Theme: 08. Metals

Topic Area: 8c. Precipitation and Stabilization of Metals

Formatted Title:

High Efficacy Two-Stage Metal Treatment Incorporating Basic Oxygen Furnace Slag and Microbiological Sulfate Reduction

Background/Objectives:

A pertinent environmental challenge at mining sites worldwide is the formation of mining-influenced water (MIW), such as rock/mine drainage, when excavated pyrite (Fe₂S)-bearing rocks are exposed to water and oxic conditions, initiating abiotic and microbial oxidation of Fe₂S. MIW streams are often acidic (pH 2−3) and commonly contain elevated concentrations of heavy metals and metalloids, such as iron (Fe), aluminum (Al), copper (Cu), zinc (Zn), lead (Pb), arsenic (As), among others. Because MIW may pose a threat to human health, wildlife ecology, and water resources, it requires treatment to limit metal(loid) mobility and exposure. Lignocellulosic sulfate-reducing biochemical reactors (SRBRs) are a type of passive treatment for MIW most often used at remote and/or abandoned mining sites. Many SRBR designs employ lignocellulose as the packing material. Lignocellulose is the major component in plant cell walls and is mainly composed of cellulose (glucose polymers), hemicellulose (xylose and other sugar polymers), and lignin (monolignol polymers). Examples of material used in SRBRs include alfalfa hay, straw, or other grasses which are high in cellulose/hemicellulose content, and sawdust, wood chips, or nut shells which are high in lignin content. Common to SRBR designs is also employing an alkalinizing agent. In this project, we evaluated basic oxygen furnace slag in combination with SRBRs for MIW treatment.

Approach/Activities:

While the potential of steel slag for chemical treatment of MIW has been clearly demonstrated, research combining slag with microbial processes, such as denitrification, fermentation, and SO₄²⁻ reduction is extremely limited and has only recently emerged. In this study, we designed a novel two-stage treatment for MIW where basic oxygen furnace slag (slag stage) and microbial SO₄²⁻ reduction (SRBR stage) were incorporated in series. The two-stage design was used to assess MIW treatment effectiveness while (i) quantitatively defining the specific contributions of slag and SRBRs metal(loid) removal from MIW and (ii) tracking the potential of the slag stage to promote the desired microbial activity in SRBRs.

Results/Lessons Learned:

The slag stage increased MIW pH from 2.6 ± 0.2 to 5.0 ± 1.0 and removed >99.9% of the total metal(loid) concentration (initial of 130 ± 30 mg L⁻¹) via precipitation or adsorption. The SRBR stage removed the bulk of SO₄²⁻ from MIW and additional metal(loid)s. Cadmium, copper, and zinc showed high removal rates in SRBRs (≥96%) and were likely precipitated as sulfide minerals. The microbial communities developed in SRBRs receiving MIW from the slag stage were enriched in hydrolytic, fermentative, and sulfate-reducing taxa. However, the SRBRs developed distinct community compositions because of the different lignocellulose sources employed. Overall, this study underscores the potential of a two-stage treatment employing steel slag and SRBRs for full-scale implementation at mining sites.

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Leonard Santisteban