3.3.5. Total Organic Matter (TOC)

Figure 6 shows that the amount of TOC was positively related to the concentration of MeHg. Column 4, with the largest percentage of OSP, released maximum MeHg during day 20–80. In a later stage from approximately day 85, however, both MeHg and TOC concentrations in Column 6 increased suddenly.

**Figure 6.** Total organic carbon (TOC) in six columns during the operation

#### **4. Discussion**

The data of material properties showed the absolutely largest BET surface area and pore volume of HAP, CB, and FeS, which may lead to its excellent performance during adsorption due to more adsorption sites. In contrast, OSP is a non-porous material with insignificant surface area and pore volume, but OSP still had appreciable adsorption performance for several metals, suggesting that the adsorption behavior of OSP is different with its derived HAP. Abundant C content in CB indicated its additional benefits as an active material [31]. HAP was rich in H and O, probably as a result of the richness of hydroxyl groups on its surface [32], contributing to its high ability as a metal scavenger [33].

In the part of batch experiments, OSP and HAP had the optimum removal efficiencies for Cr, Zn, Cu, and Ni. HAP is regarded as an ideal material with large specific surface and high stability under both reducing and oxidizing conditions [16]. It can release phosphate to interact with metals, forming metal phosphates of low water solubility [34]. OSP has been proven to be able to raise the pH value when applied to soil [35]. If the pH is above the value that promotes metal precipitation, the removal mechanisms will be related to precipitation [36]. However, it is known that the hydroxides of Ni have the highest solubility product (Ksp) among those of the five metals, making it comparatively difficult for Ni to be removed by precipitation. As for Hg, FeS and CB showed the best immobilizing ability due to their high Hg affinity. Notably, the CB used in this study was a waste tire recycled product, which contains a significant amount of S that can form chemical bonding with Hg.

The metal potential leachability could determine their environmental risks, and it has been reported to decrease in the order of Zn > Ni > Cu > Cr at pH 4 [37], which corresponded to the concentrations of the five metals in the supernatant after incubation, revealed stronger leachability of Ni and Zn, and was related to their behaviors in the microcosm.

It is generally accepted that sediment pH is an important factor in the adsorption of adsorbate on adsorbent. Under low pH condition, most metal ions are in the cationic state in the solution. The hydrogen ions can compete for the adsorption sites with the metal ions, influencing the exchange adsorption of potential toxic metals, promoting the desorption of metal ions and causing a higher release rate [38,39]. The ability of OSP to raise the concentration of hydroxide ions mentioned before may account for the increase of pH in the columns and its high removal capacity of metals. As one of the most important factors influencing the mobility of metals, the increase of ORP in sediment will correspondingly promote the oxidization of metal sulfides and the degradation of organic compounds, both accelerating the release of the adsorbed/complexing metals [40]. It appeared that ORP of Column 6 was higher than the other five capped in general, mainly due to the decrease of dissolved oxygen caused by caps in the other five columns.

In the part of the lab-scale microcosm experiments, the data from Column 5 indicated the unnecessary existence of kaolinite whether for adsorption or for stabilization in such a condition, which is inconsistent with the suggestions from some earlier studies. The reason may be due to that HAP and OSP could play the same role as kaolinite to stabilize the caps. The difference between Column 1 and Column 4 shown in Figure 5 was related to different performances of HAP and OSP to remove Ni as Figure 3a illustrated. Nevertheless, using caps mainly composed of OSP can yet be regarded as a good choice when considering costs. In batch adsorption, FeS showed the third best adsorption ability for Ni by a reduction of 10.73%, and yet CB hardly worked (Figure 3a). Thus, the release in the later stage of the experiment of Ni in Column 2 may be due to the ineffectiveness of CB. In contrast, Column 3 (5% FeS) did worse than Column 2 (5% CB) for lowering the concentration of MeHg in the long term (Figure 3f). FeS can reduce the concentration of soluble Hg species, resulting in less methylation of Hg [41]. Moreover, part of FeS was likely to be converted into insoluble HgS and taken out by the overlying water, which may explain the unsuccessful results of Column 3 to immobilize MeHg.

The relationship between TOC and MeHg during day 20–80 was likely caused by the remaining organic matter existing in OSP. It has been observed that organic matter content seemed to play a critical role for MeHg formation, acting as electron donor for Hg methylation bacteria [42,43]. The sediment in Column 4 with more OSP would likely release a greater amount of MeHg. Without capping, TOC was possibly easier to release from sediment, which then caused the uprush of MeHg in Column 6 from day 85.
