FIELD OF THE INVENTION

The invention relates to a method of removing metal ions from solution. In particular, it relates to the removal and recovery of metal ions from a solution with a biosurfactant.

BACKGROUND TO THE INVENTION

Metals and metalloids are common contaminants found in wastes from industries like mining, metal plating and the manufacturing of fertilizers, pesticides and paper. Sometimes metal- contaminated waste streams are discharged directly into the environment. The metals are harmful to organisms and may lead to reduced crop yields. Metal-contaminated food and water sources are toxic to humans and may lead to severe health problems, including cancer. Metals, and in particular heavy metals and metalloids, tend to persist in environments and can accumulate in food chains. In order to reduce contamination, metals must be extracted or alternatively converted into non-toxic compounds.

Mines commonly remove metal ions from water using alkaline precipitation. However, there is a need for an improved and more efficient method of removing metal ions from mine waters, particularly when the metals are present at low concentrations. Mining plant operation may be improved if metal ions such as copper ions, which interfere with plant processes, are removed from recycled mining water. Mine water effluent is also often outside environmental legislative requirements with respect to metal and metalloid content and requires remediation.

There are a range of methods available for extracting heavy metal and metalloid contaminants, including chemical precipitation, adsorption, ion exchange, and filtration, amongst others. Some of these methods have ecological drawbacks, such as high energy consumption, difficult waste treatment and disposal, or secondary pollutant formation. Despite its high efficiency, the application of membrane filtration in aqueous heavy metal extraction is limited due to process complexity, fouling of membranes, and high cost. Ion exchange is effective for heavy metal bearing wastewater treatment. However, the regeneration of the exhausted exchange resin can lead to secondary pollution. Additionally, the cost of the ion exchange process is high for large volumes of wastewater with dilute heavy metal concentrations. Chemical precipitation is typically a simple and cost-effective process. However, precipitation is usually only effective for high concentrations of heavy metals in solution. Adsorption is more effective when treating dilute heavy metal solutions. However, the efficiency of adsorption is dependent on the adsorbent. The increasing cost of conventional adsorbents, such as activated carbon, may be a deterrent for the use of this treatment method.

Biosorption by bio-adsorbents has been proposed as an alternative to conventional adsorbents. Biosurfactants are amphiphiles produced by a wide range of microorganisms and are used by microorganisms to assist with binding metal ions for use in metabolic cycles or for sequestering toxic metals. However, most studies using biosurfactants to recover metal ions have resulted in low recovery levels (typically less than 20%), which are unsuitable for applications such as treating industrial or mine wastewater.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of removing metal ions from a solution, the method comprising: i) adding a lipopeptide biosurfactant to a solution having a pH equal to or above 5 to form a metal lipopeptide biosurfactant complex; and ii) removing the metal lipopeptide biosurfactant complex from the solution; wherein the lipopeptide biosurfactant is selected from the group consisting of surfactin, iturin, fengycin and any combination thereof and at least 70% (mol/mol) of the metal ions are removed from the solution after steps (i) and (ii).

If the pH of the solution is lower than 5, the pH of the solution may be adjusted to be equal to or above 5 prior to step (i).

In particular, the lipopeptide biosurfactant may be surfactin.

The lipopeptide biosurfactant may be added in a 1 :1 or more than a 1 :1 molar ratio of lipopeptide biosurfactant to metal ion. The lipopeptide biosurfactant may be added in a molar ratio ranging from 1 :1 to 10:1 of lipopeptide biosurfactant to metal ion, such as a ratio of 2:1 or 3:1 . The method may include the step of mixing the solution during or after addition of the lipopeptide biosurfactant. The metal lipopeptide biosurfactant complex may be removed from the solution by centrifugation, ultrafiltration, settling, flotation or solvent extraction.

In particular, the metal lipopeptide biosurfactant complex may be removed by froth flotation. For example, step (ii) may comprise: a) causing a gas to flow through the mixture of the solution and the metal lipopeptide biosurfactant complex, thereby to float a froth fraction containing the metal lipopeptide biosurfactant complex; and b) removing the froth faction.

Alternatively, the metal lipopeptide biosurfactant complex may be removed by centrifugation, optionally followed by filtration.

The metal ions may be heavy metal ions, including any mixture of different heavy metal ions. The metal ions may be copper ions, nickel ions, cobalt ions, or any mixture thereof. The solution may be an aqueous solution. The aqueous solution may be wastewater from an industrial process.

The method may be capable of removing at least 70% (mol/mol) of metal ions from a solution having a metal ion concentration of 100 mM or lower.

The method may be capable of removing at least 70% (mol/mol) of the metal ions from a solution having a metal ion concentration of 100 /M or lower.

The method may further include a step of recovering the metal ions from the removed metal lipopeptide biosurfactant complex by dissolving the precipitate in a solution with a pH below about 4.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a flow diagram that illustrates a method of removing metal ions from solution;

Figure 2 is a flow diagram illustrating a two-stage method for removing and separating metal ions from solution;

Figure 3 is a flow diagram illustrating a method of recovering and upgrading a metallic component from a fluid mixture by flotation; Figure 4 is a Fourier Transform Infrared Spectroscopy (FTIR) spectrum of purified surfactin with the vibrational bands numbered for clarity;

Figure 5 is an FTIR spectrum of a copper-surfactin complex crystal with the vibrational bands numbered for clarity and the spectrum of Figure 4 superimposed and shown as a broken line for comparison;

Figure 6 is an FTIR spectrum of a nickel-surfactin complex crystal with the vibrational bands numbered for clarity and the spectrum of Figure 4 superimposed and shown as a broken line for comparison;

Figure 7 is an FTIR spectrum of a cobalt-surfactin complex crystal with the vibrational bands numbered for clarity and the spectrum of Figure 4 superimposed and shown as a broken line for comparison;

Figure 8 is a graph of the concentration of copper ions remaining in an aqueous solution following the precipitation of copper-surfactin complexes using increasing initial concentrations of surfactin relative to a constant initial copper ion concentration of 100 pM and at pH values of 2, 3, 4, 5, 6, 7 and 8, respectively;

Figure 9 is a graph of the concentration of nickel ions remaining in an aqueous solution following the precipitation of copper-surfactin complexes using increasing initial concentrations of surfactin relative to a constant initial nickel ion concentration of 100 pM and at pH values of 2, 3, 4, 5, 6, 7 and 8, respectively;

