The Use of Phosphorus in Sequestration of Lead and Cadmium in a Smelter Slag
M. Lambert1 , G. Pierzynski2 , and G. Hettiarachchi2
1Department of Geology, Kansas State University, Thompson Hall, Manhattan, KS 66506; 2Department of Agronomy, Kansas State University, Throckmorton Hall, Manhattan, Kansas 66506
The site of an abandoned lead smelter near the village of Dearing in southeastern Kansas is a continuing environmental and health concern because of high levels of heavy metal contamination, including lead and cadmium. Phosphate amendment of lead-contaminated soils is known to precipitate highly insoluble lead pyromorphite (Pb5(PO4)3Cl), effectively reducing its bioavailability. In this study, samples of Dearing slag were incubated with two different forms of soluble phosphate (apatite and potassium phosphate). Lead content of the amended slag decreased in the exchangable, carbonate, iron- and manganese-oxide, and organic fractions of the slag, while increasing in the residual fraction. At the same time, cadmium content of the amended slag decreased in the exchangable and iron and manganese fractions, while increasing in the carbonate and residual fractions. X-ray diffractometry shows that lead pyromorphite abundance increased in the phosphate-amended slag, suggesting that lead was precipitated as pyromorphite. Cadmium may have been precipitated as relatively insoluble octavite (CdCO3). Of the two phosphate amendments, potassium phosphate was more effective in reducing soluble lead and cadmium, and increasing pyromorphite abundance. Also, rate effect was more important than the effect of time in remediating the slag.
Key words: lead pyromorphite, bioavailability, phosphate amendment
Galena (PbS) and sphalerite (ZnS) are the most important ores of lead and zinc, respectively (Berry and Mason, 1959), and cadmium often substitutes for zinc in sphalerite (Klein and Hurlbut, 1993). For this reason, lead, zinc, and cadmium pollution are often spatially associated (Lambert, et al., 1997). Over the course of many years, lead and zinc ore from the Tri-State region of southeastern Kansas, southwestern Missouri, and northeastern Oklahoma was processed in a smelter near the village of Dearing, in Montgomery County, southeastern Kansas. Although the smelter is no longer in operation, heavy metals such as lead and cadmium that are contained in the slag are a matter of ongoing environmental concern, especially considering the effort and expense involved in the currently recommended remediation treatment of digging up contaminated soil and treating it as toxic waste.
Sequestering heavy metals in insoluble phosphate minerals has been suggested as an in situ remediation technique for heavy metals (Nriagu, 1973, 1974 and 1984). This method of remediation has the advantage of not involving the removal of contaminated soil. Most published studies concerning phosphate remediation of heavy metals have dealt with lead (Rabinowitz, 1993; Ma, Traina, and Logan, 1993; and Ruby, Davis, and Nicholson, 1994). This is no doubt due to widespread concern about lead poisoning, as well as the availability of highly insoluble lead pyromorphite (Pb5(PO4)3Cl) as a repository for the metal. In the present study, phosphorus amendments in the form of apatite (Ca5(PO4)3Cl) and potassium phosphate (KH2PO4) treatments were used in a laboratory incubation study to determine their effectiveness in reducing the amounts of bioavailable lead and cadmium in the Dearing slag.
Untreated control samples and various amended treatments of Dearing slag were incubated at a constant 25o C temperature and 18% humidity to simulate natural field conditions. The phosphate amendments were either apatite (Ca5(PO4)3Cl), or potassium phosphate (KH2PO4), with potassium phosphate the more soluble of the two phosphate sources. Treatments were applied with replicates, and consisted of the two amendments in 2:1 and 4:1 phosphorus to lead molar ratios. At time periods of three and twelve weeks, control and treatment samples were collected, and extracts of the exchangable, carbonate, iron- and manganese-oxide, organic, and residual fractions were prepared according to the methods of Tessier, et al., (1979). These extracts were then analysed for lead and cadmium content by Inductively Coupled Plasma (ICP) spectrometry. Finally, X-ray diffractometry was used to determine mineralogical changes that occurred in the slag during the course of the incubation.
