Effects of Ni(2+) on aluminum hydroxide scale formation and transformation on a simulated drinking water distribution system.

Chemosphere xxx (2014) xxx–xxx

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Effects of Ni2+ on aluminum hydroxide scale formation and transformation on a simulated drinking water distribution system Wendong Wang a,c,⇑, Shan Song a, Xiaoni Zhang a, J. Mitchell Spear d, Xiaochang Wang a, Wen Wang a, Zhenzhen Ding b, Zixia Qiao a a

Department of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China School of Environmental Science and Engineering, Chang’an University, Xi’an 710064, China Department of Environmental Technology and Ecology, Yangtze Delta Region Institute of Tsinghua University, Zhejiang, Jiaxing 314006, China d Department of Environmental Engineering, The Pennsylvania State University-Harrisburg, Middletown, PA 17057, USA b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

2+

Without Ni

addition, there was no XRD signal observed after 400 d of aging. In the system with Ni/Al = 1:100, the formation of bayerite occurred within 3 d. In the system with Ni/Al = 1:3, the XRD spectrum strength of bayerite became weak. In the system with Ni/Al > 1:1, Ni5Al4O1118H2O was the major scale component. The adsorption of Al(OH)3 (am) for Ni2+ promoted the formation of Ni(OH)2 (c).

a r t i c l e

i n f o

Article history: Received 21 March 2013 Received in revised form 28 November 2013 Accepted 18 December 2013 Available online xxxx Keywords: Aluminum hydroxide Drinking water Pipe scale Water quality

a b s t r a c t Observations of aluminum containing sediments/scales formed within the distribution pipes have been reported for several decades. In this study, the effect of Ni2+ on the formation and transformation processes of aluminum hydroxide sediment in a simulated drinking water distribution system were investigated using X-ray diffraction spectrum (XRD), Fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), and thermodynamic calculation methods. It was determined that the existence of Ni2+ had notable effects on the formation of bayerite. In the system without Ni2+ addition, there was no X-ray diffraction signal observed after 400 d of aging. The presence of Ni2+, however, even when present in small amounts (Ni/Al = 1:100) the formation of bayerite would occur in as little as 3 d at pH 8.5. As the molar ratio of Ni/Al increase from 1:100 to 1:10, the amount of bayerite formed on the pipeline increased further; meanwhile, the specific area of the pipe scale decreased from 160 to 122 m2 g1. In the system with Ni/Al molar ratio at 1:3, the diffraction spectrum strength of bayerite became weaker, and disappeared when Ni/Al molar ratios increased above 1:1. At these highs Ni/Al molar ratios, Ni5Al4O1118H2O was determined to be the major component of the pipe scale. Further study indicated that the presence of Ni2+ promoted the formation of bayerite and Ni5Al4O1118H2O under basic conditions. At lower pH (6.5) however, the existence of Ni2+ had little effect on the formation of bayerite and Ni5Al4O1118H2O, rather the adsorption of amorphous Al(OH)3 for Ni2+ promoted the formation of crystal Ni(OH)2. Ó 2013 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: Department of Environmental and Civil Engineering, Xi’an University of Architecture and Technology, No. 13, Yanta Road, Xi’an, China. Tel.: +86 135 7254 7081; fax: +86 29 8220 2729. E-mail address: [email protected] (W. Wang). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.12.045

Please cite this article in press as: Wang, W., et al. Effects of Ni2+ on aluminum hydroxide scale formation and transformation on a simulated drinking water distribution system. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2013.12.045

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W. Wang et al. / Chemosphere xxx (2014) xxx–xxx

