Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Immobilization of Cr(VI) in soil through injection of nanoscale Fe II -Al III LDH suspension into the soil column Leila Alidokht a , Shahin Oustan a, ⁎ , Alireza Khataee b,c , Mohammad R. Neyshabouri a , Adel Reyhanitabar a a Department of Soil Science, Faculty of Agriculture, University of Tabriz, 51666-16471 Tabriz, Iran b Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666- 16471 Tabriz, Iran c Department of Materials Science and Nanotechnology Engineering, Faculty of Engineering, Near East University, 99138 Nicosia, TRNC, Mersin 10, Turkey ARTICLE INFO Editor Name: Yvan Capowiez. Keywords: Column study Cr(VI)-contaminated soil HYDRUS-1D model Injection Layered double hydroxide Unsaturated column ABSTRACT We synthesized nanoscale Fe II -Al III layered double hydroxide (LDH) and investigated its efficiency for reductive immobilization of Cr(VI) in a Cr-spiked alkaline loam soil using both batch and column experiments. Results of batch experiments indicated that addition of fresh LDH suspension to the soil at a mole ratio of Cr(VI) to structural Fe II in LDH [Fe II (s) ] = 0.2, completely immobilized Cr(VI). Column experiments, using same Cr(VI)/ Fe II (s) ratio, were conducted under four various modes of LDH suspension application to the soil. Addition of LDH suspension to the saturated and unsaturated packed soil columns at a pressure head of 2 cm was inefficient in reducing Cr(VI) to Cr(III) because of shallow penetration of LDH particles into the soil. Injection of LDH sus- pension into the soil columns greatly improved penetration of LDH particles into the soil. However, only 18.8% of leachable Cr(VI) was immobilized in the saturated soil column, while the same operation in the unsaturated column experiment increased Cr(VI) immobilization efficiency to 70.0%, a significant improvement in im- mobilization. In summary, nanoscale Fe II -Al III LDH was shown to be a fast and strong reductant, which suc- cessfully remediated a Cr(VI)- contaminated alkaline soil. 1. Introduction Naturally, chromium (Cr) occurs as chromite (FeCr 2 O 4 ), tarapacaite (K 2 CrO 4 ) or crocoite (PbCrO 4 ) in ferromagnesian rocks. It may also present as co-precipitated forms with oxide and hydroxides of other metals (Al, Fe, and Mn) in soils (Burns and Burns, 1976). However, Cr is a commonly recognized pollutant in soils and waters mainly due to its widespread industrial applications. At a global scale, it has been esti- mated that discharge of Cr in soil is 896 metric ton per year, which is considerably higher than the international allowable value of 50–100 kg per year (Shahid et al., 2017). Hexavalent Cr (Cr(VI)) oxyanions (e.g., HCrO 4 − , CrO 4 2− and Cr 2 O 7 2− ) are weakly sorbed by soil components and very mobile into groundwater. They are also strong oxidants (Eh = +1.38 V) that act as acute allergen, carcinogen and mutagen to the human body. Trivalent Cr (Cr(III)), in contrast to hexavalent form, is relatively nontoxic and due to its strong adsorption on soil particles and/or precipitation as sparingly soluble Cr(OH) 3 or mixed Cr(III)-Fe III (oxy)hydroxides, is virtually immobile under alkaline to slightly acidic conditions (Shahid et al., 2017). Hence, reduction of Cr(VI) to Cr(III) would notably sup- press the mobility and toxicity of Cr in soils and waters (Marinho et al., 2019). However, due to reoxidation of dissolved Cr(III) to Cr(VI) by manganese oxides (Bartlett and James, 1979), conversion of produced Cr(III) to insoluble products is essential to achieve success in a re- mediation technique. Naturally, soils have a capacity for chemical and biological reduction of Cr(VI) to Cr(III) (Bianco Prevot et al., 2018). However, in highly contaminated soils with relatively high pH and low organic matter content, the natural capacity of soil would not be suf- ficient for immobilization of Cr(VI) and immediate remediation actions should be taken. Several researchers have reported the use of iron-based reductants, including metallic iron nanoparticles (Alidokht et al., 2011), iron scrap (Hoseini et al., 2015), dissolved ferrous iron (Fe II ) as FeSO 4 (Zhang et al., 2019), FeSO 4 /sodium dithionite mixture (Su and Ludwig, 2005), ferrous sulfide (FeS) particles (Li et al., 2017) and Fe-bearing minerals (Doğaroğlu and Kantar, 2016) for remediation of Cr(VI)-contaminated soils and solid wastes. To date, most of these studies have been done under batch systems. Furthermore, the high risk of Cr(VI) discharge https://doi.org/10.1016/j.geoderma.2020.114648 Received 6 December 2019; Received in revised form 5 July 2020; Accepted 4 August 2020 ⁎ Corresponding author. E-mail address: oustan@hotmail.com (S. Oustan). Geoderma 380 (2020) 114648 Available online 17 August 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved. T