Screening the phytoremediation potential of native plants growing on mine tailings in Arizona, USA

Mining companies have been playing an important role for the utilization of the Earth's natural resources for the prosperity of human society. However, the mining of resources has generated solid wastes called tailings that have become a serious environmental threat worldwide. Technologies such as soil caping have been used to remediate the problem. Nevertheless, the remediation of mine tailings using conventional technologies is costly and environmentally unpleasant. Since the last few decades, a plant based technology known as phytoremediation has appeared as a promising alternative. Several plant species have demonstrated to have phytoremediation capabilities; though, best results have been obtained with indigenous species. In 1991, the former Phelps Dodge Mining Company initiated a reclamation project at a copper mine tailing in Globe, AZ, USA. The project included the deposition of 20 cm of local soil to cover the tailing. Subsequently, several species of local shrubs and grasses were sown in the soil-covered tailing. Screening the phytoremediation potential using native plants to clean up such a mine tailings as in Globe, Arizona, USA, was investigated in this research. In the present research, samples of plants, soil cover, and tailings were taken periodically since 2006. Additionally, seeds of some species growing at the site were collected for further studies. Analyses were performed to determine the element concentrations in soil media and plant tissues, the relationship between elements in tissues/elements in the growth media, physiological effects of elements in plant growth, comparison of better adaptive seeds, elemental uptake in different ages of plants, and cellular localization.

X-ray fluorescence and inductively coupled plasma-optical emission spectroscopy were used to determine the concentration of metals/metalloids in the soil cover and tailings. Based on concentration, elements were classified as high and low level elements. The concentration of Cu, Pb, Mo, Cr, Zn, and As, in tailings was 526.4, 207.4, 89.1, 84.5, 51.7, and 49.6 mg/kg, respectively. The concentration for both level elements in the soil cover was 10∼15% higher than that of the tailings except for Cu and Mo.

The concentration of Cu, Pb, Mo, Zn, As, and Cr was determined in the roots and shoots of plants grown at the site. The highest accumulation of Cu was in salt cedar root (Tamarix ramosissima) (1090 mg/kg dry weight) and mesquite shoot (Prosopis spp.) (2117 mg/kg dry weight); Pb and Mo in whitethorn acacia (Acacia constricta) root (422 and 482 mg/kg, respectively) and mesquite shoot (1157 and 3757 mg/kg, respectively); As in desert broom (Baccharis sarothroides Gray) root and shoot (45 and 37 mg/kg, respectively); Zn and Cr in mesquite root (3797 and 77 mg/kg, respectively) and shoot (8548 and 245 mg/kg, respectively). Considering all hyperaccumulating criteria, mesquite tops the list of metal hyperaccumulation followed by whitethorn acacia, desert broom, salt cedar and grasses (Vetiver zizanioides). However, a combination of all these plants would be a better option for phytoremediation of Cu, Pb, Mo, Zn, As, and Cr at the site.

The growth and element uptake by the offspring of mesquite plants grown in the tailing (site seeds, SS) and plants derived from vendor seeds (VS) were compared. Plants that originated from SS grew faster and longer than plants grown from VS. At 1 mg L^-1, the concentration of all elements in SS plants was significantly higher compared to control plants and VS plants. The results suggest that SS could be a better source of plants intended to be used for phytoremediation of soil impacted with Cu, Mo, Zn, As, and Cr. In addition, the uptake of elements between old and young plants of mesquite, whitethorn acacia, and desert broom was compared because of the phyto-harvesting application. Results showed a lower concentration of Cu in the old plants than that of the young plants of the same species.

Scanning electron microscopy studies revealed the distribution of elements within the seedlings’ tissues, dominantly accumulated in the cortical and vascular (xylem) regions of root tissues. In the stem, most of the elements were found within the xylem tissue. Infrared microspectroscopy studies showed the changes in plant structure induced by metal treatments. A high concentration of protein in the xylem and epidermis with a fairly homogenous distribution throughout the cortex of untreated root tissue was observed in the untreated root tissue.

In summary, this research has shown that native plants can efficiently be used to phytoextract and/or phytostabilize excesses of chemical elements in metals/metalloids impacted site.