Capping Materials

For the application of thin layer capping on Hg-contaminated sites, laboratory studies have been conducted using AC [14, 57, 64-66], biochar [64, 67], or surface-modified black carbon [14] to immobilize Hg. Some studies have shown that the amendment of black carbon may successfully reduce bioavailable Hg and MeHg to benthic organisms [65, 66]. Beside bench-scale studies, several field-scale studies [62, 68] have also been carried out to and observed with moderate success in reducing Hg in porewater or biotas.

Activated carbon (AC) is composed of defective graphene layers, which are formed by gasification of carbon atoms via activation (often by thermal treatment). After activation, AC can be filled with pores that greatly increase surface area (as high as 500–

1500 m²/g; [24]) and intensify van der Waals force. With its high specific surface area, AC has proven to be a promising option for remediation of contaminated sites for not only organic pollutants[47-56] but also Hg [64-66, 68, 69]. AC has several kinds of acid functional groups such as carboxyl groups on its surface, and are expected to chemically adsorb Hg [13].

Nevertheless, the sorption capacity of AC toward Hg is still limited due to the nonpolar characteristics of activated carbon, which hinder interactions between charged metal species and the solid surface [43].

The other main concern of applying black carbon to sediment remediation is its possible adverse effects on the benthic organisms itself. One-fifth of previous studies have reported adverse bio-effects using black carbon to sequestrate Hg [70]. Benthic biotas suffered from reduced species richness, biomass loss, reduced feeding rate, organ damage, or reduced growth after carbon amendment. Several studies suggested that the

possible explanations are because the black carbon may reduce nutrients’ bioavailability [71], or reduce ingestion rate by harming gut structures [72].

SAC has been verified to enhance Hg adsorption capacity than its raw AC precursor in aqueous adsorption tests (Table 2-2; [12, 13, 41, 73, 74]. Sulfurization of AC enhance Hg sorption capacity in most essays [12, 13, 41, 73, 74] and sometimes over 100% by magnitude [12, 13]. For example, Wang et al. [12] demonstrated that an increase in Hg adsorption capacity of coconut AC from 150 to 820 mg/g by sulfurization with sulfur powder (C/S ratio: 1:2) was shown. Li et al. [13] increased Hg adsorption capacity of coke from 315.8 to 694.9 mg/g by impregnated the coke with sulfuric acid (80°C, 13 h).

Also, the stability of Hg sorption to SAC was also known to increase as Hg was found stably sorbed to SAC in a broad pH range [12]. However, SAC has yet been tested to apply on Hg-contaminated sediment.

Biochar is another attractive sorbents as a potential remediation material since it is less costly than AC and may play the role of fertilizer if applied in soil [75]. Similar to AC, biochar is a porous, carbon-rich, black carbon material produced by thermally decomposing biomass under low-oxygen concentrations and temperatures between 300–

1000ºC. Gomez-Eyles et al. [64] conducted an experiment testing Hg sorption affinity of 13 kinds of biochars and 4 kinds of AC, the results showed that both AC and biochar have efficient Hg sorption affinity with AC superior in Hg²⁺ sorption.

Clay is a general term for a broad range of inorganic mineral with the size within 2 mm. With good thermal stability, cation exchange capacity, and settling clay may be the other potential capping material [76]. Clay minerals could be further modified to organoclays, and was reported to enhance sorption capacity up to 10–30 times to Cr, As,

Pb, Cd, and Zn [77-79].

Table 2-2. SAC enhances Hg sorption capacity in aqueous adsorption tests.

Iron sulfide minerals (FexSy) have been found to have good immobilizing ability to double-valent metals, such as Mn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Pb²⁺, and Hg²⁺ [80-85], basically due to their adsorption or precipitation properties. In addition, iron sulfides have also been reported to degrade organic contaminants by reduction mechanisms. The common forms of iron sulfide minerals include mackinawite (FeS), greigite (Fe3S4), pyrite (FeS2), and pyrrhotite (Fe1-xS) [86]. In earlier research,

mackinawite, pyrite, and pyrrhotite have been shown to have high potential to sequestrate Hg [87, 88]. Some of the more recent studies have been conducted using lab-made nano-mackinawite to sequestrate Hg. Liu et al. [86] reported FeS with adsorption capacity up to 1700 mg/g (by calculation) at pH = 5.6; more than 99% Hg was removed at Hg/FeS ratio < 1000, with 77% of the removal by precipitation and 23% by adsorption (Hg/FeS

= 0.22). Sun et al. [89] found that commercial pyrite had lower Hg sorption capacity (9.9 mg/g) as compared to lab-synthesized mackinawite (769.2 mg/g), probably due to the smooth surface of pyrite. Skyllberg and Drott [90] conducted a slurry batches study, discovering that the dosage of 2% FeS in 5000 μg-Hg/g organic-rich soil may outcompete O/N ligands in sediment for Hg sorption, resulting in 50% Hg sorbed on FeS to form HgS4 (metacinnabar). As the ratio of FeS increased to up to 20%, a complete outcompete with 100% Hg sorbed on FeS was observed.

To date, a promising active material in remediating Hg-contaminated sites is yet to be proven with good adsorption efficiency, stability, and eco-friendliness. Consequently, further research is needed in several areas, including (1) fabricating novel capping materials with good adsorption affinity for Hg or other heavy metals; (2) establishing competitive adsorption models of amendments in sediment condition to evaluate real adsorption outcomes in nature disturbance; (3) introducing amendments with biological tests to prove the efficiency of reducing Hg bioavailability; (4) developing efficient, low-impact capping delivery systems; (5) assessing long-term stability and ecological recovery of placing active caps; (6) conducting life-cycle analysis for thin layer capping remediation.