For instance, the half-lives of triclosan and estrone increased from 5.9 d to 8.9 d and from 0.6 d to 1.1 d, respectively, in soils previously exposed to WWTP effluent as compared to unexposed soils . Extensive microbial transformation results in the mineralization of PPCP/EDCs in soil to CO2 and hence complete decontamination. Mineralization is exclusively mediated by microbial transformations . For example, in 3 soils, 14C-estrone showed 15 – 85% mineralization after 100 d of incubation . About 15% of 14C-bisphenol A was mineralized after aerobic incubation in 4 soils for 120 d , while only 0.49 – 0.58%of sarafloxacin was mineralized after 80 d of aerobic incubation in 3 soils . After 27 d, 50% of 14C-naproxen was mineralized . This variability shows that mineralization is compound and soil specific, similar to other microbial transformation processes. However, at present there is a general scarcity of information, making it difficult to predict the relative impact of mineralization in the overall fate and risk of PPCP/EDCs in the soil-plant-human continuum. Microbial transformations may produce many intermediate products before the compound is fully mineralized or bound in soil. The formation of transformation intermediates in soil poses unknown risks as the new products may have biological activity . Due to analytical challenges in identifying unknown products in environmental matrices, very little information on transformation intermediates is available for PPCP/EDCs . A study showed that diclofenac was transformed to 5-hydroxydiclofenac and its p-benzoquinone imine in a bioreactor with river sediment, though the levels were not quantified . While the p-benzoquinone imine was formed transiently and in small quantities, it is the known to have high hepatoxic potential . In a separate study using an activated sludge bioreactor, 7 transformation products of diclofenac were found but none were identified . Ibuprofen formed hydroxyibuprofen in a pilot sewage plant and carboxyibuprofen in an oxic biofilm reactor . Overall, knowledge of PPCP/EDC transformation intermediates in the soil is extremely limited and warrants further investigation. The application of treated wastewater, bio-solids, or manure to land creates a potential for plants to take up PPCP/EDCs ,vertical hydroponics which may be beneficial in areas of phytoremediation, but in agricultural areas may contaminate food crops and create a possible route of human exposure through ingestion .
The few studies that have examined PPCP/EDC uptake by plants have reported accumulation by a variety of edible and non-edible plants, with accumulation varying among compounds, plant species, plant tissues, exposure concentrations, and exposure durations . While potential for plant uptake has been shown in laboratory settings, many of these experiments used artificially high concentrations that are not representative of environmental levels of PPCP/EDCs. The extent of plant accumulation in the environment has been scarcely studied. Calderón-Preciado et al. analyzed alfalfa and apple trees irrigated with water impacted by WWTP effluentand identified PPCP/EDCs in plant tissues at ng/kg – µg/kg levels, verifying that PPCP/EDCs are susceptible to plant accumulation under realistic agronomic conditions. Due to the extensive suite of PPCP/EDCs, it is not feasible to empirically measure plant uptake of each compound. Therefore, it is crucial to develop a mechanistic understanding of their accumulation to inform risk assessment. Many factors affect plant uptake of organic compounds, including compound hydrophobicity, ionization behavior, soil pH, soil organic matter, and plant transpiration . Uptake is generally a passive process, occurring by diffusion that is driven by water potential gradients . Due to transpiration driving the translocation of water through the plant, compounds which are neutral, polar, persistent, and non-volatile have the potential to concentrate in plants up to 100 times the concentration in soil . Most PPCP/EDCs are non-volatile , making this accumulation pathway relevant for some PPCP/EDCs. Ionic compounds, like phenoxy acid herbicides, have the possibility to be taken up by active transport, perhaps through processes designed for uptake of essential nutrients , and may reach higher concentrations than would be expected through passive diffusion . Since some PPCP/EDCs exist primarily in an ionic state , these compounds may potentially accumulate to high levels in plant tissues. The pH of the soil-water or hydroponic solution affects the fraction of ionizable compounds that is in the ionic form. For compounds that are partly ionized at environmental pH levels, basic compounds have increased uptake and acidic compounds have reduced uptake as pH increases , due to changes in the prevalence of the neutral fraction and ion-trapping effects as discussed below.
Accumulation in plant tissues is also related to the tissue composition. Hydrophobic compounds may partition to lipids, where they have the potential to accumulate. Therefore, plants with higher lipid contents may accumulate a compound to a greater degree . The partitioning of a compound to plant lipids is related to its Kow, as discussed below. In comparison, polar compounds are expected to reach equilibrium with the water present in plants and with relatively polar carbohydrates and proteins, which suggests accumulation of these compounds will likely be less extensive . No single model is currently available that accurately accounts for all of these factors , and very little validation of plant uptake models has been done for PPCP/EDCs.Compounds may be taken up by plants when plant roots reach contaminated areas and by mass flow or diffusion of dissolved compounds to roots . Entry is typically by diffusion of neutral compounds across the root membrane, and for ionizable compounds by a combination of diffusion of the neutral fraction and electrostatic interactions by the ionic fraction . A positive relationship has been shown between hydrophobicity and root uptake of neutral pesticides and other neutral compounds . The partitioning of neutral compounds to plant lipids is very similar to the partitioning to octanol, and thus uptake models use log Kow values with adjustments for other factors, including the amount of lipids in the tissue . Based on the partitioning behaviors of neutral compounds and that 1% of barley roots were lipids, Chiou et al. predicted that accumulation into root lipids compared to the rest of the root tissue accounted for 15% of uptake for compounds with log Kow ≤ 1, but ~100% of compounds with log Kow of > 3,hydroponic vertical farming systems showing that while lipids make up a very small part of plant tissue, they greatly affect accumulation behavior and may explain some uptake differences among plant species . For neutral compounds, root uptake is expected to be the greatest for compounds with high hydrophobicity and for plants with high lipid content . Models developed for neutral compounds may be inappropriate to describe the behavior of ionizable compounds, which includes many PPCP/EDCs. The accumulation of compounds in aerial tissue can occur via deposition from volatilized compounds, direct contact with irrigation or amendment materials, and translocation from root tissues . Since most PPCP/EDCs are polar and nonvolatile, volatilization and deposition is expected to be a very minor input for aerial tissue uptake . The extent of organic compound uptake by direct contact is not very well characterized and warrants further investigation. In general, it is expected to proceed by diffusion similar to root uptake. Most studies have focused on the translocation of PPCP/EDCs from roots, which is likely to become more important than direct contact with increased use of drip and other water-conserving irrigation methods that reduce the likelihood of direct contact between plant leaves and irrigation water. Aerial accumulation of neutral organic contaminants from root tissue involves movement of compounds into xylem and then translocation to aerial parts.
