**1. Introduction**

Wastewater containing potential toxic metals originating from anthropogenic activities discharged to river streams is a widespread environmental issue nowadays, processing significant toxicity to aquatic organisms and accumulating by food chain, finally causing various diseases and disorders [1]. Natural processes are frequently inadequate to deal with the elevated metal loading, therefore there is an urgent need for remediation measures [2]. Thin-layer capping has been applied as an economically-feasible in-situ method for sediment remediation, reducing contaminants release from sediment to overlying water, subsequently reducing ecological and human health risk [3]. Sediment can be seen as an important sink of various organic and inorganic compounds, resulting in the simultaneous existence of several different contaminants [4–6]. However, most of the previous studies focused on only one or two capping materials. To cope with highly complex conditions in sediment, mixed capping with multiple materials was proposed. Notably, using cheap and effective alternatives for the removal of potential toxic metals could reduce operating costs, reduce the prices of products, improve competitiveness, and benefit the environment [7].

Several relatively cheap, environment-friendly materials have been proposed and examined. Based on previous research [7–22], kaolinite, carbon black (CB), iron sulfide (FeS), oyster shell powder (OSP), and hydroxyapatite (HAP; Ca10(PO4)6(OH)2,) were considered as potentially ideal amendments to be part of the mixed caps. Previous reports provided excellent results showing that kaolinite can be used as a cheap and naturally occurring adsorbent to remove Pb2<sup>+</sup>, Cd2<sup>+</sup>, Ni2<sup>+</sup>, Co2<sup>+</sup>, Cr6<sup>+</sup>, Zn2<sup>+</sup>, and Cu2<sup>+</sup> from aqueous solution in both single and multi-metal ions [8–11]. Sulfide minerals are shown to be ideal materials to scavenge Hg2<sup>+</sup> by immobilizing it through adsorption or co-precipitation as a discrete sulfide phase (HgS) and solid solution formation with iron sulfides [12,13]. HAP jumped out recently because of its high adsorption capacity of various metals, which brings the perspective for removal of Cd2+, Zn2+, Pb2+, U6<sup>+</sup>, Co2<sup>+</sup> ions, and so on [14–17]. HAP is a nanostructured material, which was recently synthesized from some high-calcium biological wastes as raw materials, such as seashells and eggshells [18]. In the hunt of such low-cost and efficient raw materials for the production of HAP, OSP has emerged as a suitable one. As for OSP itself, owing to its low price and basicity, it has been employed widely for stabilization/solidification of many kinds of potential toxic metals (As, Pb, Ni, and Cu, etc.) [7,19–22]. The solution pH of OSP greatly affected the adsorption process towards Cu2<sup>+</sup>, with an optimum adsorption pH of 5.5 and an overall negative surface charge facilitating the adsorption process [19]. The adsorption capacities of OSP towards Cu2<sup>+</sup> and Ni2<sup>+</sup> could reach 49.26–103.1 mg g−<sup>1</sup> and 48.75–94.3 mg g<sup>−</sup>1, respectively, through physical adsorption mechanisms [7].

The present study aims to find mixed amendments with the best comprehensive benefits for immobilizing potential toxic metals in contaminated sediment. Based on the previous works, kaolinite, CB, FeS, OSP, and HAP were selected as potentially ideal amendments to immobilize metals in sediment. According to the survey report of Taiwan in 2012 [23] on main rivers with contaminated sediment, Ni, Cr, Cu, Zn, and Hg are the five major potential toxic metals present in sediments, which were selected as the targets of remediation in this study. Furthermore, methylmercury (MeHg) converted from Hg was measured in this study due to its high toxicity and bioaccumulative ability. Aqueous batch experiments were first conducted by using the five adsorbents to capture the five potential toxic metals individually at various concentrations. Then, five columns containing contaminated sediment covered by different mixed materials were set based on the obtained results of batch experiments to better comprehend the immobilization effectiveness of mixed capping for the potential toxic metals as compared to the controlled column without caps. Results from this study are helpful in designing an economically and technically feasible in−situ approach for sediment remediation, which could not only reduce the potential toxic metal release from sediment to overlying water, but also reduce the ecological and human health risk.

#### **2. Materials and Methods**

## *2.1. Adsorbents Preparation*

The kaolinite and FeS used in this study were all reagent-grade chemicals purchased from Sigma-Aldrich. The CB used was obtained from Enrestec Inc. (Tainan, Taiwan), and was considered as a low-cost, recycled materials because it was a byproduct from waste rubber tire pyrolysis for oil production.

OSP was prepared by grinding oyster shells collected as food waste materials. The oyster shells were scrubbed carefully to remove impurities, then dried for 48 h at 100 ◦C [19] and ground to homogenized powder that could pass through a 30-mesh sieve. Then this sieved material was shaken over a 60-mesh sieve and any material that passed through was rejected (ASTM D2765). [24]

To synthesize HAP, the OSP passed though the 60-mesh sieve was collected as a raw material. The synthesis was operated according to the following reaction: 10Ca2<sup>+</sup> + 6PO4 <sup>3</sup><sup>−</sup> + 2OH<sup>−</sup> = Ca10(PO4)6(OH)2, referring to previous studies [18,25–27] (Figure 1). In the synthesis process, an amount of Na2HPO4 and the corresponding amount of oyster shell powder (Ca/P molar ratio = 1.67) were used as P and Ca precursors, respectively. Firstly, 5 g of that oyster shell powder (<60 mesh) was dissolved

in 1:3 hydrochloride acid/water solution and stirred thoroughly. The supernatant of the solution filtered by 0.45 μm filter was put into 500 mL of 0.1 M EDTA. Then reaction process was carried out by drop wise addition of 0.06 M Na2HPO4 solution under continuous stirring of 200 rpm. The pH value of the reaction mixture was regulated within the range of 10.5–11.5 by 5 M NaOH solution and maintained throughout the process the dripping velocity was controlled at 2 mL min<sup>−</sup>1. After mixing, the resulting mixture was left to maturate for the next 24 h. The obtained white precipitate was washed with distilled water by centrifuge and dried in the hot air oven at 65 ◦C for 24 h to get the final product.

**Figure 1.** The process of synthesizing HAP.

All of the above materials were sieved to the range of 30–60 mesh (ASTM D2765) [24] and analyzed using a physisorption analyzer (Micromeritics Inc. ASAP 2420, Norcross, GA, USA) to obtain the 77 K N2 adsorption isotherm. The total surface area was then obtained based on the Brunauer-Emmett-Teller (BET) equation and described as BET surface area (SBET). The chemical compositions of the five materials were analyzed for the C/H/N/S contents (Elementar Vario EL cube, Langenselbold, Germany) and O content by automatic elemental analyzers (Flash 2000, Thermo Fisher Scientific, Waltham, MA, USA).
