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The hexadecyltrimethylammonium (HDTMA) bromide was used to modify the surface of natural zeolitized tuff (NZ) from partial monolayer to the bilayer coverage. The modification of NZ resulted in the change of zeta potential of the surface from negative to positive which enhanced the phosphate (P) adsorption capacity. Only in reactors containing the partial monolayer coverage of the surfactant-modified zeolites (SMZs), the P was efficiently removed from wastewater by combined adsorption onto the SMZs and bacterial uptake in the biomass. The SMZs with monolayer, partial bilayer or bilayer coverage showed the bactericidal effect.

The antibacterial effect of commercial zeolite A against the pure culture of phosphate- accumulating bacterium Acinetobacter junii was investigated. The estimated EC50 values of the zeolite for the inhibition of bacterial colony forming units was 0.33-0.37 g/L by investigated initial pH values of 6, 7 and 8. The toxic effect of zeolite is ascribed to the generation of Al (0.38-1.09 x 10-2 g/L) and Si (2.02-2.93 x 10-2 g/L) ionic species during the process of hydrolysis and dissolution of zeolite (leaching of Al, Si and Na). The toxic effect of Al was confirmed in separate experiments, where the aluminum toxicity (in the form of aluminum sulphate) was estimated as EC50 of 3.45-3.89 x 10-4 g/L. These results suggest the importance of controlling the amounts of synthetic zeolites which can be released to different compartments of environment and accumulate there as nondegradable material. Zeolite A may be the important source of Al in influent of the biological wastewater treatment systems, where they can cause unwanted antibacterial effects.

Three types of natural zeolite tuffs of particle size  0.125 mm, originating from Croatia, Turkey and Serbia were examined. The tuff from Croatia consisted of approximately 50% clinoptilolite, subordinate plagioclase feldspars and quartz (10-15% of each) and traces of K-feldspars and celadonite (app. 5% of each). The tuff from Turkey contained approximately 70% clinoptilolite, subordinate opal-CT and quartz (10– 15% of each) and traces of K-feldspars and biotite (app. 5% of each). The tuff from Serbia contained approximately 75% clinoptilolite and several minor constituents were analcime, calcite, plagioclase feldspars, biotite and quartz (app. 5% of each). The tuff from Serbia had the largest cation exchange capacity of 1.71 meq/g, followed by tuff from Croatia with 1.60 meq/g and tuff from Turkey with 1.35 meq/g. The main exchangeable cation in samples of tuffs from Turkey and Serbia was Ca, whereas Na was the main exchangeable cation in the sample of tuff from Croatia. The particles of the tuff from Croatia had the most negative zeta potential of -25.14 mV, followed by tuff from Turkey (-23.12 mV) and tuff form Serbia (-17.60 mV). The phosphate adsorption capacity varied from 22.5 mg P/kg for Serbian tuff, to 30.2 mg P/kg for Croatian tuff and 77.5 mg P/kg for Turkish tuff. The interaction of zeolite tuffs and phosphate-accumulating bacterium Acinetobacter junii in simulated wastewater was investigated. After 24h of contact with minerals, most of the bacterial population was immobilized onto minerals by adsorptive growth, while the rest of the bacteria remained as planktonic cells in supernatant. The highest number of immobilized cells was obtained on the tuff from Turkey (74.91x108 CFU/g), followed by tuff form Croatia (48.39x108 CFU/g), while the tuff from Serbia had the lowest number of immobilized cells (6.14x108 CFU/g). The zeta potential of A. junii (-18.4 mV) was negative, as well as the zeta potential of mineral particles and no correlation with the number of immobilized cells and zeta potential of minerals was obtained. The best bacterial activity measured as phosphate uptake rate per CFU was obtained with tuff from Croatia, followed by tuff from Turkey and tuff from Serbia. As the result of bacterial activity, the final percent of phosphate removal from wastewater decreased in the order: 50.23%, 46.32% and 40.21% for tuff from Croatia, Turkey and Serbia, respectively. It can be concluded that zeolite tuff can be a good carrier of phosphate-accumulating bacteria. The zeta potential of zeolite tuff was not a crucial factor which determined the immobilization of bacteria. The clinoptilolite content in the zeolite tuff was not the prevailing factor for the immobilization of bacteria. Bacterial immobilization on the zeolite tuff was defined by the original structure of material. The extent of bacterial immobilization on single zeolite tuff cannot be predicted by mineralogical and chemical analysis of mineral, its cation exchange capacity or its zeta potential. The phosphate- accumulating bacteria immobilized onto zeolite tuff as a biocarrier can find application in the bioaugmentation of activated sludge in order to achieve better phosphate removal from wastewater.

Depending on the type of zeolite, different interactions between material and examined PAB A. junii were observed. Natural clinoptilolite tuff is excellent bacterial carrier and the main mechanism of interaction is immobilization of bacteria onto the surface of material and increase of the total bacterial biomass concentration. In spite, the synthetic zeolites display the toxic effect against the same PAB. Future investigations should be done in order to investigate the influence of different types of zeolite on other bacteria.

The aim of this study was to determine the potential of phosphate (P) removal from wastewater by P-accumulating bacteria and organic bentonite. Organic bentonite was prepared by treating bentonite clay with quaternary ammonium salt – cetyltrimetylammonium (CTA) bromide. Cation exchange capacity (CEC) of the bentonite was found to be 179.0 meq/100 g of the dry bentonite. The CTA occupied ca. 175% of the CEC. The modification of bentonite with CTA in amounts higher than 55% of the CEC resulted in the change of zeta potential of particles from negative to positive. Only in reactors containing organic bentonite samples occupied with 3.5 and 28% of the CEC the P was efficiently removed from wastewater by combined adsorption and bacterial uptake in the biomass. The organic bentonite samples with higher CTA loading (from 55 to 175% of the CEC) showed the bactericidal effect. To enhance the P removal from wastewater in the aerated biological system, the organic bentonite can be used, but the special attention should be given to the configuration and location of sorbed CTA molecules and its potential desorption.