Sono-oxidation of soils: degradation of naphthalene by sono-Fenton-like process

Abstract

Purpose

Sono-Fenton-like process was tested to degrade naphthalene in spiked soil utilizing mineral iron as a catalyst to generate radical species as well as to evaluate the efficiency of the proposed method and to optimize the treatment conditions. The study assessed the relationships between the most significant process variables (initial naphthalene and hydrogen peroxide concentrations) and their response on naphthalene degradation efficiency using central composite design at various ultrasound irradiation intensities. The electrical energy per order was also calculated to illustrate economic benefits of this “hybrid” technique.

Materials and methods

Sono-oxidation was performed with naphthalene contaminated soil (200, 400, and 800 mg kg⁻¹ dry weight) to mimic industrially contaminated soil conditions in a closed Pyrex glass reactor in the presence of naturally occuring mineral iron, various ultrasound irradiations (100, 200, and 400 W), and hydrogen peroxide concentrations (100, 200, and 600 mg L⁻¹). Naphthalene samples were analyzed by gas chromatography–mass spectometry using an Agilent 6890 N chromatograph coupled to a quadrupole Agilent 5975 inert XL mass selective spectrometer. Easily exchangeable iron was extracted by citrate–bicarbonate–dithionite extraction, and total iron was extracted by acid digestion and were analyzed using an atomic absorption spectroscopy flame (Perkin-Elmer 300). The surface morphology of the soil sample was observed by scanning electron microscopy, and the specific surface area was determined using Brunauer–Emmett–Teller method on a Quantachrome Autosorb 1 analyzer. The STATISTICA 7 software was used for regression and graphical analyses of the data.

Results and discussion

Scanning electron microscopy revealed that soil was predominantly associated with quartz and coarse and fine sand fractions and contained a significant amount of iron (7.1 g kg⁻¹ of total and 2.3 g kg⁻¹ of easily exchangeable iron). Control experiments showed that, in the absence of hydrogen peroxide, naphthalene degradation up to 35% was achieved, which suggested that mineral iron was able to catalyze the production of hydroxyl radicals when ultrasound irradiation was used as an oxidizing agent. The optimum values (600 mg L⁻¹ of hydrogen peroxide and 200 mg kg⁻¹ of naphthalene concentrations) were obtained by solving the regression equations. Various kinetic constants such as k_N, k_N/OH*, k_surf, half-life (τ_1/2), and [OH*]_SS were calculated for sono-Fenton-like process. The addition of hydrogen peroxide demonstrated nearly tenfold decrease in electrical energy per order or in electrical costs from 2,063–4,210 kWh ton-order⁻¹ and 144.4–294.7 € ton⁻¹ at 0 mg L^–1 H₂O₂ to 26.4–42 kWh ton-order⁻¹ in comparison to 1.7–2.9 € ton⁻¹ at 600 mg L⁻¹ H₂O₂ addition to the system.

Conclusions

Mineral iron was able to catalyze the degradation of naphthalene in the presence of ultrasound (up to 78% at 100 W and 97% at 400 W) and various concentrations of hydrogen peroxide. The optimum values (600 mg L⁻¹ of hydrogen peroxide and 200 mg kg¹ of naphthalene concentrations at 200 and 400 W) indicated up to a maximum 97% reduction in naphthalene concentration in soil after 2 h of treatment. The Fenton-like oxidation of naphthalene in the presence of ultrasound has a potential to be used for practical purposes, however, it warrants further research regarding the use of industrially contaminated soil with multiple organic contaminants. Moreover, improvement in the reactor design is necessary to prolong the lifetime of the sonotrode which tends to wear off due to ultrasound waves that propagates back to the sonotrode from the walls and the bottom of the reactor. Despite the current difficulties, ultrasound has a bright future in on-site soil remediation field and may become one of the most feasible options and environmentally sound techniques.