE0

ð1Þ

where PRE0 and PREi are the PRE in the absence and presence of HS, respectively, %. The relation between PRE and HS concentration can be formulated by a modified Monod equation (Zhou et al., 2014).

PREi ¼ PRE0

Ki K i þ C HS

ð2Þ

where CHS is the concentration of HS, mg L1; Ki is the inhibition constant of HS, mg L1. The multiplicative inverse of PREi is linear with the concentration of HS.

1 1 C HS ¼ þ PREi PRE0 K i PRE0

ð3Þ

2.4. Chemical and instrumental analyses NH+4-N and PO3 4 -P were measured according to Nessler’s reagent and ammonium molybdate spectrophotometric methods (Chinese NEPA, 2012) using a 2802 UV/VIS Spectrometer (Unico, USA), respectively. The concentration of HS was determined using the modified Lowry method (Xu et al., 2011a). Morphology of the crystals was analyzed using XL30FEG Scanning Electron Microscope (Philips, Netherland). The precipitates were characterized by D8 Advance Powder X-ray Diffractometer (40 kV, 40 mA, step size 0.1°, Bruker Ltd., Germany). XRD diffractograms were evaluated by means of Jade 6.5. TGA was carried out on Netzsch STA 409 analyzer at a heating rate of 10 °C min1 at an air flow rate of 35 ml min1. 3. Results and discussion

2.2. Batch experiments

3.1. Effects of HS on phosphorus removal under different pH values

Batch experiments of struvite precipitation were implemented using a jar tester (ZR4-6, China) at 20 °C. The addition of MgSO4 solution was carried out under continuous stirring at a faster speed (G = 105.0 s1, GT = 94,500) for 2 min to ensure rapid mixing. Then, the stirring rate was maintained at the consigned slow value (G = 105 s1, GT = 82,620) for 15 min. The pH was recorded using an HQ30d portable meter (Hach, USA). A mixed liquor sample of 30 ml was taken from reactors at frequent intervals, and filtered by 0.45 lm cellulose acetate membrane for dissolved compound analysis. The precipitates were washed with deionized water and dried in an oven at 303 K that didn’t influence the nature of precipitates (Foletto et al., 2013). Nine initial pH values (7.0, 8.0, 8.5, 9.0, 9.5, 10.0, 11.0, 11.5 and 12.0) were chosen to compare pH effects on struvite precipitation in the presence (40 mg L1) and absence of HS at an initial Mg/P

Fig. 1 shows the effect of HS on PRE at Mg/P ratio of 1.2 under different initial pH values. In the absence of HS, the PRE increased from 9.7% at initial pH 7.0 to 90.3% at initial pH 9.5, then remained above 90% at initial pH of 9.5–11.5, and then decreased with further increase of initial pH. When pH was below 8, no visible precipitates were observed, and the PRE was very low ( 1.5. The results were consistent with Song et al. (2007), who also found that the PRE increased with the increase of Mg/P ratio. Nevertheless, the Mg/P ratio played an insignificant role on

100 25 80

PRE, HS=0 mg L-1 PRE, HS=30 mg L-1 IR

60

40

20 0.0

15

IR (%)

20

PRE (%)

remained above 82% at initial pH of 9.5–11.5, and then decreased with further increase of initial pH. Two factor analysis of variance showed that the addition of 40 mg L1 HS had a significant inhibitory effect on PRE (p < 0.01). The maximum IR value of 48.9% was achieved at initial pH 8.0, and then gradually decreased with the increase of pH. The IR values were respectively 32.1% and 19.4% at initial pH of 8.5 and 9.0, and all below 10% at initial pH of 9.0–12.0. The results were in agreement with the previously reported inhibition of HS on the precipitation of calcium phosphate, where IR also reached maximum at pH 8.0 and was very small at pH P 9.0 (Song et al., 2006). In the presence of 40 mg L1 HS, the humic substance removal efficiency (HSRE) increased from 8.6% at initial pH 8.0 to 41.9% at initial pH 9.5, and then decreased with further increase of initial pH (Fig. 1b). When pH < 9.5, struvite crystallization was promoted with the increase of pH (Huang et al., 2011), and more HS was absorbed by struvite precipitates. When pH rose from 9.5 to 11.5, the formation of fine flake-like Mg3(PO4)2 occurred instead of struvite (Yang et al., 2011), which resulted in the decrease of HSRE due to the drop of adsorption capacity, although the PRE was maintained above 90%. If the pH continued to rise, PRE and HSRE both decreased because of the generation of gelatin-like Mg(OH)2 (Yang et al., 2011). 2 The major forms of phosphate are H2PO and PO3 4 , HPO4 4 under the pH lower than 9.0, 9.0–12.0 and above 12.0, respectively. At weak alkaline pH (8.0–9.5), carboxyl groups present in the HS are available to interact with positively charged metal ions, with higher affinity for Mg2+ than phosphate in the form of H2PO 4 ; thus the struvite growth is affected by inhibition (Henry and Carole, 1990). The other reason was probably that HS was soluble under alkaline conditions, but had the tendency to form aggregates in solution at neutral and weak alkaline pH (macromolecular pattern) (Baigorri et al., 2009). The macromolecular aggregates were readily coprecipitated and inhibited crystal growth by blocking the active sites of the newly formed nuclei of struvite (van der Houwen and Vaisami-Jones, 2001; Iqbal et al., 2008). When pH > 9.5, the lower inhibitory effect of HS on PRE was attributed to the higher solubility of HS and the lower affinity of carboxyl group for Mg2+ than 3 phosphate in the form of HPO2 4 and PO4 .

