Environ. Sci. Technol. 2000, 34, 3501-3507
Bonemeal Additions as a Remediation Treatment for Metal Contaminated Soil MARK E. HODSON* AND EÄ V A V A L S A M I - J O N E S Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD England JANET D. COTTER-HOWELLS Department of Plant and Soil Science, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen, AB24 3UU Scotland
The ability of bonemeal additions (finely ground, poorly crystalline apatite, [Ca10(PO4)6OH2]) to immobilize pollutant metals in soils and reduce metal bioavailability through the formation of metal phosphates has been evaluated. Leaching column experiments were carried out on contaminated soils with pH varying between 2.7 and 7.1. Monitoring of leachates over a three month period indicated that bonemeal additions resulted in the immobilization of metals and an increase in the pH of the column leachate, the soil pore water and the soils themselves. Analytical scanning electron microscopy of the bonemeal treated soil at the end of the experiment revealed that Pb and Zn were associated with phosphorus. X-ray diffraction identified several newly formed phases in the bonemeal treated soil at the end of the experiments that had peaks of similar intensity and positions as reference Pb and Ca-Zn phosphates. Batch experiments and subsequent extraction of metals from controls and bonemeal amended soils using 0.01 M CaCl2 and DTPA indicated that bonemeal additions reduced the availability of the metals in the soils. Bonemeal amendments appear to have potential as a remediation treatment for metal contaminated soils.
Introduction
Materials Soil samples were taken from the historic mining sites of Parys Mountain (PM), Leadhills (LH) and Wanlockhead (WH) (9) in the United Kingdom (UK) and were chosen to give a range of metal contamination and pH values (Table 1). The soil was sieved to e2 mm which also homogenized the soil. The bonemeal used was obtained from a commercial supplier. It was sieved to produce a 90-500 µm fraction for use in the experiments because this size fraction was found to be the most effective of those previously tested (90-500 µm, 500-2000 µm and 2000-3300 µm) (8). Bonemeal is steam sterilized at temperatures above 100 °C which is standard procedure for termination of microbial activity. However, in the UK and further afield, there is concern over the safety of skeletal tissue of cattle. In a previous study (8) we showed that incineration of bonemeal at 400 °C had no effect on the results of bonemeal treatments to metal contaminated soils.
Methods
Remediation of metal contaminated land is an important current environmental issue. The majority of remediation strategies focus on either civil engineering methods (e.g. excavation, disposal and encapsulation) or process based technologies such as soil washing. Further development of phytoextraction technologies is required before large-scale remediation using this technique can be implemented (1). An alternative, chemical/mineralogical remediation method involving metal phosphate formation has been suggested (2, 3). Many metal phosphates (e.g. Pb, Zn, Cd) are highly insoluble (4). If pollutant metals in contaminated soils could be converted into phosphates, then the metals would be immobilized in situ and their bioavailability would be reduced. The majority of experiments examining metal phosphate formation have concentrated on Pb, which can form pyro* Corresponding author phone: +44 (0) 118 931 8911; fax: +44 (0)118 931 6660; e-mail:
[email protected]. Current address: Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading, Berkshire, UK, RC6 6DW. 10.1021/es990972a CCC: $19.00 Published on Web 07/15/2000
morphite (Pb5(PO4)3Cl) (2, 3, 5). Experiments have been carried out using highly soluble forms of phosphate, e.g. K2HPO4 (3). Pyromorphite formed but the highly soluble nature of this P source means that an eutrophication risk would be associated with such a treatment. Laboratory experiments using rock apatite have also been successful (6), but rock apatite is highly insoluble (4) and so might not release P sufficiently rapidly to remediate contaminated soil on an acceptable time scale. Experiments using finely powdered synthetic hydroxyapatite (which has a solubility intermediate between K2HPO4 and rock apatite) in soils and solutions have resulted in the formation of pyromorphite (2, 5, 7), but the use of synthetic hydroxyapatite on a field scale is economically unfavorable. It was recently suggested that poorly crystalline apatite, e.g. bone apatite (in the form of bonemeal-finely crushed bone), might represent a low-cost, readily available phosphate source that could be used to remediate metal contaminated land without causing excessive P runoff (7). Preliminary experiments (8) using a moderately contaminated soil indicated that bonemeal treatments could be a useful remediation method. The current experiments were carried out to determine whether bonemeal additions could be a suitable remediation treatment for soils heavily contaminated with metals and in the acidic to neutral pH range.
2000 American Chemical Society
Leaching Columns. Two hundred grams of either soil (controls) or a 1:50 bonemeal:soil mix, with the bonemeal evenly distributed throughout the soil, were packed into 250 mL polypropylene columns which were made from inverted bottles of 60 mm diameter, with their bases cut off (see ref 8 for experimental details). The soil was added to the columns in 50 g portions and was lightly compressed between additions. The final density of soil in the columns was approximately 1 g cm-3. A quartz wool plug at the base of the columns acted as a 6 µm filter for the leachate. The upper surfaces of the columns were covered with quartz wool, and the wool was irrigated twice daily by manual sprinkling (using a pipet) of a dilute ionic solution similar in composition to natural rain (12) (pH ) 4.4, 1.94 mg NO3- L-1, 0.49 mg NH4+ L-1, 1.87 mg Na+ L-1, 0.25 mg Mg2+ L-1, 0.29 mg Ca2+ L-1, 3.41 mg Cl- L-1, 2.65 mg SO42- L-1). The wool ensured that the “rainfall” was distributed evenly over the entire surface of the soil column. The rate of column irrigation was equivalent to rainfall of 0.9 m yr-1. The columns remained well drained and undersaturated throughout the experiments. The colVOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Metal Concentrations (µg Metal g-1 Dry Soil)a and pHb in the e2 mm Fraction of Soils and the 90-500 µm Bonemeala sample
texture
pH (H2O)
pH (CaCl2)
Zn
Pb
Cd
Cu
Ni
Parys Mountain (PM) Leadhills (LH) Wanlockhead (WH) bonemeal
silty clay loam silty clay loam silty clay loam
2.71 4.71 7.1
2.53 4.48 7.1
4872 213 14282 89
15043 9882 136260
