Hydroxyapatite is, first of all, a natural occurring mineral. As such, the use of natural-derived hydroxyapatite (HAP) is encountered in several published works. The environmental application of such materials is briefly presented.
1. Application of Natural-Derived Hydroxyapatite for the Removal of Heavy Elements
Hydroxyapatite is, first of all, a natural occurring mineral. As such, the use of natural-derived hydroxyapatite (HAP) is encountered in several published works. The natural hydroxyapatite can be obtained by the processing of several resources, including animal and fish bones, coral or egg shells
[35][1]. The natural-derived HAP is mainly used in biomedical applications, due to its intrinsic properties, such as the presence of trace elements or its natural structure. Nevertheless, natural HAP can also be encountered in a series of environmental applications, including the ones targeted by the present re
vise
warch, removal of heavy metals (
Table 31). In this category,
wresearche
rs also mention the studies presenting the separation of one component (typically CaO) from natural sources (wastes of animal origin or algae), followed by other steps to develop the final hydroxyapatite material.
Table 31.
Application of natural-derived hydroxyapatite for the removal of heavy elements (references presented in chronological order)
1
.
Natural Source |
Treatment |
Application |
Heavy Metal |
Adsorption Parameters |
Ref. |
Bovine bones |
Boiled, calcinated |
Sorption studies using hydroxyapatite/poly (acrylamide-acrylic acid) composite |
Sr(II) |
Kinetics: pseudo-first-order model; ion exchange predominant model; Qmax = 53.59 mg Sr(II)/g |
[36][2] |
Bovine bones |
Boiled, calcinated |
Sorption studies; HAP characteristics: Ca/P ratio = 2, superficial area = 4.106 m2/g |
Pb(II) |
Kinetics: pseudo-second-order model; Qmax = 89 mg Pb(II)/g |
[37][3] |
Chicken bones |
Carbonized, calcinated |
Sorption studies using HAP and HAP/Fe3O4 composites |
Pb(II) |
Kinetics: pseudo-second-order model; Qmax = 105.26 mg Pb(II)/g HAP; Qmax = 109.89 Pb(II)/g HAP-Fe3O4 |
[38][4] |
Clam shells (Ca precursor) |
Dissolved in water and nitric acid, addition of H3PO4 |
Sorption studies; HAP: micrometric particle range, SSA 188.5–139.8 m2/g |
Sr(II) |
Kinetics: pseudo-second-order model; Qmax = 45.36 mg Sr(II)/g |
[39][5] |
Clam shells (Ca precursor) |
Grinded, calcinated, addition of H3PO4 |
Sorption studies |
Pb(II), Cd(II), Cu(II) |
Kinetics: pseudo-second-order model; Qmax = 265 mg Pb(II)/g; Qmax = 64 mg Cd(II)/g, Qmax = 55 mg Cu(II)/g |
[40][6] |
Mussel shells (Ca precursor) |
Grinded, calcinated, addition of NH4H2PO4 |
Sorption studies |
Cd(II) |
Kinetics: pseudo-second-order model, Langmuir isotherm; Qmax = 62.5 mg Cd(II)/g |
[41][7] |
Eggshells (Ca precursor) |
Grinded, dissolved in HCl, addition of (NH4)2HPO4 |
Sorption studies; HAP: hexagonal, 10 nm, SSA: 113 m2/g |
Pb(II) |
Kinetics: pseudo-second-order model; Qmax = 129.1 mg Pb(II)/g |
[42][8] |
Eggshells (Ca precursor) |
Grinded, addition of H3PO4, Ca(OH)2, Na2CO3 and Na2SiO3 under ultrasounds to obtain Na-SiCHAP |
Sorption studies; hexagonal, 10 nm, SSA: 79.09 m2/g, PD 21.32 nm, PV 0.40 cm3/g |
Pb(II), Cd(II) |
Kinetics: pseudo-second-order model, Langmuir isotherm model; Qmax = 698.68 mg Pb(II)/g, Qmax = 129.60 mg Cd(II)/g |
[43][9] |
Bovine horns core |
Boiled, acetone soaking, drying, calcination |
Sorption studies, using HAP with different characteristics, dependent on the calcination temperature |
Cu(II) |
Kinetics: pseudo-second-order model; Qmax = 99.98 mg Cu(II)/g |
[44][10] |
Bovine horns core |
Boiled, acetone soaking, drying, calcination |
Sorption studies, using HAP with different characteristics, dependent on the calcination temperature |
Pb(II), Cd(II) |
Kinetics: pseudo-second-order model; Qmax = 256.41 mg Pb(II)/g, Qmax = 105.26 mg Cd(II)/g |
