Mikko Brotell
CONTINUOUS AND SEMI-BATCH PRECIPITATION OF CALCIUM PHOSPHATE IN A STRIRRED TANK AND PLUG-FLOW CRYSTALLIZER
Examiners: Professor Marjatta Louhi-Kultanen D.Sc. Bing Han
Supervisor: D.Sc. Bing Han
School of Technology
Master’s Programme in Chemical and Process Engineering Mikko Brotell
Continuous and semi-batch precipitation of calcium phosphate in a stirred tank and plug-flow crystallizer.
Master’s thesis 2016
94 pages, 56 figures, 9 tables and 9 appendices.
Examiners: Professor Marjatta Louhi-Kultanen D.Sc. Bing Han
Keywords: dicalcium phosphate dihydrate, hydroxyapatite, phosphate precipitation, semi-batch crystallizer, continuous plug-flow crystallizer, continuous stirred tank crystallizer, transformation of calcium phosphate phase
The objective of this thesis was to study precipitation of calcium hydrogen phosphate dihydrate (dicalcium phosphate dihydrate, DCPD) in semi-batch and continuous stirred tank crystallizers and continuous plug-flow reactor. The literature part deals with phosphate reserves and fertilizer production in different countries, phosphorus removal methods used in wastewater treatment, and methods for phosphorus recycle from waste streams as slow-release fertilizers. Finally, different calcium phosphate phases and their precipitation are compared to DCPD precipitation which is used as model substance. The calcium phosphate precipitation was expected to proceed in steps, from precipitation of hydroxyapatite via slow transformation to DCPD due to pH change. The changes in calcium phosphate phase were seen from suspension pH evolution. The precipitated samples were examined using X-ray diffraction (XRD) analysis and Scanning Electron Microscope (SEM). Also, volumetric size distributions (Malvern Morphologi) were determined.
The results from the continuous plug-flow crystallizer showed that initial pH of reagents affected significantly precipitation, not only to amounts precipitated, but also pH influenced on produced calcium phosphate properties as well. Furthermore, it could be deduced from pH, conductivity developments and size distributions when suspension sample reached steady-state after sampling, or when desupersaturation still occurs in solution. This is an important information for example in terms of filtration so that filtration equipment would not clog during the filtration because of precipitation. In continuous stirred tank experiments, high phosphate precipitation was achieved regardless of residence time or mixing rate.
Precipitated calcium phosphate was almost completely DCPD regardless of residence time. High phosphate precipitation rates with short residence times also changed crystal morphology during the experiments. It was also noted, that anhydrous form was often co-precipitating when unstable hydroxyapatite was transforming into more stable form.
Teknillinen tiedekunta
Kemiantekniikan koulutusohjelma Mikko Brotell
Continuous and semi-batch precipitation of calcium phosphate in a stirred tank and plug-flow crystallizer
Diplomityö 2016
94 sivua, 56 kuvaa, 9 taulukkoa ja 9 liitettä.
Tarkastajat: Professori Marjatta Louhi-Kultanen D.Sc. Bing Han
Avainsanat: kalsiumvetyfosfaatin dihydraatti, hydroksiapatiitti, fosfaatin saostuminen, jatkuvatoiminen tulppavirtaus, jatkuvatoiminen sekoitussäiliö, puoli-panos kiteytin, kalsiumfosfaatin muodonmuutos Työn tarkoituksena oli tutkia kalsiumfosfaatin saostusta kalsiumvetyfosfaatin dihydraattina (dikalsiumfosfaatin dihydraatti, DCPD) puolipanostoimisessa ja jatkuvatoimisessa sekoitusreaktorissa sekä jatkuvatoimisessa putkireaktorissa. Kirjallisuusosassa kartoitetaan fosfaatti- varantoja ja lannoitetuotantoa eri maissa sekä tarkastellaan fosfaatin erotuksessa käytettyjä tekniikoita jätevedenkäsittelyssä ja jätevirrasta saostetun fosfaatin uudelleenkäyttöä lannoitteena. Lopuksi verrataan eri kalsiumfosfaatteja ja niiden saostumista malliaineena käytettävän DCPD:n saostukseen. Kalsiumfosfaatin saostumisen odotettiin etenevän portaittain siten, että ensin saostuva hydroksiapatiitti muuntuu vähitellen dikalsiumfosfaatin dihydraatiksi pH- muutoksen johdosta. Kalsiumfosfaattimuodossa tapahtuvien muutosten havaittiin näkyvän suspension pH:n vaihtelussa. Mittauksissa saostettuja kiteitä tutkittiin röntgendiffraktiolla (XRD), elektronimikroskoopilla (SEM) sekä määritettiin kiteiden tilavuuskokojakaumat (Malvern Morphologi).
Jatkuvatoimisessa tulppavirtauksessa saatujen tulosten perusteella reagenssien pH:n säädöllä oli merkittävä vaikutus, ei ainoastaan saostuneen fosfaatin määrään, mutta myös itse saostuneeseen kalsiumfosfaattiin. Lisäksi pH:n muutoksesta, sähköjohtokykymittauksista sekä kidekokojakaumamäärityksistä voitiin päätellä, missä ajassa suspensionäyte saavuttaa tasapainotilan näytteenoton jälkeen, ts. missä ajassa ylikylläisyys purkautuu liuoksessa. Tämä on tärkeä tieto muun muassa suodatuksen kannalta, jotta suodatinlaitteisto ei tukkeutuisi saostumisen vuoksi suodatuksen aikana. Jatkuvan sekoitussäiliön kokeissa saavutettiin erittäin korkea saanto fosfaatille riippumatta viipymäajan pituudesta tai sekoitusnopeudesta.
Saostettu kalsiumfosfaatti oli niin ikään lähes kokonaan dikalsiumfosfaatin dihydraattia huolimatta viipymäajan pituudesta. Saostuneen fosfaatin määrä myös muutti kiteen morfologiaa mittauksien aikana. Useassa mittauksessa huomattiin myös kidevedettömän muodon saostuminen epästabiilin hydroksiapatiitin muuntuessa stabiilimpaan muotoon.
This Master’s thesis was a part of the FLUKI-project titled from flow to crystal properties and financed by the Academy of Finland. The experimental part of this thesis was carried out at the Laboratory of Separation Technology at Lappeenranta University of Technology from the beginning of April until the end of August 2016.
I would like to thank Professor Marjatta Louhi-Kultanen for offering thesis from extremely interesting subject, not only because it combined two of the most interesting subjects what I have found during my studies, water treatment and crystallization but also because of challenging and versatile experiments and results.
I would also like to thank my supervisor Bing Han, for helping me during the experiments and giving me helpful advices, and also pushing me towards the finish line. Special thanks to Toni Väkiparta helping with SEM-analysis, and Tuomas Nevalainen for helping with the set-up, also I want to thank you Nicolus Rotich and Mehdi Hasan.
Finally, I would like to thank you my family for endless support which they have given me during all these years. And lastly, I would want to thank you Veli-Ensio Heiniluoto, Antti Paarvio, Rasmus Peltola and Isto Sipilä for all these years. You are the best.