Figure 10 is a graph of the concentration of cobalt ions remaining in an aqueous solution following the precipitation of copper-surfactin complexes using increasing initial concentrations of surfactin relative to a constant initial cobalt ion concentration of 100 pM and at pH values of 2, 3, 4, 5, 6, 7 and 8, respectively;

Figure 11 is a graph illustrating the results of an experimental ion flotation run with surfactin for Cu ²⁺ , Ni ²⁺ and Co ²⁺ ions, with conditions of Q _a ir of 0.08 L/min, a pH level of 7 and an initial ion to surfactin ratio of 1 :3, showing the extent of ion removal, the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation, and the concentration factor of ions into the overflowed foam fraction; Figure 12 is a graph showing the concentration of metal ions remaining in the residual solution over the duration of the flotation run described for Figure 1 1 ;

Figure 13 is a set of graphs illustrating how changes in the air flowrate affect the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution, the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation, and the concentration factor of metal ions into the foam overflowed fraction after ion flotation with surfactin;

Figure 14 is a set of graphs illustrating the concentration of metal ions remaining in the residual solution during ion flotation with surfactin for a (i) Cu ²⁺ solution, (ii) Ni ²⁺ solution, and (iii) Co ²⁺ solution, under the changing conditions of air flowrate described for Figure 13;

Figure 15 is a set of graphs illustrating how changes in the pH level affect the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution, the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation, and the concentration factor of metal ions into the foam overflow fraction after ion flotation with surfactin;

Figure 16 is a set of graphs illustrating the species of (i) Cu (II), (ii) Ni (II), and (iii) Co (II) as a fraction of the total metal content, with speciation being illustrated as a function of solution pH;

Figure 17 is a set of graphs illustrating how changes in the ratio of metal ions to surfactin affect the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution, the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation, and the concentration factor of metal ions into the foam overflow fraction after ion flotation with surfactin; and

Figure 18 is a graph illustrating the concentration of metal ions remaining in the residual solution during ion flotation of the solutions of metal ions described in respect of Figure 17, at both 1 :3 and 1 :10 molar ratio of metal ions to surfactin initially in solution. DETAILED DESCRIPTION OF THE INVENTION

Lipopeptide biosurfactants (or just lipopeptides) are microbial surface-active compounds produced by a wide variety of bacteria, fungi, and yeast. Lipopeptides and lipoproteins are mainly obtained from bacteria of the Bacillus and Pseudomonas genera. Lipopeptides comprise a C12 to C18 fatty acid linked to a peptide chain of about four to twelve amino acids. Lipopeptides may have a linear hydrophilic head or a lactone ring if they are cyclic lipopeptides. Surfactins, iturins, fengycins, lichenysins, viscosins, amphisins and putisolvins are all examples of cyclic lipopeptides. Surfactin, iturin, and fengycin are produced by Bacillus subtilis.

Surfactin is a well-known lipopeptide due to its excellent surface-active and antimicrobial properties. Surfactin comprises a heptapeptide ring which is linked to a /3-hydroxy fatty acid chain. The cyclic heptapeptide is the hydrophilic head group moiety of the amphiphilic surfactin molecule. The amino acid residues in the heptapeptide ring are typically in the order L-Glu-L-Leu- D-Leu-L-Val-L-Asp-D-Leu-L-Leu. The fatty acid tail is the hydrophobic moiety and can vary in chain length between twelve and seventeen carbons, but typically between fourteen and fifteen carbons. The carboxyl groups present on the Glu1 and Asp5 residues contribute to the hydrophilicity of the heptapeptide moiety due to their negative charge when deprotonated. Once the carboxylic groups are protonated the hydrophilicity of surfactin is reduced. The pKa of the carboxyl groups are 4.5 and 4.3 for the Glu 1 and Asp5 residues, respectively, and when the pH is well-below these values, or typically below 4, surfactin precipitates from solution. Surfactin dissolves in aqueous solutions at a pH of around 5 to 5.5 and will be completely dissolved in aqueous solutions with a pH greater than 6.1 .

Surfactin can bind cations. The carboxyl groups in the head group moiety are an effective binding site for cations when deprotonated. The binding of divalent cations at these sites results in the neutralisation of both anionic charges of the carboxylic acids, leading to increased hydrophobicity of the surfactin complex. It has been surprisingly found that surfactin and other lipopeptides which include a peptide chain or ring with amino acids residues having carboxyl groups may be utilised as a precipitant in the chemical precipitation of metal ions and metalloid cations from solution, which may be a solution of the metal ions in one or more polar solvents, preferably a solution including a polar protic solvent such as water, an alcohol (methanol, ethanol, isopropanol, n- butanol), acetic acid, formic acid or nitromethane, for example. The complexation of the metal ion or metalloid cation with the peptide of the lipopeptide in solution depends on the acidity/basicity of the solution and the solution should not be too acidic to result in precipitation of the lipopeptide prior to complexation. Under very acidic conditions, the lipopeptide precipitates out of solution and is therefore not available in solution to precipitate the metal. It was determined that the pH of an aqueous solution, for example, should be equal to or above 5 for metal/metalloid-surfactin complex formation and subsequent precipitation of the complex out of the aqueous solution. The pH of an aqueous solution may be between 5 and 14 when precipitating metal ions, preferably between 5 and 8, and optionally adjusted to a pH within these ranges prior to or upon addition of the lipopeptide biosurfactant. It is believed that the neutralization of the hydrophilic moieties in the peptide ring of the lipopeptide upon metal/metalloid cation complexation results in the subsequent precipitation of the lipopeptide-metal/metalloid complex out of solution. The same extent of metal ion removal or recovery was not achieved with rhamnolipid biosurfactants which are glycolipids including a sugar moiety, rather than a peptide ring.

Accordingly, a first embodiment of a method of removing metal ions from a solution (100) is provided and illustrated in Figure 1. The method comprises ensuring the pH of the solution is equal to or above 5 by optionally adjusting the pH to 5 or above (101 ) with a base when necessary; and adding a selected amount of a lipopeptide biosurfactant to the solution having a pH equal to or above 5 (103) to form a precipitate of a metal lipopeptide biosurfactant complex or chelate. The optional pH adjustment step may be done before, after or together with the addition of the lipopeptide biosurfactant to the solution. A pH in the range of 7-9 is preferred, with a pH of about 7 being optimal.

