Formatted Title

Sustainable Management of Areas Impacted by Elemental Mercury Generated in Chlor-Alkali Processes

Background/Objectives

The study took place in a chlor-alkali unit located in the metropolitan region of São Paulo, SP, Brazil. The unit, already decommissioned, operated between 1948 and 1980. The older electrolytic processes to produce soda, hydrogen, and chlorine used metallic mercury (Hg^₀) as the cathode for reactions, resulting in occasional losses in the production process and occasional releases of this element into the physical environment (surface soils, subsurface soils, and aquifers). Investigative processes covered the entire production area, revealing residual mercury presence up to 6 meters deep in the soil. Dissolved mercury plumes were three-dimensionally delineated in the phreatic aquifer. In the human health risk assessment, risks were identified for inhalation exposure in an open environment by industrial workers due to detected Hg concentrations in surface and subsurface soil, ambient air, and soil vapors. The intervention plan was developed with the aim of adopting a technique with the least possible environmental impact, aiming to maximize soil reuse potential and make it suitable and safe for industrial use. Among the evaluated techniques, the chosen option was geotechnical capping, and the method for determining this technique will be detailed in the study.

Approach/Activities

A decision matrix was established to select the best intervention alternative for the case, with pre-established criteria applicable to the Brazilian market. Evaluated criteria included availability, performance in the impacted environment, ease of application, consequences of application, cost, history of technique or action use for similar cases, time required to achieve remediation goals, technology, and sustainability. The adopted containment technique stands out for safety, sustainability, shorter implementation time, and economic viability compared to other evaluated techniques. In addition to the mentioned factors, there is a reduction in risks for workers involved in civil construction interventions. To enhance sustainability, minimal energy and resource consumption is observed, generating less waste, eliminating the need for expensive water and steam treatment systems, and not transferring mass to other locations as some treatment options do. In addition to the mentioned points, the capping technique prevents rainwater infiltration, avoiding contamination leaching, and further reduces or inhibits atmospheric emissions. The construction process included the use of a high-density polyethylene geomembrane resistant to mercury. Additionally, a layer of clayey fill was applied to be covered with concrete or crushed stone. Analyses considered the geometric characteristics of the site, resulting in the choice of the best system for covering hazardous waste landfills, considering not only permeability but also the geotechnical stability of the system.

Results/Lessons Learned

To monitor the effectiveness of the process, a bimonthly mercury vapor sampling plan was established in the Intervention Plan (sampling through adaptation of NIOSH 6009 standard), showing results below the laboratory quantification limit, proving the effectiveness of the selected intervention until the present moment. The area can, therefore, be occupied and incorporated into open industrial activities. The project is being monitored and has been approved by the Environmental Agency of São Paulo (CETESB).