An Apatite II permeable reactive barrier to remediate groundwater containing Zn, Pb and Cd

Abstract

Phosphate-induced metal stabilization involving the reactive medium Apatite II™ [Ca_10−xNa_x(PO₄)_6−x(CO₃)_x(OH)₂], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO₄ and NO₃. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River, Idaho. Microbially mediated SO₄ reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb₁₀(PO₄)₆(OH,Cl)₂] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L⁻¹), has reduced Zn to near background in this region (about 100 μg L⁻¹), and has reduced SO₄ by between 100 and 200 mg L⁻¹ and NO₃ to below detection (50 μg L⁻¹). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).

Introduction

In the Silver Valley Mining District of northern Idaho, also known as the Coeur d’Alene Mining District, Zn is the contaminant of concern with respect to aquatic life, and Pb and Cd are the contaminants of concern with respect to human health and wildlife. In 1995, the site of the Success mine and mill was identified as the largest remaining metals loader in the Ninemile Creek drainage (Golder, 2000). Ninemile Creek is an upper basin tributary to the South Fork of the Coeur d’Alene River, which ultimately flows into Lake Coeur d’Alene. On the basis of data collected by the Idaho Department of Environmental Quality (DEQ) in 1994 and 1995, the Success site contributed approximately 37% and 87% of the total metals load to this drainage at high- and low-discharge periods, respectively. The primary source of the metals loading was identified as groundwater discharge from the toe of the Success tailings and waste-rock pile to the East Fork of Ninemile Creek (Golder, 2000). Up to this time, the removal of metal sources within this area had relied primarily on the excavation and transportation of materials to hydraulically isolated repositories. Because of the large volume of source materials in the region, it was decided to investigate the use of permeable reactive barriers to treat waters as they emerged from these sources. A permeable reactive barrier (PRB) was installed at the Success site in 2001, and the objective of this study was the evaluation of its performance over the next 4 years.

PRBs consist of a water-permeable material that has specific chemical reactivities towards one or more chemical constituents via mechanisms such as adsorption, exchange, oxidation–reduction, or precipitation. These barriers can have various ranges of specificity, e.g., adsorption of Sr and microbes by modified zeolite (Bowman, 2003), precipitation of metals by apatite (Bostick et al., 1999, Conca et al., 2000, Conca et al., 2003, Fuller et al., 2002, Matheson et al., 2002, Wright et al., 2004a), or reduction by zero-valent Fe, other Fe phases, or microbial activity (Starr and Cherry, 1994, Benner et al., 1997, Tratnyek et al., 1997, Puls et al., 1999, Goldstein et al., 2000, Naftz et al., 2000). The results of previous and ongoing work demonstrate that stabilization of contaminated soils and groundwater by apatite-group minerals has the potential to be a successful and widely applicable remediation strategy for metals and radionuclides (McArthur et al., 1990, Ma et al., 1993, Ma et al., 1995, Ruby et al., 1994, Stanforth and Chowdhury, 1994, Xu and Schwartz, 1994, Chen et al., 1997a, Chen et al., 1997, Eighmy et al., 1997, Zhang et al., 1998, Manecki et al., 2000, Wright et al., 2004a, Wright et al., 2004b, Wright et al., 2005).

Apatite-group minerals form naturally and are stable across a wide range of geological conditions for hundreds of millions of years (Nriagu, 1974, Wright, 1990a, Wright, 1990b). Work by Wright and others (Wright et al., 1984, Wright et al., 1987a, Wright et al., 1987b, Wright et al., 1990) investigated the trace-element composition of fossil apatite (teeth and bones) and sedimentary phosphorite deposits of various geological ages. It was observed that sedimentary and biogenic apatite deposited in seawater not only concentrates metals and radionuclides from the seawater to millions of times the ambient concentration, but also locks them into the apatite structure for up to a billion years with no subsequent desorption, leaching, or exchange, even in the face of diagenetic changes in the pore-water chemistry and pH, temperatures of more than 500 °C, and tectonic disruptions. Including isostructural species, more than 300 apatite-type minerals exist, with elements from the entire periodic table replacing Ca, P, and OH in the fundamental apatite crystal structure (Skinner, 1989). Their solubility products (K_sp) vary from about 10⁻²⁰ to 10⁻¹⁵⁰ (Nriagu, 1974, Manecki et al., 2000).