Use of bone meal amendments to immobilise Pb, Zn and Cd in soil: A leaching column study

Abstract

The aim of this study is to test the stabilisation of metals in contaminated soils via the formation of low-solubility metal phosphates. Bone apatite, in the form of commercially available bone meal, was tested as a phosphate source on a mine waste contaminated made-ground with high levels of Pb, Zn and Cd. Triplicate leaching columns were set up at bone meal to soil ratios of 1:25 and 1:10, in addition to unamended controls, and were run for 18 months. The columns were irrigated daily with a synthetic rain solution at pH of 2, 3, and 4.4. After 100 days, the leachate Pb, Zn and Cd concentrations of all amended columns were significantly reduced. For 1:10 treatments, release of these metals was suppressed throughout the trial. For 1:25 treatments, Zn and Cd concentrations in the leachates began to increase after 300 days. DTPA and water extractions showed that Pb and Cd were more strongly held in the amended soils. This study concludes that the complexity of soil processes and the small quantities of metals sequestered precluded determination of a metal immobilisation mechanism.

Introduction

Metals may be present in soils from a variety of sources and can represent a potential hazard to humans and the environment if they are readily released into soil solution or are otherwise available to biological processes. However, if such metals are bound up in relatively inert and insoluble compounds, the danger they represent is reduced substantially.

The phosphates of many metals (including Pb, Zn, Cd, Cu and Ni) can have exceptionally low solubilities (Nriagu, 1984) and are stable across a very wide range of environmental (Eh and pH) conditions. There now exists a significant body of research showing such compounds may readily form, given the presence of a source which releases sufficient phosphate to solution (e.g. Valsami-Jones et al., 1998, Cao et al., 2003, Hettiarachchi and Pierzynski, 2004). The amendment of soils with phosphate may have the potential to reduce significantly metal bioavailability and release to ground and surface waters.

Much of this research has largely been concerned with Pb immobilisation via the precipitation of the Pb apatite-group mineral pyromorphite [Pb₅(PO₄)₃OH] in the presence of soluble phosphate. In synthetic solutions, pyromorphite has been shown to form rapidly in conditions consistent with those found in many soil environments (Xu and Schwartz, 1994). In several experiments using metal-contaminated soils, evidence for Pb phosphate formation has been obtained (e.g. Cotter-Howells et al., 1994). Experiments where Pb uptake by plants was measured have indicated that its availability is reduced in phosphate-treated contaminated soils (Laperche et al., 1997, Brown et al., 1999).

The formation of other metal phosphates, including those of Zn, Cu, Cd and U, has also been implicated (Ma et al., 1994, Chen et al., 1997, Valsami-Jones et al., 1998) and in soils, the bioavailability of metals has been reduced significantly after phosphate addition (Hettiarachchi and Pierzynski, 1999).

Given that phosphate itself can lead to environmental problems such as eutrophication of surface waters, due to its role as a nutrient, the choice of phosphate source for soil remediation is an important one. Furthermore the presence of arsenic in many metal contaminated soils should be taken into consideration, as aqueous phosphate may compete with arsenate for adsorption sites and may cause As mobilization (e.g. Boisson et al., 1999).

The calcium phosphate mineral hydroxylapatite [Ca₅(PO₄)₃OH] has a relatively low solubility when present in its rock crystal form though work by some researchers supports its potential for remediation of toxic metal contamination in soils (Hettiarachchi and Pierzynski, 2002, Cao et al., 2003). Hydroxylapatite is also the principal mineral constituent of bone, and bone meal has been identified as a potentially appropriate source of phosphate due to the moderate solubility associated with its carbonate content (Tung, 1998, Valsami-Jones et al., 1998) and poorly crystalline structure (Cheung et al., 2002). Earlier work with bone meal amendments has been encouraging. Laboratory experiments using a variety of soils have shown release of metals to pore waters to be reduced significantly, with a corresponding reduction in metals availability to plants and soil organisms (Hodson and Valsami-Jones, 2000a, Davies et al., 2003). It has also been shown recently that incinerated bone meal is a very effective sorbing agent for As(V) (Sneddon et al., 2005). The previous work on metal immobilization by bone meal was carried out mostly on acidic soils and for short periods of time (up to 4 months). Therefore, the permanence of such reductions needs to be demonstrated further, especially in soils with high metal loadings across a range of soil component fractions.

This paper reports on the results of an extended (18 months) leaching study carried out to investigate the effectiveness of bone meal amendments in reducing the mobility of the metals Pb, Zn and Cd in a contaminated mine waste made-ground of above-neutral pH.