Vol.Materials 6, No. Research, 2, 2003 Vol. 6, No. Tailoring 2, 129-135, Activated 2003. Carbon by Surface Chemical Modification with O, S, and N Containing Molecules© 2003 129 Tailoring Activated Carbon by Surface Chemical Modification with O, S, and N Containing Molecules Rachel RibeiroVieira Azzi Rios, Dênio Eduardo Alves, Ilza Dalmázio, Sílvio Fernando Vargas Bento, Claudio Luis Donnici, Rochel Monteiro Lago* Dep. Química, ICEx, UFMG, 31270-901 Belo Horizonte - MG, Brazil Received: November 11, 2001; Revised: March 24, 2003 In this work the surface of activated carbon was chemically modified in order to introduce O, S and N containing groups. The activated carbon surface was selectively oxidized with concen- trated HNO3 under controlled conditions. Characterization by thermogravimetric analyses, infra- red spectroscopy and NaOH titration suggested the formation of mainly –COOH and small amounts of –OH groups, with concentration of approximately 4.1021 groups/g of carbon. These -COOH functionalized carbons showed high adsorption capacity for metal cations in aqueous solution in the following order: Pb+2>Cu+2>Ni+2>Cd+2~Co+2>Ca+2, suggesting a cation exchange mechanism - +2 via a surface complex [–COO M ]. These –COOHsurf groups can be reacted with SOCl2 to produce a surface acylchloride group, -COCl. This surface -COCl group proved to be a very reactive and versatile intermediate for the grafting of different S and N containing molecules onto the carbon surface, such as 1,2-ethaneditiol (EDT-, HSCH2CH2SH) 1,7-dimercapto-4-thioheptane (DMTH- HSCH2CH2CH2SCH2CH2CH2SH) or 1,2-ethylenediamine (EDA- NH2CH2CH2NH2) and triethyltetraamine, TEA (H2NCH2CH2NHCH2CH2NHCH2CH2NH2). The characterization of these materials was carried out by TG, IR and TPDMS (Temperature Programmed Decomposition Mass Spectrometry) experiments suggesting the formation of thioesther and amide surface groups, i.e. –COSR and –CONHR, with yields of approximately 50 and 75% for the reaction with DME and EDA, respectively. Preliminary adsorption experiments showed that these materials can efficiently remove metals such as Pb+2, Cu+2 and Ni+2 from aqueous medium. Keywords: activated carbon, surface modification, functionalization, cation exchange, heavy metals 9 1. Introduction high surface area and porous structure it can adsorb gases and compounds dispersed or dissolved in liquids10,11. The Surface modification1 has a fundamental role on the type of contaminant to be adsorbed and the adsorption/ application of organic and inorganic supports in industrial remotion efficiency of these carbons is strongly dependent and environmental processes, such as selective purification on their surface chemical features. Therefore, the surface processes, gas separation, solvent recovery, drinking water chemical modification of carbon is of great interest in order purification, adsorption of taste, odor and other micro-pol- to produce materials for specific applications. This modifi- lutants2, ion exchange properties for metal adsorption3, cata- cation has been mainly carried out by oxidative methods, lyst preparation4, adhesion phenomenon4,6, electrode modi- producing a more hydrophilic structure with a large number fication7 and polymer technologies8. of oxygen-containing groups. Various reagents have been Activated carbon is one of the most important industrial used as oxidants: concentrated nitric or sulfuric acid, so- adsorbent/support and can be prepared by carbonization and dium hypochlorite, permanganate, bichromate, hydrogen activation of a large number of raw materials such as coco- peroxide, transition metals and ozone-based gas mixtures12. nut shells, wood, peat and coal. These carbons show high- It was found that the type of surface structures and the ex- developed porous structure and a large internal specific sur- tent of their formation depends on the oxidizing agent, the face area, which is generally greater than 400 m2/g but of- concentration and the pH of the oxidizing solution13-15. Al- ten exceeds this value, reaching 1000 m2/g 3-5. Due to this though the creation of oxygen groups on the carbon surface *e-mail: [email protected] Trabalho apresentado no I Simpósio Mineiro de Ciências dos Materiais, Ouro Preto, Novembro de 2001. 130 Rios et al. Materials Research is relatively well known, the functionalization with S and N MS chamber (P = 6.10-6 Torr). The probe was heated at containing molecules has been much less investigated. 5 °C/min up to 275 °C and all the volatile decomposition In this work the activated carbon surface was chemi- products were analyzed by the mass spectrometer. cally modified in order to introduce oxygen, nitrogen and 2.6 Potentiometric Titration of the Carbon C/HNO with sulfur containing groups. 