November 2016, Lappeenranta Mikko Brotell
1.1 Background ... 2
1.2 Research problem and study objectives ... 3
LITERATURE PART ... 4
2 Source of phosphorus and its reserves ... 4
3 Prospects of phosphorus ... 5
4 Phosphoric acid production ... 6
4.1 Thermal process of phosphoric acid production ... 6
4.2 The wet process of phosphoric acid production ... 7
5 Eutrophication and its prevention ... 8
6 Phosphorus removal from wastewaters ... 9
6.1 Enhanced Biological Phosphorus Removal (EBPR) ... 9
6.2 Sewage sludge ... 10
6.3 Sewage sludge ash (SSA) ... 11
6.4 Chemical precipitation ... 12
6.4.1 Precipitation of aluminium and ferrous phosphates... 13
7 Recovered phosphorus as fertilizer ... 14
7.1 Phosphorus recovery as magnesium ammonium phosphate, struvite (MAP) ... 15
7.2 Process examples to recover phosphorus as struvite (MAP) ... 17
7.2.1 AirPrex ® process ... 17
7.2.2 Multiform process ... 18
7.2.3 Pearl® process ... 18
7.2.4 Phospaq™ process ... 19
7.3 Phosphorus precipitation as calcium phosphate ... 20
7.4 Process examples to recover phosphorus as calcium phosphate ... 21
7.4.1 Phostrip process ... 21
7.4.2 Crystalactor® process ... 22
8 Recovery of organic phosphorus ... 23
9 Calcium orthophosphates ... 24
9.1 Solubility of calcium phosphates ... 26
9.2 Monocalcium phosphate monohydrate (MCPM) ... 27
9.3 Monocalcium phosphate anhydrate (MCPA) ... 27
9.4 Tricalcium phosphate (TCP) ... 28
9.5 Amorphous calcium phosphate (ACP) ... 28
9.6 Octacalcium phosphate (OCP)... 30
9.7 Calcium-deficient hydroxyapatite (CDHA) ... 31
9.8 Dicalcium phosphate dihydrate (DCPD) ... 31
9.9 Dicalcium phosphate anhydrate (DCPA) ... 33
9.9.1 Transformation of DCPD into DCPA ... 33
9.10 Hydroxyapatite (HAP) ... 34
9.11 Interfacial energies ... 36
10 Characterization of different calcium phosphates by FT-Raman and X-ray diffraction ... 36
11 Precipitation of dicalcium phosphate dihydrate ... 38
12 Morphology of dicalcium phosphate dihydrate ... 41
EXPERIMENTAL PART ... 44
13 Continuous and semi-batch precipitation of calcium phosphate in a stirred tank and plug-flow crystallizer ... 44
13.1 Materials and methods ... 44
13.1.1 Chemicals ... 44
13.2 Characterization of precipitated calcium phosphate phases ... 45
13.3 Experimental setup and procedure of preliminary test using the semi-batch and batch crystallizer ... 46
13.4 Experimental setup and procedure carried out using the continuous plug-flow crystallizer ... 49
13.5 Experimental setup and procedure of the continuous stirred tank crystallizer ... 52
13.6 Theoretical considerations ... 55
13.7 Results and discussion ... 58
13.7.1 Precipitation of calcium phosphate in semi-batch crystallizer ... 58
13.7.2 Precipitation of calcium phosphate in continuous plug-flow crystallizer ... 63
13.7.3 Precipitation of calcium phosphate in a continuous stirred tank crystallizer ... 74
14 Summary and conclusions ... 90
REFERENCES ... 95
APPENDICES ... 100
B nucleation rate , #/m3 s
G linear growth rate , m s-1
I intensity , counts
kv volumetric shape factor , -
𝐿̅ mean crystal size , m
Ld dominant crystal size , m
L50 median crystal size , m
m(PO4-3)initial mass of phosphate in reagent solution , g m(PO4-3)DCPD mass of phosphate precipitated as DCPD, g
MT density of suspension , kg crystals m-3 suspension n population density distribution , m-4
n0 nuclei population density , m-1 m-3 [P]in initial phosphate concentration
in reagent solution , mg L-1 PO4-3
DCPD phosphate precipitated as DCPD , %
[P]sol solute phosphate concentration
of effluent , mg L-1
xj weight fraction , %
ΔL crystal size difference , m
θ incident angle , -
λ X-ray wavelength , nm
ƞF removal efficiency with filtration , %
ρc crystal density , kg m-3
τ residence time in crystallizer , s
ACP Amorphous calcium phosphate AOPs Advanced Oxidation Processes CDHA Calcium-deficient hydroxyapatite COD Chemical oxygen demand
DCPA Dicalcium phosphate anhydrate DCPD Dicalcium phosphate dihydrate
EBPR Enhanced Biological Phosphorus Removal HAP Hydroxyapatite
IC Ion Chromatography
KDP Potassium dihydrogen phosphate MAP Magnesium Ammonium Phosphate MSMPR Mixed suspension mixed product removal OCP Octacalcium phosphate
RAS Returned activated sludge SEM Scanning Electron Microscope TCP Tricalcium phosphate
TTCP Tetracalcium phosphate WAS Waste activated sludge XRD X-ray diffraction
1 Introduction
1.1 Background
The phosphorus is needed for fertilizers with nitrogen and potassium to sustain crop yields on agricultural fields. Currently, the main source for phosphorus is high- quality phosphate rock often combined with nitrogen, potassium and sulfuric acid in mineral fertilizers. Before, organic matter for example human excreta and manure was used to increase phosphorus content in soil for crop production, and in the 20th century because of rapid population growth the crop production relied on the guano before the use of phosphate rock [1].
It is estimated that by 2050 the phosphorus demand increases by 50 % to 100 % because of global food demand, and changing diets to which food production has to increase by 70 %, being significant challenge in the future what it already is. On the other hand, when phosphate demand is increasing, it is estimated that high- quality phosphate rock will be depleted in following 50 to 100 years, increasing production costs when lower quality phosphate rock is used [1]. These high quality phosphate rock reserves are under control of only a few countries, and over 85 % of the reserves are controlled just five countries, making other countries highly dependent on the imports, for example in Europe where only one country is considered to have its own phosphorus reserves [2].
One way to decrease to dependency on imports could be precipitation of calcium phosphate or magnesium ammonium phosphate which could be used as slow- release fertilizers. The phosphorus recovery has only been so far attractive in wastewater plants which are using biological removal, and has been so far though to be contaminant. If, phosphorus would be recovered its quality could be higher than currently used phosphate rock but because of current relative cheap phosphate rock it might become viable source of fertilizer manufacturing when high-quality phosphate rock reserves are depleted [3].
1.2 Research problem and study objectives
In the experimental part of the thesis, calcium phosphate was precipitated in semi- batch, continuous plug-flow, and continuous stirred tank crystallizers. The aim was to precipitate calcium phosphate in the form of dicalcium phosphate dihydrate (DCPD) without presence of other calcium phosphate phases. It was expected that without pH adjustment during experiment calcium phosphate precipitates at first as hydroxyapatite (HAP) before transformation into DCPD. Also, it was assumed that initial pH affected significantly on the precipitation as well as the Ca/P molar ratio.
Ageing also has impact on the calcium phosphate phases. Because of possible co- precipitation of other calcium phosphates and needed time of HAP transformation, DCPD production was thought to be challenging.
