Photocatalytic Reduction Of Oxo-anions In Water Using Titanium Dioxide Nanoparticles

Oxo-anions contaminants in water pose a great risk to human health. As a typicaloxo-anions contaminant, nitrate is prevalent in groundwater. Intake of excess nitrate maycause methemoglobinemia and cancer. So far there has been no perfect nitrate reductiontechnology for drinking water treatment. Photocatalysis is a promising technology for watertreatment. This study investigated the photocatalytic reduction of nitrate and other oxo-anionsthrough laboratory experiments mainly using photocatalysis with Evonik P90as the titaniumdioxide photocatalyst. The major conclusions of this work are presented as follows:（1） P90/Ag photocatalyst was synthesized, which showed higher photocatalytic activitythan P90. After35minutes of irradiation, nitrate was completely removed to84.5%N₂and15.5%NH₄⁺using P90/Ag. The influence of pH, catalyst dosage, initial nitrate concentrationand formic acid-to-nitrate molar ratio （IFNR） on photocatalytic nitrate reduction wasinvestigated. The results showed that acid condition was favorable for nitrate reduction;Nitrate removal was fastest at catalyst dosage of1g/L; Increasing the initial nitrateconcentration inhanced it’s absolute removal rate; Increasing IFNR did not significantlyincrease photocatalytic activity. Those results provided experimental support for choosing asuitable catalyst and seeking for optimized reaction condition.（2） Based on the shortcomings of direct photocatalytic nitrate reduction in drinking water,a technology that combining ion exchange with photocatalytic reduction was proposed toremove nitrate from groundwater. Experimental results showed that it was effective to removenitrate from synthetic and real ion exchange brines using photocatalytic reduction, and thetreated brine could be reused for regeneration of ion exchange resins. It is believed that ionexchange combined with photocatalytic reduction is a feasible way to remove nitrate.Compared with single ion exchange or photocatalysis, the combination technology can greatlydecrease the expense on brine discharge or treatment and save salt consumption, moreover, bytreating the ion exchange brine instead of groundwater, it can minimize the contamination ofgroundwater with side products（NO₂^-，NH₄⁺） and ensure the water quality. Therefore, Thiscombination technology can be a new approach for nitrate reduction from groundwater. （3） Based on laboratory research work, the scale of experiment was enlarged by70times,and photocatalytic nitrate reduction was examined under pilot scale condition usingPhotoCat@as the pilot scale photoreactor. The results showed that nitrate removal in nanopurewater was hard to achieve, while nitrate in ion exchange brine was effectively removed at arate of0.7mg-N/（L min）. Using UV254low pressure mercury lamp as the light source, bothphotolysis and photocatalytic oxidation of formic acid occurred, which made simultaneousremoval of formic acid and nitrate possible. Those results further testified the potential of thecombination of ion exchange and photocatalytic reduction, and provided scientific proof forindustrial application of photocatalytic nitrate reduction.（4） Expanding photocatalysis from removal of nitrate to bromate and other oxo-anions,this study investigated their photocatalytic removal activity, kinetics, by-products as well assimultaneous removal with formic acid. The results showed that bromate, chlorate, nitrite,dichromate, iodate and chlorite could be photocatalytically reduced, but perchlorate could notbe removed. Bromate, chlorate and dichromate were reduced to bromide, chloride andtrivalent chromium, respectively, and could be simultaneously removed with formic acid. Butfor those yielding multiple byproducts （e.g., NO₃^-、NO₂^-）, they were hardly simultaneouslyremoved with formic acid. The study of simultaneous removal of oxo-anions with formic acidprovided important reference for removal of oxo-anions contaminants directly from drinkingwater and minimization the secondary pollution of formic acid.（5） Photocatalytic removal rates of oxo-anions were compared. Halogen oxoanions weresorted as IO₃^->BrO₃^->ClO₃, and chlorates were sorted as ClO₂^->ClO₃^->ClO₄^-. Under acidiccondition, oxo-anions were sorted as BrO₃^->NO₂^->ClO₃^->NO₃^->Cr（VI）. The standardreduction potential, atomic radii, electron orbital configuration, electron affinity, and the bonddissociation energy were good indicators for the relative of reduction using photocatalysis.