Phosphate-induced metal immobilization in a contaminated site

https://doi.org/10.1016/S0269-7491(02)00283-XGet rights and content

Abstract

To assess the efficiency of P-induced metal immobilization in soils, a pilot-scale field experiment was conducted at a metal contaminated site located in central Florida. Phosphate was applied at a P/Pb molar ratio of 4.0 with three treatments: 100% of P from H3PO4, 50% of P from H3PO4+ 50% of P from Ca(H2PO4)2, and 50% of P from H3PO4+5% phosphate rock in the soil. Approximately 1 year after P application, soil and plant samples were collected to determine mobility and bioavailability of selected metals (Pb, Zn, and Cu) using sequential extraction procedure and mineralogical characterization using X-ray diffraction (XRD) and scanning electron microscope-energy dispersive X-ray (SEM-EDX) analysis. Phosphorus distribution and soil pH effects were also evaluated. Phosphate was more effective in transforming soil Pb (to 53%) from the non-residual to the residual phase than soil Zn (to 15%) and soil Cu (to 13%). This was because Pb was immobilized by P via formation of an insoluble pyromorphite-like mineral in the surface and subsurface of the soil, whereas no phosphate mineral Zn or Cu was identified. While P amendment enhanced metal uptake in the roots of St. Augustine grass (Stenotaphrum secundatum), it significantly reduced metal translocation from root to shoot, especially Pb via formation of a pyromorphite-like mineral on the membrane surface of the root. A mixture of H3PO4 and phosphate rock was effective in metal immobilization, with less soil pH reduction and less soluble P. Although H3PO4 was effective in immobilizing Pb, its use should be limited to minimize soil pH reduction and potential eutrophication risk.

Introduction

Increasing awareness of the hazard that heavy metals can cause to the environment and to humans is presently pressuring society to comply with environmental regulations and develop management strategies to minimize their adverse impacts. Among available remediation technologies, in situ immobilization using metal-binding agents could be an environmentally sustainable and cost-effective alternative, especially for contaminated industrial sites. The application of P amendments to contaminated soils has been identified as a potentially efficient in situ remediation method (Cotter-Howells and Caporn, 1996, Hettiarachchi et al., 2000, Laperche et al., 1997, Ma et al., 1994). The objective is to immobilize metals primarily through the formation of metal phosphates with reduced solubility and enhanced geochemical stability in a wide range of environmental conditions.

Phosphate has been shown to effectively immobilize Pb from contaminated soils (Boisson et al., 1999, Cotter-Howells and Caporn, 1996, Hettiarachchi et al., 2000, Laperche et al., 1996, Laperche et al., 1997, Ma et al., 1994, Ma et al., 1995, Ma and Rao, 1997). It is well documented that lead phosphates are the most insoluble and stable forms of Pb in soils, and they can form rapidly in the presence of adequate Pb and P. Among all the lead phosphate minerals, chloropyromorphite has the lowest solubility, thus, it is most stable under favorable environmental conditions. Ma et al. (1993) showed that hydroxyapatite [HA, Ca10(PO4)6(OH)2] effectively immobilizes Pb in solution by formation of hydroxypyromorphite [Pb10(PO4)6 (OH)2]. In addition, the potential of using phosphate rock [PR, primarily Ca10(PO4)5F2] to immobilize aqueous Pb from Pb-contaminated soils was demonstrated (Ma et al., 1995). Phosphate rock effectively immobilized 22–100% aqueous Pb from 13 Pb-contaminated soils. The main mechanism of Pb immobilization is via dissolution of PR and subsequent precipitation of fluoropyromorphite [Pb10(PO4)5F2]. In situ phosphate-induced formation of pyromorphite is responsible for immobilizing Pb, thereby reducing its bioavailability in soils (Hettiarachchi et al., 2000, Ruby et al., 1994).

