High Surface Area Functionalized Carbon Briquettes : A Novel Adsorbent for Contaminants from Water

Neste trabalho, as vantagens de carbonos funcionalizados com alta área superficial e a forma especial de briquete foram combinadas para a produção de um novo adsorvente de metais. Análises BET, TG, IR, sítios ácidos e EDS sugerem que a funcionalização com HNO 3 ocorre por todo o corpo do briquete. Foram estudadas também as isotermas e as cinéticas de adsorção de Cr, Zn e do corante azul de metileno.


Introduction
2][3][4][5][6][7][8] In this work it is investigated for the first time the carbon surface modification by functionalization to produce special adsorbents based on activated carbon briquettes.This chemical modification can tailor the adsorbents by combining the advantages of a briquette shape with the presence of functional surface groups for special applications.
Herein, we show that carbon briquettes can be functionalized with concentrated HNO 3 generating acidic groups, not only on the external surface of the briquette but also in the most internal layers of the carbon body.The functionalization strongly improves the adsorption of cationic species, such as Zn 2+ , Cr 3+ and of the dye methylene blue, from aqueous medium with the adsorbed species homogeneously distributed throughout the briquette.

Experimental
The briquettes (ca. 1 g, 10 mm diameter x 13 mm length) were prepared from Eucalyptus camaldulensis Dehn wood (ca. 1 wt% ash, 11 wt% moisture, 69 wt% volatiles and 19 wt% fixed carbon) and rice husk (ca.17 wt% ash, 10 wt% moisture, 63 wt% volatiles and 10 wt% fixed carbon) pyrolyzed at 500 o C under N 2 atmosphere for 4h.Concentrated grape must with density of 1.35 g mL -1 and sugar (fructose and glucose) content of 860 g L -1 was used as binder to prepare the briquettes (binder:char weight ratio of 1:4) in a mould held in a hydraulic press at 1,400 kg cm -2 .The briquettes were thermally treated at 15 o C min -1 up to 880 o C under N 2 flow and activated with steam (1.7 g H2O g carbon -1 h -1 ) for 105 min.The briquettes showed high mechanical strength with impact resistance index (IRI) 9 of 1503.The carbon samples were analyzed by BET (Autossorb 1 Quantachrome), TG (Shimadzu TGA 50H), IR (Perkin Elmer IR), EDS (energy dispersive and X-ray fluorescence spectroscopy in a scanning electron microscope Jeol JXA-8900RL, Rigaku, Geigerflex 2037).Details of the preparation and characterization of these briquettes have been published elsewhere. 8The functionalization of the carbon briquettes was carried out with HNO 3 (conc) under reflux for 60 min.The adsorption isotherms were obtained after 24 h adsorption equilibrium of 20 mg carbon with solutions (20 mL, 28±2 o C) of the cationic species dye methylene blue (measured spectrophotometrically at 533 nm), Cr 3+ or Zn 2+ (measured by AAS, Carls Zeiss Jena).The adsorption kinetics were studied using one briquette (intact or ground) in 100 mL solution of methylene blue (140 mg L -1 ) or Cr 3+ (200 mg L -1 ) at 28±2 o C under stirring.The briquettes division in layers was carefully carried out with the aid of a blade to produce an outer (ca. 1 mm thick and 0.33 g), a middle (ca.1.5 mm, 0.33 g) and an inner layer (ca.2.5 mm, 0.33 g).

