Pseudo-first-order reaction of chemically and biologically formed green rusts with HgII and C15H15N3O2: Effects of pH and stabilizing agents (phosphate, silicate, polyacrylic acid, and bacterial cells)

Abstract

The kinetics of HgII and methyl red (MR) reduction by hydroxycarbonate green rust (GR1) and by hydroxysulfate green rust (GR2) were studied in the presence of naturally occurring organic and inorganic ligands (phosphate, polyacrylic acid, bacterial cells, silicate). The reducing ability of biogenic hydroxycarbonate green rust (GR1bio), obtained after microbial reduction of lepidocrocite by Shewanella putrefaciens, was also investigated and compared to those of chemically synthesized GR1 and GR2 (GR1ab and GR2ab). Pseudo first-order rate constants (kobs) of HgII reduction (at pH 7.0, 8.2, and 9.5) and MR reduction (at pH 7.0) were determined and were normalized to the structural FeII content of GRs (kFeII) and to the estimated concentration of surface FeII sites (kS). The kS values ranged from 0.3 L mmol−1 min−1 to 43 L mmol−1 min−1 for the Hg reduction, and from 0.007 L mmol−1 min−1 to 3.4 L mmol−1 min−1 for the MR reduction. No significant discrepancy between GRab and GRbio was observed in term of reactivity. However, the reduction kinetics of MR was generally slower than the HgII reduction kinetics for all tested GRs. While a slight difference in HgII reduction rate was noted whatever the pH values (7.0, 8.2, or 9.5), the reduction of MR was significantly affected in the presence of ligands. A decrease by a factor of 2–200, depending on the type of ligand used, was observed. These data give new insights into the reactivity of GRs in the presence of co-occurring organic and inorganic ligands, and have major implications in the characterization of contaminated systems as well as water treatment processes.