The lipopeptide biosurfactant may be added in an equimolar or higher molar ratio of lipopeptide biosurfactant to metal ion. The lipopeptide biosurfactant may be added in a molar ratio ranging from about 1 :1 to about 10:1 of lipopeptide biosurfactant to metal ion. For example, the molar ratio of lipopeptide biosurfactant to metal ion may be about 10:1 , about 9:1 , about 8:1 , about 7:1 , about 6:1 , about 5:1 , about 4: 1 , about 3: 1 , about 2: 1 or about 1 :1. It was found that in the case of surfactin, the metal ion recovery approaches its maximum at surfactin concentrations just above five times the critical micelle concentration (CMC) of 7.5 zM (H. Heerklotz and J. Seelig, “Detergent-like action of the antibiotic peptide surfactin on lipid membranes,” Biophysical Journal, vol. 81 , no. 3, pp. 1547-1554, 2001 ). Therefore, solutions with very low metal ion concentrations that may still be above emission legislative requirements can be treated with an equally low concentration of a lipopeptide biosurfactant. Both the metal ion concentration and the lipopeptide concentration used for recovery may be at a parts per million (ppm) level and even less than 100 parts per million. Recoveries of 70% (MOL/MOL) and higher of metal from an aqueous solution, in particular metal-contaminated water, were achieved at ppm level.

The lipopeptide biosurfactant may be selected from the group consisting of surfactin, iturin, fengycin and mixtures thereof. The lipopeptide biosurfactant is preferably surfactin. In one embodiment, the surfactin has been substantially purified from a mixture of surfactin, iturin and fengycin that is produced by Bacillus subtilis. The surfactin may be sodium surfactin.

The metal ions may be heavy metal ions, including any mixture of different heavy metal ions. The metal ions may be copper ions, nickel ions, cobalt ions, zinc ions, lead ions, chromium ions, cadmium ions, mercury ions or any mixture thereof. The method may also be used to recover metalloid cations from solution, such as arsenic, selenium or tellurium cations.

The solution is preferably an aqueous solution and may be wastewater from an industrial process.

Surfactin, for example, may be added to a heavy metal contaminated water at a concentration of from about 1 :1 to about 10:1 , more preferably about 2:1 or about 3:1 , of surfactin to heavy metal and a metal lipopeptide biosurfactant complex allowed to occur. The metal lipopeptide biosurfactant complex may be recovered, leaving the water with lower metal concentrations, potentially within emission limits. The metal lipopeptide biosurfactant complex may be a precipitate or may be in aqueous form, or a fraction of the metal lipopeptide biosurfactant complex may precipitate out of the solution while the rest remains in solution.

The method may be integrated into wastewater treatment as a passive recovery. The lipopeptide may be added and the precipitate formed allowed to settle, with no additional energy added, unlike flotation or other energy- and equipment-requiring processes.

A second embodiment is illustrated schematically in Figure 2, showing a method (200) for removing metal ions from an aqueous solution. In a first step, a solution of the lipopeptide biosurfactant (201 ) and an aqueous solution to be treated (203) are added together. The method (200) includes a step of mixing the aqueous solution (205) during or after addition of the lipopeptide biosurfactant (203) to assist in the formation of the complex of the metal ion and lipopeptide biosurfactant. Metal-bearing wastewater and a surfactin solution, for example, may be added in a mixer and mixing initiated. Once the reaction is complete the method includes the further step of removing the metal lipopeptide biosurfactant complex from the aqueous solution (207). Figure 2 illustrates four different exemplary techniques for removing the metal lipopeptide biosurfactant complex from the aqueous solution, namely solvent extraction (209), settling (21 1 ), flotation or foam fractionation (213) and filtration (215).

Accordingly, the method may include the step of removing the metal lipopeptide biosurfactant complex from the aqueous solution by solvent extraction (209) which results in the formation of a non-polar organic phase concentrate of the metal and lipopeptide complex (217) and a stripped aqueous phase (219). Alternatively, the method may include the step of removing the precipitated complex from the aqueous solution by settling (211 ) in terms of which a precipitate rich slurry (223) is formed as the phase containing the metal and lipopeptide complex together with a stream of treated aqueous solution (221 ) or water. Further alternatively, the method may include the step of removing or separating the metal lipopeptide biosurfactant complex from the aqueous solution by flotation (213). The flotation may be ion flotation, precipitative flotation, sorptive flotation, dissolved air flotation, and foam fractionation and the type of flotation may be selected depending on the quality of the up-stream solution and the target quality of the treated solution. Flotation or foam fractionation (213) results in the formation of a froth or foam concentrate (225) as the phase containing the metal and lipopeptide complex and a treated aqueous solution or water stream (227). Or the method may include the step of removing the metal lipopeptide biosurfactant complex from the aqueous solution by filtration (215), such as micro- or ultrafiltration, which yields the metal lipopeptide complex (229) and a filtrate of treated aqueous solution (231 ) or water.

The use of the lipopeptide biosurfactant as a precipitant in terms of the method (200) is thus a first stage process or pre-treatment process introducing an additional phase (a solid phase) which can be separated via a second-stage process such as foam or froth fractionation/flotation, filtration or settling, to achieve improved recovery of the metal ions. It has been found that metal lipopeptide biosurfactant complex formation followed by flotation results in similar recoveries to lipopeptide precipitation followed by settling or centrifugation.

In one embodiment, illustrated in Figure 3, the precipitate is removed by froth flotation. This can be achieved by passing a gas such as air through the solution containing the metal lipopeptide complex, causing a froth fraction containing at least a portion of the metal lipopeptide biosurfactant complex to float. The froth fraction is then removed.

In another embodiment, the precipitated complex is removed by centrifuging the mixture of the precipitate and the solution, and optionally also filtering the solution.

Both the metal ions and lipopeptide can be recovered from the metal lipopeptide biosurfactant complex. For example, the precipitate can be redissolved in an acidic solution with a pH below 4.

The method described herein results in the efficient removal of metal ions at very low metal concentrations, such as about 100mM metal ion, about 10 mM metal ion, about 1 mM metal ion and even in the micromolar range (for example, 500 pM or lower, and even at 100 pM). The disclosed method may therefore find application as a secondary treatment process after an initial alkaline treatment used industrially, to remove more metal ions which may still be present at low concentrations.