1. Introduction Aluminum salts such as alum and poly-aluminum chlorine (PACl) are widely used as coagulants in drinking water treatment processes. Although they are effective in removing color, turbidity, particles, and natural organic matter (NOM) from the raw water, Al-based coagulants may result in high concentration of aluminum residual in the treated water (Letterman and Driscoll, 1988; Sollars et al., 1989; Cui et al., 2002; Wang et al., 2010). A survey conducted by Zimmerman (1986) found that the concentration of aluminum residual in the treated water ranged between 14 and 2670 lg L1. In addition, aluminum also may enter drinking water from the raw water and the dissolution of cement/asbestos cement pipes (Snoeyink et al., 2003). Driscoll and Letterman (1988) found that approximately 11% of the aluminum input would remain in the treated water. High aluminum content in drinking water can cause elevated turbidities (Srinivasan et al., 1999; Kvech and Edwards, 2001), lower disinfection efficiency (Letterman and Driscoll, 1988), and increase many health problems (Glynn et al., 1995; Berthon, 2002; Szatanik-Kloc and Jozefaciuk, 2007). In order to control the content of aluminum in drinking water, various strategies have been proposed. Letterman and Driscoll (1988) found that lower the pH during coagulation (6.50–7.00) combined with more efficient filtration could reduce aluminum residual effectively. This strategy is currently the most widely accepted in treatment plants using Albased coagulants addition. The solubility of aluminum salt at pH 6.5–7.5 is relatively low, which is good for its removal in coagulation and subsequent filtration process (Zimmerman, 1986; Jekel et al., 1991; Wang et al., 2007). However, it should be noted that the formation rates of aluminum containing sediments are also slow. Months or years are usually required to reach thermodynamical equilibrium (Snoeynink et al., 2003), leading to the treated water supersaturated with solids such as amorphous Al(OH)3 (Baylis, 1953), aluminosilicates (Hem et al., 1973; Swaddle, 2001), and aluminum phosphates (Havics, 2001). These minerals will precipitate gradually throughout the distribution process of drinking water, and accumulate a layer of scale in the pipeline (Fuge et al., 1992; Shea, 1993; Kriewall et al., 1996; Havics, 2001; Wang et al., 2010). The existence of aluminum containing minerals, especially Al(OH)3 will affect the distribution and transformation of many pollutants in drinking water distribution systems. Lauer and Lohman (1994) speculated that Alsilicate deposits were responsible for the low lead concentrations in the pipe water. Schock and Holm (2003) found that similar to those found in soils and environmental sediments, aluminum containing minerals formed in the pipeline were able to accumulate trace metals through sorption or co-precipitation. Conversely, the existence of pollutants can also affect the formation and composition of aluminum containing sediment/scale. Hem et al. (1973) studied the effects of silica on the precipitation of aluminum from water, and found that the presence of even small amounts of silica could considerably slow down the Al(OH)3 crystallization process. Doucet et al. (2001) also found that the

concentration of silicate was the primary factor that affects the solubility of aluminum by the secondary mineral phase formation. Exley and Birchall’s (1992, 1993) experimental results further suggest that silicic acid could either inhibit the nucleation of Al(OH)3 or inhibit the aggregation of small particles. Different from silica acid, the presence of orthophosphate will change the structure of amorphous Al(OH)3 via co-precipitation or adsorption on its surface (Goldberg et al., 1996), and cause the formation of more aluminum containing solid. Goldschmid and Rubin (1988) showed that the composition of the solid depended on the molar ratio of Al=PO3 4 , and the presence of certain anions such as sulfate will enhance the precipitation rate of amorphous Al(OH)3. Nickel is one of many trace metals widely distributed in the environment. Elevated nickel levels may exist in drinking water as a result of the corrosion of nickel-containing alloys used for pipes and other components as well as from nickel-plated fittings in the distribution process. Mansour and Hasieb (2012) proved that Al(OH)3 precipitation formed during coagulation exhibited a good adsorption capacity to Ni2+. Both Zhao et al. (2004) and Li et al. (2010) found that the existence of Al could change the structure of Ni(OH)2 through partial substitution of nickel ion in the nickel hydroxide lattice. While the effects of Ni2+ on aluminum sediment/scale formation and transformation has not received any attention. The major objectives of this paper were to (1) study the effects of Ni2+ on the formation of Al(OH)3 sediment/scale in the pipeline; (2) investigate the structure transformation characteristics of Al(OH)3 with aging time; and (3) better understand the formation and transformation process of Al(OH)3 in the presence of Ni2+ in drinking water.

