Description

Sites contaminated by accidental or intentional spills of liquid phases containing toxic organic compounds such as hydrocarbons, chlorinated solvents, or pesticides are common problems in the environment. These spills have led to hundreds of local soil and groundwater contamination hotspots, posing a risk to human health and the environment. Many of these discharges are hydrophobic, forming non-aqueous liquid phases (NAPL), some lighter than water (LNAPL), which float above the water table, and others denser than water (DNAPL), which migrate vertically by gravity, being retained in areas of lower permeability. Given their hydrophobicity, the presence of these compounds also generates a plume of groundwater contamination, as shown in Figure 1. Volatile organic compounds also generate toxic gases that can reach the atmosphere. The low biodegradability and high toxicity of these organic compounds pose a significant risk to potential receptors in soil and groundwater.

After pumping out these organic phases, a significant fraction of these compounds, whose viscosity and interfacial tension have increased with ageing in the soil, remain in the site. These residual phases are trapped in soil pores or retained in lower permeability lithologies. These organic residues are difficult to pump by conventional techniques but generate a continuous plume of groundwater contamination. Injection of surfactants into the subsurface and subsequent extraction of the injected fluid can remove a significant mass of this residual contamination from the subsurface in a shorter time.

Figure 1. Discharges of organic phases into the soil and contamination plume generated in groundwater. Left: Compounds denser than water (DNAPL). Right: Compounds lighter than water (LNAPL).

How does it work?

Surfactants are amphiphilic substances with hydrophobic and hydrophilic ends. Surfactants increase the solubilization of contaminants trapped in soil pores and decrease the viscosity of the organic phase and the interfacial tension between the organic and aqueous phases.

The treatment consists of injection of an aqueous phase containing a biodegradable surfactant into the subsoil and extraction of the injected solution. The extracted phase contains the solubilized or emulsified contaminant. The surfactant solubilizes the organic contaminants and lowers the viscosity of the organic phase and the interfacial tension between the organic and aqueous phases, favoring their extraction. Proper design of this treatment requires knowledge of the interaction between the phases (soil, water, organic). The partition equilibrium of surfactants and contaminants between phases and kinetics of these processes must be evaluated. The interaction between aqueous and organic phases and the surfactant effect on viscosity and interfacial tension must be studied. This knowledge is required to select the appropriate doses and contact times.

Figure 2. Injection of surfactants into the subsoil for solubilization-mobilization of the organic phase and remediation of subsoil contamination by NAPL extraction.

Advantages

It is particularly suitable for removing residual low soluble and low volatile organic phases if they are identified at the site. In that case, the contaminant removal achieved with this treatment is orders of magnitude greater than that achieved with traditional groundwater pump and treat technologies. The solubility of many organic contaminants in groundwater is as low as a few mg/L. The surfactants applied increases the solubility of the contaminants in the aqueous phase up to hundreds or thousands of mg/L. In addition, the organic phases discharged have aged and exhibit high viscosity and interfacial tension, making standard pumping ineffective in their mobilization and removal from the subsurface. The organic phase viscosity and interfacial tension can decrease by up to 10 times in surfactant presence. The surfactants used are biodegradable and do not interfere with subsequent bioremediation or natural attenuation treatments.

Where has it been developed?

The INPROQUIMA group (UCM, Fac CC Chemistry) has the necessary knowledge and infrastructure to select the most suitable surfactant (or mixtures) to remove the residual organic phase in the contaminated site. This phase is chemically characterized, and its interaction with the different surfactants is studied to select the optimum mixture to reduce viscosity and interfacial tension. The thermodynamic equilibria between phases and the adsorption of surfactants in the soil are modelled. The biodegradability of the injected surfactants and their effect on the microbial population is also studied. Soil column and batch tests are carried out, and analytical equipment, modelling and optimization software are available. Different treatments are also available to manage the solutions extracted with the surfactant and pollutant to selective elimination of the latter and recovery of the surfactant.

And also

The INPROQUIMA group has successfully designed the SEAR process and its application in collaboration with companies in different contaminated sites, both for light organic phases (fuel and diesel spills) and dense organic phases (chlorinated compounds), for complex organic phases and different soil lithologies.

Contact

© Office for the Transfer of Research Results – UCM Faculty of Medicine. Edificio Entrepabellones 7 y 8. C/ Doctor Severo Ochoa, 7. 28040 Madrid. comercia@ucm.es

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Classification

COMPLUTRANSFER

Science and Engineering

Chemistry

Responsible Researchers

Aurora Santos López : aursan@ucm.es

Department: Chemical Engineering and Materials Department

Faculty: Chemistry