Concentrations in xylem are lower than root concentrations due to hydrophobic partitioning to root tissues, suggesting that hydrophobic compounds will be predominantly retained by roots while a greater portion of hydrophilic compounds will move to xylem and be translocated to aerial tissues . Accumulation in aerial tissue competes with compound return to roots tissues via phloem, and occurs by hydrophobic partitioning of compounds to lignin, which usually has much greater affinity for organic chemicals than carbohydrates or cellulose do . Overall, studies suggest that the maximum leaf uptake of neutral compounds may occur at log Kow values in the range of 1.8 – 3.08 . However, many of these studies utilized hydroponic systems, and it has been suggested that in a soil-plant system where uptake is in competition with soil sorption, that the optimal log Kow value would be closer to 0.75 for soil with 6% organic matter,1.25 for 1.25%, and 2 for 0.25% . Similar to root uptake, aerial uptake of ionizable PPCP/EDCs is a combination of neutral fraction uptake, which can be described with log Dow values, and ionic fraction uptake, which is controlled by electrical interactions. Anions are repulsed from all cell membranes except the tonoplast of vacuoles in root cells, so uptake of anionic PPCP/EDCs by xylem and aerial tissue is predicted to be small, except in cases of ion trapping . Cations are electrically attracted to most cell membranes, enhancing diffusion to many plant parts and resulting in generally moderate uptake ability, which may be further enhanced in alkaline soils by ion trap effect . After PPCP/EDCs have been taken up into plant tissues, a number of biological processes may occur that will reduce the bio-available fraction of the parent PPCP/EDC. Xenobiotics in general are quickly modified in a plant cell by enzymes, such as hydrolases or cytochrome p450, to enable conjugation with glutathione or glucose . The conjugated compounds may then be catabolized, creating a variety of transformation products, which are eventually mineralized or incorporated into the plant tissue . The pathways andrates of these metabolic processes are likely specific to each compound and plant species . As organic contaminants, PPCP/EDCs may be metabolized in plants to form transformation products and non-extractable residue, but this area needs further research. In one of the few studies available, Bokern and Harms used cell suspension cultures to identify toxicity and metabolism of 14C-nonylphenol. Plant species which were resistant to toxicity were most efficient at incorporating the compound into cell walls, primarily associated with lignin but also with pectin and hemicellulose. Extractable polar metabolites were also detected, showing that plant cells metabolized the nonylphenol into transformation products and non-extractable residue. In another study, Macherius et al. incubated carrot cell cultures and whole carrots with triclosan, methyl triclosan, and triclocarban. Triclosan was taken up and converted to 8 different conjugated compounds in cells due to bonding at its phenol moiety, but triclocarban and methyl triclosan were found to be taken up and not metabolized. These results suggest that metabolism of PPCP/EDCs in plant may vary widely with the compound, and some compounds may exist principally in their original form in plant tissue. This area needs more research due to its human health implications. Due to the scarcity of information about PPCP/EDC accumulation in edible plants, especially for real environmental situations, the potential of PPCP/EDC residue to have a biological effect in humans is unknown. Matamoros et al. predicted that human consumption of vegetable crops irrigated with water containing PPCP/EDCs would cause an exposure of 500 ng/d of each compound, a level well below the therapeutic dose for individual pharmaceuticals but in an active range for EDCs. Based on the accumulation in radish and ryegrass grown in soil with 0.4 – 19 µg/kg of carbamazepine, diclofenac, fluoxetine, propranolol, and triclosan, Carter et al. calculated that humans might consume 0.01 – 0.21% of an acceptable daily intake for each compound in root vegetables and 0.09 – 3.81% for leaf vegetables. The major exception in the study was the high accumulation of triclosan, which was predicted to reach 83.8% of ADI in leaf tissues, nearing the acceptable limit. These studies focused on the extractable parent compound measured in laboratory uptake studies. As discussed above, it is likely that a large portion of the accumulated PPCP/EDC may be in the form of transformation products, conjugated compounds, and non-extractable residue. While non-extractable residues of xenobiotics have significantly reduced biological activity in plants and appear to be primarily not bio-available to animal metabolism, conjugated compounds may be cleaved during animal metabolism andpotentially exert toxic effects . The presence of conjugated and transformed PPCP/EDCs in plant tissue is poorly understood and the health risks from them are far from clear. Pharmaceuticals and other anthropogenic chemicals are increasingly used around the world .