Inhibiting mercury reemission and enhancing magnesia recovery by

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Inhibiting mercury reemission and enhancing magnesia recovery by cobalt loaded carbon nanotubes in a novel magnesia desulfurization process Lidong Wang, Tieyue Qi, Mengxuan Hu, Shihan Zhang, Peiyao Xu, Dan Qi, Siyu Wu, and Huining Xiao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03364 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Environmental Science & Technology

ACS Paragon Plus Environment


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Inhibiting mercury reemission and enhancing magnesia recovery by

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cobalt loaded carbon nanotubes in a novel magnesia desulfurization

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process

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Lidong Wang1,*, Tieyue Qi1, Mengxuan Hu1, Shihan Zhang2,*, Peiyao Xu1, Dan

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Qi1，Siyu Wu1，Huining Xiao1

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1 School of Environmental Science and Engineering, North China Electric Power

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University, Baoding 071003, China

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2 College of Environment, Zhejiang University of Technology, Hangzhou, 310014

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Abstract: Mercury re-emission because of the reduction of Hg2+ to form Hg0 by

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sulfite has been becoming a great concern in the desulfurization process. Lowering

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down the concentrations of Hg2+ and sulfite in the desulfurization slurry can retard the

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Hg0 formation and thus mitigate the mercury re-emission. To that end, cobalt-based

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carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg2+ removal and

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sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the

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Hg2+ adsorption and effect of Hg2+ adsorption on catalytic activity of Co-CNTs were

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investigated. Experimental results revealed that the Co-CNTs not only accelerated

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sulfite oxidation to enable the recovery of desulfurization by-products, but also acted

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as an effective adsorbent of Hg2+ removal. The Hg2+ adsorption rate mainly depended

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on the structure of the adsorption material regardless of the cobalt loading and

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morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation

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was not significantly affected due to the Hg2+ adsorption. Additionally, the isothermal

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adsorption behavior was well fitted to the Langmuir model with an adsorption

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capacity of 166.7mg/g. The mercury mass balance analysis revealed that the Hg0

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re-emission was decreased by 156% by adding 2.0g/L of Co-CNTs. These results can

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be used as a reference for the simultaneous removal of multiple pollutants in the wet 1


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desulfurization process.

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1. INTRODUCTION

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Magnesia desulfurization process is regarded as a promising alternative for the SO2

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removal from the industrial boilers and has been widely deployed in small and

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medium-sized boilers1. It possesses various advantages such as small footprint, high

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desulfurization efficiency, high flue gas adaptability, and low operation cost2, 3. The

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re-emission of Hg0 is one of the key challenges in wet desulfurization processes5. The

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reduction of Hg2+ to Hg0 occurs in the presence of sulfite which formed during the

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SO2 absorption. The produced Hg0 can re-emit into the atmosphere from the slurry

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due to its low boiling point and thus result in a potential secondary pollution6.

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Currently, researches on the re-emission of mercury were mainly investigated in the

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conventional limestone-based desulfurization process. Removing the Hg2+ from the

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slurry is deemed as a promising method to mitigate the Hg0 re-emission. Chemical

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reaction with a trapping agent to chelate or precipitate Hg2+ has been reported to

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remove the Hg2+. For example, Amrhein et al7. added ethylene diamine triacetic acid

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(EDTA)-type chelating agents including N-(2-hydroxyethyl)-ethylene diamine

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triacetic acid (HEDTA), diethylene triamine pentacetate acid (DTPA), and

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nitrilotriacetic acid (NTA) to a wet flue gas desulfurization system. Ochoa-Gonzalez

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et al.8 added trapping reagents such as NaHS and 2, 5- dihydro -2, 4,

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5-Trimethylthiazoline (TMT). Although the addition of all the reported additives

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inhibited Hg2+ reduction to Hg0 and hence decreased its re-emission, the Hg2+ chelates

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adsorbed on the gypsum particles and thus decreased the purity of the gypsum

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by-product. In addition, the conventional methods via dosing trapping agents are

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costly9.

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Mercury removal from the wastewater by adsorption with porous materials has 2



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been investigated. Yantasee et al.10 found that super paramagnetic iron oxide (Fe3O4)

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nanoparticles with a surface functionalization of dimercapto succinic acid (DMSA)

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were effective sorbents for the Hg2+ adsorption with a large surface area (114m2/g).

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Yardim et al.11 developed activated carbon with a specific surface area of 1100m2/g

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using concentrated sulfuric acid–treated furfural which had a Hg2+ adsorption capacity

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of 174mg/g. Carbon nanotubes (CNTs) with large specific surface area, high surface

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energy, and abundant porous structures also exhibit high Hg2+ adsorption capacity.

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Shadbad et al.14 used CNTs with a specific surface area of 280m2/g to adsorb Hg2+.

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They observed that the adsorption behavior onto the CNTs fitted a quasi-second-order

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kinetic model. Its maximum adsorption capacity (78mg/g) was reached at pH 6.5–7.5.

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Similar results were also obtained by Saleh et al.15 and Moghaddam et al.16.

