Formation and characterization of struvite crystals grown through gel

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Feb 1, 2015 - spectrum revealed the presence of N-H and P-O bonds, NH4+ ion and PO43- ion, water of hydration and Oxygen-metal bond. The EDX test ...

Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

Formation and characterization of struvite crystals grown through gelgrowth technique in gelatin

Article Info Received: 13th November 2014 Accepted: 14th January 2015 Published online: 1st February 2015

A. Salsabili1,a, Mohamad Amran Mohd. Salleh2, A. Idris1, Robiah Yunus2, Suraya A. R.1 1

Department of Chemical and Environmental Engineering, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia. 2

Institute of Advanced Technology, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia. a

[email protected]

ISSN (Online): 2232-1179 I ISSN (Print): 2314-8101

© 2012 Design for Scientific Renaissance All rights reserved

ABSTRACT Magnesium ammonium phosphate (MgNH4PO4.6H2O) which is commercially known as struvite is one of the most fascinating fertilizers which has attracted significant attention of the researchers due to its high possibility of occurrence in vast variety of environments. Struvite crystals were also found as urinary calculi in both animals and humans kidneys. Struvite crystals were grown in gelatin employing both double and single diffusion technique in order to simulate the real conditions of kidney in vitro. Struvite crystals within the size range of 0.1 mm to 2.5 mm were grown with different operational conditions and completely characterized. The combination of FT-IR and EDX plus SEM tests proved the harvested crystals are struvite crystals. Comprehensive comparison between the produced crystals revealed that the crystals resulted from double diffusion tests are bigger and their number density also outnumbers that of crystals produced with the single diffusion technique. Keywords: Magnesium ammonium phosphate, Struvite crystallization, Gelatin, Struvite crystals characterization, Fertilizer

Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

1. Introduction 1.1 Struvite Magnesium ammonium phosphate which is better known as struvite (MgNH4PO4.6H2O) has been widely investigated by researchers worldwide due to several reasons. It has been reported its spontaneous precipitation in wastewater treatment plants where nutrient-richen wastewaters are treated has caused damages to the pipelines and joints and decreased the treatment efficiency as well (Doyle & Parsons, 2002). Struvite is also known as one of the major findable stones in human and animal kidneys. There are several factors known to have influence on struvite growth and some researches have been carried out both to review (Le Corre, Valsami-Jones, Hobbs, & Parsons, 2009; Salsabili, Salleh, Zohoori, & Khosrowabadi, 2014; Salsabili, Salleh, Amran, & Zohoori, 2014) and to measure the effect of these operational parameters on the struvite size, morphology and purity through laboratory works (Wang & Burken, 2005). Struvite removal from wastewater streams is considered as a sustainable approach of wastewater treatments as this method not only removes the nutrients from the stream but also recovers them in the form of valuable marketable by-product which can be sold separately and earn some of the costs of the treatment plants (Doyle & Parsons, 2002). Struvite has the privilege of being released gradually in the environment and at the same time contains some of the plants growth requirements like phosphorous in the form of phosphate ion (Ali, 2005; Le Corre et al., 2009; Münch & Barr, 2001). Struvite spontaneous precipitation happens when the ambient water has the pH of 7 or higher (alkaline), the amount of minerals like magnesium is high and the stream is enriched with nutrients (like ammonium and phosphate) (Battistoni, Pavan, Prisciandaro, & Cecchi, 2000; Galbraith & Schneider, 2009; Le Corre et al., 2009). The favourable conditions in vivo are also the same, meaning high pH, urea existence in urine and high level of minerals. 1.2 Kidney calculi Struvite occurrence as one of the components of urinary calculi has been reported in some researches (Chauhan & Joshi, 2013). Worldwide, struvite accounts for 30% and 75% of all human and animal kidney stones respectively (Chauhan & Joshi, 2013, 2014). Struvite which is considered as one of the major constituent in renal calculi occurs as crystallites in both human and animal urine and grows as a type of kidney stone. The terms urease stone or urine sand are normally used to better (Chauhan & Joshi, 2013, 2014) describe struvite calculi as the presence of microorganisms producing urease like bacteria and yeast are requisite for the for their formation. The current study provides the researchers with the information about the effects of different parameters on the growth of struvite crystals in vitro conditions. Through this study the growth inhibitors can be recognized to have a better understanding of what happens in human and animal kidneys and probably spot the parameters playing the most important role in the growth of struvite crystals.