At least about 40%, about 50%, about 60%, about 70%, about 75%, about 80%, about 85%, about 90% (mol/mol) or even more of the metal ions may be removed from solution. For example, at a molar ratio of 1 :3 ion to surfactin, surfactin was shown to float 75.2%, 94.7%, and 98.2% of an initial 100 mM solution of copper, nickel and cobalt, respectively, in less than 1 hour. At a molar ratio of 1 :2 ion to surfactin, surfactin was shown to precipitate 100% Cu (pH 8), 82 % Ni (pH 5), 84 % Ni (pH 6) and 88 % Co (pH 7) (of an initial 100 M ion solution). These are improved metal recoveries by comparison with results reported for other surfactants.

The method described herein results in excellent recoveries of the metal ions at ppm level, with the addition of only ppm levels of lipopeptide as the precipitating agent. For example, with 4 mg/L copper ions (100 /M), which is a reasonable wastewater concentration, only about 0.4 g/L surfactin may be required for adequate complex formation, optionally precipitation, to occur. Usually in order to get recovery of metal ions at ppm levels of metals, substantially more of the precipitating agent is required.

Surfactin was shown to have high selectivity for a large range of metal cations, while also being biodegradable and non-toxic at the concentrations used. Surfactin was also shown to have suitable resistance to variations in temperature, salinity, and pH (above 4) in solution.

The invention will now be described in more detail by way of the following non-limiting examples, in which copper (Cu), cobalt (Co), and nickel (Ni) cations were removed from aqueous solutions with the lipopeptide, surfactin.

Example 1

Methods and materials

CuSC , NiSO4'(H ₂ O)6, and CoS04 (H ₂ 0) ₇ were dissolved in deionised water to produce 10 mM standard solutions of each metal ion to be tested. The standard solutions were used to prepare model solutions emulating metal ion contaminated wastewater. Surfactin powder (90% purity, Kaneka Corporation) with an assumed average molar weight of 1036 g/mol was dissolved in deionised water to produce a 1 mM standard solution. The standard solution concentration was confirmed by high pressure liquid chromatography (HPLC) performed on a Phenomenex Luna C18 250 mm x 4.6 mm column with UV detection and using acetonitrile and water mobile phases at 0.9 ml/min and 30 °C. This standard solution was used to prepare the more dilute surfactin solutions used in the experiments. Solutions of 1 mM nitric acid (HNO3) and 1 mM sodium hydroxide (NaOH) were used for pH adjustments.

Fourier Transform Infrared Spectroscopy (FTIR) analysis of pure surf actin and metal-surf actin complexes

FTIR was used to investigate which functional groups of surfactin are involved in each metal ion binding to it and to determine if the same functional groups that contribute to the water solubility of surfactin are neutralised by metal cation coordination. A solid sample of surfactin was prepared by diluting 50 ml of the surfactin standard solution up to 200 ml using deionised water and adjusting the pH to near 1 using HNO3. The mixture was stirred for 30 minutes to allow time for precipitation, after which the precipitate was filtered out using a porcelain Buchner funnel with qualitative filter paper. The precipitate was allowed to dry overnight at room temperature before reducing crystal size with a pestle and mortar and washing the precipitate with deionised water. After washing, the precipitate was again separated using a Buchner funnel with a new filter paper and allowed to dry at room temperature before repeating the washing process twice more. The solid metal-surfactin complex samples were prepared by mixing approximately equimolar quantities of each metal ion and surfactin from the 10 mM and the 1 mM respective standard solutions (50 ml surfactin solution to 5 ml of metal solution). The precipitated complex was collected, dried, and washed in the same manner as the surfactant precipitate. Samples were incorporated in KBr and crushed to the correct particle size before being compressed in a 13 mm die at 7.5 tonnes of pressure for 3 minutes. The pressure on the die was then released for 2 minutes before resuming at 7.5 tonnes for an additional 3 minutes. This was repeated twice more to produce a uniformly transparent sample-containing KBr disc. The process to produce the KBr background disc was identical, without incorporating samples in the KBr before pressing. The solid samples of surfactin and metal-surfactin complexes were analysed using a Thermo Scientific Nicolet iS10 6700 spectrometer equipped with deuterated triglycine sulfate detector. Spectra of the samples were recorded over a range of 400 cm' ¹ to 4000 cm' ¹ , averaging the results over 64 scans taken per background and sample. Background scans were repeated between each sample. The spectral data was processed using Thermos Scientific OMNIC 9.2 software.

Metal-surfactin complex precipitative extraction

The effect of pH and relative molar concentration on potential complex precipitation was investigated. Seven different pH values between 2 and 8 were tested as well as seven different metaksurfactin molar ratios against a control containing no surfactin. In each case, 350 ml of a 100 rM metal ion solution was prepared by diluting 3.5 ml of the 10 mM metal standard solution with demineralised water. The pH was adjusted to 2, 3, 4, 5, 6, 7, and 8, respectively, using the HNO3 or NaOH solutions. The pH-controlled 100 rM metal ion solutions used to prepare eight 50 ml samples containing 5 rmol of each of the respective metal ions. Approximately 0, 2.5, 3.75, 5, 7.5, 10, 15 and 25 rmol of surfactin was then added to six of the 50 ml solutions to produce solutions with approximately 0:1 , 0.5:1 , 0.75:1 , 1 :1 , 1.5:1 , 2:1 , 3:1 and 5:1 surfactin to metal ion molar ratios. These solutions were then stirred on a magnetic stirring bank for 30 minutes. Thereafter, samples were taken from the solutions and centrifuged for 30 minutes at 14000 rpm in 2 ml centrifuge tubes. After centrifuging, 1 ml of the supernatant was filtered through a 0.22 zm nylon syringe filter and diluted using deionised water to a ratio of 1 :9 in 10 ml centrifuge tubes. The supernatant samples were analysed by inductively coupled plasma mass spectrometry (ICP- MS) and HPLC. An Avio 500 ICP-MS spectrometer from Perkin Elmer was used to quantify the concentration of metal ions remaining in the supernatant after precipitation, centrifugation, and filtration.

Results

FTIR analysis

The IR spectrum of a solid sample of purified surfactin is shown in Figure 4. The IR spectra of the metal-surfactin complexes are shown in Figures 5 to 7. The absorption bands and vibrational energy assignments of the spectra are listed in Table 1 . Characteristic absorption bands indicative of secondary amine groups and carboxylate groups are present in Figure 4, which agrees with the structure of surfactin depicted by FTIR in the literature (S. Joshi, C. Bharucha, and A. J. Desai, “Production of biosurfactant and antifungal compound by fermented food isolate Bacillus subtilis 20B,” Bioresource Technology, vol. 99, no. 1 1 , pp. 4603-4608, Jul. 2008; J. F. B. Pereira et aL, “Optimization and characterization of biosurfactant production by Bacillus subtilis isolates towards microbial enhanced oil recovery applications,” Fuel, vol. 1 11 , pp. 259-268, 2013).