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Our previous work developed solid porous materials, such as molecular sieve17,

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cobalt-based carbon nanotubes (Co-CNTs)18 and Co-SBA-15 catalysts19, to fabricate

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supported cobalt catalyst for promoting sulfite oxidation to recovery sulfate. The

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developed catalysts increased the sulfite oxidation rate by 2–8 folds which are

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beneficial to downsize the desulfurization tower and solve the catalyst-recycling

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issue. However, the impact of Hg2+ adsorption onto the solid catalyst on its catalytic

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activity remains unclear. Nonetheless, the CNTs, support of Co-CNTs, provides a

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large specific surface area (CNTs: greater than 90m2/g) and numerous functional

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groups (such as carboxyls and hydroxyls) for Hg2+ adsorption. Therefore, the

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Co-CNTs may also have the potential for effective Hg2+ adsorption and hence reduce

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the re-emission of mercury. Furthermore, the information on mercury re-emission in

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the magnesium desulfurization process is limited.

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Overall, the porous material supported cobalt catalysts not only can substantially

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increase the sulfite oxidation rate in the desulfurization slurry but also have the 3


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potential to simultaneously remove the coexisting pollutant Hg2+ and hence reduce the

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mercury re-emission in the desulfurization process. Based on fixed-bed adsorption

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process, this work proposed a novel magnesium-based desulfurization process with

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the simultaneous mercury adsorption ability. In this work, Co-CNTs were prepared to

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evaluate their performance for the simultaneous Hg2+ removal and sulfite oxidation

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under the conditions of the simulated magnesia-based desulfurization process. The

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effect of Hg2+ adsorption on catalytic activity for the sulfite oxidization was

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investigated. Furthermore, the kinetics and mechanism of Hg2+ adsorption onto

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Co-CNTs were also determined. These results can provide a theoretical fundament for

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the use of the solid materials to control multiple pollutants in desulfurization process.

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2. EXPERIMENTAL METHODS

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2.1 Materials

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The CNTs (10-50 nm in length) were purchased from the Chengdu Organic

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Chemicals Co., Ltd., Chinese Academy of Sciences. Standard mercury solution was

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purchased from the National Center of Analysis and Testing for Nonferrous Metals

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and Electronic Materials. The reagents of hydrochloric acid, sodium hydroxide, and

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cobalt nitrate-were of analytical grade and were purchased from Huaxin Reagent Co.,

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Ltd.

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2.2 Preparation of Co-CNTs

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The CNTs was pretreated by the 60vol% HNO3. Typically, 2.0g of CNTs were mixed

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with 150mL of 60vol% HNO3 in an Erlenmeyer flask. The mixture was heated at 80°C,

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stirred, and condensed over a period of 5h using a reflux condenser. Hereafter, the

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product was vacuum-filtered, washed until neutral pH value, and dried at 120°C for 2h.

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The pretreated CNTs was used for the preparation of the cobalt catalyst by immersing 4



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them into 50mL of hydrous ethanol with different Co(NO3)2 concentrations (10, 20, 30,

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and 40wt%). The mixture was stirred at 30°C and subsequently dispersed using

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ultrasonic dispersion equipment for 30min. The products were dried for 2h at 120°C

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and then roasted in a tube furnace fluxed with N2. During the roasting process, the

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temperature was raised to 120°C in 1h and then maintained at 120°C for another 1h.

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After that, the temperature was further raised to 500°C in 2h and then maintained at

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500°C for another 3h18. Consequently, the Co-CNTs was achieved by natual cooling to

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room temperature.

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2.3 Co-CNTs characterization

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The specific surface area of Co-CNTs were determined by N2 adsorption–

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desorption measurements at 77K with the analyzer (SA3100, USA) using the

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Brunauer-Emmett-Teller (BET) method. The X-ray diffraction (XRD, Bruker D8

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advance) was used to characterize the crystalline phases of the Co-CNTs and the

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Co-CNTs adsorbed with mercury under the wide-angle (10°–90°) scanning with a step

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size of 0.02°, tube voltage of 20～60kV (1kV/1step) and tube current of 10～60mA.

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X-ray photoelectron spectroscopy (XPS; ESCALAB 250) was applied to analyze the

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surface elemental composition and valence of the Co-CNTs. The functional groups on

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the surface of the CNTs and the Co-CNTs adsorbed with mercury were determined by

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a Fourier transform infrared spectroscope (FT/IR-200, JASCO, Japan). For each

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sample, scans on the spent adsorbent sample of 50mg in the range 4000–400cm−1 were

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recorded with background subtraction at a resolution of 4cm−1 in transmittance mode

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and the wave number accuracy of 0.01cm−1.

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2.4 Experimental procedure

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The simultaneous removal of Hg2+ and SO32− was carried out in a bubbling tank 5


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with the total volume of 200ml. The temperature was kept at 45°C. In a typical test,

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10.0g of magnesium sulfite, 40µg of Hg2+, and 0.4g of Co-CNTs were added into the

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reactor. During the reaction, hydrogen chloride and sodium hydroxide solution with a

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concentration of 1mol/L were used to adjust the value of pH. The mixture was

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sampled and measured at the certain intervals. The concentration of sulfate was

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determined by barium sulfate spectrophotometry17. The oxidation rate of sulfite can be

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obtained by plotting its concentration versus reaction time. After the reaction was

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terminated, the product was filtrated and the dissolved Hg2+ remaining in the solution

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was measured; the adsorbed mercury on both the magnesium sulfite and carbon

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nanotube was then eluted and measured. Consequently, the reemitted mercury20, 21

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was calculated by:

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Hg = Hg( ) + Hg + Hg + Hg

137 138

(1)

The reemission ratio of Hg was defined as follows:

η = × 100%

(2)

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The sole Hg2+ adsorption experiments were conducted following the same

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procedure for the simultaneous removal of Hg2+ and sulfite without adding

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magnesium sulfite in the solution. To determine the isothermal adsorption curve, the

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performance of Hg2+ adsorption onto the Co-CNTs under various Hg2+ concentration

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(10, 50, 100, 150, 200, 300, and 500 mg/L) was evaluated.

