Removal of hydrogen sulfide and sulfur dioxide by carbons impregnated with triethylenediamine.

ABSTRACT

Activated carbon (AC) adsorption has long been considered to be a readily available technology for providing protection against exposure to acutely toxic gases. However, ACs without chemical impregnation have proven to be much less efficient than impregnated ACs in terms of gas removal. The impregnated ACs in current use are usually modified with metalloid impregnation agents (ASC-carbons; copper, chromium, or silver) to simultaneously enhance the chemical and physical properties of the ACs in removing specific poisonous gases. These metalloid agents, however, can cause acute poisoning to both humans and the environment, thereby necessitating the search for organic impregnation agents that present a much lower risk. The aim of the study reported here was to assess AC or ASC-carbon impregnated with triethylenediamine (TEDA) in terms of its adsorption capability for simulated hydrogen sulfide ([H.sub.2]S) and sulfur dioxide (S[O.sub.2]) gases. The investigation was undergone in a properly designed laboratory-scale and industrial fume hood evaluation. Using the system reported here, we obtained a significant adsorption: the removal capability for [H.sub.2]S and S[O.sub.2] was 375 and 229 mg/g-C, respectively. BET measurements, element analysis, scanning electron microscopy, and energy dispersive spectrometry identified the removal mechanism for TEDA-impregnated AC to be both chemical and physical adsorption. Chemical adsorption and oxidation were the primary means by which TEDA-impregnated ASC-carbons removed the simulated gases.

INTRODUCTION

Activated carbon (AC), because of its high adsorptive properties, has been utilized for several decades in a wide variety of applications as a safe and easy method for removing organic and inorganic contaminants. With the growing concern over environmental pollution, there has been increased interest in ACs as the means for removing pollutants from both liquid and gas streams, both in the field and in industrial processes. (1) In the United States, both the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) regard AC adsorption as the "gold standard" technology for the optimal disposal of pollutants and carcinogens on the priority list.

Unimpregnated AC is a good adsorbent for some organic vapors, but it is a poor adsorbent of low-molecular-weight or polar gases. Conversely, impregnated ACs, which have been treated with a chemical reagent, react with these types of gases, binding them onto the carbons and thereby removing them from an airstream. However, the impregnating agent is likely be a source of acute poisoning to humans and the environment. (2)

Gas molecules can be adsorbed onto the surface of AC by two methods, physisorption and chemisorption. Physisorption is a surface reaction in which an adsorbate is held onto the surface of the adsorbent by Van der Waal's and classical electrostatic interaction forces. Chemisorption results from the formation of chemical bonds between the adsorbent and the adsorbate, whereby a chemical reaction occurs at the carbon interface, changing the state of the adsorbate to produce chemically adsorbed overlayers. (3) In general, in unimpregnated or virgin AC, molecules that bind to the surface, mainly by physical adsorption, are the product of weak interactions stemming from low-energy Van der Waal's forces. Because of the weakness of the adsorption interactions between the adsorbate and the AC, the adsorbate can be easily released into the environment with various environmental consequences. Impregnated ACs have been treated with a chemical reagent that can react with contaminants, binding them onto the carbons by means of significantly stronger intermolecular bonding, from 10x to 1000x greater, and more stable bonding than the physical adsorption interactions of unimpregnated or virgin AC. In terms of industrial air pollution control applications, impregnated ACs would appear to provide the optimal solution. (4) The results from earlier studies have shown that the impregnation of AC with metalloids containing, for example, Mn, Co, Ni, Fe, Cu, Zn, Ag, Cr, Mo, and V, may produce changes in the pore size of the resulting ACs and the addition of some new functional groups as well as increase the functionality of the inside/surface carbons for removing compounds not typically removed by regular AC. (5) These studies have also revealed that a novel metalloid oxide forms in the impregnated ACs as an essential catalyst for oxidation during the adsorption process. For example, impregnated copper can typically form the metalloid compounds copper carbonate and copper (II) hydroxide, and impregnated zinc can form new species such as zinc oxide or zinc hydroxide. (6)

Virgin AC does not have a great intrinsic capacity to remove contaminants from the airstream. Consequently, processes have been devised for coating chemicals onto the carbons to provide the necessary filtering capabilities. One of the first treatments of carbon for improving the removal of a variety of gases from the airstream originated in the desire to protect military personnel in World War II. Grabenstetter and Blancet (7) first described this process, known as "Whetlerization," in which a metal solution was used to impregnate AC. (7) The special combination of physical adsorption and chemical bonding properties associated with the metal impregnation of AC has been extremely successful in terms of (mask) air filtering; at the desired concentration of Cu, Cr (hexavalent chromium), and Ag, the impregnated AC is optimized for protection against toxic vapors. AC treated in this manner is called ASC-whetlerite carbon (activated copper-silver-chromium). (8) However, public concern over the environmental impact and occupational cancer risk of hexavalent chromium is resulting in a new assessment of the risk data. (9) In addition, chromic compounds are not only powerful skin irritants, but they can also be corrosive. Human occupational experience clearly indicates that prolonged inhalation of chromate (VI) dust leads to airway irritation, airway obstruction, and possibly lung cancer. (10)

Concerns on the impact of metal-impregnated ACs present in (industrial) waste products has resulted in researchers focusing on the search for effective but low-impact impregnated materials, such as organic coatings. Several promising new impregnation agents have been developed, such as triethylenediamine (TEDA), diisopropylamine (DIPA), di-N-propylamine (DNPA), piperidine, tartaric acid, and citric acid, for the removal of contaminants. (11-13) These "new-generation" impregnated ACs can be used for various applications involving the adsorption of toxic gases. (11-13) The most successful combinations to date consist of coal carbon that is especially impregnated with TEDA for the removal of radioactive methyl iodide in nuclear testing. (14) The removal of hydrogen cyanide (HCN) and cyanogen chloride toxic vapor by TEDA-impregnated ACs has also been shown. (15) However, purely organic-impregnated AC has a relatively low removal capacity for gaseous pollutants in comparison to metalloid-impregnated ACs (16) and, consequently, there is a concerted search for the formulation of a carbon which combines the advantages of both metalloid- and organic-impregnated agents in removing various contaminants.

According to the Deutsches Institut fur Normung (DIN) proposal 3181, the use of AC in the treatment of toxic substances can be divided into four categories: A class (volatile organic gases), B class (acidic inorganic gases), E class (oxidized gases), and K class (alkaline inorganic gases). Of these, the development of an impregnated AC for the removal of acidic inorganic gases, such as HCN, hydrogen sulfide ([H.sub.2]S), and chlorine, and oxidized gases such as sulfur dioxide (S[O.sub.2]) and N[O.sub.2], is urgently needed by industry for legislated pollution control norms to be met successfully. The aim of the present study was to use [H.sub.2]S and S[O.sub.2] as modeling gases and then to formulate an AC impregnated with both a metalloid and organic agent that would upgrade the removal efficiency of existing metal-impregnated carbon and thereby be both more efficient and safer for the disposal of toxic substances.

MATERIALS AND METHODS

Preparation of the AC