Environ. Sci. Technol. 1994, 28, 1472-1480
Sorption of Zn2+ and Cd2+ on Hydroxyapatite Surfaces Yuplng Xu' and Franklln W. Schwartr Department of Geological Sciences, The Ohio State University, 1090 Carmack Drive, Columbus, Ohio 43210
Samuel J. Tralna Department of Agronomy, The Ohio State University, Columbus, Ohio 43210
This study examines the mechanisms and kinetics of Zn2+ and Cd2+ sorption onto hydroxyapatite surfaces. The concentrations of Zn2+ and Cd2+in the study range from 0 to 2.5 mmol/L. Although the sorption data follow Langmuir isotherms, a detailed examination reveals that surface complexation and coprecipitation are the most important mechanisms with possibly ion exchange and solid diffusion also contributing to the overall sorption process. pH-controlled experiments point to significant deprotonation of hydroxyapatite surface and sorption by metal complexation with surface functional groups such as r P O H . The major metal surface species are likely to be =POZn+ and =POCd+. The coprecipitation of Zn2+ and Cd2+ with Ca to hydroxyapatite is implied by significant changes in Ca and phosphate concentrations during the metal sorption processes. Cd2+coprecipitation appears to be more significant than Zn2+coprecipitation. Desorption data for the metals suggest that Zn2+is held more strongly than Cd2+ on the mineral surfaces. The observation is mainly interpreted as a result of redissolution of the coprecipitates because of more Cd coprecipitate than Zn coprecipitate. Solid diffusion of the metals into the mineral interior may also contribute to the desorption results because Zn diffusion is promoted by a much smaller size and ionic radius of Zn2+. Introduction Hydroxyapatite is the major inorganic constituent of teeth and bones. To some extent, the interaction between this mineral and toxic metals such as zinc and cadmium accounts for the behavior of human teeth and bones in moderating or constraining these metals in human body. Hydroxyapatite [formula Ca5(P04)30H,HAP] is also an important mineral in soils, sediments, and suspended particles (1). Together with fluorapatite, the mineral often affects or even controls the concentrations of phosphate, calcium, and lead (2, 3) in these environments. The character of the sorption of metal ions on HAP surfaces is important to understand the fate of these metals in terms of their uptake, release, and transport. The surface sorption of cations on calcite, oxides, and clay minerals have been extensively studied (4, 5 ) . However, the mechanisms of metal sorption on hydroxyapatite are still not well understood (1, 6). Sorption is by definition a general term describing the attachment of species from a solution to its coexisting solid surfaces. Three types of processes are identified for the sorption phenomena (7):(a) surface adsorption, which is limited to the accumulation of sorbate on the external surface of a solid; (b) absorption or diffusion into the solid;
* Address correspondence to this author at his present address: Department of Geological Sciences, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607-7059. 1472
Environ. Scl. Technol., Vol. 28, No. 8, 1994
and (c) precipitation or coprecipitation. These processes often act together, and the dominance of one specific process is often hard to be distinguished without careful chemical measurements and advanced analytical techniques, such as spectroscopy at electron microscopicscales (8). For example, the sorption of Cd2+on calcite surfaces was thought to involve adsorption, precipitation, coprecipitation or recrystallization, and solid diffusion depending on many experimental factors such as the initial concentration, solid surface site, pH, and time ( 7 , 9 , 1 0 ) . The uptake of other cations by calcite was also observed to be similar (11-14). Identifying the sorption mechanism of metals on hydroxyapatite is important because the mechanism often dictates the mobilities of the metals in their existing environments. A case in point is provided by the behavior of Pb2+in aqueous solution in relation to hydroxyapatite. It was suggested that Pb2+was removed by hydroxyapatite from aqueous solutions through ion exchange with Ca on the mineral surfaces (15-18). If Pb2+behaved in this way, adsorption alone would cause the metals to be readily remobilized following composition changes in the pore fluids. However, Xu and Schwartz (19)demonstrated that the removal of aqueous P b by hydroxyapatite was mainly a result of the precipitation of pyromorphites (lead apatites) which were much more insoluble than hydroxyapatite in aqueous solutions. By forming the more stable phases, lead was almost completely immobilized in the pyromorphites coexisting with hydroxyapatite (19, 20). Studies of the sorption of metals on hydroxyapatite surfaces have been conducted for Zn2+(21,221, Cd2+(1, 23,24),Sr2+ (25),Ni2+,and Cu2+(26)and for 45Ca(27,28). Most of these studies have suggested that the ion exchange of these metal ions with Ca on hydroxyapatite surfaces was an important mechanism for their adsorption. A critical review of these studies can be found in ref 29. However, a rigorous confirmation of the sorption mechanisms is lacking. The present study examines the sorption of Zn2+and Cd2+ on the surfaces of a synthetic hydroxyapatite in terms of sorption mechanisms and kinetics. Metal sorption was studied with and without pH controls. Experimental Methods