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2.5 Analytic methods

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The sulfate concentration was determined using a barium sulfate turbidimetric

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method and spectrophotometer17. The atomic fluorescence spectrometry (AFS-933)

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with a detection limit of 0.0002µg/L was used to determine the Hg2+ concentration in

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the solution during the adsorption process. The amount of Hg2+ adsorbed per gram of

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the Co-CNTs was calculated as follows: 6



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(

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(3)

! " )/$

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where qe (mg/g) is the adsorption capacity; c0 and ce (mg/L) are the initial and

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equilibrium liquid-phase concentrations of mercury, respectively; V(L) is the volume

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of the solution; and m(g) is mass of dry sorbent used.

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3 RESULTS AND DISCUSSION

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3.1 Characterization of CNTs

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3.1.1 N2 Adsorption–desorption isotherms

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Fig. 1 illustrates the N2 adsorption–desorption isotherm of the prepared catalysts

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(Co-CNTs). The isotherm was categorized as type IV with a type H3 hysteresis loop,

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indicating that the catalyst was a mesoporous material with ordered pore sizes.

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Experimental results showed that with the increase of the cobalt loading in the catalyst,

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the BET surface area of the prepared Co-CNTs decreased, implying that the cobalt

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species penetrated into the pore of the CNTs. For example, the BET surface areas of the

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catalysts impregnated with the different Co(NO3)2 concentrations of 10, 20, 30, and

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40wt% were 97.4, 96.7, 74.9, and 63.8 m2/g, respectively, which were in good

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agreement with Fu’s results22.

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3.1.2 XPS pattern

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The Co 2p spectra was displayed in Fig.S3, it can be seen that the spin-splitting

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energy between the Co 2p3/2 peak (780.8 eV) and Co 2p1/2 peak (796.8 eV) of all

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Co-CNTs spectra was 16 eV, indicating that the main chemical state of Co species was

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Co (II). The Co 2p3/2 peak was fitted into two peaks at 781.5 eV and 780 eV,

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corresponding to Co (II) and Co(III). Peaks at 786.8 eV and 803.5 eV are the satellite

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of Co species. Moreover, basing on the peak area, the ratio of Co(II) / Co(III) showed

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in Tab.S1 increased with the cobalt loading increased to 30% and then remained 7


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unchanged when 40%, which is consistent with the sulfite oxidation rate with

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different cobalt loading. Therefore, Co(II) was confirmed to be the main cobalt

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species in Co-CNTs.

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3.1.3 X-Ray diffraction

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Fig. 2 displays the XRD spectra of the pure CNTs, Co30-CNTs, and Co30-CNTs

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after adsorbing Hg2+. The diffraction angles 26.1º and 42.8º corresponding to the

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characteristic diffraction peaks23 of the (002) and (100) crystal planes of the graphite

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in the CNTs were observed from all the tested samples, indicating that the graphite

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structure was intact and not damaged during the preparation of Co-CNTs. The peaks

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of XRD spectrum for Co30-CNTs at 36.3°, 42.5°, and 61.7° was assigned to CoO,

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indicating that CoO was the main cobalt species in Co30-CNTs which can be

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confirmed by the Co 2p XPS analysis. The high diffraction peak intensity at 42.5° in

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Co30-CNTs can be attributed to the overlapping of the CoO peak and the typical

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graphite peak at 42.8º. The peaks corresponding to HgO were observed at 26.5°,

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42.7°, and 61.9° for the Co30-CNTs after absorbing Hg2+. Due to the low loading of

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mercury (0.062mg/g) on the Co30-CNTs, the peak intensity was low. Furthermore,

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after the adsorption of mercury onto Co30-CNTs, the intensity of the diffraction peaks

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corresponding to CoO was found weakened in Fig.2, implying that CoO was partially

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covered by the adsorbed mercury.

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The XRD patterns of Co-CNTs with dissimilar cobalt loading were shown in

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Fig.S2, the peak at 26.2º and peaks at 36.3º, 42.5º and 61.7º are corresponding to C

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and CoO according to PDF#43-1004, respectively. The diffraction peak of C at 26.2º

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reduced along with the increasing of cobalt loading. And the diffraction peaks of CoO

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gradually increased when the Co loading increased from 10% to 30% but decreased

199

when 40%. This might be attributable to that the activity of the catalyst reduced 8



200

caused by the aggregation of active cobalt species when the content of Co is

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excessively high, thereby weakening the corresponding diffraction peaks.

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3.1.4 Boehm titration

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Tab.S2 presents the results of Boehm titration which determined the number of

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acidic groups on the surfaces of the untreated pure CNTs and Co30-CNTs. After nitric

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acid pretreatment, the concentration of acidic groups increased from 1.56 to

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2.12mmol/g and thus the negative charge on the carbon surfaces increased

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dramatically which provided more lone electron pairs to adsorb the heavy metal such

208

as mercury. The increase of cobalt loading from 0 to 40% decreased the number of

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acidic groups from 2.12 to 1.85mmol/g, indicating that portion of acidic groups

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interacted with the impregnated cobalt. Overall, the presence of acidic

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oxygen-containing functional groups is beneficial to polarize the CNTs and provide

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excellent adsorption sites for the heavy metals.

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3.1.5 Fourier transform infrared spectroscopy

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Fig. 3 displays the FT-IR spectra of CNTs before and after cobalt loading.

