Elena Bernalte‡
*a,
Joanna Kamieniaka,
Edward P. Randviir*b,
Álvaro Bernalte-Garcíac and
Craig E. Banksa
aFaculty of Science and Engineering, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK. E-mail: e.m.bernalte.morgado@bath.ac.uk; Tel: +44 (0)1225385457
bWaste to Resource Innovation Network, Faculty of Science and Engineering, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK. E-mail: e.randviir@mmu.ac.uk; Tel: +44 (0)1612471188
cDepartamento de Química Orgánica e Inorgánica, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, 06006, Badajoz, Spain
First published on 30th January 2019
Calcite originating from waste treatment technologies was utilised for the chemical precipitation of hydroxyapatite (HAP). The physicochemical properties of the as-synthesised-HAP was fully characterised using FT-IR, BET, SEM and TEM, confirming its crystal structure and formation of high purity HAP by XRD. The product was employed for removal of lead from aqueous media at pH 5.0, achieving almost 80% of the adsorption in the first 5 min and a maximum adsorption capacity for Pb2+ of 224.4 mg g−1. A contact time of 40 min was required to achieve equilibrium with Pb2+ uptake of 98%. The kinetics of the cation exchange of HAP from calcite were predicted using integrated rate laws, revealing a pseudo-second order cation exchange process with a rate constant of 6.84 × 10−4 g (mg min)−1. All obtained results are benchmarked against a control HAP sample simultaneously derived from eggshells, which were demonstrated to offer slower kinetics of cation exchange (4.82 × 10−4 g (mg min)−1) and almost half the maximum adsorption capacity (129.1 mg g−1). The results showed that hydroxyapatite synthesised from calcite waste represents a low-cost material for the adsorption of hazardous Pb2+ in contaminated waters and a promising alternative for heavy metals remediation in aquatic environments.
Calcite is an example of an unwanted material formed as a result of an advanced waste management systems. It is very well-known that calcite (CaCO3) is formed in many water processes causing blockages and technology failure in heat exchangers, tanks, reverse osmosis systems, and pipework within industrial environments. It is an off-white mineral that is extremely hard, crystalline, and highly pure. The EU estimates that calcite issues in processing technology (such as heat exchangers) can cost up to 0.25% of a nation's GDP in the industrialised world.2 Developing methods to feed these materials back into the supply chain will help close the loop to the ‘circular economy’.
Hydroxyapatite (HAP, Ca5(PO4)3OH) is an inorganic material that has been extensively studied due to its valuable characteristics and has been mostly employed in biomedical applications as it exhibits structural similarities to human bones and teeth. Besides various dental, orthopaedic and drug delivery fields that utilise HAP for its recognised biocompatibility, it has been also described as an excellent catalyst, because of its hydrophilic properties, structural stability and bifunctionality, as summarised in a review by Gruselle.3 In recent years, however, attention has turned towards the utilisation of HAP as a sorbent material for environmental pollutants. In particular, HAP has been successfully investigated for the adsorption of heavy metals, due to its low water solubility and high stability under oxidising and reducing conditions, giving rise to its excellent adsorption capabilities.4,5 In view of chemical synthesis of HAP, it essentially requires a source of calcium and phosphate. Thus, exploring innovative and simpler approaches, employing inexpensive recycled calcium materials and using reduced energy consumption protocols to synthesise such a benign material presents a worthwhile research challenge. Besides conventional synthetic chemical routes, a plethora of waste materials that attain high calcium content have been successfully utilised as a calcium source in the preparation process of HAP, such as mussel shells,6–9 eggshells,10–13 kina shells,14 fish bones or scale,15,16 gypsum,17–21 ash of poultry waste,20 aquaculture wastewater,22 biomass ashes,23 oyster shells,24 calcium sulfite from production of agrichemicals,25 abalone shells,26 cattle bones27 and mystery snail shells.28
Heavy metals' pollution of aquatic environments as a consequence of anthropogenic activities has drawn international attention due to their persistency, non-biodegradability, bioaccumulation and toxicity.29 Lead (Pb2+) is one of the most ubiquitous hazardous contaminants in aquatic environments of which presence at certain concentrations may cause irreparable damage for the environment, including flora and fauna, and humankind after consuming contaminated water.30 Consequently, the development of technology for the efficient removal of heavy metals from contaminated waters is a necessity of global interest.
