| In the leather industry,chrome shavings are classified as hazardous waste,commonly containing between 3%and 5%trivalent chromium and roughly 90%collagen.These wastes are primarily recycled through hydrolysis to produce either industrial gelatin or protein.However,the uncertainty associated with the distribution of chromium in collagen fibers hinders efficient separation of collagen and chromium during actual production,while the molecular weight of the hydrolysis products is difficult to regulate.Therefore,technology that enables effective dechroming of chrome shavings and regulation of the molecular weight of hydrolyzed collagen is crucial in surmounting bottlenecks encountered in the resource utilization process.The present study investigates the effects of NaOH,H2SO4,and sodium citrate,both individually and in combination,on the leaching of chrome shavings.By examining the removal rate of chromium and changes to the structure of collagen,the study aims to explore the intrinsic connection between chromium removal and structural changes in the leather’s collagen.Through a detailed analysis of the molecular weight distribution of hydrolyzed collagen during NaOH and CaO hydrolysis of the chrome shavings,the study furthermore utilizes several characterization techniques,namely Final Integrated Technology Reviews(FITR),X-Ray Diffraction(XRD),Circular Dichroism(CD),and Scanning Electron Microscope(SEM),to identify and describe the conformational changes in collagen molecules that occur during hydrolysis.The study’s results are subsequently presented and analyzed in detail.1)The present study employed NaOH,H2SO4,and sodium citrate as chromium leaching agents.By comparing their chromium removal efficacy,it was found that sodium citrate had the highest leaching rate at 50%,while the cooperative application of NaOH and H2SO4 increased the leaching rate to 91.6%.Furthermore,additional leaching with sodium citrate led to a maximum chromium leaching rate of 97.96%.After alkali-acid leaching,the thermodynamic properties and spatial conformation of collagen molecules within the chrome shavings did not undergo significant changes.However,partial destruction of collagen fibers on some surfaces was observed,resulting in fiber-to-fiber average pore size increasing from 4.863 μm to 63.528 μm,while pores smaller than 10 μm disappeared.The resulting pore closure effect further hinders the leaching of chromium.During the chromium leaching process with sodium citrate,complex cross-linking occurred between sodium citrate,"sodium citrate-chromium," and collagen,causing incomplete separation of chromium from collagen and its stable existence in collagen fibers.2)The study compared the hydrolysis processes of NaOH at room temperature to CaO at elevated temperatures.The results indicated that hydrolyzed collagen yield achieved using NaOH at room temperature was only 14%,with residual chromium content remaining as high as 500 mg·Kg-1.Conversely,CaO hydrolysis resulted in hydrolyzed collagen yield of up to 57%,while residual chromium content decreased to as low as 12 mg·Kg-1.During NaOH hydrolysis,molecular weight distribution of hydrolyzed collagen exhibited poor control,whereas under CaO hydrolysis,regular improvements of molecular weight distribution of hydrolyzed collagen were observed.CaO concentration was found to be a critical factor in such improvement.Further research indicated that chromium in hydrolyzed collagen was primarily stabilized in the form of complexed chromium ions[Cr(H2O)n]3+and small molecule ligands.However,re-complexation of the removed chromium in the ionic state with small molecule collagen fragments was unavoidable..3)After the chrome shavings were hydrolyzed by strong alkali in NaOH at room temperature,the triple-helix structure integrity of collagen molecules was stabilized at about 0.85,while the FITR and CD spectral data showed that the secondary structure of collagen molecules was still dominated by α-helical andβ-folded structures,accounting for 60.81%and 70%,respectively.The hydrolysis process did not significantly change the collagen structure,and the hydrolysis occurred layer by layer along the collagen fiber hierarchy from outside to inside,which did not affect the spatial conformation within the collagen molecules.Therefore,there is no obvious regularity in the change of molecular weight of hydrolyzed collagen with time.The triple helix integrity of the skin collagen molecule decreased from 0.8571 to 0.5573 after the CaO hydrolysis,while the FITR and CD spectral data showed that the α-helix in the secondary structure of the collagen molecule decreased from 34.06%and 54%to 23.26%and 30%.This indicates that the warming hydrolysis under the solubilization effect of CaO can induce simultaneous thermal hydrolysis inside and outside the collagen fibrils and gradual deconvolution of the triple helix structure.Therefore,the molecular weight distribution of hydrolyzed collagen is stable and well controllable.In summary,temperature is the key factor to determine whether the hydrolysis process can achieve synchronous reaction,while the concentration of weakly basic CaO is the key to control the molecular weight distribution of hydrolyzed collagen.Under the premise of synchronous hydrolysis inside and outside the collagen molecule,the degree of hydrolysis of collagen can be effectively regulated by using different concentrations of weakly basic CaO.When controlling 100℃ and 2%concentration of CaO,the hydrolysis products were mainly large-molecule gelatin-like mixture,accounting for 88.8%and 99.53%dechromium removal;when controlling 100℃ and 20%concentration of CaO,the hydrolysis products were mainly small-molecule polypeptide mixture,accounting for 65.3%and 99.96%dechromium removal.The results of this study provide an important technical basis for the efficient dechromium removal from chromium-containing leather scraps and the synchronous control of the hydrolyzed collagen molecular weight distribution for the resource extraction technology. |
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