Effect of Phosphorus Source
Figure 1 shows the effect of of phosphorus source on lead concentration for the 4:1 molar ratio phosphorus to lead amendment after three weeks of incubation time. For apatite-amended slag, there generally were no significant differences in lead concentration for any fraction when compared to the control. However, for the KH2PO4-amended slag, lead in the residual fraction showed a significant increase from 7807 mg\kg to 19821 mg\kg compared to the control, and there were significant decreases in the in lead content for the exchangable, carbonate, iron- and manganese-oxide, and organic fractions compared to the control.
The effect of phosphorus source on cadmium concentration for 4:1 molar ratio phosphorus to cadmium after three weeks incubation time is shown in Figure 2. For the apatite-amended sample, there was a significant decrease in cadmium concentration to 10 mg\kg (down from 20 mg\kg in the control) for the exchangable fraction. There was also a slight but insignificant increase in cadmium concentration for the apatite-amended carbonate and residual fractions. For the KH2PO4-amended samples, there was a significant decrease in exchangable cadmium to 8 mg\kg (down from 20 mg\kg in the control) as well as a slight but insignificant decrease in cadmium in the Fe/Mn fraction compared to the control.
Effect of Time
The histograms in Figures 3 and 4 show the effect of time on lead concentration for 4:1 molar ratio phosphorus to lead amendments. Three and twelve weeks are the time increments shown. There are no significant differences in lead content for any fraction in either the apatite-amended samples (Figure 3), or in the KH2PO4-amended samples (Figure 4). Most of the lead in the apatite-amended samples is in the residual fraction, as is almost all of the lead in the KH2PO4-amended samples (over 9000 mg/kg and over 19,800 mg/kg, respectively).
Time effect on cadmium concentration for 4:1 phosphorus to cadmium molar ratios is shown in Figures 5 and 6, with time increments of three and twelve weeks. Results for apatite-amended samples appear in Figure 5 and results for KH2PO4-amended samples appear in Figure 6. There appears to be no effect of time on cadmium fractionation other than a significant increase in the residual fraction cadmium for KH2PO4-amended samples from 38 mg/kg to 48 mg/kg. No cadmium was in the organic fraction.
Effect of Phosphorus Rate
The rate effect on lead concentration for KH2PO4-amended samples after three weeks incubation time is shown in Figure 7 for 2:1 and 4:1 molar ratio phosphorus to lead. For each of the exchangable, carbonate, Fe/Mn, and organic fractions, there is a significant decrease in lead compared to the control, although the amount of decrease between the rates is significant only for the iron- and manganese-oxide fraction. Most of the lead for both rates of amendment is in the residual fraction, where there is a significant increase compared to the control (20,753 and 19,821 mg/kg, versus 7,807 mg/kg for the control).
Rate effect on cadmium concentration for KH2PO4-amended samples after three weeks incubation time is shown in Figure 8 for 2:1 and 4:1 molar ratio phosphorus to cadmium. Incubation time is again three weeks.There are significant decreases in cadmium concentration compared to the controls for both the 2:1 and 4:1 rates in the exchangable and iron- and manganese-oxide fractions (16 and 8 mg/kg versus 20 mg/kg; and 11 and 12 mg/kg versus 15 mg/kg, respectively). A significant increase in cadmium content occurs for the 2:1 rate in the carbonate fraction and no cadmium is found in the organic fraction for either rate. Much of the cadmium for both rates is found in the residual fraction, where significant increases occur relative to the control (44 and 38 mg/kg, versus 31 mg/kg for the control).
X-ray diffractograms for the control and KH2PO4-amended slags are shown in Figure 9. Quartz peaks are labelled A, and lead pyromorphite peaks are labelled B. Peak height is proportional to abundance. In the upper set of diffractograms, there is an increase in the relative amount of pyromorphite in the 4:1 phosphorus to lead molar ratio KH2PO4 amendment when it was allowed to incubate for three weeks, but very little further increase when the same amendment was allowed to incubate for a total of six weeks. The lower set of diffractograms show the rate effect of the 1:1 and 4:1 phosphorus to lead KH2PO4 amendments at an incubation time of three weeks. The 4:1 ratio amendment can be seen to be more effective in increasing pyromorphite abundance. Some of the decrease in the quartz peak height may be due to silica dissolution.
