Evaluation of the effectiveness of phosphate treatment for the remediation of mine waste soils contaminated with Cd, Cu, Pb, and Zn
Highlights
► Phosphate treatment reduces water solubility of heavy metals in mine waste soils. ► Phosphate rock shows lower effectiveness compared to hydroxyapatite. ► The application of phosphate rock minimizes the risk of eutrophication.
Introduction
Soil contamination by toxic metals has increased over the past few decades. Heavy metals can adversely affect water resources, agricultural productivity and endanger the health of ecosystem and human populations. Many different industries such as mining, metal finishing, electroplating, battery recycling, glass, ceramic, paper and paint manufacturing are the primary sources of heavy metal contamination (Dudka and Adriano, 1997, Hettiarachchi et al., 2001).
Currently, different technologies can be applied to remediate soils and mining wastes polluted by toxic metals, such as biological, thermal and physical–chemical ones. These techniques are generally costly and disruptive to the application sites as they require the excavation of the contaminated soil, its treatment and replacement on site or disposal in specific landfills. On the contrary, in situ treatment methods using binding agents are less expensive and disruptive. Several studies have demonstrated that in situ heavy metal immobilization using phosphate is a cost-effective and environmentally positive remediation technology (Ma et al., 1993, Cao et al., 2009). Phosphate treatment transforms reactive and bioavailable heavy metal fractions in more stable forms with reduced mobility and bioavailability (Yoon et al., 2007). Phosphate minerals, i.e. hydroxyapatite (HA) and phosphate rock (PR) can effectively immobilize heavy metals in water and soil (e.g., Boisson et al., 1999, Cao et al., 2004, Chen et al., 2007, Corami et al., 2007, Corami et al., 2008b, Corami et al., 2008c, Marchat et al., 2007, Islam et al., 2010). Although PR has lower removal capacity than synthetic HA (e.g., Cao et al., 2004, Chen et al., 2007), natural low-grade (<30% P2O5) PR, which is presently considered as waste for fertilizer industries, is relatively inexpensive and needs to be explored as a low-cost adsorbent for processing contaminated media.
The reaction mechanisms for metal immobilization by phosphate minerals include: (a) ion exchange process, (b) surface complexation, (c) dissolution of the original phosphate minerals and precipitation of new metal phosphates, and (d) substitution of Ca in phosphate by other metals during recrystallization (coprecipitation) (Jeanjean et al., 1994, Ma et al., 1994, Xu et al., 1994).
Acidic soil pH conditions increase metal and phosphate solubility. An increase in pH from 5 to 8, for example, results in a decrease of HA solubility of two orders of magnitude (Shi and Erickson, 2001). Zhang and Ryan, 1998, Zhang and Ryan, 1999 showed that conversion of anglesite, cerussite, and galena to chloropyromorphite (Pb5(PO4)3Cl) or hydroxypyromorphite (Pb5(PO4)3OH) occurred between pH 4 and 5, which also optimizes the rate of HA dissolution. Therefore, the successful use of phosphate treatment to immobilize heavy metals depends on the soil pH (Laperche et al., 1996, Zhang and Ryan, 1998). Slightly acidic conditions (pH ∼ 5) have to be induced for enhanced release of metals from minerals and solubility of phosphate source, in particular if relatively insoluble sources such as HA and PR are used.
Several studies have evaluated the effectiveness of phosphate-induced Pb immobilization in soils (e.g., Ma et al., 1993, Laperche et al., 1997, Hettiarachchi et al., 2001, Cao et al., 2004). Nevertheless, the implementation of this method in mine waste soils contaminated with multiple metals such as Cd, Zn, and Cu and the evaluation of the effectiveness of different phosphate amendments is limited (Chen et al., 2007, Chen et al., 2009). The goal of the present study was to evaluate the effectiveness of different phosphate amendments (synthetic and natural) on Cd, Cu, Pb, and Zn immobilization in heavy metal contaminated soils by mining activity.
Section snippets
Materials
Two different types of phosphate amendments were used in this study: HA and FAP. Commercial HA was supplied by Alfa Aesar (Germany) and FAP by CF Industries (Florida, USA).
Heavy metal-contaminated soils have been sampled in sulfide mine areas (tailing dumps, ore stocking areas, streams, etc.) in Tuscany and Sardinia (Italy) (Fig. 1). Soil samples were collected from the top 30–40 cm at the mine sites. The samples comprised superficial tip material which was exposed to the atmosphere for several
Phosphate amendments characterization
XRD analyses confirmed that HA is pure hydroxyapatite (ICDD 9-432) (Powder Diffraction File), whereas FAP is a carbonate fluoroapatite with lesser amounts of quartz and calcite. SEM analyses showed that HA consists of rounded particles with average diameter of 10 μm and FAP of semi-rounded particles having average diameter of 50 μm.
The chemical composition and the specific surface area of the two amendments are reported in Table 1. Total metal contents were detected in the range of mg kg−1 and
Conclusions
This study investigated the effectiveness of HA and FAP in immobilizing Cd, Cu, Pb, and Zn in mine waste soils. The phosphate treatment of the polluted soils significantly reduced the water solubility of the metals with reduction percentage ranging generally from about 84% to 99%. The proposed immobilization mechanism involves both surface complexation of the heavy metals on the phosphate grains and partial dissolution of the phosphate amendments and precipitation of heavy metal-containing
Acknowledgements
This study corresponds partly to the Ph.D. research of A. Corami. Financial support was partly from CNR-IGG, U.O. Rome. The authors thank T. Coppola and S. Stellino for their laboratory assistance. S.M. and A.C. carried out the experimental work, the collection and analysis of the data, the interpretation of results and the preparation of this paper following the suggestions of V.F.
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