3.2.4. Effect of Dissolved Organic Matter on Hg Sorption at Various Salinity Levels

DOM is widely spread in environments and may control a number of essential processes relevant for Hg cycling. In this study, artificial waters with a DOM concentration of 2.6 mg-C/L was prepared with three salinity levels, and the results are shown in Figure 5. For the addition of DOM to the salinity test, the Hg removal by sorbents was relatively decreased as compared with those in the salinity test (Figure 4). In freshwater system, the sorption capacity of AC, SAC, and FeS was decreased from 286 to 137 ng/mg, 384 to 270 ng/mg, and 431 to 287 ng/mg, respectively. As for the estuary system, the sorption capacity of AC, SAC and FeS was decreased from 401 to 286 ng/mg, 408 to 366 ng/mg, and 441 to 403 ng/mg, respectively. While the sorption capacity of AC, SAC, and FeS were decreased from 355 to 302 ng/mg, 400 to 322 ng/mg, and 427 to 363 ng/mg in the seawater system, respectively (Figure 6). In Table S3, KD values also decreased significantly with the presence of DOM. Therefore, DOM may inhibit the Hg adsorption by complexation mechanism because it can compete with sorbents and complex with Hg2<sup>+</sup> [41–44]. Hg2<sup>+</sup> may form complex with organic thiol groups in DOM [45,46]. The phenolic hydroxyl groups in DOM may also complex with Hg easily to form a stable chelate that restrained the Hg adsorption [47].

**Figure 5.** Comparison of the Hg sorption capacity at various salinity levels affected by DOM.

**Figure 6.** Comparison of the influence of various salinity levels and DOM on Hg sorption capacity in the aqueous batch experiments.
