Effect of cations and ferric-oxide/hydroxide precipitation on the removal of chlorobenzene compounds from model solutions applying ferrate treatment

The effect of cations (Ca2+, Mg2+, Zn2+, Ni2+) as common compounds of groundwater and ferric-oxide/hydroxide precipitation formed during the ferrate treatment on the removal of monochlorobenzene (MCB) and dichlorobenzene (1,2-DCB and 1,3-DCB) from model solutions were studied. The model solutions contained the three CB- compounds and sodium-hydrogen-carbonate in the concentration of 10 and 150 mg/L, respectively. To investigate the cation effect only one or simultaneously all the four cations were added to the model solutions in a molar ratio (CB-s:cations) of 1:1 or 1:4, respectively. The ferrate treatment was carried out by the addition of highly alkaline sodium ferrate solution (6 g/L) to the model solutions resulting in a CB-s:Fe(VI) molar ratio of 1:5. To adjust the pH from 12 to the optimal 7, high purity 5 M sulphuric acid was used and the freshly formed suspension was stirred for 30 min at room temperature. In the liquid phase, the concentration of CB-s, the purgeable carbon content, the cations and the chlorine as oxidation by-product were measured by headspace gas chromatography-mass spectrometry, total organic carbon analyzer, flame atomic absorption spectroscopy, and ion chromatography, respectively. Comparing the removal efficiency values of CB-s obtained by ferrate treatment in absence or presence of cation( s), it can be established that the degradation of these target pollutants increased in presence of cations in the following order: Zn2+ (3%) < Mg2+ (30%) < Ni2+ ( 50%) < Ca2+ (72%). However, the ferric-oxide/hydroxide precipitate did not affect the CB-s removal, but a considerable part of cations was removed from the liquid phase due to co-precipitation and adsorption processes. It should be emphasized that the cation removal was nearly twice higher in the case of ferrate treatment than those of ferric hydrolysis due to the smaller grain size and higher specific surface area of ferric-oxide/hydroxide particles.