Selective Sorption Of Heavy Metal Ions
O.V. Vysotska
Institute of Colloid Chemistry and Chemistry of Water, Ukrainian National Academy of Sciences, Vernadsky avenue 42, 03680 Kyiv, Ukraine
In the removal of metal ions from dilute solutions, the most efficient and selective are the inorganic materials with adsorption ability essentially determined by ion-exchange and chemical interactions, in particular, the formation of complexes. Phosphates of alkali earth metals are efficient for the sorption of heavy metal ions. Among these phosphates, the hydroxyapatite Ca10(PO4)6(OH)2 is the most stable material.
The studies were performed with two samples: (HAP‑1), the synthetic hydroxyapatite for which the structural formula is presented above; and (HAP‑2), the two-phase system which consists of the hydroxyapatite (70 %) and tricalcium phosphate (30 %). The porous structure of the samples was studied by microbalance adsorption of water and hexane vapours; the results are summarised in the Table below. It is seen from the Table that the adsorbents are mesoporous, therefore no restrictions should be expected in the diffusion of adsorbed ions.
Table. Specific area (S) and capacity of arbitrary monolayer (am) with respect to hexane and water; effective pore radius (ref) and Ca:P ratio of hydroxyapatite
Sorbent
am, mmol/g
S, m2/g
ref, nm
Ca:P
n‑C6H14
H2O
n‑C6H14
H2O
HAP‑1
0.28
1.21
84
78
2−3.8
1.66
HAP‑2
0.183
0.49
55
32
2(a),
3.5−4.5(b)
1.61
(a),(b)−two maximums of effective pore radius distribution
The sorbents were studied with respect to the removal of heavy metal ions Cd2+, Ni2+, Co2+, Zn2+ in static conditions from diluted water solutions in the concentration range 100−4000mg/dm3. While the specific area of HAP‑1 is higher than that of HAP‑2, the sorption activity of the latter with respect to heavy metals is higher. However, HAP‑2 is less chemically stable; therefore, the studies and application of HAP‑1 is more promising.
The sorption of heavy metals at both samples is decreasing in the series Zn>Co>Ni>Cd. The sorption isotherms are of the Langmuir type. From the sorption isotherms in the Henry region the distribution coefficients were calculated, which were found to be as large as 104, indicating the high selectivity of the sorbent with respect to the studied ions.
To elucidate possible mechanisms of sorption at the studied sorbents, the pH values of equilibrium solutions and the amount of Ca2+ ions desorbed into the solution were measured. The pH value corresponding to the precipitate formation of hydroxides of the metals studied was higher than the pH value of initial and equilibrium solutions, therefore the formation of heavy metal hydroxides at the HAP surface cannot take place. The transfer of Ca2+ ions instead of Zn2+ ions into the equilibrium solution indicates that the sorption is determined by the ion exchange mechanism. However, for the systems Co‑, Ni‑, and Cd‑HAP the non-equivalence between the amount of sorbed heavy metal ions and the Ca2+ ions displaced to the solution was observed. This suggests that another sorption mechanism also exists, namely the formation of complexes with the phosphate groups. This mechanism was found for sorption of heavy metal ions at inorganic sorbents modified by condensed polyphosphates [1].
To summarise, the synthetic hydroxyapatite can be regarded as the promising material for the removal of heavy metal ions from diluted solutions. This suggests its applicability for the cleaning of galvanic industrial waters.
References
1. Yu.I. Tarasevich, G.M. Klimova, Applied Clay Sci. 19 (2001) 95.
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