J O U R N A L O F C H E M I S T R Y Materials Porous hydroxyapatite monoliths from gypsum waste Sachiko Furuta,a Hiroaki Katsuki*a and Sridhar Komarnenib aSaga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga, 844-0024, Japan. E-mail: katsuki@emon.scrl.pref.saga.jp bIntercollege Materials Research Laboratory, and Department of Agronomy, The Pennsylvania State University, University Park, PA 16802-4801, USA Received 25th August 1998, Accepted 8th October 1998 Porous hydroxyapatite (HAp) monoliths were synthesized from gypsum mold waste with diammonium hydrogen phosphate solution by hydrothermal treatment at 50–190 °C and their properties studied.Gypsum waste samples could be completely converted to HAp with 0.5 mol dm-3 (NH4)2HPO4 at 50 °C in 15 days and 100 °C in 2 days.This paper describes the eVect of hydrothermal reaction conditions such as reaction temperature, time and concentration of (NH4)2HPO4 on the formation of new HAp monoliths and their porous properties. vessel of a stainless steel hydrothermal reactor. The initial pH 1 Introduction of the (NH4)2HPO4 solution was ca. 7.5. The hydrothermal Hydroxyapatite [Ca10(PO4)6(OH)2], which is well known as reactor was introduced into a drying oven which was heated a main component of bone and teeth, has been used widely in in the range 50–100 °C for 1–15 days.After hydrothermal many industrial applications and in the medical field. For treatment, reacted samples were washed with distilled water example, HAp has been found to have cation-exchange proper- to remove residual ions such as SO42-, NH4+ and PO43- and ties where Ca2+ ions of HAp can be exchanged with poisonous were dried.The drying temperature was kept below 50 °C so heavy metal ions.1,2 HAp also has the ability to adsorb organic that unreacted gypsum crystals would not be decomposed chemicals.3,4 Generally, HAp powder is synthesized by sev- during the drying treatment.To promote the formation rate eral routes such as precipitation from solution containing of HAp, 1.0 mol dm-3 (NH4)2HPO4 was used as the starting Ca2+ and PO43- ions5,6 and hydrolysis of CaHPO4 or solution and the reaction temperature was increased to 190 °C. CaHPO4·2H2O at room temperature7 and hydrothermal syn- The conversion of gypsum to HAp was estimated from the thesis of Ca3(PO4)2 at 200 °C.8 One of the problems with these ratio of X-ray intensities of the peaks of gypsum (hkl=020, processes is that HAp monoliths such as bone or tooth, etc. 2h=11.62°) and HAp (hkl=211, 2h=31.77°) by powder Xcan not be obtained from these chemicals by an in situ method. ray diVraction (XRD, Model RAD-2B, Rigaku Co., Japan). Therefore, synthetic HAp powder is molded and then sintered The HAp samples were dissolved in Na2CO3–H3BO3 solution, at 1000–1250 °C.9 and the chemical composition of HAp was analyzed by ICP A large amount of gypsum plaster (CaSO4·2H2O) is used emission spectrometry (ICPS-2000, Shimadzu Co., Japan).as molds for slip or pressure casting in the ceramic industry. The crystal morphology of the synthetic samples was observed These gypsum molds wear out after repeated use (ca. 100 by scanning electron microscopy (SEM, Model JXA-840, times) and are then discarded as waste.Gypsum is a sparingly JEOL Co., Japan) and transmission electron microscopy soluble calcium salt, and has never been used previously as (TEM, Model 2010, JEOL Co., Japan). The pore diameter, the source material to prepare HAp. Here we develop a novel porosity and pore size distribution of products were investiprocess for preparing porous HAp monoliths directly from gated by mercury porosimetry (Pore Sizer 9310, Micromeritics gypsum waste by in situ crystallization using the following Co., USA) and the surface area was measured by the BET chemical reaction.method with N2 gas adsorption (Autosorb 1, Quantachrome Co., USA). 