Table 1 . Assignments of the IR spectra of surfactin and the metal-surfactin complexes determined by comparison with typical characteristic group frequencies in ‘Infrared Characteristic Group Frequencies’ by G. Socrates (G. Socrates, Infrared Characteristic Group Frequencies: Tables and Charts, 2nd ed. Chichester: John Wiley & Sons Ltd, 1994), as well as the work by Joshi et al and Pereira et al.

There are notable differences in bond energies when comparing the IR spectrum of surfactin with the IR spectra of the respective metal-surfactin complexes.

The strong, broad peak at 331 1 cm ^-1 in the pure surfactin spectrum is assigned to the characteristic stretching mode of N-H in peptides, usually found at 3305 cm' ¹ . The shoulder towards higher wavenumbers on this peak appears to have a greater intensity for the metal complexes than for surfactin alone. There is a change in bond energies for the secondary amines in the surfactin complex which suggests that these amines are likely involved in the binding of the metal ions.

Bands between 2957 cm ^-1 to 2855 cm' ¹ are indicative of bonds in the aliphatic fatty acid chain and these bands, listed in Table 1 , are at very similar wavelengths for both the complex and pure surfactin. This is to be expected as the fatty acid moiety is not involved in binding the metal cations. The very weak band near 2360 cm' ¹ in each metal complex spectrum could possibly be attributed to a NH ⁺ stretch stemming from the secondary amine groups’ electron donation to cation binding. This again indicates amine involvement in metal ion coordination.

A band attributed to the carbonyl group is expected at 1734 cm ^-1 , and this band is seen at 1732 cm ^-1 and 1737 cm' ¹ in the pure surfactin and metal-surfactin complexes respectively. Additional characteristic bands of peptides are typically seen at 1643 cm' ¹ and 1543 cm' ¹ , attributed to the stretching mode of the C=O bond and the N-H bond’s deformation mode respectively. These characteristic bands are assigned to the bands seen at 1647 cm' ¹ and 1537 cm' ¹ in the pure surfactin, and 1647 cm ^-1 and 1540 cm ^-1 in the Cu-surfactin complex. There is a considerable amount of noise surrounding these peaks in the Co-surfactin complex and especially in the Ni- surfactin spectra, however it is not expected for these groups to be involved in metal ion complexing and as such it is assumed that these peaks should not have shifted and that the noise is not as a result of new bond formation.

Bands from 1465 cm ^-1 to 1368 cm ^-1 are usually representative of the bonds present in the aliphatic chains, and the bands in this range in the pure surfactin spectra are all assigned to -CH ₂ - and - CH ₃ bond energies. However, in the spectra of the metal-surfactin complexes, there are noticeable peaks near 1440 cm' ¹ and 1410 cm' ¹ . These bands could potentially be assigned to the COs' bond energy present in carboxylic acid salts. The characteristic absorption band of COs' typically presents as multiple bands near 1400 cm' ¹ and such bands are visible in the metal- surfactin spectra where there is normally a trough between 1450 - 1400 cm' ¹ . A shallower trough is visible in the metal-surfactin spectra at -1600 cm ^-1 , which is the region in which a peak relating to an asymmetric stretch band for the same bond is to be found. The carboxyl groups appear to be involved in binding Cu ²⁺ . The complexation of Cu ions to the carboxylic acid groups may be due to steric hinderance limiting access to the amine groups when surfactin is in water. Surfactin takes on a saddle-like conformational shape in water with the carboxylic acid residues forming a ‘claw-like’ structure at the ridges of the saddle. Complex formation above the surfactin CMC of 7.5 rM also promote binding at carboxyl group sites. In micelles, surfactin will be orientated with the hydrophilic carboxyl group ‘claw-like’ structure exposed to the aqueous solution and the fatty acid chain and amine groups orientated towards the centre of the micelle, further limiting ion access to amine group binding sites. It is therefore possible that surfactin concentration relative to the CMC may have an effect on the preferred binding site for metal cations.

From the FTIR results, it appears that the Cu, Ni, and Co ions interact with both the amine and carboxylic acid groups of surfactin. It is likely these functional groups contribute to the partial water solubility of surfactin. The carboxyl groups may contribute through its anionic charges and the amine groups through hydrogen bonding with water. Thus, it is proposed that the involvement of these functional groups in coordinating metal ions result in a decrease in water solubility of the complex relative to that of free surfactin to an extent that leads to the precipitation of these complexes from aqueous solutions.

Analysis of metal ion concentration in supernatant following precipitation with surfactin, centrifugation and filtration.

Surfactin was added to the metal ion solutions with typical concentrations of the metal ions found in industrial wastewaters. Upon addition of surfactin, precipitation of the metal-surfactin complex occurred. The precipitate was separated from the solution by centrifugation, followed by filtration. Figures 8 to 10 demonstrate that the metal ions were successfully extracted from the model wastewater solutions. 100% Cu (pH 8), 82 % Ni (pH 5), 84 % Ni (pH 6) and 88 % Co (pH 7) (of an initial 100 /M ion solution) was extracted. The concentration of metal ion remaining in the supernatant decreased with each metal ion tested. The decreasing metal ion concentrations in the supernatant indicates that the extent of metal ion extraction increases relative to increased quantities of surfactin introduced to the model solution.

An insoluble Cu hydroxide species formed at pH 8, as is evident in Figure 8, and it likely that surfactin will have little added effect on the precipitation of Cu under alkaline conditions (pH at or above 8). The Cu ions will preferentially precipitate because of hydroxide formation. This is not the case with the Ni and Co ions, as hydroxide precipitation of these metal ions does not occur at the pH range tested.

Below pH 4, the extraction efficiency is reduced with the majority of each metal ion remaining in the supernatant. This is due to the protonation and neutralisation of carboxylic acid groups in the surfactin. Neutralisation of these surfactin head group charges leads to a drastic reduction in surfactin water solubility and therefore precipitation of the surfactins independently of the metal ions that are in solution. The extent of extraction of Cu ions from solution plateaus as the initial surfactin to Cu ion molar ratio approaches 1 :1. The extent of extraction of Ni and Co ions from solution with surfactin only plateaus when the surfactin to Ni/Co ion molar ratio is nearly 2:1 . This is thought to be caused by the formation of salt bridges between the divalent Ni and Co cations and the carboxylate groups of two surfactin monomers, leading to the formation of larger Ni- surfactin or Co-surfactin complex dimers or oligomers. It has been previously determined that in some cases, intermolecular salt bridge formation is more likely than intramolecular salt bridge formation.