Experiments were started with preliminary experiments and based on the obtained results, parameters were later used in the continuous crystallizers. One important objective of this work was compare obtained results from the continuous plug-flow and stirred tank crystallizers. This included phosphate recovery efficiency, produced calcium phosphate phase, crystal morphology and influence of ageing.
The produced samples were examined with X-ray diffraction (XRD), and Scanning Electron Microscope (SEM), whether other calcium phosphates were present, or transformation of HAP into DCPD was complete. Volumetric size distributions were also determined as the main results, and were used to calculate crystallization kinetics for the experiments carried out with the continuous stirred tank crystallizer.
LITERATURE PART
2 Source of phosphorus and its reserves
Phosphate rock is primarily, around 95 % of reserves found in calcium phosphate mineral, apatite, containing normally only low concentrations of phosphorus approximately 0.1 %. In phosphate rocks, the concentration of phosphorus can be as high as 10 %-20 %. For phosphorites these high concentration reserves are under control of handful of countries. Cooper et al. [2] reported, that 86 % of the global reserves are under control of only five countries, where Morocco has the greatest phosphate rock reserves. According to the USGS (1/2011), 77 % of the phosphate rock reserves which is estimated to be around 50 billion tonnes is controlled by a state-owned group which is basically under control of one man, the king of Morocco Mohammed VI. As a result, one man controls vast amount of the global phosphate rock reserves including independent territory of Western Sahara, occupied by Morocco since 1975 which is approximately 2 % of the total rock phosphate reserves of Morocco, covering totally around 75 % of the high-quality phosphate rock on the globe [2].
Other countries, like China and the USA act differently with their reserves. The USA which is mentioned to be the largest consumer, producer, exporter and importer of phosphate fertilizers and phosphate rock has its own reserves left for 25 years importing phosphate rock in great quantities from Morocco for its needs, whereas China tries to prevent the export by imposing 135 % export tariff on phosphate in order to secure its domestic supply [1].
The highest demand for food is Africa which is on the other hand the largest exporter of the phosphate rock. Western Europe and India depend on the imports [1]. They have low-quality rock phosphate which is unsuitable for the fertilizer production. That’s why they are completely depend on the imported fertilizers. It was reported that Finland is the only Western European country which has its own commercial deposits [2].
3 Prospects of phosphorus
According to Cordell et al. (2009), around 148 million tonnes of phosphate rock is currently needed for food production every year which is 90 % of total phosphorus demand. It is predicted that by 2050 phosphorus demand will increase by 50 % to 100 % because of global food demand and changing diets. Currently, partial phosphorus recycle is being used in Europe and North America where animal manure and straw are ploughed into soil to avoid over fertilization. In these areas, phosphorus demand is decreased or stabilized because of so-called critical phosphorus levels have been exceeded due to decades of use of high-grade fertilizer, requiring only slight replacement. On the other hand, situation is different in emerging and developing economies. The increase of phosphorus demand mainly from Asia because popularity of meat- and dairy based diets, and biofuel industry are increased. According to the International Water Management Institute, by 2050 the increase of food production has to increase by 70 % to stick with the global food demand. So food production is significantly challenge in the future. Currently, over 800 million people has no sufficient access to food. In Africa over 40 % of people has lack of sufficient food on a day-to-day basis [1].
In globally, food production highly dependent on cheap energy which is mainly from fossil fuels, making transportation of food, fertilizer mining and production possible as long as cheap energy exists. According to research, high-quality of phosphate rock will be depleted in 50-100 years. The costs of fertilizer production from lower quality rock reserves will be increased [1]. According to Cooper et al.
(2011), disputes and differences of phosphate rock reserves is arising since different estimations have been given from timescale of depletion of rock phosphate reserves, varying from 50 to 100 years, or even much longer [2].
4 Phosphoric acid production
The second most important mineral acid produced in the world is phosphoric acid [4] and it is produced from phosphate rock such as fluoroapatite (Ca10F2(PO4)6), chloroapatite (3Ca3(PO4)2CaCl2) and phosphorite [5]. It is primarily used for phosphate fertilizer production or used as acid. In 2013, according to Belboom et al. (2015), the consumption of phosphorus as P2O5 reached 41.8 Mt, and is expected to be increase to 45.9 Mt in 2018. The fertilizer production covers 80 % of the total phosphoric acid production produced as phosphate fertilizer from phosphate rock and/or commercial phosphoric acid [4]. Other applications are toothpastes, detergents food products and cattle supplies [5]. Phosphoric acid production is carried out using a wet or thermal process [4], which is also known as dry or electric furnace process [5].
4.1 Thermal process of phosphoric acid production
According to Awwad et al. (2013), the use of dry process has declined over the years since its development in 1935, but used still when producing elemental phosphorus or intermediate acid production for elemental phosphorus [5].
According to Schrödter et al. (2008), the thermal process is carried out in two stages, where the first stage consists from combustion in an electric resistance furnace with excess air to obtain P4O10, and the second stage from hydration which results in formation of H3PO4. Because pure phosphorus is used combustion the produced phosphoric acid contains only small fractions of impurities resulting only to the post-treatment of arsenic [6]. The thermal process produces high quality phosphoric acid, greater than fertilizer production requires, allowing production of purified and defluorinated phosphoric acid. On the other hand, because of high energy requirements majority of European countries has abandoned the process but is still used for example in China and Kazakhstan [4]. Mentioned processes for phosphoric acid are IG process, TVA process and Hoechst process [6].
4.2 The wet process of phosphoric acid production
The wet process for phosphatic fertilizer production is primarily used comprising 85 % of the production, and can be classified depending on the form of crystallized calcium sulfate, either to dihydrate, CaSO4 ∙2H2O or to hemihydrate, CaSO4 ∙ ½H2O [5]. It depends on the temperature of acid and the phosphoric acid concentration.
According to Schrödter et al. (2008), from these two crystal forms, the dihydrate is the most common compound to use. In the process point of view, all the wet processes have two common operations, digestion in a reactor and removal of CaSO4 by filtration [6]. Digested phosphate rock using concentrated sulphuric acid in the temperature range of 70 °C and 80 °C. Simplified reaction is obtained and shown in Eq. (1) [5]:
Ca5(PO4)3(OH) + 5H2SO4 5CaSO4 (s) + 3H3PO4 + H2O (1) Compared to the thermal process, the wet process produced phosphoric acid which contains different amounts of impurities depending on the phosphate rock. For recovery, only pure acid precipitation and extraction processes are used [6]. During the process, vast amount of phosphogypsum is precipitated, containing impurities from mined phosphate rock, such as heavy metals, mercury and cadmium which can be only partly recovered which makes handling environmentally challenging.
Formed phosphogypsum is disposed to landfills, and some measurements can be done to avoid environmental pollution or improve the quality of phosphogypsum itself making its use possible in construction, industrial or agricultural sector. For example, a Belgian company, Prayon SA (European Commission and Joint Research Centre, 2007) has valorised great amount of its phosphogypsum (85 %) to the plaster industry by using di-hemihydrate recrystallization filtration after the wet process. In Europe, the wet process comprises 95 % of the total phosphoric acid production [4].
