This paper shows the simulation results of struvite thermodynamics. The thermodynamic simulation shows the complex solution chemistry of struvite. Based on the thermodynamic simulation results, the complexes and ions present in struvite solution system are MgOH+, +, MgHPO4, MgPO4-, H3PO4, H2PO4-, HPO42-, NH3, NH4+, Mg2+ and PO43-. The thermodynamic simulation results also show that the supersaturation of struvite system is a function of reactant concentration (concentration of total magnesium, ammonium and phosphate) and solution pH. The paper also includes the validation the developed thermodynamic model using Minteq (a specialized thermodynamic modeling package)

Ali, Md. Imtiaj

Schneider, Philip A.

English

Universiti Teknologi Malaysia Institutional Repository

Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   510Modelling of Struvite Chemistry  Md. Imtiaj Ali∗, Philip A. Schneider  School of Engineering, James Cook University, Townsville QLD 4811 Queensland, Australia    Abstract  This paper shows the simulation results of struvite thermodynamics. The thermodynamic simulation shows the complex solution chemistry of struvite. Based on the thermodynamic simulation results, the complexes and ions present in struvite solution system are MgOH+, MgH2PO4+, MgHPO4, MgPO4-, H3PO4, H2PO4-, HPO42-, NH3, NH4+, Mg2+ and PO43-. The thermodynamic simulation results also show that the supersaturation of struvite system is a function of reactant concentration (concentration of total magnesium, ammonium and phosphate) and solution pH. The paper also includes the validation the developed thermodynamic model using Minteq (a specialized thermodynamic modeling package).  Keywords: Struvite, Thermodynamics, Simulation, Validation   1.0 Introduction  Livestock intensification is becoming an effluent discharge issue for the last couple of decades. Occasionally, strong nutrient loading in wastewater stream forms solid deposits and causes clogging of the wastewater distribution systems [1-5]. This situation of solid encrustation occurs when the solubility limit of nutrients exceeds the saturation limit [6], leading to the occurrence of supersaturated solution [1]. Solution supersaturation and pH value are the predominant parameters of struvite crystallization. The apparent pH for struvite crystallization is documented 7.5- 11.0 [1, 7-10] depending on solution concentrations. Study on struvite thermodynamics explored better understanding on struvite solution chemistry and resultant precipitation. Solution chemistry plays the key role in struvite crystal formation. In supersaturated solution, struvite forms by the chemical reaction of the free Mg2+, NH4+ and PO43- ions (equation 1). Additionally, the formation of struvite crystal incorporates six molecules of water, as the water of hydration: therefore struvite is known as magnesium ammonium phosphate hexahydrate (MgNH4PO4.6H2O). 443442 POMgNHPONHMg ⇔++ −++   (1)   2.0 Materials and Methods  2.1 Mathematical Modelling and Formulation  Supersaturation of the solution is the key parameter leading to crystallization, which in turn depends on solution pH and reactive solution concentration. Solutions consisting of Mg, NH4 and PO4 form complexes including Mg2+, MgOH+, MgH2PO4+, MgHPO4, MgPO4-, H3PO4, H2PO4-, HPO42-, PO43-, NH3 and NH4+ [11, 12]. Thermodynamic equilibria for different                                                  ∗ Corresponding author: Tel: +61 07 4781 4346, Fax: +61 07 4775 1184, Email: md.ali@jcu.edu.au Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   511complexes and relevant equilibrium constants are presented in Table 1, provided that }{i the activity based concentrations and Ki is the equilibrium constant of the ith ion complexes (as demonstrated above).  The total constituent concentrations for Mg2+, NH4+ and PO43-, denoting CT_ Mg, CT_ NH4, CT_PO4, are the sum of the ionic concentration of their complexes and free ions, which are illustrated in equations (2)-(4). [ ] [ ] [ ] [ ][ ] [ ] [ ]−+−−−++++++=4442342442434MgPOMgHPOPOMgHPOHPOPOHPOHC POT _  (2) [ ] [ ] [ ] [ ] [ ]−+++ ++++= 44422 MgPOMgHPOPOMgHMgOHMgC MgT _  (3) [ ]++= 434 NHNHC NHT _   (4)  Table 1 Value of Equilibrium Constants for Complexes  Equilibrium Equation Ki References { }{ }{ }+−+=+ MgOHOHMgKMgOH2 10-2.56 [13] { }{ }{ }++=+434 NHNHHKNH 10-9.25 [14] { }{ }{ }−−+=− 243424 HPOPOHKHPO 10-12.35 [15] { }{ }{ }−−+=−422442 POHHPOHKPOH 10-7.20 [15] { }{ }{ }434243 POHPOHHK POH−+=  10-2.15 [16] { }{ }{ }+−+=+4242242 POMgHPOHMgKPOMgH10-0.45 [16] { }{ }{ }42424 MgHPOHPOMgK MgHPO−+=  10-2.91 [16] { }{ }{ }−−+=−43424 MgPOPOMgKMgPO 10-4.80 [16] [ ][ ]−+= OHHK w  10-14 [1]  The activity coefficient is calculated using Davis equation, as shown in equation (5). The ionization fractions of Mg2+, NH4+ and PO43- ( +2Mgα , −34POα , +4NHα ) can be defined by the quotient of free ion concentration and the total concentration of each chemical species.  