Applied remediation measures
Screening of metal-tolerant species known as hyper-accumulators applied
for remediation of a metal contaminated site has been a worldwide
hotspot of research (García et al., 2009; Tie et al., 2005; Zhang et
al., 2015). A hyper-accumulator does not only tolerate high
concentrations of a metal element in its tissue but also possesses high
under-ground to above-ground metal transfer rates (Abreu et al., 2102;
Adamo et al.. 2014). The demerits for use of most of the reported
hyper-accumulators, however, are their low levels of biomass growth
associated thus with low levels of total metal uptake. In addition, the
nature of their tolerance to a specific metal element also limits their
application on multiple metal contaminated sites. Attentions in related
fields have been paid to use of tree species ( Bidwell et al.,
2002; Yang et al., 2008; Wang et al., 2010) . As most of the
fast-growing tree species cannot survive under heavily metal
contaminated conditions, the commonly applied methods for forestation at
a metal tailing site are either to seal its upper layer with an
uncontaminated soil (Gomez-Ros et al., 2013) or to use an immobilization
reagent to reduce its upper layer’s metal toxicity (Vargas-García et
al., 2012, Yang et al., 2017). The disadvantages of soil sealing and
metal immobilization are mainly their high costs.
The experimental site under study was a manganese tailing wasteland
located at Xiangtan, Hunan, China. The site was poor in nitrogen and
phosphorus but very high in contents of manganese, lead, zinc, copper,
cadmium and chromium. The applied remediation strategy was a combination
of the above mentioned methods using tree species, metal-tolerant
species and remediation reagent. Two fast-growing tree species
(Paulownia fortunei and Koelreuteria
bipinnata ) screened from the native species naturally distributed in
the uncontaminated region nearby the tailing site were used as dominant
phyto-remediation species. Before transplanting their seedlings, a
bacteri-rich organic manure specifically prepared for the tailing site
was amended to their rhizospheric areas. The enrichment of the site’s
biodiversity was counted on natural germination of the native metal
tolerant plant species based on the soil seed bank theory (Tang et al.,
2006). To prevent the spread of metal contamination to surrounding
areas, an ecological interception belt along the border of the tailing
site was established and its outlet was connected to a constructed
wetland system for treatment of runoff.
Detailed experimental information has been presented in previous work
(Ouyang et al., 2016; Wu et al., 2017). The experiment plots, sample
analysis and equations for calculation are briefly described below.