Biodegradation tests of mercaptocarboxylic acids, their esters, related divalent sulfur compounds and mercaptans

Mercaptocarboxylic acids and their esters, a class of difunctional compounds bearing both a mercapto and a carboxylic acid or ester functional group, are industrial chemicals of potential environmental concern. Biodegradation of such compounds was systematically investigated here, both by literature search and by experiments (Closed Bottle Test OECD 301D and Manometric Respirometry Test OECD 301F). These compounds were found either readily biodegradable or at least biodegradable to a significant extent. Some related compounds of divalent sulfur were tested for comparison (mercaptans, sulfides, disulfides). For the two relevant monofunctional compound classes, carboxylic acids/esters and mercaptans, literature data were compiled, and by comparison with structurally similar compounds without these functional groups, the influence of COOH/COOR’ and SH groups on biodegradability was evaluated. Thereby, an existing rule of thumb for biodegradation of carboxylic acids/esters was supported by experimental data, and a rule of thumb could be formulated for mercaptans. Concurrent to biodegradation, abiotic processes were observed in the experiments, rapid oxidative formation of disulfides (dimerisation of monomercaptans and cyclisation of dimercaptans) and hydrolysis of esters. Some problems that compromise the reproducibility of biodegradation test results were discussed. Electronic supplementary material The online version of this article (10.1007/s11356-018-1812-x) contains supplementary material, which is available to authorized users.


Experimental details of biodegradation experiments of several parent compounds
3-Mercaptopropionic acid (3-MPA) was completely transformed in both CBT and MRT at day 0 already into transformation product 3-MPA_TP1 that proved to be the corresponding disulfide (3-MPA disulfide = DTDPA) by retention time and MS comparison with authentic material (Table S2). Further, the day 0 transformed 3-MPA samples and authentic 3-MPA disulfide behaved identically in biodegradation tests: The disulfide was not further transformed in CBT or in the sterile bottle of MRT, however, at day 28 it had completely disappeared in the test and toxicity control bottles of MRT. No further products were found. These results are in concordance with the low biodegradation in CBT and high biodegradation in MRT observed for 3-MPA.
Methyl 3-mercaptopropionate (MMP) was transformed at day 0 of CBT already into three compounds, MMP_TP1 -MMP_TP3. The major product, MMP_TP1, was identified as the corresponding disulfide by retention time and MS comparison with authentic material. The minor product MMP_TP2 was 3-MPA disulfide (comparison with authentic material), while minor product MMP_TP3 was the monomethyl ester of 3-MPA disulfide according to MS data (Table S2). At CBT day 28 MMP disulfide had completely disappeared, while 3-MPA disulfide and the disulfide monoester were still present.
In MRT at day 0 the same three products resulted from MMP, at day 28 MMP disulfide had decreased, the disulfide monoester had increased with respect to day 0, 3-MPA disulfide was still present, and another minor peak (MMP_TP4) was observed but could not be identified.
Authentic MMP disulfide also was hydrolysed in CBT into its monoester (MMP_TP3) and 3-MPA disulfide (MMP_TP2). In MRT of MMP disulfide, the monoester was found, but no 3-MPA disulfide. Since the latter is completely degraded in MRT, we assume it was formed here also but was rather quickly biodegraded.
In conclusion, MMP was quickly transformed into its disulfide by oxidation while concomitant ester hydrolysis occurred in concurrence to biodegradation.
Butyl 3-mercaptopropionate (BuMP, tR 12.4 min, Figure S1) on day 0 of CBT and of MRT was quickly but not yet completely transformed into BuMP_TP1 (tR 17.6 min) and slowly into BuMP_TP2 (tR 11.9 min), which were identified as the corresponding disulfide (BuMP disulfide) and the disulfide monobutyl ester, respectively, via MS (Table S2).
In CBT all three compounds had completely disappeared at day 28. In MRT on day 28 BuMP and BuMP disulfide had completely disappeared ( Figure S1), while the disulfide monoester and another unidentified minor peak (BuMP_TP3, tR 11.2 min) were still present. 3-MPA disulfide (BuMP_TP4, tR 3.8 min) was found as a very small peak in BuMP MRT day 0 and day 28 samples, while 3-MPA was not detected. Another peak (BuMP_TP5, tR 14.3 min) was observed but could not be identified.
Glycol bis(3-mercaptopropionate) (GDMP, tR 10.7 min) also was eliminated from biodegradation test solutions from day 0 on, at day 28 the elimination was complete in both CBT and MRT. The major product (GDMP_TP1, tR 9.6 min) is regarded as the corresponding cyclic disulfide, the product of oxidative cyclization, based on several major MS signals that are less heavy by 2 Dalton than those of GDMP (Table S2). This identification is supported by the retention time difference that is analogous to that observed between DMDS and its cyclic disulfide. Several minor peaks were found at day 28, but since some of these appeared already in the freshly prepared test solution (day 0), they may be impurities rather than degradation products, and none of these was identified.

Biodegradation of GDMP was low in CBT and intermediate in MRT in our experiments.
The dimercapto thioether DMDS (tR 15.9 min) was quickly and completely transformed in CBT and MRT at day 0 to a product (DMDS_TP1, tR 14.1 min) that was identified as 1,2,5-trithiepane by HPLC-UV comparison with authentic material (Table S2), a cyclic disulfide formed by oxidative cyclization. 1,2,5-Trithiepane was not further biotransformed up to day 28 in CBT or MRT.  Table S2. HPLC retention times (tR) and MS and MS 2 data of parent compounds and transformation products