Toxicological & Environmental Chemistry Removal of ...

3 downloads 0 Views 453KB Size Report
Sep 19, 2008 - This article was downloaded by: [Tallinn University of Technology]. On: 20 March 2015, At: 01:22. Publisher: Taylor & Francis. Informa Ltd ...
This article was downloaded by: [Tallinn University of Technology] On: 20 March 2015, At: 01:22 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Toxicological & Environmental Chemistry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gtec20

2+

2+

Removal of Cd and Mn ions from aqueous solutions by synthetic cation substituted calcium‐carbonate‐apatite a

a

Kaia T[otilde]nsuaadu , Merike Peld & Mihkel Veiderma

a

a

Institute of Chemistry , Tallinn Technical University , Ehitajate tee 5, Tallinn, EE0026, Estonia Published online: 19 Sep 2008.

To cite this article: Kaia T[otilde]nsuaadu , Merike Peld & Mihkel Veiderma (1997) 2+

2+

Removal of Cd and Mn ions from aqueous solutions by synthetic cation substituted calcium‐carbonate‐apatite, Toxicological & Environmental Chemistry, 64:1-4, 145-154, DOI: 10.1080/02772249709358545 To link to this article: http://dx.doi.org/10.1080/02772249709358545

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sublicensing, systematic supply, or distribution in any form to anyone is expressly

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

Toxicological and Environmental Chemistry, Vol. 64, pp. 145-154 Reprints available directly from the publisher Photocopying permitted by license only

© 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint. Printed in India.

REMOVAL OF Cd 2+ AND Mn 2 + IONS FROM AQUEOUS SOLUTIONS BY SYNTHETIC CATION SUBSTITUTED CALCIUM-CARBONATE-APATITE KAIA TÕNSUAADU, MERIKE PELD and MIHKEL VEIDERMA Tallinn Technical University, Institute of Chemistry, Ehitajate tee 5, Tallinn, EE0026, Estonia (Received 4 March 1997; Revised 5 May 1997) Substituted Mg and Na containing carbonate-fluorapatites were synthesized by the wet method at pH = 9 and T = 80 °C. Their removal characteristics for Cd 2 + and Mn 2 + from aqueous solutions at pH = 5 were examined by a batch method. The samples were characterized by chemical, XRD, IR, and BET analyses. The amount of Cd and Mn sorbed increases with the increase in the content of Mg in the apatite up to 44.5 mg Cd and 31.0 mg Mn per 100 mg apatite. Ion sorption proceeds either by ion exchange or by the precipitation-dissolution mechanism. Keywords: Calcium-Fluor-Carbonate-Apatite; cadmium; manganese; binding capacity

INTRODUCTION Among different possibilities of utilization of synthetic apatites, their ability to bind toxic metals from solutions has found extensive attention during the past decade [1-9]. Synthetic hydroxyapatite (HAp) has been proposed as an agent to immobilize Cd, Sr and Pb in waste water and groundwater [5-7]. The introduction of Cd into the apatite structure is also of extreme biological significance in respect to its action on calcified tissues of vertebrates. Sorption of cations on apatite can proceed by different mechanisms: surface complexation, adsorption and diffusion into the solid, dissolution — precipitation, and ion exchange. Most of the studies on this subject have suggested that the ion exchange of metal ions with Ca 2+ of HAp lattice is the most 145

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

146

K. TONSUAADU et al.

widely spread mechanism for sorption. However, the different mechanisms of sorption often act together and the dominance of one of them is hard to distinguish. The sorption process can be characterized by the mole ratio of the cations released from the apatite to the cations bound (Q). In case of Q = 1, mainly the ion exchange between the cations of an apatite and of a solution takes place. Q < 1 indicates the domination of adsorption mechanism, including the filling of cation vacancies in a nonstoichiometric apatite. When Q > 1, the dissolution of the apatite and the precipitation of a new phase occurs. Most of the studies have been carried out on synthetic HAp and only in a few cases carbonate or silicate substituted Ca-HAp have been used [3, 4]. The substituted apatites have shown higher removal ability due to the weakening of the structure. It has been established that cation substitutions in the apatite structure also influence the reactivity of apatite. The most widespread cations that substitute Ca in the biological and sedimentary apatites are Mg and Na. Moreover, it has been found that Mg locates in easily exchangeable positions [10]. The objective of this work was to examine the binding ability of Mg and Na substituted calciumfluorcarbonateapatite (CaFCAp) for the removal of Cd 2+ and Mn 2+ ions from aqueous solutions and to elucidate the mechanism of the sorption. EXPERIMENTAL Synthesis The apatite samples were prepared by the precipitation method described in our earlier works [11, 12]. The products were identified by IR and XRD spectra as B-type CAps (CO 2 " substituted for PO^"). These are weakly crystallized fine powders with a specific area of 26-180 m 2 .g~' (Table I). The chemical composition of the samples is given in Table I. TABLE I Sample No A2 A3 Fl Dl D2 D3

