Synthesis Of Supported Silver Orthophosphate Photocatalysis And Their Photocatalytic Performance In The Treatment Of Dye Wastewater

Recently, it has been shown that silver phosphate（Ag₃PO₄）semiconductor has exceptionally high and intriguing photocatalytic efficiency.Ag₃PO₄exhibits extremely high photocatalytic efficiency for organic dyedecomposition under visible light irradiation. The three supportedphotocatalysts were prepared by the wet chemical method: silverorthophosphate were immobilized on attapulgite clay （Ag₃PO₄/APT）、bentonite （Ag₃PO₄/Bentonite） and actived carbon fibers （Ag₃PO₄/ACF）respectively. The solution of azo dyes Orange II, was selected as thedegradation object in the study of its photocatalytic activity. Main results ofthis paper were as follows:（1） It can be clear seen that the composites exhibited higherphotocatalytic activities for the Orange II degradation than bare Ag₃PO₄undervisible light irradiation （the photocatalytic properties of Ag₃PO₄/ACF,Ag₃PO₄/bentonite and Ag₃PO₄/APT were increased32%、20%and10%respectively）. The enhanced photocatalytic properties can be attributed to twomain factors. First is the much smaller size and good dispersion of Ag₃PO₄immobilized on the carriers. Second is a higher adsorption capability for thecomposites compared with bare Ag₃PO₄.（2） The effect of catalysts amount on the photodegradation efficiency ofOrange II were investigated. The decolorization of dye wastewater Orange IIwere linearly increased with the increase in catalysts amount firstly. However,when the catalysts amount were more than the optimum dosage, a furtherincrease in catalysts amount led to a decrease in decolorization. The optimumdosage of the three composites （Ag₃PO₄/ACF，Ag₃PO₄/bentonite andAg₃PO₄/APT） were1.0g/L、2.0g/L and800mg/L respectively.（3） The effect of the initial pH values on the photodegradation efficiencyof Orange II were investigated. For Ag₃PO₄/APT, no matter in the acidsolution or in the basic solution, the degradation rate of dye wastewaterOrange II were decreased with the increasing of solution pH values, whichsuggesting the catalyst was more suitable for processing Orange II understrongly acidic and weak alkaline conditions. However, for the other two catalysts Ag₃PO₄/bentonite and Ag₃PO₄/ACF, the degradation rate of dyewastewater Orange II were decreased with the increasing of solution pHvalues, which suggesting these two catalysts were more suitable forprocessing Orange II under strongly acidic conditions.（4） Radical-trapping experiments were performed by using threedifferent chemicals to shed light on the predominant reactive oxygen species.The addition of hydroxyl radicals scavenger tert-butanol did no affect thedegradation rate of Orange II. The addition of superoxide radicals scavengerbenzoquinone were indeed inhibited the degradation rate in some degree. Thedegradation rate of Orange II by Ag₃PO₄/ACF, Ag₃PO₄/bentonite andAg₃PO₄/APT were decreased36%、40%and32%respectively. The presenceof holy scavenger disodium ethylenediaminetetraacetate caused theinactivation of photocatalysts. To sum up, the high photocatalytic efficient ofcatalysts were attribute to the direct photoinduced holes, rather thansuperxodie radical or hydroxyl radical.（5） It can be observed that after four cycling runs of photodegradation ofOrange II, the photocatalytic activity of the three composites did show somesignificant loss: the catalytic degradation rate of Ag₃PO₄/ACF were99%、96%、82%and59%; the catalytic degradation rate of Ag₃PO₄/bentonite were85%、74%、62%and57%; the catalytic degradation rate of Ag₃PO₄/APTwere98%、90%、80%and57%.（6） The decrease in photocatalytic efficiency of the composites may beexplained in terms of silver reduction （Ag+1-Ag0） during photodegradationpocesses. In this article, the oxidant H2O2was used to re-oxidized theelemental Ag to Ag₃PO₄successfully.