Nanofiltration methods for removal of some organic compounds from waste waters

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1 Nanofiltration methods for removal of some organic compounds from waste waters ANDRA PREDESCU, ANDREI PREDESCU, ECATERINA MATEI, ANDREI BERBECARU, CRISTIAN PREDESCU Material Science and Engineering Faculty University Politehnica of Bucharest Splaiul Independentei Street, nr. 313 ROMANIA Abstract - The progress in science and advanced engineering at nanometer scale provides a range of unprecedented opportunities to develop more efficiently (in terms of costs) and ecologically the processes of water purification. This paper provides a brief overview of the use of advanced materials to purify water contaminated with toxic metal ions, radio nuclides, inorganic and organic substances. It is tried to highlight the opportunities and challenges of using nanomaterials in purifying surface water, groundwater and industrial wastewater. Figure 1 highlights the four classes of materials at nanometer scale, which are assessed as functional materials for purifying waste waters: metal oxide nanoparticles, carbon nanotubes, zeolites and dendrites. They have a wide range of physicochemical properties which make them particularly attractive to the separation of sewage. Key-Words: nanofiltration, wastewater, treatment, organic compounds, gas chromatography 1. Introduction The progress at nanometer scale in science and engineering suggests that many current problems involving water quality could be much improved or resolved using nanosorbants, nanocatalysators, bioactive nanoparticles, nanostructured catalytic membranes, nanoparticles of filtering among other products resulted from the processes of nanotechnology development. Having in mind the serious pollution problems confronting us and the issues that are raised by industrial wastes, taking into account the natural zeolitic abundance and their insufficient capitalization is required to avoid environmental pollution and limit its effects. Actions to prevent pollution and combat the effects of pollution are required to be coordinated in all countries, based on legislation designed to protect water resources of the country. Surface water is the main source for obtaining drinking water, but most of them are discharged wastewater. Surface waters should not contain harmless pollutants that can not be eliminated in water potabilization stations. For purifying industrial waste water are using processes appropriate to the nature and specific for wastewaters. In the advanced stage of purification of the waste water, ion exchange is the specific technological process. Besides the fact that the ionic exchange allows advanced purifying waste water in the process of regeneration, heavy metal ions can be retained in the form of useful products. Using natural zeolitic with ion exchange capacity sensitively enhanced by chemical processes in advanced filtering waste water containing heavy metal ions is an effective and perceptively method. The exhausted zeolites can be used for other purposes without producing environmental pollution. ISSN: ISBN:

2 Fig. 1: Researched nanomaterials for water purification 2. Special techniques for treating water In the process of separation, besides classical separation processes (distillation, rectification, extraction, ion exchange, filtration, centrifugation, sedimentation), has started a series of processes, known as membrane processes. Membrane processes have been known since the early 70s, a spectacular development, being used at industrial level in field such as waste water treatment, medical technology, and chemical industry. The quick and various evolution of these technologies has been possible due to putting up experimental techniques for preparation and characterization of membranes. A complex system consisting of a solvent in which are dissolved ion chemical species, molecule, macromolecule and dispersed macromolecule, molecular aggregates and particles, can be in components through membrane processes. Due to the wide range of their use are highlighted five important membrane processes (microfiltration, ultrafiltration, reverse osmosis, dialysis and electro dialysis) which cover the whole field of particle-size to be. Membrane processes allow the separation of dissolved chemical species, so division of homogeneous systems, being similar with the extraction, distillation or ion exchange. As seen from Table 1, microfiltration, ultrafiltration, nanofiltration and reverse osmosis have as driving force the pressure difference, naming them baromembrane processes. Baromembrane processes occupy first place in large industrial applications. These processes are usually submitted in the category of advanced filtering techniques. Thus, reverse osmosis is similar with a hiperfiltration by dehydration, ultrafiltration techniques like concentration, purification and fractionation of colloidal dispersions or macromolecules and microfiltration separation is devoted to suspension separation. Practically every membrane process may be in a viable alternative to other separation processes. ISSN: ISBN:

