•Electrochemical oxidation of pyrite occurred by two reaction pathways.•At potential of 0.50 V, electrochemical oxidation of pyrite was diffusion-limited.•At potential of 0.60 V, amorphous S8 was slowly converted to crystalline S8.•S8 was difficult to be oxidized electrochemically even at high potential.•Oxidation of pyrite was inhomogeneous on electrode surface.The oxidation of pyrite under acidic conditions, an important process leading to the formation of acid mine drainage, has been the subject of intense research yet remains incompletely understood. In this study, the mechanism of the electrochemical oxidation of pyrite in a pH 2 electrolyte was investigated using electrochemical techniques. The morphological changes and oxidation products of pyrite were studied using atomic force microscopy (AFM), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS). At low potential of 0.50 V, electrochemical oxidation of pyrite was diffusion-limited due to a sulfur-rich layer (S0) that formed and covered the pyrite surface, resulting in surface passivation. As the potential increased to 0.60 V, diffusion-limitation and surface passivation of pyrite oxidation ceased due to the conversion of amorphous elemental sulfur (S8) to crystalline S8, resulting in the previously covered active sites being re-exposed which allowed continued oxidation of pyrite. At higher potentials of 0.70 and 0.80 V, more S8 and polysulfides (Sn2−), together with an iron-rich layer composed of Fe(OH)3, FeO and Fe2O3, formed and accumulated on the pyrite surface. These products led to a decreased rate of oxidation rather than a complete passivation of the surface. AFM imaging revealed that surface roughness increased with oxidation potential and that the oxidation of the pyrite surface was inhomogeneous. These findings provide further insight into the physical and chemical changes that pyrite undergo during electrochemical oxidation, which deepens our understanding of this important process.

•HPLC is an efficient technique to detect the S0 and SnO62 −.•S0 is formed definitely during pyrite oxidation process.•Some species of SnO62 − also formed and their concentrations increased.•ferrooxidans promoted the pyrite oxidation.•ferrooxidans accelerated KFe3(SO4)2(OH)6) and FeOOH formation.The intermediate sulfur species of pyrite chemical and biological oxidation have been the subject of controversy for some time, especially the question of whether or not elemental sulfur (S0) and polythionates (SnO62 −) are formed during the oxidation process. Acidithiobacillus ferrooxidans (A. ferrooxidans), one of the most common sulfur-oxidizing bacterial strains, has been shown to remarkably accelerate pyrite oxidation. In this study, the intermediate products of pyrite oxidation with and without A. ferrooxidans present were compared by employing different analytical techniques; i.e., high performance liquid chromatography (HPLC), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS). The HPLC results showed that the concentrations of S0, S3O62 −, S4O62 −, S5O62 − and S6O62 − increased during pyrite oxidation process in the presence of A. ferrooxidans. Secondary minerals jarosite (KFe3(SO4)2(OH)6) and iron(III) oxide-hydroxide (FeOOH) were also detected by XRD and XPS. Without A. ferrooxidans, S0 was also formed and along with S3O62 −, S4O62 − and S5O62 − but only at very low concentrations at the end of the experiment. SEM micrographs further revealed that the pyrite was severely eroded by A. ferrooxidans and some spheroidal particles covered the surfaces of pyrite residues. These particles are most likely to be KFe3(SO4)2(OH)6 based on EDS analysis. The present study has quantitatively confirmed the presence of intermediate products of S0 and SnO62 − during pyrite oxidation, information that deepens our understanding of the mechanism of pyrite oxidation.

Electrokinetic-microbial remediation (EMR) has emerged as a promising option for the removal of polycyclic aromatic hydrocarbons (PAHs) from contaminated soils. The aim of this study was to enhance degradation of phenanthrene (Phe)-contaminated soils using EMR combined with biosurfactants. The electrokinetic (EK) remediation, combined with Phe-degrading Sphingomonas sp. GY2B, and biosurfactant obtained by fermentation of Pseudomonas sp. MZ01, degraded Phe in the soil with an efficiency of up to 65.1 % at the anode, 49.9 % at the cathode after 5 days of the treatment. The presence of biosurfactants, electricity, and a neutral electrolyte stimulated the growth of the degrading bacteria as shown by a rapid increase in microbial biomass with time. The electrical conductivity and pH changed little during the course of the treatment, which benefitted the growth of microorganisms and the remediation of Phe-contaminated soil. The EMR system with the addition of biosurfactant had the highest Phe removal, demonstrating the biosurfactant may enhance the bioavailability of Phe and the interaction with the microorganism. This study suggests that the EMR combined with biosurfactants can be used to enhance in situ bioremediation of PAH-contaminated soils.

