Characterization of solids originating from the Fenton's process

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Abstract

Fenton's process has proven to be a viable alternative for the oxidation of organic pollutants in mixed waste chemicals, with a high efficiency in the removal of heavy metals from the effluent as a side effect. However, a solid residue loaded with metals is obtained as a by-product. In this paper, the solids originating from the Fenton's oxidation of mixed waste chemicals from an academic laboratory were studied. The process was applied to different wastewater cases (two composite samples generated in different periods) in order to study the effect of their characteristics on the residues composition. Residues were evaluated and classified according to their leaching properties. In addition, a most comprehensive assessment of metal mobility in the residues was obtained by chemical speciation. The Fenton's residues were classified as hazardous according to Brazilian waste regulations. The application of the sequential dissolution procedure indicated that the metals in the Fenton's residues are mainly constituted of amorphous material (over 80%). Furthermore, the reactive fractions of the residues (exchangeable and amorphous iron oxide fractions) retain most of remaining metals. Therefore, the Fenton's residues present great potential for environmental contamination, and require an administration system and control of their final disposal.

Introduction

The interdisciplinary nature and the ever-changing pattern of activities of many academic research laboratories result in the generation of small volumes of mixed waste chemicals of unknown toxicity and hazard. Although these aspects differ the academic laboratories from others waste producers, up to this moment, there is no specific Brazilian legislation for handling the resulting academic waste chemicals. Until a few years ago, waste chemicals were held indefinitely at the laboratory site waiting for the development of acceptable modes of treatment and disposal. Recently, however, Public Ministry, society and the escalation of incurred costs for waste disposal are playing an important role to pressure academic institutions throughout the country to attain sustainability. Therefore, it is necessary to implement waste management plans focused on waste minimization as well as to develop economical and efficient waste treatment methods.

In this context, the Department of Chemical Engineering of the Universidade Estadual de Maringá-Brazil examined the chemical oxidation using Fenton's reagent (H2O2/Fe2+) as a treatment option for waste chemicals generated during the operations of the Control and Environmental Prevention Laboratory. The experimental results [1], [2] indicate that the oxidation process successfully leads to the oxidative destruction of organic pollutants, with a high efficiency in the removal of heavy metals from the effluent as a side effect. However, a potentially hazardous solid residue, loaded with the metals removed from the effluent, is obtained as a by-product, and needs to be disposed of properly.

Proper waste management, satisfying environmental and health related criteria, requires information on the long-term stability of heavy metals residues. Besides composition data, this information includes data on leaching of inorganic and organic constituents under simulated environmental conditions. The leaching tests are useful to identify whether solid wastes exhibits toxicity, a characteristic of a hazardous waste.

Additionally, the ability of the metal to change chemical form into water-soluble compounds is related to their chemical form of association in the residue (i.e. its speciation) and ultimately to the crystalline structure of the residue [3], [4]. Parceling, identification and quantification of the heavy metals species accumulated in the residue are also necessary to evaluate the metals’ potential environmental influence.

Chemical speciation can be defined as the process of identification and quantification of different species, forms or phases present in a material. These species can be defined (1) functionally, for example those species that can be assimilated by plants, (2) operationally, according to the procedures or reagents used in their extraction and (3) specifically, as particular components or oxidation states of an element. Operationally, the definition of the speciation includes single and sequential extractions to relate the species associated with particular phases of the sludges [5].

During recent decades various single and sequential extraction schemes have been developed [6], [7], [8], [9], [10], [11], [12], [13]. Generally speaking, these schemes use a series of chemical reagents to dissolve one or more specific phases from soil, sediment, sewage sludge or miscellaneous solid samples, while preserving others. In the extract it is possible to speciate elements solubilized during the dissolution. By studying the distribution of the metals between the different phases, their contamination risks can be ascertained, because it is possible to divide a specific metal into fractions of increasing stability such as exchangeable, organically bound, precipitated, oxide bound and residual.

Generally, there is a lack of overall background data on the characteristics of Fenton's reagent residues. Thus, the objectives of this research were to investigate the composition and mobility of metals in solid residues originated by the Fenton's treatment of waste chemicals. The stability of Fenton's residues was evaluated by use of leaching and single and sequential dissolution procedures. The process was applied to different wastewater cases (two composite samples generated in different periods) in order to study the effect of the wastewater composition on the residue characteristics. The sequential dissolution procedure was designed based on the characteristics of the raw and oxidized effluent as well as on preliminarily investigations of the total metal content in residues and X-ray diffractometry (XRD). The solids from each extraction phase were also analyzed by XRD. Moreover, the amorphous phase diffractograms were determined by differential X-ray diffraction (DXRD).

Section snippets

Materials and methods

Chemical effluents generated during the laboratory operations over a period of 17 months were monitored, collected and stored. Later on these effluents were divided into two groups and then mixed to obtain two combined samples produced in different periods of time that were used in the experiments of Fenton's oxidation. The initial characteristics of both samples are shown in Table 1.

Throughout the monitoring period waste chemicals were generated as a consequence of chemical analyses and other

Results and discussion

The results of metals determination in residues originating from the Fenton's process (Table 3) showed that the predominant metals are silver, chromium, mercury and iron. The main source of the first three ones is COD analysis. Iron is mainly a result of the Fenton's process, once it is used as a catalyst of the reaction and undergoes precipitation after oxidation. Regarding cadmium, the results of effluent characterization (Table 2) showed that sample 1 presented an initial concentration of 0.3

Conclusions

The present study reports the chemical and mineralogical characterization of solid residues originated from the Fenton's process. Even though previous works showed that the Fenton's system is an effective technology for the treatment of waste chemicals, the resulted sludge contains heavy metals initially present on the liquid phase that requires further separation, treatment and disposal, increasing thus the overall costs. According to the leaching tests, the residues were classified as

Acknowledgements

The support provided by CAPES and by the State University of Maringá is gratefully acknowledged.

References (22)

  • B. Pérez-Cid et al.

    Analytical assessment of two sequential extraction schemes for metal partitioning in sewage sludges

    Analyst

    (1996)
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