Characteristics of synthesized apatites

Specific surface area, nP'/g

CaO

MgO

26.0 35.7 37.8 76.3 178.9 41.7

53.8 52.0 50.5 45.8 38.9 28.6

0 0 0 4.2 8.2 14.3

Chemical composition, % Na2O PlOs 0 0 1.1 0 0 0

41.0 38.6 37.5 37.2 38.5 37.5

F

CO2

3.1 3.1 3.4 3.5 3.2 3.1

0.6 1.7 1.6 2.0 1.0 3.3

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

REMOVAL OF Cd 2 + AND Mn 2 + IONS FROM AQUEOUS SOLUTIONS

147

The samples differ by the substitution in cation site as well as by CO3/PO4 mole ratio. The samples of series A are CaFCAps, of series D are CaMgFCAps and F — CaNaFCAp. The mole ratio of CO3/PO4 is almost the same for samples A3, Dl and Fl. In A2 and D2 the content of CO*" is lower, and in D3 it is the highest. In CaMgFCAps the Mg/Ca ratio increases from 0.13 to 0.69. The calculations of the molecular formulae of apatite samples were carried out, assuming that the sum of P and C atoms equals six, by the data of analyzes [11, 12] and according to the electroneutrality principle. The results of the calculations are as follows: A2Ca9.75no.25(P04)5.86(C03)o.l4Fl.65no.35 A3Ca9.57n0.43(PO4)5.49(CO3)0.5lFl.65n0.35 Fl Ca9.63Nao.37(P04)5.63(C03)o.37Fl.9l(OH)o.09 Dl Ca8.6oMgi.i6no.24(HP04)o.05(P04)5.47(C03)o.47Fl.94(OH)o.06 D2Ca7.33Mg2.l2n0.55(HPO4)0.86(PO4)4.90(CO3)0.24Fl.78(OH)0.22 D3Ca5.07Mg3.48ni.45(HPO4)2.15(PO4)3.10(CO3)0.75Fl.62(OH)0.38 Binding Experiments The binding characteristics of apatites for Cd 2+ and Mn 2+ in aqueous solutions were examined by batch method at 20 °C. 100 mg of apatite was introduced into 50 ml of the solution containing Cd or Mn and then shaken for 0.5 4105 hours. After that the suspensions were centrifuged. Solutions with different Cd and Mn concentrations (4 • 10~4, 2 • 10~3, 3.7 • 10~3 and 2 • 10~2 mol/1) were prepared from their nitrates (analytical grade) and the pH was maintained at 5 by adding HNO3. To establish the solubility of the apatite samples, a series of experiments was performed under the same conditions in diluted HNO3 solution with pH = 5. Analyses The solutions obtained were analysed for Ca, Mg, Cd or Mn, P and F content. The content of Cd, Mn, Mg and Ca in the solutions was determined by AAS method on Carl Zeiss Jena AAS IN instrument; P — spectroscopically as phosphomolybdate yellow complex; F — potentiometrically with Radelkis fluorselective electrode; pH of the solutions was measured with Radelkis pH meter. The solid phases obtained were washed with water, dried at 105 °C and analysed by XRD and IR spectroscopy. The chemical composition of the solid phases was calculated from the results of the analysis of solutions.

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

148

K. TONSUAADU et al.

IR spectra were obtained on a Carl Zeiss Jena IR 75 Spectrometer in the range of 400-4000 cm" 1 , using the KBr pellets (300 mg KBr mixed with 1 mg sample). The X-ray diffraction analysis was carried out with a DRON-4 Diffractometer using Cu K^ radiation at 40 kV, 20 mA. The samples were scanned in the range of 8-60° with step size 0.04°. The specific surface area of the samples was determined by BET method using Sorptometer EMS-53. RESULTS AND DISCUSSION Kinetics of Sorption The kinetic curves of Cd and Mn ions sorption are shown in Figure 1. The degree of sorption (KJ increases with time and reaches nearly 100% in about 1-2 hours when the content of cations in the solution is smaller than the apatite's capacity to bind them. K, is expressed by the equation _ [Cat + ],--[Cat + ] where [Cat+],- and [Cat + ] r are the initial and remaining contents of Mn or Cd in the solution, respectively.

Cd in the«olid, pmol/100 mg ap.

Mn. tjmol/100 mg ap.

9 6 Ml

150



'

, - ^

zJ—^—•* ea in

~~T- •'

i/ ..• 100 H

99 II

£---£

99 II 71 II

50

r

tf

*-^







9a i

SI 1

10 15 Time, h A 3 I — " - D1 I D 3 II - * -

-*-

D2 III ••»••

20

25

D1 II - « - D2 II D 3 III

A3.

