Acid mine drainage (AMD) is characterized by the high iron concentrations, the low pH values and amout of various toxic elements. The widely distributed carbonate react with AMD in karst area, which increases the pH. An appropriate pH existed for the oxidation of Fe(II) and generates reactive oxygen species (ROSs, eg; ·OH) through the Haber-Weiss mechanism (Koppenol, 2001). ·OH is a strongly ROSs, which can rapidly oxidize redox-sensitive elements such as As(Ⅲ) (Dutta et al., 2005; Zhang et al., 2016; Zhu et al., 2017) and is identified as an important product in the abiotic oxidation of Fe(II) by O2 in AMD (Zhu et al., 2017). The diverse sources of ·OH generation by the oxidation of Fe(II) in AMD. Under low pH conditions, the mechanisms of ·OH production have been clarified (Borda et al., 2003; Schoonen et al., 2010; Zhang et al., 2016). ·OH that is produced from ionically bound species of Fe(II) by mechanism of Fenton (Hug and Leupin, 2003). However, the yield of ·OH production from oxidation of inorganic dissolved Fe(II) by O2 or H2O2 is low under acidic conditions (Miller et al., 2013; Remucal and Sedlak, 2011). The adsorption Fe(II) benefits the production of ·OH by enhancing the reducibility of Fe(II) (Ai et al., 2013). It was documented that ·OH production from iron-bearing minerals, for example, magnetite (Fe3O4) (Ardo et al., 2015), siderite (FeCO3) (Guo et al., 2013), and pyrite (FeS2) (Pham and Waite, 2008; Zhang et al., 2016) and so on, are activated by O2. The presence of citrate (Zhang and Yuan, 2017), ethylenediamine tetraacetic acid (EDTA) (Christina et al., 2008) or dissolved organic matter (DOM) (Page et al., 2013; Rose and Waite, 2002) can increase the yield of ·OH markedly via complexation, which accelerates Fe(Ⅱ) oxidation in AMD. The complexation of Fe(II) can reduce the redox of Fe(Ⅱ)/Fe(Ⅲ) couple and accelerate the oxidation rate (Christina et al., 2008; Keenan and Sedlak, 2008).
Under acidic conditions, there are rich of microorganisms acid-philic Fe(Ⅱ)-oxidizing bacteria in AMD (Schrenk and M., 1998) which can broadly classified into directly or indirectly to promote Fe(Ⅱ) oxidation (Boon, 2001; Rodrguez et al., 2003). Recently researches have discovered microorganisms that are likely to produce extracellular ROSs, including fungi, phytoplankton or heterotrophic bacteria as potentially significant sources of ROSs in natural waters (Diaz et al., 2013; Zhang et al., 2016). Extracellular Polymeric Substances (EPS) are organic polymers secreted by microbial cells, a complex high-molecular-weight mixture of polymers, of which 70%~80% are proteins and polysaccharides (Houghton and Stephenson, 2002; K Kinzler et al., 2003). EPS are a major component of biofilms and play a key role in cell surface attachment (Vu et al., 2009). Keenan and Sedlak reported that the yield of ·OH production is increased drastically in the presence of organic ligands such as oxalate or EDTA, which can accelerate the chemical oxidation of Fe(II) by O2 and inhibit the precipitation of Fe(Ⅲ) (Keenan and Sedlak, 2008). Fe(Ⅱ)-DOM complexes in water are oxidized to promote the production of ·OH (Page et al., 2013). In addition, it is play a role in H2O2-mediated Fe(II) oxidation can produce ·OH when dissolved Fe(II) is complexed by Suwannee River fulvic acid (SRFA) (Miller et al., 2013). The presence of Low-molecular-weight organic acids (LMWOAs) or natural organic matter (NOM) can improve the efficiency of ·OH production dramatically because of complexation (Peiffer and Stubert, 1999; Rose and Waite, 2002). EPS contains abundant negatively-charged functional groups, for example, carboxyl, phosphoric, sulfhydryl, phenolic and hydroxyl groups, and efficiently adsorb irons, and then inform Fe(II)-EPS complexes. (Abzac et al., 2010; Guibaud et al., 2005a; Gutnick and Bach, 2000; Pagnanelli et al., 2009). Fe(II)-EPS complexes may effect ·OH production via affecting Fe(II) oxidation by O2. However, little reports on the production of ·OH by the Fe(II)-EPS complex. which is rich in organic functional groups. Therefore, the objective of this study was to ascertain the mechanism of Fe(Ⅱ) chemical oxidation on the production of ·OH in the presence of EPS, which is of great scientific significance for understanding the migration and transformation process of As/Sb and other pollutants in the chemical oxidation.
In this study, We extracted the EPS of microorganisms secretion and simulated AMD to carry out static experiments to explore the mechanism of the production of ·OH in the presence of EPS on Fe(II) chemical oxidation. It is noted that Fe(IV) is likely to generate at neutral conditions (Remucal and Sedlak, 2011; Wiegand et al., 2017), but is not emphasized in our study. The oxidation reaction is stimulated by the addition of CaCO3 powder. To achieve this goal, sodium benzoate (BA) was added as a probe compound to quantity the cumulative concentrations of ·OH produced from oxygenating simulated AMD (Joo et al., 2005). In batch experiments, the EPS concentration and suspension pH by the addition of different dosages of CaCO3 powder were varied to explore the mechanisms. The influence of ionically Fe3+ was ultimately tested. The main components of EPS characterized by Three-dimensional excitation-emission matrix (EEM) fluorescence spectra or Fourier transform infrared spectroscopy (FT-IR). The solid particles after oxygenation were characterized X-ray powder diffraction (XRD).