Effect of pH-buffering on Cr(VI) reduction with pyrite in the presence of various organic acids: Continuous-flow experiments

• pH buffering improved Cr(VI) removal by pyrite under acidic pH conditions.
• pH buffering had adverse effects on Cr(VI) removal by pyrite under alkaline pH conditions.
• Buffers interfered with pyrite surface under alkaline pH conditions.
• Bicarbonate resulted in a better Cr(VI) removal than MOPS under alkaline pH conditions.

The effects of pH and pH buffering on catalytic hexavalent chromium (Cr(VI)) reduction by pyrite were investigated in a series of column experiments. The experiments show a strong dependence of Cr(VI) removal by pyrite on solution pH, with Cr(VI) removal decreasing with increasing solution pH. While organic buffers e.g., (acetic acid/acetate) showed higher Cr(VI) removal by pyrite under acidic conditions (e.g., pH 4) relative to unbuffered systems, the addition of an organic buffer (MOPS) decreased Cr(VI) removal by pyrite at pH 8 relative to systems buffered with an inorganic buffer (bicarbonate). Similarly, in systems buffered with acetate buffer at pH 4, organic acids (e.g., citrate and tartrate) significantly improved Cr(VI) removal by pyrite due to removal of surface oxidation products (e.g., Fe(OH)3(s)). Unlike acidic conditions, organic acids such as citrate and EDTA adversely affected Cr(VI) removal under alkaline pH conditions relative to systems containing 0.01 M MOPS buffer only. This inhibitory effect observed under alkaline pH conditions may be explained through competitive adsorption of organic buffers (MOPS) and acids (citrate, EDTA) onto pyrite surface, which, not only, modified pyrite surface properties, but also, blocked access of Cr(VI) to pyrite surface. Overall, it is clear that while pH buffering under acidic conditions may significantly improve the process efficiency of pyrite-based Cr(VI) treatment, it may have adverse effects on process efficiency under alkaline conditions (e.g., pH 8) since organic buffers (e.g., MOPS) and acids (e.g., EDTA) may directly interact with pyrite surface, and change its surface properties.