3.1. Leaching of Active Substances
The four biocides were leached from the paints in the experiment that was based on the EN 16105 standard [15
]. As observed in earlier experiments [19
], the leached amounts of terbutryn (1.1 to 2.4% of the original amount) and carbendazim (3.3 to 4.9% of the original amount) were in similar ranges and lower than emissions of diuron (9.2 to 12.5% of the original amount) and OIT (9.5 to 14.1% of the original amount). Emission of active ingredients was reduced when test specimens were additionally exposed to UV radiation (see Figure 2
). This was expected for terbutryn, diuron, and OIT, as previous experiments [16
] demonstrated photolytic degradation of these substances when dry test specimens were exposed to UV radiation. In contrast to terbutryn, diuron, and OIT, we expected emission curves to be more similar for carbendazim in experiments with and without UV radiation, since this substance was less sensitive to photolysis in experiments on water samples [20
] and dry paints [16
]. In our experiments, leaching of carbendazim was lower also in the UV-exposed samples in this experiment. It cannot be ruled out that the combined exposure to water contact and UV radiation increased the transformation of carbendazim in the paints. Changes in the polymer matrix due to UV exposure can also be a reason for decreased leachability of carbendazim. However, structural changes within the polymer matrix and binding of the active substances in the matrix were not investigated during this study.
Former EN 16105 experiments on the leaching of active substances from coatings indicated very good repeatability of parallel experiments within one laboratory (relative standard deviations between 1 and 6%) [19
]. Therefore, it is assumed that the curves from the different experiments can be compared, although the leaching tests were performed only once for each sampling event and exposure scheme.
3.2. Terbutryn and Transformation Products in Eluates
Besides terburyn, several transformation products of this substance were detected in eluates from the immersion cycles using test specimens that were kept in a dark room at 21 ± 2 °C and 60 ± 5% relative humidity between the immersion cycles and using test specimens that were exposed to UV radiation (see Figure 3
). TBSO, TB-DesE, and TBOH were observed in eluates from both exposure schemes, whereas TBOH-DesE was observed only in eluates from test specimens that were exposed to UV radiation. As a secondary transformation product formed from the primary transformation product TBOH, this substance occurred only during later periods of the experiment. Small amounts of TBSO (mainly in the white paint eluates), TB-DesE, and TBOH (in eluates from both paints) were also leached from test specimens that were exposed only to water contact (see also the emission curves of the transformation products presented as Supplementary S5
). Terbumeton was not detected in the eluates.
The first data points of both exposure schemes represent test specimens that were immersed in water for a total of 2 h, but not exposed to UV radiation. The terbutryn concentrations in the eluates from the two test specimens from exposure scheme C and in the eluate from the twenty-two test specimens from exposure scheme B differed by 11% for the white paint and 7% for the red paint (ratio between results for scheme C and scheme B). The differences in the small amounts of TBOH were about 10% for both paints, and the difference in the small amount of TBSO that was detected in the white paint was 9%. These deviations allow us to follow trends within and between the different experimental series (see also emission curves in Supplementary S5
The patterns of transformation products clearly differ for eluates from the two exposure schemes. UV radiation resulted in considerably lower amounts of terbutryn and greater amounts of transformation products in eluates. In eluates from the red paint, the amounts of the primary transformation products TBSO, TB-DesE, and TBOH decrease faster during the experiment with additional UV exposure than in eluates from the white paint. This observation probably indicates that transformation proceeds faster in this paint.
3.4. Terbutryn and Transformation Products in Paint Extracts
Terbutryn and the investigated transformation products were detected in extracts from white and red paint both after UV radiation only and after UV radiation in combination with immersion cycles (see Figure 5
). Terbumeton was detected in small amounts in addition to substances that were already observed in eluates. The amounts that were leached from these small amounts of terbumeton that were available in the paint layers may have resulted in concentrations below LOD in the eluates. The distributions of terbutryn and transformation products were similar in test specimens before UV exposure (S) and in the “dark controls” in the UV radiation setup for both paints (dark). The contents of terbutryn and transformation products in extracts from “dark controls” that were leached by one immersion cycle at the end of the experiment (Ld) and from the test specimens that were leached by ten immersion cycles (Lr) were similar. This can be explained by the very small amounts of substances (less than 3% of the original content of terbutryn) that were leached during the ten immersion cycles which were less than the variability of the amount of paint per test specimen (8% for the white paint and 6% for the red paint).
The graphs in Figure 5
illustrate that transformation is not only caused by UV radiation, but is also strongly supported by water contact, and that it differs between the investigated white and red paints (see also Supplementary S6
). The patterns of transformation products and the amounts of terbutryn in the paint layers differ, if the test specimens are exposed to immersion cycles in addition to UV radiation.
