Ellen L. Arthur

Phytoremediation—An Overview

The use of plants (directly or indirectly) to remediate contaminated soil or water is known as phytoremediation. This technology has emerged as a more cost effective, noninvasive, and publicly acceptable way to address the removal of environmental contaminants. Plants can be used to accumulate inorganic and organic contaminants, metabolize organic contaminants, and encourage microbial degradation of organic contaminants in the root zone. Widespread utilization of phytoremediation can be limited by the small habitat range or size of plants expressing remediation potential, and insufficient abilities of native plants to tolerate, detoxify, and accumulate contaminants. A better understanding and appreciation of the potential mechanisms for removing contaminants from the root zone and the interaction between plants, microorganisms, and contaminants will be useful in extending the application of phytoremediation to additional contaminated sites.

Degradation of an atrazine and metolachlor herbicide mixture in pesticide-contaminated soils from two agrochemical dealerships in Iowa.

The fate of atrazine and metolachlor,applied as a mixture, in soil taken from twopesticide-contaminated sites in Iowa (denoted as Alphaor Bravo) were determined in laboratory studies. Atrazine and metolachlor degradation, as well asatrazine mineralization, were greater in soilcollected from Kochia scoparia L. (Schrader)rhizosphere than in soils from unvegetated areas. Theradiolabeled 14C-carbinol and14C-morpholinone metabolites were identified in14C-metolachlor-applied soil 60 d aftertreatment. The half-life for atrazine in Alpha soilwas significantly less in the rhizosphere soil (50 d)than in unvegetated soil (193 d). Quantities ofspecific atrazine degraders were one to two orders ofmagnitude greater in Bravo soils than in Alpha soils. In an experiment with plants present, significantlymore 14C-atrazine was taken up by K.scoparia (9.9% of the applied 14C) than by Brassica napus L. Significantly less atrazine wasextractable from soils vegetated with K.scoparia than from soils vegetated with B.napus or unvegetated soils.

Degradation of atrazine, metolachlor, and pendimethalin in pesticide-contaminated soils : Effects of aged residues on soil respiration and plant survival

Abstract This study was conducted to determine the effects of pesticide mixtures on degradation patterns of parent compounds as well as effects on soil microbial respiration. Bioavailability of residues to sensitive plant species was also determined. Soil for this study was obtained from a pesticide‐contaminated area within an agrochemical dealer site. Degradation patterns were not affected by the presence or absence of other herbicides in this study. Atrazine concentrations were significantly lower at 21 through 160 days aging time compared to day 0 concentrations. Metolachlor and pendimethalin concentrations were not significantly different over time and remained high throughout the study. Microbial respiration was suppressed in treated soils from day 21 to day 160. Soybean and canola were the most successful plant species in the germination and survival tests. Generally, with increased aging of pesticides in soil, germination time decreased. Survival time of plants increased over time for some treatments indicating possible decreased bioavailability of pesticide residues. In some cases, survival time decreased at the longer 160‐day aging period, possibly indicating a change in bioavailability, perhaps as the result of formation of more bioavailable and phytotoxic metabolites. No interactive effects were noted for mixtures of pesticides compared to individually applied pesticides in terms of degradation of the parent compound or on seed germination, plant survival, or microbial respiration.

Enhanced degradation of deethylatrazine in an atrazine-history soil of Iowa.

