Teresa J. Cutright

EDTA and HEDTA effects on Cd, Cr, and Ni uptake by Helianthus annuus

Phytoremediation has shown great potential as an alternative treatment for the remediation of heavy-metal-contaminated soils and groundwater. However, the lack of a clear understanding pertaining to metal uptake/translocation mechanisms, enhancement amendments, and external effects on phytoremediation has hindered its full-scale application. The objective of this research was to investigate the ability of synthetic chelators for enhancing the phytoremediation of cadmium-, chromium- and nickel-contaminated soil. Ethylenediaminetriacetic acid (EDTA) and N-(2-hydroxyethyl)-ethylenediaminetriacetic acid (HEDTA) were applied to the soil at various dosages to elevate metal mobility. Uptake into and translocation within Helianthus annuus was determined. It was found that EDTA at a rate of 0.5 g/kg significantly increased the shoot concentrations of Cd and Ni from 34 and 15 to 115 and 117 mg/kg, respectively. The total removal efficiency for EDTA was 59 microg/plant. HEDTA at the same application rate resulted in a total metal uptake of 42 microg/plant. These research demonstrated that chelator enhancement is plant- and metal-specific and is subjective to inhibition when multiple heavy metals are present. Results also showed that chelator toxicity reduced the plants biomass, thereby decreasing the amount of metal accumulation.

The investigation of enhanced bioremediation through the addition of macro and micro nutrients in a PAH contaminated soil

Respirometer studies were conducted to determine the effect of adding macro and/or micronutrients for enhancing the bioremediation of PAH and/or petroleum contaminated soils. For the biostimulation study, three of the nutrient solutions tested with this particular soil-contaminant combination were effective at enhancing the oxygen uptake. The optimal oxygen consumption resulted when a low level of macronutrients and a high level of micronutrients were used with phosphorus as the dominant macronutrient. Conversely, only one nutrient solution was effective for the bioaugmentation studies. In this instance, the bioactivity of the foreign consortium was the greatest when a high level of micronutrients was used.

Biodegradability of aged pyrene and phenanthrene in a natural soil

A study was conducted to evaluate the biodegradability of pyrene (PYR) and phenanthrene (PHE) aged in a natural soil. Both the single and binary systems were either biostimulated via a nutrient amendment or bioaugmented via an inoculation of the enriched bacteria and nutrients. Aging resulted in higher concentration of both compounds and smaller bacterial activity in the solution-phase. Surprisingly, the total biodegraded extent was greater in the aged soil system than in the freshly spiked system. As anticipated, biostimulation was not appropriate to attain an effective biodegradation in this study, and bioaugmentation achieved a substantial increase the total biodegradation extent. The above findings were attributed to indigenous Pseudomonas aeruginosa entering a stationary-phase during the 200-day aging and producing rhamnolipid biosurfactants. In addition, a different sampling technique (i.e., after vigorous hand-shaking) revealed a 15 times higher microbial population than the normal sampling from the stagnant solution. Therefore, PAH bioavailability in the aged soils can be underestimated when the microbial activity is determined only from the stagnant solution. Furthermore, cometabolism enhanced PYR degradation when PHE was present as a primary substrate.

Pyrene biodegradatin in aqueous solutions and soil slurries by Mycobacterium PYR-1 and enriched consortium

To better understand complex bioavailability issues, pyrene degradation was examined in aqueous and soil slurry solutions using pure Mycobacterium sp. PYR-1 and a microbial consortium. The intrinsic rates of the aqueous pyrene degradation were very similar, 1.3 x 10(-9) microg pyrene/CFU-h for Mycobacterium sp. PYR-1 and 1.1 x 10(-9) microg pyrene/CFU-h for the consortium. Rates were much lower with the soil-slurry experiments, ranging from 1.2 x 10(-12) to 7.8 x 10(-10) microg/CFU-h, depicting the strong negative effects of soils on bioavailability. Supernatants from the slurry experiments were found to increase the aqueous-phase pyrene solubility significantly. Pyrene solubility was increased from 120.5 to over 230 microg/l. However, the linear adsorption constants of pyrene on the soil were reduced.

Preliminary exploration of the relationships between soil characteristics and PAH desorption and biodegradation.

Desorption and biodegradation of pyrene (PYR) were investigated and their relationships to soil characteristics were addressed. The results indicated that maximum achievable desorption was 30.2, 10.4, and 1.0 mg/kg for soils that had 1.7, 2.2, and 4.4 wt.% of expandable clays (smectite and vermiculite), respectively. Neither dissolved organic matter (DOM) nor total clay amounts made a good prediction of the desorption trend. Subsequently, the ease of desorption facilitated a faster aqueous biodegradation rate. The slowest aqueous biodegradation rate, 0.02 l/h, was achieved for the soil system that had the greatest amount of expandable clays, whereas the soil containing 1.7% expandable clays only achieved 0.73 l/h. The soil with 2.2% expandable clays depicted 0.41 l/h of aqueous biodegradation rate. A good linear correlation was obtained between maximum achievable desorption and aqueous biodegradation rate (R(2)=0.92). Soil analysis revealed that the total (soil+water) biodegradation reached was 65%, 78.3%, and 81.8% of the initial concentration (100 mg/kg) for the sandy clay loam (Colombian), sandy loam (Ohio), and silty loam (New Mexico) soils, respectively. This biodegradation extent was also in good agreement of expandable clay amount. Although aqueous PYR bioavailability was limited due to the strong association with the expandable clays, microbial movement and adhesion to those clays seemed to result in a great extent of the soil-phase biodegradation.