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Vibration peaks were discovered at 3450, 2926, 1640, 1580, 1405, and 1112 cm−1.

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The peak at 3450 cm−1 corresponded to O–H stretching in hydroxyl groups, peak at

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2926 cm−1 corresponded to C-H stretch vibration originated from the surface of

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CNT16, peak at 1640 cm−1 corresponded to C=C stretch vibration, peaks at 1580 and

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1405 cm−1 corresponded to C=O stretching in carboxyl or lactone groups24, and the

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peak at 1112 cm−1 corresponded to C–O stretching in the phenolic hydroxyls25. Most

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of these oxygen-containing functional groups were hydrophilic which will be

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beneficial for the dispersion of the cobalt species in the CNTs. Overall, the FT-IR

223

analysis revealed that a large amount of oxygen-containing functional groups, such as

224

carboxyl, lactone, hydroxyl, and phenolic hydroxyl groups, were present in the CNTs 9


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even after loading the cobalt, indicating that the impregnated cobalt did not

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substantially affect the number of oxygen-containing functional groups which was in

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good agreement with the results of Boehm titration.

228 229

3.2 Simultaneous removal of Hg (II) and SO32−

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The adsorption efficiency of the Co30-CNTs for the Hg2+ removal in the absence

231

and presence of sulfite was 98% and 95%, respectively, indicating that the presence of

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sulfite did not have noticeable impact on the Hg2+ adsorption by Co30-CNTs (Fig.

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4(a)). It was also found that the removal efficiency of Hg2+ was around 55% in

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presence of MgSO3, which might be due to reduction of Hg2+ to Hg0 by sulfite

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On the other hand, the sulfite oxidation rate increased by six folds reached in the

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presence of Co30-CNTs (0.0696mmol/(L·s)) compared to that in the absence of

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Co30-CNTs (0.012mmol/(L·s)) (Fig. 4(b)), indicating that the prepared Co30-CNTs

238

exhibited an excellent catalytic performance on sulfite oxidation. In addition, the

239

leakage of the cobalt from the Co30-CNTs catalyst was not detected in the reaction

240

medium after the reclamation of the catalyst which was determined by the atomic

241

absorption spectrometry, showing that the prepared Co30-CNTs catalyst is stable and

242

robust.

[26,27]

.

243

When sulfite and Hg2+ (0.2mg/L) coexisted in the solution, the sulfite oxidation rate

244

(0.063mmol/(L·s)) decreased by 9.5%, indicating that the presence of mercury ions

245

slightly inhibited the sulfite oxidation. This may be due to the fact that, after Hg2+

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adsorbed on the CNT surfaces, portion of the active cobalt were covered by adsorbed

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Hg2+ as indicated by the XRD results (Fig. 2). With the increase of Hg2+ to 10mg/L,

248

the catalyzed oxidation rate decreased by 33.3% implying that more active cobalt was

249

covered by adsorbed mercury. Since the typical Hg2+ concentration in desulfurization 10



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slurry (0.01-0.8mg/L)28 is one to three orders of magnitude lower than 10mg/L, the

251

inhibition of the sulfite oxidation by the adsorbed Hg2+ will be not as significant as

252

that observed in this study. Overall, under the typical conditions of the desulfurization

253

slurry, the simultaneous removal of Hg2+ and sulfite can be achieved by the Co-CNTs.

254 255

3.3 Parametric tests of mercury adsorption

256

3.3.1 Effect of cobalt loading on mercury adsorption and catalytic performance of

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sulfite oxidation

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The performance of the simultaneous mercury adsorption and sulfite oxidation in

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simulated desulfurization slurry with the Co-CNTs of various cobalt loadings was

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conducted. Experimental results revealed that the Hg2+ adsorption rate was increased

261

by 4.2% after CNTs loaded with 10wt% cobalt compared with their free counterparts

262

(Fig. 5). Further increase of the cobalt loading in the CNTs from 10 to 40wt% resulted

263

in slight decrease of Hg2+ adsorption efficiency by Co-CNTs. Overall, the loading of

264

the cobalt in the CNTs did not noticeably impact their Hg2+ adsorption performance. It

265

should be noted that the BET surface area of the Co-CNTs was significantly decreased

266

with the increase in the cobalt loading, indicating that the Hg2+ adsorption did not

267

mainly depend upon the structure of the CNTs. Based on the results of Boehm

268

titration and FT-IR analysis, the oxygen-containing functional groups in the CNTs

269

were not significantly decreased after the cobalt impregnation, implying that the

270

oxygen-containing functional groups in the CNTs may play an important role in the

271

Hg2+ adsorption. Fig. 5 also showed that sulfite oxidation rate increased with the

272

increase of cobalt loading from 10 to 30wt%. With the further increase of the cobalt

273

loading from 30 to 40wt%, the oxidation rate declined. Excessive cobalt impregnation

274

probably caused aggregation of active cobalt species and made it ineffective, which 11


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can be proved by Fig.S2.

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3.3.2 Effect of residence time on mercury adsorption

277

The magnesium sulfate concentration in the conventional magnesia desulfurization

278

slurry is approximately 5%. Using the proposed process with the Co-CNT catalyst, it

279

could be increased up to 30% by oxidizing magnesium sulfite into sulfate. Taking the

280

catalyzed oxidation rate (0.069mmol/(L·s)) into account, the residence time of the

281

slurry in the catalytic oxidation reactor with this novel process was estimated to be

282

approximately 11h.