[45][11] |
Fish scales |
Soaked in HCl, treated with NaOH, heated |
Sorption studies; HAP: Ca/P ratio = 1.96 |
Pb(II) |
100% removal of 0.74 mg/L lead, after 10 min., using 4% HAP |
[46][12] |
Bovine femur bone |
Washed with water, H2O2, HNO3, bleached, calcinated |
Sorption studies, by comparison with commercial HAP; HAP: SSA: 46.8 m2/g, PD 25.5 nm, PV 0.18 cm3/g |
Pb(II), Cd(II) |
Kinetics: pseudo-second-order model; Qmax = 166.67 mg Pb(II)/g, Qmax = 138.89 mg Cd(II)/g |
[47][13] |
Bovine femur bone |
Washed with water, H2O2, HNO3, bleached, boiled, calcinated |
Sorption studies, by comparison with commercial HAP; HAP: SSA: 46.87 m2/g, PD 10 nm, PV 0.164 cm3/g |
Cu(II), Fe(III) |
Kinetics: pseudo-second-order model; Qmax = 102.35 mg Cu(II)/g, Qmax = 87.245 mg Fe(II)/g |
[48][14] |
Bovine cow bone |
Dried, pyrolyzed, milled |
Sorption studies; HAP: SSA: 313.09 m2/g, PD 6.46 nm, PV 0.4538 cm3/g |
Cd(II), Cu(II), Pb(II) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 165.77 mg Cd(II)/g, Qmax = 287.58 mg Cu(II)/g, Qmax = 558.88 mg Pb(II)/g |
[49][15] |
Chlorella powder |
Added aq. NaOH and sodium dodecyl sulfate, microwave heated |
Sorption studies in the form of hollow microspheres with multicomponent nanocores; HAP: Ca/P ratio = 1.72, PD 32.6 nm |
Cd(II) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 116.434 mg Cd(II)/g |
[50][16] |
Fish bones |
Washed, dried, pulverized, sieved |
Sorption studies; particle size 149–325 nm, PD 33–105 nm |
Cu(II), Ni(II), Zn(II) |
Langmuir isotherm (copper), Freundlich isotherm (nickel and zinc); >95% ion removal (30 mg/kg ion concentration) |
[51][17] |
Fish scales |
Sonicated, dried, grinded |
Sorption studies; HAP: SSA 102.2 m2/g, PD 9.14 nm, PV 0.28 cm3/g |
Hg(II) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 227.27 mg Hg(II)/g |
[52][18] |
Eggshells (Ca precursor) |
Grinded, calcinated, addition of H3PO4 |
Sorption studies using bentonite/CoFe2O4/HAP composite |
Pb(II) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 66 mg Pb(II)/g |
[53][19] |
Snail shells (Ca precursor) |
Boiled, grinded, calcinated, addition of (NH4)2HPO4 |
Sorption studies, using HAP and HAP-SiO2 composite; HAP: Ca/P ratio = 1.64 |
Pb(II) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 123 mg Pb(II)/g HAP; Qmax = 135.14 mg Pb(II)/g HAP-SiO2 |
[54][20] |
Eggshells (Ca precursor) |
Dried, calcinated, addition of H3PO4 |
Sorption studies, using HAP and polymeric modified HAP; HAP: Ca/P ratio = 1.63 |
Co(II), Sr(II) |
Kinetics: pseudo-second-order model; Freundlich isotherm; Qmax = 43.48 mg Co(II)/g, Qmax = 30.4 mg Sr(II)/g |
[55][21] |
Chicken thigh bones |
Boiled, carbonized, calcinated |
Sorption studies using HAP/Fe3O4/polydopamine composite; HAP: SSA: 16.722 m2/g, PV 0.008 cm3/g, PD 1.935 nm |
Hg(II), Co(II), Ni(II) |
Kinetics: Intraparticle diffusion model; Langmuir isotherm; Qmax = 51.73 mg Hg(II)/g, Qmax = 49.32 mg Co(II)/g, Qmax = 48.09 mg Ni(II)/g |
[56][22] |
Bovine bones |
Boiled, crushed, calcinated |
Sorption studies using a dynamic membrane of HAP, Sargassum glauscens nanoparticles, chitosan and polyvinyl alcohol |
Zn(II), Co(II), Ni(II) |
Over 90% removal efficiency |
[57][23] |
Fish scales |
Boiled in NaOH, dried, calcinated (800 and 900 °C) |
Sorption studies; HAP: SSA 88.73/103.46 m2/g, PV 0.38/0.36 cm3/g, PD 1.64/1.84 nm |
Ni(II) |
Kinetics: pseudo-first-order model; Langmuir isotherm; Qmax = 114.151 mg Ni(II)/g HAP (800 °C), Qmax = 181.321 mg Ni(II)/g HAP (800 °C) |
[58][24] |
Bovine cortical bones |
Carbonized, calcinated |
Sorption studies using HAP/chitosan/snail shell powder composite |
Cu(II), Zn(II) |
Kinetics: pseudo-second-order model; Langmuir/Temkin isotherms; Ion removal: 90%/60% (for 3 mg/L initial ions concentration) |
[59][25] |
Camel bones |
Dried, grinded, soaked in H3PO4, treated with HNO3 and H2O2, dried |