The results demonstrate that surfactin, a lipopeptide biosurfactant, can be used to precipitate copper, cobalt and nickel ions from aqueous solutions at ppm concentrations. Recoveries of 70% and higher were obtained at pH values above 5, and in particular between 5 and 8.

Example 2

In order to assess the use of surfactin as a collector in ion flotation, surfactin was used in experimental ion flotation runs on solutions of 100 mM Cu ²⁺ , Ni ²⁺ , and Co ²⁺ . At a pH of 7, an air flowrate of 0.08 L/min, and an initial molar ratio of 1 :3 (metal ion colligends : surfactin collector) recoveries from the foam overflow were 75.2% of Cu ²⁺ , 94.7% of Ni ²⁺ , and 98.2% of Co ²⁺ ions. A lower air flowrate resulted in reduced ion recovery, but greater concentration in the foam overflow as a result of lower water recovery. Surfactin demonstrated significantly improved metal recovery than other tested surfactants from literature.

Methods and materials

10 mM standard solutions of Cu ²⁺ , Ni ²⁺ , and Co ²⁺ were produced using 99% assay sulphate salts of each metal and diluted in deionized water. All simulated solutions emulating heavy metal contaminated water used in the experiments were produced from dilutions of these standard solutions. Metal concentrations were modelled against metal concentrations found in industrial waste waters.

Sodium surfactin (90% purity) was used. This surfactin was dissolved in deionized water to produce a 5 mM standard solution of surfactin that was used in the experiments.

For the purpose of pH control, standard solutions of HNO3 and NaOH were used. A 0.1 M sodium hydroxide solution was produced from 97% assay solid NaOH salt. A 0.1 M HNO3 solution was produced by diluting a 55% nitric acid solution.

Ion Flotation

A custom micro-flotation rig was used, similar to that described in Bradshaw, D.J. and O’Connor, C.T., ‘Measurement of the sub-process of bubble loading in flotation’, Miner Eng, vol. 9, no. 4, pp. 443-448, 1996. The rig was modified to include a sparging stone to provide a greater dispersion and density of bubbles in the cell for ion flotation as opposed to single mineral flotation. Synthetic air (0.21% O2 and 0.79% N ₂ ) was used as the flotation gas with the flowrates controlled using a needle valve. Aqueous solution within the column was continuously cycled using a peristaltic pump operating at 300 mL/min, as a method of maintaining homogeneity. A sampling port was present at the base of the column to allow sample collection.

Solutions were produced by diluting appropriate volumes of 10 mM metal standard solution and 5 mM surfactin standard solution in deionised water up to 150 mL for flotation. Values of pH for each solution were adjusted dropwise to the required pH using 0.1 M solutions of NaOH and HNO ₃ .

Table 2 tabulates the operating conditions for the ion flotation experimental runs: Table 2. Operating parameters for ion flotation experiments wherein:

• Qair refers to air flowrate;

• Me ²⁺ refers to metal ²⁺ ions; and

• Flotations were run at ambient temperature (22 ^e C).

Concentration factor (CF) and ion removal (R) from feed were calculated using equation (1 ) and equation (2), respectively: wherein Co, C<fi,t), and C( _re ,t) are the ion concentration in the initial solution, the floated fraction at time t, and the residual fraction at time t, respectively.

These factors were used as metrics to evaluate the effectiveness of the ion flotation process.

The flotation process for each run was allowed to proceed for 50 minutes, or until the surfactin concentration remaining in the residual solution was insufficient to maintain foam formation. Samples each having a volume of 1 mL were taken from the residual solution at 0, 2, 5, 10, 20, 30, 40, and 50 minutes. A further sample was taken from the final foam overflow and volume of overflow was also measured after completion of the flotation.

Characterization

Samples taken from the residual solution and from the foam overflow were adjusted to pH < 1 using a 0.1 M HNO3 solution to break the complex and precipitate the surfactin. This was done to ensure no complex was trapped during filtration of samples. The samples were filtered using 0.22 pm nylon syringe filters and the samples were sent for metal analysis. The concentration of heavy metal ions was determined by inductively coupled plasma mass spectrometry (ICP-MS). Results

Comparative behaviour of the different metals

The flotation of each metal was compared at the experimental condition of 0.08 L/min flowrate, pH 7, and an initial molar ratio of 1 :3 metal ion colligends to surfactin.

Figure 1 1 plots the extent of ion removal (primary axis), the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation (primary axis), and the concentration factor of ions into the overflowed foam fraction. For all three metals, significant recovery was observed using surfactin flotation, with 75.2% of Cu ²⁺ , 94.7% of Ni ²⁺ and 98.2% (MOL/MOL) of Co ²⁺ recovered into the foam overflow after 50 minutes of flotation.

The Cu ²⁺ ion removal of 75.2% approximates the levels of recovery seen with conventional surfactants such as SDS or saponin. The near complete extraction of Ni ²⁺ and Co ²⁺ under conditions similar to those used for conventional surfactants confirms the advantages of the disclosed method as a comparatively simple process which makes use of an environmentally benign collector.

Figure 12 illustrates the rate of ion concentration out of the solution into the foam overflow by plotting the concentration of metal ions remaining in the residual solution over the duration of the flotation process. The rate of removal of Cu ²⁺ metal ions is slower than that of Ni ²⁺ and Co ²⁺ , which both share a similar rate of removal. The slope of the Ni ²⁺ and Co ²⁺ curves flatten as the time approaches 50 minutes, meaning the rate of removal for those ions is slowing and removal is approaching completion. However, the rate of Cu ²⁺ ion removal did not appear to become slower, as shown by the fact that the Cu ²⁺ curve maintains a steady slope across the entire 50 minute flotation. This indicates that a longer flotation may allow for more complete removal of Cu ²⁺ ions, and that the lower removal efficiency for Cu ²⁺ is a result of a slower flotation process.