10

5 0.5

1.0

1.5

2.0

Mg/P Fig. 2. Effect of Mg/P on the removal of phosphorus (pH = 9.0).

PRE with Mg/P ratio higher than 1.5 (Fig. 2) in this study, which was within the reported range of 1.0–1.6 (Yoshino et al., 2003; Diamadopoulos et al., 2007). The addition of 30 mg L1 HS also showed significantly inhibitory effect on PRE (p < 0.01) under different Mg/P ratios. The PRE increased from 21.6% at Mg/P = 0.2 to 79.7% at Mg/P = 1.5, and then further increased to 86.4% at Mg/P = 2.0. The IR value reached its maximum value (25.5%) at Mg/P = 0.2, then was almost constant (12.4 ± 0.4%) when Mg/P ratio ranged from 0.5 to 1.5, and then dropped to 6.6% at Mg/P = 2.0. At low Mg/P ratio, most Mg2+ was interacted with the carboxyl groups in the HS, and a higher IR value was observed. With the increase of Mg/P ratio from 0.5 to 1.5, more precipitates were generated owing to the increase of PRE, and thus adsorbed more HS on the newly-generated crystal, which resulted in an almost constant IR value. At higher Mg/P ratio of 2.0, the residual Mg2+ was adequate for P removal and thus a lower IR was observed. 3.3. Effect of HS concentration on phosphorus removal The PRE response curve with initial HS concentration at an initial pH 9.0 and Mg/P ratio of 1.2 is shown in Fig. 3a. The addition of 6 mg L1 HS slightly reduced PRE by 2.8%, and then the PRE was inhibited by 4.7% and 6.1% at HS of 18 and 24 mg L1. The IR value further increased from 10.7% at HS of 36 mg L1 to 14.0% at HS of 60 mg L1. Fig. 3b shows there is a significant linear relationship between 1/PRE and initial HS concentration (R2 = 0.9586), indicating that

97

84 82

1.40

12

78

6 76 4

1/PRE

8

1/PRE=0.0034CHS+1.1986

14

R2=0.9586 Removed HS

12

1.35

10

80

PRE (%)

(b)

14

PRE IR

IR (%)

(a)

10 8

1.30

6 1.25

4

74 2

Removed HS (mg L-1)

Z. Zhou et al. / Chemosphere 141 (2015) 94–99

2 1.20

72

0 0

10

20

30

40

50

60

0 0

10

20

30

40

50

60

Initial HS (mg L-1)

Initial HS (mg L-1)

Fig. 3. Effect of HS concentration on the removal of phosphorus and HS (pH = 9.0).

the effect of initial HS concentration on the inhibition of PRE matched the modified Monod model. The HSRE was approximately 29% at pH 9.0 irrespective of its initial concentration. Fig. 3b also shows that there is a positive correlation between removed HS and 1/PRE with correlation coefficient of 0.9376 (p < 0.01). The results indicated that the inhibitory effect of HS on the PRE was owing to the competitive inhibition mechanism by the combination of HS with Mg2+ and blocking function of the HS to the active growth sites of the initially nucleated precipitates (Song et al., 2006). According to Eq. (3), the inhibition constant Ki of HS was 356 mg L1, which was actually the half maximum inhibition concentration (IC50) of HS on PRE. 3.4. XRD and image analyses of the harvested precipitates In order to assess effects of HS on the crystal nucleus formation and growth, the composition and morphology of the precipitated crystals were characterized by XRD and SEM analysis. The XRD patterns of the precipitates obtained in the absence and presence of HS are given in Fig. 4. From Fig. 4 it is seen no significant differences on position of the peaks between the two harvested precipitates in the absence and presence of HS. With Jade 6.5 software to analyze the XRD patterns, all of them have clearly shown a series of quite strong diffraction peaks of 2h at 14.99°, 15.81°, 20.85°, 21.45°, 30.60°, 31.91° and 33.28°, corresponding to Miller indices (1 1 0), (0 2 0), (1 1 1), (0 2 1), (2 1 1), (0 4 0) and (0 2 2), which typically belong to NH4MgPO46H2O crystal structure (PDF-15-0762#). However, the presence of HS changed the relative intensity of crystal diffraction peaks. The relative intensity of lattice plane (1 1 0), (1 1 1) and (2 1 1) tended to be weaker than that of precipitated crystals without HS, and the relative intensity of lattice plane