Lead content of the amended slag dramatically decreased in the exchangable, carbonate, iron- and manganese-oxide, and organic fractions of the slag, and became concentrated in the residual fraction. Pyromorphite increases in abundance as treatment progresses, indicating that this is the mineral phase in which the lead is being sequestered. Although the cadmium content of the exchangable fraction of the amended slag also decreased as the experiment progressed, the fate of the cadmium is less clear, although there is an indication that phosphate amendment may move some cadmium to the carbonate fraction (probably represented by the mineral octavite, CdCO3). The more soluble potassium phosphate (KH2PO4) was more efficient than apatite (Ca5(PO4)3Cl) in remediating the incubation samples, and rate effects were more important than time effects in heavy metal redistribution.
Bioavailable lead and cadmium in slag at the Dearing, Kansas, site can be reduced by amending the slag with phosphate in the form of apatite (Ca5(PO4)3Cl) or potassium phosphate (KH2PO4). This form of remediation has the advantage of being performed in situ, without the need for expensive excavation and waste removal.
The addition of phosphate is more efficient at redistributing lead than cadmium, as the insoluble phosphate mineral pyromorphite is available to sequester this metal in a non-bioavailable form. The mineral phase in which cadmium is sequestered is less obvious, but may be octavite (CdCO3). Potassium phosphate (KH2PO4), the more soluble phosphate source, is more effective in redistributing heavy metal content than apatite
Phosphate source and the rate at which the phosphate amendment is applied appears to be more important than the time the samples were incubated. Apparently, precipitation of the sequestering mineral phase(s) took place relatively quickly, prior to the taking of the first sample at three weeks into the experiment.
The work presented here was partially supported by the U.S. Environmental Protection Agency (EPA) under assistance agreement 819653 for the Great Plains/Rocky Mountain Hazardous Substance Research Center. It has not been presented to EPA for peer review and may not reflect the views of the agency, and no official endorsement should be inferred. The Kansas Agricultural Experiment Station and the Center for Hazardous Substance Research also provided partial funding.
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Figure 1. The effect of of phosphorus source on lead concentration for 4:1 molar ratio
phosphorus to lead amendments after three weeks of incubation time.
Figure 2. The effect of phosphorus source on cadmium concentration for 4:1 molar ratio
phosphorus to cadmium amendments after three weeks incubation time. Results for both
apatite and potassium phosphate amendments are shown.
Figure 3. The effect of time on lead concentration for 4:1 molar ratio phosphorus to lead
apatite amendment. Three- and twelve-week time increments are shown.
Figure 4. The effect of time on lead concentration for 4:1 molar ratio phosphorus to lead
KH2PO4 amendment. Three- and twelve-week time increments are shown.
Figure 5. Time effect on cadmium concentration for 4:1 molar ratio phosphorus to
cadmium apatite amendment. Time increments of three and twelve weeks are shown.
Figure 6. Time effect on cadmium concentration for 4:1 molar ratio phosphorus to
cadmium KH2PO4 amendment. Time increments of three and twelve weeks are shown.
Figure 7. Rate effect on lead concentration for KH2PO4-amended samples after three
weeks incubation. The rates shown are 2:1 and 4:1 molar ratio phosphorus to lead.
Figure 8. Rate effect on cadmium concentration for KH2PO4-amended samples after three
weeks incubation. The rates shown are 2:1 and 4:1 molar ratio phosphorus to cadmium.
Figure 9. X-ray diffractograms for the control and KH2PO4-amended slags. Quartz peaks (A) and lead pyromorphite peaks (B) are indicated. Heights of the pyromorphite peaks increase with greater abundance of that mineral. The upper set of diffractograms (Figure 9a) shows diffractograms for the control as well as for the 4:1 phosphorus to lead molar ratio KH2PO4 amendment at three- and twelve-week incubation times. The lower set of diffractograms (Figure 9b) shows diffractograms for the control, and for the 1:1 and 4:1 molar ratio phosphorus to lead ratio KH2PO4-amended samples after an incubation time of six weeks.