2. Experimental materials and methods Experiments were conducted in a simulated water supply modeling system (Fig. 1). Distribution water was prepared from deionized water by adding 0.50 M NaOH, 0.50 M HNO3, 1000 mg L1 Al2(SO4)3, and 50.0 mg L1 Ni(NO3)2 into the tank in which a stirring device was fixed to guaranty complete mixing. The synthetic water was pumped from a water tank into the distribution system using a constant flow pump (BT-102, Zhisun Instrument, China). To exclude the effects of pipe corrosion on concentrations of trace metal elements, polyvinyl chloride pipes were used. The water quality was simulating the range of actual drinking water distribution systems. To accelerate the formation of Al(OH)3 scale, alum dosage in the system was maintained at approximately 20.0 mg L1. The concentration of Ni2+ in the pipe water was adjusted according to its molar ratio to aluminum. To insure that the added metal elements were fully mixed with Al(OH)3 scale, the velocity of the simulated distribution water was maintained at 0.5 mL s1, much lower than the designed value in drinking water distribution systems. Reagent grade chemicals were used except where noted. Water temperature was controlled at 15 °C by placing the tank within an incubator (SPX-250B, Tianjin Taisite Instrument, China).

Reagents

Pipe effluent

Pump

Water tank

Pipe system

Return to the tank

Fig. 1. Water supply modeling system used in the experiment.


3


3. Results and discussion Experiments were conducted applying synthetic water with pH and temperature maintained at 8.5 and 15 °C, respectively. The effects of Ni2+ on the formation of aluminum containing pipe sediment/scale were studied after 13 d of operation. It was determined that in the system without Ni2+ addition, no signal was observed in the XRD spectrum of the pipe scale, suggesting that they were in an amorphous form. In the system with Ni2+ addition, however, the scales formed on the pipe surface were quite different. With Ni/Al ratio at 1:100, spectrum peaks contributed by bayerite were observed, indicating the presence of Ni2+ even when in a small amount could promote the formation of bayerite or the transformation of amorphous Al(OH)3 to bayerite. Increasing the ratio of Ni2+ from 1:100 to 1:10, the amounts of

1800

(a)

1:100

1500

1:10

Intensity

1200

1:3 900

1:1 600

3:1 300

3:0 0

θ

60

80

1064

1455

1289

3:0

(b)

839

40

621 532 450

20

3:1 3649

1.6

1386

1:1 1.2

1:10

527

730

975 1020

3420

3545

0:3 0.4

774

539

975

0.8

737 620 539 436

1386

971 920

1:3

3420

Transmittance /%

To investigate the formation mechanism of bayerite, the water supply modeling system was originally operated with the synthetic water containing 20.0 mg L1 aluminum. After the formation of pipe sediment/scale, the aluminum addition was removed and only 4.0 mg L1 Ni2+ was added. Drinking water and pipe sediment/scale samples were taken simultaneously at different time intervals. Water samples were obtained directly from the water tank. Aluminum containing pipe scale/sediment samples were obtained from both the bottom of the water tank and the pipe interface. As the sediments formed in system were not tightly bound to the pipe surface, samples could be easily obtained by scraping or filtrating (tank sediments), and then dried naturally under atmospheric conditions for static adsorption experiments and structure analysis. Static batch adsorption experiment was examined to determine the adsorption capacity of the pipe scale formed under different conditions. 0.05 g of air-dried pipe scales (obtained from the modeling water distribution systems with a Ni/Al molar ratio of 1:10) were placed into 250 mL tubes which were stirred using a magnetic stirrer (JZG9-S-6A, Xihuayi Technology Corporation, China). Then, 50.0 mg L1 Ni(NO3)2 were added after 0.0, 6.0, and 14.0 min of adsorption to maintain the total Ni2+ inputs at 0.1, 0.5, and 1.0 mg L1, respectively. The mixtures were sampled before and after the increase of adsorbate contents and centrifuged at 5000 rpm for 20 min to analyze the concentration of Ni2+ residual in the solution. Meanwhile, Ni(II) speciation including the solids that might precipitate from solution was also calculated applying the VISUAL MINTEQ software, by defining concentrations of all components, pH, water temperature, and ion strength. All conditions were the same as that controlled in the experiments. The concentrations of aluminum and nickel in the distribution water were determined using an ICP-AES (IRIS intrepid-II, Thermo Scientific, USA). Sample aliquots were digested with trace metal grade nitric acid to pH < 2 for 12 h prior to analysis. pH was determined using a pH meter (Model 828, Thermo Orion, USA), temperature was determined with a thermometer (Model TTM1-JM6200IM, Yuan-Da Technology Corporation, China). To observe the surface characteristics of aluminum containing pipe scale, scanning electron microscopy (SEM) (JSM-6490LV, JEOL Ltd., Japan) and an X-ray diffractometry (Ultima IV, Rigaku Corporation, Japan) were applied. Infrared analysis was carried out with a Bruker IFS 55 spectrometer in transmission mode. 200 scans collected at 2 cm1 resolution in the 4000–400 cm1 range. The SEM samples were sputter-coated with gold before conducting surface scanning and elemental component analysis. Specific area was measured using a Micromeritic Gemini model VII 2390 and an adsorption volumetric system with a pressure transductor (MKS Baratron 170 M). 100 mg of the sample was kept under N2 flow at 60 °C for 4 h. The assays were performed with N2 as the adsorbate at 77 K.

