2.1

Resin composites used

In the present study, three different resin composites were used: an ormocer (Ceram X®; Dentsply Sirona, Bensheim, Germany), a nanofilled resin composite (Filtek™ Supreme XTE; 3M ESPE, Seefeld, Germany) and a dual-cure core build-up material (Core-X® flow; Dentsply Sirona, Bensheim, Germany). Detailed information about the resin composites is given in Table 1 .

2.2

Preparation of resin composite samples and eluates

A dental unit (KaVo Estetica E70T, KaVo Dental GmbH, Biberach/Riß, Germany) was used in order to obtain the wastewater samples; it was disconnected from the waterflow for the experimental procedures. Resin composite samples (diameter 5 mm × 2 mm depth) were prepared in a metal mold plate. The resin composite material was inserted into the mold in one increment, flattened with a plugger and covered by a matrix strip. The polymerization of the samples was carried out by using a LED light-curing unit (bluephase®, Ivoclar Vivadent AG, Schaan, Lichtenstein) with constant light intensity (950 mW/cm ² ) according to the manufacturers’ instructions (20 s curing time). Grinding was carried out under standardized conditions, to simulate the clinical procedures in the dental practice. Each resin composite sample was ground for 90 s by using a diamond bur with a standard grit size of 106 μm (Komet, Lemgo, Germany) at 200,000 rpm with a red contra-angle handpiece. To avoid a possible contamination of the water samples with BPA-containing particles from previous experiments, a wash-out phase of the dental unit for 5 min took place between each grinding procedure. The water flow was kept stable, after setting the water flow of the motor at maximum, so that after grinding each sample 750 mL of water were collected in a 1 L brown glass bottle (Carl Roth GmbH + Co. KG, Karlsruhe; Germany), respectively. In addition, 10 negative control samples were prepared, by collecting water samples after performing the same procedure without grinding a composite sample. For this, all the parameters were the same as the ones used for grinding the resin composite samples. The bottles were stored in darkness at 7 °C for 6 months, to simulate a long storage of composite dust in wastewater.

After the storage period, the water bottles were dispersed at 23 °C room temperature in an ultrasonic bath for 20 min. For reliable qualitative and quantitative determination of BPA by HPLC analysis, all water samples were concentrated. Preliminary tests showed that a five-fold concentration of the wastewater samples was necessary for a valuable data analysis; being able to measure BPA concentrations above the quantification limit. This concentration step was performed using solid phase extraction (SPE) with Chromabond® Easy cartridges with polar-modified polystyrene-divinylbenzene matrix (Macherey-Nagel, Düren, Germany). For each single sample a new cartridge was used. The cartridges had to be preconditioned before use. For this step, first, 6 mL of methanol (MeOH) were added to the cartridge followed by rinsing with 6 mL ultrapure H ₂ O. After this prewash step, 5 mL of the dispersed water samples were applied to the preconditioned SPE cartridges using a pipette (Eppendorf, Hamburg, Germany). After complete passage of the water samples, the cartridges were dried for 30 min at reduced pressure (0.2 bar). At the end of the drying period, the adsorbed BPA was eluted from the SPE matrix with 4 mL of methanol and collected by applying a slight vacuum (approximately 0.2 bar) in a brown threaded bottle (4 mL). The further procedure took place in a test tube holder. The samples were evaporated in their threaded bottles with nitrogen to complete dryness. To recover the dried powder, 1 mL of pure ethanol (EtOH) was added. Then the bottles were sealed and shaken for 1 min using a vortex mixer (Bender and Hobein GmbH, Freiburg, Germany) followed by retrieving the liquid from the 4 ml threaded bottles in 1 mL HPLC threaded bottles. The finished samples were analysed by HPLC–FLD (high performance liquid chromatography–fluorescence detector). The used method was verified in preliminary studies by using reference BPA samples prepared with the method mentioned above. This testing revealed for the method a good accuracy and a sufficient recovery rate of 87–99%. Between measurements, the samples were stored in darkness at a constant temperature (4 °C).