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Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

1.3 Gel-growth technique The term gel growth technique refers to the growth of any types of crystals in the gel medium where the reactants are supposed to react in the gel medium (Henisch, 1986; Irusan, Arivuoli, & Ramasamy, 1990). In other words the reaction zone is located inside the gel and one of the reactants which mostly its supernatant form is used would be diffused in the gel which already contain foreign ions (mostly single diffusion) or is plain gel (only double diffusion). In this technique the growth happens as the results of reaction between the reactants or by achieving supersturation by diffusion in gel medium. This technique is mainly suitable for growing compounds which are fairly soluble in pure water and decompose at low temperatures. Phosphorous ions are mostly insoluble in pure water and this technique is recognized as a suitable technique to have the phosphate ions precipitated and crystallized while other techniques like evaporation and melt technique cannot be used. The similarity of gel to the mucus of living organisms (Chauhan & Joshi, 2013) make it an ideal medium for growing biological crystals. The other benefit of this method is that the gel supports the crystals to suspend and grow in all dimensions. As the chemicals react step by step (not all at the same time) the crystals grow more slowly compare to precipitation techniques. This benefit is the main difference of precipitation and crystallization meaning to control the number of precipitates and improve and increase the crystallization of the existing tiny precipitates in a way to enhance their quality and size (morphology). 2. Material and Methods Experimental Struvite crystals were grown through both single and double diffusion technique in gelatine as the medium. All glassware was autoclaved for 15 minutes at 120 Celsius degree. The water used for gel and chemicals preparation was double distilled water which was prepared in the laboratory and the chemicals’ grade was analytical reagent. The gelatine formation is divided in two major categories here, the gel containing Ammonium Dehydrogen Phosphate (ADP) and the plain gel which is only applicable for double diffusion tests. That is due to the stabilization of the gel medium via formaldehyde. It was noticed that the gelatine containing the foreign ions cannot be stabilized by aldehydes any more. Among all controlling parameters like pH, supersaturation, temperature, molarity and molar ratios and the gel specific gravity, here in the current study the effects of specific gravity, pH and molar ratios has been investigated. 2.1 Prparation of the gel For the preparation of gelatin containing ADP, different amount of gelatin powder was added to 100 ml of double distilled water in a beaker to reach the needed specific gravity (equation 1) (Banks, Chianelli, & Pintchovsky, 1973). The mixture was then stirred vigorously for 2 hours at 55 degree Celsius with the speed of 800 round per minute using a magnet stirrer to form a uniform medium for struvite crystals production and to avoid premature or localized gelation. At this time, ADP with different molar ratios (1M and 0.5M) was added to gel and the compound was stirred well at 600 RPM with no temperature change. By the addition of ADP 18

Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

the colour of the mixture turned a bit light allowing us to better observe the probable localized gel. In the present study the specific gravity of gelatin was set from 1.04 to 1.11 for different samples. Aqueous solutions of Ammonium dehydrogen phosphate (ADP) with different molarities (0.5 and 1) was added to the gelatin and then transferred to the test tubes and Ushaped tubes to have the samples ready for the experiments. Specific gravity (SG) = (MS-M0) / (MW-M0)

eq. (1)

