In situ chemical oxidation using persulfate is one alternative to remediate polycyclic aromatic hydrocarbon-contaminated soil; however, oxidation can lead to the formation of toxic and persistent by-products, and treatment efficiency can be dependent on environmental conditions. Temperature and soil matrix properties can dictate reaction rates and pathways, promoting oxidant activation or scavenging the free radicals generated. This research investigated the ability of persulfate to degrade anthracene in tropical environmental conditions. Batch tests were conducted for various persulfate systems (naturally and chelated-iron-activated), with an Oxisol contaminated with anthracene. Electron paramagnetic resonance (EPR) was used to identify free radicals formed. Naturally activated persulfate degraded more than 96% of the anthracene and its by-product anthraquinone after 90 days, considered more toxic and persistent, while the chelated-iron-activated persulfate system used was able to remove 70% of the anthracene. EPR measurements showed the coexistence of SO₄·⁻ and ·OH radicals. Sulfate radicals were formed by thermal activation at ambient temperatures (mean of 23.7 °C), and ·OH was formed by propagation reactions and hydrolysis in acidic conditions that lead to peroxide formation. In the naturally activated system, anthracene degradation was observed and SO₄·⁻ radicals were abundant, indicating that this treatment system can be effective in a typical tropical soil environment.

Peroxidases (EC 1.11.1.7) have enormous biotechnological applications. Usage of more abundant, basic isoforms of peroxidases in diagnostic kits and/or in immunochemistry has led to under exploitation and disregard of horseradish peroxidase (HRP) acidic isoforms. Therefore, acidic horseradish peroxidase (HRP-A) isoenzyme was used for the preparation of a biocatalyst with improved ability in dye decolorization. Ten biocatalysts were prepared by covalent binding of enzyme to chitosan and alginate, adsorption followed by cross-linking on inorganic support (aluminum oxide), and encapsulation in spherical calcium alginate beads via polyethylene glycol. Model dyes of 50 to 175 mg l⁻¹ were removed by the biocatalysts. Among the tested biocatalysts, the three with the highest specific activity and biodegradation rate were further studied (Chitosan-HRP, Al-Gel-HRP and Al-HRP-Gel). The impact of hydrogen peroxide concentration on dye decolorization was examined on the Chitosan-HRP biocatalyst, since the HRP is susceptible to inhibition/inactivation by high H₂O₂. On the other hand, H₂O₂ is needed as a co-substrate for the HRP, and the H₂O₂/dye ratio can greatly influence decolorization efficiency. Concentrations of H₂O₂ ranging from 0.22 to 4.4 mM showed no difference in terms of impact on the biocatalyst decolorization efficiency. The high decolorization efficiency of the biocatalysts was validated by the removal of 25 and 100 mg l⁻¹ anthraquinone (Remazol Brilliant Blue R (RBBR)), triphenylmethane (Coomassie Brilliant Blue (CBB)), acridine (Acridine Orange (AO)), and formazan metal complex dye (Reactive Blue 52 (RB52)). After the seven consecutive decolorization cycles, the decolorization was still 53, 78, and 67% of the initial dye for the Al-HRP-Gel, Al-Gel-HRP, and Chitosan-HRP immobilizate, respectively. The results obtained showed potential of otherwise neglected acidic HRP isoforms as a cost-effective biocatalyst with significant potential in wastewater dyestuff treatment.

In this study, performance of hydrolysis acidification process treating simulated dyeing wastewater containing azo and anthraquinone dyes in different stages was investigated. The decolorization ratio, CODCᵣ removal ratio, BOD₅/CODCᵣ value, and volatile fatty acids (VFAs) production were almost better in stage 1 than that in stage 2. Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) confirmed the biodegradation of Reactive Black 5 (RB5) and Remazol Brilliant Blue R (RBBR) in hydrolysis acidification process. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analyses revealed that significant difference of microbial community structures existed in stage 1 and 2. The dominant species in stage 1 was related to Bacteroidetes group, while the dominant species in stage 2 was related to Bacteroidetes and Firmicutes groups. From the results, it could be speculated that different dyes’ structures might have significant influence on the existence and function of different bacterial species, which might supply information for bacteria screening and acclimation in the treatment of actual dyeing wastewater.