Phytoextraction: Where's the action?

Many articles concerning phytoextraction of trace elements state that it is an emerging technology that can be used for the low-cost clean-up of contaminated land.... Given the lack of commercial phytoextraction operations or even successful field trials, we sought to determine whether phytoextraction could ever compete with existing technologies to clean up soil within a realistic time-frame, say <25 years. We also investigate why phytoextraction has not found commercial use for the phytomining of valuable metals. Calculations reveal that bioaccumulation coefficients of >10 are required to reduce the total metal concentration in soil by 50% within 25 years, under conditions that are ideal for phytoextraction. Heterogeneity of both the target element, nutrients, and water in soil, as well as heterogeneity of plant roots has a large, but as-yet unquantified effect on remediation time. Variations in climatic conditions, including drought and flooding can also reduce metal extraction rates. Unlike phytoextraction for soil cleansing, phytomining could theoretically produce valuable crops of metal. However, phytomining suffers from a low efficiency of metal extracted per unit of land. Ironically, phytomining may have a larger ecological footprint than conventional mining. Currently, lack of infrastructure limits its implementation. While our review shows that phytoextraction for soil cleansing and phytomining is currently impractical, it is not our intention to discourage research in this area. The best rebuttal of our analyses would be full-scale field operations. However, investigations of new plants/soils/soil conditioner combinations should at least demonstrate how phytoextraction could work by providing convincing basic mass-balance calculations. (C) 2015 Elsevier B.V. All rights reserved.