László Weiser

Characteristics of thin-film nanofiltration membranes at various pH-values

Abstract Salt rejection and ion selectivity of NF-255 and NF-45 nanofiltration (NF) membranes were investigated. The rejection of two cations (Na+, Ca2+) and two anions (Cl−, SO42−) which are common in natural and in industrial wastewater, were studied as a function of pH at permanent pressure and temperature. The ion rejection of NF membranes were investigated in single salt solutions like NaCl, CaCl2, Na2SO4, CaSO4, and in multicomponent systems that contained all the previous ions. We found that, there is a minimum rejection of the Na+ and Cl− ions between pH 4–5 in NF-255 and between pH 7–8 in NF-45. The rejection of calcium ions were increased in each case at lower pH in both membranes. However the pH value where the ion rejection behaviour of membranes changed, were different: pH 4 in NF-255 and pH 8 in NF-45. In NF-45 the chloride ion has negative rejection which depends on the quality of ions and the pH. We found that below pH values of 4 the selectivity of mono- and multivalent cations considerable increased in NF-255. This phenomena may be used for separation of calcium ions from sodium ions from weakly acidic (hydrochloric and sulfuric acid) solution, e.g. regeneration solution of sodium form softening ion exchangers.

Study of calcite scaling and corrosion processes in geothermal systems

Prediction of scaling in geothermal systems is important in order to be able to take preventive action. Scaling and corrosion processes are correlated in geothermal systems so that the factors influencing both processes are discussed in this paper. Pilot-scale equipment to model dissolution and scaling, and investigate corrosion, has been constructed. The construction of the pilot-scale equipment and the design of the experiments were based on the results of earlier laboratory model experiments. In order to check the measurement results we used a computer programe that was developed earlier, and which is suitable for the calculation of the equilibrium solubilities. Based on the results of the equilibrium experiments, we developed a modelling method for scaling in the pilot-scale equipment, and studied corrosion during scaling on the surfaces of five different structural materials.

Development of selective cobalt and cesium removal from the evaporator concentrates of the PWR Paks

Summary At the PWR Paks (Hungary) the diluted radioactive waste water is converted to a concentrate by evaporation and the concentrate is stored in tanks. The most important radionuclides in these solutions are 134Cs, 137Cs and 60Co. In this research we studied granular potassium nickel hexacyanoferrate(II) as a Cs-selective ion-exchanger. Its capacity depended on the preparation method, temperature of pretreatment and age of the ion exchanger. We investigated also the effect of metal ions (Fe2+, Fe3+, Mn2+, Mg2+, Ca2+, Co2+, Ni2+, Cu2+) on the Cs-capacity in the presence of complex forming compounds citrate, oxalate and EDTA. The cesium ion exchange capacity increased with addition of inactive cobalt or nickel salts. Additionally we studied filtration, adsorption and ultrafiltration separation processes for cobalt removal. The results showed that only adsorption by active carbon could be successfully used for the cobalt removal from evaporation concentrates. Experiments were performed both in the laboratory and at the PWR Paks.

Radioactive wastewater treatment using selective ion exchangers

It is well known that in the Hungarian PWR-type nuclear power plant Paks the radioactive waste waters are collected in common tanks. These water streams contain radioactive isotopes in ultra-low concentration and inactive compounds as major components (borate 1.7 g/dm3, sodium-nitrate 0.4 g/dm3, sodium-hydroxide 0.16 g/dm3, and oxalate 0.25 g/dm3). These low salinity solutions were evaporated by adding sodium-hydroxide, until 400 g/dm3 salt content is reached. There is about 6000 m3 concentrated evaporator bottom residues in the tanks of the PWR. We have developed a complex technology for the selective separation of the long live radionuclides and for the partial recycle of boric acid from this evaporator bottom residue. A wastewater treatment system has been developed by using a cesium selective inorganic ion exchanger. The selective separation of cesium (137Cs, 134Cs) from high salt concentration and strongly alkaline evaporator bottom residue in Paks Nuclear Power Plant has a volume reduction factor about 1800-3500 at the value of the decontamination factor DF > 100, for the samples of four tanks of the Hungarian PWR Paks.

Analysis and Selective Treatment of Radioactive Waste Waters and Sludges

In the Hungarian PWR-type nuclear power plant Paks (four 500 MWe capacity VVER440/213 blocks) the radioactive waste waters are collected in common tanks. These water streams contain radioactive isotopes in ultra-low concentration and inactive compounds as major components (borate 1.7 g/dm3, sodium-nitrate 0.4 g/dm3, sodium-hydroxide 0.16 g/dm3, and oxalate 0.25 g/dm3). Up to the present the low salinity solutions were evaporated (by adding sodium-hydroxide) till 400 g/dm3 salt content (pH~13) and after solidification by cementing buried. There is about 6000 m3 concentrated evaporator bottom residue in the tanks of the PWR. In order to separate the inactive salt content before cementing a Liquid Wastewater Treatment Technology (LWT see Figure 1.) was developed to treat this wastewater before solidification and burial (Patzay et al., 2006). The long-life radionuclides are present in very low concentration (10-9-10-12 mol/dm3) as ions, suspended, colloid particles and in complex (EDTA, oxalate, citrate) form. In this technology the SELION CsTreat cesium selective ion exchanger is used for the selectice separation of radiocesium isotopes (134Cs, 137Cs). The SELION CsTreat cyanoferrate based cesium-selective ion exchanger is not stable at pH>11 (see reaction equation below), so the use of CsTreat needs partial neutralisation of the evaporator bottom residue to pH~9-11, and during neutralisation sodium-borate crystals precipitate with about 15-30% of the radioactivity. [ ] 4 2 6 6 2 ( ) 2 2 [ ( ) ] ( ) K Co Fe CN OH K Fe CN Co OH − + − + ⇒ + + (1)