10CaSO4·2H2O+6(NH4)2HPO4�Ca10(PO4)6(OH)2 +6(NH4)2SO4+4H2SO4+18H2O 3 Results and discussion This concept will alleviate the environmental problem from the following two standpoints: (1) eVective recycling of indus- 3.1 Synthesis of HAp from gypsum waste trial waste materials, and (2) use of this new porous HAp as Gypsum waste used in this study was composed of fine needle- a purification system of waste water containing heavy metal like crystals of length 5–10 mm and 1–2 mm thickness as shown ions or organic chemicals. in Fig. 1. This gypsum has a porous structure with 2.4 mm In this study, porous HAp monoliths were produced by in median pore diameter, 60% porosity and 1.6 m2 g-1 surface situ crystallization from gypsum mold waste by a conventional area. The eVect of reaction temperature on the formation of hydrothermal treatment and its characteristics were HAp was investigated with 0.5 mol dm-3 (NH4)2HPO4.Fig. 2 investigated. shows XRD patterns of the as-received gypsum and the samples synthesized for 1, 2 and 3 days at 100 °C, and Fig. 3 2 Experimental shows XRD patterns of the samples synthesized at 50, 75 and 100 °C for 6–15 days. Fig. 4 summarizes the conversion rate Gypsum waste mold was washed and cut into rectangular pieces of 5×10×20 mm. The surface layer of gypsum mold from gypsum to HAp based on the calculation from XRD intensity ratios of gypsum and HAp. At 100 °C, ca. was removed since small amounts of some impurities such as Na and Si components from the ceramic raw materials are 58–59 mass% gypsum was hydrothermally converted into HAp after 1 day, and the conversion of the gypsum to HAp reached deposited on the surface of the repeatedly used gypsum mold.The gypsum and 0.5 mol dm-3 diammonium hydrogen phos- 100 mass% after 2 days. HAp monoliths could be hydrothermally prepared by in situ crystal growth from gypsum waste phate [(NH4)2HPO4] solution were placed in a 50 ml Teflon J. Mater.Chem., 1998, 8, 2803–2806 2803Fig. 1 Morphology of gypsum waste. Fig. 4 The conversion rate of gypsum to HAp based on the calculation from the X-ray intensity ratio under the hydrothermal reaction with 0.5 mol dm-3 (NH4)2HPO4. &: 50°C, $: 75°C and +: 100 °C. involves slight leaching of Ca2+ ions from gypsum crystals and these ions then react with PO43- and OH- ions in solution to form HAp until all the gypsum crystals are converted.These results show that the novel HAp monoliths can be synthesized by in situ crystallization of gypsum waste with (NH4)2HPO4 solution under conventional hydrothermal treatment at 50–100 °C. 3.2 Properties of new HAp crystals Fig. 2 XRD patterns of (a) gypsum waste and samples synthesized by To characterize the chemical composition of synthesized HAp hydrothermal reaction with 0.5 mol dm-3 (NH4)2HPO4 at 100 °C for crystals, the Ca/P molar ratio of the crystals was analyzed by (b) 1 day, (c) 2 days and (d) 3 days.ICP. The Ca/P molar ratios of crystals obtained at 50 °C for 15 days and 100 °C for 6 days were calculated to be 1.25 and 1.30, respectively. These new HAp crystals were nonstoichiometric compared to theoretical hydroxyapatite [Ca10(PO4)6(OH)2] whose Ca/P ratio was 1.67. HAp is known to be a stable substance even though its composition is nonstoichiometric. There are several previous reports11–13 on explaining why the Ca/P ratio of apatite can be varied over a wide range: (1) other crystal phases such as CaH(PO4)3·2H2O and Ca4H(PO4)3·3H2O can co-exist,11 (2) phosphoric acid can be adsorbed on the surface of HAp,12 (3) there may be a deficiency of Ca2+ in the crystal lattice of apatite.13 In our study, only the HAp phase was observed by XRD after complete conversion.Thus it is considered that residual phosphoric acid is adsorbed on the surface of HAp because the concentration of (NH4)2HPO4 in the starting solution is stoichiometrically higher than that of gypsum.The crystallinity Fig. 3 XRD patterns of HAp synthesized from gypsum waste and of the HAp crystals increased with reaction temperature as 0.5 mol dm-3 (NH4)2HPO4 under the following hydrothermal reaction can be deduced from peak widths of the XRD patterns conditions: (a) 100 °C6 days; (b) 75 °C, 7 days; (c) 50 °C, 15 days. in Fig. 3. Fig. 5 and 6 show the morphology of HAp synthesized at 50, 75 and 100 °C.After reaction at 50 °C for 7 days, unreacted and (NH4)2HPO4 at 100 °C in 2 days. To prepare smaller HAp crystals, the growth of HAp was examined at lower gypsum crystals were still present within the body of the monolith, but the surface was coated with fine HAp crystals temperatures. The conversion of gypsum to HAp was 100 mass% at 75 °C after treatment for 7 days and after 15 of diameter 10–30 nm as shown in Fig. 5(b). Almost all gypsum crystals were completely converted into the HAp days at 50 °C. Monoliths of HAp resulted under all reaction conditions, phase after 15 days leading to an HAp monolith which was composed of crystals of 80–120 nm. With increasing reaction and the size of the HAp monoliths were the same size as the starting gypsum piece in all cases.During this preparation temperature, the crystal growth of HAp was enhanced and the crystals grew to 3–8 mm in length at 100 °C for 3 days. process, HAp is produced in a solution under neutral or mildly alkaline condition with the solubility product To promote the formation rate of HAp, the gypsum waste was reacted with 1.0 mol dm-3 (NH4)2HPO4 at 190 °C.