283

The effect of residence time on the Hg2+ adsorption by Co-CNTs was investigated

284

with the initial Hg2+ concentration of 10mg/L. Fig. 6a illustrates that the removal

285

efficiency of Hg2+ by Co-CNTs was a function of residence time. The removal

286

efficiency reached 90% during the first 10min, and became approximately invariable

287

after 60min. A similar phenomenon was observed by Gupta et al29. A large number of

288

vacant surface sites were probably available for adsorption during the initial stages,

289

after which the adsorption decreased because of repulsive forces between mercury in

290

solid and liquid phases. The Co-CNTs used in the experiment absorbed more than

291

90% of the Hg2+ in the wastewater within 10-20min and more than 92% within 19h,

292

demonstrating a strong adsorption ability that meets the requirements for industrial

293

wastewater discharge.

294

3.3.3 Effect of pH on mercury adsorption

295

In a typical magnesia-based desulfurization process, the pH value of

296

desulfurization slurry ranged from 6 to 7. To simulate such conditions, the

297

experiments were conducted in the pH ranges of 4-8 to investigate its effect on

298

Co-CNTs’ adsorption of Hg2+. The results showed that the Hg2+ removal efficiency

299

was initially increased but then decreased slightly as the pH value of the solution was 12



300

increased; the removal efficiency peaked at pH 5-7 (Fig. 6b). This differed from the

301

work reported by Wang30 et al. using polyaniline as adsorbent. They found that the pH

302

values had a significant impact on mercury adsorption performance by PAN and the

303

maximum removal efficiency was achieved at the pH value of 5.5. With the pH values

304

ranging from 4 to 5 in this study, H+ ions in the solution might compete with the

305

positively charged Hg2+ ions for the active sites on surface30,31, resulting in the low

306

adsorption efficiency. With the further increase of the pH value, the Hg2+ removal

307

efficiency did not noticeably decrease and kept over 90%. The experimental results

308

demonstrated that Co-CNTs have excellent Hg2+ adsorption performance under the

309

typical pH value of desulfurization slurry.

310

3.3.4 Effect of initial concentration of mercury ion

311

Fig. 6c presents the adsorption isotherms of Hg2+ onto Co-CNTs under various

312

initial concentration of Hg2+ ranging from 50 to 800mg/L where the obtained

313

equilibrium concentration of Hg2+ (ce) was from 20.8 to 530.3mg/L. The equilibrium

314

adsorption amount increased sharply with the initial Hg2+ concentration increasing

315

from 50 to 150mg/L. After that, the increasing tendency became flattened. The

316

adsorption capacity of the Co-CNTs reached a maximum of 177.3mg/g which was

317

much higher than that by the other carbon-based adsorbents such as sulfur

318

incorporated SWC-NTs (131mg/g)32, activated carbon prepared from sago waste

319

(55.6mg/g)33, and activated carbon prepared from peanut shells (12.8mg/g)34. This can

320

be attributed to enormous adsorption active sites on the surface and inner pore of

321

CNTs, which features large specific area and abundant pore structure with graphite,

322

like gap and multi cylindrical plane of hexagonal surface14,22,29. Moreover,

323

oxygen-containing functional groups like OH groups are existed in Co-CNTs catalysts

324

which can be confirmed by FTIR spectra in Fig.3 and Boehm titration in Tab.S2. And 13


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the complexation of OH groups in catalysts and Hg2+ contributes to the high

326

adsorption capacity of Co-CNTs catalyst11,16,31.

327

3.4 Kinetic analysis

328

To determine the rate-limiting steps of the Hg2+ adsorption onto the Co-CNTs, both

329

pseudo-first-order and pseudo-second-order kinetic models integrated with the

330

intraparticle diffusion were employed to simulate the Hg2+ adsorption process. The

331

pseudo-first-order kinetic model35 can be expressed as:

332 333 334

ln( − ( ) = ln − )* +

(4)

The pseudo-second-order kinetic model36 can be expressed as: (

,-

=

*

./ ,"/

+

(

(5)

,"

335

where qe and qt are the amounts of Hg2+ adsorbed on the Co-CNTs at equilibrium and

336

time t(mg/g), respectively; k1 is the equilibrium rate constant of the pseudo-first-order

337

adsorption (min−1), which is determined from the slope of the plot of ln(qe / qt) versus t

338

(Fig. 7(a)); and k2 is the equilibrium rate constant of the pseudo-second-order

339

adsorption(g/(mg·min)), which is similarly determined from the slope of the plot of t/

340

qt versus t (Fig. 7(b)).

341

For the pseudo-second-order kinetic model, the correlation coefficients were

342

calculated to be greater than 0.999, which was higher than those determined by the

343

pseudo-first-order

344

pseudo-second-order kinetic model was 4.95(mg/g) which was close to the

345

experimental data. These results suggest that the Hg2+ adsorption onto Co-CNTs

346

follows the pseudo-second-order kinetics, implying that the adsorption process was

347

controlled by the chemical adsorption.

model

(0.828).

The

theoretical

348 349

3.5 Adsorption isotherms 14


qe

obtained

from


Page 16 of 32

350

The Hg2+ adsorption isotherms by the Co-CNTs were fitted by both Langmuir and

351

Freundlich models in an attempt to understand the Hg2+ adsorption mechanism. The

352

following equations describe the Langmuir and Freundlich models37, 38： 0"

=,

*

0

+ ,"

353

Langmuir:

354

Freundlich: ln = ln)3 +

,"

1 .2

(6)

1

*

45

ln6

(7)

355

where ce(mg/L) is the equilibrium concentration of Hg(II); qe(mg/L) is the Hg2+

356

content adsorbed under equilibrium; qm(mg/g) is the theoretical maximum adsorption

357

capacity of the adsorbent; kL(L/mg) is a Langmuir binding constant related to the

358

energy of adsorption; and kF and n are the Freundlich empirical constants. The data

359

illustrated in Fig. 8 were applied in the Langmuir and Freundlich models.