3 NaOH 2. Experimental The carbon (0.1 g) suspended in 25 ml of distilled water was titrated with a 0.02 M NaOH solution with continuous 2.1 HNO Treatment (C/HNO ) 3 3 stirring. The titration was carried out under nitrogen atmos- 2 The activated carbon (20 g, Aldrich Norit, 930 m /g) phere to avoid interference of CO2 from the air. NaOH so- lution was added stepwise in 0.5 ml in intervals of was treated with 100 ml of concentrated HNO3 under re- flux for 2, 4, 8, 16 or 38 h. After reflux, the resulting mate- 5-10 min to allow the reaction to take place and the pH to rial was filtered and extensively washed with hot distilled stabilize. water until the cleansing water pH was approximately 7. The carbons were dried in vacuum at 60 °C for 24 h. 3. Results and Discussion 2.2 Reaction of the Activated Carbon with Thionyl 3.1 Treatment with HNO Chloride (C/HNO /SOCl ) 3 3 2 The controlled oxidation of the activated carbon was The HNO treated carbon (0.80 g) was dried under vacuum 3 carried out with concentrated HNO3 under reflux for differ- at 80 °C to remove water and other substance which could ent periods. After this treatment the obtained material was interfere on the reaction. The reaction was carried out with extensively washed with hot water and carefully dried in 5 ml benzene and 5 ml of SOCl2 under reflux for 24 h. The vacuum at 60 °C. For comparison, the untreated original mixture was dried in a rotatory-evaporator and extensively activated carbon was also washed with water and dried un- washed with benzene to eliminate residual SOCl2. der vacuum. The mass balance with the weight losses ob- served after different reflux times is shown in Fig. 1. 2.3 Reaction with Ethaneditiol (EDT, HSCH2CH2SH) or 1,7-dimercapto-4-thioheptane (DMTH, It can be observed that the weight loss increases with the reflux time, probably due to the oxidation of the acti- HSCH2CH2CH2SCH2CH2CH2SH) vated carbon to gaseous products, e.g. CO2, and water solu- After treatment with SOCl the material C/HNO /SOCl 16 2 3 2 ble derivatives such as mellitic acid . (0.80 g) was reacted with 9 mmol of 1,2-ethaneditiol or Infrared spectroscopic analyses of the HNO3 treated car- 1,7-dimercapto-4-thioheptane in 5 ml of chloroform under bons (Fig. 2) showed strong absorption at approximately reflux for 48 h. The product was washed extensively with 1720 cm-1 (-C=O), 1550 cm-1 (-COO-), 1250 cm-1 (-C-O) and CHCl3 and separated by centrifugation. 2.4 Reaction with Etilenodiamina (EDAH2NCH2CH2NH2) or Triethyltetraamine (H2NCH2CH2NHCH2CH2NHCH2CH2NH2) After treatment with SOCl2 the material (0.80 g) was reacted with of the amine (2.6 ml) in 6 ml of chloroform under reflux for 48 h. The product was extensively washed with CHCl3 and separated by centrifugation. Thermogravimetric analyses were carried out in a Mettler TA 4000 System under N2 or air flow with a heating rate of 10 °C/min. Infrared spectroscopic analyses were obtained in a FTIR Mattson Instrument using KBr. BET surface areas were obtained in a Quantachrome Nova 1200 equipment with the carbon samples treated at 200 °C under vacuum for 3 h. 2.5 Temperature Programmed Decomposition Coupled with a Mass Spectrometer (TPDMS) The TPDMS experiments were carried out in a HP 5989 II mass spectrometer using 5 mg sample in a direct probe Figure 1. Weight losses after refluxing activated carbon with con- equipped with a heating system which was inserted in the centrated HNO3 for different periods. Vol. 6, No. 2, 2003 Tailoring Activated Carbon by Surface Chemical Modification with O, S, and N Containing Molecules 131 3450 cm-1 (-OH) which can be assigned to carboxylic acid To estimate the surface concentration of carboxylic groups16. groups, TG experiments were carried out keeping the tem- The HNO3 treated carbons were also analyzed by perature at 750 °C until no weight loss was observed. The thermogravimetry. Figure 3 shows the TG profiles for the total weight loss obtained from 200 °C was considered to different HNO3 treated carbons. be related to the amount of CO2 formed. It was also carried It can be observed that the untreated activated carbon out potentiometric titrations of the carbons with NaOH to showed very small weight decrease during TG analysis. On obtain the number of acid sites. the other hand, the treated carbons showed significant weight The number of acid (COOH) sites for the different re- loss after 200 °C. These weight losses observed for HNO3 flux time obtained by both TG and NaOH titration is dis- treated carbons have been assigned mainly to the decompo- played in Fig. 4. sition of carboxylic surface groups and in a lesser exten- It can be observed that up to 8 h reflux with HNO3 sion to –OH groups according to the following processes2,16: the –COOH concentration increases to ca. 2-4 . 1021 sites/g, but it remains approximately constant even if the carbon is → -COOHsurf CO2 → -OHsurf CO Figure 4. Acid sites concentration (as –COOH) on carbon surface Figure 2. IR spectra of the HNO treated carbons.