Study On Photocatalytic Reduction Of Cr （Ⅵ） By TiO₂ And Ag/TiO₂ Under Visible Light And Its Mechanism

In recent years,the ecological environmental danger caused by wastewater containing Cr（Ⅵ）has become worse and worse with the rapid development of the chemical industry,so it is urgent for us to hunt for a green energy-saving and efficient method for Cr（Ⅵ）.TiO2 and Ag/TiO2 photocatalysts were prepared and characterized firstly,and photocatalytic reduction for Cr（Ⅵ）were investigated,and lastly reduction mechanism for Cr（Ⅵ）with TiO2 and Ag/TiO2 was studied,respectively.Photocatalytic reduction experimental conditions including photocatalyst dosage,initial p H of solution,initial concentration of K2Cr2O7,reaction atmosphere and photogenerated hole sacrificial agent were analyzed,and life time of photocatalysts were tested.Photocatalytic reduction kinetics and reaction rate constant were established and calculated.Experimental results display that the optimal preparation conditions for TiO2 photocatalyst are as follows: ethanol: tetrabutyl titanate is 56.60:1,p H is 3.86,calcination temperature is 622.45 ℃,calcination time is 56.67 min,and the optimum experimental conditions for Ag/TiO2 photocatalyst are as follows: ethanol: tetrabutyl titanate is 56.60:1,Ag: Ti value is 2.0 at.% to 3.0 at.%,p H is 4,calcination temperature is 400 ℃ and calcination time 1 h.The characterization results show that the prepared TiO2 photocatalyst is anatase crystal phase,particle size is about 500 nm and absorption bandgap is about 387 nm.TEM displays that Ag is doped on the surface of TiO2 and particle size is about 8 nm,and redshift occurs on Ag/TiO2 photocatalytst and its absorption bandgap is about 420 nm.Under the optimal conditions,removal for Cr（Ⅵ）is 75 % and 100 % in TiO2 and Ag/TiO2 photocatalytic reduction in visible light irradiation,respectively.Apparent reaction rate constants ranges with reaction conditions.It can be expressed by the expressionk=2.994×10-8C0-1.4259n0.5415CH+-0.1889 when TiO2 dosage is lower than 2 g/L,p H value is higher than 4.And it can be expressed by the following expression:k=5.983×10-7C0-1.4259n0.5415CH+0.1336 when TiO2 dosage is lower than 2 g/L,p H value is higher than 4.And it can be expressed by the following expression: k=5.808×10-8C0-1.4259n-0.3429CH+-0.1889 when TiO2 dosage is more than 2 g/L,the p H value is higher than 4.And it can be expressed by the following expression: k=1.161×10-6C0-1.4259n-0.3429CH+0.1336 when TiO2 dosage is more than 2 g/L,the p H value is higher than 4.Apparent reaction rate constants also change with reaction conditions in Ag/TiO2 photocatalytic reduction.It can be expressed by the expression k=7.089×10-7C0-1.473n1.1605CH+-0.1568 when Ag/TiO2 dosageis less than 2 g/L and the p H value is lower than 4.And it can be expressed by the expression k=8.690×10-6C0-1.473n1.1605CH+0.1542 when Ag/TiO2 dosageis less than 2 g/L and the p H value is higher than 4.And it can be expressed by the expression k=2.014×10-6C0-1.473n-0.1916CH+0.1568 when Ag/TiO2 dosage is more than 2 g/L and p H value is lower than 4.And it can be expressed by the expression k=2.469×10-5C0-1.473n-0.1916CH+0.1542 when Ag/TiO2 dosage is more than 2 g/L and p H value is higher than 4.Photocatalytic reduction mechanism for Cr（Ⅵ）by TiO2 and Ag/TiO2 was analyzed,respectively.A new composite HCrO4-/TiO2 catalyst forms in TiO2 and Cr（Ⅵ）homogenerous system.HCr O4-/TiO2 produces photoinduced electrons in visible light irradiation,causing to the reduction for Cr（Ⅵ）to Cr（III）.However,redshif of Ag/TiO2 is more overwhelming than that of TiO2,and at the same time,the doping of Ag on TiO2 improves the transfer of electrons,inhibiting the recombination of photoinduced carriers and increasing photocatalytic reduction efficiency.