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Treatment of Phenolic Pollutants by UV/H2O2: Influence of Operating Parameters and Pollutants’ Structure on Degradation Kinetic and Sum Ecological Parameters

Phenolic compounds are among the most widespread forms of vast array of chemical pollutants in industrial wastewater streams originated from oil refineries, coal conversion plants, petrochemicals, polymeric resins, coal tar distillation, pharmaceuticals, dye industry, etc. Phenols, especially chloro-and nitro-substituted, are designated as priority pollutants by US Environmental Protection Agency (USEPA) and European Commission (EC) due to their bio-recalcitrance and high toxicity, even at low concentrations. These pollutants are also characterized to be potentially carcinogenic and mutagenic. Thus, their recovery or elimination prior to discharge into natural effluents is highly required. Advanced oxidation processes (AOPs) represent an alternative when common wastewater treatment technologies are insufficiently effective, particularly for the non- or low-biodegradable pollutants. Among various types and combinations of AOPs, photochemical processes based on usage of various oxidants and UV irradiation shown potential as destructive and low- or non-waste generating technologies. The aim of the study was to investigate the influence of substituent type of phenolic pollutants on their degradation kinetic, as well as on biodegradability and toxicity during the treatment by UV/H2O2 process. The structural influence (type and position of substituent) along with the influence of process parameters (initial pH and concentration of oxidant) on the rate of their elimination was investigated using design of experiments approach, studying the cross-factor effects by applying the full factorial design (FFD) combined with response surface modeling (RSM). The degradation kinetic, as well as the changes in biodegradability and toxicity were correlated with the structural, physical and chemical properties of studied pollutants using statistical approach by calculating the Spearman rank coefficients (SRC). The ANOVA results on RSM showed high direct influence of both the type (-OH, -Cl, -NO2, -CH3) and the position (ortho and para) of phenol substituents on their degradation kinetic, as well as their indirect influence through the interactions with the process parameters (particularly operating pH). Simplified mechanistic model describing the elimination of phenolics as well as the mineralization of overall organic content was developed. The low SRC between calculated first order and corresponding second order (with hydroxyl radicals) degradation rate constants indicate on high contribution of formed by-products to the overall degradation rate. Biodegradability of all studied phenolics was significantly improved ; from non-biodegradable before the treatment (0.06<BOD5/COD<0.34) to even readily biodegradable after mineralization of 50% of initial organic content of studied phenolics (0.44<BOD5/COD<0.89). It was determined that the pattern of changes of biodegradability and toxicity during the treatment strongly depends on the structure of parent pollutants, and subsequently formed by-products.

wastewater; phenolic pollutants; treatment; modeling; toxicity; biodegradability

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