Material. The hydroxyapatite (HAP) used in this study was commercially synthesized HAP (Bio-Rad HTP). The surface area of the HAP sample was 77.0 f 0.4 m2/g, measured by the single-point BET (N2) method. More information on this hydroxyapatite can be found in Xu and Schwartz (19). The deionized water used in all the experiments was obtained from a Milli-RO Plus 30 system (Millipore Corp., Marlborough, MA). The water was further purified by two Universal and two Research ion-exchanger columns (Cole-Parmer Instrument Co., Chicago, IL). The zinc and cadmium nitrate crystals used were of 99.999% purity 0013-936X/94/0928-1472$04.50/0
0 1994 American Chemlcal Society
(Aldrich Chemical), and all other chemicals used in the study were of reagent grade. Sorption Isotherm. A 0.1-g sample of HAP was weighed into 50-mL Sepcor polycarbonate centrifuge tubes equipped with O-ringseals (VWR Scientific, Inc.) to isolate the tubes from atmospheric CO2. An aliquot of 30 mL of 0.1 M KN03 solution was added to each centrifuge tube containing HAP. The slurry was then shaken on a reciprocating shaker (Eberbach Corp., Ann Arbor, MI) at 180 oscillations/min for 20 h at 26 f 1 "C to predissolve HAP in the solution and to form a uniform suspension. After that, various quantities of 0.1 M and 0.01 M Znsolutions were added to corresponding (NO& or Cd(N03)~ tubes to provide initial Zn2+or Cd2+concentrations ranging from 0 to approximately 2.5 X 10-3 M. The slurry was then shaken for 30 h before it was centrifuged by a Beckman GS-6R centrifuge at 1500 rpm for 1h. No pH control was imposed during these experiments. A 25-mL supernatant of each slurry was decanted to a 30-mL polycarbonate bottle. A subsample (5 mL) was used for pH measurement (EA920,Orion Research Institute), and the rest was acidified by 2 drops of 37 % HC1. Flame atomic absorption spectrometry (Perkin-Elmer 3030B) quantified the total Ca, Zn, or Cd concentrations in the supernatant; while a photospectrometer (Beckman DU-6) was used to determine total phosphate concentration using an ascorbic acid method (30). Desorption Experiment (Sequential Extraction). Desorption experiments were carried out with the solid suspensions in the tubes containing initial Zn or Cd and 2.5 X lo4 M. After the concentrations of 2.5 X sorption experiments, the solid residue in a tube was thoroughly washed three times by deionized water, and the supernatant was decanted after centrifuging. In the first sequence, the tube was filled with 30 mL of 0.5 M KN03solution and then shaken for 36 h before the slurry was centrifuged and its supernatant was decanted as described above. In the second sequence, the solid was washed, followed by the addition of 30 mL of 0.5 M CaC12. The slurry was shaken for 30 h and then centrifuged and decanted as described above. A similar desorption experiment was carried out for the solid suspensions in the tubes containing initial Zn or Cd concentrations of 1.0 X 10-3 M. Only one extraction sequence was used with 30 mL of 0.5 M MgC12 solution. Kinetic Experiments. A total of 0.5 g of HAP was weighed into 250-mL plastic bottles. A 150-mL sample of 0.1 M KN03 electrolyte solution was added to each bottle, and the slurry was shaken for 24 h in a Eberbach shaker for the predissolution, as described above. The solution to HAP solid ratio was 300 mL/g, the same one used in the sorption isotherm experiments described above. Next, concentrated Zn(NO3)zand Cd(N03)~ solutions were added to each of the four bottles so that the starting solutions and 2.5 X 10-3 M Zn(N03)~and Cd(N03)2, were 1.0 X respectively. The bottles containing these slurries were shaken at 26 f 1"C for 10 days. For each bottle, a 10-mL aliquot of the well-mixed slurry was collected occasionally with a plastic syringe and filtered immediately through a 0.1-pm polycarbonate filter (Nuclepore Co.) mounted on a 13-mm filter holder (Gelman Co.). The filtrate was analyzed for pH, total Ca, Zn, Cd, and PO4 concentrations as described previously. A blank without the addition of any Zn and Cd was also monitored.
