# Native Plant Capacity for Gentle Remediation in Heavily Polluted Mines
## 1 Introduction  
Industrial activity, such as mining, has generated large amounts of waste over time, giving rise to major environmental problems worldwide [1]. In the Sulcis-Iglesiente region (Sardinia, Italy), after more than 2000 years of mining activities, numerous spoil heaps of various types of waste material persist from the extraction process as well as the chemical and electrolytic treatments [2]. The trace-metal content is highly variable, as well as the characteristics of the contaminated soils [3]. The most abundant elements in the spoil heaps include Fe, Zn, Mn, Pb, and Cu. The bioavailability of these elements relates to their total concentration and to soil characteristics such as pH, clay content, soil organic matter, CaCO\({}_{3}\), Al, Mn and Fe oxides and hydroxides [4]. The use of native plants can be a solution to stabilize these sediments, avoid or minimize erosion and eventually accumulate and eliminate part of the trace elements. Phytoremediation techniques have been widely studied, giving rise to diverse results and conclusions. Some authors consider this approach coofriendly and cost efficient, forming part of a new industrial ecology [5, 6]. Phytoremediation is a recovery strategy for contaminated soil, where certain plant species (specifically their roots and associatedmicrobial communities) are used to absorb and eliminate trace elements in the soil by transferring them to different plant tissues [7]. According to the part of the plant involved or the procedure used, the technique is called phytoplankton, phytoextraction, phytotransformation, phytodegradation, phytostimulation, phytofiltration, etc. [6]. Occasionally, plants have also been used to extract valuable trace metals, such as gold and mercury, which are found in the soil in such small quantities that they do not allow traditional mining. This technique, called phytomining [8], has the curiosity, according to [PERSON] et al. [7], of being potentially more polluting even than conventional mining. However, these authors point out the need for new studies that combine the knowledge of the plant-soil relationships by performing a biomass balance in order to demonstrate the effectiveness of phytoextraction or phytomining techniques. Phytoextraction consists in the absorption of considerable amounts of toxic elements in plant tissues, especially in the aboveground biomass [9]. In this sense, [PERSON] et al. [10] use the term \"hyper-accumulating plants\" for those that reach values of 100 ppm for Cd; 1000 ppm for Ni, Cu, Co, and Pb; and 10,000 ppm for Zn and Mn in leaves. In addition, these plants must be able to grow under harsh climatic conditions, such as those that characterize the Mediterranean area [11] and develop a large biomass [7]. The capacity of plants to accumulate trace elements in its tissues is usually weighted by concentration factors, also called bioaccumulation factors by some researchers [12; 13; 14]. This is calculated as the ratio between the concentration of the trace element in the plant vs. the concentration in the soil. However, some researchers [15] have pointed out that the costs of techniques based on the use of plants will not be fully known until the technology matures and is widely applied. [PERSON] et al. [7] pointed out that under the most optimal phytoextraction growing conditions, a plant with bioaccumulation coefficients \(>10\) would need some 25 years to reduce the total amount of pollutant in a soil by 50%. However, the use of plants can be oriented towards what has been called \"gentle remediation\" [16; 17], which involves the use of a set of in situ techniques to avoid the spread of contamination by accumulating trace elements in its biomass (phytoextraction) or phyto-stabilizing metals [18]. Gentle Remediation Options (GROs), such as biostimulation, bioaugmentation, phytoremediation, and vermieremediation, are cost-effective and environmentally-friendly solutions for soils simultaneously polluted with organic and inorganic compounds [19].