2.3

Analysis of BPA using HPLC–FLD

The BPA measurements were carried out on a HPLC unit (Shimadzu Deutschland GmbH, Duisburg, Germany). The system consisted of the following components: CBM20A System Controller, Degasser DGU-20a5, two LC20AT pumps, a SIL20AC auto-sampler, a CTO20AC column oven, a M20A diode array detector and a RF10AXL fluorescence detector. The LC Solution software from the same company was used. For BPA analysis, a HPLC column 125/4 Nucleodur 100-5 C18ec (pore size: 100 Å) (Macherey-Nagel, Düren, Germany) was used. The HPLC column and the entire remaining HPLC unit were calibrated once a day according to a fixed procedure. This step was performed using acetonitrile (ACN) and formic acid (0.1%) containing water (H ₂ O and CH ₂ O ₂ ). Both reagents were used additionally as eluents during the measurements. First, a rinse was carried out at a rate of 99:1 (ACN/CH ₂ O ₂ ) for 30 min, then at 55:45 (ACN/CH ₂ O ₂ ) for 30 min, and finally at 35:65 (ACN/CH ₂ O ₂ ) for another 30 min for calibration of the system immediately before the measurements. Detection of BPA was carried out with an Altus A-30 Fluorescence detector (FLD). The excitation wavelength was at 275 nm and the emission wavelength at 313 nm. Using these HPLC measurement parameters, BPA showed a retention time of 7.15 min. For each measurement, 20 μL of the water sample were injected, at a constant flow rate of 0.75 mL/min and a temperature of 35 °C.

For the qualitative and quantitative determination of BPA, a dilution (100 mg/L) of pure BPA powder and pure ethanol (≥99%) was prepared as stock solution. This solution was the first data point for all dilution series. Seven calibration reference standards were determined: 0.005/0.01/0.05/0.1/0.5/0.75 and 1[mg/L] ( Fig. 1 ). Based on the reference standards, the linearity of the chromatographic analysis could be evaluated as shown in Fig. 2 . The detection limit (LOD) was set at 0.001 mg/L and the quantification limit (LOQ) at 0.005 mg/L. The fluorescence diagrams ( Fig. 3 ) were evaluated by the LC Solution Software from Shimadzu. Each measurement result was – despite automatic, software-supported evaluation – re-integrated and controlled manually.

Graphical representation of the chromatograms (HPLC–FLD) of the calibration reference standards (LC-Solution Software, Shimadzu).

Linear calibration after HPLC measurement of the reference samples ( x -axis: BPA concentration [mg/L], y -axis: integral surface for fluorescence measurement).

HPLC chromatogram of BPA analysis of (a) the reference substance (conc. 0.75 mg/L) and (b) a representative dental unit wastewater sample using an Nucleodur 125/4 100-5 C18ec column (Machery Nagel, Düren, Germany). FLD settings: excitation at 275 nm and emission at 313 nm.

2.4

Filtration materials

For the second part of the study, three different materials were used for filtration of BPA-containing water samples: Zeosorb, Katalox Light and Catalytic Carbon (Watch GmbH, Mannheim, Germany) ( Fig. 3 ). The materials differ in particle size, matrix, kind of reactive surface and separation process. Detailed properties and information about the three filtration materials are given in Table 2 .

Table 2

Characterization of filtration materials used.

	Zeosorb	Katalox light	Catalytic carbon
Manufacturer	SZ Water Treatment System GmbH (Bubenheim, Germany)
Batch number	Not given	KL2013456789	3-44-15-0008
Filtering matrix	Alumino-silicate and green sand	MnO 2 coated Zeosorb + Calcium hydroxide	Activated carbon + catalyst FeOH
Surface structure	Microporous, crystalline(Hydrophilic)	Microporous, crystalline (Hydrophilic)	Microporous (Hydrophilic)
Color	Green/gray	Gray/black	Black
Density	820 kg/m 3	1060 kg/m 3	630–640 kg/m 3
Reaction surface	<270 m 2 /g	Not given	2000–2500 m 2 /g
Target substances	Ammonium, nitrogen, heavy metals, radionuclides, dyes, microorganisms	Hydrogen sulfide, heavy metals, radionuclides (e.g., radium, uranium)	Hydrogen sulphide, heavy metals, tannins, phenols, chloramines, trihalomethanes, fragrances and dyes
Flow rate	≤10 m/h	≤20 m/h	Not given
Separation process	Filtration	Filtration	Adsorption
Surface charge	Negative	Negative	Positive
Particle filtration up to	<3 μm	<3 μm	≤1 μm
Particle size	50–150 μm	60–140 μm	60–240 μm

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