M0 = Mass of the dry empty bottle MS = Mass of the bottle filled with gelatin MW = Mass of the bottle filled with distilled water The pH of the mixture containing ADP was reported to be 6.5 at this step with no addition of chemicals changing the pH value of the sample like Sodium hydroxide (NaOH). The pH of the plain gel (U-shaped samples) were reported neutral (7). The mixture was then filtered with a Whatman qualitative filter paper of 11 micrometer pore size into another beaker to have the final gel absolutely uniformed and purified. The gelatin specially the one containing the ADP took days to form in normal room and laboratory conditions. Aldehydes are proved to work as stabilizer on gelatin (Banks et al., 1973). Here, formaldehyde is used to stabilize the final gel for both plain gel and ADP mixed gel (Banks et al., 1973). Unfortunately, the gel containing ADP is not stabilized with formaldehyde which left us no other way to stabilize it but to keep it refrigerated. The samples stabilized with formaldehyde remained at room temperature for 1 day to make sure the gel is completely formed and then they received the Magnesium acetate supernatant as the source of magnesium needed for struvite production. The gelatin samples containing ADP were transferred to straight tubes and then moved to refrigerator. The samples remained in the refrigerator at 14 Celsius degrees for 2 days. Then these samples also received Magnesium acetate supernatant as source of Mg and then moved back to refrigerator and remained there for couple of weeks. The pH value of the samples was maintained to the range of 6.5 to 8 by the addition of sodium hydroxide (NaOH). It was noticed that in the samples with high pH values like 10 and above a thick milky whitish layer forms and prevents the magnesium supernatant from penetrating the gel. 2.2 Preparation of magnesium acetate supernatant and ADP Different magnesium acetate solutions with different specific gravities were prepared using the equation 1. Table 1 better explains the range of operational conditions changed during the samples preparation.

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Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

Table 1 Different growth parameters used to grow struvite crystals pH range of Gel

ADP Concentration

Mg Concentration

1

Specific gravity of Gel 1.04

6.5 to 8

0.5, 1

1

2

1.06

6.5 to 8

0.5, 1

1

3 4

1.08 1.11

6.5 to 8 6.5 to 8

0.5, 1 0.5, 1

1 1

Set Number

Different amount of magnesium acetate [Mg(CH3COO)2.4H2O] and ADP was added to 100 ml water to reach the desired molarity based on its molecular weight. Then the solution was stirred for 1 hour via magnet stirrer at 800 RPM. Then the solution was filtered in order to get rid of any possible foreign impurities. The expected reaction in the test tubes is as followed: NH4H2PO4.2H2O + Mg(CH3COO)2.4H2O

MgNH4PO4.6H2O + 2CH3COOH

eq.(2)

In the straight tubes the ADP is dissolved in the gel and after the gelatin formed the Magnesium acetate would be added and in the U-shaped tubes from one side the ADP is added and from the other side the magnesium acetate would be added then the chemicals will reach each other in the reacting zone where the reaction will take place. The reactant will meet somewhere at the bottom of the curve in the U-shaped tubes which is the reaction zone and there would be no crystals near the solution and gel interface where in the straight tubes normally the crystals forms in the middle of the gel height a bit closer to the bottom of the tubes as the crystals are heavy and gelatin cannot bear their weight. 2.3 Characterization techniques The grown crystals in the form of crystals were taken to be analysed by characterizing sets namely, Scanning Electron Microscope (SEM), Energy-Dispersive X-Ray spectroscopy (EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). SEM analysis was carried out employing Hitachi S~3400N to find out the molecular shape of the produced crystals and to better monitor the molecules surface area and porosity. The FT-IR spectrum revealed the presence of N-H and P-O bonds, NH4+ ion and PO43- ion, water of hydration and Oxygen-metal bond. The EDX test also proved the existence of magnesium as the mineral, nitrogen, phosphorous and oxygen plus some trace of gold (the coting material used for the test). All the results are reported in the form of tables and chart in the result and discussion section of the present article. 3. Results and discussions Here gelatin has been chosen as the medium in which the struvite crystals are to grow. As discussed in earlier parts of this article the plain gelatin samples were stabilized by formaldehyde and used for double diffusion tests and gelatin containing ADP has been used for both single and double diffusion tests and were stabilized by the reduction of temperature (in

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Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

fridge). The following pictures better illustrate the struvite crystals formation in gelatin (figure 1) and SEM results at different zoom levels (figure 2).