2+

2+

- D1

- * - D2

-B- D3

FIGURE 1 Sorption kinetics of Cd (a) and Mn (b) on fluorcarbonateapatites. Solution concentrations: I — 4 • 10~ 4 , II — 2 • 1(T 3 , III — 3.7 • 10" 3 M. Number in the line shows the sorption degree (K s ).

REMOVAL OF Cd 2 + AND Mn 2 + IONS FROM AQUEOUS SOLUTIONS

149

When the amount of Cd or Mn ions in the initial solution is higher than the apatite capacity to bind them, 24 hours or more is needed to obtain equilibrium. Therefore, the experiments for establishing the apatite binding capacity were carried out for 24 hours, and with the sample D3 additionally for 105 h with 2 • 10~2 M Cd and Mn solutions.

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

Binding Capacities of Apatites The binding capacities of the apatite samples are given in Table II. The results affirm that the binding capacity of apatite depends strongly on the substitutions in its structure. With the increase in C O ^ / P O ^ mole ratio in CaFCAps (A2, A3) from 0.02 to 0.09 the sorption of Cd increases from 1.8 to 3.8 mg and Mn sorption from 0.4 to 0.5 mg per 100 mg of apatite. CaNaFCAp (sample Fl with Na/Ca = 0.03) binds less Cd and Mn from solution than CaFCAp (A3) with approximately the same CO^/PO^" mole ratio. This could be explained by the stabilizing effect of Na ions on the FCAp structure [10]. The growing Mg-content in CaMgFCAp (Dl, D2, D3) increases remarkably the sorption of Cd and Mn, reaching the binding capacities of 44.5 mg of Cd and 31.0 mg of Mn per 100 mg of apatite for sample D3.

Solid Phase Analyses The XRD analysis confirms that the solid products of the binding experiments of all apatite samples except D3 (with the highest Mg-content) retain their CaFCAp structure. In the final solids with Cd/Ca > 0.63 and Mn/Ca > 0.25, XRD analysis reveals, beside CaFCAp, new solid phases such as C d s I ^ P O . ^ 4H 2 O (d = 8.2, 2.97, 3.07 A, ASTM 14-400) and Mn3(PO4)2- 3H2O (d = 3.34, 2.60, 3.01, ASTM 3-0426). TABLE II Sample No A2 A3 Fl Dl D2 D3

The binding capacities of apatites and the mole ratios in final solids

Cd, mg/100 mg Ap

Mole ratio Cd/Ca

Mn, mg/100 mg Ap

Mole ratio Mn/Ca

1.8 3.8 1.8 9.7 18.0 44.5

0.02 0.03 0.02 0.11 0.26 1.35

0.4 0.5 0.3 3.1 5.7 31.0

0.01 0.01 0.01 0.08 0.16 2.26

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

150

K. TONSUAADU et al.

The analysis of IR absorption patterns of apatites before and after uptake of Cd 2+ and Mn 2+ ions also shows that the B-type CAp structure was maintained. The changes in the intensities in the domain of CO2," absorption at 1450T1475, 1425-1435 and 860-880 cm" 1 for the CaFCAp and CaNaFCAp have not been established. For the CaMgFCAp the decrease of CO 2 " absorption bands is noticeable when the uptake of Mn 2+ is more than 5.7 mg and the uptake of Cd 2+ is more than 18 mg per 100 mg of apatite. When the Cd/Ca mole ratio in the final solid exceeds 0.26 (D2), the new absorption bands appear at 950-955 and 1250 cm" 1 , which indicate the formation of Cd 5 H 2 (PO 4 ) 4 - 4H 2 O. In the final solid with Mn/Ca mole ratio >0.25 there are new bands at 1090, 930, 730 and 665 cm" 1 , which belong to Mn3(PO4)2- 3H2O [13]. These results indicate that the binding of Cd and Mn from aqueous solutions causes partial dissolution of Mg-substituted apatite and precipitation of the new phases takes place. The established lower sorption level for Mn 2+ in comparison with Cd 2+ is in a good agreement with the previous studies on different introduction of Cd and Mn into the apatite structure [3, 4, 14, 15]. Solubility of Apatites The bigger the extent of substitutions in the structure of apatite is, particularly that of Mg, the higher is the solubility of apatite samples in diluted aqueous

P in the solution, umol/100 mg ap

P in the solution. pmol/100 mg ap. 14C

120 100

• 03

eo EO

40' ^b. D2

20 DI

^^tr-——

0

100 200 300 400 500 Mn in the solid, pmol/100 mg ap A3

—•— F1

-o-

D2

-•»-

n

100 200 300 Cd in the solid, pmol/100 mg ap.

