The phytoremediation potential of tamarisk at different-aged ash ponds at the fly ash deposit site of ‘TENT A’ (Obrenovac, Serbia)

Abstract

Fly ash is a solid, toxic by-product of the combustion of coal and its deposition in the open ash ponds has a serious and adverse environmental impact. Due to its fine texture, fly ash particles are easily dispersed into the surrounding area and leach into soil and ground water, meaning the establishment of vegetation cover, i.e. phytoremediation, is the best way of ensuring the physical and chemical stabilization of this mobile substrate [1]. For the remediation of polluted habitats such as fly ash deposits, the application of phytostabilization technologies achieves the best results, using excluder plants, which avoid excessive transport of metal ions from the roots to the shoots, [bioconcentration factor (BCF) in the root higher or lower than 1, translocation factor (TF) less than 1], [2,3]. Our previous research showed that suitable plants for the phytoremediation of this type of habitat were perennials with an extensive root system, capable of vegetative reproduction and with the capacity to tolerate high pH, salinity and toxic chemical elements, as well as extremely high temperatures and drought [4,5,6]. This study focused on examining the phytoremediation potential of a planted tamarisk (Tamarix tetrandra Pall) on the fly ash (FA) deposits of the ‘Nikola Tesla A’ thermoelectric power plant (‘TENT A’) in Obrenovac, formed on fertile agricultural land, 30 km south-west of Belgrade, on the right bank of the River Sava. The total As, B, Se and Cr content in the ash from the ash ponds weathered for 3 (L1) and 11 (L2) years was found to be elevated, falling in a range higher than the average values for sandy soils [7] (Fig. 1). Analysis of the root and leaf tissues of the tamarisk at L1 showed that concentrations of As were between 4.86-3.52 μg g−1, B between 12.82-35.75 μg g−1, Se between 1.62-6.35 μg g−1, and Cr between 1.47-0.76 μg g−1. At L2, concentrations of the examined elements were between 5.00-3.63 μg g−1 for As, 8.93-66.29 μg g−1 for B, 2.43-16.56 μg g−1 for Se, and 1.29-1.05 μg g−1 for Cr. Se concentrations in leaves from both ash ponds were within the toxic range, whilst As and Cr were in a range higher than the average concentrations for these elements in plant tissues, but not toxic. B concentrations in leaves increased over time, with levels being in the normal range at L1, but in the toxic range at L2 [7]. The bioconcentration factor (BCF) in the roots and leaves for As, B and Cr was less than 1, while the translocation factor (TF) for As and Cr was also less than 1, but higher than 1 for B. For Se, the BCF in roots was less than 1, but the BCF in leaves and the TF were higher than 1 (Fig. 2). The results of this research showed that tamarisk exhibited good phytostabilization potential for substrates with high levels of As and Cr, moderate potential for substrates with high B concentrations, and low potential for substrates with high levels of Se. The relatively good stabilization potential of tamarisk at L1, its deep root system, and its tolerance to high temperatures, drought and increased salinity show that the decision to use this species at the start of the phytoremediation process, when environmental conditions were least favourable, was completely justified.