Sorption studies; HAP consisting material: SSA 19.29 m2/g, PV 0.054 cm3/g, PD 11.18 nm |
V(V) |
Kinetics: pseudo-second-order model; Langmuir isotherm; Qmax = 19.45 mg V(V)/g |
[60][26] |
Chicken bones |
Dried, carbonized, calcinated |
Sorption studies using HAP, HAP/Fe3O4 and polydopamine/HAP/Fe3O4, composites |
Zn(II) |
Kinetics: pseudo-second-order model; Freundlich isotherm; Qmax = 37.57 mg Zn(II)/g HAP, Qmax = 40.07 mg Zn(II)/g Hap-Fe3O4, Qmax = 46.37 mg Zn(II)/g poly-Hap-Fe3O4 |
[61][27] |
Eggshells (Ca precursor) |
Washed, dried, calcinated, addition of H3PO4 |
Sorption studies; HAP: Ca/P ratio = 1.65, SSA 63.7 m2/g, PV 0.1512 cm3/g |
Pb(II) |
Kinetics: pseudo-second-order model; Sips isotherm; Qmax = 518.46 mg Pb(II)/g |
[62][28] |
Eggshells (Ca precursor) |
Washed, dried, calcinated, pulverized, addition of HNO3 and (NH4)2HPO4 |
Sorption studies; HAP: Ca/P ratio = 1.74, SSA 32 m2/g |
Cu(II), Ni(II) |
Kinetics: pseudo-second-order model; Freundlich isotherm; Qmax = 10.58 mg Cu(II)/g, Qmax = 9.53 mg Ni(II)/g |
[63][29] |
Hassan et al.
[36][2] presented the sorption of Sr(II) on a gamma-radiation composite including natural-derived hydroxyapatite and poly(acrylamide-acrylic acid). The
resea
uthorchers proposed a complex mechanism for the Sr(II) uptake, including ion exchange (between the Ca and Sr ions) and surface complexation. The equilibrium data presented suggested that the best fitting isotherm models were Langmuir (for 298 K) and, respectively, Freundlich for 318 and 328 K. In addition, the uptake capacity of the composite was superior to other popular sorbents, including carbon nanotubes, kaolinite, or biosorbents, in the studied concentration range (10–50 mg/L).
2. Specifics
HAP sorbent also obtained from bovine bones was applied by Caballero et al.
[37][3] for the removal of Pb(II) in a wide range of concentrations (400–1400 mg/L). Their results suggested that the best fitting isotherm for the adsorption process is represented by the Freundlich isotherm, thus supporting an adsorption in a monolayer. The
resea
uthorchers also suggested that the optimum adsorbent concentration was 0.7 g/L, for this concentration being reached a removal efficiency of approx. 100%. Adsorption of Pb(II) was also evaluated by Vahdat et al., using both chicken-derived HAP and a magnetic HAP composite, at lower concentrations (1–10 g/L). The obtained results also suggested a pseudo-second-order model kinetic and the Freundlich isotherm being the best model to describe the process. The higher uptake capacity observed can, in
ourresearchers' opinion, be assigned on the one hand to the different origin of the HAP, and, on the other hand, to the different lead concentration used.
Removal of Cu(II) was studied by Ngueagni et al.
[44][10], using as sorbent material hydroxyapatite obtained from the core of ox horns. The
res
tudyearch involved metal concentration in the range 100–500 mg/L, while the characteristics of the sorbent were determined by the calcination temperature (400–1100 °C), with the Ca/P ratio varying between 1.22 and 1.61, while the specific surface between 130 and 1 m
2/g. The copper adsorption process for the sample calcinated at 400 °C was best described by the Langmuir isotherm, reaching a maximum adsorption capacity of 99.98 mg/g, at room temperature and a pH of 5, the phenomenon being governed, according to the
resea
uthorchers, by a cation exchange process. The adsorption capacity recorded was superior to other sorbents, such as hazelnut activated carbon or bentonite, but inferior to other more complex materials. The same group utilized the material to study the adsorption of lead and cadmium ions, obtaining similar results, the most promising adsorbent being the sample calcinated at 400 °C, in a process best described by the Langmuir isotherm.