5 Eutrophication and its prevention
According to Rittman et al. (2011), approximately 80 % of the used phosphorus obtained from phosphate rock reserves is used for agricultural fields, from which 40 % is mentioned to ends up into harvesting crops. From this amount, 23 % is incorporated to our food, and 16 % is consumed by us. Still, only small fraction of consumed phosphorus is retained to human body and the rest end up to municipal wastewater treatment plants. Almost half of the amount (7 %) is then disposed to landfills as sludge, and another half of it (8 %) is discharged by wastewater treatment plants entering directly to waterways [7].
The lost phosphorus, primarily from runoff and soil erosion (46 % of mined P), and animal wastes (40 % of mined P) then enters to surface waters enhancing eutrophication which eventually leads to colour, odour and turbidity changes, declining amounts of dissolved oxygen, and perishing of fish habitat [7]. According to Cordell et al. (2009), it is estimated that a quarter of billion tonnes phosphate rock mined since 1950 has ended up to landfills or in water bodies [1]. The annual losses because of these for phosphorus is estimated to be 8-30 Mt [8].
To treat these waters, anaerobic treatment is used to remove nutrients from agro- industrial and domestic wastewaters. In the case of phosphorus, this means conversion of ions into a solid fraction in the form of a microbial mass in an activated sludge, insoluble precipitate, or a plant biomass in constructed wetlands.
Even though, the phosphate removal is usually compulsory, it is not performed in many cases. These untreated effluents then lead to contamination in worldwide scale. According to de-Bashan and Bashan (2004), treatment is needed when domestic and agro-industrial wastewaters are being processed, which consists of large quantities of phosphate and nitrogen [3]. Compared to soil erosion and runoff, the animal wastes have smaller volumetric rates but instead the phosphorus concentration and organic material are significantly high, approximately 380 mg P L-1 and 14,000 mg COD L-1 respectively. Much greater amounts can be still obtained in their peaks, 2,400 mg P L-1 and 90,000 mg COD L-1 respectively. The animal wastes are sometimes recirculated back to agricultural fields but because of lack of regulations in some cases towards animal wastes phosphorus ends up to surface waters [7].
6 Phosphorus removal from wastewaters
Phosphorus enters into wastewater in different ways such as suspended or dissolved, organic and inorganic compounds. In the form of food residues, human excreta (urine, feces), industrial emissions, and at least in the form of detergents in Europe as well, which results to around 1.5-2 g load cap-1 of phosphorus coming to WWTPs daily [9]. Common approaches to limit eutrophication involves chemical precipitation of the inorganic phosphate anion by addition of calcium, magnesium, ammonium iron, aluminium or iron cation producing sludge which is often discarded to landfills after treatment [7]. In Europe, chemical precipitation of phosphorus using Fe or Al are common methods to guarantee that the concentration values in effluent are following the values determined by the legislation. When phosphorus is removed by precipitation and followed by biological removal up to 95 % of the influent phosphorus can be transferred into sewage sludge [9]. Another used method is enhanced biological phosphorus removal (EBPR), involving enrichment of phosphorus using bacteria The phosphorus containing organic biosolids are either disposed to landfills or used as soil amendment. Both of these methods are primarily used for the decrease of the phosphorus amount which enters to waterways instead of recycling it, and have been also developed wastewaters where phosphorus concentration is 5 to 10 mg P L-1[7]. To date, the main purpose of the removal of nutrients and organics has been to ensure human and environmental health by ensuring reliable process efficiency [9].
Potential methods for phosphorus recovery from wastewater includes the separate urine collection, sewage sludge, sewage sludge ash, digester supernatant, and the secondary treatment effluent from WWTP, all of which differ from each other in terms of phosphorus concentration, volume, source characteristics and pollutant content, and the theoretical recovery potential. Direct phosphorus recovery is possible from separated waste streams such as, urine and digester supernatant [9].
6.1 Enhanced Biological Phosphorus Removal (EBPR)
In enhanced biological phosphorus removal (EBPR) as wastewater treatment method, phosphate rich sludge is taken out as excess sludge from the system. The EBPR is based on the selective bacteria enrichment where bacteria are
accumulating to inorganic polyphosphate which they are using for ingredient for the cells. The enrichment involves cycling of microbial metabolic via several microbial-accumulated biopolymers where anaerobic incubation is followed by aerobic incubation. In aerobic stage, microbes are depleting the organic matter and carbon sources from wastewater are accumulating mainly PHAs (polyphosphate- accumulating organisms) and glycogen, and releasing orthophosphate. The produced activated sludge contains higher concentrations of phosphate which is usually discarded but from which a small fraction is recycled, and used as an inoculum for upcoming wastewater [3].
6.2 Sewage sludge
Being the residue from the wastewater treatment process sewage sludge contains not only plant nutrients but also POPs (persistent organic pollutants), heavy metals, and also pathogens, pharmaceuticals and endocrine disrupting compounds. Because of environmental and health risks its use for agriculture is either prohibited or restricted. Currently, the use of sewage sludge for fertilizing purposes is mentioned to be low or decreasing in many European countries, where alternative disposal methods are focusing on co-incineration leading inevitably loosing phosphorus. On the other hand, in some parts of the world, direct agricultural use of wastewater and sewage sludge is being used, primarily because of its simplicity to recycle phosphorus [9].
In order to recover phosphorus and simultaneously decrease concentration of contaminants pH must be adjusted to under 2.0 with a strong acid which dissolves phosphorus over 80 % but simultaneously is interfered by other ions with various dissolving rates. To dissolve, and later to recover phosphorus different ways can be chosen. This includes anaerobic treatment, thermal hydrolysis and wet chemical leaching. Digested sludge is preferred over to raw sludge because of decrease of complex organic compounds during digestion. Two different wet chemical approaches for dissolution have been released the Seaborne process also known as the Gifhorner process in full-scale, and the pilot scale Stuttgart process. Theoretical phosphorus recovery in these processes is 99.9 % but their recovery potential is limited by the cost, decreasing the recovery to 35 %-45 % depending on the WWTP
influent. The alternative option for these processes is the Budenheim process that treat sludge with CO2 at pH 4-5 and consuming lower amounts of chemicals than two other processes mentioned [9].
6.3 Sewage sludge ash (SSA)
Sewage sludge ash (SSA) is organic matter which is closely completely oxidized during thermal treatment in temperatures between 800 °C and 900 °C. During the thermal treatment, 97 %-99.9 % of phosphorus accumulates to SSA including heavy metals with an exception of mercury which evaporates. The produced SSA is also water insoluble. The governing mineral phosphates present in the SSA are low- solubility minerals such as whitlockite, sometimes also minerals such as DCPD, estanite, and sometimes natural apatite can be present. Due to chemical precipitation during wastewater treatment, aluminium and iron phosphates are also present in the SSA, resulting to concentration of phosphorus from 50-100 g P kg-1 TS. Number of different approaches for phosphate recovery after SSA have been invented, the fluidized bed reactor for incineration the sludge is the most preferred method which produce a powdery ash and thus simplifies further treatments.
Different methods such as wet-chemical leaching, acidic leaching (e.g. EcoPhos®, BioCon®), alkaline leaching (LOTUS-project), and thermos-chemical (AshDec®) have been invented for phosphorus recovery. It reported that 70 %-85 % of wastewater plant influent can be recovered [9].
