Environ. Sci. Technol. 1994, 28,408-418
Effects of Nos-, CI’, F-, S042-, and Cos2- on Pb2+ Immobilization by Hydroxyapatite Qi Ying Ma,’ Terry J. Logan, and Samuel J. Traina Department of Agronomy, The Ohio State University, Columbus, Ohio 43210
James A. Ryan RREL, U S . EPA, Cincinnati, Ohio 45268
Remediation of Pb-contaminated wastes has received considerable attention recently. We have previously shown that hydroxyapatite [Ca5(P04)30Hl can reduce Pb2+concentrations below the EPA action level (72.4nmol L-l) and, thus, has the potential for in situ Pb2+immobilization against leaching. This research investigated the effects of Nos-, C1-, F-, so42-, and C032-on hydroxyapatite-Pb2+ interactions. Solutions containing initial Pb2+ concentrations of 24.1-482 pmol L-I were reduced to below 72.4 nmol L-I after reaction with hydroxyapatite, except in the presence of high levels of C032-and Pb2+. Concentrations of C1-, F-, and sod2-decreased, whereas NO3-and C032-concentrations were unchanged after reaction with hydroxyapatite. Hydroxypyromorphite [Pb5(P04)30Hl precipitated after the reaction of hydroxyapatite with Pb2+ in the presence of NO3-, S042-,and C032-,while chloropyromorphite [Pbs(P04)3C1] and fluoropyromorphite [Pbs(P04)3F] formed in the presence of C1- and F-, respectively. The ability of hydroxyapatite to rapidly remove Pb2+from solution in the presence of high levels of NO3-, C1-, F-, S042-, and C032- demonstrates its great potential for reducing the environmental impact of Pb2+-contaminated wastes.
Introduction Lead is a widespread constituent of the earth’s crust. Its concentration in soil ranges from 2 to 200 mg kg-l and averages 16mg k g l ( I ) . Due to its long-term and extensive use, Pb2+is a major contaminant of solid wastes and soils, especially in landfills. Existing and abandoned disposal sites are a potential source of groundwater and surface water pollution, According to The Conservation Foundation (2),Pb2+ levels in 10% of the nation’s rural wells exceeded the EPA’s drinking water standard of 241 nmol L-l. Concerns over water pollution from heavy metalcontaminated landfills have generated tremendous interest in developing technologies that can cleanup these contaminated waste sites. Much of the attention in recent years has focused on Pb2+among all the heavy metals due to its potential hazard to the environment and human health. Phosphate forms insoluble lead orthophosphates after reaction with Pb, and both aqueous P and hydroxyapatite (HA) [Ca~(P04)30H]have been used to treat Pb2+contaminated wastes and water (3-7). Among all inorganic P sources, apatites are the most economical to use because of their ready availability and low cost. Hydroxyapatite has been studied as a cation exchanger to remove heavy metals from wastewater (5-7). We have recently shown * Address correspondence to this author at her present address: Soil and Water Science Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 326114510. 408
Environ. Sci. Technol., Vol. 28, No. 3, 1994
that HA can effectively attenuate aqueous Pb2+, exchangeable Pb2+,and Pb2+in contaminated soil material (8). Hydroxyapatite reduced initial dissolved Pb2+ concentrations of 24.1-2410 pmol L-1 to below 96.5 nmol L-1 after 0.5 h. Aqueous Pb2+in Pb-contaminated soil material was reduced from 11.0 to 0.17 wmol L-l after reacting with HA for 5 h. Solution pH, initial aqueous Pb2+concentrations, and especially aqueous P concentrations are important factors in the Pb2+immobilization process (8). Hydroxypyromorphite (HP) precipitation and HA dissolution were the main mechanisms for removal of aqueous Pb. Before HA can be successfully used as a Pb-immobilizing material, three factors need to be considered. Hydroxyapatite has