Decontamination of water co-polluted by copper, toluene and tetrahydrofuran using lauric acid

Abstract

Co-contamination by organic solvents (e.g., toluene and tetrahydrofuran) and metal ions (e.g., Cu²⁺) is common in industrial wastewater and in industrial sites. This manuscript describes the separation of THF from water in the absence of copper ions, as well as the treatment of water co-polluted with either THF and copper, or toluene and copper. Tetrahydrofuran (THF) and water are freely miscible in the absence of lauric acid. Lauric acid separates the two solvents, as demonstrated by proton nuclear magnetic resonance (¹H NMR) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The purity of the water phase separated from 3:7 (v/v) THF:water mixtures using 1 M lauric acid is ≈87%v/v. Synchrotron small angle X-Ray scattering (SAXS) indicates that lauric acid forms reverse micelles in THF, which swell in the presence of water (to host water in their interior) and ultimately lead to two free phases: 1) THF-rich and 2) water-rich. Deprotonated lauric acid (laurate ions) also induces the migration of Cu²⁺ ions in either THF (following separation from water) or in toluene (immiscible in water), enabling their removal from water. Laurate ions and copper ions likely interact through physical interactions (e.g., electrostatic interactions) rather than chemical bonds, as shown by ATR-FTIR. Inductively coupled plasma-optical emission spectrometry (ICP-OES) demonstrates up to 60% removal of Cu²⁺ ions from water co-polluted by CuSO₄ or CuCl₂ and toluene. While lauric acid emulsifies water and toluene in the absence of copper ions, copper salts destabilize emulsions. This is beneficial, to avoid that copper ions are re-entrained in the water phase alongside with toluene, following their migration in the toluene phase. The effect of copper ions on emulsion stability is explained based on the decreased interfacial activity and compressional rigidity of interfacial films, probed using a Langmuir trough. In wastewater treatment, lauric acid (a powder) can be mixed directly in the polluted water. In the context of groundwater remediation, lauric acid can be solubilized in canola oil to enable its injection to treat aquifers co-polluted by organic solvents and Cu²⁺. In this application, injectable filters obtained by injecting cationic hydroxyethylcellulose (HEC +) would impede the flow of toluene and copper ions partitioned in it, protecting downstream receptors. Co-contaminants can be subsequently extracted upstream of the filters (using pumping wells), to enable their simultaneous removal from aquifers.

Figure 1

Separation between THF and water with 0.125 M lauric acidic, at basic, acidic and circum-neutral pH after 24 h (bottom row) and after 5 min (top row). The THF-water mixture contains 30% THF relative to water, v/v. Images collected 5 min after mixing with 1 M lauric acid are in the supporting information (Fig. S2).

Figure 2

Separation between THF and water at circum-neutral pH (pH = 6.5), with different lauric acid concentrations ranging from 0.2 to 1 M (as indicated on the x axis). Volumes were measured after centrifuging samples for 10 min at 3500 RPM. The THF percentages reported in the legend are relative to the water phase (v/v). The deviation on the ordinate axis was measured using Eq. (1) as follows (V_{water,measured}-V_water,used/V_water,used)*100(%).

Figure 3

SAXS patterns of lauric acid in either 100% THF, or in THF-water mixtures, with 95% THF or 90% THF (at either acidic or circum-neutral pH) relative to water (v/v) and 500 g/L lauric acid. The experimental data are shown in red, and fit to the Steubner and Strey model (continuous black line) or a Gaussian (dashed line). The Ornstein and Zernicke fit (dotted line) is poor, and shown as a reference. The best fitting parameters of the Ornstein and Zernicke model are as follows: 100% THF: a₂ = 0.6572, c₁ = 152.8; 95% THF, neutral pH: a₂ = 0.6366, c₁ = 167.0; 90% THF, neutral pH: a₂ = 0.6402, c₁ = 173.0; 90% THF, acidic pH: a₂ = 0.6249, c₁ = 175.9. The Teubner and Strey best fitting parameters are as indicated in Table 4.

Figure 4

Proposed mechanism of THF-water separation by lauric acid. Note that this mechanism occurred when mixing lauric acid in its dry form to THF-water mixtures. Mixing dry lauric acid in water is an option when treating surface water. Delivery of lauric acid in polluted aquifers requires that its is dissolved in a liquid carrier, e.g., canola oil, as discussed later and schematized in Fig. 11.

Figure 5

Spectra of the top (THF-rich) layer of THF-water mixtures separated using 1 M lauric acid under acidic (black), circum-neutral (red) and basic (green) conditions. THF-water mixtures were prepared using 30:70 THF:water (v/v) mixtures. The spectra of 30:70 THF:water (v/v) mixtures (without lauric acid) (blue) and the spectra of pure water (cyan) are also shown for comparison. The inset shows the carbonyl and water bending region of the spectra. The grey dashed line indicates the shift in the THF C–O stretch peak.

Figure 6

Bottle tests conducted using aqueous solutions of 0.03 M CuCl₂ and CuSO₄, using 30% or 50% THF v/v relative to water, at circum-neutral pH. Copper ions partition in the THF phase only at circum-neutral, while they remain solubilized in water at pH = 2 and precipitate in part out of solution at pH = 13 (Fig. S10, supporting information file). Images were taken after leaving vials to sit overnight.

Figure 7

ATR-FTIR spectra of samples prepared with 1 M lauric acid, and 3:7 THF: 100 mM CuSO₄ in water, at circum-neutral and acidic pH. These samples were separated into a top and a bottom phase. The color coding is as follows: bottom layer (black), top layer (red) and top layer at acidic pH (green). Red and green lines are identical in shape and intensity. Inset shows the sulfate absorbing region, indicating that there is no sulfate in the top layer of separated THF-water mixtures, in the presence of copper.

Figure 8

ATR-FTIR spectra of samples prepared with lauric acid in canola oil and toluene and DI water with 10 mM NaOH, with or without copper salts, and with lauric acid in toluene alone. ATR-FTIR spectra were collected for the top phase (i.e., the oil phase) of samples prepared using 5 mL of either 30 mM CuCl₂ or CuSO₄ solutions, and 2 mL of oil (containing 1:1 toluene to 0.25 M lauric in canola oil solution).

Figure 9

Optical microscopy images of water in oil emulsions sampled following bottle tests conducted using toluene (10% v/v), 0.25 M lauric acid in canola oil (10% v/v) and either 30 mM CuCl₂ solutions (70% v/v, a) or 30 mM CuSO₄ solutions (70% v/v, b), with 10 mM NaOH. The scalebar is 100 μm.

Figure 10

Sample compression isotherm measured at the oil water interface. The aqueous and oil phases were obtained starting from samples prepared with 2:1 toluene: 0.25 M lauric in canola oil solution and copper salt solutions (30 mM CuCl₂ + 10 mM NaOH or 30 mM CuSO₄ + 10 mM NaOH), as described in “Static interfacial tension measurement”. Samples were separated and each phase was re-introduced in the trough, to create a planar interface.

Figure 11

Schematics of the proposed approach, which combines HEC + injectable filters and lauric acid to simultaneously prevent copper ion and toluene migration. Once the flow of toluene and copper is arrested upstream of the filters, these contaminants can be extracted from the aquifer (e.g., using a pumping well). Note that while HEC + filters retain toluene in which Cu²⁺ partitions, they allow water flow. Also note that while mixing lauric acid in canola oil is required to enable its injection in polluted aquifers, lauric acid can be introduced in wastewater to be treated at the surface in its dry form (e.g., as a powder).