Polycyclic aromatic hydrocarbon biodegradation and kinetics using Cunninghamella echinulata var. elegans

Bioremediation is a process technology that uses microorganisms to degrade specific organic chemicals. In recent years, bioremediation has proven to be successful for the remediation of soils contaminated by polycyclic aromatic hydrocarbons (PAHs). Even though bioremediation has had a high success rate, the associated kinetics are still not fully understood. The kinetics become even more complicated when fungi are used for the remediation. The primary objective of this research was to determine the specific degradation rates for the bioremediation of PAH contaminated soils. Specifically, the kinetics associated with the fungi Cunninghamella echinulata var. elegans in conjunction with three supplemental nutrient solutions were investigated.

Simultaneous Hyperaccumulation of Multiple Heavy Metals by Helianthus Annuus Grown in a Contaminated Sandy-Loam Soil

Phytoremediation is a promising means for the treatment of contamination arising from heavy metal spills. Although several species have been identified as hyperaccumulators, most of the studies were performed with only one heavy metal. Experiments were conducted with two cultivars of H. annuus exposed to different combinations of metal contamination (30 mg/kg Cd, Cr, Ni, As, and/or Fe). Cultivar efficiency was based on total metal uptake, as well as translocation and selectivity of each metal. The results for each cultivar were also compared after 0.1 g/kg or 0.3 g/kg EDTA was added to enhance metal bioavailability. The key finding was that H. annuus achieved hyperaccumulator status for multiple metals simultaneously: Cd, Cr, and As.

Impact of Clay Minerals and DOM on the Competitive Sorption/Desorption of PAHs

Two model compounds were used to investigate sorptive phenomena of a silty-sand soil under single and binary solute systems at different concentrations. In the sorption isotherms, the presence of phenan-threne (PHE) exhibited a statistically significant (P<0.05) sorption competition over pyrene (PYR), regardless of the concentration. PYR influenced the PHE isotherms only when it was present at 15 mg/L. The concentration dependence in sorption competition was only evident for the more hy-drophobic PYR. In the presence of PHE, the fraction of desorbed PYR was significantly increased with an increase in PYR concentration. PHE desorption enhancement was the most observable with the higher initial concentration (15 mg/L). However, the presence of PYR did not affect PHE desorption. This study found that, based on equivalent solid mass, soils containing only clay minerals sorbed 12.2% more PHE than soils with only soil organic matter (SOM) for 3 mg/L PHE. Clay minerals also impacted desorption as evidenced by a 65% decrease in desorbed PYR fraction compared with when the soil only contained SOM. The dissolved organic matter (DOM) amendment did not increase desorption. Instead, PHE desorption was significantly inhibited by the added DOM. For this study, co-sorption was found to be the mechanism for the inhibited desorption.

Nutrient Amendment for the Bioremediation of a Chromium-Contaminated Soil by Electrokinetics

This article presents the results of a preliminary laboratory investigation wherein electrokinetics was used for the delivery of nutrients to metal-reducing micro-organisms in a low permeability clayey soil. In particular, the micro-organisms were used to reduce a toxic and mobile heavy metal (hexavalent chromium or Cr(VI)) to a less toxic and immobile form (trivalent chromium or Cr(III)). Three bench-scale electrokinetic experiments were conducted using kaolin as a model low permeability clayey soil, and the kaolin was artificially contaminated with Cr(VI) at an initial target concentration of 1,000 mg/kg. All the experiments were conducted with a constant electrical voltage gradient of 1.0 V/cm and included a control test without micro-organisms or nutrients, a test with micro-organisms but without nutrients, and a test with micro-organisms and supplemental nutrients, specifically acetate, phosphate, and ammonium. The results showed that acetate and phosphate amendment by electrokinetics was effective because both nutrients electromigrated into the soil. Moreover, the results indicate that employing the micro-organism cultures improved Cr(VI) reduction. These results suggest that nutrient amendment by electrokinetics for the bioremediation of heavy metals has great potential; however, the microbial strains responsible for Cr(VI) reduction must be identified so the electrokinetic system can be engineered to provide the optimal nutrient, pH, and environmental conditions for these strains.

The Role of Soil Properties in Pyrene Sorption and Desorption

Soil type will greatly affect the sorption and subsequent desorptionof hydrophobic contaminants. To gain a better understanding of theimpact of soil type on sorptive behavior, the sorption-desorption of pyrene (PYR) with three different soils was studied. The first soil originated from Colombia and is classified as silty sand with3.54% soil organic matter (SOM) and 18% clay materials (<2 microns). The New Mexico soil is a sandy lean clay comprisedof 8.4% SOM and 10% clay. The last soil originated fromOhio and is a silty sand with 1.84% SOM and 9.6% clay. Based on soil mineralogy and sorption-desorption isotherms,the Colombia soil had the greatest binding potential followedby the New Mexico and Ohio soils. The Freundlich model couldfit both the Colombia and New Mexico soils. For the Ohiosoil, a two-stage Freundlich model was required. For allthree soils, PYR desorption was slow and resistant, anddepicted an apparent hysteresis. The extent of sorption-desorption for each soil was attributed to its individual classification.For instance, the SOM present in the New Mexico soil (8.4%) enabled a relatively easy desorption in comparison to the other two soils. For the Ohio and Colombia soils, the interaction with the clay fractions rendered a stronger sorptive bond.