3 Filtration methodes for pollutant removal from waste waters Table 1 Method Separated phase type Porosity Selectivity origin Pressure gradient Unitary operation Clearing, separation Clearing, purification, concentration Purification, separation, concentration, demineralization Microfiltration Ultrafiltration liquid -liquid 0,1 10 µm 1 nm 1 µm Unequal particle dimension of the 1-3 bar 3-10 bar Nanofiltration < 2 nm Diffusion difference of the bar Gas filtration Gas separation gas - gas 0, µm 50 nm < 2 nm Unequal dimension of the particle to be Diffusion difference of the 0,1-5 bar 0,1-50 bar Separation, dust exhaust Separation, extraction, depolution 3. Nanofiltration (NF) Nanofiltration operates at lower pressure 3,5-15 bar and higher permeate recovery (80-90%). Nanofiltration membranes usually made of synthetic polymers, remove all insoluble particles present in the water supply with a series of inorganic dissolved salts and organic molecules. The application of membranes (NF) in the potable water treatment is designed especially for groundwater with total dissolved salts relatively small but shows a high hardness and a high quantity of coloured substances and THM precursors. The specific flow passing through the membrane depends on temperature, ph and concentration. The water passing through the membrane and containing rejected solutions, called concentrated, leaves the plant by reducing the relatively high hydraulic pression. The relatively high pression can change the polymer membrane structure which may affect also the specific flow passage. The efficiency of a nanofiltration system requires optimization of the permeate recovery and rejected solution. The regular operation of a nanofiltration system requires the optimization of the chemical pre-treatment and chemical cleaning frequency in order to minimize the pollution without affecting the production needs. Long-term performance of NF systems depends on a proper maintenance, the most important factor being the pretreatment. 4. Ultrafiltration (UF) Ultrafiltration differs from the nanofiltration in the way that the membrane pore size is about one order higher than the nanofiltration. Ultrafiltration has been used in the past for water treatment in manufacturing industries and wastewater treatment. The use of nanofiltration to obtain drinking water is a recent concept. The detection of some complex organic compounds is made by using the gas chromatography as detection method. 5. Gas chromatography method Gas-chromatography method (GC) uses a suitable detector, being one of the most sensitive detection methods and thus for the trace analysis of many substances, especially non-polar organic substances, such as chloroform, TCE (Trichlorethylene), PCE (Perchlorethylene) but also aromatic substances such as chlorobenzene is the best method. In most cases, it is used a flame ionization detector (FID), which for a trace analysis is not sensitive enough. For this purpose, ISSN: ISBN:

4 it is used a MS-sensitive detector (mass spectrometer), which is also universally applicable and often suites for the standardised method. An Electron Capture Detector (ECD) is suitable especially for the detection of nitro aromatics, particularly chlorinated hydrocarbons. With such a detector can be determined the concentrations of TCE and PCE. How our own measurements showed and after a confirmation from the Konik Firm, is suitable an ECD for the determination of PCE and TCE. A disadvantage of the gas-chromatography is that next to the method even also the sample preparation is quite costly. Most GC divided columns are unfit for the injection of aqueous solutions so that is necessary a transfer of the substances into an organic phase. This is achieved in the simplest case by extraction. It is usually selected a volume ratio of about 10:1 in favour of the aqueous phase, so that it is done a concentration of substances to be determined. An even greater enrichment of up to a factor of 250 is achieved by solid phase extraction (SPE). For the extraction of TCE and PCE has been used n-hexane as solvents, toluene proved to be unsuitable. The sample volumes were 1 to 3ml, after every three extraction of 3 to 8ml with n-hexane and drying with sodium sulfate, and then the solutions were filled up to 10 or 25ml. The GC analysis was equipped with a gas chromatograph HRGC 4000 B Konik company. As make-up gas and the cooling of the injector was used N2, and H2 as carrier gas with a flow rate of 13 and 1.3 ml / min. The sample injection was done through a 10ml Hamilton syringe with V = 4ml in the form of an injection on Column to fuse silica capillary column DB5. The program temperature was adjusted at: T 1 = 37 C, t 1 = 3 min, R 1 = 7 C / min, T 2 = 70 C, t 2 = 3 min, R 2 = 20 C / min, T 3 = 200 C, t 3 = 3 min, R 3 = 20 C / min, T 4 = 240 C, t 4 = 15 min. The retention times under these conditions were 1.8 min for TCE and 2.6 min for PCE. Fig. 1: GC / ECD calibration curves for TCE and PCE in n-hexane with V=4l In fig. 1 are presented the GC / ECD calibration curves of TCE and PCE. Due to the measured concentration range of almost five sizes (10 ng / l to 1mg / l = 0.1nM to 5mm), was chosen a double logarithmic display. The concentrations are in mg / l given, because these investigations have been carried out under the conditions of water cleaning and the water maximum allowable concentration of organic substances is 10mg / l. How it is shown in fig. 1, the results of the double logarithmic application point each approximately a straight line. The curves can be as a whole but can also be described by a polynomial of 2nd and 4 grades. For TCE is apparently the detection limit at 1mg / l reached, while for PCE is also a ten power lower concentration recorded. Due to the lack of theoretical model for the calibration curves, were found to be determined TC and PCE concentrations graphically, using only the straight line and curves. Fig. 1 shows that with the GC / ECD analysis, the organochlorine hydrocarbons very sensitive concentrations as TCE and PCE are very sensitive can be measured, so that ISSN: ISBN:

