Heavy metal contamination of surface soil has posed a serious threat to plants, animals, and humans. Conventional technologies, which are energy and cost intensive, are not economically feasible for this problem. Phytoremediation, the use of vegetation for the remediation of contaminated sediments, soils, and ground water, is an emerging technology for treating several categories of persistent, toxic contaminants. However, the lack of clear understanding of metal uptake/translocation mechanisms as well as external effects on phytoremediation has hindered its development. The shortage of effective enhancement strategies has also limited its success in full-scale application. Current research has investigated the effectiveness of phytoremediation of contaminants by hydroponics (water based) on soil-based plants.
This research focused on the ability of plants to hydroponically treat water contaminated with cadmium, chromium, nickel, radionuclides (cesium and strontium), and/or phthalates. Specifically, the effectiveness of dwarf sunflowers (Helianthus annuus) and two strains of mustard seed (Brassica juncea) were investigated. The selectivity, on a mass basis, for the sunflowers and Indian mustard was Cd > Cr > Ni and Cr > Ni > Cd, respectively. This selectivity order did not change when cesium was present. Nickel was the metal least affected by inhibition.
When phthalates were the sole contaminant, the selectivity was DBP > BBP > DOP. This accumulation occurred even though phthalates are hydrophobic. For each compound, the major accumulation in the plant tissue was in the leaves with very little in the root mass. However, when metals and phthalates were both present, the majority of the contaminants were located in the roots. Thus indicating that mass transfer through the root tissues is the rate limiting step. The greatest inhibition was experienced with the metal-phthalate solution. Metal uptake was diminished and translocation from the roots to other tissues was severely limited. No detectable phthalate uptake occurred.
The experiments showed that the sunflowers have a better uptake rate and higher concentration of contaminants in the plant tissue than the mustard. This suggests that the sunflowers would be effective at remediating water contaminated with heavy metals and radionuclides. The process feasibility is evident by the fact that the contaminants used in this study are not indicative of normal tap water or plant matter (i.e., chromium and cesium). Therefore, the presence of the contaminants in the plant tissue is a direct result of accumulation via uptake from the contaminated water through the roots. As anticipated, the rate limiting step for uptake and translocation was the transfer of the contaminant through the root center across the endodermis.
As with all treatment technologies, the concentration present at a site will impact the final remediation efficiency of the selected process. Only the minimum effective concentration was determined by studying the effects of initial concentration. Both the Indian mustard and dwarf sunflowers were ineffective at treating solutions that had a contaminant concentration below 0.7 ppm. However, maximum concentration that could be tolerated was not determined by this study. The sunflowers were able to successfully uptake over 100 ppm of phthalates and 15 ppm of heavy metals during a four day study. The concentration that would restrict plant growth and normal cellular functions was not determined. Furthermore, plant exhaustion was not ascertained.
This phase of the research investigated the efficiency of phytoremediation for remediating a soil contaminated with Cd2+, Cr3+, and Ni2+. A special emphasis was placed on evaluating the effects of multiple heavy metal interactions, co-existing organic contaminant, and soil properties on the metal removal efficiency. This research also studied the effectiveness of synthetic chelator addition, fertilizer amendment, and bacterial inoculation on facilitating phytoremediation of heavy metal contamination.
It was determined that the presence of multiple heavy metals in soil significantly affected phytoremediation in that plants exhibited an uptake selectivity over a specific metal. In this study, Helianthus annuus showed a selectivity of Cd (II) > Ni (II) >> Cr (III). It was also determined that synthetic chelators such as HEDTA and EDTA could enhance Cd and Ni shoot accumulation by more than 2- and 4-fold, respectively. For example, when EDTA was added the shoot concentrations of Cd and Ni were significantly increased from 34.21 mg kg-1 and 14.48 mg kg-1 to 115.11 mg kg-1 and 117.10 mg kg-1, respectively. However, the total biomass of plants was drastically decreased by 56.53%, a negative effect on plant biomass yield that, was too significant to be ignored.
The combination of EDTA amendment and plant inoculation with rhizobacteria was found to produce the best results on Cd, Cr, and Ni removal with a total removal rate of 610 (g kg-1 soil. As a result, synthetic chelators could be used in conjunction with rhizobacteria as a new strategy in phytoremediation applications. In addition, the amendment of commercial fertilizer was also demonstrated to have a significantly positive effect on metal removal in phytoremediation.
The presence of organic compounds not only depressed the plant growth, but also decreased the metal uptake rate. When metal contaminated soil was spiked with DEHP, the root tissue biomass was significantly decreased by more than 40% as compared with the plants grown in non-DEHP spiked soil. The addition of DEHP also slightly decreased the shoot biomass, although no significant reduction on shoot yield could be observed. The Cd, Cr, and Ni accumulation by Helianthus annuus was dramatically depressed in both shoot and root tissues as a result of DEHP addition. For instance, in the absence of DEHP, the root concentration of Cr was more than 7 mg/kg. Once more than 50 mg/kg of DEHP was added to the soil, no Cr root uptake could be detected.
Soil type will greatly affect the sorption and subsequent desorption of contaminants. To gain a better understanding of the impact of soil type on sorptive behaviour, the sorption-desorption of pyrene with three different soils was studied. The first soil originated from Colombia and is classified as silty sand with 18% clay materials. The New Mexico soil is a sandy-lean clay comprised of 10% clay. The last soil originated from Ohio and is a sandy loam with 11% clay material. Based on soil mineralogy and sorption-desorption isotherms, the Colombia soil had the greatest binding potential followed by the New Mexico and Ohio soils. The Freundlich model could fit both the Colombia and New Mexico soils. In case of the Ohio soil, a two-stage Freundlich model was required. For all three soils, desorption was slow, resistant, and depicted an apparent hysteresis. The extent of sorption-desorption for each soil was attributed to its individual classification. For instance, the soil organic matter present in the New Mexico soil enabled a relatively easy desorption of pyrene in comparison to the other two soils. For the Ohio and Colombia soils, the interaction with the mineral content of the clay fractions rendered a tighter sorptive bond.
Sorptive interactions between PAHs and soil matrix will directly affect the treatment efficiency and are a function of physical-chemical factors and time. Literature results based on =3 ring PAHs report achieving equilibrium after one to two days. This will not be the case for higher ringed PAHs. Batch experiments with pyrene was conducted to quantify the effect of contact time on its sorption and desorption behavior in sandy loam Ohio soil. True sorption equilibrium was never obtained. Initial results depicted pseudo-equilibrium after 48-hr contact time. Therefore a 24- and 48-hr, contact time were chosen for non-equilibrium studies, and 240 hours for pseudo-equilibrium study. The fluctuation, non-linearity and hysteresis of pyrene sorption behavior were attributed to both internal and external interactions with SOM and the expandable clay minerals. The extent and dominance of their respective roles were time-dependent. The non-linear, pseudo-equilibrium sorption isotherm was fit to a two-stage Freundlich model (3,000 - 7,000 ppb and 7,000 - 15,000 ppb). As expected, desorption required a longer time when the sorption contact time was increased. Care should be taken when investigating sorption/desorption of high molecular weight PAHs, such as pyrene, in subsoil environments since the contact time will greatly affect the sorptive behavior.
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| Figure 1. Proposed Research Plan |