The fraction of water entrained in the foam and partitioned into the froth overflow, or water recovery, was 31.7%, 34.9%, and 39.3% respectively for the Cu ²⁺ , Ni ²⁺ , and Co ²⁺ solutions. The lower entrainment of water in the Cu ²⁺ ion flotation suggests that Cu-surfactin complex hydrophobicity may reduce the froth stability. Surfactin produced a fine wet foam similar to that produced by saponin biosurfactant collectors. A defrother may be advantageous to also use with surfactin to reduce water recovery in the overflowed foam fraction. Alternatively, a longer column may allow greater water drainage from the rising foam due to increased foam residence time. Effect of air flowrates

Figure 13 illustrates the impact of changes made to the rate of air flow. The graphs plot the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution (primary axis), the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation (primary axis), and the concentration factor of metal ions into the foam overflow fraction after ion flotation with surfactin at Q _a ir of 0.06, 0.08 and 0.1 L/min, respectively. The pH was maintained at 7 and the ion to surfactin ratio was initially 1 :3 in all cases.

An increase in the rate of flow of air through a flotation cell may cause an increase in metal ion removal. The results confirm that this was the case initially when increasing Q _a ir from 0.06 L/min to 0.08 L/min, as removal of Cu ²⁺ increased from 42.7% to 75.2%, removal of Ni ²⁺ increased from 80.1 % to 94.7%, and Co ²⁺ removal increased from 89.4% to 98.2%.

An increase in Q _air may lead to an increased quantity of bubbles, and hence a greater bubble surface area ascending through the flotation cell at any given time, assuming flowrates have little effect on bubble size. This greater bubble surface area may explain the increase in ion removal seen when Q _air was increased from 0.06 to 0.08 L/min. The increase in water recovery seen when Qair was increased to 0.08 L/min was also likely a result of increasing bubble velocity through the flotation cell.

A further increase of Q _air to 0.10 L/min had little impact on the extent of ion removal for both Ni ²⁺ and Co ²⁺ . However, this was likely as a result of the ion removal being near complete. As such, the percentage of Ni ²⁺ removed only increased by 1 .3% to 96.0%, and the removal of Co ²⁺ was still within 1 % of the removal at 0.08 L/min. This could indicate that as extraction of ions by ion flotation approaches completeness, the impact of increasing air flowrate decreases.

Figure 14 illustrates the effect of changing air flowrate on the rate of extraction during the flotation process. The graphs show the concentration of metal ions remaining in the residual solution during the ion flotation of a (i) Cu ²⁺ solution, (ii) Ni ²⁺ solution, and (iii) Co ²⁺ solution, performed at Qair of 0.06 L/min, 0.08 L/min and 0.10 L/min, respectively. The pH was controlled at 7 and the initial ratio of metal ions to surfactin was 1 :3 in all cases.

Figures 14 (ii) and 14 (iii) show that the flotation rate appears to have little dependence on the air flowrate, with the extraction at 0.08 L/min and 0.10 L/min occurring at similar rates for Ni ²⁺ and Co ²⁺ , and the rate of ion extraction only being slightly slower for both ions at 0.06 L/min. The notable decrease in ion removal seen for Cu ²⁺ ions can be explained by Figure 14 (i), which shows that the flotation of Cu ²⁺ ions at Q _air = 0.10 L/min effectively stopped after 30 minutes. At this point the froth was no longer stable and foam overflow ceased, which in turn suggests that all frothforming surfactin had overflowed at this point. This did not occur with the other metal ion solutions, and the extent of extraction was not as great as that achieved at flowrates of 0.06 L/min and 0.08 L/min, suggesting extraction of Cu-surfactin complexes was not complete.

The concentration factor is dependent not only on the extent of partitioning of ions into the foam fraction, but also the entrainment of water in the foam phase. With greater volumes of water entrained in the foam overflow, the ion concentration becomes more dilute. This is demonstrated in Figure 13 by the lower concentration factor seen in cases where the percentage of water in the foam overflow is higher. For this reason, the greatest concentration factor for all three metal ion solutions was achieved at the lowest air flowrate of 0.06 L/min, where the least water was recovered in the overflow, despite the ion removal being greater at higher air flowrates.

In view of these results, the preferred C for a flotation run can be selected dependent on the desired outcome of the run. Lower rates of air flow typically gave a greater concentration factor (and therefore a more concentrated metal product) at the cost of having a slower flotation process and, in some cases, lower extent of ion removal. Lower rates of air flow may accordingly result in reduced ion recovery but greater concentration in the foam overflow as a result of lower water recovery. If more extensive ion removal is required, higher rates of air flow may be more effective, although this comes at the cost of increased volumes of water becoming entrained in the foam fraction and overflowing, reducing the concentration factor. In cases where water is being treated to reuse or recycle, or if entrainment of other components in aqueous solution into the foam is undesirable, the flowrate can be adjusted to allow enough ion extraction without undesirably partitioning large volumes of water into the metal ion rich foam overflow phase.

Effect of solution pH

The chemistry and activity of surfactin in solution is greatly affected by solution pH. Below around pH 5, the surfactin is protonated and becomes neutral and unavailable for ion binding or froth formation, thereby simultaneously losing its water solubility and metal ion chelating ability. Basic pH resulted in the formation of hydroxide species of metals; however, metal species continued to be concentrated in the overflow even under basic conditions. This suggests that the surfactin collector may adsorb to neutral hydroxide species and allow flotation.

Figure 15 shows the impact of pH changes. The graphs plot the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution (primary axis), the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation (primary axis), and the concentration factor of metal ions into the foam overflow fraction after ion flotation with surfactin at pH 5, 7 and 10, respectively. C was maintained at 0.08 L/min and the ion to surfactin ratio was initially 1 :3.

The loss of water solubility and chelating ability is clear from the graphs. At pH 5 there is no extraction of metal ions from the solution following flotation with any of the heavy metals. No foam formation was observed due to the precipitation of the surface active surfactin from the solution. As a result, the entire water volume was present in the residual phase after sparging. Lower pH and higher concentration of H ⁺ ions may reduce recovery in ion flotation due to displacement of the colligend from complexes by H ⁺ . This is therefore not a symptom of the use of a biosurfactant but rather typical of ion flotation using anionic collectors. The protonation of the collector imposes a lower limit on metal recovery.

The greatest extent of ion removal was achieved at pH 7 for all three tested heavy metal ions. Thereafter, as pH increased to 10, the ion removal decreased notably for both Ni ²⁺ and Co ²⁺ , while it increased slightly for Cu ²⁺ . The lower ions removal is due to the hydrolysis of metal ions at basic conditions.

