Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng Phyto-extraction of zinc, lead, nickel, and cadmium from zinc leach residue by a halophyte: Salicornia europaea Misagh Khanlarian a , Melina Roshanfar a , Fereshteh Rashchi a, ⁎ , Babak Motesharezadeh b a School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b Department of Soil Science, College of Agriculture and Natural Resources, University of Tehran, Tehran, Iran ARTICLE INFO Keywords: Salicornia europaea Phyto-extract Zinc leach residue Citric acid Arbuscular mycorrhizal fungi Nano-silica 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 ap- proach—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. 1. Introduction Currently, 80–85% of the world's zinc (Zn) demand is produced through conventional hydrometallurgical processes of oxidative roasting, acid leaching, purification, and electrowinning (RLPE). The main drawback of this method is the considerable quantity of zinc leach residue (ZLR, 0.5–0.9 ton per ton of zinc) which is produced in the leaching process (Sethurajan et al., 2016; Tang et al., 2018; Yan et al., 2014; Zheng et al., 2016). ZLR contains huge amounts of heavy metals such as zinc, lead (Pb), nickel (Ni), cadmium (Cd), and cobalt (Co) (Jiang et al., 2017; Tang et al., 2018). Toxicity characteristic leaching procedure (TCLP) classifies ZLR as hazardous and toxic waste (Sethurajan et al., 2016). Releasing of these heavy metals, through acid rain, wind, and soil erosion, as well as accumulating of these non-bio- degradable metals in living organisms result in critical environmental problems besides huge loss of valuable metals (Chandra and Yadav, 2011; Çoruh and Ergun, 2010; Tang et al., 2018). Generally, there are some pyrometallurgical and hydrometallurgical processes to recover valuable elements from ZLR. Nonetheless, there are some considerable shortages that incapacitate these methods to solve the problem. The typical pyrometallurgical methods—Waelz and Ausmelt—suffer two main drawbacks of high energy consumption due to the high operating temperature (1100–1300 °C) and secondary high iron-bearing residue generation, which make these methods econom- ically impractical (Jiang et al., 2017; Zheng et al., 2016). Hydro- metallurgical processes are more economical because of lower capital and operation costs; nevertheless, the purification processes are com- plicated because of the high leaching rate of iron. Moreover, managing the large quantities of liquid effluents of the hydrometallurgical pro- cesses is the other challenge (Jiang et al., 2017; Yan et al., 2014). https://doi.org/10.1016/j.ecoleng.2020.105797 Received 26 October 2019; Received in revised form 1 February 2020; Accepted 11 March 2020 ⁎ Corresponding author. E-mail address: rashchi@ut.ac.ir (F. Rashchi). Ecological Engineering 148 (2020) 105797 0925-8574/ © 2020 Published by Elsevier B.V. T