Metallic anion recovery from aqueous streams and removal agent recycle in the polymer–surfactant aggregate process
Graphical abstract
Introduction
As a result of industrialisation, the use of metallic anions has increased dramatically. Chromate, as one such example, is commonly used in the pigment and painting industries. Some of the effluent containing chromate will contaminate water bodies following discharge and is usually present in a low concentration. However, it can accumulate in living organisms; in the case of the human body, serious health disorders can arise. Apart from contaminating the water body, this also represents a waste of valuable resources. Thus, a separation and purification process is needed to both treat the industrial effluent and turn the waste into a valuable resource. However, treating a dilute wastewater in a cost effective manner still remains as a challenge.
To overcome this challenge, the polymer–surfactant aggregate process has been developed and applied to remove metallic anions from dilute aqueous solutions [1]. This process uses a anionic polymer, such as poly(acrylic acid) (PAA), and a cationic surfactant, such as myristyl trimethyl ammonium bromide (MTAB), to form micelle-like aggregates on the oppositely charged polymer chains at critical aggregation concentration, thus forming polymer–surfactant aggregates (PSAs) [2], [3], [4]. The PSA has two main characteristics: nano-meter size and containing both positive and negative charges. The nano-meter size PSA has a high surface to volume ratio, so it is effective in binding metallic anions from dilute effluents. When metallic anions are bound to PSAs, because they contain both positive and negative charges and because of the bridging effects from anions, these metallic anion loaded PSAs also associate with each other and eventually form flocs. The flocs can be separated by coarse filtration or gravity settling which obviate the need for an ultrafiltration membrane to separate metallic anion loaded PSAs from solution. Previous results show that 99% of 0.1 mM Fe(CN)63 − and 80% of 0.2 mM CrO42 − are removed under the optimum dosages in the presence of inorganic and organic contaminants [1]. In previously published work, the polymer–surfactant aggregate process has been applied to remove and recover heavy metal cations from dilute aqueous solutions [5], [6]. The cationic removal process is similar to that for the anionic removal process, except that it uses polymer and surfactant with exactly reversed charges due to the reversed charge of the target ions [1], [6], [7]. In the cation recovery stage, acid leaching is applied to recover and concentrate the bound heavy metal cations, and then basification is used to recover the removal agent for recycling [5].
The conventional physico-chemical method to treat chromate is to reduce Cr(VI) into Cr(III) using a reducing agent such as sodium bisulphite (NaHSO3) and then to increase the pH to alkaline conditions to precipitate the Cr(III) as Cr2O3 or Cr(OH)3. This method generates a large amount of Cr(III) contaminated sludge, and does not allow chromium recovery. Various treatment processes have been applied to recover chromate directly from aqueous solutions, such as adsorption [8], [9], solvent extraction [10], [11], ion exchange [12], [13], nanofiltration [14] and electrodialysis [15]. The following are examples for each process. A reusable adsorbent, ethylenediamine grafted poly(glycidyl methacrylate-co-methyl methacrylate) (poly(GMA-co-MMA)), was investigated to remove chromate under acidic conditions. The results suggested that, at the optimum pH of 2, > 95% of 100 ppm chromate was removed by the polymeric adsorbent [16]. The adsorbent was regenerated using 0.1 M NaOH and/or 2 M NaCl, and reused for 5 cycles with an only 3% deterioration in the adsorption capacity [17]. A sequence of non-dispersive solvent extraction and ion exchange was proposed to treat chromate in a high concentration. The non-dispersive solvent was used to recover the high concentration of chromate, and then ion exchange was used to meet the discharge limit of 0.5 ppm [18]. A two-stage electrodialysis process using a monovalent selective membrane was applied to purify chromate from electroplating effluents. The first step was to remove all the monovalent ions from the acidic effluents containing chromium, which was in the monovalent form (HCrO4−). The second step was to increase the pH of the permeate from 2.2 to 8.5 to transform HCrO4− into CrO42 −, so that the other monovalent ions can pass through the membrane, while the purified and concentrated CrO42 − is retained. It was reported that chromate was concentrated by a factor of 191% [19]. The cost of adjusting pH for a large volume of effluent, the cost of ion exchange membranes and the problem of membrane fouling may be the barriers for industrial applications. In short, it seems that strongly alkaline or saline solutions are commonly used in the chromate recovery process, either for regenerating the adsorbents and ion exchange resins or for changing the chemical formulation of chromium.
In the present work, a ‘mirror image’ of the cation recovery process is developed and applied. It uses basification to recover and concentrate the metallic anions, and then uses acidification to recover and recycle the polymer and surfactant. The concentrated anions may be considered to be a valuable resource to recycle back into the manufacturing process stream. The regenerated polymer and surfactant are recycled to the next cycle, with a small addition of surfactant to makeup the surfactant leakage at the removal stage. The whole anion treatment process is presented, and the important process variables including pH, concentration factor and residence time are investigated and optimised. Finally, the treatment theory, performance, procedure and application for the cation and anion treatment processes are compared and contrasted.
Section snippets
Materials
Poly(acrylic acid) (PAA) solutions were prepared by diluting stock PAA solution (Sigma Aldrich, average MW < 100,000, 35 wt% in H2O). Sodium dodecyl sulphate (SDS) (purity ≥ 99.9%) and myristyl trimethyl ammonium bromide (MTAB) (purity ≥ 99%) were obtained directly from Sigma Aldrich. Potassium chromate and potassium chloride were purchased from Fisher Scientific (all purity ≥ 99%). Sulphuric acid (ACS reagent, 95–98%) and sodium hydroxide (reagent grade, ≥ 98%, pellets) were obtained from Sigma
Overview of the whole anion treatment process
A flow sheet of the whole metallic anion treatment process is showed in Fig. 1, including the three stages: removal, recovery and recycle. The chromate is used as an example metallic anion in the process. At the removal stage, which is reported in the previous contribution [1], the PAA and MTAB are added at the optimum dosage ratio (100 ppm PAA and 2.5 mM MTAB) to bind the dilute chromate ions, thus forming yellow flocs. The flocs are then separated from the treated water by a coarse filter,
Conclusions
A new application of the polymer–surfactant aggregate process was previously presented to remove metallic anions from dilute aqueous solutions [1]. According to the recovery method for the cation treatment process [5], a parallel metallic anion recovery and polymer–surfactant recycle method has been developed. This method uses a small amount of base solution to recover the anions from the flocculated PSAs, and then coarse filters separate the highly concentrated anion solution from the
Acknowledgements
We acknowledge the support of SPORE (the Singapore-Peking-Oxford Research Enterprise), a collaborative DPhil scholarship funding scheme set up between the National University of Singapore, Peking University and the University of Oxford. We also thank Prof Robert Thomas and Dr. Peixun Li at the University of Oxford for their valuable and helpful advice.
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