Optimization of magnetic powdered activated carbon for aqueous Hg(II) removal and magnetic recovery
Highlights
► Thermal oxidation of MPAC decreased the amorphous characteristic of iron oxides. ► Thermal oxidation did not influence magnetic recovery or Hg removal performance. ► At all thermal oxidation temperatures, the 3:1 MPAC achieved the highest Hg removal.
Introduction
Mercury (Hg), a toxin that has been shown to bioaccumulate, can enter the environment from anthropogenic sources such as chlor-alkali wastewater and has severe health effects on humans, animals, and the environment [1]. The treatment of mercury-contaminated water remains a challenge, particularly due to the very low regulatory concentrations. Due to its listing as a toxic pollutant under section 307(a) of the Clean Water Act (CWA), site-specific technology-based aqueous Hg effluent limits are regulated through the National Pollutant Discharge Elimination System (NPDES) permitting system. Any discharge to impaired water must not exceed the Total Maximum Daily Load (TMDL), the maximum allowable amount of a pollutant that a particular body of water can receive and still meet water quality standards. States have the power to require lower effluent limits, as is the case in the Great Lakes region where the limit has been set to less than 1 μg/L. The EPA has determined the water quality criteria for the protection of wildlife and for the protection of human health to be 1.3 ng/L and 1.8 ng/L, respectively [2], [3]. Adsorption can be used as a polishing technique to reach lower wastewater effluent concentrations [4].
Activated carbon is known to remove Hg(II) from aqueous solutions [5], [6], [7], [8], [9], [10]. MPAC has the potential to lower wastewater effluent mercury concentrations from industries such as chlor alkali and coal-fired power plants utilizing flue gas desulfurization to less than 0.2 μg/L (the analytical detection limit using cold vapor atomic absorption (CVAA) spectroscopy) while being magnetically recoverable from solution. Traditional filtration methods to separate dispersed activated carbon from aqueous solution are susceptible to filter blockages and head loss. Magnetic recapture allows for simple separation of the sorbent from the waste stream and increases the ease of residuals management according to the cradle to grave responsibility of the Resource Conservation and Recovery Act (RCRA).
Magnetic adsorbents are an attractive solution for metallic and organic aqueous pollutants, particularly due to the simple magnetic separation process. Magnetic iron oxides have been used to synthesize new adsorbents utilizing multiwalled carbon nanotubes for Pb, 1-napthol, Ni, Sr, and Eu adsorption [11], [12], [13], zeolites for Cr, Cu, and Zn adsorption [14], activated carbon for phenol, chloroform, and chlorobenzene adsorption [15], and dimercaptosuccinic acid for Hg, Ag, Pb, Cd, and Tl adsorption [16]. However, these adsorbents have been applied to only a limited number of contaminants and mercury has thus far been largely overlooked. The available literature does not discuss the ratio of sorbent to iron oxide for either optimal adsorption or optimal magnetic recovery. Additionally, the potential for increased magnetic recovery from thermal oxidation of the iron oxides has not been investigated.
In this study, the adsorption of Hg(II) onto MPAC was studied in a batch system with respect to the synthesis variables of C:Fe mass ratio and thermal oxidation temperature and duration. Thermal oxidation was performed on the synthesized MPACs with the purpose of converting amorphous iron oxides formed during synthesis to magnetic iron oxides such as magnetite or maghemite. The goal of this study was to identify the synthesis variables for both optimal aqueous Hg removal and optimal sorbent recovery.
Section snippets
Materials
Solutions were prepared using ultrapure Type I water with a resistivity of 18.2 MΩ and a conductivity of 0.055 μS. A commercially available bituminous coal-based powdered activated carbon (Calgon WPH) with a surface area of 1020 m2/g was oven-dried at 100 °C for a minimum of 24 h prior to use. Hg(II) solutions were prepared by diluting 1000 mg/L stock Hg(NO3)2 (Fisher Scientific) in ultrapure water. The oxidizing purge trap to capture volatilized Hg was prepared using 4%, w/v potassium permanganate
Adsorbent characterization: porosity
The process of iron impregnation onto the carbon was expected to reduce the available surface area relative to the virgin activated carbon due to the minimal surface area of the iron oxides (1.9 m2/g). As expected, the 1:1 C:Fe resulted in approximately a 50% reduction of surface area from 1020 m2/g to 551 m2/g while the 2:1 and 3:1 C:Fe showed surface areas reduced by the expected ∼33% and 25% to 709 m2/g and 790 m2/g, respectively. As predicted, the available surface area increased as the loading
Acknowledgments
We thank Valenciun Craciun, Ph.D. and the Major Analytical Instrumentation Center (MAIC) at the University of Florida for support. We express particular appreciation to Katrina Indarawis, Carmen Ceron and Kelsie Timpe for their assistance.
References (24)
- et al.
The removal of mercury(II) from dilute aqueous-solution by activated carbon
Water Res.
(1984) - et al.
Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II)
J. Hazard. Mater.
(2009) - et al.
Magnetic zeolites: a new adsorbent for removal of metallic contaminants from water
Water Res.
(2004) - et al.
Activated carbon/iron oxide magnetic composites for the adsorption of contaminants in water
Carbon
(2002) - et al.
Mercury mass balance at a wastewater treatment plant employing sludge incineration with offgas mercury control
Sci. Total Environ.
(2008) - et al.
Influence of surface ionization on the adsorption of aqueous mercury chlorocomplexes by activated carbons
Carbon
(1998) - U.S. Environmental Protection Agency, Mercury study report to congress, volume II: an inventory of anthropogenic...
- Code of Federal Regulations Water Quality Criteria for the Protection of Wildlife, 2010; 40CFR132 Table...
- Code of Federal Regulations Water Quality Criteria for Protection of Human Health, 2010; 40CFR132 Table...
- US Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation, Treatment technologies...
Improving mercury (II) removal by activated carbon
J. Environ. Eng. Div.-ASCE
Activated Carbon Adsorption
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