Soil and Mineral Nanopores and Their Role in Contaminant Fate and Transport

There is often a fraction of soil or groundwater contaminant that resists cleanup or transformation. Even after decades of treatment or natural attenuation, this recalcitrant fraction may persist. The contaminant may resist breakdown because it is stored in nanopores – microscopic cavities in soil and rock particles that are created by weathering or cracking (Figure 1). Although these pores are less than ten nanometers across, the tiny voids are large enough to store molecules of contaminants and isolate them from potential reactants. WRHSRC researcher Martin Reinhard and his research team at Stanford University are studying nanopores. Their focus is on halogenated hydrocarbons and understanding how nanopores influence their residence times in soils and aquifers.

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Experimental Design

Reinhard and his graduate student, Hefa Cheng, are carrying out their experiments with an innovative setup that combines a soil column with a gas chromatograph (Figure 2). They introduce a known quantity of test material that will react in the column or be sorbed into soil nanopores. After the column has equilibrated, they purge it with helium. The purged material is fed directly into the gas chromatograph where they measure concentrations of parent and daughter compounds. Contaminant sorption and desorption are studied as a function of temperature, humidity, and competitive cosorbates or cosolvents. The slowly desorbing fraction of test material is assumed to be sorbed in soil nanopores. The procedure can be calibrated using sorbents with known porosity (silica gel) and sorbates with known reaction rates. Reinhard and Cheng have carried out two sets of experiments. One set uses trichloroethylene (TCE), a compound which adsorbes but does not react with water, and the second is 2,2-dichloropropene (DCP), a compound which reacts with water.

The TCE experiments investigate whether TCE molecules can be forced out of soil nanopores by adding other compounds that compete for the sorption sites. They added water vapor, methanol vapor or methane to the purging stream, and measured whether these compounds caused faster TCE release. They found that adding water increased the desorption flux by 1.5 times, and adding methanol increased the desorption flux by 8.2 times. They hypothesize that this difference indicates the presence of two types of soil nanopores – ones that are hydrophilic and ones that are hydrophobic. Methanol increases the desorption flux more than water, because it is able to displace TCE from both types of pores, whereas water is repelled from the hydrophobic pores and cannot displace the TCE sorbed in them. The team is collecting more data to enhance their interpretation of these results.

The team’s set of experiments with DCP was designed to investigate how nanopores affect the ability of halogenated hydrocarbons to hydrolyze or react with water. They worked with DCP because it hydrolyzes quickly – if a sample is added to water at 50 degrees C, nearly all will transform to its daughter product, 2-chloropropene, within 10 hours. In their experiments, Cheng and Reinhard added DCP to soil columns under a variety of moisture and temperature conditions. The found that even under fully wet, heated conditions, a significant proportion of DCP did not transform. They believe that the unhydrolyzed DCP was sequestered in hydrophobic nanopores (Figure 3).

Reinhard and Cheng’s work shows that microscopic cavities in soil and rock can isolate and sequester contaminants and prevent their break down. They hypothesize that the properties of the pores is important – hydrophobic pores can preserve contaminants that would normally degrade in the wet conditions of soils and aquifers. The team’s future work will substantiate and quantify their findings with column experiments using a wider range of materials and conditions.