Comparative evaluation of short-term leach tests for heavy metal release from mineral processing waste

doi:10.1016/j.scitotenv.2005.10.021

Science of The Total Environment

Volume 364, Issues 1–3, 1 July 2006, Pages 14-23

https://doi.org/10.1016/j.scitotenv.2005.10.021 Get rights and content

Abstract

Evaluation of metal leaching using a single leach test such as the Toxicity Characteristic Leaching Procedure (TCLP) is often questionable. The pH, redox potential (E_h), particle size and contact time are critical variables in controlling metal stability, not accounted for in the TCLP. This paper compares the leaching behavior of metals in mineral processing waste via short-term extraction tests such as TCLP, Field Leach Test (FLT) used by USGS and deionized water extraction tests. Variation in the extracted amounts was attributed to the use of different particle sizes, extraction fluid and contact time. In the controlled pH experiments, maximum metal extraction was obtained at acidic pH for cationic heavy metals such as Cu, Pb and Zn, while desorption of Se from the waste resulted in high extract concentrations in the alkaline region. Precipitation of iron, caused by a pH increase, probably resulted in co-precipitation and immobilization of Cu, Pb and Zn in the alkaline pH region. A sequential extraction procedure was performed on the original waste and the solid residue from the E_h–pH experiments to determine the chemical speciation and distribution of the heavy metals. In the as-received waste, Cu existed predominantly in water soluble or sulfidic phases, with no binding to carbonates or iron oxides. Similar characteristics were observed for Pb and Zn, while Se existed mostly associated with iron oxides or sulfides. Adsorption/co-precipitation of Cu, Se and Pb on precipitated iron hydroxides was observed in the experimental solid residues, resulting in metal immobilization above pH 7.

Introduction

Heavy metal contamination of natural environments due to hazardous waste leaching is a global problem. The mobility of toxic metals released from wastes is assessed using a variety of risk assessment procedures including the Toxicity Characteristic Leaching Procedure (TCLP), among others (van der Sloot, 1996, Barna et al., 1997, USEPA, 1997). The TCLP test, widely used by state and national agencies, was designed to simulate leaching of heavy metals and organics from industrial wastes to be co-disposed in municipal solid landfills. Over several years, severe limitations of using TCLP in assessing contaminant leaching from industrial wastes such as mine and mineral processing wastes (MPW) have come to light due to its specificity only to landfill conditions and other physicochemical parameters (USEPA, 1995, USEPA, 1999). The main reasons contributing towards the inaccurate prediction of heavy metal leaching from MPW included: (1) choice of correct extraction fluid (Hooper et al., 1998); (2) pH or alkalinity of the wastes (Batchelor, 1998, van der Sloot, 1996); (3) particle size (Janusa et al., 1998, Karius and Hammer, 2001) and; (4) redox potential (Meng et al., 2001). Due to variation in metal leachability with these physical conditions in short-term experiments, standardization of the results and procedures is critical in the risk assessment of hazardous waste disposal.

Field experiments on mine wastes suggest that metal mobility depended on the pH and redox potential, as they influence the speciation and association of the metals in the waste matrix (Jurjovec et al., 2002, Pagnanelli et al., 2004). While metals are immobilized in the waste due to secondary precipitation or adsorption reactions, changes in pH or a pH induced redox potential (E_h) decrease could enhance metal mobility.

Logically, speciation analysis is of major importance in environmental research as it provides crucial evidence on the mineral phases and chemical form of target metals present in the waste matrix (Scheckel et al., 2003). As a consequence, sequential extraction procedures (SEP), using extraction fluids of increasing strengths, have been used to characterize the physical and chemical properties of the wastes. Although the results from the SEP are often variable and should be accompanied by in-depth investigation including the use of EXAFS, they are still useful in providing information on contaminant partitioning within an environmental matrix.

Despite the emphasis on the need for a more integral and scientific approach towards the determination of waste leaching mechanisms, very few studies have correlated the results from standardized batch extraction tests to characteristics of the waste. To aid in the understanding of these mechanisms, the main objective of this paper is to: (1) compare leaching of Cu, Pb, Zn and Se from MPW in different short-term batch extraction procedures; (2) quantify release of the selected metals under different pH and redox conditions and; (3) correlate metal mobility to its speciation in the waste matrix. The mineral processing waste was selected due to its high metal content and to also evaluate its applicability in landfill disposal. In addition to the TCLP, short-term procedures such as the Field Leach Test (FLT) used by the United States Geological Survey (USGS), and deionized water (DI) extraction tests were performed. Here, “short-term extraction test” is operationally defined as test conducted within a 48-h period. This work aims to illustrate the effect of pH on the characteristics of waste, which subsequently controls metal mobility.

Section snippets

Waste sampling

The mineral processing waste was obtained from an abandoned copper mine facility in Nevada, USA. Details of the sampling location have been reported elsewhere (USEPA, 2004). After collection, the samples were homogenized, air dried to remove moisture and sieved. Particles with sizes less than 9.5 mm were used for the TCLP test, while those with sizes less than 2 mm were used for the DI, FLT and controlled pH experiments. Ideally, the waste should be homogenized preferably to pass through 125 μm

Waste characterization

The waste sample was observed to have high concentration of iron (Fe, 66,300 mg kg^− 1), copper (Cu, 1755 mg kg^− 1), lead (Pb, 936 mg kg⁻¹), zinc (Zn, 429 mg kg^− 1) and selenium (Se, 183 mg kg^− 1), with the total organic content less than 0.1 wt.% (Table 2). Fig. 1 presents the data on the metal associations in the waste matrix, determined by the sequential extraction procedure. The metal concentration in the supernatant collected after the rinsing procedure, denoted by Step A rinse, Step B rinse

Waste mineralogy

Even though the sequential extraction of the metals may not provide accurate determination of its association in the waste matrix, it could be used selectively to understand impact of pH on metal leaching. The sequential extraction procedure results reveal that Cu, Fe and Zn existed predominantly in sulfidic or oxide phases (Fig. 1). The abandoned mine site was used for copper extraction and consisted of tailings piles, waste rock and mineral processing residues. The mineral ores present in the

Conclusion

High concentration of iron (Fe, 66,300 mg kg^− 1), copper (Cu, 1755 mg kg^− 1), lead (Pb, 936 mg kg^− 1), zinc (Zn, 429 mg kg⁻¹) and selenium (Se, 183 mg kg^− 1) were found in the waste. These metals existed either as pyrites or adsorbed on iron oxides as determined by the sequential extraction procedure. Nearly half the amount of total Cu was readily exchangeable, resulting in high extract concentration. Comparison between TCLP, FLT and deionized water extraction test indicated strongly that heavy

Acknowledgements

This paper has not been subjected to internal policy review. Therefore, the research results presented herein do not, necessarily, reflect the views of the USEPA or its policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors would like to express their appreciation to Mr. David Reisman of the USEPA for his help in obtaining the waste sample and his input during the course of this research.

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Comparative evaluation of short-term leach tests for heavy metal release from mineral processing waste

Abstract

Introduction

Section snippets

Waste sampling

Waste characterization

Waste mineralogy

Conclusion

Acknowledgements

J Hazard Mater

Geoderma

J Hazard Mater

Microchem J

Geochim Cosmochim Acta

Sci Total Environ

J Colloid Interface Sci

Environ Pollut

Geochim Cosmochim Acta

Appl Geochem

Adv Environ Res

Waste Manag