5 the control of the maximum allowable concentration of 10 mg / l is quite possible. Through a concentration of the sample can even quantities in ppt range (ng/ l) determined. However, it should be remembered that at such low concentrations with impurities, can occur large measurement error. A problem of gas chromatography is that strong polar substances hardly grasp leave, since they are already in the sample even when using a very polar solvent such as ethyl which is poorly extractable. This is especially important for the degradation products, when it should handle with substances with a higher number of hydroxyl groups or even to carboxylic acids. The determination should be directly from the aqueous phase, for instance through ion chromatography, which is, however significantly less sensitive than GC / MS analysis. One possibility to determine polar substances with higher sensitivity is gas chromatographic method using a water-resistant column. 7. References [1] Obare S.O. & G.J. Meyer - Nanostructured materials for environmental remediation of organic contaminants in water. in J. Environ. Sci. Health A. 39(10), 2004, pp [2] Meyer D.E., K. Wood, L.G. Bachas & D. Bhattacharyya - Degradation of chlorinated organics by membrane-immobilized nanosized metals, in Environ. Prog. 23(3), 2004, pp [3] Rodny Penafiel Ayala - Photokatalytische Behandlung von biologisch schwer abbaubaren Wasserverunreinigungen mit Titandioxid und simuliertem Sonnenlicht, Technischen Universität Berlin, Berlin 2002 [4] Roshan Guttikonda - Nanotechnology and clean water ECG 653 Project Report, 2006 [5] X X X - Zukunftspotenziale der Mikro-und Nanotechnologie als Sclusseltechnologie fur die Umwelttechnik in Baden-Wurttemberg, Ministerium fur Umwelt und Verkehr, Stuttgart, Germany, Conclusions The waste water treatments are used for purifying and protection of environment and the ion exchange method is one of the advanced technological processes. In addition, in the process of separation, besides classical separation processes (distillation, rectification, extraction, ion exchange, filtration, centrifugation and sedimentation) has been developed a series of processes, known as membrane processes. The application of membranes for water treatment is designed especially for groundwater with total dissolved salts relatively small but shows a high hardness and a high quantity of coloured substances and THM precursors. The gas-chromatography method (GC) is used as detection method for the trace analysis of many substances, especially non-polar organic substances, such as chloroform, TCE (Trichlorethylene), PCE (Perchlorethylene) but also aromatic substances such as chlorobenzene is the best method. The best detector is an electron capture detector (ECD). The organochlorine hydrocarbons very sensitive concentrations as TCE and PCE are very sensitive, so that the control of the maximum allowable concentration of 10 mg / l is quite possible. ISSN: ISBN: