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Remediation of Heavy-Metal-Contaminated Soil by a CAM Plant
Li, T., Tao, Q., Han, X. and Yang, X. 2013. Effects of elevated CO2 on rhizosphere characteristics of Cd/Zn hyper-accumulator Sedum alfredii. Science of the Total Environment 454-455: 510-516.

The authors write that "anthropogenic activities in modern society including mining, smelting, land application of sewage sludge, fertilization and reclaimed irrigation water have accelerated soil contamination by heavy metals," citing Khan et al. (2000) and Terzano et al. (2007). And they say that the remediation of such contaminated soil "is necessary not only to preserve the soil resource but also to safeguard human health," citing Kramer (2005) and Vangronsveld et al. (2009).

What was done
In doing their part to help achieve this worthy goal, Li et al. conducted a pot experiment to contrast the effects of elevated CO2 (800 vs. 350 ppm) on rhizosphere characteristics of a hyper-accumulating ecotype (HE) and a non-hyper-accumulating ecotype (NHE) of Sedum alfredii, which is the only Crassulacean acid metabolism (CAM) species that is known to be a Zn/Cd hyper-accumulator.

What was learned
The four researchers say their results "clearly showed that HE has a great potential for removal of Cd and Zn from contaminated soil," for "after a growth period of 45 days, phytoextraction efficiency of Cd and Zn by HE was increased significantly (by 52.8% for Cd and 42.6% for Zn, respectively) by elevated CO2."

What it means
Noting that their results are in accordance with those of previous reports (Zheng et al., 2008; Wu et al., 2009; Jia et al., 2010), Li et al. say they collectively demonstrate that "elevated CO2 can be developed as a potential useful phytoremediation tool to clean up soils contaminated with heavy metals by enhancing the hyper-accumulation of metal."

Jia, Y., Tang, S., Wang, R., Ju, X., Ding, Y., Tu, S. and Smith, D.L. 2010. Effects of elevated CO2 on growth, photosynthesis, elemental composition, antioxidant level, and phytochelatin concentration in Lolium mutiforum and Lolium perenne under Cd stress. Journal of Hazardous Materials 180: 184-194.

Khan, A.G., Kuek, C., Chaudhry, T.M., Khoo, C.S., and Hayes, W.J. 2000. Role of plants, mycorrhizae and phytochelators in heavy metal contaminated land remediation. Chemosphere 41: 197-207.

Terzano, R., Spagnuolo, M., Vekemans, B., De Nolf, W., Janssens, K., Falkenberg, G., Fiore, S. and Ruggiero, P. 2007. Assessing the origin and fate of Cr, Ni, Cu, Zn, Pb, and V in industrial polluted soil by combined micro-spectroscopic techniques and bulk extraction methods. Environmental Science and Technology 41: 6762-6769.

Vangronsveld, J., Herzig, R., Weyens, N., Boulet, J., Adriaensen, K., Ruttens, A., Thewys, T., Vassilev, A., Meers, E., Gent, U., Nehnevajova, E., van der Lelie, D. and Mench, M. 2009. Phytoremediation of contaminated soils and groundwater: lessons from the field. Environmental Science and Pollution Research 16: 765-794.

Wu, H.B., Tang, S.R., Zhang, X.M., Guo, J.K., Song, Z.G., Tian, S.A. and Smith, D. 2009. Using elevated CO2 to increase the biomass of a Sorghum vulgare x Sorghum vulgare var. sudanense hybrid and Trifolium pratense L. and to trigger hyper-accumulatio of cesium. Journal of Hazardous Materials 170: 861-870.

Zheng, J.M., Wang, H.Y., Li, Z.Q., Tang, S.R. and Chen, Z.Y. 2008. Using elevated carbon dioxide to enhance copper accumulation in Pteridium revolutum, a copper-tolerant plant, under experimental conditions. International Journal of Phytoremediation 10: 161-172.

Reviewed 16 October 2013