[Ca2+]5[PO43-]3[OH] exceeding that of HAp.6 The solubility of CaSO4·2H2O in water in the present reaction is ca. However, under these conditions the gypsum could not be completely converted into the HAp phase because the surface 0.16–0.21% at 50–100 °C.10 The mechanism of HAp formation 2804 J. Mater. Chem., 1998, 8, 2803–2806Fig. 6 Morphology of HAp crystals synthesized from gypsum waste Fig. 5 Morphology of HAp crystals synthesized from gypsum waste and 0.5 mol dm-3 (NH4)2HPO4 by hydrothermal reaction. (a) and and 0.5 mol dm-3 (NH4)2HPO4 by hydrothermal reaction. (a) 75 °C, 3 days; (b) 75 °C, 7 days; (c) 100 °C, 1 day; (d) 100 °C, 3 days. (b): 50 °C, 7 days; (c) and (d): 50 °C, 15 days. were thus not carried out using this concentration of (NH4)2HPO4.of gypsum rapidly reacted with (NH4)2HPO4, and a tight HAp layer of about 1 mm thickness was formed on the surface 3.3 Porous properties of HAp monoliths after reaction for 3 days and prevented further conversion of gypsum crystals to HAp. Therefore, unreacted gypsum crystals In this study, novel HAp monoliths were easily synthesized from the reaction of gypsum waste with (NH4)2HPO4 and it remained within the body of the monolith.Further reactions J. Mater. Chem., 1998, 8, 2803–2806 2805at 100 °C for 3 days. The surface area strongly depends on the morphology of HAp, and decreased with increased crystal size. The porosities of the HAp samples which were synthesized using 0.5 mol dm-3 (NH4)2HPO4 at 75 °C for 1–10 days were in the range 75–78% while the porosity of starting gypsum waste was 60%.Pore size distributions of the starting gypsum and products at 75 °C are shown in Fig. 8. The bimodal pore distribution in Fig. 8(b) and (c) is attributed to the diVerent pore sizes of gypsum waste and HAp. Pore diameters of products obtained after treatment for 1–7 days centered at 2–3 mm were apparently from unreacted gypsum, and the pore distribution of HAp aggregates was between 0.01 and 1 mm with a median pore diameter of around 0.3 mm.The pore size distribution of gypsum centered at 2–3 mm remained in the sample treated up to 7 days at 75 °C, and the sample had double-pore structures, one from gypsum and the other from HAp. A single pore distribution derived from HAp crystals was obtained after 10 days at 75 °C.While it was not possible to obtain a single pore structure at 50 °C over short times, the sample prepared at 100 °C for 2 days showed only the pore size distribution of HAp. Fig. 7 The surface area of synthesized HAp with 0.5 mol dm-3 of (NH4)2HPO4. &: 50°C, $: 75°C and +: 100 °C. Conclusion In this study, porous HAp monoliths were synthesized and characterized from gypsum mold waste and (NH4)2HPO4 solution by hydrothermal reaction.The main results are as follows. (1) The conversion of gypsum to HAp was aVected by reaction time and temperature taking about 2 weeks for complete conversion at 50 °C, but only 2 days at 100 °C when 0.5 mol dm-3 (NH4)2HPO4 was used. The crystallinity of the synthesized HAp was enhanced by increasing the reaction temperature.SEM observation revealed that HAp monoliths formed at 50 °C in 15 days and were composed of fine crystals of HAp of 80–120 nm diameter and 0.5–1 mm length while HAp formed at 100 °C for 3 days showed larger crystals of size 3–8 mm. (2) The surface area of synthesized HAp monoliths ranged from 20–45 m2 g-1 at 50–100 °C and was related to the morphology of the HAp crystals.The pore size distribution of gypsum was ca. 2–3 mm and decreased in intensity during the hydrothermal reaction while the pore size distribution of HAp, which was between 0.01 and 1 mm, increased. Under appropriate conditions of time and temperature of reaction, porous HAp monoliths with single pore structure resulted from the reaction of gypsum waste and (NH4)2HPO4.Fig. 8 The pore size distributions of (a) starting gypsum and HAp synthesized with 0.5 mol dm-3 (NH4)2HPO4 at 75 °C for (b) 1 day, (c) 7 days and (d) 10 days. 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