Stabilization/solidification (S/S) of mercury-contaminated hazardous wastes using thiol-functionalized zeolite and Portland cement
Introduction
Mercury is of great concern receiving a major focus due to its unique high toxicity, volatility, and persistence in the environment and easiness of bio-accumulation. Organic forms of mercury are generally more toxic than inorganic forms, but it is possible for inorganic mercury to be biologically methylated. Methyl mercury has high affinities for fatty tissues in organisms and can accumulate through food chain to higher toxic levels within those organisms. Therefore, it is important to strictly control inorganic mercury leaching from mercury-containing wastes [1], [2]. A wide variety of industries can generate mercury-containing solid wastes, including chlor-alkali manufacturing, weapons production, copper and zinc smelting, gold mining, painting application, etc. [3]. Heavy metals, such as Pb, Cd and Cr, have been successfully immobilized by using stabilization/solidification (S/S) technologies. The conventional S/S technologies, however, cannot effectively reduce the leachability of mercury [4]. It is reported that mercury tends to hydrolyte to form a red precipitate of HgO in Portland cement, but the mercury still has a strong potential to volatilize from cement-solidified sludge [5], [6]. It was known that utilizing the combination of the chemical and physical isolation processes can effectively immobilize mercury [7], [8].
Natural zeolite has recently been found as cost-effective material due to its low-cost and high capability to cation exchange. Many researchers have studied the use of natural zeolite for the adsorption of mercury from the gas phase and aqueous solutions [9], [10]. The effect of grafting of the thiol group (–SH) to the support matrixes containing inorganic oxides (e.g. silica, alumina, or clay) has also been investigated [11], [12], [13]. The coupling of –SH to the support matrixes results in great enhancement of mercury adsorption capacity. This enhancement in mercury adsorption is attributed to the presence of the surface-bound thiol, which has strong binding affinities for mercury and can form mercuric sulfide on the matrixes surface. It has been tested that the reaction between the mercury and -SH is the most favorable thermodynamically [14] and it has been shown that the metal sorption is directly related to the amount of thiol groups available in the sorbent [15]. Haile used cysteamine hydrochloride to modify natural clinoptilolite zeolites, the mercury adsorption capacity was enhanced greatly upon modification and the amount of –SH in the modified zeolite is 0.375 mmol g−1 [16]. Many researchers have found that the amount of –SH in the sorbent is highly dependent on the method to introduce –SH into the sorbent, which then can have great effect on mercury adsorption capacity. 3-mercaptopropyltriethoxysilane has been successfully used to graft –SH to many support matrixes containing inorganic oxides, such as silica, alumina and clay et al., the amount of –SH could be as high as 0.55–1.5 mmol g−1. But so far 3-mercaptopropyltriethoxysilane has never been used to graft –SH to natural zeolite, and the mercury sorption characters have never been studied.
The major objective of this study was to find a cost-effective method to treat and dispose mercury-containing solid wastes. In this study, 3-mercaptopropyltriethoxysilane was used to graft the –SH to the natural clinoptilolite zeolites and the mercury adsorption capacity onto the thiol-functionalized zeolite from the aqueous solutions was investigated. The thiol-functionalized zeolite was used to stabilize mercury in solid wastes, and then the stabilized solid wastes were subjected to cement solidification to test the effectiveness of the whole S/S process.
Section snippets
Thiol-functionalized zeolite (TFZ) preparation
TFZ was used in this study. The natural zeolite mineral comes from a quarry in Jiutai city, Jilin Province of China and was supplied by the company “Jiutai Zeolite Mining Company”. To improve the adsorption capacity of the natural zeolite, it was treated with 3-mercaptopropyltriethoxysilane and TFZ was prepared as below. 5.0 g natural zeolite was refluxed in 250 mL of toluene containing 5.0 g 3-mercaptopropyltriethoxysilane at 110°C for 24 h. The zeolite was then recovered by filtration and washed
Hg2+ adsorption isotherms
Fig. 1 shows the isotherms obtained for the mercury retention by TFZ and NZ. Langmuir and Freundlich isotherm models were used to assess the adsorption of mercury on the zeolites and the adsorption data was showed in Table 1.
The Freundlich adsorption isotherm is represented as:
The Langmuir adsorption isotherm is represented as:
Where Qe is the amount of mercury adsorbed at the equilibrium concentration (Ce), Qmax is the maximum adsorption capacity of
Conclusions
From the results presented above, it is evident that stabilization of mercury in solid wastes by thiol-functionalized zeolite is successful. Grafting of thiol group to the natural zeolite effectively improves the mercury adsorption capacity of the zeolite mineral, possibly due to the action between SH group and mercury on the thiol-functionalized zeolite surface. TFZ has a high level of SH content (0.562 mmol g−1) and the adsorption of mercury by TFZ conforms to the Freundlich adsorption
Acknowledgements
The authors would like to acknowledge the support of the National Basic Research Program (973) of China (No. 2004CB418507) and National water pollution control and management of major special Technology (2008ZX07208-009).
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2020, Environmental PollutionCitation Excerpt :The Langmuir maximum sorption capacity (Qmax) of SRHB for Hg has been shown to be ∼80% higher than that of RHB (O’Connor et al., 2018). The increased amount of sorption is attributed to the strong binding affinity between S and Hg, which forms the highly stable compound HgS (cinnabar) (O’Connor et al., 2018; O’Connor et al., 2018b; Xing et al., 2019; Zhang et al., 2009). SRHB is considered a particularly promising option for treating Hg polluted surface soils (O’Connor et al., 2018).