Abstract:
Phytoremediation has emerged as a promising treatment approach for rehabilitating mine degraded lands. Heavy metals such as Cu and Zn are by-products of smelting which can cause serious health problems when ingested in substantial amounts from drinking contaminated water. Plant hyperaccumulators are used to inactivate heavy metals in the soil preventing them from leaching into surface and ground water and translocating them into aerial parts of the plant. Over the past 20 years, several metal-hyperaccumulating plants have been discovered but are mostly endemic to the metalliferous soils in a specific region. There are less studies published on non-endemic and high biomass plants which are essential in barren and contaminated mine soils. There are even fewer studies in hyperaccumulation of essential trace elements such as Cu and Zn in plants due to homeostatic control and high regulation of these elements in plant cell membranes. This research presents two plant species Helianthus annuus (sunflower) and Medicago sativa (alfalfa) and their Cu and Zn accumulation capacity, focusing on root-to-shoot translocation, soil-to-plant bioconcentration and hyperaccumulation criteria recently proposed in the scientific community. Upon bioavailable metals in the soil, sunflowers showed 95-97% increase in Cu concentration reaching up to 3813 ppm while there was 6% increase in Zn concentration with accumulation reaching up to 7298 ppm. Meanwhile, alfalfa had up to 96% increase in Cu accumulation reaching up to 6479 ppm with recorded Zn accumulation up to 7881 ppm. Both plants exceeded the minimum threshold criteria of hyperaccumulation for Cu and Zn. Previous studies have also indicated that metal ions can be bound by chelators to increase the accumulation in plants. Chelators contain varying numbers of functional groups or binding sites that are capable of complexing heavy metals sorbed in the finer soil particles. In this research, the capacity of synthetic chelation was determined and the effects of Ethylenediaminetetraacetic acid (EDTA) was investigated in two plant species. EDTA-chelate increased Cu accumulation in sunflowers by up to 20% and Zn accumulation by up to 7%. While in alfalfa, chelation exhibited 66% increase in Cu accumulation and no significant increase for Zn accumulation.