(592f) Double the Pleasures: Perennial Grasses As Soil Phytoremediation Agents and Renewable Carbon Resources for Fuels and Chemicals

Biomass is the only renewable source of carbon that, when utilized efficiently and optimally, can significantly help reducing our reliance on fossil-based carbon resources. Lignocellulosic biomass is abundant, inexpensive and does not compete with the production of food crops. Nevertheless, the complex nature of biomass and many ill-defined issues related to biomass utilization pose a substantial challenge to large-scale biomass utilization. Growing biomass for energy crops means that more land needs to be utilized. It is desired that biomass for energy crops would be grown in land areas that are considered marginal and not currently being utilized for anything productive.

Certain types of fast-growing perennial grasses, such as switchgrass, have great potential to be used as renewable feedstock to produce bioenergy due to their relatively high yields of biomass per acreage of land. These grasses can grow well in land areas which are otherwise not suitable for conventional farming. They also can resist a wide range climate temperature, thus can grow in many parts the world including in cold regions. The fast-growing characteristic, along with the ability to grow in marginal land, make these perennial grasses highly potential sources for renewable carbon.

Another unique characteristic of switchgrass is that the plant spreads by short rhizomes and its roots can go deep more than three meters below the surface. The rhizomes and deep roots make switchgrass a potential plant to be grown in soils that are contaminated with heavy metals. As a phytoremediation agent, switchgrass can extract these heavy metals from the contaminated soil, which in turn can improve the quality of the soil. This method is potentially much less expensive than the conventional soil remediation method, such as ex-situ excavation method, which is a highly energy intensive process.

In this study, the potentials of switchgrass as soil remediation agent and as feedstock for a biorefinery system were evaluated. A pot study of growing switchgrass in soils that were doped with lead at various concentration demonstrated that the grass could grow in the contaminated soils and extract the heavy metal. Germination and growth rates of the grass were not affected when the lead concentration in the soil was between 0 – 200 ppm. Based on data, it was estimated that it will only require 3.5 years to reduce the lead concentration in the soil from 500 mg Pb/Kg of soil to 10 mg Pb/Kg of soil. The lead-contaminated switchgrass plants were harvested, dried, and comminuted into millimeter-size particles to be used as a feedstock for a biorefinery process system. Four process routes consisted of enzymatic hydrolysis, acid hydrolysis, fast pyrolysis and their combinations were evaluated to produce sugars, bio-oil, and biochar.

The study showed that the presence of lead in the grassy feedstock did not seem to affect the yields sugars and bio-oil from the hydrolysis and the fast pyrolysis steps, respectively. An LCA study on growing switchgrass for soil phytoremediation and using the switchgrass as feedstock for the biorefinery system was performed. Results from the study showed that the combination of using switchgrass as soil phytoremediation agent and feedstock for producing energy in a biorefinery decreases GHG emissions and lowers the use of energy produced from fossil fuels when compared with a combination of a refinery system and ex-situ excavation for soil remediation.