Effect Of Chemicals Used In Tanning And Post-tanning Processes On Chrome Precipitation

Chromium-containing wastewater is usually discharged from tanning andpost-tanning processes which include tanning, retanning, neutralization, filling,fatliquoring, dyeing and other processes. And tanning, retanning processesadded chromium, while chrome of other processes are mainly released from theblue skins. Different chemicals added to each section make the morphology ofchromium more complex, which makes it difficult to remove chromium stablyby conventional precipitation. In this study, organic chemicals from chrometanning, retanning and dyeing sections was chose to investigate their interactionand precipitation behavior with chrome. Organic compounds used in theexperiment were:1representative masking agents formic and citric acid fromtanning section, the hydrolyzate of collagen-gelatin and the broad-spectrumchelator disodium ethylene diamine tetraacetate(EDTA);2the new retannagevinyl polymer/nano-ZnO composite tanning agent (PDM/ZnO-I and PDM/ZnO-II) from retanning section;36kinds of leather dyes and auxiliaries fromdyeing section.(1) Effect of organics from tanning section on precipitation behavior ofCr(III)Formic acid, citric acid and gelatin were chose in this study withUV-visible spectroscopy and gel permeation chromatography(GPC) onidentifying changes of complexes under different pH conditions, and thermalgravimetric analysis on corresponding alkali precipitates. The results showedthat: three organic ligands could complexed with Cr(III) at different degrees; thestability of the complexes formed were quite different, while the stability andalkali resistance of the complexes were similar in terms of sequence: citricacid> gelatin>formic acid. The complexation of Cr(III) with masking agentcontaining hydroxy carboxylic group makes it difficult to remove chromium inleather wastewater. Furthermore, the efficiency of precipitation would be affected by gelatin hydrolysate as well; meanwhile, these organic ligands couldcoprecipitate with Cr(III) during alkali precipitation.To obtain the complex law of small molecule organic chemicals andchromium, EDTA was selected as the reference. Equimolar series methodcombined with UV spectroscopy were used to study the coordination relationsbetween EDTA and Cr(III) and its impact on alkali precipitation behavior ofCr(III) under100mg·L-1Cr(III). It was suggested that: interference on thealkaline precipitation significantly enhanced with the increased amount ofEDTA. Additionally, EDTA and Cr(III) could form stable complexes at ligandratio of1:1; Based on that, UV-visible spectroscopy of n(Cr:EDTA)=1:1underdifferent pH conditions and the effluent of Cr(III) and Cr-EDTA through anionand cation resin were analyzed. It is well known that absorption peaks occurredvarying degrees of redshift with the increase of pH，and the stability ofcomplexes decreased as well. The spectra of effluent of Cr(III), Cr-EDTA weresignificantly different, which conclusively proving that the complexes actuallyexisted and interfered the precipitation of Cr(III) seriously.(2) Effect of organics from post-tanning processes on precipitation of Cr(III)We investigated the interaction between Cr(III), the concentration of whichwas ranged from0-120mg·L-1, and PDM/ZnO-I and PDM/ZnO-II at industryamount. And the impact of PDM/ZnO-II addition on the stability of Cr(III) wasalso analyzed in this work. The results showed that: the different preparationmethods of PDM/ZnO-I and PDM/ZnO-II led to the differences of uniformity,stability and particle size distribution between the two tanning agents. Theprecipitate formed during the reaction between Cr(III) and them, however, thedeposit amount of chromium were quite different. The ratio PDM/ZnO-II andCr(III) and PH value had a great influence on the stability of Cr(III). Theremoval efficiency for Cr(III) was initially increased and then decreased withincreasing of PDM/ZnO-II amount, and reached to its maximum50%when VII=0.5mL; the removal rate chromium was improved with increasing of pH.The interactive effect on alkali precipitation behavior of100mg·L-1Cr(III)by leather chemical materials used in the dyeing process was studied, while theadding proportion of the volume of which was2%. Classified the leatherchemical materials according to the appearance and function result of Cr(III) and them, as well as the effect on alkali precipitation behavior. Finally discussedmechanism of action between the leather chemical materials acrylic polymers,fats, oils and Cr(III).All results indicated that the interaction between the chemicals andchromium in tannery processes have the impacts on precipitation of chromium,whereas the technique for removing chromium in wastewater need to optimizeacording to different effects.