360

Compared to Freundlich model, the Langmuir model much better described the

361

behavior of the Hg2+ adsorption onto the Co-CNTs (R2=0.996), indicating that Hg2+

362

adsorption on Co-CNTs can be considered as a monolayer adsorption process. Based

363

on the Langmuir model, the maximum adsorption capacity (qm) was calculated to be

364

166.7mg/g, which was close to the experimental data (177.3mg/g) in Fig. 6(c).

365 366

3.6 Inhibition mechanism of Hg0 reemission with Co-CNTs

367

In the typical desulfurization slurry, the concentration of Hg2+ was reported to range

368

from 0.01 to 0.8mg/L29 and the impurities such as SO4, Cl-, NO3-, and other heavy

369

metals coexisted. The reemission of mercury, in the form of elemental mercury (Hg ),

370

was induced by the reduction of sulfite26,27,39:

371

: Hg + SO: 9 + H O → Hg +SO< + 2H

(8)

372

Based on Eq. 8, it is crucial to decrease the reduction rate between the Hg2+ and

373

sulfite to inhibit the formation of Hg0 which can be achieved by decreasing the 15


Page 17 of 32


374

reactant concentration. Since the developed dual functional material can remove both

375

sulfite and Hg2+ in the slurry, the dosage of the dual functional material can thus

376

decrease the reduction rate and hence inhibit the formation of elemental mercury.

377

Furthermore, the mass balance of the mercury shown in Eq. 1 was determined to

378

figure out the fate of the mercury after the dosage of the developed Co-CNTs. Fig.9

379

indicated that in presence of MgSO3, the Hg2+ was removed due to the reemission of

380

Hg0. It was primarily aroused from the reduction of Hg2+ by sulfite according to Eq.8

381

and the adsorption capability of undissolved MgSO3 was negligible. The experimental

382

results as shown in Fig. 9 revealed that the reemission of mercury can be reduced by

383

156% with the dosage of 2.0g/L of Co-CNTs compared with that in the absence of the

384

Co-CNTs. Fig. 9 also showed that the presence of the impurities, especially Cl-, was

385

beneficial to inhibit the Hg0 reemission. In the presence of the impurities, the

386

reemission of mercury was decreased by 253%, which may be caused by the

387

combination of Cl- with Hg2+ and hence the re-emission of mercury was further

388

inhibited.

389

In summary, the oxidation rate of sulfate catalyzed by Co-CNTs was slightly

390

affected by the adsorption of mercury onto the Co-CNTs. Therefore, the sulfite and

391

mercury ions in the desulfurization slurry could be simultaneously removed by the

392

Co-CNTs. The adsorption kinetics of Hg2+ adsorption onto the Co-CNTs well fitted

393

the pseudo-second-order kinetic model. The adsorption isotherm was better described

394

by Langmuir model. The Hg0 reemission was inhibited by the addition of the

395

Co-CNTs. In the industrial application, the Co30-CNTs will be deposited and covered

396

on the surface of support mold, such as cordierite, in order to facilitate its recovery 16



397

and regeneration. After the prepared dual-functional material approached saturation

398

for mercury adsorption, it can be regenerated by either heating or elution. For elution,

399

the Co-CNTs can be immersed in the eluent which deserves attention in the further

400

research.

401 402

ASSOCIATED CONTENT

403

Supporting Information

404

Additional information as noted in the text. This information is available free of

405

charge via the Internet at http://pubs.acs.org/.

406 407

AUTHOR INFORMATION

408

Corresponding Authors

409

*(L.D.W.) Tel: +86 312 752 5511; E-mail address: [email protected]

410

*(S.H.Z.) Tel: +86 571 8832 0853; E-mail address: [email protected]

411

Notes

412

The authors declare no competing financial interest.

413 414

ACKNOWLEDGEMENT

415

The present work is supported by the National Key Research and Development

416

Program of China (No. 2016YFC0204102), the National Natural Science Foundation

417

of China (No. 51378204 and 51379077), and the Natural Science Foundation of Hebei

418

Province (No. E2016502096).

419 420

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Page 18 of 32

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ultrasonic-assisted removal of Hg(II) ions on graphene oxide modified with

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2-pyridinecarboxaldehyde thiosemicarbazone: Adsorption isotherms and kinetics

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17. Wang, L.; Cui, S.; Li, Q.; Wang, J.; Liu, S., Kinetics and mechanism of

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20. Yao, H.; Luo, G.; Xu, M.; Kameshima, T.; Naruse, I., Mercury Emissions and

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21. Luo, G. Q.; Yao, H.; Xu, M. H., Partitioning behavior of mercury during coal 19


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22. Fu, T.; Liu, R.; Lv, J.; Li, Z., Influence of acid treatment on N-doped multi-walled

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carbon nanotube supports for Fischer–Tropsch performance on cobalt catalyst. Fuel

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23. Zhu,

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nanotube/polyaniline composite films as supports of platinum for formic acid

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electrooxidation. Appl. Surf. Sci. 2008, 254, (10), 2934-2940.

