Influence of dissolved organic matter on carbonyl sulfide and carbon disulfide formation from cysteine during sunlight photolysis

Abstract

Sunlight photolysis of cysteine and dissolved organic matter (DOM) from freshwater to open ocean water sources generates carbonyl sulfide (COS) but not carbon disulfide (CS2).

Influence of dissolved organic matter on carbonyl sulfide and carbon disulfide formation from dimethyl sulfide during sunlight photolysis

“…Fresh stock solutions of DMS were prepared daily by placing 10 μl of pure liquid DMS ( ρ = 840 kg/m 3 ) into 10‐ml ACN to reach a final concentration of 13.5 mM. Stock solutions of SRFA and phenol were prepared at final concentrations of ~368 mg C/L and 1.0 M, respectively, following a similar approach detailed previously (Modiri Gharehveran et al, 2020). A Thermo Scientific water purification system was used to obtain reagent grade water (≥18.2 MΏ cm at 25°C).…”

“…Four DOM isolates were tested in this study and were identical to those used previously (Modiri Gharehveran et al, 2020). The methods behind how they were collected and characterized were discussed therein (Modiri Gharehveran et al, 2020) but will be briefly described here.…”

“…This study attempted to close these uncertainties by evaluating how DMS photochemically generated COS and CS 2 in the presence of various DOM types and concentrations, while further aiming to identify the key RIs involved. These results were also directly cross‐compared with results obtained previously when CYS served as the organic sulfur precursor instead and when DMS was spiked into natural waters, because in both cases, nearly identical experimental regimes were used (Modiri Gharehveran et al, 2020; Modiri Gharehveran & Shah, 2018). Here, the experimental regime included conducting photochemical experiments where synthetic solutions containing DMS (14 μM) and one DOM isolate over varied concentrations (0–20 mg C/L) were irradiated using simulated sunlight for up to 4 h. DOM isolates were collected from four different sources that included freshwater DOM, brackish water DOM, and open‐ocean water DOM.…”

“…COS formation from various DMS‐spiked natural waters was previously confirmed to correlate well with these waters' inherent dissolved organic carbon (DOC) concentrations when irradiated by simulated sunlight for 4 h (Modiri Gharehveran & Shah, 2018). In addition, the presence of various types of DOM isolates ranging from freshwater (e.g., Suwanee River Fulvic Acid [SRFA]) to open‐ocean DOM increased COS formation in synthetically derived waters spiked with cysteine; however, formation was relatively independent of DOM type (Modiri Gharehveran et al, 2020). DMS is proposed to form COS by way of specific DOM‐generated RIs that initially trigger formation of a sulfur‐based or a carbon‐based radical (Scheme 1).…”

“…Certain solutions were also amended with phenol to selectively quench 3 CDOM* to better evaluate its role on COS and CS 2 formation. In addition, all these solutions were modified in two important ways, as executed previously (Modiri Gharehveran et al, 2020; Modiri Gharehveran & Shah, 2018). First, the solutions were deoxygenated prior to sunlight exposure to extend 3 CDOM* lifetimes, because O 2 quenches excited triplet states (McNeill & Canonica, 2016), and to extend DMS‐derived radical lifetimes, because O 2 can also quench sulfur‐based radicals (Karoui et al, 1996).…”