Influence of the simultaneous calibration of multiple ring dosimeters on the individual absorbed dose

Ring dosimeters for personal dosimetry are calibrated in accredited laboratories following ISO 4037-3 guidelines. The simultaneous irradiation of multiple dosimeters would save time, but has to be carefully studied, since the scattering conditions could change and influence the absorbed dose in nearby dosimeters. Monte Carlo simulations using PENELOPE-2014 were performed to explore the need to increase the uncertainty ofHp0.07in the simultaneous irradiation of three and five DXT-RAD 707H-2 (Thermo Scientific) ring dosimeters with beam qualities: N-30, N-80 and N-300. Results show that the absorbed dose in each dosimeter is compatible with each of the others and with the reference simulation (a single dosimeter), with a coverage probability of 95% (k= 2). Comparison with experimental data yielded consistent results with the same coverage probability. Therefore, five ring dosimeters can be simultaneously irradiated with beam qualities ranging, at least, between N-30 and N-300 with a negligible impact on the uncertainty ofHp0.07.

An optimization model for the treatment of perfluorocarboxylic acids considering membrane preconcentration and BDD electrooxidation

Treatment of persistent perfluorocarboxylic acids in water matrixes requires of strong oxidation conditions, as those achieved by boron doped diamond (BDD) electrooxidation (ELOX). However, large scale implementation of ELOX is still hindered by its high energy consumption and economical investment. In this work, we used process systems engineering tools to define the optimal integration of a membrane pre-concentration stage followed by the BDD electrolysis of the concentrate, to drastically reduce the costs of treatment of perfluorohexanoic acid (PFHxA, 100 mg L^-1) in industrial waste streams. A multistage membrane cascade system using nanofiltration (NF90 and NF270 membranes) was considered to achieve more sophisticated PFHxA separations. The aim was to minimize the total costs by determining the optimal sizing of the two integrated processes (membrane area per stage and anode area) and the optimal process variables (pre-concentration operating time, electrolysis time, input and output concentrations). The non-linear programming model (NLP) was implemented in the General Algebraic Modelling System (GAMS). The results showed that for a 2-log PFHxA abatement (99% removal), the optimal two membrane stages using the NF90 membrane obtains a 75.8% (6.4 $ m^-3) reduction of the total costs, compared to the ELOX alone scenario (26.5 $ m^-3). The optimized anode area and the energy savings, that were 85.3% and 88.2% lower than in ELOX alone, were the major contributions to the costs reduction. Similar results were achieved for a 3-log and 4-log PFHxA abatement, pointing out the promising benefits of integrating electrochemical oxidation with membrane pre-concentration through proper optimization for its large-scale application to waters impacted by perfluorocarboxylic acids.

Calibration of a thermoluminescent dosimeter worn over lead aprons in fluoroscopy guided procedures

Fluoroscopy guided interventional procedures provide remarkable benefits to patients. However, medical staff working near the scattered radiation field may be exposed to high cumulative equivalent doses, thus requiring shielding devices such as lead aprons and thyroid collars. In this situation, it remains an acceptable practice to derive equivalent doses to the eye lenses or other unprotected soft tissues with a dosimeter placed above these protective devices. Nevertheless, the radiation backscattered by the lead shield differs from that generated during dosimeter calibration with a water phantom. In this study, a passive personal thermoluminescent dosimeter (TLD) was modelled by means of the Monte Carlo (MC) code Penelope. The results obtained were validated against measurements performed in reference conditions in a secondary standard dosimetry laboratory. Next, the MC model was used to evaluate the backscatter correction factor needed for the case where the dosimeter is worn over a lead shield to estimate the personal equivalent dose H _p (0.07) to unprotected soft tissues. For this purpose, the TLD was irradiated over a water slab phantom with a photon beam representative of the result of a fluoroscopy beam scattered by a patient. Incident beam angles of 0° and 60°, and lead thicknesses between the TLD and phantom of 0.25 and 0.5 mm Pb were considered. A backscatter correction factor of 1.23 (independent of lead thickness) was calculated comparing the results with those faced in reference conditions (i.e., without lead shield and with an angular incidence of 0°). The corrected dose algorithm was validated in laboratory conditions with dosimeters irradiated over a thyroid collar and angular incidences of 0°, 40° and 60°, as well as with dosimeters worn by interventional radiologists and cardiologists. The corrected dose algorithm provides a better approach to estimate the equivalent dose to unprotected soft tissues such as eye lenses. Dosimeters that are not shielded from backscatter radiation might underestimate personal equivalent doses when worn over a lead apron and, therefore, should be specifically characterized for this purpose.

Efficient treatment of perfluorohexanoic acid by nanofiltration followed by electrochemical degradation of the NF concentrate

The present study was aimed at the development of a strategy for removing and degrading perfluorohexanoic acid (PFHxA) from industrial process waters at concentrations in the range 60-200 mg L^-1. The treatment train consisted of nanofiltration (NF) separation followed by electrochemical degradation of the NF concentrate. Using a laboratory-scale system and working in the total recirculation mode, the DowFilm NF270 membrane provided PFHxA rejections that varied in the range 96.6-99.4% as the operating pressure was increased from 2.5 to 20 bar. The NF operation in concentration mode enabled a volume reduction factor of 5 and increased the PFHxA concentration in the retentate to 870 mg L^-1. Results showed that the increase in PFHxA concentration and the presence of calcium sulfate salts did not induce irreversible membrane fouling. The NF retentate was treated in a commercial undivided electrochemical cell provided with two parallel flow-by compartments separated by bipolar boron doped diamond (BDD) electrode, BDD counter anode, and counter cathode. Current densities ranging from 20 to 100 A m^-2 were examined. The electrochemical degradation rate of PFHxA reached 98% and was accompanied by its efficient mineralization, as the reduction of total organic carbon was higher than 95%. Energy consumption, which was 15.2 kWh m^-3 of treated NF concentrate, was minimized by selecting operation at 50 A m^-2. While most of the previous research on the treatment of perfluoroalkyl substances (PFASs) focused on the removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), these compounds have been phased out by chemical manufacturers. Our findings are relevant for the treatment of PFHxA, which appears to be one of the present alternatives to long-chain PFASs thanks to its lower bioaccumulative potential than PFOA and PFOS. However, PFHxA also behaves as a persistent pollutant. Moreover, our results highlight the potential of combining membrane separation and electrochemical oxidation for the efficient treatment of PFAS-impacted waters.