Terbutryn content in the paint extracts was lower if the test specimens were exposed not only to UV radiation, but also to immersion cycles. This decrease is not necessarily caused solely by leaching, but can also be caused by transformation reactions that depend on water contact. This is supported by the fact that the amount of terbutryn and its transformation products in eluates are lower than the difference in the remaining contents in the paints (see also Figure 2
and Figure 3
, Supplementary S5
and Supplementary S6
). TBSO contents were greater in the white paint than in the red paint and increased with water contact in both paints, but especially in the white paint. Under the chosen test conditions, TBSO and TBOH tend to accumulate in the paints. This was especially pronounced for TBSO in the white paint (see Supplementary S6
). Small amounts of terbumeton were observed mainly in the red paint after combined exposure to UV radiation and water contact. Results for TB-DesE from LC-MS analysis were affected by a matrix signal, so that the reported data probably overestimate the real values. The amounts of TB-DesE were greater in the extracts from the red paint, especially from test specimens that were exposed only to UV radiation, but were in similar ranges in the course of the experiments. Amounts of TBOH slightly increased during UV radiation in both paints, but to a greater extent when UV radiation was combined with immersion cycles. This increase was greater in the red paint than in the white paint. TBOH-DesE was detected only after at least four days of UV radiation. The amounts were increased if test specimens were exposed to immersion cycles in addition to UV radiation, and they were greater in the red paint than in the white paint. This is not necessarily caused by water contact (see also Figure 3
for the results in the eluates from test specimens without UV radiation), but probably depends on the availability of the primary transformation product TBOH.
Greater amounts of TBSO in the white paint than in the red paint were also observed for UV-exposed test specimens under the same experimental conditions [16
]. Similar relative amounts of TBOH, TBOH-DesE, and terbumeton and the presence of TBOH-DesE only during later stages of the experiment were also observed in the previous study. However, smaller amounts of TB-DesE were detected in both paints in the former experiments. A certain amount of TB-DesE was probably already present in the red paint at the beginning of the current experiment (see data on test specimens before start “S” in Figure 5
) and caused the observed differences between white and red paint.
A number of transformation products were detected by LC-MS and LC-HRMS in photolysis experiments in which we exposed aqueous solutions of terbutryn, TBOH, TBOH-DesE, TB-DesE, TBSO, and terbumeton (each as a single substance in water) to UV radiation at either 254 nm or to a spectrum with its maximum at 350 nm for 48 h. In addition to the investigated substances, detected mass signals correspond to terbutryn transformation products that were previously reported: desbutyl-terbutryn [1
], desthiomethyl-terbutryn [9
], desthiomethyl-desethyl-terbutryn [9
], desthiomethyl-desbutyl-terbutryn [9
], desbutyl-2-hydroxy-terbutryn [9
], 2,4-diamino-1,3,5-triazin [21
], 2-hydroxy-4,6-diamino-1,3,5-triazin [22
], and terbutryn-TP-210 [1
]. Desethyl-terbumeton is mentioned as a transformation product of terbumeton [23
]. Furthermore, signals that correspond to the masses of desethyl-terbutryn sulfoxide and desbutyl-terbutryn sulfoxide were detected. The distribution of these substances differed between the two applied maxima of UV radiation. Not all signals could be assigned (unpublished data).
However, the transformation products that were selected for quantification in this study seem to include the main reactions of terbutryn, since the sum of all substances remains rather stable and approximately in the range of the original amount of terbutryn. The data presented in Figure 3
and Figure 5
represent the detected amounts of the transformation products. The results for the transformation products were not recalculated in relation to the original amount of terbutryn. If large parts of terbutryn are transformed, this affects the calculated sum of all substances, since the molecules of most of the transformation products—except TBSO—are lighter than those of terbutryn (see Supplementary S4
) and therefore lead to lower masses than the initial mass of terbutryn. The uncertainty of the measurement of transformation products is also influenced by the recovery rates, which had to be determined by a rather complex procedure. The results for the two test specimens that were analyzed in parallel for each sampling event were in similar ranges, i.e., the results for each sample differed from the mean values in ranges between 1 and 5% for terbutryn, 5 and 11% for TBSO, 3 and 5% for terbumeton, 3 and 8% for TB-DesE, 2 and 7% for TBOH, and 7 and 11% for TBOH-DesE in the four experimental series (white and red paint, UV radiation only and UV radiation combined with immersion cycles).
The results of this study are summarized in Figure 6
. TBSO, TB-DesE, and TBOH are generated by water contact as well as by UV radiation, whereas the reaction due to UV radiation is increased by occasional water contact. The secondary transformation product TBOH-DesE was observed only after UV radiation. Terbumeton was not observed in eluates from test specimens that were exposed only to water contact, but it was generated by exposure to UV radiation and was increased in the paint extracts after occasional water contact.
