Comparative study for the removal of methylene blue via adsorption and photocatalytic degradation

doi:10.1016/j.jcis.2007.02.013

Journal of Colloid and Interface Science

Volume 310, Issue 2, 15 June 2007, Pages 498-508

https://doi.org/10.1016/j.jcis.2007.02.013 Get rights and content

Abstract

Physically and chemically activated carbons were prepared from date pits and olive stones. Titania and WO_x-TiO₂/MCM-41 were prepared as photoactive catalysts. Surface characterizations were investigated from ash content, pH, base neutralization capacities and FT-IR techniques. The textural characteristics, namely specific surface area ( $S_{BET}$ ) and pore texture, were determined from low temperature adsorption of N₂ at 77 K. The decolorization of aqueous solution of methylene blue was performed by means of two alternative methods. Steam-activated carbons own higher surface area compared with ZnCl₂-activated carbons, and the micropore surface area represents the major contribution of the total area. Steam-activated carbons were the most efficient decolorizing adsorbents owing to its higher surface area, total pore volume and the basic nature of the surface. The calculated values of $Δ G^{0}$ , $Δ H^{0}$ and $Δ S^{0}$ indicate the spontaneous behavior of adsorption. The photocatalytic degradation is more convenient method in decolorizing of methylene blue compared with the adsorption process onto activated carbons.

Graphical abstract

Adsorption/desorption isotherms of N₂ adsorption onto activated carbons at 77 K.

Introduction

Textile dyes represented the main source for aquatic pollution with colored compounds. Sauer et al. report that approximate amounts of 1–15% are discharged in waste water during the dyeing process and textile industries [1]. The contamination of waste water with these dyes consumes the dissolved oxygen and therefore affects the aquatic life, causing thus environmental problems [2], [3]. Methylene blue is cationic dye, used extensively for dying cotton, wool and silk. The risk of the presence of this dye in waste water may be arisen from the burns effect of eye, nausea, vomiting and diarrhea.

Adsorption has been reported as an efficient method for the removal of different toxic pollutants in waste effluents, among of which heavy metals, dyes, odors, oils and organic pollutants were. Activated carbons are widely used as powerful adsorbents for most pollutants in waste water or at the supplying drinking water. These higher adsorption capacities were correlated to the porous nature of activated carbons and its high internal surface area and therefore favored as excellent adsorbent among the others. The use of different wastes of lignin origin to prepare the activated carbons, e.g. date pits and olive stone, is one of the aims of the present investigation. Such selection of younger fossil materials is based on the potential of obtaining high quality activated carbons, the huge volume as agriculture by-product solid wastes and their uses as cheapest precursor for the active carbons.

On the other hand, photocatalytic degradation has received a great attention as an alternative method in the removal of environmental pollutants in aqueous as well as gaseous media [4], [5]. Anatase-type titania has been proven as environmental friendly catalysts because of its capability to decompose the different organic and inorganic pollutants [6], [7], [8], [9], [10]. The efficiency of photocatalytic degradation could be enhanced by anchoring of photoactive catalysts on suitable support particularly that of large surface area [11]. The adsorption of pollutants on a high surface area supports, e.g. active carbons [12], [13], increases its concentration around supported titania; its diffusion to titania and thereby promotes the photocatalytic process [14], [15], [16]. The use of MCM-41 as a support for photoactive catalyst has not been studied yet.

Herein, the present study aimed to: prepare powerful adsorbents from cheapest agriculture solid wastes, prepare titania and WO_x-TiO₂/MCM-41 as a photoactive catalyst, characterize active carbons investigated as well as the home-made catalyst and compare the removal efficiency of methylene blue via adsorption and photocatalytic degradation.

Section snippets

Active carbons

Zinc chloride (Z)-activated carbons (C) were prepared by soaking the dried raw materials, namely date pits (DP) or olive stones (OS), in aqueous zinc chloride solution containing the appropriate amount of activating agents for 96 h at room temperature. The raw materials, activating agent ratio of 2:1 (I), 1:1 (II) and 1:2 (III), were prepared. The mixture was then dried at 393 K and then carbonized at 873 K for 6 h in the presence of N₂ atmosphere. The rate of heating was adjusted to 10 °C/min.

Ash content, pH and base neutralization capacities

Table 1 lists the estimated ash content, pH of aqueous suspended carbons and the base neutralization capacities. Inspection of Table 1 reveals that: (i) for date pits and olive stones, the ash content of steam-activated carbons is higher than those of ZnCl₂-activated carbons. The ash content of steam-activated carbons increases with the increase of % burn-off, leading to a decrease of carbon content. (ii) The ash content of ZnCl₂-activated carbons is generally low compared with the

Conclusion

Steam-activated carbons prepared from date pits or olive stones are basic, while zinc chloride-activated carbons are acidic. The activation process of the carbonized precursors increased the porosity and therefore increased the surface area. In all cases, the micropore volume is predominant and the surface area of micropore represents the major contribution of the total surface area. Steam-activated carbons own a higher surface area compared with zinc chloride-activated ones.

Non-activated

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Comparative study for the removal of methylene blue via adsorption and photocatalytic degradation

Abstract

Graphical abstract

Introduction

Section snippets

Active carbons

Ash content, pH and base neutralization capacities

Conclusion

J. Photochem. Photobiol. A

Environ. Pollut. Ser. A

J. Mol. Catal. A Chem.

Surf. Coat. Technol.

J. Mol. Catal. A

Ultrason. Sonochem.

Appl. Catal. A

Catal. Today

J. Mol. Catal. A

Surf. Coat. Technol.

Catal. Today

J. Anal. Appl. Pyrolysis

J. Anal. Appl. Pyrolysis

Micropor. Mesopor. Mater.

Fuel

J. Catal.

J. Photochem. Photobiol. A

J. Colloid Interface Sci.

J. Hazard. Mater. B

Micropor. Mesopor. Mater.

J. Colloid Interface Sci.

Micropor. Mesopor. Mater.

Micropor. Mesopor. Mater.

Catal. Today

Catal. Commun.