| Abstract [eng] |
Excess release of phosphate into the surface water even at very low concentrations could still lead to eutrophication. Phosphorus is an essential nutrient in agriculture, phosphate rock is a finite resource and significantly increasing demand for it is observed. As a consequence, more sustainable use of phosphorus and development of technologies to recover it from waste streams are encouraged. Unfortunately, current methods used for phosphorus removal such as dosing metal salts cannot meet the environmental standards and to recover phosphorus from formed products is not economical. Stricter environmental regulations promote the search for more efficient removal of phosphate in wastewater treatment plants (WWTPs) and reversible adsorption could be a solution. Successful application of chemically formed iron oxides (ChFeO) adsorbents for phosphate removal is attributed to a good adsorption capacity and high selectivity towards phosphate. Adsorption capacity can be improved by increasing surface area. It could be done by coating nanoparticles on supporting material, but this is hard to implement. Therefore, their application has some problems, which could be solved by using biogenic iron oxides (BioFeO). BioFeO are known for their wide occurrence, good phosphate adsorption capacity and that they occur as nanoparticles. This encouraged the study to investigate the applicability of BioFeO on phosphate removal from water and compare this with ChFeO. Microbial mats of Leptothrix sp. were collected in iron-rich seepage areas for phosphate removal by batch adsorption experiments. For experiment also used microbial mats of Gallionella sp. formed in the laboratory. For comparison with ChFeO an engineered adsorbent (GEH) was used. Characterization of adsorbents was done using microwave digestion analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy, light microscope and X-ray diffraction analysis. Adsorbents efficiency for phosphate removal was carried out using the kinetic and isotherm studies. Non-washed microbial iron oxyhydroxides of Leptothrix sp. and GEH have reached equilibrium in four days. Unwashed microbial mats of Leptothrix sp. have lower affinity, but 27% higher phosphate removal capacity in comparison to GEH. The high adsorption capacity is related to the combination of phosphate removal by different mechanisms – adsorption on BioFeO and surface precipitation with soluble iron. In order to avoid iron release, the microbial mats were washed with MQ (deionized) water. Washed microbial iron oxyhydroxides of Leptothrix sp. had much slower kinetics, equilibrium was not reached in seven days. MQ-washed microbial iron oxyhydroxides of Gallionella sp. and Leptothrix sp. have lower affinity and respectively 1.2 and 2.1 times lower adsorption capacity than GEH. Stalks of Gallionella sp. and Leptothrix sp. have different structures, but it does not have an influence on affinity and adsorption capacity. Iron content plays key-role on phosphate adsorption. Microbial iron oxyhydroxides can contain up to 50% of organic matter (OM), while synthetic FeO are pure iron oxyhydroxides. Microbial mats of Leptothrix sp. have restrictions for its application as adsorbents for phosphorus removal in WWTP, but iron-oxidizing bacteria (FeOB) is relevant for various research disciplines – for controlling phosphorus, heavy metals and other substances in the natural and engineered systems. In order to be able to make predictions about the bioavailability and transport of nutrients and contaminants in natural systems and also about their behavior in engineered systems a detailed knowledge is required about the influence of exopolymeric substances of BioFeO on interaction with Fe (e.g. type of bond and strength), which determines sorption mechanisms. |
