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Removal of heavy metals from waste water using a hybrid membrane process

Sabo S., Bakalar T., Bugel M., Pavolova H.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

130--131

EN

Soil and water pollution by heavy metals is currently a very important problem in environmental and other sectors. The origin of metals in water may be of either natural character (erosion of rocks and sediments, leaching of mineral resources) or anthropogenic character (mining and extraction of metals, industry, agriculture, etc.) (Martins et al. 2010). Many heavy metals, such as Pb, Cd, Cu, Zn, etc., are the most polluting factors in industrial wastewater and may get into the ground water. Subsequently they are bio-accumulated in living organisms and cause various diseases and disorders (Jamil et al. 2010). These problems need to be responded by developing new and more efficient methods for waste-water treatment (Martins et al. 2010). In practice several methods are used for removal of heavy metals from water. One of the promising methods for the removal of metal ions from water is a hybrid membrane processes. This method includes two processes – adsorption of metal ions on the natural zeolite and microfiltration of zeolite suspension through ceramic membrane. Experiments were carried out using model solutions containing Cu2+ions (from CuSO4∙5H2O and Cu(NO3)2∙3H2O) a nd Zn2+ions (from ZnSO4∙7 H2O and Zn(NO3)2∙6H2O).In the experiments zeolite from Nižný Hrabovec localization (Zeocem JSC Bystré), Slovakia was used. Zeolite is mainly composed of mineral clinoptilolite (84%), other mineral are cristobalite (8%), clay (4%) and plagioclase (3–4%). Its structure is formed by three-dimensional network. Clinoptilolite is composed from silicate tetrahedron SiO44−bound together by oxygen atoms, where part of the Si atoms is replaced with aluminum AlO45−. This creates space structures with a number of cavities and channels, in which are accommodate metal cations and water molecules. The total volume of cavities is 24–32%. Zeolite has a bulk density of 1600–1800 kg∙m−3, a specific gravity of 2200–2440 kg∙m−3and the specific surface of 30–60 m2∙g−1 (www.zeocem.com). Adsorption experiments on model solutions were performed with the zeolite with particle size 20 microns. Before and after the experiments the concentrations of Cu and Zn were determined by atomic absorption spectrophotometry (AAS) using iCE 3300 AA Spectrometer Thermo Scientific. Solutions with concentrations of 10–5000 mg∙L−1 were prepared from each of the chemicals. The solutions were shaken with 1 g of zeolite in 100 ml PET flasks on a shaker for 2.5 hours at 25°C and 220 rpm. The amounts of metals (Cu or Zn) in solutions were measured after stabilization, filtration and required dilution by AAS. The equilibrium between the concentration in solution and the adsorbed substance was evaluated using Langmuir, Freundlich and Redlich-Peterson models. According to the results of the adsorption experiments zeolite adsorbed of both the nitrates ions (Cu2+and Zn2+) (equilibrium concentrations 1.48 mg∙g−1and 1.49 mg∙g−1, respectively) best and the sulfate ions (0.34 mg∙g−1and 0.85 mg∙g−1, respectively) less. Due to better adsorption capacities of zeolite for ions derived from nitrates, further experiments were made from chemicals Cu(NO3)2∙3H2O and Zn(NO3)2∙6H2O. The next step was microfiltration of suspension of zeolite since a hybrid process for removal of Cu and Zn ions was used. Microfiltration was carried out at a constant pressure of 50 kPa, the f low rate of suspension 2.2 m ∙ s−1 and various concentrations of zeolite (1–6 g ∙ L−1). Tubular Membralox ceramic membrane was used, with length of 25 cm, internal diameter of 0.5 cm, external diameter of 0.7 cm and porosity of 50 nm. Active surface of the membrane was 49.48 cm2. Zeolite was added to the solution of ions circulating in the cross-flow microfiltration system. Metal ions were adsorbed by zeolite and the suspension was filtered by the membrane. The adsorbed metal remained circulating in the system and the permeat was purified water. Using this method, at suitable selection of the experimental conditions, up to 90–100% of metal ions can be removed from the solution.

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Influence of zeolite suspension concentration on microfiltration characteristics

Bakalar T., Sabo S., Bugel M., Loch B.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

59--60

EN

Crossflow microfiltration is a very effective and energy efficient separation method allowing separation of very fine particles from liquids. It is mainly used for separation of particles from 0.1 to 10 microns. These membrane processes are used for separation of solids from liquids in pharmaceutical, chemical, food, and dairy industries as well as in environmental protection and water treatment (Fadaei et al. 2007, Chellam et al. 2011). Ceramic membranes are often used for treatment of water. These membranes are preferred for their higher chemical, mechanical, and thermal resistance compared with organic membranes. Membranes form a physical barrier to the sludge, bacteria and suspended particles. Microstructural parameters as pore size, pore density, and porosity of the membrane have a great influence on the permeate f lux (Ogunbiyi et al. 2008, Altunkaynak et al. 2010). Zeolites occur in a number of variations such as clinoptilolite and chabazite. Clinoptilolite is the most abundant zeolite and is easily available in more than 40 varieties. Natural and also synthetic zeolites have unique physical, chemical and structural properties. Therefore zeolites are widely used in technological, environmental and agricultural processes. One of the most studied zeolites, clinoptilolite showed the highest selectivity for some heavy metals ions such as Pb2+, Cd2+, Zn2+ and Cu+ Babel et al. 2003). In the experiments natural zeolite was used from Nižný Hrabovec in the Slovak Republic. The structure is composed from three-dimensional grid, the main mineral is clinoptilolite. Clinoptilolite is composed of a tetrahedron (SiO4)4- connected by oxygen atoms, and a part of the silicon atoms is replaced with aluminum atoms. The total volume of the cavities of the zeolite is from 24 to 32%. The volume density is from 1600 to 1800 kg∙m-3, the density from 2200 to 2440 kg∙m -3 and the specific surface is from 30 to 60 m 2∙g -1 (www.zeocem.com). For the experimental measurements special laboratory microfiltration apparatus was used. The filtered suspension was pumped using a membrane pump from a reservoir (4 liters volume) into membrane module, in which the ceramic membrane with porosity of 50 nm is placed. Magnetic flow meter measured the flow of the suspension. The values of the input and output transmembrane pressure were recorded from gauges. From the membrane module permeate flowed to collecting bottle. The collecting bottle was placed on a labbalance. The monitored data (pressure, suspension flow rate, permeate flow and temperature) were processed by a computer program. The temperature of the suspension was maintained at 25°C, the pressures were in the range from 40 to 100 kPa and the flow rate of the suspension was 2.2 m∙s-1. The measurements were focused on determination of the stability of the microfiltration system using different pressures and different concentrations of suspensions of zeolite. Experiments were performed to determine the permeate fluxin the microfiltration of suspensions of zeolite at concentration of 3 g∙L-1, 6 g∙L-1 and 9 g∙L -1, at a constant pressure of 100 kPa and a constant rate of suspension flow of 2.2 m∙s-1. After stabilization of the system, and after the addition of a given concentration of zeolite to the apparatus in all three cases it is possible to see a decrease in membrane flux to a value of about 310 L∙m-2∙h-1. However this flux has not remained constant, but in the course of the experiment continued to decrease to a final value of 280 L∙m-2 h-1. This decrease indicates that under these conditions there is some fouling of the membrane at relatively low concentrations of the zeolite suspension. Due to membrane fouling experiments with gradual increasing and then decreasing of pressures were performed. In the experiments at a constant suspension f low rate of 2.2 m∙s-1 and a constant concentration of zeolite suspension of 3 g∙L-1 the pressure in the apparatus was gradually varied from 40 to 70 kPa and then back to 60 kPa and 50 kPa. For each of these pressures the system was allowed to stabilize for 30 min. From the comparison of the levels of flow rates at 60 kPa and 50 kPa it may be seen that in both cases a decrease in flux occurs after reduction of pressure. Therefore, it can be concluded that in the microfiltration experiments with the membrane and zeolite used it is preferably to use lower operating pressure of 70 kPa. In the following experiments therefore the pressure of 50 kPa was used. These experiments were carried out at constant pressure and constant rate of 2.2 m∙s-1. Concentrations of zeolite were varied by 1 g∙L-1 to a final concentration of 30 g∙L-1. The system was stable at low concentrations of zeolites and there was no initial decrease of flow. In the experiments with higher zeolite concentrations has also been shown that even at the highest concentration of used zeolite suspension of 30 g∙L-1 there was no decrease in membrane flux. This flux was approx. 150 L∙m-2∙h-1 and further did not decrease.

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Use of bentonite for MSWI fly-ash stabilisation

Bakalar T., Kozakova L., Zelenak M., Sabo S.

Geology, Geophysics and Environment

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2014

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Vol. 40, no. 1

78

EN

Fly-ash produced during combustion processes is usually considered to be hazardous waste. Therefore, its effective treatment is necessary. Stabilization of hazardous waste by solidification is one of the effective methods of immobilization of pollutants. A solid structure resistant to leaching is created by stabilization. The created stabile structure has to resist the influence of natural factors such as rainfall causing possible leaks of pollutants in the form of leachates. Having fulfilled the required properties, the stabilized structure can be used as raw material in the construction or reconstruction of pavements as well as in the construction industry. All around the world, the stabilization processes are used for solidification of municipal solid waste incineration (MSWI) including fly-ashes. The MSWI plant in Kosice is planning to introduce a technology of solidification as a possible treatment stage before deposition in a landfill. In the process of stabilization/solidification, several ingredients to achieve the desired result are used. The created solidified structures must be characterized by sufficient compressive strength with a minimum probability of release of pollutants in the event of disruption of its structure. The basic ingredients are water, cement, and other natural materials, such as bentonite. Bentonite is a residual clay, which was created by mechanical and chemical weathering of parent rock in an alkaline medium, mainly volcanic tuffs, rhyolites, basalts, andesites and other predominantly Tertiary rocks. It is plastic rock that has a high sorption capacity. Its chemical and mineral composition depends on the bearing formation. Bentonite contains mainly montmorillonite. Other major components are mainly beidellite, kaolinite, and illite. World reserves of bentonite deposits are estimated as more than 2 millions tons per year mined and discovery of new deposits is assumed (Bentonite 2013). In the experimental part of the work, the possibility of solidification of MSWI fly-ash was studied using fly-ash from filters (hereinafter referred to as F), and fly-ash from cyclone (hereinafter referred to as C). Several experiments of solidification with different ratios and various combinations of materials were conducted. The results confirm that the method of solidifying of both fly-ashes F and C from the MSWI plant appears to be an effective procedure for their stabilization. The next experiments were concentrated on leaching of heavy metals from the stabilised structures. Atomic absorption spectrometry enabled to detect only small concentrations of heavy metals in the leachates of the stabilized structures. The comparison of measured concentrations of heavy metals in the leachates from solidified fly-ashes with limit values given by valid legislation shows that none of the concentrations of assessed heavy metals achieved or exceeded the limit values.

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Influence of aeration on zeolite suspension microfiltration

Sabo S., Bakalar T., Bugel M.

Geology, Geophysics and Environment

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2014

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Vol. 40, no. 1

125--126

EN

The effects of air injection under various conditions on the experimental results were investigated on microfiltration apparatus. In the experiments zeolite from the area of Nizny Hrabovec was used. Different patterns of flow in the microfiltration system under changing conditions were monitored. The particular flows were photographed and evaluated. If the particles in suspension are of larger dimensions than the membrane pores, the particles settle slowly and accumulate on the membrane surface and form a highly concentrated layer. Due to the formation of this layer the accumulation of solute at the membrane wall and thus sometimes irreversible clogging of the membrane occurs (Vera et al. 2000). The sedimentation of particles on the surface and in the pores of the membrane results in a reduction in the flow and growth of energy consumption. However, there is great potential to reduce this concentration polarization and fouling. At present, two-phase flow studies are of experimental nature and focus mainly on improving the conditions in filtration (Mikulasek et al. 2002). The injection of gas helps to reduce concentration polarization layer so that it improves the hydrodynamic conditions near the surface of the membrane. The results of the study suggest that the improvement of the flow should be achieved by providing appropriate conditions, using the multi-phase microfiltration, low liquid velocity, by mean speed of the gas injected (Qian etal. 2012). The use of a two-phase flow is considered to be the preferred method to overcome the concentration polarization. During the process gas is directly injected at a given pressure and speed into the feed (Bakalar 2013). Cui & Wright (1996) observed improvement in flow of up to 320% if used two-phase flow with gas injected into the suspension in their experiments. Under normal circumstances gas-liquid two-phase vertical flow exhibits different flow patterns including dispersed bubble flow, slug-flow, churn flow, and annular flow as the gas to liquid ratio increases. Bubble flow occurs when the bubbles are significantly less than (e.g. < 60%) the tube or channel size. The bubble behaviour is similar to the bubbles in a stationary liquid. Slug-flow (also called plug-flow) occurs when the gas flows as large bullet-shaped bubbles approaching the diameter of the tube or channel size (Cui & Wright 1996, Vera et al. 2000).