Methods of Solid Examinations. The samples for X-ray diffraction analyses were prepared in the following way. A 0.40-g sample of HAP was mixed with 200 mL of 0.0154 M Cd(N03)~solution for 2.5 hours by a magnetic stirrer, and the solid suspension was separated by filtration. Another 0.40 g of HAP was mixed with 150 mL of 4.54 X 10-3 M Zn(NO3)zfor 4.5 h before the solid suspension was separated by filtration. The concentrations of Cd2+and Zn2+ per gram of HAP in the two experiments were approximately 10 and 2 times the highest concentrations used in all the sorption experiments, respectively. The solids from both experiments were examined with an X-ray diffractometer (Philips Electronic) using CuKa radiation at 35 kV and 20 mA. The diffraction was digitally recorded in the 28 range of 20-60" with a O.05-deg interval and a 4-s count for each 28 angle. The X-ray diffraction patterns were compared to that of HAP. The solid residues from the 10-day kinetic experiments with the highest initial concentrations of Zn2+and Cd2+ (2.5 x lO-3M) were also examined by infrared spectroscopy using a Matson Polaris FT-IR. The procedure for the sample preparation and analysis is described elsewhere (31). All IR spectra were recorded as an average of 100 scans made at 2 cm-1 resolution in the 400-4000-~m-~ range. The blank sample from the 10-day HAP dissolution was also examined in the same way for reference. The same samples used in the IR analyses were also examined with a scanning electron microscope (SEM, JEOL JSM-A20). The SEM was equipped with a energy dispersive X-ray analyzer (EDS). The powder samples for the SEM analyses were mounted on stainless steel stubs using double-stick tapes and were coated with gold. Sorption with pH Controls. Two sets of experiments were designed to elucidate pH effects on metal sorption. Static pH control was achieved by acid (HN03) and base (KOH) titration using a Mettler DL70ES autotitrator (Mettler-Toledo, Inc., Hightstown, NJ). In the first set of experiments, 0.1 g of hydroxyapatite was allowed to react with 30 mL of 5.0 X lo4 M Zn2+or Cd2+solutions. Neither predissolution nor 0.1 M KN03 background electrolyte was used. During the 30-min reaction, pH was controlled to 6.61 f 0.05, 6.24 f 0.05, and 5.88 f 0.05 for different runs with 0.1 N HN03titrant. Immediately after a reaction run, the slurry was vacuum filtered through a 0.2-pm polycarbonate membrane filter. A blank run with no Zn2+and Cd2+was processed in the same way for each pH value. The Ca, Zn, and Cd concentrations of the filtrate were measured using the flame AA described previously. The experiments were intended to address the effects of Zn and Cd sorption on the aqueous Ca concentrations at controlled pH. In the second set of experiments, 0.2 g of hydroxyapatite was added to 63 mL of 0.1 M KN03 electrolyte solution in a 100-mL plastic beaker with a sealed lid. The slurry was shaken for 20 days. During the prolonged dissolution, the slurry was manually titrated to a pH of 5.64,5.88,6.34, 6.61, and 7.22 by adding trace amounts of concentrated solutions. The pH of each slurry was KOH or "03 monitored and found to have no detectable changes in the last few days of the dissolution. Therefore, it was assumed that hydroxyapatite dissolution achieved equilibrium at each targeted pH. Each sample was then reacted with Zn2+ and Cd2+ (1.0 X lo-" M total concentration) for a total of 30 min. An autotitrator was employed throughout the reaction period to prevent changes in solution pH Envlron. Sci. Technol., Vol. 28, No. 8, 1994 1473
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Figure 1. Langmuir sorption isotherms of Zn2+ and Cd2+ on HAP surface (a) and their linear flttings (b) after the transformatlon by the Langmuir equation (eq 1). The data are from the 30-h sorption experiments. M= maximum sorption capacity: b = adsorption constant.
resulting from the sorption. After a 30-min reaction, the slurry was immediately vacuum filtered, as described above. The filtrate was then analyzed for its Ca concentration by the flame AA and Zn and Cd concentrations by a graphite furnace AA (Varian Spectraa-20). A blank spiked with no Zn and Cd was also run for each pH.