The degradation of deethylatrazine (DEA), a major metabolite of atrazine, was studied by using radiotracers in soils with two different atrazine histories. DEA degradation was enhanced in soils which had received long-term exposure to atrazine (atrazine-history soil) compared with soils that had not received long-term atrazine exposure (no-history soil). After 60 days of incubation, mineralization of DEA to 14CO2 in the atrazine-history surface soil was twice that in the no-history surface soils, with 34% and 17% of the applied 14C-DEA as CO2, respectively. In surface soils, 25% of the applied 14C remained as DEA in the atrazine-history soil, compared with 35% in the no-history soil. Microbial plate counts indicated an increase in numbers of bacteria and fungi in soils incubated with DEA compared to control soils. No significant difference in total microbial respiration was seen among atrazine-history and no-history soils incubated with DEA, but DEA-treated soils had greater microbial respiration than untreated control soils after 6 days. A 14C-most-probable-number procedure was used to enumerate specific DEA degraders. A greater number of DEA degraders were indicated in atrazine-history subsurface soil compared with all other soils in this study (p < 0.05). From this study, it appears that an increase in microbial activity contributes to decreased persistence and increased degradation of DEA in soils that have had long-term exposure to atrazine at field application rates, compared to soils with no long-term exposure. Decreased persistence of this major metabolite of atrazine in atrazine-history soils is important in that there will be less available for movement in surface runoffs.

Evaluation of microbial inoculation and vegetation to enhance the dissipation of atrazine and metolachlor in soil

Four greenhouse studies were conducted to evaluate the effects of native prairie grasses and two pesticide-degrading bacteria to remediate atrazine and metolachlor in soils from agricultural dealerships (Alpha site soil, northwest Iowa, USA; Bravo site soil, central Iowa, USA). The Alpha soil contained a low population of atrazine-degrading microorganisms relative to the Bravo soil. Each soil freshly treated with atrazine or metolachlor was aged for a short or long period of time, respectively. An atrazine-degrading bacterium, Agrobacterium radiobacter strain J14a; a metolachlor-degrading bacterium, Pseudomonas fluorescens strain UA5-40; and a mixture of three native prairie grasses-big bluestem (Andropogon gerardii Vitman), yellow Indian grass (Sorghastrum nutans [L.] Nash), and switchgrass (Panicum virgatum L.)-were added to the soils after the soils were aged for long periods of time. The soils aged for short periods of time were treated with J14a, the prairie grasses, or both after aging. The J14a and the grasses significantly reduced the concentration of atrazine in Alpha soil when the soil was aged for a short period of time. However, these treatments had no statistically significant effect when the soil was aged for a long period of time or on atrazine in Bravo soil. Inoculation with UA5-40 did not enhance metolachlor dissipation in either soil, but vegetation did increase metolachlor dissipation. Our results indicate that the dissipation of atrazine by J14a is affected by the presence of indigenous atrazine-mineralizing microorganisms and probably by the bioavailability of atrazine in the soil.

Sorption of Two New Sulfonylaminocarbonyltriazolinone Herbicides and their Metabolites on Organic and Inorganic Exchanged Smectites

The sorption capacity and possible mechanisms of sorption of two new sulfonylaminocarbonyltriazolinone herbicides, methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1 H -1,2,4-triazol-1-yl)carbonyl]amino]sulfonyl]benzoate (MKH 6561) and (4,5-dihydro-3-methoxy-4-methyl-5-oxo- N -[[2-(trifluoromethoxy)phenyl]sulfonyl]-1 H -1,2,4-triazole-1-carboxamide) (MKH 6562), and their potential metabolites, 2,4-dihydro-4-methyl-5-propoxy-3 H -1,2,4-triazol-3-one (triazole) and 2-trifluoromethoxybenzenesulfonamide (phenylsulfonamide) on octadecyl (C18) and dioctadecyldimethylammonium (DOD) saturated and Fe 3+ and Na + saturated smectites has been investigated. Sorption of MKH 6561 on inorganic clays and C18 organoclays was much higher than for MKH 6562, but was similar on DOD saturated clays. For C18 saturated clays, sorption of both parent compounds increased with decreasing layer charge of the organosmectite. Sorption of triazole metabolite, which was much lower than for its parent compound MKH 6561, was higher on inorganic and C18 saturated clays than on DOD saturated clays or inorganic clays. Phenylsulfonamide metabolic sorption, which was also lower than the corresponding parent compound MKH 6562, was higher on DOD saturated clays than on C18 or inorganic saturated clays. These differences in sorption behaviour are related to the diverse relative contribution of hydrophobic and polar interactions for the various compound-clay systems.