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24. Pamula, E.; Rouxhet, P. G., Bulk and surface chemical functionalities of type III

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PAN-based carbon fibres. Carbon 2003, 41, (10), 1905-1915.

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25. Zhou, J. M.; Li, H. Y.; Lin, G. D.; Zhang, H. B., Purification of multiwalled

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carbon nanotubes and characterization of their oxygen-containing surface groups.

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Acta Phys. Chim. Sinica 2010, 26, (11), 3080-3086(7).

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26. Heidel, B., Klein, B., Reemission of elemental mercury and mercury halides in

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wet flue gas desulfurization. Int. J. Coal. Geol. 2017, 170 ,28–34.

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27. Zhao, S., Duan, Y., Yao, T., Liu, M., Lu, J., Tan, H., Wang, X., Wu, L., Study on

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the mercury emission and transformation in an ultra-low emission coal-fired power

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plant. Fuel 2017, 199, 653–661.

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28. Pudvay, M., Operating experience on the treatment on FGD scrubber blowdown

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from existing generating stations. http://www.degremont-technologies.com/, May 26,

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29. Gupta, A.; Vidyarthi, S. R.; Sankararamakrishnan, N., Enhanced sorption of

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mercury from compact fluorescent bulbs and contaminated water streams using

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functionalized multiwalled carbon nanotubes. J. Hazard. Mater. 2014, 274, 132-144.

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30. Wang, J.; Deng, B.; Chen, H.; Wang, X.; Zheng, J., Removal of aqueous Hg(II)

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by polyaniline: sorption characteristics and mechanisms. Environ. Sci. Technol. 2009,

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43, (14), 5223-5228.

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31. Li, Y.; Wang, S.; Wei, J.; Zhang, X.; Xu, C.; Luan, Z.; Wu, D.; Wei, B., Lead

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adsorption on carbon nanotubes. Chem. Phys. Lett. 2002, 357, (3–4), 263-266.

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32. Yu, Y.; Addai-Mensah, J.; Losic, D., Functionalized diatom silica microparticles

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Z.; Wang,

Z.; Li,

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L.,

Functional multi-walled carbon

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for removal of mercury ions. Sci. Technol. Adv. Mater. 2012, 13, (1),

510

15008-15018(11).

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33. Kadirvelu, K.; Kavipriya, M.; Karthika, C.; Vennilamani, N.; Pattabhi, S.,

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Mercury (II) adsorption by activated carbon made from sago waste. Carbon 2004, 42,

513

(4), 745-752.

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34. Namasivayam, C.; Periasamy, K., Bicarbonate-treated peanut hull carbon for

515

mercury (II) removal from aqueous solution. Water Res. 1993, 27, (11), 1663-1668.

516

35. Ahmed, M. J.; Dhedan, S. K., Equilibrium isotherms and kinetics modeling of

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methylene blue adsorption on agricultural wastes-based activated carbons. Fluid

518

Phase Equilib. 2012, 317, (317), 9-14.

519

36. Ho, Y. S.; McKay, G., Pseudo-second order model for sorption processes. Process

520

Biochem. 1999, 34, (5), 451-465.

521

37. Kundu, S.; Gupta, A. K., Arsenic adsorption onto iron oxide-coated cement

522

(IOCC): Regression analysis of equilibrium data with several isotherm models and

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their optimization. Chem. Eng. J. 2006, 122, (1–2), 93-106.

524

38. Pan, Y.; Wang, F.; Wei, T.; Zhang, C.; Xiao, H., Hydrophobic modification of

525

bagasse cellulose fibers with cationic latex: Adsorption kinetics and mechanism.

526

Chem. Eng. J. 2016, 302, 33-43.

527

39. Chen, C. M.; Jiang, L. X.; Liu, S. T.; Jiang, Y. Z., Control of Hg0 re-emission

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from simulated wet flue gas desulfurization liquors by sodium dithiocarbamate. Adv.

529

Mater. Res. 2012, 613, 1473-1477.

530

21


Page 22 of 32

Page 23 of 32


531

Figure captions:

532

Fig.1 Comparison of N2 adsorption–desorption isotherms of CNTs impregnated with

533

dissimilar [Co2+], Co10-CNTs: Carbon nanotube impregnated with 10% cobalt,

534

Co20-CNTs: Carbon nanotube impregnated with 20% cobalt, Co30-CNTs: Carbon

535

nanotube impregnated with 30% cobalt, Co40-CNTs: Carbon nanotube impregnated

536

with 40% cobalt.

537

Fig.2 XRD patterns of pure CNTs, Co30-CNTs, and Co30-CNTs after Hg2+

538

adsorption.

539

Fig.3 FTIR spectra of pure CNTs and Co30-CNTs

540

Fig.4 Hg2+and SO32- simultaneous removal experiment. qe: amount of Hg2+ adsorbed

541

at equilibrium time. ccat=2g/L, Co wt%=30%, V=200mL, cHg2+=0.2mg/L, pH=6, t=2h.

542

Fig.5 Effect of impregnated Co on Hg2+ adsorption and oxidation rate of MgSO3.

543

Cowt%=0%, 10%, 20%, 30%, and 40%, ccat=2g/L, V=200mL, cHg2+=10mg/L, t=2h,

544

pH=6.

545

Fig.6 Parametric tests of mercury absorption under the optimized dosage levels of

546

Co-CNTs: a) Effect of residence time; b) Effect of pH; c) Effect of equilibrium

547

concentration of Hg2+.

548

Fig. 7 Kinetic analysis of mercury adsorption: a) pseudo-first order; b) pseudo-second

549

order.

