Article pubs.acs.org/est
Reaction of Gadolinium Chelates with Ozone and Hydroxyl Radicals Maike Cyris,†,‡ Wolfgang Knolle,§ Jessica Richard,† Elke Dopp,†,∥ Clemens von Sonntag,‡ and Torsten C. Schmidt*,†,‡,∥ †
IWW Water Centre, Moritzstraße 26, 45476 Mülheim an der Ruhr, Germany University of Duisburg-Essen, Instrumental Analytical Chemistry, Universitätsstraße 5, 45141 Essen, Germany § Leibniz-Institut für Oberflächenmodifizierung (IOM), Permoserstraße 15, 04318 Leipzig, Germany ∥ Centre for Water and Environmental Research (ZWU), University of Duisburg-Essen, Universitätsstraße 2, 45141 Essen ‡
S Supporting Information *
ABSTRACT: Gadolinium chelates are used in increasing amounts as contrast agents in magnetic resonance imaging, and their fate in wastewater treatment has recently become the focus of research. Oxidative processes, in particular the application of ozone, are currently discussed or even implemented for advanced wastewater treatment. However, reactions of the gadolinium chelates with ozone are not yet characterized. In this study, therefore, rate constants with ozone were determined for the three commonly used chelates GdDTPA, Gd-DTPA-BMA, and Gd-BT-DO3A, which were found to be 4.8 ± 0.88, 46 ± 2.5, and 24 ± 1.5 M−1 s−1, respectively. These low rate constants indicate that a direct reaction with ozone in wastewater is negligible. However, application of ozone in wastewater leads to substantial yields of •OH. Different methods have been applied and compared for determination of k(•OH+Gd chelate). From rate constants determined by pulse radiolysis experiments (k(•OH+Gd‑DTPA) = 2.6 ± 0.2 × 109 M−1 s−1, k(•OH+Gd‑DTPA‑BMA) = 1.9 ± 0.7 × 109 M−1 s−1, k(•OH+Gd‑BT‑DO3A) = 4.3 ± 0.2 × 109 M−1 s−1), it is concluded that a reaction in wastewater via •OH radicals is feasible. Toxicity has been tested for educt and product mixtures of both reactions. Cytotoxicity (MTT test) and genotoxicity (micronuclei assay) were not detectable.
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refractory compounds can also be transformed via •OH formed as secondary oxidant with the matrix. However, again, competition between wastewater matrix and target compounds is a limiting factor. The •OH rate constants strongly influence the efficiency of target compound oxidation.3,10 To understand the oxidation of target compounds in a wastewater matrix it is necessary to investigate both the reaction via ozone and •OH separately. The reactions of chelates with ozone and hydroxyl radicals are rarely studied.11 There are few studies on the reaction of metal EDTA complexes12−15 and on metal DTPA complexes,16,17 but to the authors’ knowledge, no other chelates are covered in these studies. Here, we focus on gadolinium chelates (cf. Figure 1), used as contrast agents in magnetic resonance imaging. They have a very high complex stability, and are excreted without metabolization,18 hence they reach WWTPs without any modification. Total concentrations of gadolinium in European WWTP effluents range from the low ng L−1 to 1 μg L−1 at maximum (unpublished results of a European monitoring campaign). Conventional wastewater
INTRODUCTION Ozone treatment and advanced oxidation processes are long known for disinfection and removal of trace substances in drinking water treatment.1−3 In recent years, these processes have been discussed for application in wastewater treatment and are in some places already established in full scale.4 This is due to frequent detections of wastewater-derived micropollutants in surface and ground waters.5,6 The removal of these substances is often inefficient in conventional wastewater treatment plants (WWTPs). According to the European water framework directive the water quality shall reach a “good chemical status” as well as a “good ecological status”.7 This aim is potentially threatened by the presence of many trace substances, especially those with biological activity such as (xeno-)hormones, antibiotics, and other pharmaceuticals. For further elimination of such compounds in an additional treatment step, the application of ozone is one of the preferred methods. Ozone action in wastewater can be divided into two reaction pathways.3 Mainly it reacts with the wastewater matrix forming •OH radicals to substantial yields.3,8−10 In wastewater the application of ozone generates approximately 13 mol % • OH of the applied ozone dose.9 In competition to the •OH forming reaction, ozone-reactive target compounds are oxidized directly. In many cases, the biological activity of the target compounds is eliminated upon a single ozone attack.3 Ozone © 2013 American Chemical Society
Received: Revised: Accepted: Published: 9942
May 17, 2013 July 23, 2013 July 26, 2013 July 26, 2013 dx.doi.org/10.1021/es402219u | Environ. Sci. Technol. 2013, 47, 9942−9949
Environmental Science & Technology
Article
Figure 1. Molecular structures of tested Gd chelates including trade names and molar weights, examples for the reactive center for the reaction with • OH are marked as circles, examples for the reactive center for the reaction with ozone are marked as squares.
treatment does not remove them substantially.19 It is assumed that they remain intact, and no free Gd(III) is released.19 This may be different in oxidative (waste)water treatment processes, which are potentially able to destroy Gd chelates, and Gd(III) ions, which are toxic in contrast to the chelated form (LD50 in mice for GdCl3 = 0.1 mmol kg−1 and for GdDTPA-BMA = 12 mmol kg−1 20), may be liberated. Hence, not only the reaction of the chelates with oxidants was analyzed in this work, but also toxicity of reaction products was studied.
The para-chlorobenzoic acid (pCBA) and para-nitrobenzoic acid (pNBA) (both purchased from Sigma Aldrich, p.a.) stock solutions (4 mM) were prepared in ultrapure water alkalinized with sodium hydroxide (Sigma Aldrich, p.a.) to yield pH 7 in the reaction solutions. At this pH pCBA and pNBA are present in their dissociated form (pKa = 3.9822 and 3.41,22 respectively. Therefore, rate constants of the dissociated forms have been utilized. Rate Constants. All rate constants of the chelates with ozone and •OH have been determined in triplicate experiments. Rate constants for the reaction with ozone have been determined under pseudo-first-order conditions ([O3] = 50 μM and [Gd] = 500 μM). Ozone decrease was followed by the indigo method.23 As radical scavenger tBuOH (20 mM) was used and the reaction was buffered with phosphate (10 mM) at pH 7. Ozone rate constants were calculated from the slope resulting from plotting the natural logarithm of the ratio of the residual ozone concentration [O3] over the initial ozone concentration [O3]0 vs the reaction time. Hydroxyl Radical Rate Constants: Method 1. The •OH rate constants were determined by pulse radiolysis experiments, which is the ultimate reference method for their determination. Very reliable values, within an error margin of
