Cellulose (2018) 25:1353–1364
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minimum silver content introduced into the zeolite structure. The material should also prevent the silver from migrating into the ecosphere.
equipped with two anticathodes Mg/Al (power Mg/ Al = 400/600 W), and the x-ray monochromator with the radiation source (single anticathode–Al). XPS spectra were calibrated to the carbon component C1 s binding (with energy of 285.0 eV). High resolution spectra were fitted using CasaXPS software. Two types of analysis were performed, according to the AgNP characterisation. At first we monitored the de novo formation of Ag NPs by UV–Vis spec- troscopy. We observed the characteristic band assigned for AgNPs, hence we were sure of the NPs’ presence in suspension. This observation was con- firmed by DLS experiments.
Experimental
Zeolite preparation
Y-type zeolite (Faujasite, FAU) in powder form was purchased from Zeolyst International (CBV-100). After conditioning at 23.5 C and RH = 50% for 12 h, the zeolite in sodium form was used further for ion exchange from silver nitrate solutions. Silver nitrate was obtained from Avantor Performance Materials. Disodium ethylenediaminetetraacetate dihydrate (Na 2 EDTA 2H 2 O) from Sigma-Aldrich was used for another treatment to remove the external silver oxide NPs (those which were not exchanged with sodium cations). Faujasite exchanged with silver (AgFAU) was used as an additive to paper pulp. AgFAU samples were prepared by suspending 1.00 g of zeolite in 100 cm 3 of 0.1 M AgNO 3 solution in deionised water. Due to the light sensitivity of silver, suspensions were stirred in the darkness (300 rpm, 1 h). After ion exchange, AgFAU samples were filtered and washed with deionised water. After preparation, the obtained samples were dried in an oven at 60 C for 8 h. In order to remove the unexchanged (non-bonded) silver from the external surfaces of the zeolite, the prepared samples were washed with 10 cm 3 of0.01M Na 2 EDTA solution and 10 cm 3 of deionised water (the procedure was repeated five times in total) for reference. The washing procedure was repeated until the concentration of silver cations in the eluent reached a minimum constant value. After each step, the zeolite suspension was centrifuged (4000 rpm). The ion exchange capacity and EDTA washing efficiency were monitored by energy dispersive x-ray fluorescence spectroscopy (ED-XRF, ARL QUANT’X; Thermo Scientific, USA). Based on calibration, the Ag content in the zeolites was determined. The silver that remained on the zeolite surface was analysed by x-ray photoelectron spectroscopy (XPS) using the ESCA Prevac spectrometer equipped with a hemispheric analyser of charged XPS and AES particles (VG Scienta R3000). The x-ray tube was
Modified paper preparation
In order to obtain paper sheets containing AgFAU, cellulose from Whatman filter paper was used. Prior to preparation, the Whatman paper sheets were condi- tioned according to the ASTM D685 norm, and under these conditions they were cut into 4 9 4 cm pieces following disintegration in 400 cm 3 of deionised water using a dispersing instrument (IKA T18 Ultra- Turrax with stainless steel dispersing elements). Cellulose suspension (paper pulp) content is shown in Table 1. Elemental analysis of the obtained paper was performed with an XRF spectrometer (Thermo Sci- entific ED-XRF, thick Cu filter, K a = 22.36 eV). Prior to XRF analysis, the obtained samples were digested in boiling 65% nitric acid for 15 min. The paper sheets containing AgFAU, along with the reference materials (Ag NPs, Ag ? ), were formed on a custom-built papermaking vacuum table. Wet paper sheets (about 15 9 20 cm) were dried on the glass surface at ambient temperature. From these paper sheets, small circle samples were cut by hole punch- ing, and then used for microbial tests. Apart from the final samples of paper containing AgFAU washed with EDTA (PZAg ? _EDTA), four kinds of reference samples were prepared: • P—cellulose without additives • PAg ? —paper with Ag ? cations prepared from cellulose suspensions in 0.1 M AgNO 3 • PAg 0 —paper with Ag NPs prepared from AgNO 3 suspensions as above, but then treated by sonica- tion (10 min) of the AgNO 3 solution (0.1 M) containing a low amount of ethanol (1.5 vol.%).
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