In wet-chemical treatment, after addition of acid such as sulfuric acid or phosphoric acid, resulting in over 90 % of phosphorus can be obtained from SSA when pH is below 2.0. This depends on the ratio of acid to ash which was found to increase from 66.5% to 99.4 % when the amount of acid (6 %) changed from 0.3 to 0.68 kg/kg SSA. The phosphorus recovery on the other hand depends on the chemical composition, especially from fractions of iron, aluminium and calcium present in the ash [10].
Dissolution of phosphorus is not the only compound which ends up to the leachate, as some heavy metals also dissolved. After removal of heavy metals pH is increased which precipitates phosphorus mainly as aluminium phosphate which is not suitable for fertilizer components. To solve this problem SESAL-Phos process has been
developed based on acidic pre-treatment to convert alkaline-insoluble calcium phosphate into alkaline soluble aluminium phosphate to produce calcium phosphates by precipitation after alkaline leaching which can be recovered in wastewater treatment plants with aluminium. Heavy metals are not extracted during the process. According Krüger and Adam (2015), recovery of heavy metals have been tested using sequential precipitation and nanofiltration. But so far data from metal recovery from SSA by acidic leaching does not exist [10].
In thermo-chemical treatment the aim is to remove heavy-metals and enhance the phosphorus bioavailability in one process step. The heavy metals removal via the gas phase take place when heated to high temperatures enhancing the phosphorus bioavailability in the same time by mineral phase destruction present in SSA and after formation of new phosphorus-bearing mineral phases which are available for plants. Different process for thermal-chemical treatment have been developed. For example, Mephrec and AshDec pilot plant which latter was operated for two year separated 70 %-90 % of heavy metals, and had capacity of 200-300 kg h-1. Currently, AshDec has not been yet implemented in industrial scale [10].
6.4 Chemical precipitation
The reasons for the use of chemical precipitation in wastewater treatment are to remove heavy metals and phosphorus, to improve the performance of primary settling facilities, and improve quality of reused water by softening. The main chemicals for phosphorus removal for wastewater are ferrous iron Fe(II), ferric iron Fe(III), aluminium Al(III), and calcium Ca(II) [11].
According to Piekema and Giesen (2001), conventional phosphate precipitation processes are based on the formation of calcium or iron salts or fixation in activated sludge. The last method usually generates huge amount of a water-rich sludge and requires continuous disposal, and results in increase of operation costs. To decrease disposal costs, mechanical dewatering of the sludge is often used before disposal.
But water content in the sludge is still relatively high (between 60 to 85 %). As a result, main part of the disposal costs comes from dewatering. Because of its insufficient high quality with very fine microscopic particles, the reuse of phosphate is not economically attractive. Furthermore, the use of mechanical dewatering
equipment is mentioned to be often troublesome and conventional processes requires a large operation area because of needed process steps consisting from coagulation, flocculation, sludge or water separation and sludge dewatering which are performed serially [12].
6.4.1 Precipitation of aluminium and ferrous phosphates
To precipitate phosphorus as iron phosphates, FeSO4 or FeCl2 can be used as precipitant producing a non-soluble ferrous phosphate, called vivianite [3]. Other chemicals used to precipitate iron phosphates are ferric chloride (FeCl3), and ferric sulfate [Fe2(SO4)3] which unfortunately may produce hydrogen sulphide during the digestion process [11]. Iron precipitation is also mentioned to be able to eliminate common phosphorus pollutants, such as mono-and polyphosphates when working with Zn, Cu and Ca ions [3].
For phosphorus removal aluminium salts used in chemical precipitation are alum, Al2(SO4)3 ∙ 18H2O or Al2(SO4)3 ∙ 14H2O, and aluminium chloride (AlCl3) [11]. De- Bashan and Bashan (2004) mentions that, the type of organic matter and its concentration in the wastewater affects the precipitation of aluminium compounds formation, which depends on the components present in the wastewater. As a result, different surface properties containing solid aluminium species are formed.
According to precipitated AlPO4 and theoretical analysis, when normal amount of alum is added to wastewater, the removal of orthophosphate is achieved via aluminium hydroxide phosphate precipitation instead of AlPO4 precipitation [3].
Historically the precipitation reactions have been written in Eq.(2) and (3) for aluminium and iron, respectively:
𝐴𝑙3++ 𝐻𝑛𝑃𝑂43−𝑛 ↔ 𝐴𝑙𝑃𝑂4(𝑠) + 𝑛𝐻+ (2)
𝐹𝑒3++ 𝐻𝑛𝑃𝑂43−𝑛 ↔ 𝐹𝑒𝑃𝑂4(𝑠) + 𝑛𝐻+ (3)
According to Sedlak (1991) and Water Environmental Federation (2011) overall reaction is more complicated, which is shown in Eq. (4):
𝑟𝑀𝑒3++ 𝐻𝑃𝑂4−+ (3𝑟 − 1)𝑂𝐻− → 𝑀𝑒𝑟 𝐻2𝑃𝑂4(𝑂𝐻)3𝑟−1 (𝑠) (4) where r=0.8 for Al(III) and 1.6 for Fe(III) [11].
7 Recovered phosphorus as fertilizer
The used cations for chemical precipitation and recycling as fertilizers are calcium and magnesium-based, and iron-and aluminium cations are commonly used for the purpose of phosphorus removal by precipitation from wastewater. The drawback of the latter as phosphorus recycling purposes is that aluminium is toxic for many plants and also to some soil organisms where iron phosphates are considered to be improper for plants. In addition, phosphorus recovery from iron and aluminium compound solutions is stated to be challenging [7].
According to de-Bashan and Bashan (2004), phosphate recovery has to date been attractive only in the wastewater treatments plants using biological nutrient removal. The produced activated sludge which contains notable amount of phosphorus is currently though to be contaminant instead of resource and threw away in fields. According to authors, 10 %-80 % of the phosphorus coming to treatment plant would be economically feasible to recover, and the quality of the recovered phosphate could be superior compared to currently used phosphate rocks.
The main source of phosphate for production of fertilizers comes from mined rocks since they are relatively cheap and abundant. However, high quality portion of sources founded so far are mentioned to be almost exhausted in 100 years.
Recovered phosphate from wastewater might become viable source for manufacturing of phosphate fertilizers, especially if low-grade phosphate rock has to be used for the fertilizer production, which can increase remarkably the production costs [3].
7.1 Phosphorus recovery as magnesium ammonium phosphate, struvite (MAP)
According to de-Bashan and Bashan (2004), excluding intentional struvite precipitation, magnesium is the most rarely used cation for phosphorus precipitation, and used for sludge stabilization assisting sludge digestion. In MAP (MgNH4PO4 ∙ 6H2O) precipitation, ammonia can be removed with phosphorus from wastewater when magnesium salt is added. This is mentioned to be the most promising compound to be recovered from wastewater treatment plants. In some processes precipitation is spontaneous [3]. According to Jaffer et al. (2002), spontaneous MAP precipitation in wastewater treatment plants is often seen as a disadvantage situation since it increases operational costs and reduces efficiency when precipitation is taking place in pipes and containers. This can be seen frequently for example in digester liquor discharge line, in anaerobic digester units, in centrifuge dewatering units and heat exchangers as reported by Bhuiyan et al.
