Suitability of four woody plant species for the phytostabilization of a zinc smelting slag site after 5 years of assisted revegetation

Abstract

Zinc smelting activity generates large volumes of highly toxic waste slags and poses a potential extreme environmental risk for the surrounding areas. The establishment of a vegetation cap for the phytostabilization of abandoned mine tailing heaps using plants is usually considered a beneficial approach. This study aimed to evaluate the suitability of phytostabilization of zinc smelting slag using four woody plants combined with organic amendments, to investigate the distribution of heavy metals in the slag–plant system, and to better understand how the direct revegetation of a zinc smelting slag site can influence the mobility and geochemical fraction of heavy metals. Slags were collected from the areas planted with vegetation (Arundo donax, Broussonetia papyrifera, Robinia pseudoacacia, and Cryptomeria fortunei) and a bare area in a zinc smelting waste slag site using an indigenous method. Physicochemical properties were determined with the usual procedures. The geochemical fraction and bioavailability of heavy metals was determined using the three-step modified European Community Bureau of Reference (BCR) sequential extraction and diethylene triamine pentaacetic acid (DTPA) sequential extraction schemes. Heavy metal concentrations (Cu, Pb, Zn, and Cd) in the slag and plant samples were also measured. Vegetation planted directly in the zinc smelting waste slag significantly enhanced the nutrient accumulation and reduced the bioavailability of heavy metals (Cu, Zn, and Cd) with the exception of A. donax for Zn and Cd. The presence of four woody plants increased the bioavailability of Pb. Sequential extraction revealed that revegetation reduced the acid-soluble extractable fraction and increased the fraction of heavy metals associated with the Fe/Mn oxy(hydr)oxides or organic matter. This is attributed to the establishment of plant-enhanced weathering of minerals in the waste slag that resulted in the formation of an amount of dissolved metals, and the amount of dissolved metals was partly redistributed into the soluble extractable fraction of the zinc smelting waste slag. The final concentration of metals (Cu, Pb, Zn, and Cd) in the soluble extractable fraction is dependent on the dynamics of metals induced by root activity in the rhizosphere. Much lower levels of heavy metals with lower translocation factors accumulated in the four woody plants than in the associated slags. We conclude that the studied four woody plants showed a beneficial vegetation cover and phytostabilization potential within 5 years of revegetation. These woody plants have the potential for high heavy metal tolerance and low heavy metal accumulation. Therefore, these woody plants could be used for revegetation and phytostabilization of zinc smelting slag sites under field conditions.