600

400

D3

FIGURE 2 Changes in the amount of phosphorus released as a function of the amount of Cd 2 + (a) and Mn 2 + (b) sorbed.

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

REMOVAL OF Cd 2 + AND Mn 2 + IONS FROM AQUEOUS SOLUTIONS

151

solution of HNO3 (pH = 5) (Figure 2-4). The dissolution rate of apatite reached its peak in the case of sample D3 amounting to 26% of P, Ca 2+ — 2%, and Mg 2 + — 29%. In sample Fl the dissolution rates are: for P 1%, Ca 1% and Na 13%. Therefore, incongruent dissolution of apatite takes place, at which more weakly bound cations (Mg and Na) tend to leave the apatite structure. As a result of sorption of Cd 2+ and Mn 2 + ions from the solution the solubility of P decreases with the increase in the amount of cations sorbed (Figure 2), which refers to the formation of insoluble Cd- and Mn-phosphates found also by XRD and IR analyses. The amount of Ca released from apatite is higher than the amounts of Cd 2+ and Mn 2+ ions sorbed (Q = 1 4- 1.4) for CaFCAps (samples A) and CaNaFCAp (sample F). For CaMgFCAps (samples D), with an exception of Mn sorption on Dl, the ratio Qca* is lower than 1. The value of Qc a decreases with the increase in the initial Mg content (Figure 3). The solubility of Mg in the sorption process is higher than that of Ca. The extent of Mg dissolution depends on its initial content in the sample and rises from 28% (Dl) to 57% and 85% for D2 and D3, respectively. Since the dissolution of Mg is relatively high, then in the case of a small sorption extent a Ca in the solution, pmol/100 mg ap.

Ca in the solution, pmol/100 mg ap. X' 250

-

200

-

150

-

100

1/ /

03

50 3 F1 n

0

100 200 300 Cd in the solid, iimol/100 mg ap A3

-+-F1

-*-01

-S-D2

•«- D3

400

.-'

r

0

100 200 300 400 500 Mn in the solid, |«nol/10O mg ap.

— A3

—*— Fl

-*-O1

-e-D2

600

•«• 03

FIGURE 3 Changes in the amount of calcium released as a function of the amount of Cd 2 + (a) and Mn 2 + (b) sorbed. * Qca presents the mole ratio of Ca 2+ ions released from the apatite to the cations (Cd 2+ and Mn 2 + ) sorbed from a solution.

152

K. TONSUAADU et at.

Mg in the solution, umol/100 mg ap.

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

Mg in the solution, (imol/100 mg ap.

5° I-/

0

100

200

300

Cd in the solid, pmol/100 mg ap.

400

0

100 200 300 400 500 Mn in the solid, pmol/100 mg ap.

600

FIGURE 4 Changes in the amount of magnesium released as a function of the amount of Cd 2 + (a) and Mn 2 + (b) sorbed.

the ratio Q Mg is higher than 1 (Figure 4). With the increase of the sorption extent the value of QMg decreases and becomes smaller than 1. The comparison of Ca and Mg solubilities leads to the assumption that the sorption of Cd and Mn on CaMgFCAps proceeds preferably by the ion exchange with Mg ions. The amount of Na released from sample Fl is 7-9%, while the dissolution level of Ca is not more than 3% and the mole ratios CjNa for Cd is 0.1 and for Mn is 0.3. The correlations between the amounts of cations released from apatite in sum to the amount of Cd or Mn sorbed are presented in Figure 5. In the experiments with Cd the mole ratio Q is close to 1 and does not depend extensively on the amount of Cd sorbed. This result suggests that the sorption of Cd proceeds mainly by the ion exchange mechanism. In the experiments with Mn the ratio Q is higher than 1 for all apatites and increases with the increase in the extent of sorption, except for D3 (Figure 5). Therefore the sorption of Mn 2+ ions proceeds partly by the dissolution-precipitation mechanism. In comparison with the other apatites, the behaviour of CaMgFCAp D3 (with the highest initial Mg content) is exceptional. The solubility of this sample is relatively high and depends considerably on the amounts of cations sorbed, which leads to the conclusion that the binding of Cd and Mn proceeds mainly through the formation of insoluble Mn- and Cd-phosphates. This is also

REMOVAL OF Cd 2 + AND Mn 2 + IONS FROM AQUEOUS SOLUTIONS

(Ca+Mg+Na) in the solution, pmol/100 mg.

(Ca»Ma*Na) in the solution, umol/100 mg.

Downloaded by [Tallinn University of Technology] at 01:22 20 March 2015

153

.C*"-'D1 ot_.



0

100

200

300

400

Cd in the solid, pmot/100 mg ap.

500

0

100

200

300

400

5O0

600

Mn in the solid, pmol/100 mg ap. -*