This paper explores the utilisation of calcite waste, which is already in existence as common waste by-product, as a calcium source in the synthesis of hydroxyapatite (HAP). Calcium carbonate was extracted from waste calcite in acidic media and directly employed in the standard wet precipitation protocol to obtain stoichiometric HAP. The obtained material was benchmarked against eggshells-derived HAP that is widely reported in the literature. Resulting materials together with raw calcium precursors were fully characterised for their physicochemical properties and applied to remove hazardous Pb2+ from aqueous solutions.
Calcite is a common mineral found to build up inside water treatment technologies as a result of high levels of calcium ions within hard water. The process of calcium spontaneously depositing on a surface is commonly known as scale deposition, and can affect chemical technology significantly if not managed effectively. The resulting “scaling” of treatment technologies can cause severe process hindrance and downtime through increasing pipe pressure and blockages. This work takes advantage of a waste treatment process suffering from a “downtime” period, where calcite was responsible for its closure. The calcite was extracted from a process water tank suffering from severe calcite scale deposition. The tank was part of an industrial anaerobic digestion process for the treatment of municipal solid waste. The mineral was sheet-like and very brittle, hence was extracted from the tank walls using a hammer and chisel. The calcite was dried at 105 °C for 18 hours before use.
Pre-treated waste powders (8.51 g) were dissolved in a 100 mL solution of HCl (37%) and water in the molar ratio 1:3 and stirred for 2 hours at room temperature to extract calcium. The obtained solutions were filtered in a vacuum system to eliminate any possible residue and the resultant filtrates were kept at room temperature until used as calcium precursors to perform the synthesis of HAP.
(1) |
The kinetics of cation exchange on HAP were calculated using the function method of simple integrated rate laws. The pseudo-first order kinetic model was investigated using the Lagergen equation,10 expressed as eqn (2), which describes the adsorption rate of the material based on the adsorption capacity:19
(2) |
(3) |
Attention was turned to the investigation of the chemical composition of raw eggshells and calcite waste materials via XRF analysis. As summarised in Table 1, the results obtained confirmed an expected high calcium content in eggshells, which corresponded to 41.9%. A near identical high concentration of calcium was also detected in the waste mineral that was found to be 40.6%. Apart from calcium, minor impurities were identified in both materials, including Mg, Al, Si, P, S, Fe, Cl and Mn that are present at insignificant concentrations compared to Ca. It is also noted that Pb was not observed in significant levels within either material (0.0024 wt%).
Ca (%) | Mg (%) | Al (%) | Si (%) | P (%) | S (%) | Fe (%) | Cl (%) | Mn (%) | |
---|---|---|---|---|---|---|---|---|---|
Eggshells | 41.90 ± 0.097 | 0.204 ± 0.025 | 0.088 ± 0.0202 | 0.177 ± 0.002 | 0.175 ± 0.001 | 0.12 ± 0.000 | 0.010 ± 0.000 | 0.077 ± 0.000 | 0.004 ± 0.000 |
Calcite | 40.60 ± 0.092 | 0.554 ± 0.037 | 0.165 ± 0.003 | 0.472 ± 0.003 | 0.163 ± 0.001 | 0.491 ± 0.001 | 0.327 ± 0.003 | 0.066 ± 0.000 | 0.191 ± 0.003 |
Following chemical characterisation, XRD analysis was next performed to investigate the crystal structure of the raw materials. As presented in Fig. ESI 2,† the obtained diffraction patterns for both materials showed all peaks that clearly corresponded to pure calcite (CaCO3) (JCPDS 05-0586). This was an expected result for eggshells, however great similarities were identified in the pattern obtained for the extracted calcite, and a higher diffraction intensity was observed for the calcite when compared to eggshells. Furthermore, the observed patterns did not display any secondary phases present in both materials, at the same time excluding any major impurities present that could cause a detrimental effect in further synthesis.
Subsequently, spectral characterisation of the waste products was performed via FT-IR studies, using attenuated total reflectance (ATR) to indicate chemical bonding within the materials. As shown in Fig. ESI 3,† the most intense peak present at 1398 cm−1 corresponded to ν3 asymmetric stretching mode of carbonate. The peak observed at 872 cm−1 indicated asymmetric deformation of CO32− followed by the peak at 712 cm−1 associated with symmetric deformation of CO32−.31 The absence of any other significant peaks is in agreement with other studies performed that excluded any other phases or impurities and confirmed that calcite is the main component of the tested waste materials.
