Abandoned metal mines and the large quantity of tailings deposited around these mines, which are rich in heavy metals, represent a significant threat to terrestrial, aquatic and atmospheric environments. The heavy metal content of mine tailings is a serious contamination issue worldwide. These elements can be transferred through the food chain and accumulate in the human body, causing considerable harm due to their carcinogenicity, mutagenicity and teratogenicity. Thus, it is extremely important to remove heavy metals from polluted media (tailings, soils and water) and/ or to inactivate them within these media in order to conserve human health and permit continued use and development of urban and rural land. Therefore, the remediation of multi heavy metal-contaminated sites was investigated using different approaches and under various conditions to reduce the phytoavailability of cadmium (Cd), zinc (Zn), lead (Pb) and copper (Cu) in mine tailings collected from Parys and Britannia copper mines, and Pandora lead mine located in North Wales, UK.
A sequential extraction protocol was applied to examine the distribution of studied heavy metals among different geochemical fractions and to evaluate the effectiveness of the remediation strategies in reducing target metal availability. The metal fractionation results obtained showed that the chemical dynamics of target metals were similar in Parys and Britannia copper mines with the majority of studied metals associated with the residual fraction. However, the exchangeable fraction retained high proportions of investigated metals in Pandora tailings. Organic and inorganic amendments individually and in combination with the phytoremediation technique were examined in terms of their ability to reduce the mobility and phytoavailability of target metals to plant and soil biota.
Application of 2% of several organic and inorganic amendments (zero valent iron (Fe0), zeolite, green waste biochar and green waste compost) to Pandora and Parys tailings was investigated during a 1-year pot experiment under uncontrolled conditions in Henfaes research station. The application of 2% of amendments modified several physiochemical properties of treated tailings particularly highly acidic tailings with various effects on the bioavailability of target metals. The added amendments reduced the metal availability at the beginning of study, but with time the mobility of investigated metals fluctuated and was higher than in the control treatments on several occasions. We concluded that 2% of amendments was not sufficient to reduce the mobility of several metals.
A greenhouse trial using different mixtures of tailings and uncontaminated soil (US) was conducted using ryegrass as a bio-indicator of phytoremediation technology. The results
showed that applying US enhanced plant biomass production and ryegrass roots accumulated higher levels of heavy metals than shoots with lower levels of heavy metals toxicity indicated e.g., shortness of plant shoots and roots and chlorosis. Ryegrass roots accumulated higher levels of target metals with low translocation of heavy metals to the vegetative parts, with the exception of Zn. Thus, ryegrass may be an effective plant to use for the phytostabilisation of heavy metals in both sites, and a hyperaccumulator plant appropriate for the treatment of Pb in Pandora mine tailings. The metal content in grass tissue depends on the phytoavailable fraction of the metal. Plants grown in Pandora tailings accumulated higher concentrations of heavy metals in their roots than grass planted in Parys tailings, reflecting the higher occurrence of studied metals in the exchangeable fraction in Pandora tailings compared with Parys tailings.
The co-amending mine tailings experiment was conducted to examine the effect of applying phytoremediation (ryegrass) in combination with two types of amendment (Fe0 and green waste compost, added separately to target tailings). Results indicated that the applied amendments increased the pH of severally acidic tailings (Parys tailings) more effectively and compost was more effective than Fe0 particularly at a higher dosage to promote ryegrass biomass productivity. Fe0 did not show significant enhancement of grass biomass production even at a higher dosage. The amendments had various effects on of target metal phytoavailability, metal uptake and translocation within plant tissues. Compost reduced the translocation of Cu, Zn, Pb and Cd within grass tissues in treated Parys mine tailings. For Pandora tailings both amendments increased Cu translocation within grass tissues, but Zn immigration from roots to shoots decreased. Also, the uptake of Zn and Cd by grass roots increased, but Pb uptake was reduced by the addition of amendments to Pandora tailings.
Pb isotopic studies suggested that geogenic routes seemed to be the main source of Pb fingerprinting in Parys tailings, but in Pandora mine tailing the ore processing of Pb appeared to be the major route of Pb pollution. In addition, the applications of compost and Fe0 did not result in a significant alteration in the Pb isotopic content of ryegrass tissues.
Overall, application of green waste compost supported the establishment of ryegrass cover on mine sites, thus reducing the erosion of mine tailings by wind and rain. This approach also promoted the growth of deep grass roots, which increased the ability of ryegrass to take up heavy metals for storage in the roots or translocate them to aboveground parts prior to harvesting and safe disposal. The application of ryegrass and compost can be used to amend soils exposed to heavy metal contamination by any source including mine tailings. Further studies are necessary in controlled and uncontrolled conditions to examine the feasibility of
using several native plant species supported by organic matter application for phytoremediation of mine tailings.