Study On The Complexation And Removal Behavior Of Polymer-Cr In Leather Retanning Wastewater

The combination of vegetable and chrome tanning is widely used in leather industry,which makes chrome and polymer tanning agents exist in the form of complex in retanning wastewater.The stable "polymer-Cr" complex has also become a major problem of Cr emission reduction.Up to now,there are few reports about the complexation behaviour,transformation and removal of"polymer-Cr".In this study,the stability and removal rule of typical chemicals and Cr in retanning process were investigated.Meanwhile,the complex structure of tannic acid with Cr(Ⅲ)was studied.The distribution characteristics of TOC/Cr were analyzed by the method of "microfiltration+ultrafiltration".Furthermore,the mechanism of alkali precipitation and Fenton oxidation of tannin-Cr(Ⅲ)complex were discussed in detail.The main contents of this research are as follows:1)The polymer tanning agents such as DHN,RST and NHK were selected for simulation,and the removal of TOC and Cr after alkali precipitation,coagulation precipitation and Fenton oxidation were investigated separately.The results showed that Cr was still stable in solution due to the complexation with polymer tanning agents after alkali precipitation.Coagulation precipitation could remove Cr from the solution effectively,but the sludge was rich in organic matter and Cr,which leads to high treatment cos.The removal rate of TOC by Fenton oxidation was higher than coagulation precipitation,but the residual Cr concentration was still about 10 mg·L-1 after oxidation.2)Tannic acid,the main component of vegetable tanning agent,was used as the polymer to complex with Cr(Ⅲ).It was found that TOC of tannic acid was mainly distributed in the range of particle size<1 KDa,which was due to the hydrolysis of tannic acid.Meanwhile,TOC and Cr of tannin-Cr(Ⅲ)were mainly distributed in the range of larger particle size(0.1 μm～100 KDa and 100 KDa～50 KDa).For tannin-Cr(Ⅲ)complex,the removal rate of Cr was the highest when the pH is 7～8,The residual TOC and Cr were concentrated in the range of 0.1μm～100 KDa and 100 KDa～50 KDa after alkali precipitation.3)At the optimal condition of Fenton oxidation,the residual concentrations of TOC and Cr in the solution were 46.82 mg·L-1 and 23.04 mg·L-1,respectively,It was worth noting that the formation and accumulation of Cr(Ⅵ)were negligible.Additionally,TOC and Cr were mainly distributed in the range of 0.45 μm-0.22 μm and<1 KDa.Combined with the combination spectrum analysis,it was found that a highly conjugated product with complex structure were formed during the oxidation process of tannin-Cr(Ⅲ).Meanwhile,there were a lot of C=O,C-O,-OH and other functional groups in the oxidation products,which showed a chemical bond with Cr.Finally,the removal path of tannin-Cr(Ⅲ)complex during Fenton oxidation was proposed.Firstly,·OH radicals which generated by Fenton’s reagent transformed Cr(Ⅲ)into Cr(Ⅵ).Then the intermediate of tannic acid products could reduce the oxidized Cr(Ⅵ)to Cr(Ⅲ)and complex with Cr(Ⅲ)again,which hindered the removal of Cr.