IIIAZLog ii 3.01 2/12/12 −⎥⎦⎤⎢⎣⎡+=− γ   (5) Where γi = Activity of solution Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   512I = Ionic strength in mol/l Zi = Valency of the corresponding elements A = DeBye-Hückel constant, has a value of 0.493, 0.499, 0.509 and 0.519 at 5, 15, 25 and 35°C, respectively [17].   Two types of solubility products describe the solubility status, which include the Conditional Solubility Product (Pcs) and the Product of Analytical Molar Concentration (Pso) [1]. The conditional Solubility Product (Pcs) relates to the solution properties, including ionization fraction (αi), activity coefficients (γi) and the minimum struvite solubility product (Kso) [7]. The Product of the Analytical Molar Concentration relates to the total concentrations of reactive constituents (CT,i), where i represents the concentration of magnesium, ammonium, and phosphate, as required [1]. The mathematical definition of the minimum solubility product of struvite is shown in equation (6). The mathematical formula of the Conditional Solubility Product (Pcs) and the Product of the Analytical Molar Concentration (Pso) are demonstrated in equations (7) and (8), respectively. ++−−++=44343422 4_4__ .. NHNHNHTPOPOPOTMgMgMgTso CCCK γαγαγα  (6) −−++++=34344422 POPONHNHMgMgsocsKP γαγαγα   (7) 44 NHTPOTMgTso CCCP ___ ..=   (8) Solutions with a higher value of the Concentration Product than the Conditional Solubility Product (Pso > Pcs) refers to a supersaturated solution. Equal numerical values of Pcs and Pso characterize the saturated condition of solution, where as Pso < Pcs demonstrates the undersaturated condition of a solution [1]. The negative logarithmic value of the minimum struvite solubility product (pKso) value applied in this thermodynamic modeling is 13.26 [7].   The active mass of each reactant (free ion concentration) and the rate of chemical reaction for struvite precipitation are proportional to the degree of supersaturation [7, 17]. Often, the degree of supersaturation is expressed by the critical supersaturation ratio (Sc) relating to the Thermodynamic Conditional Solubility Product (Pcs) and the Concentration Product of reactants (Pso), as shown in the equation (10) [1, 7]. Based on the chemical formation of struvite, the number of species in anhydrous struvite is ν = 3. ν1⎟⎠⎞⎜⎝⎛=cssoc PPS   (9) 314_4__34423442 ⎟⎟⎟⎟⎟⎠⎞⎜⎜⎜⎜⎜⎝⎛⎟⎟⎠⎞⎜⎜⎝⎛=−++−++ PONHMgPONHMgsoPOTNHTMgTcKCCCSαααγγγ (10)  The solution thermodynamic properties specify the state of saturation, free ion concentrations, molar concentration of ion complexes and the state of precipitation. The precipitation of struvite occurs in supersaturated solutions, which is particularly influenced by solution pH and reactant concentration [17, 18].    Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   5133.0 Result and Discussion  3.1 Struvite Chemistry  The thermodynamic modeling was derived using Cabarlah Park primary pond data, investigated by Queensland DPI. The respective concentration of magnesium, ammonium and phosphate of the Cabarlah Park primary pond effluent are 26, 199.7 and 34.1 mg/l.  The ionic interactions between Mg2+ and the different PO43- ions (PO43-, HPO42- and H2PO4-) cause the formation of magnesium phosphate ion (MgPO4-), aqueous magnesium mono-hydrogen phosphate (MgHPO4) and magnesium di-hydrogen phosphate (MgH2PO4+). The rest of the total magnesium remains as free Mg2+ ion.    Figure 1 Presence of different Magnesium complexes in Struvite system  Figures 1 and 2 show the log concentration of different magnesium, phosphate and magnesium-phosphate complexes over the pH range 5 to 14. It is observed from the simulation response that in compare to free Mg2+ ion, small amount of magnesium remains as magnesium-phosphate complexes. At higher pH (above 10.5) most of the total magnesium forms MgOH+ complex. A validation of the model response for magnesium/ magnesium-phosphate complexes is made using vMinteq (a specialized thermodynamic modeling package). Very close similarities have been observed of the thermodynamic responses between gPROMS output and vMinteq output, which shows the acceptability of the thermodynamic modeling using gPROMS process simulation software.  Over the range of pH<10, the free PO43- ion concentration is reasonably small when compared to Mg2+ ion (Figure 2), since the major portion of phosphate remains as MgPO4-, MgHPO4 (aq), MgH2PO42-, H2PO4-, H3PO4 and HPO42-. However, above pH 10.5, the concentration of free PO43- increases considerably due to the de-protonation of HPO42-, H2PO4- and H3PO4.          