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Photocatalytic decomposition of humic acids on TiO2: Part I: Discussion of adsorption and mechanism

https://doi.org/10.1016/S1010-6030(02)00022-9Get rights and content

Abstract

In this paper we present the results of the photocatalytic degradation of humic acids (HAs) in aqueous solution. Bench scale experiments were carried out using titanium oxide as photocatalyst. We studied the adsorption of HAs on TiO2 surface and we describe the kinetics of their photocatalytic degradation. The adsorption study of HA at three pH solution (1.9/7.5/11) by FT-IR DRIFT technique indicated that at acidic pH, HA are adsorbed on TiO2 mainly by carboxylate groups. We determined the isotherms of HA and it was shown that Langmuir model cannot be applied in this case. However, if the asymptotic values of the isotherms were assumed for these analytical methods the close values were obtained. The kinetics of degradation was followed with TOC and pH. There are no important pH evolutions. It was obtained 88% of TOC removal after 6 h of irradiation with optimum TiO2 loading 1.0 g/l. We observed the presence of two domains of HA degradation kinetics. In the first step, slight TOC decreased possibly due to the photodepolymerization of adsorbed HA on TiO2. In the second one, the photodegradation followed pseudo-first-order kinetics. Organic intermediates were determined by high performance liquid chromatography (HPLC) (PDA-detector) analysis as well as biological oxygen demand (BOD5). It was shown that photocatalysis process improved the biodegradability of HA nevertheless mineralization process mainly occurred and there was no large amount of easy biodegradable species present in the reaction solution.

Introduction

The humic acids (HAs) have a significant role in the aquatic systems. They can complex heavy metals and organic pollutants such as pesticides, insecticides and herbicides [1]. But they are especially precursors of mutagenic products [1], [2]. Indeed, they will react with the chlorine used for the disinfection of water, to give organochloride compounds which are well-known to be carcinogenic products [3]. The HA generally form the major fraction of the dissolved organic matter in surface water that represents 90% of dissolved organic carbon [4]. They are formed during the decomposition of plants and dead animals. The HA have structures which are not yet well identified. Usually they contain carboxylic, carbonyl, methoxyl, hydroxyl and phenolic function groups [4].

In many countries, the HA is eliminated from water before chlorination by coagulation with aluminium sulphate and filtration. However, coagulation process brings two main disadvantages. Firstly, is the maintenance of residual sludge with high aluminium concentration. Secondly, treated water needs high quality monitoring of aluminium concentration. While the water contains more than 0.2 g/l it is not recommended for drinking [3]. Moreover, it needs to be mentioned that during this conventional treatment process only 10–50% of the TOC are eliminated [5].

The heterogeneous photocatalysis can be an effective alternative solution for the elimination of the HA from aqueous solution [6], [7], [8], [9]. With this process it is possible to degrade the majority of the organic molecules, without adding of additional chemicals except the photocatalyst (e.g. titanium dioxide). The basic process of photocatalysis consists of ejecting an electron from the valence band (VB) to the conduction band (CB) of the TiO2 semiconductor creating a h+ hole in the VB. This is due to the UV irradiation of TiO2 with an energy equal or superior to the band gap (>3.2 eV):TiO2+hν→eCB+hVB+This is followed by the formation of extremely reactive radicals (like OHradical dot) at the semiconductor surface and/or a direct oxidation of the polluting species. According to Corin et al. [4] direct photolysis of the HA leads to the formation of low-molecular-weight carboxylic acids (oxalic, succinic, formic, acetic, etc.).

During this work the photomineralization and photodecomposition of commercial HA in water solution have been studied. The mineralization kinetics was controlled by total organic carbon (TOC) and pH changes determination. Organic intermediates were studied with high performance liquid chromatography (HPLC) analysis, as well biodegradability (measuring biological oxygen demand, BOD5) evolution was determinate. In order to investigate the HA degradation mechanisms, we have studied the adsorption of these compounds on TiO2 in aqueous solution by diffuse reflectance infrared spectroscopy (DRIFT) of the solids and by determination of kinetic and adsorption isotherms. The DRIFT is perhaps one of the most valuable and informative methods among the different IR analysis techniques [10], [11], [12], [13].

Section snippets

Products and analysis

HAs sodium salts was supplied by Sigma–Aldrich company and titanium dioxide P25 from Degussa (70% anatase, 99.8% purity, average particle size 30 nm and specific surface of 50 m2/g) was used as received. The chemicals purchased to assist in HPLC analysis were obtained from Fluka. Milli-Q water was used throughout for the preparation of aqueous solutions or as a component of the mixed water–acetonitrile–phosphoric acid mobile phase.

The degradation of HA as well as formation and disappearance of

HA adsorption studied by DRIFT spectroscopy, UV-light absorption and by TOC analysis

Adsorption could play a prominent role in catalytic photodegradation of organic molecules [11], [14], [15] and, e.g. can explain selective oxidation in the case of mixtures treatment [16]. In this part, the measurements of HA adsorption were carried out by UV-light adsorption at two wavelengths (254 and 400 nm) and in parallel by TOC analysis. Some saturated TiO2 were characterised by IR spectroscopy in mode DRIFT. In all cases, the adsorption kinetics were fast and equilibrium is reached within

Conclusion

The results obtained in a bench scale experiments were encouraging and photocatalytic treatment of HAs solution appears as an efficient process.

We found that adsorption play a prominent role in photocatalytic degradation HA. Study of the HA adsorption on the TiO2 surface by DRIFT spectroscopy indicates that at the acidic pH, HA were adsorbed on TiO2 mainly as carboxylate surface groups. DRIFT spectra show also presence of free carboxylic groups. It was shown that Langmuir model of adsorption

References (24)

  • G.S. Wang et al.

    Chemosphere

    (2001)
  • B.R. Eggins et al.

    Wat. Res.

    (1997)
  • N. Corin et al.

    Chemosphere

    (1996)
  • G.S. Wang et al.

    Wat. Res.

    (2000)
  • D. Robert et al.

    Appl. Surf. Sci.

    (2000)
  • C.H. Giles et al.

    J. Colloid Interf. Sci.

    (1974)
  • M. Bekbolet et al.

    Wat. Sci. Technol.

    (1996)
  • A.J. Motheo et al.

    Sci. Total Environ.

    (2000)
  • C. Pulgarin et al.

    Catal. Today

    (1999)
  • E. Lichtfouse et al.

    Analysis

    (1999)
  • M. Schiavello (Ed.), Heterogeneous Photocatalysis, Wiley, New York,...
  • D. Bahnemann, in: P. Boule (Ed.), Handbook of Environmental Photochemistry, Springer, Berlin, 1999, p....
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