Trace Cobalt-Mediated Activation Of Peroxymonosulfate To Degrade Azo Dye Acid Orange7: Role Of Bicarbonate Ions

Advanced Oxidation Technologies （AOTs） based on the generation of sulfateradical （SO₄^â†'） is a recently emerging way to remove the nonbiodegradable organicpollutants. These technologies gain their prominence owing to their excellentproperties in some aspects of solving environmental problems, such as dyes andpharmaceuticals wastewater treatment, groundwater pollution remediation anddrinking water disinfections.As a commonly existent ion in water, bicarbonate receives much attention ofresearchers in AOTs that based on hydroxyl radical, and it is considered as one of theexcellent radical inhibitors that could impede the contaminant degradation process,which is realized as one of these technologies’ Achilles heel. Cobalt-mediateddecomposition of peroxymonosulfate, named as CP system, is a new type of AOTsbelongs to Fenton-like reaction. CP system catches much focus because of someproperties such as ability to produce SO₄^â†', which has a high redox potential of+2.5³.1V, vs. NHE, and could effectively degradate organic pollutants in acidic, neutralas well as faintly basic conditions.In this study, CP system is considered as a fundament, acid orange7（AO7） isused as a model contaminant, and the effect of a series of bicarbonate concentrationsfrom1.7×10-5mol L^-1to1.7×10^-1mol L^-1is studied as a coexistence with trace cobalt(10μg L^-1) that realized before as the main catalysis to decompose peroxymonosulfatein CP system. Scavengers are used to identify the main activities, and the initialmechanism of the new cobalt-biocarbonate-comediated peroxymonosulfate system,named as CBP system, is also discussed. Specific studies and results are as follows:（1） The ion effect of bicarbonate is positive in trace cobalt-mediatedperoxymonosulfate system, which could increase the azo dye AO7decoloration. The increased degree is depond on the concentration of bicarbonate, appeared as a trend ofreduse after the first increase, and then increase again. When the concentration ofbicarbonate is below5mmol L^-1, the positive effect increases first then decrease, andbecomes a maximum when the concentration of bicarbonate is1mmol L^-1, theincreased decoloration degree is¹23.6%. When the concentration of bicarbonate ismore than5mmol L^-1, the positive effect continuously increases and becomes anothermaximum when the concentration of bicarbonate reached1.7×10^-1mol L^-1, and theincreased decoloration degree is¹50%. The decoloration of AO7followed apseudo-first-order kinetics in CBP system, and the reaction rate constant has apositive correlation with the decoloraton degree.（2） Bicarbonate could activate peroxymonosulfate to degradate azo dye AO7without any cobalt in the solution. This process also followed a pseudo-first-orderkinetics. However, bicarbonate-mediated peroxymonosulfate, named as BP system,followed different mechanism with CBP system. In BP system, the decoloration rateof AO7monotonously increases with the concentration of bicarbonate. When it isbelow10mmol L^-1, the activation could be neglected, but when it is more than10mmol L^-1, bicarbonate could activate peroxymonosulfate well and fast until itsconcentration reaches100mmol L^-1, and this trend would become gentle. Thedecoloration of AO7is more than90%in4h, when the concentration of bicarbonateexceeded50mmol L^-1, and the reaction rate increases from0.0002min^-1to0.013min^-1, which is⁶5times.（3） In CBP system, the decoloration mechanism of AO7is related to the molarratio of bicarbonate and cobalt, which could explain by two phase mechanism.（4） The main activities of the system could be identified by using differentinhibitors. The inhibition degree is connected with the concentration of inhibitors. Theconcentration of50%inhibition, IC₅₀, could be used to reflect this relationship. Themain activities of CBP system is related to the molar ratio of HCO₃^-and Co²⁺, whichcould explain by twe phase mechanism. When n（HCO₃^-）: n(Co²⁺) is small, the mainactivities are S₄^â†'and1O2, and when it is big enough, the degradation process wouldfollow HCO4oxidation mechanism containing a series of intermediate radicals produced by unimolecular homolysis of HCO4, and mainly the contribution of1O2.（5） The self-decomposition of peroxymonosulfate in basic condition coulddegradate organic pollutant like AO7. The concentration of hydroxyl ion is the key ofthe decoloration rate. The decoloration of AO7in buffered solution followed apseudo-first-order kinetics.（6） The degradation of contaminant by CBP system should contain the effect ofthe self-decomposition of peroxymonosulfate in basic condition. In the twe phasemechanism, when the concentration of bicarbonate is not high enough to maintain thealkalinity of solution, the effect mentioned above could be neglected. While theconcentration of bicarbonate is high enough to buffer the solution with a constant pHabout8.5or even higher than that, the effect mentioned above should be consideredcarefully and there should exist an optimum pH value, which is near to the pKa2ofcarbonic acid.