Study On Cr(?)Reduction By Carbon Dioxide Anion Radical Based On UV-Acticated Small Molecule Organic Acid

Hexavalent chromium is mainly derived from the discharge of industrial activities such as electroplating,metallurgy and tanning.The high mobility makes Cr(?)easier to accumulate in organisms and human bodies,causing serious damage to the ecological environment and human health.The current treatment methods for chromium-containing wastewater mainly include chemical method,physicochemical method and biological method,etc.However,a large number of practices have shown that these treatment methods still have problems such as high treatment cost,practical operation difficulty and serious secondary pollution.In this paper,three different advanced reduction processes based on carbon dioxide anion radical were established,and the reduction efficiency of Cr(?)and influencing factors in different advanced reduction systems were investigated.The specific research results are as follows:(1)The UV-activated persulfate/formic acid system was used to reduce Cr(?)in water.The active free radicals generated in the system were identified by electron spin resonance technology.The activation mechanism of the system and the reduction of Cr(?)were analyzed.The effects of persulfate dosages,initial pH,humic acid,inorganic anions(Cl-,HCO3-,NO3-)and the initial concentration of Cr(?)on removal of Cr(?)were systematically investigated.The result showed that when the initial concentration of Cr(?)was 200 mg·L-1,the concentration of sodium persulfate and formic acid were 20 mmol L-1 and 40 mmol·L-1 respectively,the initial pH was 2.4(without the adjustment)and the Cr(?)was almost completely reduced within 50 min.In addition,the reduction rate of Cr(?)increased with the increase of persulfate concentration.Under acidic conditions,the system was with the highest reduction efficiency of Cr(?).The reduction efficiency decreased significantly with the increase of pH.Further studies revealed that Cl-,HCO3-and NO3-inhibited the reduction of Cr(VI),the degree of inhibition was HCO3->NO3->Cl-,and humic acid was also an inhibition factor for the removal of Cr(?)at the same concentration.Ultraviolet activated persulfate/formic acid system was well fitted with the zero-order kinetic equation,and its kinetic constant is 78.467?M·min-1.(2)The UV-activated formic acid system was used to reduce Cr(?)in water.The activation mechanism of the system and the reduction of Cr(?)were studied by electron spin resonance technique and free radical quenching experiment.Moreover,the main influencing factors,including initial Cr(?)concentration,dosage of formic acid,initial pH,reaction temperature,organic containments and common anions were also discussed.The experimental results showed that percent Cr(?)removal of 100%was obtained in 60min when the initial concentration of formic acid and Cr(?)was 40mmol/L and 10mg/L,respectively.It was further revealed that percent Cr(?)removal efficiency increased with the increase of formic acid concentration and reaction temperature,and appeared to be an acid-pH-favoured condition(pH?3.5).NO3-could remarkably promote the reduction of Cr(VI),while Cl-,HCO3-showed an inhibitory effects.From the fitting results of the removal kinetics data at different temperatures,it is clearly exhibited that the reduction of Cr(?)by the UV-activated formic acid system followed the pseudo-first order reaction kinetics after 40min.Arrhenius activation energy Ea were calculated as 1 5.9kJ/molaccording to the rate constants k at different temperatures.(3)The UV-activated Cu?(cit)system was used to reduce Cr(?)in water.The main free radicals components in the system were identified by contrast test and electron spin resonance technology.The decomplexation mechanism of Cu?(cit)and the reduction of Cr(?)were analyzed.The effects of Cu?(cit)dosage,initial pH,initial concentration of Cr(?)and inorganic anions(Cl-,NO3-)on removal of Cr(?)were systematically investigated.The result showed that percent Cr(?)removal of 100%was obtained in 80min when the initial concentration of Cr(?)and Cu?(cit)was 20mg/L and 16mg/L,respectively.what's more,total chromium and total copper in the effluent were not detected via adding alkali and precipitation(pH=10.0),The effluent can meet the maximum allowable emission concentration of total chromium and Cr(?)and the limit of the primary copper primary emission standard specified in the "Integrated Wastewater Discharge Standard"(GB 8978-1996).Increasing the concentration of Cu?(cit)in UV/Cu?(cit)system can significantly improve the reduction efficiency of Cr(VI).Under acidic conditions(pH<4.0),the system was with the highest reduction efficiency of Cr(VI).The reduction efficiency decreased significantly with the increase of pH.When the reaction time is within 40 min,it is found that the presence of Cl-can promote the decomplexation of Cu?(cit),thereby increasing the removal rate of total chromium and total copper in water.NO3-simultaneously inhibits the reduction of Cr(?)and the decomplexation of Cu?(cit),resulting in a significant decrease in the removal rate of total chromium and total copper.