Figure 16 illustrates how, as the pH approaches 10, all metals begin to form neutral hydroxide species that are insoluble in aqueous solution. As these hydroxide species are neutral, it is expected that little interaction with the anionic surfactin will occur. The extraction seen at pH 10 may have been due to the entrainment of their hydroxide species in the foam. However, the concentration factors of the metals being between 1 .75 and 2.25 suggests that some selective flotation of the hydroxide species may have been occurring.

The results suggest that the operating pH of ion flotation with surfactin is more favourable near neutral pH, as acidic pH sees the precipitation of surfactin from solution without binding colligends, while basic pH leads to formation of neutral metal species that do not interact with the surfactin collector and thereby reduce the ion removal achievable by ion flotation. This means that in cases where aqueous solutions treated by ion flotation are at acidic or basic pH, some pH adjustment may be necessary in order to achieve maximum ion removal or concentration into the froth overflow.

Effect of metal ion to surfactin ratio

The volume of water entrained in the froth, or water recovery, is strongly dependent on the concentration of foaming surfactant in the solution. The influence of collector structure and frothing ability on water recovery in ion flotation is known, and increased frother dosage in conventional froth flotation leads to increased water recovery by increasing froth stability. Therefore, the increases in excess surfactin not involved in collecting available as a frother has a strong influence on froth stability and water recovery, where increasing the concentration of the surfactin available as a frother may be expected to increase water recovery.

Decreasing the concentration of surfactin to an equimolar concentration to the metal ions resulted in negligible froth formation, as a result of all surfactin being involved in metal binding and precipitation. This shortcoming can be addressed by providing an excess of surfactin to act as frother, or by employing one or more additional frothing or foaming agents to create and stabilise the foam. Increasing the concentration of surfactin to 10x that of the metal ions resulted in near complete removal of the Ni ²⁺ and Co ²⁺ ions, and greatly improved Cu ²⁺ recovery. However, water recovery in the overflowed foam fraction approximately doubled as a result of the excess surfactin.

Figure 17 illustrates the influence of changing concentrations of excess surfactin. The graphs plot the extent of (i) Cu ²⁺ , (ii) Ni ²⁺ , and (iii) Co ²⁺ ion removal from aqueous solution (primary axis), the distribution of water recovery in the overflowing foam phase and in the residual solution after flotation (primary axis), and the concentration factor of metal ions into the foam overflow fraction after ion flotation with surfactin at initial metal ion to surfactin ratios of 1 :1 , 1 :3 and 1 :10, respectively. C was maintained at 0.08 L/min and pH ratio was maintained at pH 7.

Typically, an increase in the ratio of collector to colligend results in improved ion removal. When the initial ratio was 1 :1 , the collector and colligend were present in the theoretical stoichiometric proportions, and as such there was little to no surfactin in excess that could act as a frother and form a stable foam. Without the formation of a stable foam or froth, the flotation process was inhibited and so there was negligible concentration of metal ions out of the solution.

When the metal ion to surfactin ratio was initially 1 :3, the water recovery in the overflow fraction for each metal ion solution was between 30% and 40%.

Figure 18 illustrates that a significant increase in excess surfactin present in the solution has little influence on the dynamics of the metal ion extraction. As seen in Figures 17 (i) and 18, however, by comparison with the removal of Ni ²⁺ and Co ²⁺ ions, there was an increased removal of Cu ²⁺ ions as the ratio was adjusted from 1 :3 to 1 :10 (in addition to surfactin concentrating the copper). This outcome can potentially be attributed to the increased water recovery at the ratio of 1 :10 ion to surfactin, leading to greater ion extraction as a result of ion entrainment in overflow water. It may also be a result of the excess surfactin providing a more stable foam. Cu-surfactin complexes may be more hydrophobic and agglomerate more strongly than those of Ni- and Co-surfactin. As such, Cu-surfactin complexes may destabilise foam more than the complexes of other metals, as hydrophobic complexes lead to bubble coalescence.

While the extraction of metal ions at the ratio of 1 :10 is closer to complete, with extraction of Cu ²⁺ , Ni ²⁺ , and Co ²⁺ reaching 94.4%, 99.7%, and 99.4%, respectively, the increase in water recovery in the overflow fraction results in a far lower concentration factor as the concentrate overflow is diluted. This means that in applications where nearly complete heavy metal extraction is necessary, a greater excess of surfactin may be desirable. However, in cases where a concentrated overflow fraction is desired, such as if the metal ions were to be recovered after flotation, it may be preferable to optimise the ratio of ions to surfactin to a point where a sufficient fraction of the metal ions are concentrated without encouraging a large water recovery. Alternatively, the flotation cell may be optimised by providing a taller column allowing more space for water drainage from the froth.

The results of this study demonstrate that surfactin has the capabilities for both collection (e.g., via chelation) and foam fractionation.

Those skilled in the art will appreciate that further improvements in the recovery of metal ions may be achieved by optimising the flotation cell to increase foam residence time or by introducing a defrother to reduce water recovery and allow greater concentration of metal ions in the overflow. Modifications of these types, amongst others, also fall within the scope of the invention.

The impacts of air flowrate, molar ratio, and pH as operating conditions provide insights into key industrial operating criteria. The results of the experiments indicated that greater air flowrates and higher concentrations of surfactin can improve recovery of metal ions from solution. Increasing the gas flow rate modifies the float by increasing the carryover of water into the overflow.

The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. For example, the method may be used to remove metal ions from any polar solvent and the pH of the polar solvent may be optionally adjusted to an acidity equivalent to a pH of 5 of an aqueous solution. An absolute or unified pH scale may be used in this regard to determine the appropriate pH value for polar organic solvents. It will further be appreciated that any metal or metalloid cation can be precipitated out of solution by carrying out the method described herein and that the higher the valency or charge of the metal or metalloid cation, the more lipopeptide precipitant must be added to the solution. Any amount of lipopeptide can be added to the aqueous solution, but to obtain the best recovery of the metal ions, a molar concentration of 1 :1 or above of lipopeptide precipitant to metal ion should be used. It was found that percentage recovery of the metal ions may stay constant at molar concentrations over 2:1 of lipopeptide to metal ions. The amount of lipopeptide biosurfactant added to the aqueous solution will depend on the sum of concentrations of all the different metal ions and/or metalloid cations present in the aqueous solution.

The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.

Finally, throughout the specification and accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.