Results Sorption Isotherms. The sorption isotherms for both Zn and Cd are shown in Figure la. These isotherms suggest that Cd2+is sorbed to a slightly greater extent on the HAP surfaces than Zn2+in the same experimental conditions. The shape of these curves suggests that the Langmuir isotherm should provide a suitable quantification of the experimental data. The Langmuir isotherm has the following general form:
where C is the equilibrium concentration of Zn2+or Cd2+ in mmol/L, m is the Zn or Cd sorbed on the HAP surfaces in mmol/g, M is the maximum sorptive capacity of the surface in mmol/g, and b is the adsorption constant in L/mol. Because no additional complexing ligands were added during the experiments, phosphate ions were the major ligands that could form complexes with Zn2+and Cd2+. By using the measured total phosphate concentrations under the experimental conditions, the calculations by Environ. Sci. Technol., Vol. 28, No. 8, 1994
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Figure 2. Relation between the amounts of Zn and Cd sorbed on HAP surface and the total calcium concentration, [Ca],, in the solutions after 30-h reactions. The metals sorbed are in mmoi/L of solution based on the solution to solid ratio of 300 mL/g.
5.7 5'4 Cd: M= 0.592 mmoVg (7.9 mmol/m2) b = 32.2xlW l/mol
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Sorbed Zn or Cd (mmol/l) Flgure 3. Relation between the sorbed Zn2+ and Cd2+ and pH after 30-h interaction between the cations and HAP.
the geochemical code MINTEQA2 (32) show that phosphate complexation with Zn and Cd is insignificant. Therefore, the measured total Zn and Cd concentrations, [ Z n l and ~ [ C d l ~in, the solutions were approximately that of Zn2+ and Cd2+. With eq 1, the Langmuir linear isotherms are plotted in Figure lb. After the least-square fitting, the calculated parameters are as follows: M = 0.568 mmol/g and b = 1.73 X lo4 L/mol for Zn2+ sorption and M = 0.592 mmol/g and b = 3.22 X lo4L/mol for Cd2+.The fit of the data to a Langmuir isotherm is generally good with the slight offset of the last concentration point of Zn or Cd. Similar isotherms have been reported for Zn2+ (221, Cd2+(23),Sr2+(251,and Ni2+(26),but onlyonestudy (23) was conducted at a controlled pH in the sorption experiments. During the experiments, pH and the total concentrations of calcium [ c a ] and ~ phosphate [Po& in the solutions were also measured. Figure 2 depicts the [ C a l ~ changes in response to the sorption of ZnZ+or Cd2+ onto HAP surfaces. The linear relation between the two is apparent. For example, the linear regression for Cd sorption results in a Ca:Cd atomic ratio of 0.857 (see Figure 2). The plot of the solution pH versus sorbed Zn or Cd in Figure 3 also results in a linear relationship similar to that in Figure 2. The simultaneous changes of pH and [ C a l ~make it difficult to determine whether the [ C a l ~increases were the consequences of ion exchange of Zn2+(or Cd2+),further HAP dissolution due to the pH decrease, or other mechanisms.
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Table 1. Saturation Indices of Final Sorption Solutions solution (initial concn)
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Figure 4 shows the final [P04]T changes in relation to the sorbed Zn2+ or Cd2+. With pH decreases during these sorption experiments (Figure 31, increases in [POIT would be expected. However, the Zn sorption to HAP apparently maintained [ P 0 4 ] T regardless of the final pH, whereas the Cd sorption resulted in significant decreases in phosphate concentration. The decreases in [ P 0 4 ] T by the Cd sorption seem to be dependent upon the initial concentrations until [ C d 2 + ]was ~ greater than about 1.0 mmol/L. Overall, the final [PO4]T of the Cd solutions was about 27% less than that of the Zn solutions. Saturation Indices of Potential Solids. The least soluble zinc and cadmium phosphates in the pH range studied are Zn3(P04)~4HzO(hopeite) and Cd3(P04)2, respectively (2). The saturation index (SI) is defined as SI = log(Q/K),where Q is the reaction quotient and K is the equilibrium constant. MINTEQA2 (32)was used to calculate the saturation indices (SI) of HAP, Zn3(P04)y4H20 (hopeite),and Cd3(P04)2for a HAP dissolution blank and the final solutions of the two highest initial Zn2+and Cd2+concentrations, respectively. The calculation utilized the measured pH, [ C a l ~[PO~IT, , and [ Z n l ~ or [ C d l ~concentrations in each final solution with the ionic strength of 0.1 M. The results of the calculation are listed in Table 1. Calculations show that the blank solution is supersaturated with respect to HAP. Because the HAP was prepared using the method of Tiselius et al. (33)and the sample had relatively low crystallinity and high surface area of 77 m2/g (19),the apparent supersaturation of the blank solution is expected with respect to the well-crystallized HAP represented in the database of MINTEQA2. Ultimately, both final solutions of Zn and Cd were undersaturated with respect to HAP. In other words, HAP dissolution equilibrium was not achieved even at the end of the reactions. Table 1also shows that the final solutions were undersaturated with respect to either Zn3(PO4)~4H20 (hopeite) or Cd3(P04)2 for Zn and Cd, respectively, although initial solutions of some higher Zn or Cd were supersaturated with respect to these solid phases.
Results of Solid Examinations. The X-ray diffraction spectra were identical for the original HAP and the solid residues that were produced in the reactions of HAP with Zn2+ and Cd2+ concentrations much higher than 2.5 X 103M. In other words, no other solid phases were detected from the sorption reactions given the sensitivity of the X-ray diffraction method. Identical infrared spectra were also obtained for the 10-day HAP blank and the solid residues involved in the sorption of highest Zn2+and Cd2+ concentrations. OH and PO4peaks corresponded well with the peak positions documented in Farmer (34) for hydroxyapatite. No displacements and magnitude differences of the peaks were found to be significant among these solids. SEM micrographs of the solid residues from the sorption of the highest initial Zn2+and Cd2+concentrations were compared with HAP blank, and no significant morphological differences were found among them. An extensive search through secondary electron (SE) imaging and detailed X-ray dispersion (EDS) analysis was unable to identify new solid phases on the surfaces of HAP grains. Uniform coverage of Zn or Cd was detected with EDS analyses on the surfaces of respective solid residues. Desorption Experiments. The results from the desorption experiments are presented in Table 2. The percentages of Zn and Cd desorbed were calculated based on their concentrations in solutions after the desorption. Table 2 indicates that neither Zn or Cd desorbed significantly in a KC1 solution. For example, the calculations indicated that less than 1.7 % of Zn was desorbed from the most Zn-enriched HAP (0.567 mmol/g). With the 0.5 M CaClz solution, the desorption of Zn from HAP was more complete ranging from 37% to 47% for Zn and 69% to 75% for Cd. In other words, Cd desorbs from HAP more extensively than Zn. This point was reinforced by desorption experiments with a 0.5 M MgC12 solution in which 12.8% Zn and 22.4% Cd desorb from HAP. Sorption Kinetics. A necessary prerequisite for explaining the sorption kinetics for Zn and Cd is some understanding of the kinetics of HAP dissolution because the dissolution and metal sorption processes may interact during the experiments. The changes of pH, [ C a l ~and , [ P 0 4 ] T in the solution are shown in Figure 5 for HAP dissolution without Zn and Cd. Figure 5 provides a blank comparison to the data shown later. After 4 h of dissolution, [ C a l ~leveled off, whereas pH and [P04]T continued to increase. pH increased very slowly up to about 100 h and became stable. However, [P04]T increased slowly and steadily with respect to a logarithmic time scale even after 100 h of dissolution. Therefore, the dissolution of HAP resulted in nonstoichiometric releases of calcium and phosphate ions to the solution. Environ. Scl. Technol., Vol. 28, No. 8, 1994
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The changes in solution chemistry during Zn2+sorption are shown in Figure 6a for the initial Zn2+concentration M and in Figure 6b for the initial Zn2+ of 1.0 X M. In Figure 6a, [ C a l became ~ concentration of 2.5 X stable after approximately 0.5 h, and pH leveled off at 6.12 after 100 h. [ Z n l ~ decreased rapidly from 1.0 X to 3.44 X M in the first 30 min. [POllT declined initially in the first 30 min and rose back slowly and steadily thereafter. Similar trends in concentration changes are 1476
Environ. Sci. Technol., Vol. 28, No. 8, 1994
1
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Time (hours)
t
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Flgure 7. pH and total concentration changes during 217-h sorption of Cd2+ on HAP surface. The total Cd2+ concentrations were 1.O X M (a) and 2.5 X M (b). The solutlon to HAP ratio was 300 mL/g, and the background electrolyte solution was 0.1 M KNOB.
found for [ Z n l ~in Figure 6b. Where the initial Zn concentration waslarger (Figure 6b), pH, [ C a l ~and , [Znl~ changed significantly within the time frame of the experiment (217 h). Figure 6b demonstrates no clear cut of the equilibrium state. The comparison between Figure 6, panels a and b, reveals that the time to reach the sorption equilibrium appears to depend upon the initial Zn concentration. The time variation in solution chemistry during the Cd2+ sorption is shown for the initial Cd2+concentrations of 1.0 X M and 2.5 X M in Figure 7a,b, respectively. Figure 7a is very comparable to Figure 6a, except that the [ P 0 4 ] T is much lower during Cd2+sorption. This observation is consistent with that shown in Figure 4. In Figure 7b, Cd2+ concentration behaved similarly to Zn2+ concentration in Figure 6b. Again, the final [POllT values were lower in Figure 7b. No sign of equilibrium is revealed during the 217-h experiments for the case of larger initial concentrations (Figure 7b). Sorption with pH Controls. Figure 8 shows the calcium concentrations resulting from HAP dissolution and Zn and Cd sorption for the first set of pH-controlled experiments. The experiments used the initial Zn and Cd concentrations of 5.0 x lo4 M with no background electrolyte and no predissolution of hydroxyapatite. Under the pH controls, the results apparently indicate that the Zn and Cd sorption lowered [ C a l ~with respect to the pure HAP dissolution under all tested pH conditions. The results eliminate the possibility of a simple ion-
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Figure 10. Changes in Ca concentrations in the solutions after the 30-min titration discussed in Figure 9.
8 and 10 imply that the metal sorption causes the net removal of Ca from the solutions.
Discussion
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Flgure 9. Removal percentagesof Zn and Cd from solutions and base (OH-) consumption after 30-min titration with the metal sorption. 0.1 M KNOJ electrolyte was used in the solutions, and hydroxyapatite underwent 20 days predissoiution before the sorption experiments.
exchange reaction between the metals and Cd on HAP surfaces because with ion exchange [ C a l should ~ increase proportionally to quantities of sorbed metals. Figures 9 and 10 illustrate the results of the second set of pH-controlled experiments with 0.1 M KN03 background electrolyte, 20-day HAP predissolution, and 1.0 X lo4 M initial concentrations of Zn2+ and Cd2+. The experimental conditions are similar to the sorption isotherm and kinetic experiments discussed previously. As shown in Figure 9, Zn and Cd were more than 99% removed at all pHs from 5.64 to 7.22. At pH L 6.99,100% removal of Zn and Cd from the solutions was achieved. Meanwhile, the OH- consumption necessary to maintain the solution pH decreased from 4.9 (Zn) or 3.3 pmol (Cd) at pH 5.64 to approximately 2.0pmol at pH 6.61and leveled off at higher pH. The significant OH- consumption during the experiments indicates that the sorption processes for both Zn2+ and Cd2+ result in proton production in the solutions. Figure 10 depicts the final calcium concentration after the metal sorption. Again, [ C a l ~is either identical or lower than that due to the pure HAP dissolution. The observation reinforces the finding that the Zn or Cd sorption does not cause net [ C a l ~increase under the pH-controlled conditions. In fact, both Figures
The concentration changes of Zn and Cd during the sorption experiments indicated at least two distinct stages: a rapid decrease in the first 30 min followed by a slow and nearly linear decrease as a function of the logarithm of the reaction time (Figures 6 and 7). Conventionally, only these metal concentrations were examined and modeled in the literature. Our interpretation will utilize virtually all of the chemical data as well as the results from X-ray diffraction, FTIR, and SEM to discuss the possible mechanisms involved in the Zn2+and Cd2+ sorption on HAP surface. Adsorption Processes. Adsorption is defined as the accumulation of matter at the interface between the aqueous solution phase and a solid adsorbent (35). However, adsorption processes may be due to several mechanisms (7) such as ion exchange and the formation of surface complexes by surface function groups. Because of the nature of the adsorption, whether physical or chemical, it occurs fast and is often involved in the early metal uptake. The Langmuir isotherm in Figure 1 and the linear relationship between [ C a l and ~ sorbed Zn and Cd in Figure 2 appear to be consistent with the ion-exchangemechanism between these metals and Cd on HAP surfaces. In fact, previous studies (22, 23, 25, 26) have observed similar isotherms and suggested a 1:l ion-exchange mechanism. An ion-exchange reaction can be described by the Langmuir isotherm. However, the reverse logic is not always true. Figure 3 demonstrates that the Zn and Cd sorption processes themselves cause pH decreases, which in turn will increase [ C a l ~in the solutions by more HAP dissolution. With pH controlled during the sorption experiments, no proportional [ C a l increases ~ are observed from the possible 1:l ion exchange (see Figures 8 and 10). While ion exchange may still occur, it is unlikely a dominant mechanism for Zn or Cd removal by HAP. The extent of such ion exchange is also difficult to evaluate in the systems studied because deprotonation, coprecipitation, and possibly other reactions may mask the apparent [ C a l in ~ the final solutions. Environ. Scl. Technoi., Vol. 28, No. 8, 1994