HS=0 mg L-1

Intensity

HS=30 mg L-1

MgNH4PO4

10

20

30

40

50

60

70

2-Theta (°) Fig. 4. XRD patterns of the harvested precipitates in the absence and presence of HS.

(0 2 1) was strengthened. Therefore, the addition of HS obviously changes structure of the precipitated struvite crystals. During the SEM examination, special attention was paid to the surface of the crystal to illustrate details of the struvite microstructure and to confirm results of the XRD analysis. Fig. 5a is the image of struvite precipitates in the absence of HS. It can be noted that the struvite particles retained prismatic shape, corroborating with those reported by other researchers (Wang et al., 2010; Hutnik et al., 2013; Liu et al., 2013). With the presence of HS, the shape of crystal was frustum of pyramid (Fig. 5b). The pyramid type and other morphologies of struvite (prismatic type, coffin shaped, feather shaped, needle type, etc.) were observed by Chauhan and Joshi (2013), who also found that the morphology of struvite was strongly dependent on growth parameters. The pyramid-type struvite in this study was probably because the addition of HS obscured the active sites of struvite crystals, resulting in the blocking of growth of specific crystal faces (Freche and Lacout, 1992). In order to further understand the crystal morphology, the magnification was increased. Compared to the dense regular crystal surface of porous struvite precipitates in the absence of HS (Fig. 5c), the crystal surface was loose and only few holes were observed in the presence of HS (Fig. 5d). In Fig. 5d, there were also more nucleations, leading to a rougher surface than the precipitates in the absence of HS. The phenomena further proved the blocking of active sites of the newly formed nuclei of struvite by adsorption of HS on crystal surface.

3.5. TGA of the harvested precipitates TG analysis was done for the struvite obtained at pH 9.0 to validate the results obtained from XRD and SEM analysis (Fig. 6). At around 100 °C, water was removed from hydrated products (Katsioti et al., 2008). The theoretical mass loss for the struvite formula (MgNH4PO46H2O) upon heating is 51.42%, which is composed of mass losses of water (contributing 44.08%) and ammonium (contributing 7.34%) (Bhuiyan et al., 2008; Wang et al., 2010; Foletto et al., 2013). In the absence of HS, the mass loss of struvite precipitates (53.63%, Fig. 6a) matches well with the theoretical and the reported values (52.49% (Wang et al., 2010); 51% (Bhuiyan et al., 2008)). The mass losses were 56.85% and 57.78% for samples obtained at HS concentration of 30 and 60 mg L1, respectively. The observed mass losses for samples in the presence of HS were higher than the pure struvite, indicating the coprecipitation of HS with struvite by adsorption on crystal surface and the interaction with magnesium ions on the surface. At HS of 30 and 60 mg L1, the theoretical yield of MgNH4PO46H2O was 590 and 540 mg L1 according to PRE values in Fig. 3a, and the removed HS (Fig. 3b) accounted for 1.72% and 2.69% in the coprecipitate.

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(b) HS=60 mg L-1

(a) HS=0 mg L-1

(d) HS=60 mg L-1

(c) HS=0 mg L-1

Fig. 5. SEM image of the harvested precipitates in the absence and presence of HS.

100

crystal surface. TGA results revealed that the presence of HS could compromise struvite purity.

HS=0 mg L-1 HS=30 mg L-1

90

HS=60 mg L-1

Acknowledgement

Mass/%

80 70

This work was financially supported by Chinese National 863 Program [2012AA063403].

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4. Conclusions The present study demonstrated that HS had a significant inhibitory effect on P removal by struvite precipitation. Inhibition ratio of HS reached its maximum value of 48.9% at pH 8.0, and was below 10% at pH > 9.0. The increase of Mg/P ratio could enhance PRE, and thus reduce the influence of HS on struvite precipitation. At pH 9.0, the inhibitory effect of initial HS concentration on PRE matched the modified Monod model with half maximum inhibition concentration of 356 mg L1, and 29% HS was removed in conjunction with struvite crystallisation. XRD analysis revealed that the crystal form of struvite precipitates was changed in the presence of HS. The morphology of harvested struvite was transformed from prismatic to pyramid owing to the coprecipitation of HS on

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