Theme: 08. Metals

Topic Area: 8a. Managing Chromium-Contaminated Sites

Formatted Title:

Immobilization and On-Site Reuse of Soils Contaminated with Arsenic and Chromium: A Circular Economy Approach

Background/Objectives:

A former wood treatment plant in an industrial area in Adelaide, South Australia was being upgraded to new commercial/industrial land use. During the site works, it was discovered that arsenic and chromium were present at levels which exceeded the State based maximum soil landfill disposal guidelines for both solid phase and leachate. The site owner assessed several different management options for the soil and decided that immobilization followed by on-site reuse of the treated soil was the most cost-effective and sustainable option, consistent with a circular economy approach. Landfill disposal was cost prohibitive, unsustainable, and not in accordance with State guidance on waste management (reduce, reuse, recycle, etc.). Due to shallow groundwater, on-site reuse without immobilization would not be an acceptable environmental outcome for the site due to the potential for contaminant leaching to or inundation with groundwater and associative potential impact to down hydraulic gradient receptors.

Approach/Activities:

Several soil samples were collected from stockpiles at the site and sent to a local laboratory for bench-scale feasibility testing. Soils were treated with 1% to 5% of a carbon/mineral sorbent and an alum sludge from a drinking water treatment plant. Portland cement (5%) treatments were also tested. The carbon/mineral sorbent contains positively charged and hydrophobic surfaces, while the alum sludge contained iron and aluminium oxyhydroxides which present a high surface area of positively charged surfaces. Portland cement has been shown to enhance some heavy metal leaching through solidification.

The sorbents were mixed with the soil and water was added to assist the binding process. After a fixation period of 3 days, treated soils were sent to an accredited commercial laboratory for soil leachate preparation using the Australian Standard Leaching Procedure (ASLP, pH 5) and ICP analysis of chromium and arsenic in the leachates.

After successful bench-scale feasibility testing, around 1,600 metric tonnes of soil were treated in the field at dosage rates of 5% for the carbon/mineral sorbent and 5% for the alum sludge. Validation samples were sent for analysis as per the above methodology.

Results/Lessons Learned:

Bench-scale feasibility testing showed that 5% addition of the carbon-mineral sorbent reduced arsenic leachability by 81% (from 4.25 mg/L to 0.8 mg/L), and 5% addition of the alum sludge reduced chromium leachability by 95% (from 0.75 mg/L to 0.04 mg/L). Portland cement had little or negative effect, probably due to the increase in pH which can actually sometimes hinder binding of amphoteric metals.

Validation sampling and analysis confirmed that the treatment was successful, with reduction of arsenic leachability by 87% and chromium leachability >99%, and approval was granted to reuse the soil under a paved area on site. This was a sustainable and cost-effective outcome for the site owner and was consistent with the principles of a circular economy; landfill disposal was avoided and the alum sludge, that would otherwise have been disposed to landfill, was reused as a beneficial remediation material. Finally, the immobilization process has a relatively low carbon footprint compared to most other traditional remediation technologies.

This project demonstrated a cost-effective and sustainable alternative to landfill for soil contaminated with chromium and arsenic and is consistent with a circular economy approach, including the avoidance of landfill disposal and the beneficial reuse of an alum sludge byproduct.

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James Fox