(a)

(b)

(c) (d) Fig. 1. (a) Struvite crystals in gelatin (SG 1.06) 7 days after Mg addition, pH 7.5; (b) Struvite crystal in gelatin (SG 1.06), 7 days after Mg addition, pH 9; (c) Gelatin stabilized with formaldehyde (SG 1.06), pH 7, 7 days after Mg addition; (d) Struvite crystals in gelatin, 10 days after Mg addition pH 6.5. Figure 1 shows how pH affects the struvite crystallization through gel growth technique. By any increase in the gel pH from 6.5 to 9, we can observe that the number density is decreasing but the crystal size is increasing. In the U-shaped sample which was stabilized by formaldehyde and remained in the laboratory situation for a week the number of the crystals are limited but at their size is considerable (2.1 mm). In the last picture (d) we can observe that in straight tubes with the pH of 6.5 which were not stabilized, the gel and solution are mixed but the crystals are accumulated at the bottom. In the other samples the gel-solution interface is quite clear (a, b were stabilized by curing in fridge). 21

Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

Totally, there was no dominant difference between the struvite crystals grown in U-shaped and strait tubes in the same operational conditions. The growth is slow at the beginning. The very first crystals occur by the second day after Mg addition and the diffusion continues until approximately the 10th day when the chemicals are almost finished. The maximum growth happens during the second 2 days and after that there is no more crystal birth but development of the existing ones. Samples with very high pH like 9 and above seems to be the least favourite operational conditions for the growth of struvite crystals as there would be an instant milky layer formation by the addition of Mg to the gel which inhibits the diffusion process. The layer stops the solution from penetrating into the gel phase which results in no more reaction and crystal growth. As a result, the best pH range was recognized as 6.5 to 7.5. Specific gravity also plays an important role in struvite crystallization through this technique (Chauhan & Joshi, 2013, 2014). The harder gel (SG over 1.1) was considered too stiff which does not let the crystals grow normally and limits both number and size of the crystals. The low specific gravities (below1.04) were also too loose to hold the crystals even at the very first moments of crystal birth. The best operational conditions, however, was recognized in the U-shaped samples, stabilized with formaldehyde at the pH range of 6.5 to 7.5 with the specific gravity of 1.04 to 1.08. In the mentioned conditions with varying other parameters like molar ratios the best crystals are supposed to form.

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Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

(a)

(b)

(c) (d) Fig. 2. Struvite crystals pictures capture by SEM. (a) 35 times bigger; (b) 200 times bigger; (c) 30K times bigger; (d) 60K times bigger. Figure 2 shows the porosity and surface area of grown struvite crystals in gelatin captured by SEM set at different zoom levels from 35 to 60000 enlargement. The EDX test revealed the presence of the following elements in the crystals, nitrogen (N), oxygen (O), magnesium (Mg) and phosphorous (P). The table below (table 2) describes the EDX test results better.

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Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

Table 2. EDX study results Element

Weight %

Weight %

Atom %

Error

Atom %

Compnd %

Error

Norm. Compnd%

N

14.98

+/- 1.11

22.37

+/- 1.65

14.98

14.98

O

48.33

+/- 1.03

54.20

+/- 1.15

48.33

48.33

Mg

13.71

+/- 0.22

10.12

+/- 0.17

13.71

13.71

P

22.98

+/- 0.42

13.31

+/- 0.24

22.98

22.98

Total

100.00

100.00

100.00

100.00

The FT-IR spectrum revealed the presence of N-H and P-O bonds, NH4+ ion and PO43ion, water of hydration and Oxygen-metal bond (figure 3).

Fig. 3. FT-IR result.

