In this study an innovative electrochemical system for the removal of hexavalent
chromium from dilute acidic solution is described. This was achieved using a tubecell, which is based on a flow-through process. The cell was used for the examination
of redox processes occurring at a porous carbon-felt electrode to study model waste
solutions of hexavalent chromium. The prospect of removing Cr (VI) in a single stage
process and without the addition of any chemical was realised.
The kinetics of the reduction of Cr (VI) were evaluated under static and flow
conditions as a function of pH, flow rate, current density and electrode material. The
performance of different type of felts was evaluated.
Firstly, the importance of controlling the pH for the reduction and subsequent
precipitation of chromium in an appropriate form for its recovery was investigated. It
was shown that the reduction process of Cr (VI) reduction occurred under kinetic
control. Hydrodynamic voltarnrnetry studies of the Cr (VI) reduction process showed
that the mass transport should be carefully controlled. The results obtained
demonstrated that the Cr (VI) reduction at a porous carbon felt electrode occurs under
a mixed control regime.
In the second part of the work, a comparison of the different electrodes used was
carried out using Electrochemical Impedance Spectroscopy. The high-density
electrodes were shown to improve the electron transfer through better conductivity
and larger surface area.
Finally, the electrolysis of the Cr (VI) solution was investigated under different
conditions of flow rate, current density and electrode material. Slow flow rates, pH
values between 3 and 4 and high current densities were essential for good removal
efficiency. Surface analysis techniques confirmed that Cr(OH) 3 was the major
precipitate at the carbon and graphite felt electrodes.