Igh initial phosphate concentration results in elevated zeolite P-loadings in equilibrium (Section 3.2.2). FIIN-1 Autophagy Figure five shows the influence of pH on N- and P-removal at higher initial phosphate concentration by comparing experiment (d) and (f). Experiment (d) reached the highest P-loading of all experiments at initial phosphate concentration of 245 mg PO4 3- L-1 and pH 8. The exact same initial N and P concentrations have been utilised in experiment (f) at pH 9. As determined in Section three.2.3 larger pH results in decrease N-loading on the zeolite and overall reduced N-removal, which have been in (d) 66 and in (f) 46 in state of equilibrium. Within the 1st three h P-removal at pH = 9 is faster and much more efficient than P-removal at pH = eight. In an equilibrium state, P-loadings of zeolite are six.78 mg PO4 3- g-1 in (d) and four.09 mg PO4 3- g-1 in (f), which reveals the relative P-removal prices of 56 and 34 , respectively. In summary, the following results were obtained from the laboratory experiments:Calcium is the preferred exchange ion in ammonium sorption; P removal is really a precipitation reaction combined with ammonium sorption; High initial phosphate concentrations bring about higher P-loadings; Speed of phosphate precipitation is controlled by pH; Ammonium AVE5688 site sorption reduces at pH 9.three.two.five. Observations Concerning the Zeolite Surface Two zeolite samples were analyzed by FEG-SEM. The surface of an unloaded organic zeolite is shown in Figure 6a. Right after simultaneous removal experiments, surface changed substantially. A fine textured crystal phase precipitated all more than zeolite surface (Figure 6b). The second sample features a P-loading of six.59 mg PO4 3- g-1 , which can be equal for the loading of experiment (d).Figure 6. FEG-SEM images of: (a) unloaded natural zeolite and (b) N P loaded zeolite with precipitated CaP-phases.ChemEngineering 2021, 5,9 of4. Discussion As shown in Figure 1, calcium could be the preferred, desorbed cation in ammonium sorption and so, larger initial ammonium concentration leads to greater amounts of desorbed calcium ions. Analysis on the regeneration solution revealed that only calcium and phosphate had been dissolved in the zeolite. Consequently, phosphate and calcium ions type a compound that is soluble in acid options. Important concentrations of other ions for example NH4 + , K+ , Mg2+ or Na+ were not identified inside the regeneration solution, so the formation of magnesium ammonium phosphate (MAP) as a prospective P-precipitate may be excluded. A doubled initial ammonium concentration outcomes within a 19 larger P-removal price at the identical initial phosphate concentration. In brief, higher initial ammonium concentration potentially leads to larger calcium desorption and additional to higher P-removal (Figure 2), correlating to the findings of Lin et al. [23]. Larger initial phosphate concentration impacts larger P-loading on zeolite (Figure three), however the removal rate is reduced than in experiment (c). Ammonium sorption reveals precisely the same N-loadings in both experiments (c) and (d). In comparison with the single ammonium experiment (a2 ), simultaneous N and P removal has no influence on N-removal. An critical acquiring in Figure three is the fact that P-loading is faster and greater in (d) with an equal calcium availability in case of equal initial ammonium concentrations. Hence, phosphate is the limiting issue in Figure two, for the reason that a enough level of calcium was desorbed as a consequence of higher ammonium sorption. Rising the initial pH worth from 8 to 9 features a considerable effect on the rate of phosphate removal in the beginning of t.
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