1E -181E -161E -141E -121E -101E -081E -060.00010.014 6 8 10 12 14Solution pHLog molar concentrationM gH2P O 4+ (M inteq)M gH2P O 4+ (gP RO M S )M gHP O 4 (aq)(M inteq)M gHP O 4 (aq)(gP RO M S )M gP O 4-(M inteq)M gP O 4-(gP RO M S )M g+ 2(M inteq)M g+ 2 (gP RO M S )M gO H+ (M inteq)M gO H+ (gP RO M S )Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   514 Figure 2 Presence of different Phosphate complexes in Struvite system  Figure 3 Presence Of different ammonium complexes in struvite system  With the given concentration (in the previous paragraph), the thermodynamic simulation results show that the major portion of total ammonium remain as free NH4+ ion below pH 8.5 (Figure 3), and decreases rapidly due to the transformation of NH4+ to NH3 in alkaline phase.                Figure 4 State of saturation at different pH value (based on the critical supersaturation ratio, Sc)  Figure 4 shows the computed response of the struvite supersaturation over the pH range 5 to 14 for a solution concentration of 26mg/l magnesium, 199.7mg/l ammonium and 34.1mg/l 1E-251E-221E-191E-161E-131E-101E-070.00014 6 8 10 12 14Solution pHLog molar concentrationH2PO4-(Minteq)H2PO4-(gPROMS)H3PO4(Minteq)H3PO4 (gPROMS)HPO4-2(Minteq)HPO4-2(gPROMS)PO4-3(Minteq)PO4-3 (gPROMS)0.00000010.0000010.000010.00010.0010.010.14 6 8 10 12 14Solution pHLog molar concentrationNH3 (aq)(Minteq)NH3 (aq)(gPROMS)NH4+(Minteq)NH4+(gPROMS)0.00.51.01.52.02.54 6 8 10 12 14Solution pHValue of SupersaturationSupersaturationSaturationSaturation SupersaturatioA B Proceedings of the 1st International Conference on Natural Resources Engineering & Technology 2006 24-25th July 2006; Putrajaya, Malaysia, 510-515   515phosphate. As described in section 3, saturation of solution reflects that the value of conditional solubility product (Pcs) and analytical concentration product (Pso) is equal. At the supersaturation of solution, analytical concentration product (Pso) is higher than the conditional solubility product (Pcs). Based on Figure 4, model input solution remains in supersaturated state between pH ranges 7.8 to 12 (AB), with maximum supersaturation at pH 10.1.  It is worthwhile pointing out that the struvite thermodynamic and solution chemistry, described in the previous paragraphs, is based on the effluent pond data of magnesium, ammonium and phosphate along with the relevant chemical equilibria. For the additional chemical species of the pond data, an extension of the gPROMS coding and simulation must be included.   4.0 Conclusions  Solution chemistry plays an important role in struvite precipitation. Thermodynamic simulation result shows that both solution pH and free ion concentration of dissolved species (magnesium, ammonium and phosphate) plays most significant role in struvite crystallization.   References  [1] Snoeyink V.L. and Jenkins, D. 1980 Water Chemistry. John Wiley and Sons, USA. [2] Sharpley A.N. 2000 Agriculture and Phosphorus Management - The Chesapeake Bay. Blvd, Florida: Lewis     Publisher, CRC Press LLC. [3] Seckler M.M. and Bruinsma, O.S.L. 1996. Calcium Phosphate Precipitation in a Fluidised Bed in Relation of Process Condition: A Black Box Approach. Wat. Res. 30(7): 1577-1585 [4] Piekema P. and Giesen, A. Phosphate Recovery by the Crystallization Process: Experience and Developments. 2001. in Second International Conference on the Recovery of Phosphorus from Sewage and Animal Wastes. Noordwijkerhout, The Netherlands  [5] Piccini S. and Bortone, G. 1991. The Fertilizer Value of Agricultural Manure: Simple Rapid Methods of assessment. J. Agri. Eng. Res. 49: 197-208 [6] Ali M.I. and Schneider, P.A. 2006. A Fed-Batch Design Approach of Struvite System in Controlled Supersaturation. Chem. Eng. Sci. 61(12): 3951-3961 [7] Ohlinger K.N. Kinetics Effects on Preferential Struvite Accumulation in Wastewater.1999.PhD Thesis. School of Science and Engineering, California State University [8] Stumm W. and Morgan, J.J. 1996 Aquatic Chemistry. New York: John Wiley and Sons, USA. [9] Buchanan J.R., Mote, C.R., and Robinson, R.B. 1994. Struvite Control by Chemical Treatment. American Soc. Agri. Eng. 37(4): 1301-1308 [10] Doyle J.D., Oldring, K., Churchley, J., and Parsons, S.A. Fundamental Chemistry and Control of Struvite Precipitation. 2001. in Second International Conference on the Recovery of Phosphorus from Sewage and Animal Wastes. Noordwijkerhout, The Netherlands  [11] Ohlinger K.N., Young, T.M., and Schroeder, E.D. 2000. Postdigestion Struvite Precipitation Using a Fluidised Bed Reactor. J. Env. 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Modelling of struvite chemistry

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Modelling of struvite chemistry

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