Enhanced removal of chromate by graphene-based sulfate and chloride green rust nanocomposites

Green rust, a double layered (Fe^II, Fe^III) hydroxide, can impact the mobility and toxicity of highly reactive contaminants such as chromate through reduction of Cr^VI to Cr^III. The reduction of Cr^VI by sulfate and chloride green rusts (GRs) was comparatively studied for a range of initial Cr^VI concentrations and pH values. At an equal molar ratio of Cr^VI to Fe^II in GR, sulfate green rust (GRSO₄) removed higher amounts of Cr^VI from solution than chloride green rust (GRCl). The rate of the reactions with respect to initial concentration of Cr^VI is well described by pseudo-second order model. The GRCl reacted with chromate much faster than GRSO₄. The obtained pseudo-second order rate coefficients for the GRCl and GRSO₄ ranged from 99.53 to 14.24 and 35.45 to 4.53 mmol/s, respectively. Graphene-based green rust nanocomposites were prepared using partially reduced graphene (PRG) and examined for removal of Cr^VI. The efficiency of Cr^VI removal by GRSO₄/PRG and GRCl/PRG nanocomposites was dependent on the mass ratio of PRG to total Fe precipitated as GR (PRG:Fe_(s)) and the highest removal was observed at PRG:Fe_(s) mass ratios of 1:2. In comparison with bare GRs, the GR/PRG nanocomposites removed higher amounts of Cr^VI from solution. However, the reduction rates of Cr^VI by GR/PRG nanocomposites were ∼2 to 3-fold lower than those observed by bare GRs in both sulfate and chloride systems.