Toscanesi, Maria Recalcitrant COD: characterization and abatement in the wastewater derived from the tanning industry. [Tesi di dottorato]

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Abstract

The wastewater from the tanning industry represents a serious environmental problem, as it has high concentrations of organic and inorganic contaminants. Some of these compounds are not degraded by the wastewater treatments and, for this reason, are called recalcitrant. The nature of these contaminants is not always known, and the research project focuses on their characterization. The first results showed the presence of COD values higher than the limits set by the legislation and strongly correlated with the TOC values, denoting an exclusively organic origin; furthermore, the low values of the BOD/COD ratios confirm that these compounds are not very biodegradable. The result of chemical analyses for the research of organic compounds showed, in particular, the absence of persistent organic pollutants such as polycyclic aromatic hydrocarbons and pesticides, the presence of organic substances of various kinds such as fatty acids, alcohols, alkyl-benzenes, phthalates, and terpenes. Among the recalcitrant organic compounds that arouse greater interest in the tanning sector are polyphenolic compounds, alkylphenols and their ethoxylated derivatives. The analyses, carried out by gas chromatography technique coupled with mass spectrometry, confirmed the presence of organic compounds belonging to the class of alkylphenols and ethoxylated alkylphenols in relatively low concentrations, while the analyses carried out by MALDI-TOF/TOF, showed the presence of residues of polyphenolic molecules. A method based on the UNI EN ISO 18857 standard was developed for the quantification of alkylphenols and their derivatives; the method was also validated as described by the UNI EN ISO 17025: 2015 standards. In this case, the method involved solid phase extraction (SPE), followed by instrumental analysis in gas chromatography coupled with mass spectrometry (GC-MS); in the validation phase, the detection limit (LOD) and the quantification limit (LOQ), the repeatability uncertainty, the calibration uncertainty, the extended uncertainty, as well as the recovery values of the method were determined. Finally, the last part of the project focused on the characterization of some of the synthetic and natural tannins, present on the market and used in the tanning industry, during different stages of the tanning process. The chemical characterization of these compounds was carried out by MALDI-TOF/TOF analysis, which provided innumerable information about the main molecules of which both synthetic and vegetable tannins are made, but also the degree of polymerization. These compounds are not very biodegradable, as demonstrated by the chemical analyzes and the literature, it turned out that these compounds cannot be eliminated by the classic methods of biological oxidation. In this contest, the project focused on the study of advanced oxidation processes that can be used for the abatement of vegetable and synthetic tannins and therefore for the abatement of recalcitrant COD. The processes studied are heterogeneous photodegradation and adsorption. Both processes were carried out in batches and using TiO2 Aeroxide P-25 both as a catalyst and as an adsorbent material. The various experiments were conducted to verify the influence of temperature, the amount of catalyst and the amount of initial COD concentration, on the recalcitrant COD reduction resulting from the presence of natural and synthetic tannins in the wastewaters. TiO2 Areoxide P-25 was used as catalyst, irradiated with UV light, which proved to be an effective catalyst for COD abatement with a maximum value of 60%. From the collected evidence, an activation energy of Ea = 28 KJ/mol was obtained comparable with the literature values obtained for persistent organic molecules. In every experiment, COD reached a plateau value, independent of the temperature value but linearly dependent with both the catalyst loading and the initial COD value. This effect was already observed in the literature, where Jouali et al. attributed the phenomenon to the shield effect due to the tanning residual content. Higher is the catalyst load, more efficient is the photodegradation process, while using high values of COD0 could lead to non-optimal reaction conditions as it inhibits the reaction itself, suggesting a surface mechanism. In preliminary study of adsorption, different experiment was conducted to verify the influence of the temperature and the quantity of adsorbent. The influence of the reaction temperature was investigated. It was observed that as the temperature increases, a reduction in the adsorbed quantity of contaminant was observed. The effect of COD concentration was evaluated by varying its value from 50 mg/L to 200 mg/L. The results showed a degradation efficiency of about 60% for COD concentrations of 50 mg / L, while for concentrations above 100 mg/L, the efficiency decreases up to a maximum value of 40%. In the equilibrium analysis the experimental data were fitted with classical isotherm Freundlich equations.

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