Transformation of terbutryn depends not only on water contact and UV radiation. It can also be affected by the type of radiation and by the surrounding matrix. Terbutryn can also be metabolized by the enzymatic reactions of organisms [13
]. Photodegradation of triazines, e.g., terbutryn, depends on the applied energy [24
]. Different irradiation sources that cause either direct or indirect photolytic reactions produced different degradation rates of terbutryn in water [25
]. Different patterns of transformation products were observed when the transformation of terbutryn was investigated either in water under laboratory conditions or in renders exposed to natural weathering. TBSO, TBOH, TB-DesE, and TBOH-DesE were the main transformation products in runoff from the renders, while desthiomethyl-terbutryn, TBOH, and TBSO were the main transformation products in the laboratory experiment on terbutryn in water. Terbutryn, TB-Des E, TBOH, TBOH-DesE, and a small amount of TBSO were detected in the render at the end of the experiment. Differences were observed between an acrylate-based and a silicone-based render [9
]. Certain reactions of terbutryn were dominant in paints that contained titanium dioxide and iron oxide pigments [16
]. Terbutryn sulfoxide was formed by biotic degradation in activated sludge [13
3.5. Diuron and Transformation Products in Paint Extracts
The amount of diuron in the test specimens decreased during the experiments due to UV radiation (see Figure 7
). This was already observed under the same experimental conditions in the previous study [16
]. In the previous study, the remaining concentrations of diuron were slightly lower in the white paint than in the red paint. The current study could not confirm this, because analytical data for diuron in extracts from the red paint were inconsistent. Therefore, comparing this observation from both studies is not justified.
The amounts of diuron in the paint extracts were lower if the test specimens were additionally exposed to immersion cycles. As for terbutryn, the differences between the diuron content of the test specimens were greater than the amount of diuron and the two demethylation products in the leachates—at least for the white paint. The inconsistent data on diuron from one experiment on the red paint (see also Figure 2
and Figure 4
, Supplementary S5 and S6
) prohibit drawing conclusions here. It can be assumed that this difference is explained not only by leaching, but also by transformation.
DCPMU content increased in the test specimens during all four experiments with slightly greater amounts in the white paint that was exposed to UV radiation. As mentioned earlier, DCPU was already present in the red paint before the experiment started. This content decreased during the first four days of the experiments with this paint. After that, the contents remained in a similar range. The amounts were slightly lower in the test specimens that were exposed to immersion cycles. This is consistent with the amounts of DCPU that were found in eluates at the beginning of the experiments with immersion cycles (see Figure 4
and Supplementary S5
). The amounts of DCPU in the white paint increased only after 16 days of UV exposure, and only after 23 days of UV exposure in combination with immersion cycles. This is probably caused by the formation of this secondary transformation product from its precursor DCPMU.
Moreover, for diuron and its transformation products, the results for the duplicate test specimens were in similar ranges, i.e., the results for each sample differed from the mean values in ranges between 2 and 5% for diuron, between 5 and 13% for DCPMU, and between 1 and 3% (31% in one series) for DCPU in the four experimental series.
In addition to DCPMU and DCPU, numerous signals were observed in aqueous solutions of diuron, DCPMU, monuron, dichloroaniline, and 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea that were exposed to UV radiation at either 254 nm or to a spectrum with its maximum at 350 nm for 48 h both by LC-MS and LC-HRMS analysis. In addition to the investigated substances, detected mass signals correspond to substances that were already reported or can be assumed as transformation products of diuron, i.e., 3-(4-chloro-3-hydroxyphenyl)-1,1-dimethylurea, 3-(3-chloro-4-hydroxyphenyl)-1,1-dimethylurea [1
], 1-(3-chloro-4-hydroxyphenyl)-3-methylurea [30
], 3,4-dichlorophenyl isocyanate [29
], 3-(3-hydroxyphenyl)-1,1-dimethylurea (fenuron), and 3-(3-hydroxyphenyl-1,1-dimethylurea (hydroxyfenuron). The formation of different dimers from fenuron and monuron [26
] was also indicated by the observed mass signals. Different distributions of substances were observed for the two maxima of applied UV radiation. Several signals could not be assigned to an assumed structure. Monuron and 3-(4-chloro-3-hydroxyphenyl)-1,1-dimethylurea or 3-(3-chloro-4-hydroxyphenyl)-1,1-dimethylurea (hydroxyfenuron) was derived from diuron under these experimental conditions (unpublished data).
However, the sum of the amounts of diuron, DCPMU, and DCPU decreased considerably during the experiments and can be explained only partly by leaching during the immersion cycles.
Studies of transformation products of diuron in runoff from construction products have considered mainly DCPMU and DCPU. So far, the occurrence of 3-(4-chloro-3-hydroxyphenyl)-1,1-dimethylurea, 3-(3-chloro-4-hydroxyphenyl)-1,1-dimethylurea in runoff [1
] and in eluates from façade coatings [22
] was also reported. Repeated analysis of the extracts from the white paint after long-term storage indicated that dichloroaniline and monuron were also formed during the experiments. The formation of dichloroaniline was apparently supported by water contact during the immersion cycles. In addition, small amounts of monuron were identified in the samples. The amounts of monuron were greater in the samples that were exposed only to UV radiation, whereas these small amounts of monuron were probably effectively leached during the immersion cycles. Comparison of the results for diuron, DCPMU, and DCPU from the analysis directly after the experiment and of methanolic extracts after long-term storage indicates that data for the later analysis are still reliable. Only the very small amounts of DCPU were not observed during the repeated analysis (see Supplementary S6
The results on diuron during this study are summarized in Figure 8
. The reasons for different observations in the different experimental procedures were not investigated. It can be assumed that the occurrence of substances is influenced by different stabilities and further reactions under the actual experimental conditions.