Emerging contaminants require investment in the knowledge and applicability of remediation techniques for detection and remediation.
LITOCLEAN is developing several subsurface remediation projects where 1,4-dioxane is the main remediation compound. The treatment of this compound is a challenge in itself due to its particularities. The company’s commitment to R&D&I, the collaboration with university entities and the development of innovation projects make it possible to respond to the contamination of sites by this compound.
It is a man-made heterocyclic organic compound, classified as an ether, which is liquid at room temperature. It is colorless, highly flammable and characterized by high solubility and miscibility in water and most organic solvents, as well as instability at elevated ambient temperatures and pressures, with the possibility of forming explosive mixtures with prolonged exposure to light or air.
It has a high stability once dissolved in the aqueous medium and a high volatilization potential when on dry surfaces, as well as a low affinity for organic carbon and soil adsorption processes, being easily photodegradable when in vapor form in the atmosphere. This means that its presence in soils and air is of little relevance, since its low affinity for organic carbon and soil adsorption processes means that retention within the vadose zone is very limited, being easily leached into groundwater, while in air photodegradation gives it a half-life of only a few days.
On the other hand, its high stability in the aqueous medium favors its rapid migration within an aquifer, potentially moving at a speed similar to that of groundwater, leading to rapid plume expansion in highly permeable media with high hydraulic gradients.
1,4-dioxane is a compound used in a multitude of industrial processes, with special emphasis on its application as an additive to chlorinated solvents, due to its role as a stabilizer of these.
The main sources of contamination of the natural environment in relation to 1,4-dioxane are, in the first place, the industrial sites where the different industrial processes in which this compound is involved are carried out, as a consequence of historical direct discharges into the environment without treatment and accidental leaks and spills occurring during the development of their activity.
Another major source of contamination is the discharge of waste effluents from urban wastewater treatment plants, given the presence of 1,4-dioxane in consumer and industrial products, due to its presence as a by-product in production processes. In this regard, it should be noted that urban wastewater treatments are not efficient in the elimination of 1,4-dioxane since it is a compound of recent consideration, the elimination of which was not contemplated at the time of designing the plant’s treatment equipment. On the other hand, the presence of 1,4-dioxane in some pesticides also makes its presence possible in areas of intensive agriculture.
The treatment of this compound is a challenge in itself due to its particularities. The high solubility in water and, in turn, its low volatilization pose a very specific remediation strategy. As a consequence, some of the conventional remediation methodologies and treatments, such as air stripping or Soil Vapor Extraction (SVE) are ineffective unless the volatilization of 1,4-dioxane is increased. Therefore, when selecting the most appropriate remediation methodology, the subsurface area to be treated must be taken into account.
ONGOING PROJECT
Currently, in response to the problems presented by emerging contaminants, LITOCLEAN is immersed in the remediation process of the subsoil of an industrial plant mainly affected by 1,4-dioxane, in addition to other compounds such as acetone, phenol and, to a lesser extent, TPH.
After the initial subsoil characterization through the execution of a Base report, the remediation process has been developed through a sequence of stages whose execution is critical to obtain a satisfactory result.
Once the problem was detected, which originated in the poor condition of the plant’s industrial network, the first step was the development of several characterization campaigns, through the execution of boreholes and water and soil sampling, to delimit the contaminant plume. This process is key for the future sizing of the remediation system, allowing to establish the Conceptual Model of the project.
As shown in Figure 2, the contaminant, when it reached the subsurface through the leaks from the industrial water collection system, advanced vertically through the unsaturated zone until it reached the water. Once it reached the saturated zone, it moved in the direction of the subsurface flow.
Treatability tests and remediation design: during this stage, pilot treatment and sizing tests were carried out to determine the best methodology for remediation of the subsurface environment. Among these, laboratory tests were carried out (chemical oxidation by sodium persulfate, chemical oxidation by ozone, etc.), as well as on-site pilot treatment and sizing tests (treatment by stripping and ISCO, calculation of pumping and gas collection influence radii, etc.).
Remediation system: after developing the sizing of the remediation system, two phases of treatment of the medium were established. A first phase of action on the water by means of mechanical extraction by a Pump&Treat system and treatment by stripping, proceeding to the advanced chemical oxidation by ozone of the discharge. Also, to act in the unsaturated zone, a Soil Vapour Extraction (SVE) system. Finally, in relation to the particularities of 1,4-dioxane treatment, especially because of its difficulty of remediation in the unsaturated zone due to its low volatilization, once a certain asymptotic evolution was reached during Phase I, a complete action of the subway environment was established by means of a Phase II of ISCO (In situ chemical oxidation).
Remediation progress: at present, after an 18-month performance through Phase I of Pump&Treat and SVE, as well as a subsequent ISCO campaign, the results are excellent, having achieved water impact reductions of more than 90% for all compounds subject to remediation, as well as containment and reduction of the contaminant plume.
Fig.4: 1,4-dioxane isoconcentration evolution
As shown in the evolution graph, as well as in the isoconcentration maps, the entire process of study and development of the environment and its remediation has borne fruit, showing a highly positive evolution, practically reaching the objective values established for the project in question.