(2007). Ohlinger et al. (1998) mentioned that methods for preventing struvite scaling might be effective but costly [13]. On the other hand, spontaneous precipitation of struvite acquires presence of high concentrations of phosphate and ammonia, high pH (>7.5), and low concentration of suspended solids. Furthermore, molar ratio of Mg2+/NH4+/PO43- as, 1:1:1 is needed. It means that addition of magnesium is required when great amounts of ammonium are often present in wastewaters [3]. According to the literature, the commonly used magnesium sources for MAP precipitation in chemical precipitation processes are MgO, MgCl2, MgSO4, and alternative sources such as sea water, bittern and wood ash are under investigation [13].
According to Kozik et al. (2011), suitable molar ratios of Mg2+: NH4+: PO43- would be 1.15:1:1, which can effectively remove ammonium and thus decrease phosphate concentration in the effluent. As reported by many researchers, size of struvite crystals increased with increase of [PO43-]:[Mg2+] from 1:1 to 1:1.2. According to Hao et al. (2008), struvite precipitates at pH range of 7.5-11.5, but suitable range is 7.5-9.0. Adnan et al. (2003), found that pH should be increased to 8.3 or higher to obtain high phosphorus recovery rate (>90 %) [14]. Typically, pH is adjusted to 8.0-8.8 and it can limit formation of other solids such as calcium phosphate or
calcium carbonate, minimize addition of chemicals, and limit the supersaturation degree [15].
Many different technological innovations for MAP recovery from biological processes appeared. On the other hand, common method for MAP recovery from these processes is lacking [3]. The most common method for ammonium and phosphate recovery in the form of MAP is mentioned to be upflow fluidized bed reactor [11]. It is also worth noting that operation concerning many wastewater processes do not meet the requirements for MAP crystallization. The best medium for precipitation and recovery is mentioned to be the sludge supernatant, obtained after EBPR process. But it may have only marginal effect on the net concentration of phosphorus in the sludge. On the other hand, struvite has potential to replace diammonium phosphate which can be produced by neutralizing phosphoric acid with ammonia. By mixing MAP with phosphoric acid can produce significantly more cost-effective fertilizer which might be superior compared to conventional diammonium phosphate fertilizers consisting slow-release MgHPO4, and fast- release ammonium phosphate (NH4)2HPO4. Even though use of MAP as fertilizer has potential value, it has not been proven to be commercially profitable [3].
According to Kataki et al. (2016), Dockhron (2009) estimated the cost of phosphorus recovery as struvite by chemical precipitation was approximately
$3500 tonne-1 of P, while the market value during that time was remarkably smaller which was only $765 tonne-1 [13].
According to Kataki et al. (2016), so far commercial scale MAP recovery has been demonstrated that only sludge and human urine as sources for wastewater treatment. Even though, its recovery has been widely studied as wastewater treatment method reported by Munch et al. (2001), its potential use as fertilizer has been only realized recently [13].
7.2 Process examples to recover phosphorus as struvite (MAP)
7.2.1 AirPrex ® process
Developed by Berliner Wasserbetriebe in collaboration with Berliner Institute of Technology AirPrex® technology crystallizes struvite directly from the sludge stream coming from an anaerobic digester instead of sidestream, preventing struvite formation in the sludge dewatering process. The process configuration includes a dual-stage aeration tank configuration which is divided to separate parts by wall if only one tank is used. The hydraulic retention time (HRT) in single tank is approximately 8 hours. Aeration is induced from the bottom of the reactor to remove carbon dioxide while solution is mixed which increases the solution pH.
Magnesium chloride is added to precipitate struvite which settles to the bottom of the conical section. Formed product is then withdrawn from tank continuously or at intervals, and washed. Additional struvite is possible to recover after second stage of the process where aerated sludge overflows are directed to sedimentation tank or the dewatering process [15]. According to Kataki et al. (2016), removal of CO2 by aeration increases pH to around 8.0 and results in struvite precipitation. Recovery rate of AirPrex® process is mentioned to be around 90 %-95 %, and restricted to wastewater treatment plants which are mentioned to operate in Germany and in the Netherlands [13].
Figure 1. AirPrex® process [15].
7.2.2 Multiform process
Developed by Multiform Harvest Inc., Multiform technology consists a conical section and a solid-liquid separation zone located at the top of the crystallizer which dimensions are selected to obtain desired range of superficial upflow velocities.
Injection system, locating at the bottom of the cone is used for addition of sodium hydroxide and magnesium chloride to increase pH to desired level and provide supersaturation conditions in the system [15]. Struvite crystals are removed from the bottom of the cone shaped fluidized bed crystallizer recovering 80 % of phosphate and 20 % of nitrogen from wastewater. The Multiform which situated after stages of anaerobic digestion and dewatering, to run wastewater through only once with a retention time of 15 minutes, or less for struvite crystallization is between two or three days. The technology can be used also to for swine farm wastes or food processing wastes, which are under investigation at two dairies [13].
7.2.3 Pearl® process
Pearl® technology was developed at the University of British Columbia and introduced by Ostara Nutrients Recovery Technologies Inc. (USA) at full-scale.
The process consists fluidised bed crystallizer with a segmented construction where the zone diameter decreases from top to bottom which reduces the upflow liquid velocity gradually when the segment diameter increases, retaining various sizes of struvite crystals within each zone. The solid/liquid separation section is located at the top of the reactor. To maintain the upflow velocity profile within the desired level, effluent is recirculated to the bottom of the reactor. When diameter of struvite pellets increases they drop from one zone to next, and are finally removed from the bottom zone after which it is screened dried and bagged. The circulation rate of effluent and retention time of struvite are adjusted to control pellet size in the final product [15]. The technology is suitable for side-stream treatment for high concentration of ammonium and phosphate containing effluents. If Pearl technology is incorporated with WASSTRIP® process approximately 80 % phosphate recovery efficiency, and between 10 % and 15 % to nitrogen can be obtained. In WASSTRIP® process, phosphate is stripped in an anaerobic zone from
activated sludge and added to the reject water [13]. In Fig. 2 Pearl® process is shown.
Figure 2. Pearl® process [15].
7.2.4 Phospaq™ process
Phospaq™ technology was developed in the Netherlands by Paques, and used commercially at Lomm, Olburgen and in the UK at Severn Trent’s Stoke Bardolph wastewater treatment works [13]. The Phospaq™ process consists a proprietary solids-liquid-air separation device located in the upper section of the vessel and an aerated reaction zone. Aeration design is used to strip CO2 from the solution and increase pH and provide efficient mixing. And dissolved oxygen can be used for biological treatment [15]. MgO is typically used as a magnesium source for struvite precipitation. Struvite can be collected from the bottom of the reactor [13, 15] and separated from mixed liquor through a hydrocyclone. Smaller struvite particles and biomass are allowed to settle and return to the reaction zone [15]. According to Kataki et al. (2016), up to 80 % of PO4-P can be recovered using this technology [13]. In Fig. 3 PhospaqTM process is shown.
Figure 3. PhospaqTM process [15].
