Toxicity: Degradation by-products may collect in ground water or bio-accumulate in animals. Additional research is needed to figure out the exact resting place of various compounds that may have an adverse effect on the environment. Scientist need to determine if contaminants in plant droppings and products will have a negative impact; for example toxins can be collected in the leaves or wood of trees and could potentially be released when leaves fall in the autumn or when wood in the form of firewood or mulch is put to use. Also, the disposal of harvested plans with large concentrations of heavy metal can be a major problem. Merely throwing the plant by-products would only re-add the toxins to the environment and make the entire phytomediation process useless. However, other methods of disposal can be costly and/or ineffective
Bioavailability:
Depth Limits:The location of the contaminants sometimes limits treatment. Each plant's treatment zoine is dependent of the type of root and to which depth the roots reach. In most cases, treatment is limited to shallow soils, streams and groundwater.
Possible Solutions: Pumping the contaminated water out of the ground and use as irrigation sometimes treats contraminated groundwater that is too deep to reach. In the case of contaminated soil, deep tilling, where practical, brings heave metals closer to the roots. Scientists have also started looking into using trees instead of small plants since trees have a deeper root reach.
Concentration Limits:For most plants successful phytomediation occurs in areas of low contamination. There are limitations to the amount of toxins each plant can be exposed to and still survive. In contaminant concentrations that are too high plants often die. Also, phytoremediation is not particularily effective for strongly absorbed contaminants like polychlorinated biphenyls (PCBs).
Environmental and Other Limits: The success of phytoremediation may be seasonal and dependent of location. Other climatic factors will influence effectiveness as well. For example, attempting to cacti in a wetland environment in the winter is obviously not going to be hightly successful. In addition, the success of remediation depends on establishing a selected plant community. Introducing new species may have an adverse effect on the environment if for example the species is invasive.
Future Advances:
Trangenetic and other advances in interspecies breeding brings about a whole new chapter of phytoremediation. In the future, using the technology of Genetically Modified Organisms (GMOs) plants may have multiple traits that would help them deal with the evironment, depth and/or concentration limits. For example, say Plant A has very deep roots but not a high tolerance for high concentrations of Uranium and Plant B has a very high tolerance of Uranium but incredibly shallow roots. By the successful implantation of traits of one in the other you would have a plant with deep roots and tolerance of Uranium.