Agricultural Soil Remediation — Pilot Treatment for Mercury, Arsenic and Cadmium Impact

Project summary

iEnvi Principal Michael Nicholls and team were engaged by the Gansu and Baiyin government (as part of an Australian Foreign Affairs expert delegation) to design a rapid agricultural soil remediation strategy for irrigated cropland impacted by smelter and industrial wastewater. The site had elevated mercury (Hg), arsenic (As) and cadmium (Cd) across a large area planted to corn and wheat. A pilot treatment area requiring active (non-blending) remediation was 26,000 m3, within a broader impacted volume estimated at 2,400,000 m3. The design focused on treating the crop root zone to 1.0 m depth and on enabling future agricultural production with reduced crop uptake and acceptable toxicology results.

Key technical approach

The preferred approach combined excavation, soil washing and engineered immobilisation/soil amendment. The design used a mobile Matrix Enhanced Treatment System (METS) to increase soil–wash contact area and injection control to accelerate processing compared with conventional soil washing.

1. Excavation and staged treatment: target the crop root zone (≈1.0 m depth) for soils where plant uptake risk was highest.
2. METS + chelation: metered dosing of EDTA via the METS to chelate Cd and transfer into the liquid wash phase.
3. Chelator recovery and wash treatment: proposed recovery of EDTA from washate using ferric chloride (FeCl3), disodium hydrogen phosphate (Na2HPO4) and calcium hydroxide (Ca(OH)2) to precipitate or complex metals for separation.
4. Arsenic and mercury removal: dimercaprol (2,3‑dimercapto‑1‑propanol) was proposed to mobilise As and Hg into the wash, followed by ferric chloride / ferric sulphate flocculation to precipitate these metals from solution.
5. Blending and depth management: lower‑concentration soils below 0.6–1.0 m were to be blended rather than actively treated, to balance cost, risk and soil volumes.
6. Soil amendments for residual risk control and fertility: a recipe of locally available materials was proposed: red mud (~750 kg/mu), Cd‑free lime for pH adjustment, clean organic manure/compost (~500 kg/mu), plant straw powder (~200 kg/mu) and clay minerals (bentonite or zeolite, ~300 kg/mu) to immobilise residual metals and restore fertility.

Processing capacity and scale

Typical soil‑washing processing rates are often reported at ~100 m3/day in many programs. The METS approach was presented as a way to increase soil surface contact and reduce contact time with injection dosing, with reported potential processing rates up to ~6,000 m3/day for the METS configuration used in this project. The design therefore aimed to reduce program duration for the pilot volumes (26,000 m3) and to provide a framework for scaling to larger affected volumes (2,400,000 m3).

Target outcomes and verification

• Suggested Cd target range for treated soils: 3–6 mg/kg (to be validated through a small plot toxicology and crop uptake trial against relevant food safety thresholds).
• Post‑treatment verification to include soil chemistry, residual chelator concentration, washate characterisation, and crop uptake testing under Food Administration thresholds.

Practical risk, compliance and delivery considerations

• Regulatory and food safety verification: any target concentrations must be validated with toxicology and crop uptake trials and agreed with local food safety authorities before re‑introducing food crops.
• Washate and residual handling: chemical mobilisation of metals generates a contaminated liquid phase requiring treatment, disposal or secure containment that can add significant cost and regulatory burden.
• Chelator persistence and downstream risk: residual EDTA and other chelators can remobilise metals if not fully recovered; monitor residuals and consider alternative ligands or stabilisation where appropriate.
• Worker and community safety: controls for dust, chemical exposure and transport of contaminated materials are essential during excavation and treatment.
• Local materials and logistical benefits: using nearby organic manures and immobilising additives reduces transport costs and supports soil recovery, but quality control of amendments is required.

Practical takeaways for developers and land managers

• Targeting the root zone (≈1 m) reduces treated volumes and can deliver agricultural outcomes more quickly than whole‑profile remediation.
• METS can increase throughput versus conventional washing, shortening project duration; however, validate throughput claims, treatment effectiveness and washate management in a pilot program before scaling.
• Combine active removal (chelation/washing) with immobilising amendments to manage both total soil concentrations and bioavailability.
• Budget and plan explicitly for washate handling, reagent recovery, residuals testing and long‑term monitoring — these are often material cost drivers.

Recommended next steps

1. Implement a controlled small‑plot pilot to confirm target soil criteria (3–6 mg/kg Cd suggested), monitor crop uptake and demonstrate food safety compliance.
2. Confirm METS processing rates and reagent dosages on site, and fully characterise washate to develop an appropriate treatment/disposal pathway.
3. Engage regulators early to confirm acceptable target levels, monitoring requirements and waste management obligations.
4. Prepare a clear residual risk and long‑term monitoring plan, including potential follow‑up immobilisation if residual chelators persist.

Closing

This project demonstrates an integrated approach for accelerated remediation of metal‑impacted agricultural soils using a mobile METS soil‑washing system combined with chemical treatment and immobilising amendments. It is suited to clients seeking to re‑establish agricultural production while managing human health and regulatory risk, but requires careful pilot verification and planning for washate and residuals management.

Contact iEnvi for advice and project support: call 13000 43684 or use our contact page to discuss feasibility, pilot design and compliance pathways.