*2.1. Solubility in Water*

The solubility of GAP was higher than 1000 mg/mL at 24 ± 1 in UPW and snSW, being classified according to the USP Pharmacopeia as a "very soluble" compound [29]. The high water solubility of GAP is in accordance with the reduced value (−7.02) of its *n*-octanol/water partition coe fficient (logKow) previously calculated with EPISUITE [24]. This suggests that water will retain the compound in solution, reinforcing the low a ffinity of GAP for sediments and fatty tissues, which makes GAP an eco-friendly candidate to be incorporated in coatings [22,24].

### *2.2. Optimization of the IP-RP-HPLC Method*

An IP-RP-HPLC method was developed to quantify GAP in UPW and snSW. First, several mobile phases containing di fferent proportions of acetonitrile and acidified water (0.1% acetic acid) were investigated. However, the retention factor (k) was not satisfactory (less than 1), with GAP overlapping with the solvent front. As the retention of this charged molecule was not achieved by regular reversed-phase, a mobile phase containing an aqueous solution of 25 mM of tetrabutylammonium bromide (TBA-Br) and acetonitrile (38:62 *v*/*v*) was used to favor ion pairing, and consequently suitable k values between 1-10 could be reached [28]. The ion pairing of GAP dissolved in UPW was accomplished, with an acceptable k value of 1. However, in snSW a k of 0 was still observed, even using di fferent proportions of the mobile phase. In order to assure retention (k - 0), using the same aqueous mobile phase containing TBA-Br and acetonitrile (38:62 *v*/*v*), a pre-treatment of the sample was carried out before injection, diluting GAP dissolved in snSW with 60 mM of TBA-Br in a proportion of 1:3, leading to ion pairing with a desirable k value of one.

### *2.3. Stability of Sulfate Groups in Water*

To assess the stability of GAP in water, its half-life (DT50) was evaluated in UPW and snSW. GAP was exposed to several stress conditions (4 ◦C, 18 ◦C and r.t. (24 ± 1) in the absence and presence of natural light) in order to mimic di fferent natural environmental conditions and its stability was determined at 0, 2 and 9 months.

In UPW, the concentration of GAP significantly decreases after the first 2 months (except at 4 ◦C in darkness), but it remains approximately constant during the following consecutive months, for a total of 9 months (Figure 2), suggesting the chemical stability of the sulfate linkage (without significant di fferences (*p* < 0.05) between the initial time and the ninth month).

**Figure 2.** Stability of gallic acid persulfate (GAP) dissolved in ultra-pure water (UPW) (200 μM) and exposed to several stress conditions over a period of 9 months. \* Indicates significant differences at *p* < 0.05 (Dunnett test) against the negative control (initial time (T0M), blue bar). T0M: initial time; T2M: two months; T9M: nine months).

After 9 months in snSW, GAP was abiotically degraded, reaching a significant 50% degradation (*p* < 0.05) from which a half-life (DT50) value of 7 months in all tested stress conditions was calculated (Figure 3). This study also suggests that photolysis may not accelerate GAP degradation, since the degradation rate did not increase in the presence of light and there are non-significant differences (*p* < 0.05). In the future, the identification of transformation products by liquid chromatography associated to high resolution Mass Spectrometry (LC-MS), as well as their ecotoxicological evaluation, will be performed, providing information to regulatory authorities.

**Figure 3.** Stability of gallic acid persulfate (GAP) dissolved in sterilized natural seawater (snSW) (200 μM), exposed to the several stress conditions over a period of 9 months. \* Indicates significant differences at *p* < 0.05 (Dunnett test) against the negative control (T0M, blue bar). T0M: initial time; T2M: two months; T9M: nine months).