The HAP obtained from bovine femur was applied by Ramdani et al.
[47][13] in sorption studies, using as heavy metals Pb(II) and Cd(II). The obtained results were compared with those resulted for the application of commercial HAP, with superior results for both ions. The natural HAP had superior pore size distribution and pore volume, compared with the commercial sample, and similar specific area. The adsorption processes were found to fit the Langmuir isotherm model for the natural HAP and the Freundlich isotherm model for the commercial sample. This could be explained, in
ouresearcher
s' opinion, by the differences in terms of morphological characteristics between the two samples. A similar approach (with similar results) was applied by the same group
[48][14], who compared the efficiency of natural HAP with the one of commercial HAP for the removal of copper and iron(III) ions. The adsorption process was found to be best fitted by the Langmuir isotherm, and the results were superior for the natural HAP in terms of maximum adsorption capacity.
Fish scales were used by Sricharoen et al.
[52][18] to obtain HAP using an ultrasound-assisted method, with application in the uptake of Hg(II). The material obtained had a high uptake capacity for the targeted ion, superior to several types of complex adsorbents, as presented by the
resea
uthorchers, a phenomenon assigned by the
authoresearchers to the ion exchange with the Ca
2+ in HAP structure, as well to the electrostatic interactions between the positively charged Hg
2+ and the HAP surface.
An interesting approach was presented by Bi et al.
[50][16]. Using
Chlorella powder and a microwave-assisted method, the
resea
uthorchers obtained hollow microspheres with multicomponent nanocores, in which the dominant phase was HAP, with the presence of whitmoreite, magnetite and chlorapatite. The material was used for the removal of cadmium ions, with a relatively high adsorption capacity. In addition, the magnetite phase present in the composites allowed the magnetic removal of the composite.
A composite based on natural materials (HAP obtained from bovine cortical bone, chitosan obtained from shrimp shells and snail shell powders) was applied by Bambaeero et al.
[59][25] for the removal of copper and zinc ions. The process obeyed a pseudo-second- order model, while the isotherms that best fitted the experimental data were the Langmuir and Temkin isotherms. The
resea
uthorchers reached an ion removal of 90% and 60%, respectively, for 3 mg/L initial ions concentration, 0.02 g of adsorbent and a pH of 5.5.
A particular case regarding the natural-derived hydroxyapatite is represented by the use of natural material to obtain the precursors for the hydroxyapatite synthesis. Typically, the material obtained is CaO (noted in
Table 31 as
Ca precursor), which is used to obtain, by the addition of phosphoric acid (or other P-containing precursors), HAP. This approach was used by Xia et al.
[39][5], HAP being applied for the adsorption of Sr(II). According to their results, the process is best fitted by the Liu isotherm model, reaching a maximum adsorption capacity of over 45 mg/g. Núñez et al.
[40][6] applied HAP obtained by a similar recipe for the removal of lead, cadmium and copper ions. The process was better fitted by the Langmuir model, but most importantly, the
resea
uthorchers also performed a selectivity study, suggesting a competitive effect between different ions. The lead ions were preferably adsorbed by HAP, most probably due to their high electronegativity and ionic radius closer to Ca(II). Elsanafeny et al.
[55][21] applied a similar method for obtaining the calcium precursor from eggshells and applied the synthesized HAP as such or in the form of polymer modified HAP, in order to propose a method for the treatment of wastewater containing radioactive cobalt and strontium. The obtained results showed superior ion uptake capacity for the HAP. Using egg shells as a source of CaCO
3, Zeng et al.
[43][9] obtained an anionic/cationic substituted HAP by an ultrasound-assisted procedure. The partial substitution of Ca
2+ and PO
43− with Na
+, SiO
44− and CO
32− led to a macroporous structure with a relatively high pore volume compared to the other presented data. The material was applied for the adsorption of lead and cadmium ions, with superior registered maximum adsorption capacity, by comparison with HAP previously presented, and the
resea
uthorchers described the uptake process as a mixture of ion exchange, precipitation and electrostatic interactions. The stability of the adsorbent was also established by regeneration studies, the materials being effective after four regeneration cycles.