Removal of metallic anions from dilute aqueous solutions by polymer–surfactant aggregates
Graphical abstract
Introduction
Metallic anions are widely used in the plating and pigment industries. Like metal cations (e.g. chromium and iron), metallic anions can also be present in a stable and multivalent form in aqueous solutions. Examples of metallic anions include oxyanions and metals bound with cyanide ions, such as CrO42 −, Cr2O72 − and Fe(CN)63 −. They are usually found at dilute concentrations within a wide range of solutions, and can accumulate in ecological systems causing health disorders. Their presence in aqueous effluents has thus attracted a great deal of attention with respect to the development of mitigation techniques.
All metallic anion removal techniques have inherent advantages and limitations in their various applications, depending upon the characteristics of the wastewater and the treatment requirements. Ion exchange is highly effective in the removal of small amounts of contaminants at high concentrations [1], [2], [3], but the cost and secondary pollution when regenerating the resin are critical limiting factors. Thus, it is not economically feasible to treat large amounts of dilute metallic anion wastewater. Electrochemical techniques are regarded as rapid and well-controlled methods to remove both metal cations and anions. They are also associated with fewer chemical additions and less sludge production [4]. The drawbacks are high capital and running costs. Adsorption is an alternative method to treat dilute systems, but achieving a balance between the cost and effectiveness of the physico-chemical adsorbent remain a challenge [5], [6], [7]. Biosorption has proven to be a promising and sustainable removal method. It is advantageous in that it is able to treat a large capacity of wastewater and that it produces a concentrated sludge. However, the capital and maintenance costs are high [8], [9]. Finally, the application of membrane filtration technology is an effective treatment method, but high capital and operating costs, membrane fouling and low permeate flux are limitations [10], [11], [12], [13], [14], [15]. Therefore, treating dilute anion contaminated aqueous streams remains as a challenge with no clear and effective solution.
In response to the unsolved challenge and to fill the resulting niche, polymer–surfactant aggregates have already been applied for the efficient removal and recovery of cations from dilute aqueous streams [16], [17]. This process uses a structure called a polymer surfactant aggregate (PSA), which is formed under a specific range of dosage ratios between oppositely charged polymer and surfactant ions [18], [19]. The application of polymer and surfactant in combination results in the removal of heavy metal ions from dilute aqueous solutions, and surface tension measurements have been used to show that the PSA is indeed responsible for removing the metal ions [16]. Measuring the electrical conductivity of a solution containing polymer and surfactant is one of the common methods used for investigating the interactions between them in the bulk solution [20], [21]. For example, the critical micellar concentration (CMC) of a surfactant can be measured by the break point in the increase of conductivity of the surfactant aqueous solution with concentration [22]. The PSAs form at low surfactant and polymer concentrations (˂250 ppm cationic surfactant; ˂100 ppm anionic polymer) as a result of electrostatic and hydrophobic interactions, and contain both positive and negative charges [23], [24], [25], [26]. In the removal process, individual nano-scale PSAs with a high surface–volume ratio bind to dilute anions via electrostatic and chelation forces. While binding with the anions, and due to having both positive and negative charges, they also associate inter-molecularly with each other. This causes charge neutralisation and leads to the formation of large flocs which precipitate out and settle under gravity. After allowing the solution to settle or after coarse filtering, the PSA-anion precipitates can be recovered; with further treatment, the PSA precipitates may be recycled, and the anions are recovered in a concentrated form. The polymer surfactant aggregate technique has the benefit of using a small amount of recyclable substrates to remove anions from dilute aqueous streams and does so without the need for expensive processing.
In this paper, the change of conductivity divided by the change in surfactant concentration has been studied as a means of investigating the correlation between PSA formation and anion removal. The effects of charge density of the polymer on the treatment performance, and the kinetics of the process as a function of polymer, surfactant and anion are also studied. Finally, the effects of the separation method, pH, temperature, and the presence of salt and organic contaminants on the anion removal efficiency are investigated.
Section snippets
Materials
Poly(acrylic acid) (PAA: (C2H3COOH)n) solutions were prepared by diluting stock PAA solutions (Sigma Aldrich, average MW < 100,000, 35 wt.% in H2O). Poly(sodium 4-styrenesulfonate) (PSS: (C2H3C6H4SO3Na)n) (average MW 1,000,000), sodium dodecyl sulphate (SDS: CH3(CH2)11OSO3Na) (purity ≥ 99.9%) and myristyl trimethyl ammonium bromide (MTAB: CH3(CH2)13N(CH3)3Br) (purity ≥ 99%) were obtained from Sigma Aldrich, and used directly without further purification. Potassium chromate (K2CrO4), potassium
Studies of conductivity measurements
The electrical conductivity of the PSS–MTAB system is studied to investigate directly the polymer–surfactant interactions in bulk solutions. The PSS–MTAB system contains anionic polymer and cationic surfactant to form PSAs for the removal of metallic anions. When the conductivity increases with increasing concentration of pure MTAB, the CMC is found as the transitional point of the slope, but this transitional point can be difficult to locate. Thus, some researchers have tried various ways of
Conclusions
A novel effluent treatment process is reported which use uses polymer–surfactant aggregates (PSAs) to remove anions from dilute aqueous solutions. This process uses PSS or PAA as an anionic polymer and MTAB as a surfactant to form micelle-like aggregates on the polymer backbone, and then adsorb free multivalent anions, such as Fe(CN)63 − and CrO42 −, onto the aggregates. At the same time, the anion-bound PSAs flocculate to form larger flocs under the hydrophobic and electrostatic forces. These
Symbol glossary
- CMTAB filtrate
the concentration of MTAB in the filtrate (unit: mM)
- CPAA filtrate
the concentration of PAA in the filtrate (unit: ppm)
- Carbon contentMTAB
molecular weight of carbon in MTAB = 204 g/mol
- Carbon molecular weight by percentagePAA
the percentage of carbon in the molecular weight to the total molecular weight of each segment of PAA = 0.5
- CFC
critical formation concentration
- CMC
critical micellar concentration
- MTAB
myristyl trimethyl ammonium bromide
- PAA
poly(acrylic acid)
- PSA
polymer surfactant aggregate
- PSS
Acknowledgements
We acknowledge the support of the Singapore–Peking–Oxford Research Enterprise which is a collaborative scholarship funding scheme between the National University of Singapore, Peking University and the University of Oxford and provided L.C. Shen with a DPhil scholarship.
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