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The point of zero charge (PZC) is defined as the pH at which the total surface charge becomes zero (36). It is an important parameter in surface complexation models. By using a {-potential measurement, Somasundaran and Wang (6) obtained a PZC of 7.0 for a synthesized hydroxyapatite. Dissolution experiments can also be used to estimate PZCs of apatite minerals (37). Because a dissolution steady state often corresponds to an equilibrium of surface protonation, the pH at the steady state is approximately the PZC at which the equilibrium of surface protonation is achieved with no addition of acid or base. The 9-day dissolution results shown in Figure 5 reveal that the HAP used in the study has a PZC of approximately 6.85. The surface complexation of Zn2+and Cd2+with HAP surface functional groups is apparently reflected in the results in Figure 9, which show that significant quantities of H+ enter into the solutions during the pH-controlled sorption. Presumably, these H+ are partially displaced from HAP surfaces by the adsorbed Zn and Cd and partially generated from coprecipitation reactions discussed subsequently. The total quantities of the displaced H+ (4.9-2.0 pmol) are comparable to the total Zn2+and Cd2+ present initially in the solutions (6.3 pmol). The surface protonation at fluorapatite surfaces was examined by Wu et al. ( 3 9 , who proposed the following as the dominant reactions: =PO-
+ H+ = =POH
=CaOHz+ = =CaOH
log &ol(int) = 6.6 f 0.1 (2)
+ H+
log Pllo(int) = -9.7 f 0.1 (3)
With Zn2+ (or Cd2+)present in the reactions, the following reactions may occur: =POH
+ Zn2+= =POZn+ + H+
(4)
+ Zn2+= EPOZn+
(5)
=PO=CaOH
+ Zn2+= =CaOZn+ + H+
(6)
Wu et al. (37) showed that at pH near PZC (e.g., f1 pH unit) =PO- and =CaOH2+ dominate over other species. Within this pH range near PZC, =POH becomes significant at pH < PZC whereas ECaOH becomes significant at pH > PZC. In this study, the pH range considered is 5.64-7.22 with PZC = 6.85. Over the experimental pH range, reaction 5 may dominate the metal complexation with contribution from reaction 4 in pH 5.64-6.85 and possibly reaction 6 in pH 6.85-7.22. Reactions 5 and 6 involve deprotonation whereas reaction 4 does not. Precipitation and Coprecipitation. The X-ray diffraction and SEM results did not reveal the formation of new phases after the sorption reactions. The IR results suggest similarly that new solid phases such as Zn3(P04)2’4H20 (hopeite) or Cd3(P04)2were not present in the solid residues after the sorption reactions. This conclusion is also reinforced by the negative saturation indices of these phases in Table 1. However,the analytical methods are unable to detect hydroxyapatite with a minor substitution of calcium by other metals (38, 39). Thus, coprecipitation of Zn2+ and Cd2+ into the hydroxyapatite structure cannot be excluded on the basis of the measurements. In the experiments, the chances of such coprecipitation were increased by the fact that HAP was 1478
Envlron. Scl. Technol., Vol. 28, No. 8, 1994
predissolved for approximately 1day or longer before the Zn2+and Cd2+sorption started. As seen in Figure 5, the dissolution of HAP after 1 day provided relatively large concentrations of phosphate and calcium ions in solution. Coprecipitation has been argued to explain the “slow” uptake often observed in metal sorption experiments (9). In fact, solid solutions of Sr-Cu-Ca, Ca-Pb-Cu, and CaPb-Cd hydroxyapatite have been synthesized in room temperatures in the laboratory (38,39).Xu and Schwartz (19)demonstrated that Pb2+precipitated or coprecipitated rapidly with phosphate ions. The following observations from the present study suggest the coprecipitation in the initial “fast” uptake stage concurrently with other adsorption processes. (a) Significant decreases of [ P 0 4 ] T were found in the first 30 min of sorption (Figures 6 and 7). The equilibrium results in Figure 4 also indicate that coprecipitation of Cd may be much more significant than that of Zn. This observation is consistent with the result that more Cd2+was removed from the solution to HAP surfaces than Zn2+. Similar observations were also made by Suzuki et al. (15) for these two metal ions. The concentration decreases are in accordance with coprecipitation. The gradual increases of [P04]T after the initial decrease are also seen in Figures 6 and 7, more so in Zn systems than in Cd systems. The [ P O ~ I T recovery is presumably due to further dissolution of HAP (see Figure 5). (b) With pH controls, Figures 8 and 10 demonstrate that Ca concentrations decreased significantly during the Zn and Cd sorption. With the possibility of ion exchange, which can result in Ca concentration increases, these figures clearly indicate that there were other mechanisms that removed Ca in the solutions. The coprecipitation of Cd2+with Ca2+can be represented by the following general reactions:
which also deliver protons to the solution. I t is likely that part of the OH- consumption shown in Figure 9 was used to neutralize these protons. The speciation calculation using MINTEQAB reveals that HzPO4- dominates the aqueous phosphate concentration at the lower pH range in Figure 9 (5.62-6.24), whereas HP042- dominates at the higher pH range (26.99). Therefore, Cd coprecipitation follows predominantly reaction 7 in the pH range of 5.626.24, which produces more protons than reaction 8, representing the higher pH range. The pH effect on the coprecipitation coincideswith the more OH- consumptions at lower pH range in Figure 9. With more Cd2+being coprecipitated than Zn2+under no pH controls,the solution pH would decrease more in the Zn2+ sorption. These arguments are apparently supported by the results shown in Figures 1 and 3. The substitution of Ca by other cations in HAP crystal structure is widely known in geological and biological apatites (40). LeGeros and LeGeros (41)concluded that cations whose ionic radii were smaller than Ca2+(0.99 A) may be incorporated in the apatite structure to a much lesser extent than those cations of larger ionic radii.