•Saline surfactant shows lower CMC values than fresh surfactant.•The lower β values of saline surfactants, the higher pyrene solubilization.•STPP reduces surfactant sorption onto soil and enhances pyrene desorption.This study investigates the influence of salinity on micellar behavior, enhanced solubilization and desorption of pyrene within single and mixed saline anionic–nonionic surfactants. Various interaction parameters linked to the micellarization and solubilization process have been correlated through theoretical treatments to quantify micellar characteristics and solubilization capabilities of the systems. Results show that the experimental critical micelle concentration (CMC) values of the saline system were lower than that of non-saline system, indicating more attractive interactions of individual surfactants and higher stability of mixed micelles in the presence of salts. Pyrene solubility in saline surfactant solutions exceeded that in a non-saline solution. Less electrostatic repulsion of saline mixed micelles may be responsible for the reduced CMC of the mixed micelles, the increase of molar solubilization ratio (MSR) and micelle–water partition coefficient (Km). The saline micelles enhanced desorption of pyrene from kaolin-rich soil, and the pyrene desorption increased with the increase of salinity. Furthermore, the presence of salinity reduced the sorption of surfactant onto kaolin-rich soil; e.g. from 15.34 to 11.07 mg of surfactant/g of soil at a mass ratio of 5:5 mixture of anionic and nonionic surfactants. The results of this study provide new insights for estimating the utility of mixed surfactants treatments for soil remediation.

A fusant strain F14 with high biodegradation capability of phenanthrene was obtained by protoplast fusion between Sphingomonas sp. GY2B (GenBank DQ139343) and Pseudomonas sp. GP3A (GenBank EU233280). F14 was screened and identified from 39 random fusants by antibiotic tests, scanning electron microscope (SEM) and randomly amplified polymorphic DNA (RAPD). The result of SEM analysis demonstrated that the cell shape of fusant F14 different from parental strains. RAPD analysis of 5 primers generated a total of 70 bands. The genetic similarity indices between F14 and parental strains GY2B and GP3A were 27.9 and 34.6 %, respectively. F14 could rapidly degrade phenanthrene within 24 h, and the degradation efficiency was much better than GY2B and GP3A. GC–MS analysis of metabolites of phenanthrene degradation indicated F14 had a different degradation pathway from GY2B. Furthermore, the fusant strain F14 had a wider adaptation of temperatures (25–36 °C) and pH values (6.5–9.0) than GY2B. The present study indicated that fusant strain F14 could be an effective and environment-friendly bacterial strain for PAHs bioremediation.

The fusant strain (F14), which produced by protoplast fusion between Sphingomonas sp. GY2B (GenBank DQ139343) and Pseudomonas sp. GP3A (GenBank EU233280), was tested for phenanthrene biodegradation at 30 °C and pH of 7.0. The kinetics of phenanthrene biodegradation by F14 was investigated over a wide range of initial concentration (15–1,000 mg l−1). The rate and the extent of phenanthrene degradation increased with the increase of concentration up to 230 mg l−1, which indicated negligible inhibition effect at low concentrations. The non-competitive inhibition model was found to be fit for the process. GC–MS analysis showed that biodegradation of phenanthrene by F14 was via dioxygenation at both 1,2- and 3,4-positions and followed by 2-hydroxy-1-naphthoic acid and 1-hydroxy-2-naphthoic acid. The relative intensity of 2-hydroxy-1-naphthoic acid was approximately 3–4 times higher than that of 1-hydroxy-2-naphthoic acid, indicating the 2-hydroxy-1-naphthoic acid was the predominant product in the phenanthrene degradation by fusant strain F14.