0.0 3500

3000

2500

2000

Wavenumber/cm

1500

1000

500

-1

Fig. 2. X-ray powder diffractogram (a) and FT-IR spectra (b) of pipe scale formed in solutions with different Ni/Al molar ratios after 13 d of operation at pH 8.5. } theophrastite (Ni(OH)2), N nickel aluminum oxide hydrate (Ni5Al4O1118H2O), and d bayerite intensity peaks.

bayerite accumulated on the pipeline increased accordingly (Fig. 2a). While in the system with Ni/Al molar ratio at 1:3, the signal strength corresponding to the presence of bayerite became weaker. However, the signals contributed by nickel aluminum oxide hydrate (Ni5Al4O1118H2O) indicating greater stability compared to bayerite in the system with high Ni2+ concentration (reaction 4 Scheme 1). In the system with Ni/Al molar ratio above 1:1, the spectrum signals of bayerite were absent; while the signal strength contributed by Ni5Al4O1118H2O increased. To complement the XRD measurements, FT-IR spectra for the scales formed under the same experimental conditions were made and the results were shown in Fig. 2b. In the curve formed by amorphous aluminum hydroxide, the broad and smooth absorption bands in the ranges of 400–839 and 3200–3700 cm1 related respectively to the Al–O and O–H stretching vibration (Meher et al., 2005) reveal the formation of amorphous structures. The two weak bands at about 1630 cm1 and 1386 cm1 are due to the angular deformation of adsorbed water molecule and stretching vibrations of adsorbed anions, respectively (Shek et al., 1997).


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W. Wang et al. / Chemosphere xxx (2014) xxx–xxx + Ni2+

(1) Ni2+

Al(OH)3 (am)

Ni(OH)2 (C)

Al(OH)3

Boehmite Bayerite Al(OH)3(am) Ni(OH)2(am) Ni(OH)2(c)

-2

hydrolysis

pH =6.5~7.5

+ (2) Ni2+ + 2OH-

Ni(OH)2 (C) pH >8.5

ng agi

Al(OH)3 (am)

(6)

-4

Log C

+ Bayerite

+ pH

5 >7. pH

pH >7.5

>8 .5 (5 )

(4)

Al3+ + 3OH-

N i

-6

2+

Al Ni/

:3 1:3

-10

Scheme 1. Proposed reaction processes for the formation of bayerite, Ni5Al4O11 18H2O, and Ni(OH)2 crystal in solutions at different pH and Ni/Al molecular ratio.

5

6

7

8

9

pH Fig. 3. Log C versus pH in solutions with possible solids formed in the pipe line as calculated using Visual MINTEQ software. d aluminum, and } nickel determined in the modeling drinking water distribution system with Ni/Al ratio at 1:10.

