Enhanced Degradation of TCE on a Superfund Site Using Endophyte-Assisted Poplar Tree Phytoremediation

Abstract

Trichloroethylene (TCE) is a widespread environmental pollutant common in groundwater plumes associated with industrial manufacturing areas. We had previously isolated and characterized a natural bacterial endophyte, Enterobacter sp. strain PDN3, of poplar trees, that rapidly metabolizes TCE, releasing chloride ion. We now report findings from a successful three-year field trial of endophyte-assisted phytoremediation on the Middlefield-Ellis-Whisman Superfund Study Area TCE plume in the Silicon Valley of California. The inoculated poplar trees exhibited increased growth and reduced TCE phytotoxic effects with a 32% increase in trunk diameter compared to mock-inoculated control poplar trees. The inoculated trees excreted 50% more chloride ion into the rhizosphere, indicative of increased TCE metabolism in planta. Data from tree core analysis of the tree tissues provided further supporting evidence of the enhanced rate of degradation of the chlorinated solvents in the inoculated trees. Test well groundwater analyses demonstrated a marked decrease in concentration of TCE and its derivatives from the tree-associated groundwater plume. The concentration of TCE decreased from 300 μg/L upstream of the planted area to less than 5 μg/L downstream of the planted area. TCE derivatives were similarly removed with cis-1,2-dichloroethene decreasing from 160 μg/L to less than 5 μg/L and trans-1,2-dichloroethene decreasing from 3.1 μg/L to less than 0.5 μg/L downstream of the planted trees. 1,1-dichloroethene and vinyl chloride both decreased from 6.8 and 0.77 μg/L, respectively, to below the reporting limit of 0.5 μg/L providing strong evidence of the ability of the endophytic inoculated trees to effectively remove TCE from affected groundwater. The combination of native pollutant-degrading endophytic bacteria and fast-growing poplar tree systems offers a readily deployable, cost-effective approach for the degradation of TCE, and may help mitigate potential transfer up the food chain, volatilization to the atmosphere, as well as direct phytotoxic impacts to plants used in this type of phytoremediation.

Bacterial Endophyte Colonization and Distribution within Plants

“…Because of their rapid and hardy growth, they are considered important plants for biomass production in the bioenergy industry. In addition, they provide various environmental services including conservation of soil, water, and biodiversity, and help to clean the contaminated sites by toxic chemicals [ 135 , 167 , 168 ]. Many endophyte strains are known to colonize and reside in poplar trees.…”

Application of Floating Aquatic Plants in Phytoremediation of Heavy Metals Polluted Water: A Review

“…The negative impacts of chelators can be minimized by the application of a proper amount of the chelators, cautious application, and proper understanding of the water seepage mechanism [84]. The organic acids have advantages over synthetic chelators being economical and easily biodegradable and environment-friendly [85,86].…”

A review of the performance of woody and herbaceous ornamental plants for phytoremediation in urban areas

“…The inoculated trees showed an increased growth and a reduced toxic effect compared to control, excreting 50% more chloride ions into the rhizosphere, a good signal of an increased TCE metabolism in planta. A significant decrease in the concentration of TCE and its derivatives from the tree-associated groundwater plume was also detected (Doty et al 2017). With hybrid poplar clones associated to Arthrobacter strains, the possibility to rhizoremediate soils contaminated with the insecticide exachlorocyclohexane (HCH) isomers was demonstrated, stressing the importance of in situ pre-selection of the best candidate plants and bacteria strains (Bianconi et al 2011).…”