7.3 Phosphorus precipitation as calcium phosphate
According to de-Bashan and Bashan (2004), calcium-phosphorus precipitation is commonly used because of its ease handling and low cost. The direct precipitation of hydroxyapatite can be obtained using calcite as seeding material, also calcium silicate hydrate can be used for phosphorus removal by crystallization [3]. In wastewater treatment, HAP is achieved by addition of lime (Ca(OH)2) and the recovered calcium phosphate is primarily used in food, dairy and industrial applications [11].
When HAP crystallization takes place between pH values of 8.0 and 8.5, 75 %- 85% phosphorus removal was obtained, without co-precipitation of calcium carbonate precipitation which was mentioned to have negative effect on the process, and taking place at pH 8.0. To avoid this solution pH can be increased. If, pH is increased to the range of 9.0 to 11.0, carbonates were co-precipitated. This not only competes with phosphate from calcium ions, but also produces phosphate a relatively low content as well [3].
The spontaneous precipitation of hydroxyapatite is also possible achieved if the solution is supersaturated. The supersaturation ratio can be increased using citrate which inhibiting HAP precipitation [3].
7.4 Process examples to recover phosphorus as calcium phosphate
7.4.1 Phostrip process
Phostrip process was specifically developed to enhance biological phosphorus removal (EBPR), and it is one of the first processes where phosphate recovery takes place in mainstream process instead of in sidestream. In the process, a portion of returned activated sludge (RAS) from EBPR process is subjected to anaerobic conditions causing stripping or release of orthophosphate from sludge to bulk water resulting to production of readily biodegrable COD (rbCOD). The retention time of the sludge in the EBPR process is between 12 and 20 hours [15]. In the process, the phosphate stripper tank consisting typically of a gravity thickener with thickened sludge recycling which are used to separate released phosphorus to the thickener overflow from the sludge. Also, a pre-stripper tank is used to denitrify the (RAS) flow and optimize phosphate release before phosphate stripping, after which it is directed back to mainstream process or part of it is sent as waste activated sludge (WAS) to the solids processing facility [15].
According to Metcalf and Eddy (2014), traditionally lime has been used for chemically precipitate stripper overflow at pH 9.0-9.5 to remove phosphate from the solution. Because of the presence of bicarbonate in the stripper overflow calcium carbonate formation takes place which results to a mixed solid composition. Formed chemical solids are then separated in the primary sedimentation tanks or separated individually [15].
It is also possible to mix the overflow of the stripper tank with side stream coming from post-digestion dewatering and fed the combined stream to a crystallization process to produce struvite. This concept can be adapted for waste activated sludge coming from EBPR process where waste activated sludge is derived to anaerobic conditions resulting to phosphate release when conditions allows solids hydrolysis and fermentation. Phosphate release from waste activated sludge before anaerobic digestion has significant decreased in struvite formation in the digester and its overflow. The activated sludge stripper of fermenter can be configured as a WAS thickener with sludge recycle in the similar way than in the Phostrip process. The magnesium release ratio in the WAS process is approximately 0.25g Mg/g PO43-–
P which reduces required amount of the magnesium hydroxide or oxide in the struvite crystallizer [15].
7.4.2 Crystalactor® process
Crystalactor® process consists a fluid-bed crystallizer where phosphate is removed and recovered as high puirity phosphate pellets. A major advantage of the crystallizer is that the formed crystal pellets, diameter around 1 mm can be re-used.
Also, the crystallizer design is compact since the needed space for the process has been reduced. The Crystalactor® process can be used successfully to recover phosphate from wastewater in the form of calcium phosphate, magnesium ammonium phosphate and magnesium phosphate [12]. The process recovery rate depends on the source of the processed wastewater, for wastewater coming from agricultural source where phosphate availability varies moderate phosphate recovery can be obtained with Crystalactor® process, whereas industrial source of phosphorus can be easily recycled with high recovery value. The phosphate content in the formed pellets is approximately 40 % to 50 %, when seed material content is around 30 % to 40 %. In the process no additional sludge is produced when phosphorus is crystallized almost exclusively as calcium phosphate pellets and seed material [16]. After atmospheric drying very low water content pellets (1 %-5 %) are obtained which consist 90 % to 98 % phosphate product and under 5 % of pellets consist from seed material [12]. Also, the process can be adjusted to most types of sewage treatment works. To promote the phosphate crystallization process conditions must be adjusted by addition of milk of lime or caustic soda. Because of high crystallization rate, short residence time is needed which can decrease the crystallizer size itself. Furthermore, requirements for pre-degasification and post- filtration may need. It is possible to reduce formation of calcium carbonate by adding sulphuric acid [16].
The formed calcium phosphate pellets have been used in the Netherlands by phosphate processing industries, and has been proved to be clean and attractive secondary phosphate source because of reusability and lack of residual wastes.
Pellets can be also used as slow-release fertilisers, raw material for kettle food, as
intermediate product for fertilizer production and raw material for phosphoric acid production [12]. Detailed picture of the Crystalactor®is showed in the figure 4.
Figure 4. The Crystalactor® process [12].
8 Recovery of organic phosphorus
Organic phosphorus is presented in natural waters in the form of animal or plant tissue, nuclei acids, phospholipids and nucleotides in the bodies of aquatic organisms. In addition, it can also be found due to its fixation into cellular material from agricultural, municipal and animal biosolids [7]. According to Qiu et al.
(2011), the main wastewater pollutants are organic phosphorus intermediates, by- products, and finished products such as epoxypropenyl phosphoric acid, propargyl phosphonic acid etc., and solvents like EDTA, anilines and alcohols [17].
According to Morse et al. (1998), techniques to recover organic phosphorus from an organic medium has not been invented by the phosphate industry, meaning that without major adaptation of sludges they cannot be used as a feedstock. Possible exception to this when phosphate is extracted from sludge ashes, and followed by biological phosphorus removal. In this technique phosphate is likely to band together with magnesium or calcium, instead of aluminium or iron salts [16].
Organic phosphorus has been transformed into the form of inorganic phosphorus, because it has largely been grouped in the “non-bioavailable” and “non-reactive”
component of total phosphorus. Above all, organic phosphate is not sensitive to the removal technologies used to separate inorganic phosphorus, which is why the transformation of organic into inorganic phosphorus is typically required to separate phosphorus from biosolids. In streams, such as animal wastes or organic solids digestion in the anaerobic processes, both of which contain high concentrations of organic phosphorus which reported by Salerno et al. (2009). A number of innovations have been applied, to extent microbial conversion in anaerobic digestion and enhance the kinetics. This includes pre-treatments using high temperature, enzymes, acid, base, oxidants, microwaves, or pulsed electric fields [7].
In low concentration streams of organic phosphorus, promising methods to transform organic to inorganic phosphorus are Advanced Oxidation Processes (AOPs). These include Fenton’s reaction, titanium dioxide photocatalysis, ozonation, ozone/peroxide, and UV/peroxide. AOPs have been typically evaluated for specific P-based contaminant destruction, such as organophosphorus pesticides relying on non-specific free radical species such as hydroxyl radicals (∙OH) which attack quickly to the organic compound structure [7], and degrade the organic pollutant, and minerlized to water, carbon dioxide and mineral salts [18]. On the other hand, because of non-specific nature of AOPs the technology might be impractical against high-strength streams like animal wastes in direct use. Because of vast amount of organic matter present, reactions with free radicals result in increasing amount of oxidant consumption which is not practical or economical.