After full characterisation, both powders were separately treated with hydrochloric acid to extract the calcium into solution. The final concentration of Ca was accurately determined using ICP-OES. Based upon those results, the exact amount of phosphate precursor ((NH4)2HPO4) required to precipitate stoichiometric HAP was calculated in each case in order to maintain molar ratio of Ca/P of 1.67 during the synthesis process.
5CaCO3 + 3(NH4)2HPO4 + 2NH4OH → Ca5(PO4)3OH + 4(NH4)2CO3 + CO2 + 2H2O | (4) |
This novel approach utilised calcite materials directly from the source, without further purification or pre-treatment, instead of using refined material since this work recycles the waste calcite in order to obtain an advantageous material that can be employed in many different ways. This allows insights into the direct valorisation of a waste material at a reduced cost. Moreover, this synthetic approach is attractive due to an extremely high yield obtained from both products, which corresponded to 99.2% and 95.7% when calcite and eggshells were utilised, respectively.
The obtained HAP materials were characterised via SEM, which revealed a highly agglomerated crystalline HAP structure, as observed in Fig. 1. It is interesting to note, that as exhibited in SEM micrographs smaller particles were formed on the surface of HAP synthesised from eggshells. EDX analysis was subsequently performed to detect impurities formed within the precipitated HAP products. Excluding elements anticipated in HAP i.e. Ca, P and O, there were insignificant presence of other species within the products. In the mineral derived HAP the main impurities were Fe, Cl, Mg, Mn, S, Al and Si. The chlorine species were associated with residues from HCl employed to extract Ca, whereas all other elements were present in the raw mineral before any treatments (Table 1) and are likely intercalated within the structure due to trace ions in the waste water process. However, such low wt% of all elements (<1 wt%) were considered as insignificant. The eggshells-derived HAP exhibited Mg and Cl from the same origins. The structure of HAP was further studied by HR-TEM. As depicted in Fig. 2C, the obtained HAP particles were highly ordered hexagonal shaped when HAP was prepared using calcite. At higher magnification (shown within Fig. 2D), very clear repeated structural arrangements of HAP were observed, that indicated columns of calcium ions and oxygen atoms originating from phosphate groups, that are situated parallel to the hexagonal axis. This is a typical feature of HAP, which is demonstrated previously appearing as “light” coloured lines, circled on the HR-TEM images (Fig. 2). The eggshells-based HAP exhibited the same surface characteristic, shown in Fig. 2A and B, however the particles appear to exhibit a shorter range order (∼10 nm) compared to the calcite-based HAP (∼40 nm).
Following microscopic imaging, XRD analysis was performed to confirm phase purity of the prepared HAPs. As shown in Fig. 3B, the calcite based HAP was highly crystalline with sharp peaks that resembled the standard diffraction pattern (JCPDS 09-0432). Furthermore, no secondary phases were observed, indicating a highly efficient synthetic process. It is worth noting that the HAP prepared by employing eggshells (Fig. 3A) revealed a less crystalline structure, observed in Fig. 3A as broader diffraction peaks. Note in this particular application, reduced crystallinity may be more beneficial, since according to Hashimoto and Sato less crystalline materials promote enhanced adsorption capabilities.20 FT-IR studies revealed in both cases the characteristics of the HAP phase (shown in Fig. 4), with the main peaks at 1030 cm−1, corresponding to the stretching mode of PO, a small band at 870 cm−1 indicating HPO42− ions, and broad band at 1420 cm−1 typical of CO32− species.32 It is also worth noting that while the OH band typical in HAP appears reduced, this is actually a result of the increased intensity of the PO stretching mode. The peaks have been clearly labelled on Fig. 4 for ease of identification.
