(18e) Phytoremediation of Deicing Salt from Roadside Soils

Rock salt, consisting of primarily sodium chloride (NaCl), is a cheap and effective deicing agent for winter weather events in northern climates. However, most of the salt dissolves in melting snow and ice and eventually finds its way into soil, streams, lakes, wetlands and groundwater. The Minnesota Pollution Control Agency (MPCA) found that 39 of 340 lakes tested for chloride were above the chronic exposure limit that has been shown to be lethal to aquatic life. Sodium accumulates in soil where it can negatively impact soil structure and plant growth. Contaminated soil can be up to 10 meters from the side of the road. Sodium concentrations of roadside soils measured in the Twin Cities Metro Area ranged from around 150 ppm to exceptionally high levels of roughly 8200 ppm. Many forms of soil remediation are costly and disruptive to the soil. Phytoremediation, on the other hand, uses salt tolerant plants to uptake salt into their biomass, which is then harvested. Although phytoremediation can require multiple seasons or harvests to remediate a site, it is an inexpensive and environmentally non-disruptive method. Also, the harvested plant biomass may provide further uses in industrial applications like serving as animal feed or sources of energy.

A species inventory of potential plant species was created to determine the most promising species for salt phytoremediation of roadside soil. Greenhouse experiments were then performed with the highest rated species, including common sunflower (Helianthus annuus), pitseed goosefoot (Chenopodium berlandieri), and big bluestem (Andropogon gerardii). Plants were grown for roughly 7 weeks in 10 cm diameter pots with 75 g of fresh Sungrow potting soil and watered weekly with 100 mL of 0, 50, or 100 mM NaCl saltwater solution for a total of 4 weeks during the experiment. Common sunflower showed the highest total salt uptake into aboveground biomass with 51.4 and 58.5 mg Na⁺ and 162.4 and 185.0 mg Cl^- for the 50 and 100 mM NaCl treatments respectively. Pitseed goosefoot accumulated 19.6 and 29.4 mg Na⁺ and 89.9 and 101.3 mg Cl^- while big bluestem showed the lowest salt uptake with only 2.92 and 1.65 mg Na⁺ and 23.6 and 9.36 mg Cl^- for the 50 and 100 mM treatments. According to the amount of salt added in the 50 mM treatment, common sunflower removed 17.8% and pitseed goosefoot removed 9.15% of the total salt from the soil. Therefore, common sunflower could theoretically remove most of the salt within 5 to 6 harvests and it would take around 10 to 11 harvests for pitseed goosefoot. These results show that common sunflower and pitseed goosefoot may be great candidates for salt remediation on salt contaminated roadsides and should be further explored with field studies on roadsides contaminated with deicing salt.

Keywords: phytoremediation, road salt, deicing salt