Unfortunately, most of these treatments have issues concerning at least one of following drawbacks such as energy consumption, capital costs, chemical usage, odour and corrosion [7].
9 Calcium orthophosphates
Consisting from calcium, phosphorus and oxygen, calcium orthophosphates (PO43-
), includes in many calcium orthophosphates hydrogen either as an acidic orthophosphate anion such as H2PO4- or HPO42-, also it can incorporate with water as in dicalcium phosphate dihydrate (CaHPO4 ∙ 2H2O). Important parameters to
distinguish between calcium phosphate phases are the molar ratios of [Ca]:[P] and the solubilities. Calcium orthophosphates generate producing more soluble and acidic calcium phosphate phase in lower [Ca]:[P] molar ratio [19].
The crystallization of different calcium phosphates involves in many cases the formation of metastable precursor which dissolves later when the precipitation reactions proceeds. Because of presence of metastable phases, complex intermediate phases can participate in the crystallization reaction. Other inorganic additives than phosphate and calcium have significant influence on crystallization which makes predicting of forming phases difficult [19].
By means of precipitation calcium phosphate can be divided to two groups, calcium phosphate precipitates, and calcium phosphate calcinates. Precipitate calcium phosphate group depended on pH of the medium, whereas the calcinate group is a function of calcination temperature which consists only TCP (tricalcium phosphate) and TTCP (tetracalcium phosphate) [20].
Studies show that formation of apatite mineral is complicated because of possibility of forming several calcium phosphate phases. Hydroxyapatite which is the least soluble calcium phosphate phase is preferentially formed in neutral or basic conditions, or even beyond ideal conditions. The formed precipitates are generally nonstoichiometric which means the formation of calcium-definient apatites. In acidic solutions phases, octacalcium phosphate (OCP) and dicalcium phosphate dihydrate, (DCPD) are the common phases. And they are both implicated to be possible precursors for apatite formation. This may occur if DCPD or OCP is initially precipitated [19]. However, the formation of actual calcium phosphate phase is often dictated by kinetic factors instead of thermodynamic considerations under any given conditions. In addition, because of complicated precipitation process of calcium orthophosphates it is possible to crystallize number of different phases [21].
Great amount of studies has been carried out for crystalline hydroxyapatite (HAP), octacalcium phosphate (OCP), dicalcium phosphate anhydrate (DCPA) and dicalcium phosphate dihydrate (DCPD) due to biological mineralization [21]. In general, calcium phosphates are important materials in agriculture, chemistry, bioengineering and biochemistry. It has significant role in biological calcification
(teeth and bones formation), and is used for bone and tooth implant. On the other hand, they are causing problems in vital organs when formed in pathological conditions [22].
9.1 Solubility of calcium phosphates
All calcium orthophosphates have low solubility in water. HAP has the lowest solubility among them and can be as its natural priority. The solubility is greatly affected by pH of the medium which significantly affects their preparation and biological behaviour. Chow and Eanes (2001) have shown in their calculations that DCPA at pH < 4.2 and 25 °C being the least soluble in the Ca2+, H+/ OH-, PO43-//
H2O system, and at higher pH values HAP has the lowest solubility. Also, DCPD is the most soluble when pH exceeds 8.2, whereas TTCP being the most soluble at lower pH values than 8.2. In the pH range of 7.3 to 7.4 at 25 °C, the solubility of calcium phosphates can be put to in the order, TTCP > α-TCP > DCPD > DCPA >
OCP ~β-TCP > HAP [20]. Solubilities of different calcium phosphates are showed in the Figure. 5 and in Table I show also their stability range and Ca/P molar ratios.
Figure 5. Solubility of different calcium phosphates in different pH values [23].
Table I. Different calcium phosphates and their solubilities and stable pH ranges [24].
9.2 Monocalcium phosphate monohydrate (MCPM)
Monocalcium phosphate monohydrate, Ca(H2PO4)2 ∙ H2O, precipitates from solutions which are highly acidic, and are used normally in the production of phosphorus containing fertilizers (“triple superphosphate”). MCPM is the most water-soluble calcium phosphate compound and also the most acidic compound.
MCMP might be produced from precipitation of phosphoric acid and calcium carbonate in ambient temperature in aqueous and acetone medium might. Because of the nature of the compound, high solubility and acidity MCPM has not been found in biological calcifications, and as a pure compound MCPM is not biocompatible with bones. The main use of technical-grade MCPM is for agriculture where it is used as a fertilizer, but it is also used as a component of a several self-hardening calcium orthophosphate cements in medicine. In addition, it has also applications such as a nutrient, mineral supplement and acidulates for food, feed, some beverages, and dry baking powders or food additive [25].
9.3 Monocalcium phosphate anhydrate (MCPA)
MCPA, Ca(H2PO4)2, is anhydrous form of MCPM crystallized temperature higher than 100 °C. For example, during fertilizer production it can be obtained from highly concentrated mother liquors. MCPA, the same as MCPM, has high acidity
and is not biocompatible, and does not appear in calcified tissues. Compared to MCPM, anhydrous form has not been used as medicine because of highly hydroscopic properties which reduces its commercial application [25].
9.4 Tricalcium phosphate (TCP)
Four types of tricalcium phosphate polyforms β-TCP, α-TCP, α’-TCP and γ-TCP are known. Both α and β forms are precipitated by solid state reaction or by thermal decomposition. β-TCP can be obtained when stoichiometric mixture is heated close to 1000 °C for 30-60 minutes. α-TCP on the other hand, can be obtained when temperature is increased over 1200 °C, which transforms to α’-TCP if it is further heated to 1430-1470 °C. TCP production by thermal decomposition from ACP can be carried out by heating to 700 °C, or above 1200 °C to obtain α-TCP, whereas to obtain β-TCP temperature should be between 900 °C and 1150 °C. γ-TCP can be obtained when β-TCP is heated under a high pressure of 4.0 GPa to 950 °C, or under pressure of 15 GPa and 1200 °C if prepared using HAP [26].
In biological calcifications, β-TCP never form as pure compound. Instead, Mg- substituted form, whitlockite is found in dental calculi, and also in dentinal caries, urinary stones, salivary stones, arthritic cartilage, and in some soft tissue deposits.
On the other hand, no β-TCP has been found in bone, dentine or in enamel. In biomedicine, β-TCP is used in calcium orthophosphate bone cements and also other types of substitution bioceramics for bone. Other applications for β-TCP are for example in porcelain, pottery and polymer stabilizer [25].
In medicine, α-TCP is used in calcium orthophosphate cement, whereas in biological field it has not gained much attention. In acidic soils, technical grade α- TCP like β-TCP might be used as a slow-release fertilizer [25].
9.5 Amorphous calcium phosphate (ACP)
Amorphous calcium phosphate (CaxHy(PO4)z ∙ nH2O , n=3 – 4.5, 15 %-20 % H2O) precipitates as a precursor from neutral or basic solution through interaction with calcium, and phosphate during the synthesis of HAP crystals. Precipitation of amorphous calcium phosphate takes place at sufficient high supersaturation and pH