550

Fig. 8 Adsorption isotherm model: a) Langmuir equilibrium isotherms; b) Freundlich

551

equilibrium isotherms.

552 553

Fig.9 The mercury speciation analysis under different conditions

22



Impregnated Co%

10%

20%

30%

Page 24 of 32

40%

2

Volume, cc/g (STP)

Specific surface areas (m /g) 97.37 96.74 74.89 63.81

Co40-CNTs Co30-CNTs Co20-CNTs Co10-CNTs

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/P0 554 555

Fig.1 Comparison of N2 adsorption–desorption isotherms of CNTs impregnated with

556

dissimilar [Co2+], Co10-CNTs: Carbon nanotube impregnated with 10% cobalt,

557

Co20-CNTs: Carbon nanotube impregnated with 20% cobalt, Co30-CNTs: Carbon

558

nanotube impregnated with 30% cobalt, Co40-CNTs: Carbon nanotube impregnated

559

with 40% cobalt.

23


Page 25 of 32




pure CNTs

C ∇ CoO ♦ HgO



Intensity(a.u.)



∇ Co30-CNTs



∇

∇



Co30-CNTs 2+ absorb Hg

∇♦

∇

0

10

20

30

40

♦∇

50

60

70

2θ(deg) 560 561

Fig.2 XRD patterns of pure CNTs, Co30-CNTs, and Co30-CNTs after Hg2+

562

adsorption.

563 564

24



565 566 567 568

Fig.3 FTIR spectra of pure CNTs and Co30-CNTs

25


Page 26 of 32

Page 27 of 32


569

Oxidation rate of MgSO3 (mmol/L⋅s)

0.10

(b) 0.08

0.069 0.063 0.06

0.046 0.04

0.02

0.012

0.00 Uncatalyzed

Co30-CNTs

Co30-CNTs/Hg2+ Co30-CNTs/Hg2+ 0.2mg/L

10mg/L

570 571

Fig. 4 Hg2+ and SO32- simultaneous removal experiment. (a) Hg2+ removal under

572

different conditions, ccat =2g/L, cMgSO3 =50g/L, Co wt%=30%, V=200mL,

573

cHg2+=0.2mg/L, pH=6, t=2h; (b) Co30-CNTs catalytic oxidation of sulfite, ccat=2g/L,

574

cMgSO3 =50g/L, Co wt%=30%, V=200mL, pH=6, t=2h.

575

26


Page 28 of 32

100

0.10

80

0.08

60

0.06

40

0.04

20

0.02

0

0%

10%

20%

30%

40%

0.00

Oxidation rate of MgSO3,mmol/L⋅s

2+

Hg removal efficiency, %


Co loading of Co-CNTs 576 577

Fig. 5 Effect of impregnated Co on Hg2+ adsorption and oxidation rate of MgSO3. Co

578

wt%=0%, 10%, 20%, 30%, and 40%, ccat=2g/L, cMgSO3 =50g/L, V=200mL,

579

cHg2+=10mg/L, t=2h, pH=6.

580

27


Page 29 of 32


100

2+


(a) 80 60 40 20 0

0

20

40

60

80

1000

Time, min

581

100

2+


(b) 95 90 85 80 75

4

5

6

7

8

pH

582

200

(c) qe(mg/g)

160 120 80 40 0

0

100

200

300

400

500

600

700

Ce (mg/L) 583 584

Fig.6 Parametric tests of mercury adsorption under the optimized dosage levels of

585

Co-CNTs (ccat=2g/L, cMgSO3 =50g/L, Co wt%=30%, V=200mL): a) Effect of residence

586

time (pH=6, cHg2+=10mg/L); b) Effect of pH (pH=4~8, cHg2+=10mg/L); c) Effect of

587

equilibrium concentration of Hg2+ (pH=6, cHg2+=50~800mg/L) 28



(a)

Page 30 of 32

-0.8

lnqe-qt

-1.2

-1.6

-2.0

-2.4 0

20

40

60

80

t (min)

588

(b)

16

t/qt

12

8

4

0 0

20

40

60

80

t (min)

589 590

Fig. 7 Kinetic analysis of mercury adsorption: a) pseudo-first order; b)

591

pseudo-second order.

592

29


Page 31 of 32


(a) 0.020

1/qe

0.015

0.010

0.005

0.00

0.05

0.10

0.15

0.20

1/Ce

593

(b) 5.5

lnqe

5.0

4.5

4.0

3.5 -1.5

0.0

1.5

3.0

4.5

lnCe

594 595

Fig. 8 Adsorption isotherm model: a) Langmuir equilibrium isotherms; b) Freundlich

596

equilibrium isotherms

597 598

30



60

Amount of mercury species, µg

2+

50

Hg in solution 2+ adsorbed Hg 0 reemission of Hg

40 30 20 10 0 I: MgSO3

II: MgSO3/Co-CNTs

III: MgSO3/Co-CNTs with impurities

599 600

Fig.9 The mercury speciation analysis under different conditions: ccat=2g/L, c

601

MgSO3=50g/L,

602

(MgSO3/Co-CNTs)+Hg2+; III: (MgSO3/Co-CNTs)+Hg2++impurities (cCl- =20g/L, c

603

SO4

2-

cHg2+=0.2mg/L, V=200mL, t=2h, pH=6. I: MgSO3+Hg2+; II:

=3g/L, c NO3- =50mg/L).

604

31


Page 32 of 32

Inhibiting mercury reemission and enhancing magnesia recovery by

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