4. Conclusion The current study revealed that samples are highly pH dependent. It was also noted that the gel with lower specific gravity provide the crystals with better environment for growth. The crystal size range varies by depth. At the gel-solution interface the crystal size are so tiny at the beginning. 24

Journal of Purity, Utility Reaction and Environment Vol.4 No.1, February 2015, 16-26

Their growth rate is also very limited as the time passes. As the depth increases the crystals become bigger. It was mentioned that the crystals formed separately and sometimes on the wall of the test tubes but eventually they form colonies of crystals. The morphology of the struvite crystals was hardly affected. Normally the crystals are flower-shaped in the range of 0.1 to 2.5 mm excluding the struvite powder form crystals. References Ali, M. (2005). Struvite crystallization from nutrient rich wastewater. Retrieved from http://eprints.jcu.edu.au/148 Banks, E., Chianelli, R., & Pintchovsky, F. (1973). The growth of some alkaline earth orthophosphates in gelatin gels. Journal of Crystal Growth, 18(2), 185–190. doi:10.1016/0022-0248(73)90198-X Battistoni, P., Pavan, P., Prisciandaro, M., & Cecchi, F. (2000). Struvite crystallization: a feasible and reliable way to fix phosphorus in anaerobic supernatants. Water Research, 34(11). Retrieved from http://www.sciencedirect.com/science/article/pii/S0043135400000452 Chauhan, C. K., & Joshi, M. J. (2013). In vitro crystallization, characterization and growthinhibition study of urinary type struvite crystals. Journal of Crystal Growth, 362, 330– 337. doi:10.1016/j.jcrysgro.2011.11.008 Chauhan, C. K., & Joshi, M. J. (2014). Growth and characterization of struvite-Na crystals. Journal of Crystal Growth. doi:10.1016/j.jcrysgro.2014.01.052 Doyle, J. D., & Parsons, S. a. (2002). Struvite formation, control and recovery. Water Research, 36(16), 3925–40. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/12405401 Galbraith, S., & Schneider, P. (2009). A review of struvite nucleation studies. … Streams: May 10-13, 2009, the …, (1). Retrieved from http://books.google.com/books?hl=en&lr=&id=3gM5ixYKYFIC&oi=fnd&pg=PA69&dq =A+review+of+struvite+nucleation+studies&ots=15NkJqZSj&sig=Ce4NzSVA3cpDtK5R8vljNccCRLA Henisch, H. (1986). Liesegang ring formation in gels. Journal of Crystal Growth, 76, 279–289. Retrieved from http://www.sciencedirect.com/science/article/pii/0022024886903726 Irusan, T., Arivuoli, D., & Ramasamy, P. (1990). Growth of struvite crystals from gel. Crystal Research and …, 25(5), 104–107. Retrieved from http://onlinelibrary.wiley.com/doi/10.1002/crat.2170250532/abstract Le Corre, K. S., Valsami-Jones, E., Hobbs, P., & Parsons, S. a. (2009). Phosphorus Recovery from Wastewater by Struvite Crystallization: A Review. Critical Reviews in Environmental Science and Technology (Vol. 39, pp. 433–477). doi:10.1080/10643380701640573 Münch, E. V, & Barr, K. (2001). Controlled struvite crystallisation for removing phosphorus from anaerobic digester sidestreams. Water Research, 35(1), 151–9. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11257869 Salsabili, A., Salleh, M. A. M., Zohoori, M., & Khosrowabadi, E. (2014). Understanding the Fundamentals and Concepts of Wastewater Treatment through Struvite Precipitation. In 25

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International Conference on Agriculture, Environment and Biological Sciences (pp. 34– 37). Salsabili, A., Salleh, M., Amran, M., & Zohoori, M. (2014). Evaluating the Effects of Operational Parameters and Conditions on Struvite Crystallization and Precipitation with the Focus on Temperature. Advances in Natural & Applied Sciences, 8(April), 175–179. Retrieved from http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Evaluating+the+Effects +of+Operational+Parameters+and+Conditions+on+Struvite+Crystallization+and+Precipit ation+with+the+Focus+on+Temperature#0 Wang, J., & Burken, J. (2005). Engineered struvite precipitation: Impacts of component-ion molar ratios and pH. Journal of Environmental …, (October), 1433–1440. Retrieved from http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9372(2005)131:10(1433)

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