Volume of algal amendment (mL)
4. Research on the application of Nanofiltration 1. Introduction
4.7. Discussion and conclusions
The initial screening experiments with the batch cell showed that the type of separation achieved for the UME was very dependent on the membrane type. The two membranes chosen for the cross-flow cell work showed good potential for the separation of sulphate, manganese, uranium and radium. The performance was influenced by a number of factors, including membrane charge, ion speciation, and the operating conditions of the cross-flow rig.
4.7.1. Membranes and ion speciation
The rejection levels achieved for all the multivalent cations in solution were generally above 95%. The Desal DK and Nanomax 50 membranes gave very similar results for these species,
with differences between the two less than half a percent with few exceptions. For these solutions there were, however, differences observed for the rejection of other cations. The rejection levels of sodium and potassium from the UME solution were lower for the Nanomax 50 membrane by around 20%.
For raffinate, there was some difference in the rejection results for phosphorus and silica, which may be due to a slight difference in the membrane pore size. In the raffinate at pH 1.2, the speciation of these two elements is probably H3PO4 and Si(OH)4. Phosphoric acid has a pKa of 2.13, so the ratio of H3PO4:H2PO4- is around 11:1. Since both H3PO4 and Si(OH)4
have no net charge in solution, rejection takes place on the basis of size. Slightly larger pores in the Nanomax 50 membrane would result in lower rejection levels for the neutral species, although both species were retained over 90% for both membranes.
Rejection levels differ between the UME and raffinate, due to the differing charge on the membrane. Cation rejection was enhanced in the raffinate filtration at pH 1.2, for the same pressure, due to the high repulsive forces between the ions and the positively charged membrane surface. This effect counters the decrease in rejection that would be expected due to the significantly higher concentrations in the raffinate solution. Rejection of sulphate was much lower in the raffinate filtration as a result of a number of factors. Firstly, the membrane charge in the UME filtration is negative, and the electrostatic repulsion between the membrane and the sulphate ions ensures high rejection of these ions, while in the raffinate solution, the membrane is positively charged. The speciation of the sulphate changes as the pH changes. At neutral pH, the sulphate will be present as SO42-, while at pH 1.2 the ratio of SO42-: HSO4- is around 1:6.6, and hydrogen ions, which permeate the membrane more easily than metal cations, perform the role of counter ions for these anions. The higher concentration in the raffinate solution also leads to lower rejection.
4.7.2. Operating conditions
The driving force for the solute across the membrane has two components: convective (solute carried through by the solvent flux) and diffusive (solute diffusing through the pores of the membrane. At low fluxes, that is, low pressures, the diffusive component, which is driven by the concentration gradient, increases relative to the convective component. This results in lower solute rejections.
Increasing operating pressure and cross-flow velocity resulted in an increase in the rejection levels in most cases, although the dependence on pressure was less for the species in the UME. The effect of pressure was most apparent in the sulphate rejection data for both solutions. There was a higher dependence of rejection on pressure for the raffinate solutions, than for the UME solution, and the rejection at the lower pressures was much lower for sulphate in the raffinate. As the sulphate concentration is much higher in the raffinate, it would be expected that the diffusive component of the solute permeation driving force would be proportionately larger for the raffinate than for the UME. This would lead to the lower rejections observed at lower pressure for the raffinate, and also to the higher dependence of rejection on operating pressure as the concentration driven diffusive transport mechanism is reduced relative to the convective component. Increasing rejection at higher fluxes (that is, at higher pressures) is generally reported. However, increasing flux through the membrane promotes concentration polarisation. For this condition and low cross-flow velocities, low rejection of ions is observed, which decreases with increasing pressure (Afonso & de Pinho, 1998). The rejection of acid by Desal DK membranes shows a decrease in rejection from 1000 kPa to 1300 kPa for all flow rates.
There are two mechanisms by which increasing concentration at the membrane surface, induced by concentration polarisation, affects solute rejection. As the concentration increases, the electrostatic forces between the membrane and the ions in solution become weaker. Also, the concentration driving force for solute transport increases, increasing the diffusive component of the transport through the membrane. Both of these can lead to decreased solute rejection. The increased osmotic pressure may also lead to lower permeate fluxes.
Concentration polarision may be minimized by enhancing the mixing in the cell, by increasing the flow rate or modifying the feed spacer, to promote turbulent flow. Variation of the rejection levels with cross-flow velocity is most apparent in this work at lower pressures, that is, where the rejections are lower.
The separation properties of these membranes for metal ions in these solutions are good and the low acid rejection, coupled with high cation rejection under acid conditions, shows great potential for acid recovery from process streams. Further work needs to be carried out to determine the long-term membrane performance, water recovery and fouling characteristics in order to properly evaluate nanofiltration as a treatment method.
ACKNOWLEDGEMENTS
The Scenedesmus and Carteria algae were isolated by Karyn Wilde and identified by John Ferris. The technical assistance of Karyn Wilde is gratefully acknowledged. Sulphate determination by Ion Chromatography was conducted by Andrea Li, Grant Spindler and Agness Knapik with supervision from David Hill. A large portion of this work constituted part of a collaborative project lead by Dr. David Jones of CSIRO, and was funded by Energy Resources of Australia Ltd.
REFERENCES
[1] TAPSELL, G., RING, R.J., COLLIER, D., Treatment of uranium waste liquors by the high density sludge process, Australian Chemical Engineering Conference (Chemeca 98), Port Douglas, Qld, Australia, 28–30 September, (1998).
[2] KOSTENBADER, P.D., HAINES, G.F., High density sludge treats acid mine drainage.
Coal Age, September, p. 90–97, (1970).
[3] KUIT, W.J., Mine and tailings effluent treatment at the Kimberley, B.C. operations of Cominco Ltd. CIM Bulletin, 73 (824), p. 105–112, (1980).
[4] DEMOPOULOS, G.P., ZINCK, J.M., KONDOS, P.D., – Production of Super Dense Sludges with a Novel Neutralization Process, Can. Waste Process – Recycl. Miner.
Metall. Ind. II, Proceeding of International Symposium, 2nd, 401–11 (1995).
[5] ATLAS, R.M., Handbook of Microbiological Media. CRC Press, Tokyo, (1993).
[6] HEDIN, R.S., Passive mine water treatment in the Eastern United States. In: Minewater Treatment using Wetlands. P.L. Younger (ed.) Proceedings of a CIWEM National Conference, 5 September, University of Newcastle, Newcastle Upon Tyne NEI 7RU, (1997).
[7] HEDIN, R.S., HAMMACK, R., HYMAN, D., Potential Importance of Sulphate Reduction Processes in Wetlands Constructed to Treat Coal Mine Drainage. In:
Proceedings of the International Conference on Constructed Wetlands, Hammer D.A.
(ed.). Lewis Publishing, Ann Arbor, MI, U.S.A. pp 508–514, (1989).