With the rapid development of the world’s economy and industrialization, the demand for copper resources has progressively increased. At the same time, a large amount of acid wastewater will be generated during the mining of copper mines. In this context, the High-density Sludge (HDS) process is the main effective method used to treat acid mine wastewater and thus improve the recovery rate of mineral resources. The HDS process flow is shown in Fig. 1. In the HDS treatment process, many heavy metal elements in the ore will enter the HDS sediment with the wastewater. After their introduction into HDS sediment, heavy metal elements affect the physicochemical properties and microorganisms of HDS sediment directly or indirectly. As the accumulation time of HSD sediment increases, the heavy metal elements in it will gradually transfer to the soil of the dumpsite. Then, the pollution caused by heavy metals is a severe risk to the surrounding ecosystem because of its toxicity, non-biodegradability, and bioaccumulation. Heavy metals in soil may transport and accumulate in the human body along the food chain. In this case, heavy metal pollution has become one of the most serious environmental problems in mining areas.
Nowadays, a variety of remediation approaches, including physical remediation (guest land methods, electro-remediation methods and thermal desorption methods), chemical remediation (Add fixative or acid-base leaching agents containing porous structure material, etc.), and biological remediation (microbial remediation and phytoremediation) approaches have been developed to reclaim heavy metal-polluted soils. The physical repair method is widely used in engineering, but it is featured with a large amount of engineering, short-time effect, and high cost. The chemical remediation method has a large amount of precipitated sludge and is environmentally damaging. Whereas, phytoremediation adopts the ability of plants to remove the heavy metal of soil which is regarded as the most cost-effective and environmentally friendly technique for remediation of heavy metal-contaminated soils; besides, it uses plants to extract and transport elemental pollutants in the soil, thus stabilizing the fertility of plant growth substrates. Actually, more than 400 plants have been found to have the ability to super-enrich heavy metals. For example, the ryegrass possesses a strong cadmium ion enrichment ability, and there are differences between different types of ryegrass. B. papyrifera has an excellent enrichment ability for lead, cadmium and zinc ions, and different ions feature different enrichment levels in different parts of the plant. Pteris vittata L. is also called "As hyperaccumulator plant" due to its high ability to accumulate arsenic. In recent years, applications of phytoremediation technology to the heavy metal pollution of solid waste in mining areas have been rapidly advancing. Solid waste in mining areas includes tailings generated during beneficiation and HDS sediment generated in mine wastewater treatment. Solanum viarum Dunal is used for solving heavy metal problems in tailings, and shows potential applicability for the treatment of areas contaminated by heavy metals in the mining area. Quercus spp. and Salix spp. were planted into the tailings containing lead and zinc ions. Due to the different mechanisms operated to confer tolerance of heavy metals, the root systems for heavy metals transhipment showed obvious differences. When being planted in mine tailings containing copper, manganese, zinc and lead, even if the concentration of heavy metals in the tailings is different, ryegrass can still treat heavy metals there. Clearly, it is found that little research is carried out on the use of phytoremediation techniques to repair HDS sediments contaminated by heavy metals.
As adaptability to the growth environment varies with plants, the growth environment has a great influence on the effect of phytoremediation. Therefore, it is the key to phytoremediation technology to find heavy metal-rich plants suitable for growth in contaminated soil. Native plant species are generally superior to introduced plants in terms of growth and reproduction under environmental stress. The use of native plant species for phytoremediation is of great importance, which is a key element in the efficiency of phytoremediation strategies. After testing and analysis, the bottom mud contains multiple heavy metals such as Cu, Cd, Zn, Pb, Cr and Hg, when four heavy metals namely Cu, Cd, Zn, and Pb have the highest content, so that the subsequent tests are all about these four heavy metals. In fact, the heavy metals in the sediment are easy to form and accumulate, which will eventually threaten the ecological environment of the mining area. Considering that, in this experiment, three native plant species, including slash pine (Pinus elliottii Engelmann), Chinese white poplar (Populus tomentosa Carr.) and black locust (Robinia pseudoacacia), grown in the experimental area were used to remediate heavy metals in HDS sediments. Besides, the accumulation and transportation of heavy metals in different plants were analyzed, and its rules were also explored. At the same time, the thermogravimetric analysis on the sediments after different phytoremediation was carried out to identify the changes in the properties of the sediments before and after the improvement, which provides data support for the phytoremediation of heavy metals in the experimental area and similar areas.