Exhaustive depletion of recalcitrant chromium fraction by electrocogulation in a continuous flow pilot plant.

Nowadays there is a great concern about the increasing release of heavy metals into environment, since these substances can have harmful effects on many form of life. In this frame, Cr release represents a menace to natural life and public health for its mutagenic and carcinogenic properties. Wastewaters containing Cr are discharged into the aquatic environment from various industries like those of metal finishing, electroplating, pigments, tannery and chemical manufacturing. In particular, the leather industry which uses Cr for the tanning of animal skins and hides, produces a large amount of wastewater containing Cr concentrations much higher than those allowed for the more stringent environment requirements. Conventional treatments generally assure quite satisfactory Cr removal; the rising problems are the requirements of expensive equipments and monitoring systems, the use of additional chemicals that cause a secondary pollution, high energy consumption and large production of toxic sludge which require disposal. Electrochemical processes have been proved to be competitive methods allowing high removal efficiency without requiring supplementary addition of chemicals and reduced volume of sludge produced. Electrocoagulation and electroprecipitation, in particular, seem to be promising technologies when wastewater with high Cr concentrations are considered. The limit for these processes is represented by the presence, in wastewater, of stable Cr complexes with the organic substances. Recent studies [1] have shown that recalcitrant Cr abatement may be improved by using an ozonization pre-treatment followed by an electrocoagulation with Fe electrodes. In order to evaluate the possibility to develop this process on industrial scale, electrocoagulation treatment was investigated using a specifically designed pilot plant. Working conditions were first defined by experiments done on laboratory apparatus. Then tests were carried out in the pilot plant which operates in batch recirculation mode until selected working conditions are reached and then as a continuous flow system, in steady-state conditions. Mathematical models to represent the behaviour of the whole system in the different operating mode were developed. Comparison of experimental data against simulation results of the whole system operating in batch recirculation mode validated the proposed model. [1] C. Durante et al. Chemosphere, 78, 2010, 620.