Results and Discussion
The high surface area briquettes for application as adsorbents have been prepared by a careful procedure involving steps of chars production, binder addition, press moulding and thermal activation carefully established in a previous work. 8Another important feature of these briquettes is that they are produced from low cost agricultural by-products, i.e. eucalyptus waste, rice husks and grape most, available in different areas of Brazil and Argentine.
The carbon functionalization was carried out by the controlled oxidation with concentrated nitric acid. 10,11In order to obtain an analytical depth profile the briquette bodies were separated into three layers, i.e. outer, middle and inner, as shown in Figure 1.
The different layers of the functionalized briquettes, were analyzed by IR, TG, BET and acid titration.The IR spectroscopic analyses of the different layers clearly showed the presence of strong bands at ca. 3440 and 1720 cm -1 characteristic of COOH groups formed by the oxidation with HNO 3 , and bands located at ca. 1595 cm -1 likely related to NO 2 groups produced by the nitration of the carbon.These results agree with a large number of studies which show that the functionalization of activated carbon with concentrated nitric acid produces mainly surface carboxylic acid groups, -COOH, and -OH and -NO 2 in less extension. 10,11TG analysis of the original briquette does not show any significant weight loss up to 400 o C. On the other hand, oxidized carbons showed from ca. 4 up to 20% weight losses in the temperature range 120-400 o C due to the decomposition of the COOH and -NO 2 groups to produce CO 2 and NO 2 .The acid site concentrations determined by titration 12 in the different layers of the briquettes treated with concentrated HNO 3 are shown in Figure 1 and Table 1.
It can be observed that the outer layer of the original briquette shows approximately 0.32 mmol g -1 acid sites which decrease to 0.24 and 0.13 mmol g -1 for the middle and inner layers, respectively.It is also observed for the outer layer a slightly higher BET surface area of 410 m 2 g -1 compared to the middle and inner layers.When the briquettes are treated with HNO 3 at the ratio of 2 mL acid/ g briquette a large increase in the acid sites to 2.01, 1.89 and 1.68 mmol g -1 is produced in the outer, middle and inner layers, respectively.Under reflux with HNO 3 at 20 mL acid/ g briquette the acid sites further increased and a significant increase in the surface area is observed.The data presented in Table 1 suggest that this increase in surface area is related to an increase in the microporosity of the material.If the reaction is carried out with 100 mL HNO 3 /g briquette for 60 min under reflux, a strong increase in the acid site concentration to approximately 4 mmol g -1 is produced (Figure 1).However, the briquette becomes very fragile with low mechanical resistance.In order to investigate the effect of the functional groups on adsorption processes, experiments with Cr 3+ (aq) , Zn 2+ (aq) and the dye methylene blue (aq) , a positively charged organic molecule, were carried out.The adsorption isotherms of the dye methylene blue and Cr 3+ (aq) for the original and functionalized briquettes are shown in Figure 2.
It can be observed that the original briquette adsorbs a maximum of approximately 140 mg dye g -1 adsorbent.On the other hand, upon functionalization, the adsorption capacity increases up to 330 mg g -1 .A more significant effect is observed for the adsorption of Cr 3+ which increases from approximately 1 mg g -1 for the original briquette up to 78 mg g -1 for the functionalized carbon.These results suggest that the presence of the surface carboxylic groups strongly improve the adsorption of the cationic species from the aqueous medium.
In order to investigate if a preferential adsorption on the more external part of the briquette occurs or if the metal can diffuse and adsorb into the briquette body, EDS analyses of Zn 2+ adsorbed briquette were carried out.The adsorption experiments with Zn 2+ (aq) were carried out by exposing the functionalized briquettes to the metal aqueous solution for 24 h at 28 ±2 o C.After drying, the briquette was cut transversally and analyzed from the centre to the border by EDS/SEM with approximately 6 spectra mm -1 .The EDS spectra showed signals related to the elements Si, S, Ca, Al, K and Mg, all originated from the precursors used and two strong signals at 8.6 and 1.6 keV related to Zn.The relative areas of the Zn signal peak for the spectra obtained at different points of the briquette are shown in Figure 3.
It can be observed that the Zn EDS signals from the centre to the border of the briquette showed similar relative intensities, suggesting that Zn 2+ does not adsorb preferentially on the more external surface of the briquette.These results suggest that the briquette possesses a porous structure large enough to allow facile diffusion of the hydrate Zn 2+ species and its homogeneous adsorption throughout the briquette body.
It was also investigated if the adsorption on the briquette was limited by diffusion of the adsorbate into the inner part of the briquette body.The adsorption kinetics  of methylene blue and Cr 3+ on the functionalized carbon as a briquette and as a powder were compared.The results are displayed in Figure 4.
It can be observed that the adsorption of the dye onto powder functionalized carbon takes place with a first order dependent rate constant k ads of 1.5x10 -1 h -1 whereas the adsorption onto the briquette is a much slower process with k ads of 1.6x10 -2 h -1 .On the other hand, for the adsorption kinetics of Cr 3+ on carbon powder a k ads of 6.8x10 -2 h -1 was observed whereas the adsorption onto the briquette was a slightly slower process with rate constant of 4.5x10 -2 h -1 .This suggests that in the case of the relative large organic molecule methylene blue the diffusion significantly affects the rate of the adsorption process whereas the adsorption of the smaller metal cation Cr 3+ is much less affected by diffusion processes.
In conclusion, the functionalization of activated carbon briquettes with HNO 3 can be carried out homogeneously throughout the briquette body, suggesting that the HNO 3 molecule can penetrate the porous structure to oxidize the carbon surface to produce mainly carboxylic groups in the most internal surface of the briquettes.The presence of these carboxylic groups strongly promote the adsorption of Zn 2+ , Cr 3+ and of the dye methylene blue, which are homogeneously adsorbed throughout the briquette body.Adsorption kinetic measurements suggest that the adsorption of metallic cations onto the briquette is not seriously controlled by diffusion whereas for large organic molecules a significant diffusion limitation is observed.

Figure 2 .
Figure 2. Adsorption isotherms of (a) methylene blue dye and (b) Cr 3+ on the original and functionalized briquettes.

Figure 3 .
Figure 3. Relative intensities of the EDS Zn signals at 8.6 keV from the centre to the border of the functionalized briquette exposed to a Zn 2+ solution.

Table 1 .
Acid site concentration and BET surface areas for the different layers of briquettes treated with different volumes of HNO 3 per g briquette at 120 o C for 60 min