Although P amendments have mainly been applied to remediate Pb-contaminated soil, they are also applicable to other metals such as Zn, Cu, and Cd (Xu and Schwartz, 1994). phosphate rock effectively removes Zn and Cd from aqueous solutions, but the removal mechanism is different from that of Pb. Concerning the immobilization of other metals by hydroxyapatite, several mechanisms have been proposed: (1) ion exchange processes at the surface of HA (Middelburg and Comans, 1991, Xu and Schwartz, 1994); (2) surface complexation (Xu and Schwartz, 1994); (3) precipitation of some amorphous to poorly crystalline, mixed-metal phosphates (Ma et al., 1994), and (4) substitution for Ca in HA by other metals during recrystallization (coprecipitation) (Jeanjean et al., 1994, Xu and Schwartz, 1994). The last mechanism (coprecipitation) appears more significant for Cd sorption than for Zn (Xu and Schwartz, 1994). This might be due to the fact that the ionic radius of Cd2+ (0.109 nm) is closer to that of Ca2+ (0.114 nm) than that of Zn2+ (0.088 nm). The isomorphic substitution by Cu2+ (ionic radius 0.087 nm) would be less favorable than both Cd2+ and Zn2+. Direct evidence that Cd can occupy Ca crystallographic sites has been given using extended X-ray absorption fine structure (EXAFS) (Sery et al., 1996). It is difficult to quantitatively determine the proportions of metal removal due to specific mechanisms and it appears that the above four mechanisms more or less all work together (Xu and Schwartz, 1994). Nevertheless, the effectiveness and mechanisms of using phosphate to immobilize other metals such as Zn, Cu and Cd are not well understood.

Accordingly, soil acidic conditions may play an important role in metal immobilization using P amendments. Under neutral soil pH environments, observed solubility of metals and phosphate remains low, causing a major limitation for the efficient metal immobilization. For example, dissolution of the initial Pb phase has been reported to be the limiting factor in the formation of hydroxypyromorphite at pH values between 5 and 8 (Laperche et al., 1996). The conversion of PbO to Pb5(PO4)3OH was the most rapid at pH 5. In a similar experiment, Zhang et al. (1997) found that dissolution of anglesite (PbSO4) and cerussite limited the rate of the formation of Pb5(PO4)3OH. Therefore, effective metal-immobilization using P amendments requires enhanced solubility of metals by inducing acidic conditions.

In addition to soil pH, different P sources, with different solubilities, may also impact the effectiveness of metal immobilization. Our previous work (Cao et al., 2001) suggested that using soluble H3PO4 in combination with Ca(H2PO4)2 or PR effectively immobilized Pb from a Pb-contaminated soil. The role of H3PO4 in the mixture was to solubilize cerussite and PR, thereby increasing the readily available Pb in the soil, which facilitated more precipitation of Pb phosphate compounds.

Although the immobilization of heavy metals using P amendments has been very successful in the laboratory, the implementation of this technology in the field is very limited. Thus, a field demonstration of this technology was conducted at a site heavily contaminated with Pb, Cu and Zn. The main objectives of this research were (1) to demonstrate the effectiveness of P amendments on immobilization of Pb, Cu, and Zn in the field; (2) to compare the effectiveness of different P sources on metal immobilization; and (3) to assess the effects of P amendments on metal phytoavailability.

Section snippets

Site characterization and experimental plot establishment

The P-treated plots were established in a highly contaminated sit in Jacksonville, Florida. Past industrial activities had contaminated this site, including a gasoline station, salvage yard, auto body shop, and recycling of lead batteries. Selected physicochemical properties of composite soil samples from different plots of the site are given in Table 1. The plots are circular with areas of approximately 4 m2. phosphate amendments were applied to each plot, at a 4.0 P/Pb molar ratio based on

Soil characteristics at the contaminated site

Selected physical and chemical properties of surface soil (0–10 cm) are listed in Table 1. Organic matter (∼4.0%) and soil pH (∼7.0) were high compared to the average values for Florida soils, which average 2.9% and 5.04, respectively (Chen et al., 1999). Elevated organic matter mainly originated from the urban waste. Lime addition to neutralize acidity from lead battery probably resulted in the increase of soil pH (Nedwed and Clifford, 1997). Lead is the main contaminant with an average

Conclusions

This pilot-scale field study indicates that P amendments are efficient in transforming lead from non-residual into residual forms. The effective immobilization of lead is attributed to the in situ formation of insoluble pyromorphite-like minerals after application of P amendment. Phosphate amendment is effective for Pb immobilization, but less for Zn and Cu. Although P amendment enhanced the accumulation of Pb, Zn, and Cu in the roots of Stenotaphrum secundatum, it significantly reduced metal

Acknowledgements

This research was supported in part by the Florida Institute of Phosphate Research (Contract No. 97-01-148R). The authors would like to thank Mr. Thomas Luongo for his assistance in chemical analysis. We also gratefully acknowledge Drs. George O'Connor and Yongcong Li for reviewing this manuscript.

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