1.0 2+

-1

2+

Ni concentration/mg L-1

Ni dosage 0.1 mg L

-1

2+

Ni dosage 0.5 mg L

-1

Ni dosage 1.0 mg L

0.8

0.6 pH8.5 pH7.5 pH6.5

0.4

0.2

(a) 0.0 5

10

15

20

Reaction time/min

Soluble Al concentration/mg L-1

For the solid formed in the system with Ni:Al = 1:10, the band at 527 cm1 related to the stretching mode of Al–O in the octahedral (Barroso et al., 2006), the bands at 730 and 774 cm1 related to the symmetric stretching vibration of the AlO4 tetrahedron (Barroso et al., 2006), the bands at 975 and 1020 cm1 related to bending vibrations of O–H, and the bands at 3545, and 3420 cm1 related to characteristic stretching vibrations of O–H (Meher et al., 2005) can be observed, indicating the formation of bayerite. When the Ni/Al ratio were 1:1 and 3:1, however, the FT-IR spectra (especially at low wavenumbers) of the solids were quite different. The newly formed bands at 436, 539, 620, 737, and 920 cm1 corresponding to the existence of nickel aluminum oxide hydrate (Fig. 2a). For the system only with Ni, the couple bands at about 524 cm1 and 465 cm1 are associated with Ni–O–H bending and Ni–O stretching vibrations respectively, and the sharp band at about 3649 cm1 correspond to the stretching vibrations of hydroxyl groups in the nickel hydroxide lattice (Ramesh and Vishnu Kamath, 2006). Observations of aluminum containing solids on the pipe wall of drinking water distribution systems had been reported for several decades. An extensive review of the literature conducted by Snoeyink et al. (2003) showed that aluminum was frequently a major metal in the composition of the lead pipe scales. However, nearly all of the literatures did not observe the existence of crystalline aluminum trihydroxides including gibbsite, bayerite, and nordstrandite, indicating that its conversion from amorphous to crystal state requires a long aging time. Without any other co-existing ions, the formation of bayerite was only observed after a 400 d of aging. In the existence of Ni2+, however, the formation process of bayerite could complete within 13 d (Fig. 2a). To verify if the newly formed bayerite was converted from amorphous Al(OH)3 or deposited directly from the supersaturated drinking water, the system was initially operated without Ni2+. After the formation of pipe scale, the water was changed to include 4.0 mg L1 of Ni2+. It was determined that amorphous Al(OH)3 did not convert to bayerite after 13 d of aging, suggesting that the presence of Ni2+ could not accelerate the transformation of amorphous Al(OH)3 to bayertie but promoted the formation of bayerite directly (reaction 3 Scheme 1). The concentrations of Ni (II) equilibrium with amorphous and crystal Ni(OH)2 were calculated using Visual MINTEQ software. It was determined that Ni2+ did not precipitate in the form of amorphous or crystal Ni(OH)2 at pH 7.5; while, the determined concentration of Ni2+ residual in drinking water was much lower than the applied dosage, indicating that the Ni2+ was partially adsorbed by the pipe scale (Fig. 3). The contribution of amorphous Al(OH)3 on the removal of Ni2+ was further investigated in a static shaking flasks experiment. Increasing Ni2+ input from 0.1 to 0.5 mg L1

(b)

0.4

0.3

0.2

0.1

0.0 0

2

4 2+

6

-1

Total Ni dosage/mg L

Fig. 4. The concentrations of (a) Ni2+ residual with adsorption time, and (b) aluminum residual in drinking water with total Ni2+ dosage at pH 7.5.

after 6 min of adsorption, the residual concentration of Ni2+ increased from 0.02 to 0.15 mg L1 immediately (Fig. 4a), further supporting that the adsorption of Ni2+ on Al(OH)3 was a fast process. Most of the Ni2+ input would be adsorbed or co-precipitated in the formation process of amorphous Al(OH)3 scale. Compared with the solution at pH 6.5, residual concentrations of Ni2+ were higher at pH 7.5 and 8.5, indicating that the adsorption capacity of Al(OH)3 for Ni2+ was weaker under basic conditions. In solutions at pH > 7.5, however, the surface of amorphous Al(OH)3 has a neutral or net negative charge (Parks, 1967), which should promote