to be able to immobilize Pb2+in the presence of interfering cations, anions, and dissolved organic matter; the reaction products have to be stable in the contaminated environment; and the reaction should be rapid. Numerous studies have investigated the effects of ions on the properties of HA, but little information is available on H P (9-11)- There are three types of substitutions that can occur in HA or H P structures. The cations Pb2+,Ba2+, Zn2+,Fe3+, and Mg2+can substitute for Ca2+,while the oxyanions A s O ~ ~V0d3-, -, C032-, and S042- can replace structural Pod3-. Additionally, anions such as F- and C1can exchange with OH- (12). Thus, when HA reacts with Pb2+in the presence of C1- or F-, the following minerals can form: chlorapatite (CA) [Ca5(P04)3Cll,fluorapatite (FA) [Ca5(P04)3Fl,chloropyromorphite (CP) [Pb~(P04)3Cl], and fluoropyromorphite (FP) [Pb5(P04)3F]. In the case of C032-and so42-, incomplete replacement of P043by C032- or S042- can result in the formation of Cas(P04,COhOH, CadP04,SO4)30H, PbdP04,C03)30H, or Pbj(P04,S04)30H. In the present study, HA was reacted with Pb2+in the presence of NO3-, C1-, F-, so42-, or c032-. Thus, both substitution and dissolutionlprecipitation reactions involving these anions were possible. Hydroxyapatite is noted for its isomorphous substitution. Both C1- and F- can substitute for OH- in apatite. We found in previous work that NO3- had no effect on Pb-HA interaction and did not interact with HA, and thus it was included as a control in this experiment (8). Fluorapatites are the most abundant mineral apatites and have received extensive study, They have larger crystallite size and are thermaly and chemicaly more stable than HA (12). No significant effects on the crystallinity of apatites have been observed as a consequence of C1- incorporation; however, the large unit cell volume of CA suggests that C1contributes to the instability of apatite (12). Among all apatite substitutions, that by C032-is considered the most common. This is attested to by the abundance of carbonate in nature (13). apatite [ (Ca,Na,Mg)5(P04,C03,S04,F)3Fl Carbonated apatite has smaller crystallite size, higher solubility, and lower thermal stability than HA (12). 0013-936X/94/0928-0408$04.50/0
0 1994 American Chemical Society
Table 1. Initial Pb2+and Anion (NOS-,C1-, F-, Sod2-, and CO+'-) Concentrations at Different Anion/PbZ+ Molar Ratios anion concn (wmol L-9
initial Pb*+concn (@molL-1)
2
4
8
12
24.1 121 241 482
48.2 241 482 964
96.4 482 964 1930
193 964 1930 3860
289 1450 2890 5780
Substitution of S042- for P043- is generally much less important than C032-substitution, and such substitution does not occur to a significant extent (12). One example of S042--substituted apatite is wikeite [Ca5(PO4,SO4,Si04)3(F,OH)](13). Sulfate-substituted apatite has lower chemical stability than HA and has similar crystal size. Hydroxypyromorphite, like HA, shows a remarkable tendency to form solid solutions with respect to anions by the substitution of F- or C1- for OH-. The stability sequence for the pyromorphite series is as follows: Pb5(P04)3C1> Pb5(P04)30H> Pb5(P04)3F(14). In contrast, the stability sequence for apatite is as follows: Cab(P04)3F > Cab(P04)3C1> Ca5(P04)30H (15). Such differences in stability are probably related to the crystal-chemical characteristics of these two groups of compounds, which suggest that the results obtained by studying apatite may not apply to pyromorphite. Isomorphous substitution has been suggested as the process for the formation of CP and F P from the reaction of aqueous Pb2+ with HA in the presence of C1- and F- (5-7). However, HA dissolution followed by CP or F P precipitation may also occur, as we have previously observed for H P (8). There are few reports on the substitution of S042- or C032- for Po43- in pyromorphite. Such substitution is considerably less in the pyromorphite series of related minerals than the apatite series (16). In the pyromorphite series, C1- is the major halide ion, as compared to F- and OH- in the apatite series (16). This research was conducted (1)to determine the effects of NOS-,Ck, F-, Sod2-,and Cos2- on Pb2+immobilization by HA; (2) to examine the reaction products; and (3) to investigate the mechanisms of those reactions. Experimental Section
