Phyto-extraction of zinc, lead, nickel, and cadmium from zinc leach residue by a halophyte: Salicornia europaea

Abstract

The aim of the present study is to phyto-extract zinc, lead, nickel, and cadmium from zinc leach residue (ZLR)—as a metallurgical waste—by using Salicornia europaea plants. This study tried to introduce a new approach—which was common for remediation of contaminated soil and water—in order to recover and remove heavy metals from ZLR, which can provide more environmental sustainability as compared to conventional pyrometallurgical and hydrometallurgical processes. The experiments were performed at two stages (screening and modification). Due to high heavy metal concentrations and salinity of ZLR, at the screening stage, the ZLR/soil ratio was investigated and determined to be 25 wt% ZLR + 75 wt% soil, providing sufficient plant growth and heavy metal uptake. At this ratio, the BCF results were obtained 0.1, 0.12, 0.99, and 0.14 for zinc, lead, nickel and cadmium, respectively. At the modification stage, to enhance metal bio-availability and plant growth, the effects of three modifiers—citric acid (0.005 & 0.1 M), arbuscular mycorrhizal fungi (AMF) (50 g/kg), and nano-silica (50 & 250 mg/kg)—were investigated on the ZLR/soil optimum ratio. The results have revealed that AMF achieved the most plant growth. The plants treated by nano-silica accumulated metals in the root and did not translocate them from root to shoot. However, the optimum condition was treating the mixture containing 25 wt% ZLR by the high level citric acid (0.1 M), providing the most extraction, translocation factor, and sufficient plant growth. The result revealed that the BCFs of zinc, lead, nickel, and cadmium were obtained 0.23, 0.3, 2.11, and 0.35, respectively. Also, the TFs of zinc, lead, nickel, and cadmium reached to 1.04, 1.14, 3.5, and 1.1, respectively. At this optimum condition, 5.68% zinc, 7.44% lead, 52.63% nickel, and 8.82% cadmium were extracted. Therefore, in the pot containing 25 wt% ZLR, by applying 0.1 M citric acid, the most percentage of phyto-extraction was achieved.