The wastewater released from industry often contains high levels of toxic heavy metals, which can kill organisms, damage ecosystems, and accumulate in the foodchain. Electroplating, lead smelting, mining, and countless other processes produce enormous volumes of such wastewater.
In a perfect world, remediation would be powered by a renewable energy supply, there would be no solid waste to dispose of, and the heavy metal contaminants could be recycled back into the industrial process with minimal losses. That would be industrial Utopia, of course, but something close might exist if scientists can genetically modify aquatic plant species to grow quickly and soak up heavy metal ions from wastewater.
So-called phytoremediation technology has been used as an economical and eco-friendly option for treating wastewater for several years. It could have an even more significant impact on industry in the developing world, as genetic engineering programs mature.
“Phytoremediaton technologies involving the use of aquatic plants can be a better alternative to traditional/conventional technologies for treating wastewater in terms of low capital investment, minimum human power, less damage to environmental resources and limited energy consumption,” says Bhupinder Dhir of the Department of Environmental Biology, at the University of Delhi, India. He points out that the biosorption potential of aquatic plants should be more keenly explored for developing remediation methods.
Crucial to success is to grow wetland and other aquatic plants that produce a large biomass quickly even in highly toxic wastewater, but also express high levels of metal-sequestering proteins and other factors. For instance, over expression of the plant enzymes cystathionine gamma synthase and selenocysteine methyltransferase in aquatic plants could quickly soak up heavy metals. Species such as Spartina and Typha are already under investigation as transgenic wetland plants carrying Mer genes, which can absorb mercury from contaminated aquatic ecosystems.
However, all this work, still leaves one major problem: what to do with the contaminated plant biomass once the wastewater has been cleaned up?
“The decomposition of metal loaded plant biomass with passage of time raises an issue of major concern among the scientific community,” says Dhir. “Appropriate treatment of plant biomass retaining high levels of heavy metals before disposal is important to prevent further threat to the environment.” He suggests that post-harvest treatment is essential, perhaps involving composting and the associated reduction in volume.
Phytoremediation may eventually offer a leafy green possibility for cleaning up industrial wastewater even if we have not quite reached Utopia. However, there also has to be a way to retrieve the heavy metal leachate from this process. Incineration, pyrolysis, and biogas production are all being considered for the end products of phytoremediation. The technology will only become acceptable once a safe way to extract the heavy metals from the biomass and then safely dispose of the residue is found. And to be environmentally worthy that also has to be both energetically and economically viable.
Bhupinder Dhir (2010). Use of aquatic plants in removing heavy metals from wastewater Int. J. Environmental Engineering, 2 (1/2/3), 185-201