Experimental Procedures. Different concentrations of Pb2+were reacted with HA in the presence of various levels of NO3-, C1-, F-, S042-,or C032- to test the effects of these anions on Pb2+immobilization by HA. Solutions of PbC12 and PbFz were used as Pb2+sources for the C1and F- treatments, and Pb(NO& solution was used for the NOa-, sod2-, and COS2-treatments. Sodium salts were used to supply NOa-, 61-, F-, so42-,and C032-. A sample of 0.1 g of HA (Bio-Rad) [see Ma et al. (ref 8) for a description of these materials] was reacted with 200 mL of solution containing 24.1,121,241, and 482 pmolof Pb2+ L-1 as Pb(NO&, PbC12, and PbF2, respectively. A t each Pb2+level, four different anion concentrations were used; 2, 4, 8, and 12 times the respective initial Pb2+ molar concentrations (Table 1). All solutions were adjusted to pH 6 with dilute HN03, HC1, HF, or NaOH. The suspensions were shaken for 2 h and then filtered with 0.2-pm Nucleopore polycarbonate membrane filters. The filtrates were analyzed for total Pod3-,PbZ+, Ca2+,NO3-, C1-, F-, S0d2-and c03'-, and solution pH. The solid phases
were analyzed by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Measured total P04%,Ca2+, Pb2+, NO3-, C1-, F-, S042- and C032-concentrations and solution pH were used as inputs to the MINTEQA2 equilibrium speciation model (17). Due to the variation reported for the equilibrium constant K , 1/20 log K around the saturation index = 0 was used as the accepted uncertainty (18). Equilibrium constants of 10-143.3, 10-106.2, and 10-44.35were added to the MINTEQA2 database for FP [Pb5(P04)3F],CA [Ca5(P04)3Cll,and glauberite [NazCa(S04)21,respectively (12, 17, 19, 20). We hypothesize that Pb2+ was precipitated as CP or F P when reacted with HA in the presence of C1- and F-, and thus HA simply acted as the P source. To elucidate such a mechanism, aqueous P was reacted with P b ( N 0 3 ) ~in the presence of C1- and F- a t controlled pH. A total of 0.08 g of NaH2P04 was mixed with 0.07 g of NaCl or 0.05 g of NaF in 50 mL of distilled deionized water. Solution pH was adjusted to 3, 5, 7 , and 9 with a Mettler DL 70 autotitrator set in the pH-stat mode using dilute HC1, HF, or NaOH. Then 50 mL of 19.3 mmol L-l P b ( N 0 3 ) ~ solution was added slowly to the above solution while the pH was maintained at a desired level and the solutions were stirred for 10 min. The suspensions set overnight before being washed and oven dried at 110 "C. The samples were then analyzed by XRD. To further demonstrate that CP and FP were formed mainly through precipitationldissolution instead of substitution, both C1- and F-of the same initial concentrations as PbCl2 or NaCl and PbFz or NaF were reacted with 0.1 g of HA. The initial C1- and F- concentrations were 48.2, 96.4, 193, and 289 pmol L-l, respectively. The filtrates were analyzed for total C1-, F-, Po43-, Pb2+,and pH a t the end of 2 h of shaking. Analytical Methods. A Perkin-Elmer 3030B atomic absorption spectrophotometer was used to analyze total Ca2+concentrations and Pb2+ concentrations >4.8 pmol L-l; a Varian SpectrAA-20atomic absorption spectrometer equipped with a graphite furnace atomizer was used to measure Pb2+concentrations