Therefore, Zn2+(0.69A) may become much more difficult to coprecipitate with Ca2+into apatite structure than Cd2+ (0.97A). This argument is consistent with the experiment results. Overall Sorption. If we assume that all the Zn2+and Cd2+ removed from the solutions (see Figure 2) was incorporated to the Ca site in hydroxyapatite through coprecipitation, these metals could make up to 5.7and 6.0 atomic % at the site, respectively. Bigi et al. (42)showed that as low as 5% Mg in the Ca sites could cause a significant broadening in the X-ray diffraction peaks and a significant reduction of IR absorption band of OH for the Mg-Ca coprecipitated HAP. The identical X-ray diffraction patterns and IR spectra observed in this study argue against the dominance of coprecipitation at the end of the sorption reactions. The discussion above reveals that adsorption of Zn2+and Cd2+through surface complexation and possibly ion exchange also plays a critical role in the overall removal of these metals from the solutions. Although it is difficult to quantitatively estimate the proportions of the removal due to any specific mechanism, it appears that these mechanisms all work together to a significant extent, especially the coprecipitation and surface complexation. The coprecipitation reaction appears to be more significant for Cd2+sorption than Zn2+. The difference in surface affiliation between Zn and Cd is also revealed in their desorption results. Table 2 shows that the percentage of desorption due to 0.5M CaC12 varied from 37 to 47% for Zn2+and from 69 to 75% for Cd2+.The desorption percentages due to 0.5 M MgClz were 12.8% for Zn2+and 22.4% for Cd2+,following the same pattern. As the ionic radii of Ca2+and Cd2+are similar, so are the radii between Zn2+and Mg2+ (43). In other words, the sorbed Zn2+ was less desorbable than that of Cd2+ regardless of the desorption solutions. The results contradict the finding of Zachara et al. (14) for the sorption of these cations onto calcite. Zachara et al. (14)proposed that Cd2+formed a dehydrated surface phase much more rapidly than Zn2+ did because of the higher hydration energy of Zn2+. We propose two possible reasons to interpret our desorption results. Because Cd2+ may coprecipitate to a greater extent than Zn2+,the presence of 0.5 M CaC12 desorption solution produces a redissolution of these coprecipitates and the exclusion of more Cd2+ due to the overwhelming Ca2+ concentration. Another explanation is that Zn2+diffused further into the HAP hydration shell and/or crystal than Cd2+. Both the size and mass of Zn2+ are significantly smaller than that of Cd2+. Thus, the faster diffusion rate of Zn2+is expected. Acknowledgments
The authors wish to thank Qiying Ma, Jerry Bigham, and Uby Soto for their laboratory assistance. The study is supported in part by the Ohio Eminent Scholar fund to F.W.S. The senior author wishes to thank The Ohio State University for the Presidential fellowship. Thanks also go to the anonymous reviewers whose critical comments greatly improved the manuscript. Literature Cited (1) Middelburg, J. J.; Comans, R. N. J. Chem. Geol. 1991,90, 45-53. (2) Lindsay, W. L. Chemical equilibria in soils; John Wiley & Sons: New York, 1979; 449 pp.
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(40) McConnell, D. Apatite, Its Crystal Chemistry, Mineral-
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* Abstract published in Advance ACS Abstracts, June 1, 1994.