the adsorption of Ni2+. The weak adsorption capacity of Al(OH)3 as suggested in our research might be related to the formation of bayerite or other aluminum containing crystals under the basic conditions tested. Compared with Ni2+, the concentration of soluble aluminum in drinking water was mainly influenced by the structure of the pipe sediment/scale. Without the presence of Ni(II), the concentration of soluble aluminum was relatively high, about 0.41 mg L1 (Fig. 4b). In the drinking water with 0.2 mg L1 Ni2+ addition, its concentration decreased to 0.35 mg L1 at the same aging time. When the concentration of Ni2+ was 2.0 mg L1 (Ni/Al = 1:20), the concentration of soluble aluminum decreased further (0.1 mg L1). Basing on thermodynamic calculation results using Visual MINTEQ software, the maximum concentration of amorphous Al(OH)3 dissolved at pH 7.5 and 15 °C was approximately 0.22 mg L1 (calculated as Al), further suggesting the existence of bayerite. In fact, although the concentrations of soluble aluminum was higher than 0.22 mg L1, XRD experimental results also support the formation of bayerite in the system with Ni2+ dosage less than 1.2 mg L1 (Fig. 2a). However, as amorphous Al(OH)3 was the predominant phase and its low precipitation reaction constant, soluble aluminum was in a supersaturated condition. In the systems with Ni2+ dosage above 2.0 mg L1, the determined concentration of aluminum was lower than the dissolved equilibrium value of amorphous Al(OH)3, but still higher than that of bayerite and nickel aluminum oxide hydrate. To observe the surface characteristics of the aluminum containing solids, scanning electron microscopy pictures of the pipe sediment/scale were observed (Fig. 5). The system without Ni2+

5

addition, the pipe scale had a porous structure and with a large specific surface area of 160 m2 g1. In the system with Ni/Al molar ratio at 1:10, large amounts of triangular and rectangular crystals were found; while the specific area of the pipe scale decreased to 122 m2 g1. De Vicente et al. (2008) also found that the removal of orthophosphate from solutions by Al(OH)3 sorption decreased gradually upon Al(OH)3 aging. Combining the XRD spectrum obtained under the same water quality condition, the crystal mineral formed in the pipe line was determined to be bayerite (Fig. 2a); while the amorphous phase mainly contributed by Al(OH)3 still constituted more than approximately 80% of the total solids. Yoldas (1973) found that the hydrolysis of aluminum in the cold water (20 °C) would produce a hydroxide which was largely amorphous and contained boehmite and scattered small crystals of bayerite. Large triangular and rectangular single crystals of bayerite start to form within a few hours of aging; and after 24 h of aging bayerite became the predominant phase. While Yoldas (1973) found the formation of bayerite was mainly converted from amorphous Al(OH)3, our results do not support this reaction pathway. The effects of aging time on the composition of pipe scale were investigated fixing the Ni/Al ratio at 1:10. It was determined that at the initial time of operation, only crystalized Ni(OH)2 was detected in the system at pH 7.5 (Fig. 6a). However, after 3 d of operation, the spectrums contributed by crystal Ni(OH)2 decreased, which increased the bayerite forming within the pipe scale. However, from both its XRD spectrum strength and the SEM results, the percentage of bayerite in the pipe scale was minimal. With the exception of crystal Ni(OH)2, there were no other detectable spectrum of bayerite or other aluminum hydroxide minerals observed at pH

(a)

(b)

(c)

(d)

Fig. 5. Aluminum hydroxide scale formed in the pipe line from the synthetic drinking water without Ni2+ addition after 13 d (a) and 400 d (d) operation, and from the synthetic drinking water with 4.0 mg L1 Ni2+ addition after 13 d operation (b) and (c).


6


(a)

bayerite Ni ( OH ) 2

750

Intensity

23 d

500

13 d

3d

250

0

0d

20

40

60

80

θ

(b)

bayerite Ni ( OH )2

750

Many studies showed that aluminum commonly exists in an amorphous state in the scale of drinking water distribution systems (Baylis, 1953; Costello, 1984; Havics, 2001). The authors also found that the presence of phosphate, silicate acid, calcium, and magnesium would inhibit the formation of bayerite (not shown in the paper). This might be one of the major reasons that led to aluminum existing predominantly in the amorphous states within the pipe scale. However, in the system even with very low concentrations of Ni2+, the formation rate of bayerite was enhanced (reaction 3 Scheme 1). Similar experimental results had not been observed in the systems coexisting with other trace metal elements, except Ni2+ including iron and cobalt which has the same periodicity as nickel. The reaction mechanism between Ni(II) and Al(III) was still not clear based on present experimental results. Notwithstanding its limitation, the results obtained in this research does prove that bayerite, Ni5Al4O1118H2O, and other aluminum containing crystals can form in drinking water distribution systems in a short time of operation and will permit proper water treatment adjustments with respect to aluminum control and drinking water distribution system maintenance.