Subsequently, a nitrogen adsorption isotherm was employed to examine the porosity and pore size distribution of both materials. The BET surface areas of the HAPs corresponded to 45.4 ± 0.1 m2 g−1 and 113.4 ± 0.3 m2 g−1 for calcite and eggshells-derived HAPs, respectively. This demonstrates that even though there were no additional templates employed in the process, thus keeping the procedure simple, the obtained materials revealed relatively high porosity when compared to that reported in the literature.33 In consequence, as presented within Fig. 5A, BJH pore size distribution showed very uniform pores obtained from eggshells-derived HAP with the average diameter of 10 nm. Much bigger and irregular mixture of pores were obtained in HAP that utilised calcite (Fig. 5B) with the average of 53 nm; however the full range of pores present involved both meso- and macro-sized pores. It is worthy to note that even though calcite-derived HAP exhibited a lower surface area, it revealed higher pore volume when compared to eggshells-derived HAP and corresponded to 0.47 cm3 g−1 and 0.33 cm3 g−1, respectively. This suggests that during the synthesis calcite promoted formation of an open structure of HAP, which in consequence can be more beneficial for an ion exchange mechanism.12
Fig. 5 Pore size distribution and BET surface area HAP synthesised from (A) eggshells and (B) calcite. |
As represented in Fig. 6, a rapid kinetic reaction of Pb2+ adsorption by both HAPs occurred within the first 5 min. This behaviour was more drastically observed in HAP from calcite, where the removal of around 80% (187.4 mg g−1) of Pb2+ took place in the first 5 min. Differences in the capability of both HAPs to adsorb aqueous Pb2+ were also clearly observed in Fig. 6. Unexpectedly, HAP synthesised from calcite had a greater rate of adsorption and removed 98% of Pb2+ (218.6 mg g−1) in 40 min, whilst HAP synthesised from eggshells just achieved 58% (128.2 mg g−1) within 300 min. These results were unpredicted considering the fact that eggshells are a well-known and reported waste source to perform this synthesis, and also the higher purity of precursor from eggshells and subsequent HAP were assumed more favourable for this application. Also, as reported by Hashimoto and Sato,20 poorly-crystalline HAP showed greater capacity for Pb2+ removal than crystalline, so better effectiveness of HAP synthesised from eggshells in the adsorption experiments was expected in contrast with the experimental results obtained. It can thus be proposed from this experiment that the poorer crystallinity of the eggshells-derived HAP cannot explain previous experiments due to the fact that the more crystalline calcite-derived HAP has significantly out-performed the eggshells-derived HAP in terms of cation exchange. This observation is also supported by the sorption kinetic rates calculated for both materials using eqn (2) and (3). As depicted in Fig. 6B, the R2 value of the correlation coefficients indicated that the adsorption of Pb2+ onto both eggshell and calcite-derived HAPs fit the expected pseudo-second order model, where greater linear correlation obtained for calcite HAP explains enhanced efficiency of this material for the adsorption process. Also, the characteristic rate constants (K2) estimated for both materials after the adjustment to the pseudo-second order model revealed a higher value for calcite-derived HAP. Corresponding calculated values using the pseudo-first order and the pseudo-second order models for the adsorption of Pb2+ on calcite-derived HAP and eggshells-derived HAP are summarised in Table 2.
Metal ion | HAP | Pseudo-first order model | Pseudo-second order model | ||||
---|---|---|---|---|---|---|---|
qe | K1 | R2 | qe | K2 | R2 | ||
Pb2+ | Calcite | 12.68 | 8.52 × 10−3 | 0.2142 | 178.57 | 6.85 × 10−4 | 0.9942 |
Eggshells | 59.53 | 1.66 × 10−2 | 0.9647 | 105.26 | 4.82 × 10−4 | 0.9850 |
The efficacy of HAP synthesised from calcite compared to eggshell is also demonstrated in Fig. 7, where the time-evolution of the concentration of Pb2+ and Ca2+ in solution is monitored during the adsorption process by using ICP-OES. It was observed that the cation exchange reaction took place during the adsorption process of Pb2+ on HAP synthesised from both waste materials because of a decrease in the concentration of Pb2+ in solution was accompanied with an increase in concentration of Ca2+ from HAP in a similar proportion (Fig. 7A and B). Substitution of Pb2+ in the structure of HAP was remarkably more effective in HAP from calcite (Fig. 7B) where a concentration of 5 mg L−1 of Pb2+ was only remaining in solution after 40 min, demonstrating that the cation exchange process took place favourably on this material. This may be related to the pore volume of the calcite-derived HAP, which was demonstrated to be larger than the eggshells-derived HAP, thus allowing movement of ions in and out of the open structure of HAP more efficiently.
Finally, the maximum adsorption capacities of HAP synthesised in the present study from calcite and eggshells were calculated using eqn (1) and corresponded to 224.4 mg g−1 and 129.1 mg g−1, respectively. They were similar to those reported in the literature for the removal of Pb2+ using HAP obtained from recycled eggshells (101 mg g−1),10 incinerated ash of poultry waste (277 mg g−1)20 or flue gas desulfurisation gypsum waste (227.8 mg g−1).29
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra04701d |
‡ Current affiliation: Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK. |
This journal is © The Royal Society of Chemistry 2019 |