Immobilization of Cr(VI) in soil through injection of nanoscale Fe^II-Al^III LDH suspension into the soil column

Highlights

• Fe^II-Al^III LDH completely immobilized soil Cr(VI) at Cr(VI)/Fe^II_(s) = 0.2 in a batch system.

• In column system, soil saturated/unsaturated conditions and method of LDH application were highly determinative.

• LDH injection into the unsaturated soil column immobilized 70% of leachable Cr(VI)

• Cr(VI) immobilized in soil via its reduction to Cr(III) and subsequent precipitation.

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 suspension 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 immobilization. In summary, nanoscale Fe^II-Al^III LDH was shown to be a fast and strong reductant, which successfully remediated a Cr(VI)- contaminated alkaline soil.

Introduction

Naturally, chromium (Cr) occurs as chromite (FeCr₂O₄), tarapacaite (K₂CrO₄) or crocoite (PbCrO₄) 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 estimated 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., HCrO₄⁻, CrO₄²⁻and Cr₂O₇²⁻) 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 suppress 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 remediation 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 sufficient 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 FeSO₄ (Zhang et al., 2019), FeSO₄/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 from contaminated soils, especially from those with alkaline reaction into the groundwater has been addressed in some soil column experiments (Matern and Mansfeldt, 2016, Yolcubal and Akyol, 2007). However, until now, there are only few promising treatment options to resolve the problem (Banks et al., 2006, Tang et al., 2011). It has been shown that Fe^II can be highly effective in reductive removal of Cr(VI) from aqueous solutions and soils running batch systems. Reduction of Cr(VI) by Fe^II leads to formation of sparingly soluble Cr(III)-Fe^III (oxy)hydroxide according to Eq. (1) (Eary and Rai, 1988):

$$\mathit{xCr}(VI)+{(1-x)Fe}^{\mathit{II}}+H_{2}O={\mathit{Cr}(III)}_x{\mathit{Fe}}_{\left(1-x\right)}^{\mathit{III}}{(OH)}_3+3H^{+}$$

where, the best mole ratio of Cr(VI) to Fe^II for complete reaction was reported to be 0.33.

In a research conducted to immobilize Cr(VI) in the chromite ore processing residue (COPR), FeSO₄ was added to the infiltrating solution in a column experiment (Geelhoed et al., 2003). Their findings illustrated the inefficiency of Fe^II for Cr(VI) immobilization in COPR. They explained that high pH value and large buffering capacity of CORP caused rapid oxidation of Fe^II and its precipitation as Fe^III hydroxides at the entrance into the column.

In another study, effectiveness of carboxymethylcellulose-stabilized FeS nanoparticles (CMC-FeS NPs) for remediation of a Cr(VI)-contaminated soil has been investigated (Wang et al., 2019). Authors reported that after passing NPs suspension through the saturated soil column, Cr(VI) concentration in the effluent decreased to a minimum value of 5 μg L⁻¹. Concentration of Cr(VI), determined by toxicity characteristic leaching procedure (TCLP), was 4.58 mg L⁻¹ after elution of 0.15%-CMC solution in non-treated soil, which decreased to 63.75 μg L⁻¹ after elution of CMC-FeS NPs suspension in the treated soil. From breakthrough curves presented in their article, accumulative concentration of Cr(VI) and total leached Cr in both treated and non-treated columns, were almost the same. Decreasing the Cr(VI) concentration in soil column after treatment with CMC-FeS NPs may be due to the leaching of Cr(VI) from column and not because of the reduction to Cr(III). In fact, the utilized NPs were unable to immobilize Cr(VI) in the soil column.

To benefit from the high redox reactivity of Fe^II toward Cr(VI) in alkaline contaminated soils, in the present study we suggested the application of nanoscale Fe^II-Al^III LDH as a source of Fe^II. LDHs are two-dimensional inorganic materials consisting of positively charged brucite-like layers with the general formula [M^II_1−xM^III_x(OH)₂]^+x [$\frac{x}{n}$A⁻ⁿ·mH₂O]^−x; where, M^II and M^III are divalent and trivalent metal cations, respectively, x is mole fraction of trivalent cation and Aⁿ⁻ is the interlayer anion along with varying amount of water molecules (Génin and Ruby, 2004, Gholami et al., 2020). Placement of Fe^II in nanoscale LDH structures (compounds with large specific surface area, high pH buffering capacity and relatively high stability in near natural pH) allows their application as powerful reductants for treating many redox-responsive pollutants (Chitrakar et al., 2011, Alidokht et al., 2018, Safarpour et al., 2020) and it is anticipated to react more effectively with Cr(VI) in soils with high pH. This work investigated the efficiency of nanoscale Fe^II-Al^III LDH with the 33% substitution of Al^III for Fe^II positions (x = 0.33) for Cr(VI) immobilization in an artificially spiked soil through batch and column experiments. For the first time, we employed various modes of LDH application to the contaminated soil column and designed an injection system to attain the highest efficiency of Cr(VI) immobilization and minimize Cr(VI) concentrations in the column leachate.