Intensity

pH 8.5

500

pH 7.5

4. Conclusions

250 pH 6.5

0

20

40

60

80

θ Fig. 6. X-ray powder diffractogram of aluminum hydroxide scales formed in the solution with Ni/Al = 1:10 (a) at different aging time at pH 7.5 and (b) at different pH condition after 3 d of operation.

6.5 within in the experimental period tested (Fig. 6b). In contrast to the experiments tested at pH 6.5 and 7.5, the strength of the spectrum contributed by bayerite was much higher at pH 8.5. This was especially evident after 23 d of aging suggesting that the formation of bayerite was promoted under basic conditions. Sato (1962) also found that the transformation of bayerite was slow in acidified waters, but more rapidly in alkaline solution. Based on thermodynamic calculation results, the total concentration of Ni2+ added in drinking water (4.0 mg L1) much lower than the saturate values in the presence of crystal Ni(OH)2 at pH 6.5 and 7.5, approximately 2000 and 22 mg L1 respectively (Fig. 3). The formation of crystal Ni(OH)2 might occur during the hydrolysis of the adsorbed Ni2+ (reaction 1 Scheme 1). In solutions at pH below 7.5–8.5, the surface of amorphous Al(OH)3 has a net positive charge (Parks, 1967), where precipitation using cation exchange reactions proved difficult. The adsorption of Ni2+ on Al(OH)3 was mainly contributed by its large surface energy. Although there were still a large amount of Ni2+ adsorbed by Al(OH)3, their combining force was weak. Wang et al. (2012) also found that because of H+ inhibition, the adsorption of Mn2+ on Al(OH)3 changed gradually from chemical coordination to physical adsorption at pH < 7.5. The weak adsorption force of amorphous Al(OH)3 for Ni2+ made the formation of crystal Ni(OH)2 possible under the effect of hydroxylation. In the system at pH 8.5, the dosage of Ni2+ was higher than the thermodynamic equilibrium value of crystal Ni(OH)2 (Fig. 3); it could precipitate directly at pH 8.5 (reaction 2 Scheme 1); while its XRD signal disappeared after 3 d, which suggested the conversion of process of crystal Ni(OH)2 and bayerite to Ni5Al4O1118H2O.

Aluminum salts are widely used as coagulants in drinking water treatment process. Although they are effective in removing color, turbidity, particles, and natural organic matter (NOM) in the raw water, observations of aluminum containing solids formed in the pipe s of the distribution systems have been reported for several decades. The existence of aluminum containing minerals, especially aluminum hydroxide sediments were often reported to affect the transportation of many pollutants in drinking water distribution systems; however this affect has not received much attention in present literatures. Applying a novel approached of using XRD/ SEM observation and thermodynamic calculation, the effects of Ni2+ on the formation process of Al(OH)3 scale were investigated in this study. In the system without Ni2+ addition, no XRD signal was observed. Solid scale formed on the pipes was mainly in a porous structure; its specific surface area was about 160 m2 g1. However in the presence of minimal Ni2 the formation of bayerite in approximately 3 d at pH 8.5 was promoted. Further increasing the molar ratio of Ni/Al from 1:100 to 1:10, the amounts of bayerite formed on the pipeline increased notably. Meanwhile the specific area of the scale formed decreased to 122 m2 g1 suggesting this effect was caused by a difference in crystalline structure formed during these different molar ratios. In the system with Ni/Al molar ratio at 1:3, the XRD signal strength contributed by bayerite became weak, and when the Ni/Al molar ratio was higher than 1:3, the spectrum signals of bayerite could not be observed, while the Ni5Al4O1118H2O became the major component in the pipe scale. Without Al(OH)3 co-existing, Ni2+ would not precipitate from solution either in the form of amorphous or crystal Ni(OH)2 at pH 6.5–7.5. In the system with amorphous Al(OH)3, however, would promote the formation of crystal Ni(OH)2, which was likely formed by hydrolyzation of the Ni2+ adsorbed on Al(OH)3 sediment/scale. Meanwhile, the presents of Ni2+ had minimal effects on the formation of bayerite or Ni5Al4O1118H2O especially under acidic condition. Compared with Ni2+, the concentration of soluble aluminum in drinking water was mainly controlled by the major component of the pipe scale, and the formation of bayerite and Ni5Al4O1118H2O was promoted under basic condition. However, their formation mechanisms should be studied further in systems coexisting with Ni2+.



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